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Webcam Magnification

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  • teafornosps
    Greetings! Got my webcam working and am able to record videos. But I have a question that I haven t seen addressed and the vendor was of no help. So the
    Message 1 of 16 , Nov 1, 2012
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      Greetings!
      Got my webcam working and am able to record videos. But I have a question that I haven't seen addressed and the vendor was of no help. So the answer must be obvious to the most casual observer. However I don't 'see' the answer and would appreciate a little help.
      Normally the scope's magnification is the FL of the OTA divided by the FL of the eyepiece. However, with a webcam there is no eyepiece, so what determines the magnification? What if I want to do an image at 40X.......how do I get that magnification. I'm sure the answer is quite simple; but it escapes me. Thanks in advance.
    • Thomas D. Dean
      ... I think roughly the same as a 6mm eyepiece. http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4092449/Main/4087390 Tom Dean
      Message 2 of 16 , Nov 1, 2012
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        On 11/01/12 16:31, teafornosps wrote:
        > Greetings!
        > Got my webcam working and am able to record videos. But I have a question that I haven't seen addressed and the vendor was of no help. So the answer must be obvious to the most casual observer. However I don't 'see' the answer and would appreciate a little help.
        > Normally the scope's magnification is the FL of the OTA divided by the FL of the eyepiece. However, with a webcam there is no eyepiece, so what determines the magnification? What if I want to do an image at 40X.......how do I get that magnification. I'm sure the answer is quite simple; but it escapes me. Thanks in advance.
        >

        I think roughly the same as a 6mm eyepiece.

        http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4092449/Main/4087390

        Tom Dean
      • John Mahony
        Comparing to a 6mm EP only compares FOV (and only if this is a Plossl (or similar) EP with about 50 deg apparent FOV, and only if the webcam chip has 640X480
        Message 3 of 16 , Nov 1, 2012
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          Comparing to a 6mm EP only compares FOV (and only if this is a Plossl (or similar) EP with about 50 deg apparent FOV, and only if the webcam chip has 640X480 9u pixels, which is a very common size for webcam chips).  Another issue is resolution.

          But more importantly, the term "magnification" doesn't mean much when imaging.  Telescope "magnification" for visual use refers to magnification of angular size, or "how much closer the object looks" (those two definitions are nearly identical, unless the object has large apparent size when magnified, where the relation starts to break down).  But when imaging, the angular size depends on how big your monitor (or printed image) is, monitor display settings, how close your eyes are to the screen/paper, etc.  

          "Magnification" of microphotos in a biology textbook refers not to angular size, but to the ratio of image size to "actual size".  But that definition gives some very disappointing numbers when used for astrophotography.

          -John



          ----- Original Message -----
          > From: Thomas D. Dean <tomdean@...>

          > On 11/01/12 16:31, teafornosps wrote:
          >> Greetings!
          >> Got my webcam working and am able to record videos.  But I have a question
          > that I haven't seen addressed and the vendor was of no help.  So the answer
          > must be obvious to the most casual observer.  However I don't 'see'
          > the answer and would appreciate a little help.
          >> Normally the scope's magnification is the FL of the OTA divided by the
          > FL of the eyepiece.  However, with a webcam there is no eyepiece, so what
          > determines the magnification?  What if I want to do an image at 40X.......how do
          > I get that magnification.  I'm sure the answer is quite simple; but it
          > escapes me.  Thanks in advance.
          >>
          >
          > I think roughly the same as a 6mm eyepiece.
          >
          > http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4092449/Main/4087390
          >
          > Tom Dean
          >
          >
          > ------------------------------------
          >
          > Yahoo! Groups Links
          >
          >
          >
        • autostaretx
          Another way to look at it is by primary focal length. If you re shooting through an F/10 10 LX200gps, 2500mm focal length, it s the same as putting a 2500mm
          Message 4 of 16 , Nov 1, 2012
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            Another way to look at it is by primary focal length.
            If you're shooting through an F/10 10" LX200gps, 2500mm focal length,
            it's the same as putting a 2500mm telephoto lens on a camera.
            The imaging chip size tells you how much acceptance angle you're going to have.

            If memory serves, the equivalency to a "times 1" magnification is a 30 degree field of view. I vaguely remember the Meade LSI as having about a 5 arcminute field of view. So (if you must speak of magnification on the imaging sensor) that's about 360x.

