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Re: Assistance with flat field calibration

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  • Stan
    ... IMHO learn from mistakes and move on. But of course there is Photoshop (dodge and burn). Stan
    Message 1 of 20 , May 26, 2013
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      --- "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 2 of 20 , May 27, 2013
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        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 3 of 20 , May 27, 2013
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        • 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 4 of 20 , May 28, 2013
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            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 5 of 20 , May 28, 2013
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              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 6 of 20 , May 28, 2013
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                --- "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 7 of 20 , May 28, 2013
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                  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 8 of 20 , May 28, 2013
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                    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 9 of 20 , May 28, 2013
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                      --- 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 10 of 20 , May 28, 2013
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                        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 11 of 20 , May 28, 2013
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                          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 12 of 20 , May 28, 2013
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                            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 13 of 20 , May 28, 2013
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                              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 14 of 20 , May 28, 2013
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                                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 15 of 20 , May 28, 2013
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                                  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 16 of 20 , May 28, 2013
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                                    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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                                    >>
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