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Assistance with flat field calibration

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  • jerry2kam
    Folks, I am in need of a bit of guidance/advice on solving an issue I run into when calibrating my astro images. Most of the images I take with my C14 SCT
    Message 1 of 20 , May 25, 2013
      Folks,

      I am in need of a bit of guidance/advice on solving an issue I run into when calibrating my astro images. Most of the images I take with my C14 SCT generally have a very noticeable central hot spot. I assume that this anomaly is caused by the SCT's central obstruction.

      Issue: When I calibrate my images using CCDStack the central hot spot is not fully removed. This is particularly noticeable in the RGB images.

      Optical setup: C14/Optec 3"FR/Apogee F16m-Astrodon GenII LRGBHa filters
      Images taken using MaximDL; Calibration with CCDStack; Final adjustments with PS5

      I take flat fields using a light box, and my target ADU for flats is 20K-25K.

      Guidance as to how I can resolve this problem would be most appreciated.

      Thanks
      Jerry K
    • Bernard Miller
      Jerry, If you have a 16-bit camera, you can try boosting the target ADU to somewhere between 30K-40K. That should still be in the linear range of the camera.
      Message 2 of 20 , May 26, 2013
        Jerry,



        If you have a 16-bit camera, you can try boosting the target ADU to
        somewhere between 30K-40K. That should still be in the linear range of the
        camera. You may also want to post a picture to help people see the problem.
        A picture is worth a thousand words.



        Bernard





        From: ccd-newastro@yahoogroups.com [mailto:ccd-newastro@yahoogroups.com] On
        Behalf Of jerry2kam
        Sent: Saturday, May 25, 2013 1:42 PM
        To: ccd-newastro@yahoogroups.com
        Subject: [ccd-newastro] Assistance with flat field calibration






        Folks,

        I am in need of a bit of guidance/advice on solving an issue I run into when
        calibrating my astro images. Most of the images I take with my C14 SCT
        generally have a very noticeable central hot spot. I assume that this
        anomaly is caused by the SCT's central obstruction.

        Issue: When I calibrate my images using CCDStack the central hot spot is not
        fully removed. This is particularly noticeable in the RGB images.

        Optical setup: C14/Optec 3"FR/Apogee F16m-Astrodon GenII LRGBHa filters
        Images taken using MaximDL; Calibration with CCDStack; Final adjustments
        with PS5

        I take flat fields using a light box, and my target ADU for flats is
        20K-25K.

        Guidance as to how I can resolve this problem would be most appreciated.

        Thanks
        Jerry K





        [Non-text portions of this message have been removed]
      • Stan
        ... All scopes are brightest at the center, in part because any aperture presents less area for collecting off-axis (slanted) rays. Central obstruction does
        Message 3 of 20 , May 26, 2013
          --- "jerry2kam" <jpkam2@...> wrote:
          > ... C14 SCT generally have a very noticeable central hot spot.
          > I assume that this anomaly is caused by the SCT's central
          > obstruction.

          All scopes are brightest at the center, in part because any aperture presents less area for collecting off-axis (slanted) rays. Central obstruction does not produce a hot spot. But I've observed such artifacts due to reducers, likely due to lens reflections. Filters can increase reflections.

          Such reflections usually flat out for the most part. But a significant difference between image and flat lighting scope and camera conditions can leave a residual defect. So evaluate your flat acquisition techniques. If you use a light box or "t-shirt" then verify it via twilight flats (flat the artificial flat with the sky flat). Always take separate flats for each filter and camera orientation. The camera should be in focus. Expose within the linear zone of the camera.

          > When I calibrate my images using CCDStack the central hot spot
          > is not fully removed.

          Both the image and the flat must be dark (or bias) subtracted; failing to do so can result in under or over correction. Confirm the master flat by using a single flat frame to (instead of the master) to observe any significant differences in a calibrated image.

          Stan
        • jerry2kam
          Hi Bernard, Thanks for the feedback. As you suggest, I am in the process of creating a set of test flats that cover a wide range of ADU values. I ll see how
          Message 4 of 20 , May 26, 2013
            Hi Bernard,

            Thanks for the feedback. As you suggest, I am in the process of creating a set of test flats that cover a wide range of ADU values. I'll see how that goes.

            After posting my query, I went back into the message archives and found several other posts that touch on this same issue. Most related to using and setting up lightboxs. The sense I gathered from these discussions is that obtaining flats, that will calibrate well, can be a real challenge.