            As John wrote, that number does not take into account what you're then doing with the image. In the "film" days, at least on a slide, the 360x was real... the image was 360x bigger on that piece of film than the original image would have been in a pinhole camera.

            But in terms of what you do with the camera's signal...
            Do you consider the screen-filling image on your iPhone the same "magnification" as the exact same image screen-filling a 27 inch monitor?

            have fun
            --dick

            --- In LX200GPS@yahoogroups.com, John Mahony <jmmahony@...> wrote:
            >
            > Comparing to a 6mm EP only compares FOV (and only if this is a Plossl (or similar) EP with about 50 deg apparent FOV, and only if the webcam chip has 640X480 9u pixels, which is a very common size for webcam chips).  Another issue is resolution.
            >
            > But more importantly, the term "magnification" doesn't mean much when imaging.  Telescope "magnification" for visual use refers to magnification of angular size, or "how much closer the object looks" (those two definitions are nearly identical, unless the object has large apparent size when magnified, where the relation starts to break down).  But when imaging, the angular size depends on how big your monitor (or printed image) is, monitor display settings, how close your eyes are to the screen/paper, etc.  
            >
            > "Magnification" of microphotos in a biology textbook refers not to angular size, but to the ratio of image size to "actual size".  But that definition gives some very disappointing numbers when used for astrophotography.
            >
            > -John
            >
            >
            >
            > ----- Original Message -----
            > > From: Thomas D. Dean <tomdean@...>
            > > 
            > > On 11/01/12 16:31, teafornosps wrote:
            > >> Greetings!
            > >> Got my webcam working and am able to record videos.  But I have a question
            > > that I haven't seen addressed and the vendor was of no help.  So the answer
            > > must be obvious to the most casual observer.  However I don't 'see'
            > > the answer and would appreciate a little help.
            > >> Normally the scope's magnification is the FL of the OTA divided by the
            > > FL of the eyepiece.  However, with a webcam there is no eyepiece, so what
            > > determines the magnification?  What if I want to do an image at 40X.......how do
            > > I get that magnification.  I'm sure the answer is quite simple; but it
            > > escapes me.  Thanks in advance.
            > >>
            > >
            > > I think roughly the same as a 6mm eyepiece.
            > >
            > > http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4092449/Main/4087390
            > >
            > > Tom Dean
            > >
            > >
            > > ------------------------------------
            > >
            > > Yahoo! Groups Links
            > >
            > >
            > >
            >
          • Barry
            The FOV of a web cam is aproximately the same as the FOV of a 6.4mm focal length eyepiece when viewed through a given telescope. To increase the
            Message 5 of 16 , Nov 2, 2012
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              The FOV of a web cam is aproximately the same as the FOV of a 6.4mm focal length eyepiece when viewed through a given telescope.

              To increase the "magnification" just place a barlow or two in the light path to the web cam.

              Barry

              --- In LX200GPS@yahoogroups.com, "Thomas D. Dean" <tomdean@...> wrote:
              >
              > On 11/01/12 16:31, teafornosps wrote:
              > > Greetings!
              > > Got my webcam working and am able to record videos. But I have a question that I haven't seen addressed and the vendor was of no help. So the answer must be obvious to the most casual observer. However I don't 'see' the answer and would appreciate a little help.
              > > Normally the scope's magnification is the FL of the OTA divided by the FL of the eyepiece. However, with a webcam there is no eyepiece, so what determines the magnification? What if I want to do an image at 40X.......how do I get that magnification. I'm sure the answer is quite simple; but it escapes me. Thanks in advance.
              > >
              >
              > I think roughly the same as a 6mm eyepiece.
              >
              > http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4092449/Main/4087390
              >
              > Tom Dean
              >
            • don
              The magnification is determined by your focal length, e.g., f/10 ,etc. You can only change it with Barlow. The camera field of view is fixed. You can only
              Message 6 of 16 , Nov 2, 2012
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                The magnification is determined by your focal length, e.g., f/10 ,etc. You
                can only change it with Barlow. The camera field of view is fixed. You can
                only change it by changing size of chip.



                Here is one source for info.
                http://www.astro-imaging.com/Tutorial/Calculations.html



                You should google for others and find the calculations you need.



