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Re: [ccd-newastro] Demystifying bias frames?

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  • Ron Wodaski
    Yes, if you take darks of the same duration as your lights, you do not need bias frames. The purpose of a bias frame is to remove the initial count of
    Message 1 of 12 , Jul 18, 2013
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      Yes, if you take darks of the same duration as your lights, you do not need bias frames. The purpose of a bias frame is to remove the initial count of electrons from a dark frame in order to scale only the dark current.

      However: if you are making flats, then you need bias frames, because the flat is always scaled when it is applied.

      (So: anything that is scaled needs a bias.)

      It's not unusual that you are being asked to include a bias in your flats, therefore. That is normal. (A flat is not like other types of calibration frames, in that it is always scaled.)

      I'm writing a book on PixInsight. It will help you get that poker out of your eye. :-)

      Ron Wodaski



      On Jul 18, 2013, at 1:59 PM, Stuart Forman <s24man@...> wrote:

      > Hi all,
      >
      > I am a new member to this group, and in the spirit of the "classroom" nature, I'd like to dive in with a series of questions about bias frames.
      >
      > I have been a DSLR imager for a couple of years, and have used DSS. I diligently took my lights, darks, dark-flats, flats, and flat frames and plugged them into the program.
      >
      > But I'm upgrading to a QSI 683, I've started using CCDstack, and I'm dipping my toe into Pixinsight. I'm realizing that I need to understand these calibration frames much more.
      >
      > I've studied these sections in Ron's book, and I understand the concepts behind his bias chapter. From my understanding of what Ron wrote, if you take darks that are the same time as your lights, you don't need bias frames. Is this the current thinking?
      >
      > Taking CCDstack as an example.
      > CCDstack asks you to make a master bias, and a master dark. But in the creation of the master dark, it doesn't ask you to subtract the bias.
      >
      > Where it asks you to subtract the bias is in the creation of the master flat. And it actually asks you twice--once in the creation of the master flat, and the second in the routine to calibrate the light frames.
      >
      > Pixinsight seems to have a similar routine (although frankly learning Pixinsight to me is about as fun as sticking a hot poker in my eye so I haven't explored it very deeply and am missing something for sure).
      >
      > Can someone please help me deystify this?
      >
      > Regards,
      > Stuart Forman
      > TEC 140
      > A-P Mach1 GTO
      >
      >



      [Non-text portions of this message have been removed]
    • Stuart Forman
      Thank you Ron, I m looking forward to your PI book. At the moment I m still struggling with the somewhat inscrutable online tutorials before my trial license
      Message 2 of 12 , Jul 18, 2013
      • 0 Attachment
        Thank you Ron,

        I'm looking forward to your PI book. At the moment I'm still struggling with the somewhat inscrutable online tutorials before my trial license runs out.

        Last follow up question, and I thank you for your forbearance:

        I understand better now about the bias and the flats. And I assume that I don't subtract the bias from the flats twice.

        Do I subtract the bias from the lights in the final light calibration step before registering?