            That said, it begs the question, If calibrating images does leave artifacts is there a general consensus on how to use processing techniques to remove them. In my case, getting rid of the central hot spot artifact, is my primary goal


            Jerry K

            --- In ccd-newastro@yahoogroups.com, "Bernard Miller" <bgmiller011@...> wrote:
            >
            > Jerry,
            >
            >
            >
            > If you have a 16-bit camera, you can try boosting the target ADU to
            > somewhere between 30K-40K. That should still be in the linear range of the
            > camera. You may also want to post a picture to help people see the problem.
            > A picture is worth a thousand words.
            >
            >
            >
            > Bernard
            >
            >
            >
            >
            >
            > From: ccd-newastro@yahoogroups.com [mailto:ccd-newastro@yahoogroups.com] On
            > Behalf Of jerry2kam
            > Sent: Saturday, May 25, 2013 1:42 PM
            > To: ccd-newastro@yahoogroups.com
            > Subject: [ccd-newastro] Assistance with flat field calibration
            >
            >
            >
            >
            >
            >
            > Folks,
            >
            > I am in need of a bit of guidance/advice on solving an issue I run into when
            > calibrating my astro images. Most of the images I take with my C14 SCT
            > generally have a very noticeable central hot spot. I assume that this
            > anomaly is caused by the SCT's central obstruction.
            >
            > Issue: When I calibrate my images using CCDStack the central hot spot is not
            > fully removed. This is particularly noticeable in the RGB images.
            >
            > Optical setup: C14/Optec 3"FR/Apogee F16m-Astrodon GenII LRGBHa filters
            > Images taken using MaximDL; Calibration with CCDStack; Final adjustments
            > with PS5
            >
            > I take flat fields using a light box, and my target ADU for flats is
            > 20K-25K.
            >
            > Guidance as to how I can resolve this problem would be most appreciated.
            >
            > Thanks
            > Jerry K
            >
            >
            >
            >
            >
            > [Non-text portions of this message have been removed]
            >
          • Stan
            ... IMHO learn from mistakes and move on. But of course there is Photoshop (dodge and burn). Stan
            Message 5 of 20 , May 26, 2013
              --- "jerry2kam" <jpkam2@...> wrote:
              > If calibrating images does leave artifacts is there a general
              > consensus on how to use processing techniques to remove them...

              IMHO learn from mistakes and move on.
              But of course there is Photoshop (dodge and burn).

              Stan
            • JoeMize
              Jerry, the following is what I gleaned from Ron s 2002 The New CCD Astronomy book. Don t try a hit or miss technique making Flats, this formula has worked
              Message 6 of 20 , May 27, 2013
                Jerry, the following is what I gleaned from Ron's 2002 "The New CCD Astronomy" book. Don't try a 'hit or miss' technique making Flats, this formula has worked very well for me many years.
                Source: "The New CCD Astronomy", Ron Wadaski, 2002, pg. 263-264.

                pg-263: Find Saturation value by dividing Full Well by eGain.
                pg-264: Aim at a Flat Brightness 35-50% of Saturation.
                Derived formula:

                Full Well / eGain x %Brightness = Flat ADU Target.

                a.. Obtain Full Well from manufacturer specifications.
                b.. Obtain eGain from FITS Header, Not manufacturer specifications.
                a.. 'Each' chip is different even within the same camera model
                c.. %Brightness - recommended 35-50% of Saturation Value.
                a.. I chose 42.74%, or (0.4274), the mid-point of the range.
                To determine if you are achieving your Flat ADU Target based upon your Exposure Time, using Maxim, open the Information Panel, set to Area, and watch the Average field for each Flat.

                I use "SkyFlats Assistant", a MaximDL Plug-In, for making Flats using my self-built ELP. SkyFlats does an excellent job, don't forget to change Filters in Maxim before starting the next Flat sequence. HTH...joe :) http://winfij.homeip.net/maximdl/skyflats.html


                "May You Go Among The Imperishable Stars"
                Joe Mize www.cav-sfo.com
                Chiefland Astronomy Village (CAV), Fla
                StarFields Observatory, (SFO).