                DonW



                [Non-text portions of this message have been removed]
              • Richard Michels
                I would like to thank everyone for your most informative inputs.  I learned a great deal.  I m off now in search of my next point of confusion.  :-)  
                Message 7 of 16 , Nov 2, 2012
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                  I would like to thank everyone for your most informative inputs.  I learned a great deal.  I'm off now in search of my next point of confusion.  :-)

                   
                  Sincerely,
                  Richard
                  NOSPS, Past Commander
                  D16 Webmaster
                  www.calmseas.org Webmaster
                  1-360-670-5418


                  "No matter how hard you push the envelope, it will always be stationery"


                  ________________________________
                  From: autostaretx <rseymour@...>
                  To: LX200GPS@yahoogroups.com
                  Sent: Thursday, November 1, 2012 8:22 PM
                  Subject: [LX200GPS] Re: Webcam Magnification


                   
                  Another way to look at it is by primary focal length.
                  If you're shooting through an F/10 10" LX200gps, 2500mm focal length,
                  it's the same as putting a 2500mm telephoto lens on a camera.
                  The imaging chip size tells you how much acceptance angle you're going to have.

                  If memory serves, the equivalency to a "times 1" magnification is a 30 degree field of view. I vaguely remember the Meade LSI as having about a 5 arcminute field of view. So (if you must speak of magnification on the imaging sensor) that's about 360x.

                  As John wrote, that number does not take into account what you're then doing with the image. In the "film" days, at least on a slide, the 360x was real... the image was 360x bigger on that piece of film than the original image would have been in a pinhole camera.

                  But in terms of what you do with the camera's signal...
                  Do you consider the screen-filling image on your iPhone the same "magnification" as the exact same image screen-filling a 27 inch monitor?

                  have fun
                  --dick

                  --- In LX200GPS@yahoogroups.com, John Mahony <jmmahony@...> wrote:
                  >
                  > Comparing to a 6mm EP only compares FOV (and only if this is a Plossl (or similar) EP with about 50 deg apparent FOV, and only if the webcam chip has 640X480 9u pixels, which is a very common size for webcam chips).  Another issue is resolution.
                  >
                  > But more importantly, the term "magnification" doesn't mean much when imaging.  Telescope "magnification" for visual use refers to magnification of angular size, or "how much closer the object looks" (those two definitions are nearly identical, unless the object has large apparent size when magnified, where the relation starts to break down).  But when imaging, the angular size depends on how big your monitor (or printed image) is, monitor display settings, how close your eyes are to the screen/paper, etc.  
                  >
                  > "Magnification" of microphotos in a biology textbook refers not to angular size, but to the ratio of image size to "actual size".  But that definition gives some very disappointing numbers when used for astrophotography.
                  >
                  > -John
                  >
                  >
                  >
                  > ----- Original Message -----
                  > > From: Thomas D. Dean <tomdean@...>
                  > > 
                  > > On 11/01/12 16:31, teafornosps wrote:
                  > >> Greetings!
                  > >> Got my webcam working and am able to record videos.  But I have a question
                  > > that I haven't seen addressed and the vendor was of no help.  So the answer
                  > > must be obvious to the most casual observer.  However I don't 'see'
                  > > the answer and would appreciate a little help.
                  > >> Normally the scope's magnification is the FL of the OTA divided by the
                  > > FL of the eyepiece.  However, with a webcam there is no eyepiece, so what
                  > > determines the magnification?  What if I want to do an image at 40X.......how do
                  > > I get that magnification.  I'm sure the answer is quite simple; but it
                  > > escapes me.  Thanks in advance.
                  > >>
                  > >
                  > > I think roughly the same as a 6mm eyepiece.
                  > >
                  > > http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4092449/Main/4087390
                  > >
                  > > Tom Dean
                  > >
                  > >
                  > > ------------------------------------
                  > >
                  > > Yahoo! Groups Links
                  > >
                  > >
                  > >
                  >




                  [Non-text portions of this message have been removed]
                • l_d_bonnie
                  ... Barlows or focal reducers depending on the object and optics. If you want to see what some are doing with video check out the Night Sky s Network.
                  Message 8 of 16 , Nov 2, 2012
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                    --- In LX200GPS@yahoogroups.com, "don" <cdonwaters@...> wrote:
                    >
                    > The magnification is determined by your focal length, e.g., f/10 ,etc. You
                    > can only change it with Barlow. The camera field of view is fixed. You can
                    > only change it by changing size of chip.
                    >
                    >
                    >
                    Barlows or focal reducers depending on the object and optics. If you want to see what some are doing with video check out the Night Sky's Network.

                    http://www.nightskiesnetwork.com/

                    Click on view as a guest. Depending on the weather, there is usually someone broadcasting at night.

