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RE: Re: Is this RBI and how does it affect your imaging strategy

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  • lmbuck2000
    kjcphoto -- thanks so much for jumping in. i thought i needed to take rbi-flood bias and flats. you are correct, it appears to be unnecessary. i looked at my
    Message 1 of 16 , Sep 18, 2013
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      kjcphoto -- thanks so much for jumping in.  i thought i needed to take rbi-flood bias and flats.   you are correct,  it appears to be unnecessary.  i looked at my regular bias frames vs. rbi bias frames and they are nearly identical.  same goes for the flats.


      i calibrated an rbi-flood light frame using an rbi-dark with regular bias and light and it looks great.


      appreciate the info (and makes things simpler - and faster)


      Lee


    • kjcphoto2001
      Lee, you re welcome. Preflashing flats does add a lot of time to your work flow. Also, if you are not scaling your darks, and making darks that are the same
      Message 2 of 16 , Sep 19, 2013
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        Lee, you're welcome.  Preflashing flats does add a lot of time to your work flow.  Also, if you are not scaling your darks, and making darks that are the same time exposure as the lights you are calibrating, you don't need to apply bias frames.  Adam block covers this in his teaching series on using CCDStack. 


        I have been told that some people with RBI prone chips do not preflash because it adds noise.  Kevin's point about it not being necessary if you don't move the telescope is a workable solution.  I used my camera without preflash for awhile because I didn't have the driver to engage the feature.  I made some good images, but I did see ghost images of bright stars that I had to clean up.  Bright areas in galaxies and nebula may not appear to show any RBI, but it is there.  It's hidden in the brightness of the object and it may not be detrimental to your final image.  


        The main problem I noticed was when I made my darks without preflash.  I noticed long swirl marks on my darks when I didn't preflash.  It looked like I had light leaking in from the back of the scope in the observatory.  I tried blocking the back and front of the scope but they still persisted.  And they were the same consistent pattern.  I realized that the chip had trapped charges in the substrate from previous exposure.  I would make my flats first, since they were quick to do and then I would leave the scope to make darks on it's own, since it takes much longer.  The flat frame exposures were preflashing the chip causing RBI to appear in my darks.  I had to wait at least 24 hours with no light exposure for the chip to get back to the point where I could make some clean dark frames.





        --- In ccd-newastro@yahoogroups.com, <lmbuck2000@...> wrote:

        kjcphoto -- thanks so much for jumping in.  i thought i needed to take rbi-flood bias and flats.   you are correct,  it appears to be unnecessary.  i looked at my regular bias frames vs. rbi bias frames and they are nearly identical.  same goes for the flats.


        i calibrated an rbi-flood light frame using an rbi-dark with regular bias and light and it looks great.


        appreciate the info (and makes things simpler - and faster)


        Lee


      • lmbuck2000
        one reason i thought i needed to preflash the bias frames is i do use them to scale darks AND i use them to dark subtract the flats. fact is, the non
        Message 3 of 16 , Sep 19, 2013
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          one reason i thought i needed to preflash the bias frames is i do use them to scale darks AND i use them to "dark subtract" the flats.  fact is, the non preflash bias frames look identical to the pre-flashed bias and same for the flats.   (your comment made me examine them more carefully - than just my assumption --  we know what assumptions do) 


          because of image shift from dithering and data shift from filter changes i have decided for cameras that matter, using the RBI pre-flood can only help.  but i do need to get some time to create an image (same object) with and without pre-flash to see if it is actually noticeable in "pretty pictures"


          Lee

          --- In ccd-newastro@yahoogroups.com, <ccd-newastro@yahoogroups.com> wrote:

          Lee, you're welcome.  Preflashing flats does add a lot of time to your work flow.  Also, if you are not scaling your darks, and making darks that are the same time exposure as the lights you are calibrating, you don't need to apply bias frames.  Adam block covers this in his teaching series on using CCDStack. 



        • Gregory
          Forgive my ignorance, what is a “flash dark?” Gregory From: stan_ccd@yahoo.com Sent: Monday, September 16, 2013 9:36 AM To: ccd-newastro@yahoogroups.com
          Message 4 of 16 , Sep 21, 2013
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            Forgive my ignorance, what is a “flash dark?”
             
            Gregory
             
            Sent: Monday, September 16, 2013 9:36 AM
            Subject: [ccd-newastro] RE: RE: Re: Is this RBI and how does it affect your imaging strategy
             

            CORRECTION:

             

            Here is a corrected and more insightful method:

             

            For a given exp time:

             

            1) take 2 virgin darks. Subtract one from the other and measure STD. virgin_noise = STD*gain/sqrt(2);

             

            2) take 2 flashed darks (flash each dark). Subtract one from the other and measure STD. flashed_noise = STD*gain/sqrt(2);

             

            noise from flashing = sqrt(flashed_noise^2 - virgin_noise^2)

             

            Stan

          • stan_ccd
            A flashed dark is an otherwise normal dark frame taken immediately after flashing. Flashing floods the CCD with infrared light just prior to the exposure,
            Message 5 of 16 , Sep 22, 2013
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              A "flashed dark" is an otherwise normal dark frame taken immediately after flashing.

               

              "Flashing" floods the CCD with infrared light just prior to the exposure, in order to saturates the substrate.  The CCD substrate can retain electrons; i.e. the pixel well is not completely emptied by readout. Those retained electrons bleed out during the next exp(s) to produce a "ghost image" from the prior exp. Flashing destroys ghost images by making the entire CCD a uniform ghost.

               

              If the camera does not have a built-in pre-flash device then another method is to avoid super-cold temps (colder CCDs retain more electrons in the substrate) or warm and re-cool the CCD (esp prior to taking darks).

               

              But don't worry about this unless you have a an obvious problem.  Usually ghosts are not a problem unless Vega or other very bright star is the field.

               

              Stan



              --- In ccd-newastro@yahoogroups.com, <fyrframe@...> wrote:

              Forgive my ignorance, what is a “flash dark?”
               
              Gregory
               
              Sent: Monday, September 16, 2013 9:36 AM
              Subject: [ccd-newastro] RE: RE: Re: Is this RBI and how does it affect your imaging strategy
               

              CORRECTION:

               

              Here is a corrected and more insightful method:

               

              For a given exp time:

               

              1) take 2 virgin darks. Subtract one from the other and measure STD. virgin_noise = STD*gain/sqrt(2);

               

              2) take 2 flashed darks (flash each dark). Subtract one from the other and measure STD. flashed_noise = STD*gain/sqrt(2);

               

              noise from flashing = sqrt(flashed_noise^2 - virgin_noise^2)

               

              Stan

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