                ----- Original Message -----
                From: "jerry2kam" <jpkam2@...>
                To: <ccd-newastro@yahoogroups.com>
                Sent: Sunday, May 26, 2013 2:57 PM
                Subject: [ccd-newastro] Re: Assistance with flat field calibration


                > Hi Bernard,
                >
                > Thanks for the feedback. As you suggest, I am in the process of creating a set of test flats that cover a wide range of ADU values. I'll see how that goes.
                >
                > After posting my query, I went back into the message archives and found several other posts that touch on this same issue. Most related to using and setting up lightboxs. The sense I gathered from these discussions is that obtaining flats, that will calibrate well, can be a real challenge.
                >
                > That said, it begs the question, If calibrating images does leave artifacts is there a general consensus on how to use processing techniques to remove them. In my case, getting rid of the central hot spot artifact, is my primary goal
                >
                >
                > Jerry K
                >
                > --- In ccd-newastro@yahoogroups.com, "Bernard Miller" <bgmiller011@...> wrote:
                >>
                >> Jerry,
                >>
                >>
                >>
                >> If you have a 16-bit camera, you can try boosting the target ADU to
                >> somewhere between 30K-40K. That should still be in the linear range of the
                >> camera. You may also want to post a picture to help people see the problem.
                >> A picture is worth a thousand words.
                >>
                >>
                >>
                >> Bernard
                >>
                >>
                >>
                >>
                >>
                >> From: ccd-newastro@yahoogroups.com [mailto:ccd-newastro@yahoogroups.com] On
                >> Behalf Of jerry2kam
                >> Sent: Saturday, May 25, 2013 1:42 PM
                >> To: ccd-newastro@yahoogroups.com
                >> Subject: [ccd-newastro] Assistance with flat field calibration
                >>
                >>
                >>
                >>
                >>
                >>
                >> Folks,
                >>
                >> I am in need of a bit of guidance/advice on solving an issue I run into when
                >> calibrating my astro images. Most of the images I take with my C14 SCT
                >> generally have a very noticeable central hot spot. I assume that this
                >> anomaly is caused by the SCT's central obstruction.
                >>
                >> Issue: When I calibrate my images using CCDStack the central hot spot is not
                >> fully removed. This is particularly noticeable in the RGB images.
                >>
                >> Optical setup: C14/Optec 3"FR/Apogee F16m-Astrodon GenII LRGBHa filters
                >> Images taken using MaximDL; Calibration with CCDStack; Final adjustments
                >> with PS5
                >>
                >> I take flat fields using a light box, and my target ADU for flats is
                >> 20K-25K.
                >>
                >> Guidance as to how I can resolve this problem would be most appreciated.
                >>
                >> Thanks
                >> Jerry K
                >>
                >>
                >>
                >>
                >>
                >> [Non-text portions of this message have been removed]
                >>
                >
                >
                >
                > ------------------------------------
                >
                >
                >
                >
                >

                [Non-text portions of this message have been removed]
              • Stan
                ... http://www.dpreview.com/articles/2268382985/photoshop-gradient-tool-part-2-adjusting-images
                Message 7 of 20 , May 27, 2013
                • Larry Phillips
                  I have some friends who made homemade light boxes and had problems with flats. When they took sky flats the problem went away. It was decided that the light
                  Message 8 of 20 , May 28, 2013
                    I have some friends who made homemade light boxes and had problems with flats. When they took sky flats the problem went away. It was decided that the light box construction did not provide a flat field.

                    Larry

                    --- In ccd-newastro@yahoogroups.com, "jerry2kam" <jpkam2@...> wrote:
                    >
                    >
                    > Folks,
                    >
                    > I am in need of a bit of guidance/advice on solving an issue I run into when calibrating my astro images. Most of the images I take with my C14 SCT generally have a very noticeable central hot spot. I assume that this anomaly is caused by the SCT's central obstruction.
                    >
                    > Issue: When I calibrate my images using CCDStack the central hot spot is not fully removed. This is particularly noticeable in the RGB images.
                    >
                    > Optical setup: C14/Optec 3"FR/Apogee F16m-Astrodon GenII LRGBHa filters
                    > Images taken using MaximDL; Calibration with CCDStack; Final adjustments with PS5
                    >
                    > I take flat fields using a light box, and my target ADU for flats is 20K-25K.
                    >
                    > Guidance as to how I can resolve this problem would be most appreciated.
                    >
                    > Thanks
                    > Jerry K
                    >
                  • JoeMize
                    Larry, very good input. When I made my first LED Lightbox I had the same problem. Researching why I came across the Royal Greenwich Observatory s PDF about
                    Message 9 of 20 , May 28, 2013
                      Larry, very good input. When I made my first LED Lightbox I had the same
                      problem. Researching why I came across the Royal Greenwich Observatory's
                      PDF about their high-end Lightbox which in Section-3 discusses proper
                      spacing of the light source to the diffuser. If spacing is incorrect the
                      light wavefront when striking the diffuser will not be correct...joe :)

                      http://www.ing.iac.es/Astronomy/observing/manuals/ps/tech_notes/tn108.pdf

                      "May You Go Among The Imperishable Stars"
                      Joe Mize www.cav-sfo.com
                      Chiefland Astronomy Village (CAV), Fla
                      StarFields Observatory, (SFO).