                    LdB

                    http://www.mts.net/~lmlod/Observatory.html
                  • paul valleli
                    Angular Magnification = angular size of image divided by real angular size in the sky and , angular size of image = physical size of image on the detector
                    Message 9 of 16 , Nov 2, 2012
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                      Angular Magnification = angular size of image divided by real angular size
                      in the sky
                      and ,
                      angular size of image = physical size of image on the detector divided by
                      EFL

                      Must use same units of measure ie: mm/mm or
                      inches divided by EFL in inches.

                      Image size can also be determined by # of pixels that the object covers
                      times
                      pixel size , 1 micron = 1/1000 of a mm. or +1 *10^6 Meter

                      Starman Paul


                      [Non-text portions of this message have been removed]
                    • mweasner@mac.com
                      You might check out Paul Rodman s AstroAid for iOS devices: https://itunes.apple.com/us/app/astroaid/id541448395 There is a setting for a Philips webcam.
                      Message 10 of 16 , Nov 2, 2012
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                        You might check out Paul Rodman's AstroAid for iOS devices:

                        https://itunes.apple.com/us/app/astroaid/id541448395

                        There is a setting for a Philips webcam. (Paul is the developer of the well known AstroPlanner.)

                        I'll have a full review online on my ETX Site in a few days.

                        Mike Weasner Email: mweasner@... / Twitter: @mweasner
                        LX200-ACF-8" ETX-125AT ETX-105PE ETX-90RA ETX-70AT PST GPOD-XL5
                        Cassiopeia Observatory: http://www.weasner.com/co
                      • John Mahony
                        ... Again, that only works if you specify the AFOV of the EP.  A 6mm Ethos will show a much wider field than a 6mm Plossl.  The common comparison of webcam
                        Message 11 of 16 , Nov 2, 2012
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                          ----- Original Message -----

                          > From: Barry <barrygastro@...>

                          >T he FOV of a web cam is aproximately the same as the FOV of a 6.4mm focal length
                          > eyepiece when viewed through a given telescope.

                          Again, that only works if you specify the AFOV of the EP.  A 6mm Ethos will show a much wider field than a 6mm Plossl.  The common comparison of webcam to 6mm EP assumes a Plossl (50 deg AFOV) and a standard 640X480 9u pixel webcam chip.

                          -John



                          >
                          > --- In LX200GPS@yahoogroups.com, "Thomas D. Dean" <tomdean@...>
                          > wrote:
                          >>
                          >> On 11/01/12 16:31, teafornosps wrote:
                          >> > Greetings!
                          >> > Got my webcam working and am able to record videos.  But I have a
                          > question that I haven't seen addressed and the vendor was of no help.  So
                          > the answer must be obvious to the most casual observer.  However I don't
                          > 'see' the answer and would appreciate a little help.
                          >> > Normally the scope's magnification is the FL of the OTA divided by
                          > the FL of the eyepiece.  However, with a webcam there is no eyepiece, so what
                          > determines the magnification?  What if I want to do an image at 40X.......how do
                          > I get that magnification.  I'm sure the answer is quite simple; but it
                          > escapes me.  Thanks in advance.
                          >> >
                          >>
                          >> I think roughly the same as a 6mm eyepiece.
                          >>
                          >>
                          > http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4092449/Main/4087390
                          >>
                          >> Tom Dean
                          >>
                          >
                          >
                          >
                          >
                          > ------------------------------------
                          >
                          > Yahoo! Groups Links
                          >
                          >
                          >
                        • Richard Michels
                          Concerning the F/10 number of the scope; what is the significance of the F number.  In doing imaging, is a high F number better or worse than a lower F
                          Message 12 of 16 , Nov 3, 2012
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                            Concerning the F/10 number of the scope; what is the significance of the 'F' number.  In doing imaging, is a high F number better or worse than a lower F number?  Also, what does "better" or 'worse mean' with respect to the captured image?
                             