        Stuart


        --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
        >
        > Yes, if you take darks of the same duration as your lights, you do not need bias frames. The purpose of a bias frame is to remove the initial count of electrons from a dark frame in order to scale only the dark current.
        >
        > However: if you are making flats, then you need bias frames, because the flat is always scaled when it is applied.
        >
        > (So: anything that is scaled needs a bias.)
        >
        > It's not unusual that you are being asked to include a bias in your flats, therefore. That is normal. (A flat is not like other types of calibration frames, in that it is always scaled.)
        >
        > I'm writing a book on PixInsight. It will help you get that poker out of your eye. :-)
        >
        > Ron Wodaski
        >
        >
        >
        > On Jul 18, 2013, at 1:59 PM, Stuart Forman <s24man@...> wrote:
        >
        > > Hi all,
        > >
        > > I am a new member to this group, and in the spirit of the "classroom" nature, I'd like to dive in with a series of questions about bias frames.
        > >
        > > I have been a DSLR imager for a couple of years, and have used DSS. I diligently took my lights, darks, dark-flats, flats, and flat frames and plugged them into the program.
        > >
        > > But I'm upgrading to a QSI 683, I've started using CCDstack, and I'm dipping my toe into Pixinsight. I'm realizing that I need to understand these calibration frames much more.
        > >
        > > I've studied these sections in Ron's book, and I understand the concepts behind his bias chapter. From my understanding of what Ron wrote, if you take darks that are the same time as your lights, you don't need bias frames. Is this the current thinking?
        > >
        > > Taking CCDstack as an example.
        > > CCDstack asks you to make a master bias, and a master dark. But in the creation of the master dark, it doesn't ask you to subtract the bias.
        > >
        > > Where it asks you to subtract the bias is in the creation of the master flat. And it actually asks you twice--once in the creation of the master flat, and the second in the routine to calibrate the light frames.
        > >
        > > Pixinsight seems to have a similar routine (although frankly learning Pixinsight to me is about as fun as sticking a hot poker in my eye so I haven't explored it very deeply and am missing something for sure).
        > >
        > > Can someone please help me deystify this?
        > >
        > > Regards,
        > > Stuart Forman
        > > TEC 140
        > > A-P Mach1 GTO
        > >
        > >
        >
        >
        >
        > [Non-text portions of this message have been removed]
        >
      • tpiccian
        I agree with all you said. Then again, if you have bias frames that you took for your flats, you might as well use them IMHO. Besides, having a dark master
        Message 3 of 12 , Jul 18, 2013
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          I agree with all you said. Then again, if you have bias frames that you took for your flats, you might as well use them IMHO. Besides, having a dark master made up of images taken over many sessions and the master may need to be scaled even if its of the same temp and duration. I've seen this already with ccdstack. My images were 5 minutes as was my darks. Temps were almost identical. but the darks were scaled to roughly 94% on most of my images. Interesting action but it seemed to be correct.

          I have a feeling ccdstack has some routines in there that are more complex than just subtracting the pedestal.

          Anyway, if you're gonna shoot the biases anyway, might as well create a bias master and use it.

          But on top of all of this is to not use good imaging time for calibration data gathering. If the sky is clear and you're aligned and ready to shoot, then shoot sky, not black.

          Tom P.



          --- In ccd-newastro@yahoogroups.com, "Stuart Forman" <s24man@...> wrote:
          >
          > Hi all,
          >
          > I am a new member to this group, and in the spirit of the "classroom" nature, I'd like to dive in with a series of questions about bias frames.
          >
          > I have been a DSLR imager for a couple of years, and have used DSS. I diligently took my lights, darks, dark-flats, flats, and flat frames and plugged them into the program.
          >
          > But I'm upgrading to a QSI 683, I've started using CCDstack, and I'm dipping my toe into Pixinsight. I'm realizing that I need to understand these calibration frames much more.
          >
          > I've studied these sections in Ron's book, and I understand the concepts behind his bias chapter. From my understanding of what Ron wrote, if you take darks that are the same time as your lights, you don't need bias frames. Is this the current thinking?
          >
          > Taking CCDstack as an example.
          > CCDstack asks you to make a master bias, and a master dark. But in the creation of the master dark, it doesn't ask you to subtract the bias.
          >
          > Where it asks you to subtract the bias is in the creation of the master flat. And it actually asks you twice--once in the creation of the master flat, and the second in the routine to calibrate the light frames.
          >
          > Pixinsight seems to have a similar routine (although frankly learning Pixinsight to me is about as fun as sticking a hot poker in my eye so I haven't explored it very deeply and am missing something for sure).
          >
          > Can someone please help me deystify this?
          >
          > Regards,
          > Stuart Forman
          > TEC 140
          > A-P Mach1 GTO
          >
        • Ron Wodaski
          I wouldn t advise just use it. Every calibration step does two things: it removes some repeatable error; it adds noise. Every master calibration frame is a
          Message 4 of 12 , Jul 18, 2013
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            I wouldn't advise "just use it." Every calibration step does two things: it removes some repeatable error; it adds noise.