                      ----- Original Message -----
                      From: "Larry Phillips" <llp41astro@...>
                      To: <ccd-newastro@yahoogroups.com>
                      Sent: Tuesday, May 28, 2013 9:49 AM
                      Subject: [ccd-newastro] Re: Assistance with flat field calibration


                      >I have some friends who made homemade light boxes and had problems with
                      >flats. When they took sky flats the problem went away. It was decided
                      >that the light box construction did not provide a flat field.
                      >
                      > Larry
                      >
                      > --- In ccd-newastro@yahoogroups.com, "jerry2kam" <jpkam2@...> wrote:
                      >>
                      >>
                      >> Folks,
                      >>
                      >> I am in need of a bit of guidance/advice on solving an issue I run into
                      >> when calibrating my astro images. Most of the images I take with my C14
                      >> SCT generally have a very noticeable central hot spot. I assume that this
                      >> anomaly is caused by the SCT's central obstruction.
                      >>
                      >> Issue: When I calibrate my images using CCDStack the central hot spot is
                      >> not fully removed. This is particularly noticeable in the RGB images.
                      >>
                      >> Optical setup: C14/Optec 3"FR/Apogee F16m-Astrodon GenII LRGBHa filters
                      >> Images taken using MaximDL; Calibration with CCDStack; Final adjustments
                      >> with PS5
                      >>
                      >> I take flat fields using a light box, and my target ADU for flats is
                      >> 20K-25K.
                      >>
                      >> Guidance as to how I can resolve this problem would be most appreciated.
                      >>
                      >> Thanks
                      >> Jerry K
                      >>
                      >
                      >
                      >
                      >
                      > ------------------------------------
                      >
                      >
                      >
                      >
                      >
                    • Stan
                      ... Thanks for that link. Robust light box design and construction is not simple. Stan
                      Message 10 of 20 , May 28, 2013
                        --- "JoeMize" <jmize@...> wrote:
                        > http://www.ing.iac.es/Astronomy/observing/manuals/ps/tech_notes/tn108.pdf

                        Thanks for that link.
                        Robust light box design and construction is not simple.

                        Stan
                      • tpiccian
                        I made a thin EL panel lightbox that I ve tested to make sure the image is OK. I have yet to test it in the field or apply it to images though. The image was
                        Message 11 of 20 , May 28, 2013
                          I made a thin EL panel lightbox that I've tested to make sure the image is OK. I have yet to test it in the field or apply it to images though. The image was bright in the center and faded at the corners.

                          If you have Facebook, look for my light box at: https://www.facebook.com/TomsAstronomyStuff

                          Tom P.

                          --- In ccd-newastro@yahoogroups.com, "jerry2kam" <jpkam2@...> wrote:
                          >
                          >
                          > Folks,
                          >
                          > I am in need of a bit of guidance/advice on solving an issue I run into when calibrating my astro images. Most of the images I take with my C14 SCT generally have a very noticeable central hot spot. I assume that this anomaly is caused by the SCT's central obstruction.
                          >
                          > Issue: When I calibrate my images using CCDStack the central hot spot is not fully removed. This is particularly noticeable in the RGB images.
                          >
                          > Optical setup: C14/Optec 3"FR/Apogee F16m-Astrodon GenII LRGBHa filters
                          > Images taken using MaximDL; Calibration with CCDStack; Final adjustments with PS5
                          >
                          > I take flat fields using a light box, and my target ADU for flats is 20K-25K.
                          >
                          > Guidance as to how I can resolve this problem would be most appreciated.
                          >
                          > Thanks
                          > Jerry K
                          >
                        • Mark Striebeck
                          What is the best way to evaluate flat images? I usually take Tom s approach: if it s bright in the middle, fades out to the corners and shows some specks of
                          Message 12 of 20 , May 28, 2013
                            What is the best way to evaluate flat images? I usually take Tom's
                            approach: if it's bright in the middle, fades out to the corners and shows
                            some specks of dust, then I'm OK. Is there a more analytical approach?