                            Sincerely,
                            Richard
                            NOSPS, Past Commander
                            D16 Webmaster
                            www.calmseas.org Webmaster
                            1-360-670-5418


                            "No matter how hard you push the envelope, it will always be stationery"


                            ________________________________
                            From: autostaretx <rseymour@...>
                            To: LX200GPS@yahoogroups.com
                            Sent: Thursday, November 1, 2012 8:22 PM
                            Subject: [LX200GPS] Re: Webcam Magnification


                             
                            Another way to look at it is by primary focal length.
                            If you're shooting through an F/10 10" LX200gps, 2500mm focal length,
                            it's the same as putting a 2500mm telephoto lens on a camera.
                            The imaging chip size tells you how much acceptance angle you're going to have.

                            If memory serves, the equivalency to a "times 1" magnification is a 30 degree field of view. I vaguely remember the Meade LSI as having about a 5 arcminute field of view. So (if you must speak of magnification on the imaging sensor) that's about 360x.

                            As John wrote, that number does not take into account what you're then doing with the image. In the "film" days, at least on a slide, the 360x was real... the image was 360x bigger on that piece of film than the original image would have been in a pinhole camera.

                            But in terms of what you do with the camera's signal...
                            Do you consider the screen-filling image on your iPhone the same "magnification" as the exact same image screen-filling a 27 inch monitor?

                            have fun
                            --dick

                            --- In LX200GPS@yahoogroups.com, John Mahony <jmmahony@...> wrote:
                            >
                            > Comparing to a 6mm EP only compares FOV (and only if this is a Plossl (or similar) EP with about 50 deg apparent FOV, and only if the webcam chip has 640X480 9u pixels, which is a very common size for webcam chips).  Another issue is resolution.
                            >
                            > But more importantly, the term "magnification" doesn't mean much when imaging.  Telescope "magnification" for visual use refers to magnification of angular size, or "how much closer the object looks" (those two definitions are nearly identical, unless the object has large apparent size when magnified, where the relation starts to break down).  But when imaging, the angular size depends on how big your monitor (or printed image) is, monitor display settings, how close your eyes are to the screen/paper, etc.  
                            >
                            > "Magnification" of microphotos in a biology textbook refers not to angular size, but to the ratio of image size to "actual size".  But that definition gives some very disappointing numbers when used for astrophotography.
                            >
                            > -John
                            >
                            >
                            >
                            > ----- Original Message -----
                            > > From: Thomas D. Dean <tomdean@...>
                            > > 
                            > > On 11/01/12 16:31, teafornosps wrote:
                            > >> Greetings!
                            > >> Got my webcam working and am able to record videos.  But I have a question
                            > > that I haven't seen addressed and the vendor was of no help.  So the answer
                            > > must be obvious to the most casual observer.  However I don't 'see'
                            > > the answer and would appreciate a little help.
                            > >> Normally the scope's magnification is the FL of the OTA divided by the
                            > > FL of the eyepiece.  However, with a webcam there is no eyepiece, so what
                            > > determines the magnification?  What if I want to do an image at 40X.......how do
                            > > I get that magnification.  I'm sure the answer is quite simple; but it
                            > > escapes me.  Thanks in advance.
                            > >>
                            > >
                            > > I think roughly the same as a 6mm eyepiece.
                            > >
                            > > http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4092449/Main/4087390
                            > >
                            > > Tom Dean
                            > >
                            > >
                            > > ------------------------------------
                            > >
                            > > Yahoo! Groups Links
                            > >
                            > >
                            > >
                            >




                            [Non-text portions of this message have been removed]
                          • paul valleli
                            1. Since F/No. is just shorthand for the ratio of EFL divided by Aperture. It is unitless. The F/No. has a direct influence on the imaging of faint nebulosity.
                            Message 13 of 16 , Nov 3, 2012
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                              1. Since F/No. is just shorthand for the ratio of EFL divided by Aperture.
                              It is unitless.
                              The F/No. has a direct influence on the imaging of faint nebulosity. An
                              F/1 system responds
                              to a faint extended object much faster than an F/10.

                              2.The detection of point sources such as faint stars is strictly related to
                              the active area of the
                              Clear Aperture ( subtract area of obscurations such as the Secondary
                              mirror). ( and of course,
                              the sensitivity of the detector, whether CCD or film.)

                              3. The Scale Factor is determined strictly by the Total EFL, the size of
                              the detector, and size of pixel
                              elements. Same as MAGNIFICATION.

                              4. Resolution is determined by the nu mber of resolution elements (pixel
                              size), the OTF -
                              Optical Transfer Function, and the EFL. OTF is strictly determined by the
                              F/No. (a ratio).