            Every master calibration frame is a collection of measurements; measurement always entails error (noise). Now you may have a huge stack of bias frames, say, to drive error down, but it does not drive down to zero.

            So you should only use them if you need them (although the software may well make that decision correctly, and not use them even if they are included when they are not essential).

            it's easy to apply common sense to noise, but it's often the wrong approach; noise is very non-intuitive.

            It does sound like CCDStack does some scaling above and beyond others, so you would want to pay attention to that. The more one understands about whatever software they are using, the better.

            Ron Wodaski



            On Jul 18, 2013, at 6:44 PM, tpiccian <tpicciani@...> wrote:

            > I agree with all you said. Then again, if you have bias frames that you took for your flats, you might as well use them IMHO. Besides, having a dark master made up of images taken over many sessions and the master may need to be scaled even if its of the same temp and duration. I've seen this already with ccdstack. My images were 5 minutes as was my darks. Temps were almost identical. but the darks were scaled to roughly 94% on most of my images. Interesting action but it seemed to be correct.
            >
            > I have a feeling ccdstack has some routines in there that are more complex than just subtracting the pedestal.
            >
            > Anyway, if you're gonna shoot the biases anyway, might as well create a bias master and use it.
            >
            > But on top of all of this is to not use good imaging time for calibration data gathering. If the sky is clear and you're aligned and ready to shoot, then shoot sky, not black.
            >
            > Tom P.
            >
            > --- In ccd-newastro@yahoogroups.com, "Stuart Forman" <s24man@...> wrote:
            > >
            > > Hi all,
            > >
            > > I am a new member to this group, and in the spirit of the "classroom" nature, I'd like to dive in with a series of questions about bias frames.
            > >
            > > I have been a DSLR imager for a couple of years, and have used DSS. I diligently took my lights, darks, dark-flats, flats, and flat frames and plugged them into the program.
            > >
            > > But I'm upgrading to a QSI 683, I've started using CCDstack, and I'm dipping my toe into Pixinsight. I'm realizing that I need to understand these calibration frames much more.
            > >
            > > I've studied these sections in Ron's book, and I understand the concepts behind his bias chapter. From my understanding of what Ron wrote, if you take darks that are the same time as your lights, you don't need bias frames. Is this the current thinking?
            > >
            > > Taking CCDstack as an example.
            > > CCDstack asks you to make a master bias, and a master dark. But in the creation of the master dark, it doesn't ask you to subtract the bias.
            > >
            > > Where it asks you to subtract the bias is in the creation of the master flat. And it actually asks you twice--once in the creation of the master flat, and the second in the routine to calibrate the light frames.
            > >
            > > Pixinsight seems to have a similar routine (although frankly learning Pixinsight to me is about as fun as sticking a hot poker in my eye so I haven't explored it very deeply and am missing something for sure).
            > >
            > > Can someone please help me deystify this?
            > >
            > > Regards,
            > > Stuart Forman
            > > TEC 140
            > > A-P Mach1 GTO
            > >
            >
            >



            [Non-text portions of this message have been removed]
          • Ron Wodaski
            Bias is only ever subtracted from darks and flats for scaling purposes. Your software should do it correctly if you supply them. Ron Wodaski ... [Non-text
            Message 5 of 12 , Jul 18, 2013
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              Bias is only ever subtracted from darks and flats for scaling purposes. Your software should do it correctly if you supply them.