                            MarkS


                            On Tue, May 28, 2013 at 10:19 AM, tpiccian <tpicciani@...> wrote:

                            > **
                            >
                            >
                            > I made a thin EL panel lightbox that I've tested to make sure the image is
                            > OK. I have yet to test it in the field or apply it to images though. The
                            > image was bright in the center and faded at the corners.
                            >
                            > If you have Facebook, look for my light box at:
                            > https://www.facebook.com/TomsAstronomyStuff
                            >
                            > Tom P.
                            >
                            >
                            > --- In ccd-newastro@yahoogroups.com, "jerry2kam" <jpkam2@...> wrote:
                            > >
                            > >
                            > > Folks,
                            > >
                            > > I am in need of a bit of guidance/advice on solving an issue I run into
                            > when calibrating my astro images. Most of the images I take with my C14 SCT
                            > generally have a very noticeable central hot spot. I assume that this
                            > anomaly is caused by the SCT's central obstruction.
                            > >
                            > > Issue: When I calibrate my images using CCDStack the central hot spot is
                            > not fully removed. This is particularly noticeable in the RGB images.
                            > >
                            > > Optical setup: C14/Optec 3"FR/Apogee F16m-Astrodon GenII LRGBHa filters
                            > > Images taken using MaximDL; Calibration with CCDStack; Final adjustments
                            > with PS5
                            > >
                            > > I take flat fields using a light box, and my target ADU for flats is
                            > 20K-25K.
                            > >
                            > > Guidance as to how I can resolve this problem would be most appreciated.
                            > >
                            > > Thanks
                            > > Jerry K
                            > >
                            >
                            >
                            >


                            [Non-text portions of this message have been removed]
                          • Stan
                            ... Take a proper sky flat and use it to flatten the light-box flat (be sure to dark or bias subtract both). Unless there are defects in the light-box then the
                            Message 13 of 20 , May 28, 2013
                              --- Mark Striebeck <mark.striebeck@...> wrote:
                              > What is the best way to evaluate flat images?

                              Take a proper sky flat and use it to flatten the light-box flat (be sure to dark or bias subtract both). Unless there are defects in the light-box then the result will be completely "flat" (no vignette or anything else).

                              Properly made sky flats are the "gold standard".

                              Stan
                            • redmondjohn48
                              Jerry, I have been researching the solution to the same basic problem with my C11. Though no expert, I ve found that flat fielding doesn t eliminate the hot
                              Message 14 of 20 , May 28, 2013
                                Jerry, I have been researching the solution to the same basic problem with my C11. Though no expert, I've found that flat fielding doesn't eliminate the hot spots. I found a discussion of the difference between hot spots and other sorts of problems that can be eliminated with flats in the book The Handbook of Astronomical Image Processing by Richard Berry and James Burnell. Ron W. mentions and recommends this book in his book.

                                The gist of it is that hot spots are caused by non-celestial light contamination and can't be addressed with flat frame calibration. His unwelcome advice is to correct the problem (frequently introduced in a focal reducer or t-adapter extension tube) at the source with baffles, flocking, etc.

                                I do wonder what others think about this conclusion. I'm not very gifted with manual dexterity, and don't look forward to messing around in this way.

                                John


                                --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@...> wrote:
                                >
                                > --- Mark Striebeck <mark.striebeck@> wrote:
                                > > What is the best way to evaluate flat images?
                                >
                                > Take a proper sky flat and use it to flatten the light-box flat (be sure to dark or bias subtract both). Unless there are defects in the light-box then the result will be completely "flat" (no vignette or anything else).
                                >
                                > Properly made sky flats are the "gold standard".
                                >
                                > Stan
                                >
                              • Ron Wodaski
                                Well, the one thing you cannot do is to evaluate it by looking at it. :-) The measure of a good flat is whether it captures the brightening and darkening of
                                Message 15 of 20 , May 28, 2013
                                  Well, the one thing you cannot do is to evaluate it by looking at it. :-)

                                  The measure of a good flat is whether it captures the brightening and darkening of the optical field. You have to know the brightening and darkening of the optical field to see if it's been corrected - so the only way to evaluate a flat is to apply it to an image and then evaluate the result.

                                  If it's a good flat, it will remove the brightening and darkening within a few percent of correct. A really good flat should have a gradient of < 3% in the background areas (even sky flats with small fields can easily be this far off for a variety of reasons), and should accurately remove shadows from obstructions (e.g., dust motes, cleaning residue, etc.).

                                  Such tests should be performed when it is known that there are no off-axis light sources during the image exposure - no moon, no light leaks in the telescope or camera, no porch lights, no clouds, etc.