                              5. The field of view , FOV, varies considerably with the detector size and
                              Total EFL.
                              Eyepiece performance ( if used) such as Apparent Field of View - AFOV may
                              vignette a
                              large detector leading to loss of faint objects at the perimeter of the
                              image.

                              Starman Paul ( semi-retired optical engineer)


                              [Non-text portions of this message have been removed]
                            • John Mahony
                              ... The f number (aka focal ratio) is the ratio of focal length to aperture (the / symbol is used to remind you it s a ratio). The aperture determines the
                              Message 14 of 16 , Nov 6, 2012
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                                ----- Original Message -----


                                > From: Richard Michels <teafornosps@...>

                                > Concerning the F/10 number of the scope; what is the significance of the
                                > 'F' number.

                                The f number (aka "focal ratio) is the ratio of focal length to aperture (the "/" symbol is used to remind you it's a ratio).

                                The aperture determines the amount of light entering the scope from a given object.  The image scale at the focal plane is determined by the focal length.  So the main practical meaning of f ratio in imaging is that this ratio is directly related to the "surface brightness" (light per area) of the image at the focal plane.  Lower f ratio gives more light per pixel per second, so the exposure time is shorter.

                                Two other factors are affected: for a given aperture, lower f ratio means the image (at the focal plane) of a given object is smaller, and each pixel covers a wider area of sky, so you get a wider field (useful for large objects) but lose image size (of a given object) and resolution. 

                                (The practical interpretation is different in conventional photography, where (for a given lens) the f ratio is normally adjusted by stopping down the lens (changing aperture) by adjusting the iris diaphram, rather than by changing focal length (although in zoom lenses you can do both).  Conventional photographers may even use the terms "aperture" and f ratio (or f stop) interchangeably, whereas in astrophotography the full aperture is almost alway used, and f ratio is normally changed by using a focal reducer or barlow to change the effective focal length.  Also, in terrestrial imaging, the f ratio affects "depth of field", but this is not an issue in astrophotography, where everything is (practically) at infinite distance.)        

                                >  In doing imaging, is a high F number better or worse than a
                                > lower F number?  Also, what does "better" or 'worse mean' with
                                > respect to the captured image?


                                That all depends on what you want.  It's a trade-off.  For a given scope, for large and/or faint DSOs, using a lower f ratio means you can capture the whole object in the FOVand reduce exposure time.  Small DSOs like planetary nebulas and glob clusters are small, so you don't need the wide FOV, and are often bright, so you don't need the shorter exp time.  Planets are very small but very bright, so you can go the other way and use a barlow to increase f ratio to capture more detail. 

                                -John
                              • Keith Marley
                                that s the best description I ve heard yet! ... -- Clear Skies K [Non-text portions of this message have been removed]
                                Message 15 of 16 , Nov 6, 2012
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                                  that's the best description I've heard yet!


                                  On Tue, Nov 6, 2012 at 8:10 PM, John Mahony <jmmahony@...> wrote:

                                  > **
                                  >
                                  >
                                  > ----- Original Message -----
                                  >
                                  > > From: Richard Michels <teafornosps@...>
                                  >
                                  > >
                                  > > Concerning the F/10 number of the scope; what is the significance of the
                                  > > 'F' number.
                                  >
                                  > The f number (aka "focal ratio) is the ratio of focal length to aperture
                                  > (the "/" symbol is used to remind you it's a ratio).
                                  >
                                  > The aperture determines the amount of light entering the scope from a
                                  > given object. The image scale at the focal plane is determined by the
                                  > focal length. So the main practical meaning of f ratio in imaging is that
                                  > this ratio is directly related to the "surface brightness" (light per area)
                                  > of the image at the focal plane. Lower f ratio gives more light per pixel
                                  > per second, so the exposure time is shorter.
                                  >
                                  > Two other factors are affected: for a given aperture, lower f ratio means
                                  > the image (at the focal plane) of a given object is smaller, and each pixel
                                  > covers a wider area of sky, so you get a wider field (useful for large
                                  > objects) but lose image size (of a given object) and resolution.
                                  >
                                  > (The practical interpretation is different in conventional photography,
                                  > where (for a given lens) the f ratio is normally adjusted by stopping down
                                  > the lens (changing aperture) by adjusting the iris diaphram, rather than by
                                  > changing focal length (although in zoom lenses you can do both).
                                  > Conventional photographers may even use the terms "aperture" and f ratio
                                  > (or f stop) interchangeably, whereas in astrophotography the full aperture
                                  > is almost alway used, and f ratio is normally changed by using a focal
                                  > reducer or barlow to change the effective focal length. Also, in
                                  > terrestrial imaging, the f ratio affects "depth of field", but this is not
                                  > an issue in astrophotography, where everything is (practically) at infinite
                                  > distance.)
                                  >
                                  >
                                  > > In doing imaging, is a high F number better or worse than a
                                  > > lower F number? Also, what does "better" or 'worse mean' with
                                  > > respect to the captured image?
                                  >
                                  > That all depends on what you want. It's a trade-off. For a given scope,
                                  > for large and/or faint DSOs, using a lower f ratio means you can capture
                                  > the whole object in the FOVand reduce exposure time. Small DSOs like
                                  > planetary nebulas and glob clusters are small, so you don't need the wide
                                  > FOV, and are often bright, so you don't need the shorter exp time. Planets
                                  > are very small but very bright, so you can go the other way and use a
                                  > barlow to increase f ratio to capture more detail.
                                  >
                                  > -John
                                  >
                                  >
                                  >