                                  Ron Wodaski



                                  On May 28, 2013, at 10:21 AM, Mark Striebeck <mark.striebeck@...> wrote:

                                  > What is the best way to evaluate flat images? I usually take Tom's
                                  > approach: if it's bright in the middle, fades out to the corners and shows
                                  > some specks of dust, then I'm OK. Is there a more analytical approach?
                                  >
                                  > MarkS
                                  >
                                  >
                                  > On Tue, May 28, 2013 at 10:19 AM, tpiccian <tpicciani@...> wrote:
                                  >
                                  >> **
                                  >>
                                  >>
                                  >> I made a thin EL panel lightbox that I've tested to make sure the image is
                                  >> OK. I have yet to test it in the field or apply it to images though. The
                                  >> image was bright in the center and faded at the corners.
                                  >>
                                  >> If you have Facebook, look for my light box at:
                                  >> https://www.facebook.com/TomsAstronomyStuff
                                  >>
                                  >> Tom P.
                                  >>
                                  >>
                                  >> --- In ccd-newastro@yahoogroups.com, "jerry2kam" <jpkam2@...> wrote:
                                  >>>
                                  >>>
                                  >>> Folks,
                                  >>>
                                  >>> I am in need of a bit of guidance/advice on solving an issue I run into
                                  >> when calibrating my astro images. Most of the images I take with my C14 SCT
                                  >> generally have a very noticeable central hot spot. I assume that this
                                  >> anomaly is caused by the SCT's central obstruction.
                                  >>>
                                  >>> Issue: When I calibrate my images using CCDStack the central hot spot is
                                  >> not fully removed. This is particularly noticeable in the RGB images.
                                  >>>
                                  >>> Optical setup: C14/Optec 3"FR/Apogee F16m-Astrodon GenII LRGBHa filters
                                  >>> Images taken using MaximDL; Calibration with CCDStack; Final adjustments
                                  >> with PS5
                                  >>>
                                  >>> I take flat fields using a light box, and my target ADU for flats is
                                  >> 20K-25K.
                                  >>>
                                  >>> Guidance as to how I can resolve this problem would be most appreciated.
                                  >>>
                                  >>> Thanks
                                  >>> Jerry K
                                  >>>
                                  >>
                                  >>
                                  >>
                                  >
                                  >
                                  > [Non-text portions of this message have been removed]
                                  >
                                  >
                                  >
                                  > ------------------------------------
                                  >
                                  >
                                  >
                                  >
                                • Ron Wodaski
                                  I would say that that statement really isn t something that lends itself to opinion, in that it s simply a true statement. :-) As I alluded to briefly in my
                                  Message 16 of 20 , May 28, 2013
                                    I would say that that statement really isn't something that lends itself to opinion, in that it's simply a true statement. :-)

                                    As I alluded to briefly in my other email in this thread, off-axis light sources will contaminate both a flat and an image. So getting rid of them is very important.

                                    (Why can't you expect to simply subtract effects from off-axis sources? Typically, they are constantly changing with your telescope's position. A porch light is obvious: the angle to the optical axis is constantly changing. The porch light is still, but the scope moves, and so the result of that off-axis light (via light leaks, internal reflections, direct off-axis light paths, etc.) varies over time.)

                                    Such off-axis light is technically referred to as "unwanted signal." The noise from that unwanted signal is quite real, and reduces your signal to noise wherever it occurs. To an engineer, off-axis light is just part of instrument calibration: you track down the cause of each one, and you figure out how to eliminate it. Astronomy is mostly an engineering exercise, after all. ;-)

                                    Ron Wodaski



                                    On May 28, 2013, at 12:16 PM, redmondjohn48 <jredmond17@...> wrote:

                                    > Jerry, I have been researching the solution to the same basic problem with my C11. Though no expert, I've found that flat fielding doesn't eliminate the hot spots. I found a discussion of the difference between hot spots and other sorts of problems that can be eliminated with flats in the book The Handbook of Astronomical Image Processing by Richard Berry and James Burnell. Ron W. mentions and recommends this book in his book.
                                    >
                                    > The gist of it is that hot spots are caused by non-celestial light contamination and can't be addressed with flat frame calibration. His unwelcome advice is to correct the problem (frequently introduced in a focal reducer or t-adapter extension tube) at the source with baffles, flocking, etc.
                                    >
                                    > I do wonder what others think about this conclusion. I'm not very gifted with manual dexterity, and don't look forward to messing around in this way.
                                    >
                                    > John
                                    >
                                    >
                                    > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@...> wrote:
                                    >>
                                    >> --- Mark Striebeck <mark.striebeck@> wrote:
                                    >>> What is the best way to evaluate flat images?
                                    >>
                                    >> Take a proper sky flat and use it to flatten the light-box flat (be sure to dark or bias subtract both). Unless there are defects in the light-box then the result will be completely "flat" (no vignette or anything else).
                                    >>
                                    >> Properly made sky flats are the "gold standard".
                                    >>
                                    >> Stan
                                    >>
                                    >
                                    >
                                    >
                                    >
                                    > ------------------------------------
                                    >
                                    >
                                    >
                                    >
                                  • Robert Woodard
                                    Apologies if this has already been mentioned. This article from the SBIG site might be of some help with hot spots:
                                    Message 17 of 20 , May 28, 2013
                                      Apologies if this has already been mentioned. This article from the SBIG site might be of some help with hot spots:

                                      https://www.sbig.com/blog/flat-fields-the-ugly-truth/

                                      Woody

                                      On May 28, 2013, at 3:16 PM, "redmondjohn48" <jredmond17@...> wrote:

                                      >
                                      > The gist of it is that hot spots are caused by non-celestial light contamination and can't be addressed with flat frame calibration. His unwelcome advice is to correct the problem (frequently introduced in a focal reducer or t-adapter extension tube) at the source with baffles, flocking, etc.
                                      >



                                      [Non-text portions of this message have been removed]
                                    • Mark de Regt
                                      I just want to highlight what Stan and others have said: Light boxes, while sounding attractive because you don t have to wake up at dawn, often cause more
                                      Message 18 of 20 , May 28, 2013
                                        I just want to highlight what Stan and others have said:

                                        Light boxes, while sounding attractive because you don't have to wake up at
                                        dawn, often cause more problems than they solve.

                                        My experience with sky flats is that they rarely have problems; they're
                                        fairly easy to take, and they just work.

                                        There are software packages that automatically take sky flats.

                                        Mark

                                        -----Original Message-----
                                        From: ccd-newastro@yahoogroups.com [mailto:ccd-newastro@yahoogroups.com] On
                                        Behalf Of Stan
                                        Sent: Tuesday, May 28, 2013 7:48 AM
                                        To: ccd-newastro@yahoogroups.com
                                        Subject: [ccd-newastro] Re: Assistance with flat field calibration

                                        --- "JoeMize" <jmize@...> wrote:
                                        > http://www.ing.iac.es/Astronomy/observing/manuals/ps/tech_notes/tn108.pdf

                                        Thanks for that link.
                                        Robust light box design and construction is not simple.

                                        Stan



                                        ------------------------------------
                                      • redmondjohn48
                                        Ron, I figured you d say that! Anyway, I have just been evaluating a few flats and test images I took last night with the benefit of a new, well-fitting metal
                                        Message 19 of 20 , May 28, 2013
                                          Ron,
                                          I figured you'd say that! Anyway, I have just been evaluating a few flats and test images I took last night with the benefit of a new, well-fitting metal dew shield, and much of the hot spot so evident in all my previous efforts seems to have been eliminated.

                                          Also, thanks to Robert for the SBIG blog link. I hate to paint anything with Rustoleum, since I fear making an awful mess. But I guess it may be necessary.

                                          John

                                          --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
                                          >
                                          > I would say that that statement really isn't something that lends itself to opinion, in that it's simply a true statement. :-)
                                          >
                                          > As I alluded to briefly in my other email in this thread, off-axis light sources will contaminate both a flat and an image. So getting rid of them is very important.
                                          >
                                          > (Why can't you expect to simply subtract effects from off-axis sources? Typically, they are constantly changing with your telescope's position. A porch light is obvious: the angle to the optical axis is constantly changing. The porch light is still, but the scope moves, and so the result of that off-axis light (via light leaks, internal reflections, direct off-axis light paths, etc.) varies over time.)
                                          >
                                          > Such off-axis light is technically referred to as "unwanted signal." The noise from that unwanted signal is quite real, and reduces your signal to noise wherever it occurs. To an engineer, off-axis light is just part of instrument calibration: you track down the cause of each one, and you figure out how to eliminate it. Astronomy is mostly an engineering exercise, after all. ;-)
                                          >
                                          > Ron Wodaski
                                          >
                                          >
                                          >
                                          > On May 28, 2013, at 12:16 PM, redmondjohn48 <jredmond17@...> wrote:
                                          >
                                          > > Jerry, I have been researching the solution to the same basic problem with my C11. Though no expert, I've found that flat fielding doesn't eliminate the hot spots. I found a discussion of the difference between hot spots and other sorts of problems that can be eliminated with flats in the book The Handbook of Astronomical Image Processing by Richard Berry and James Burnell. Ron W. mentions and recommends this book in his book.
                                          > >
                                          > > The gist of it is that hot spots are caused by non-celestial light contamination and can't be addressed with flat frame calibration. His unwelcome advice is to correct the problem (frequently introduced in a focal reducer or t-adapter extension tube) at the source with baffles, flocking, etc.
                                          > >
                                          > > I do wonder what others think about this conclusion. I'm not very gifted with manual dexterity, and don't look forward to messing around in this way.
                                          > >
                                          > > John
                                          > >
                                          > >
                                          > > --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
                                          > >>
                                          > >> --- Mark Striebeck <mark.striebeck@> wrote:
                                          > >>> What is the best way to evaluate flat images?
                                          > >>
                                          > >> Take a proper sky flat and use it to flatten the light-box flat (be sure to dark or bias subtract both). Unless there are defects in the light-box then the result will be completely "flat" (no vignette or anything else).
                                          > >>
                                          > >> Properly made sky flats are the "gold standard".
                                          > >>
                                          > >> Stan
                                          > >>
                                          > >
                                          > >
                                          > >
                                          > >
                                          > > ------------------------------------
                                          > >
                                          > >
                                          > >
                                          > >
                                          >
                                        • Ron Wodaski
                                          Hey, that s me: making nightmares come true for more than a decade. Ron Wodaski
                                          Message 20 of 20 , May 28, 2013
                                            Hey, that's me: making nightmares come true for more than a decade. <g>

                                            Ron Wodaski



                                            On May 28, 2013, at 5:25 PM, redmondjohn48 <jredmond17@...> wrote:

                                            > Ron,
                                            > I figured you'd say that! Anyway, I have just been evaluating a few flats and test images I took last night with the benefit of a new, well-fitting metal dew shield, and much of the hot spot so evident in all my previous efforts seems to have been eliminated.
                                            >
                                            > Also, thanks to Robert for the SBIG blog link. I hate to paint anything with Rustoleum, since I fear making an awful mess. But I guess it may be necessary.
                                            >
                                            > John
                                            >
                                            > --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
                                            >>
                                            >> I would say that that statement really isn't something that lends itself to opinion, in that it's simply a true statement. :-)
                                            >>
                                            >> As I alluded to briefly in my other email in this thread, off-axis light sources will contaminate both a flat and an image. So getting rid of them is very important.
                                            >>
                                            >> (Why can't you expect to simply subtract effects from off-axis sources? Typically, they are constantly changing with your telescope's position. A porch light is obvious: the angle to the optical axis is constantly changing. The porch light is still, but the scope moves, and so the result of that off-axis light (via light leaks, internal reflections, direct off-axis light paths, etc.) varies over time.)
                                            >>
                                            >> Such off-axis light is technically referred to as "unwanted signal." The noise from that unwanted signal is quite real, and reduces your signal to noise wherever it occurs. To an engineer, off-axis light is just part of instrument calibration: you track down the cause of each one, and you figure out how to eliminate it. Astronomy is mostly an engineering exercise, after all. ;-)
                                            >>
                                            >> Ron Wodaski
                                            >>
                                            >>
                                            >>
                                            >> On May 28, 2013, at 12:16 PM, redmondjohn48 <jredmond17@...> wrote:
                                            >>
                                            >>> Jerry, I have been researching the solution to the same basic problem with my C11. Though no expert, I've found that flat fielding doesn't eliminate the hot spots. I found a discussion of the difference between hot spots and other sorts of problems that can be eliminated with flats in the book The Handbook of Astronomical Image Processing by Richard Berry and James Burnell. Ron W. mentions and recommends this book in his book.
                                            >>>
                                            >>> The gist of it is that hot spots are caused by non-celestial light contamination and can't be addressed with flat frame calibration. His unwelcome advice is to correct the problem (frequently introduced in a focal reducer or t-adapter extension tube) at the source with baffles, flocking, etc.
                                            >>>
                                            >>> I do wonder what others think about this conclusion. I'm not very gifted with manual dexterity, and don't look forward to messing around in this way.
                                            >>>
                                            >>> John
                                            >>>
                                            >>>
                                            >>> --- In ccd-newastro@yahoogroups.com, "Stan" <stan_ccd@> wrote:
                                            >>>>
                                            >>>> --- Mark Striebeck <mark.striebeck@> wrote:
                                            >>>>> What is the best way to evaluate flat images?
                                            >>>>
                                            >>>> Take a proper sky flat and use it to flatten the light-box flat (be sure to dark or bias subtract both). Unless there are defects in the light-box then the result will be completely "flat" (no vignette or anything else).
                                            >>>>
                                            >>>> Properly made sky flats are the "gold standard".
                                            >>>>
                                            >>>> Stan
                                            >>>>
                                            >>>
                                            >>>
                                            >>>
                                            >>>
                                            >>> ------------------------------------
                                            >>>
                                            >>>
                                            >>>
                                            >>>
                                            >>
                                            >
                                            >
                                            >
                                            >
                                            > ------------------------------------
                                            >
                                            >
                                            >
                                            >
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