                                  --
                                  Clear Skies
                                  K


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                                • Richard Michels
                                  John - Thanks so much for the excellent explanation.  Your explanation was easy to understand, covered all the salient points and stated in a way that makes
                                  Message 16 of 16 , Nov 7, 2012
                                  • 0 Attachment
                                    John -

                                    Thanks so much for the excellent explanation.  Your explanation was easy to understand, covered all the salient points and stated in a way that makes it easy to remember!!  Kudos.
                                     
                                    Sincerely,
                                    Richard
                                    NOSPS, Past Commander
                                    D16 Webmaster
                                    www.calmseas.org Webmaster
                                    1-360-670-5418


                                    "No matter how hard you push the envelope, it will always be stationery"


                                    ________________________________
                                    From: John Mahony <jmmahony@...>
                                    To: "LX200GPS@yahoogroups.com" <LX200GPS@yahoogroups.com>
                                    Sent: Tuesday, November 6, 2012 5:10 PM
                                    Subject: Re: [LX200GPS] Re: Webcam Magnification


                                     
                                    ----- Original Message -----

                                    > From: Richard Michels <teafornosps@...>

                                    > Concerning the F/10 number of the scope; what is the significance of the
                                    > 'F' number.

                                    The f number (aka "focal ratio) is the ratio of focal length to aperture (the "/" symbol is used to remind you it's a ratio).

                                    The aperture determines the amount of light entering the scope from a given object.  The image scale at the focal plane is determined by the focal length.  So the main practical meaning of f ratio in imaging is that this ratio is directly related to the "surface brightness" (light per area) of the image at the focal plane.  Lower f ratio gives more light per pixel per second, so the exposure time is shorter.

                                    Two other factors are affected: for a given aperture, lower f ratio means the image (at the focal plane) of a given object is smaller, and each pixel covers a wider area of sky, so you get a wider field (useful for large objects) but lose image size (of a given object) and resolution. 

                                    (The practical interpretation is different in conventional photography, where (for a given lens) the f ratio is normally adjusted by stopping down the lens (changing aperture) by adjusting the iris diaphram, rather than by changing focal length (although in zoom lenses you can do both).  Conventional photographers may even use the terms "aperture" and f ratio (or f stop) interchangeably, whereas in astrophotography the full aperture is almost alway used, and f ratio is normally changed by using a focal reducer or barlow to change the effective focal length.  Also, in terrestrial imaging, the f ratio affects "depth of field", but this is not an issue in astrophotography, where everything is (practically) at infinite distance.)        

                                    >  In doing imaging, is a high F number better or worse than a
                                    > lower F number?  Also, what does "better" or 'worse mean' with
                                    > respect to the captured image?

                                    That all depends on what you want.  It's a trade-off.  For a given scope, for large and/or faint DSOs, using a lower f ratio means you can capture the whole object in the FOVand reduce exposure time.  Small DSOs like planetary nebulas and glob clusters are small, so you don't need the wide FOV, and are often bright, so you don't need the shorter exp time.  Planets are very small but very bright, so you can go the other way and use a barlow to increase f ratio to capture more detail. 

                                    -John



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