              Ron Wodaski



              On Jul 18, 2013, at 6:40 PM, Stuart Forman <s24man@...> wrote:

              > Thank you Ron,
              >
              > I'm looking forward to your PI book. At the moment I'm still struggling with the somewhat inscrutable online tutorials before my trial license runs out.
              >
              > Last follow up question, and I thank you for your forbearance:
              >
              > I understand better now about the bias and the flats. And I assume that I don't subtract the bias from the flats twice.
              >
              > Do I subtract the bias from the lights in the final light calibration step before registering?
              >
              > Stuart
              >
              > --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
              > >
              > > Yes, if you take darks of the same duration as your lights, you do not need bias frames. The purpose of a bias frame is to remove the initial count of electrons from a dark frame in order to scale only the dark current.
              > >
              > > However: if you are making flats, then you need bias frames, because the flat is always scaled when it is applied.
              > >
              > > (So: anything that is scaled needs a bias.)
              > >
              > > It's not unusual that you are being asked to include a bias in your flats, therefore. That is normal. (A flat is not like other types of calibration frames, in that it is always scaled.)
              > >
              > > I'm writing a book on PixInsight. It will help you get that poker out of your eye. :-)
              > >
              > > Ron Wodaski
              > >
              > >
              > >
              > > On Jul 18, 2013, at 1:59 PM, Stuart Forman <s24man@...> wrote:
              > >
              > > > Hi all,
              > > >
              > > > I am a new member to this group, and in the spirit of the "classroom" nature, I'd like to dive in with a series of questions about bias frames.
              > > >
              > > > I have been a DSLR imager for a couple of years, and have used DSS. I diligently took my lights, darks, dark-flats, flats, and flat frames and plugged them into the program.
              > > >
              > > > But I'm upgrading to a QSI 683, I've started using CCDstack, and I'm dipping my toe into Pixinsight. I'm realizing that I need to understand these calibration frames much more.
              > > >
              > > > I've studied these sections in Ron's book, and I understand the concepts behind his bias chapter. From my understanding of what Ron wrote, if you take darks that are the same time as your lights, you don't need bias frames. Is this the current thinking?
              > > >
              > > > Taking CCDstack as an example.
              > > > CCDstack asks you to make a master bias, and a master dark. But in the creation of the master dark, it doesn't ask you to subtract the bias.
              > > >
              > > > Where it asks you to subtract the bias is in the creation of the master flat. And it actually asks you twice--once in the creation of the master flat, and the second in the routine to calibrate the light frames.
              > > >
              > > > Pixinsight seems to have a similar routine (although frankly learning Pixinsight to me is about as fun as sticking a hot poker in my eye so I haven't explored it very deeply and am missing something for sure).
              > > >
              > > > Can someone please help me deystify this?
              > > >
              > > > Regards,
              > > > Stuart Forman
              > > > TEC 140
              > > > A-P Mach1 GTO
              > > >
              > > >
              > >
              > >
              > >
              > > [Non-text portions of this message have been removed]
              > >
              >
              >



              [Non-text portions of this message have been removed]
            • larryl
              Can we sign up for the book now? ;-) Larry Leitch
              Message 6 of 12 , Jul 19, 2013
              • 0 Attachment
                Can we sign up for the book now? ;-)
                Larry Leitch

                --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
                >
                > Yes, if you take darks of the same duration as your lights, you do not need bias frames. The purpose of a bias frame is to remove the initial count of electrons from a dark frame in order to scale only the dark current.
                >
                > However: if you are making flats, then you need bias frames, because the flat is always scaled when it is applied.
                >
                > (So: anything that is scaled needs a bias.)
                >
                > It's not unusual that you are being asked to include a bias in your flats, therefore. That is normal. (A flat is not like other types of calibration frames, in that it is always scaled.)
                >
                > I'm writing a book on PixInsight. It will help you get that poker out of your eye. :-)
                >
                > Ron Wodaski
                >
                >
                >
                > On Jul 18, 2013, at 1:59 PM, Stuart Forman <s24man@...> wrote:
                >
                > > Hi all,
                > >
                > > I am a new member to this group, and in the spirit of the "classroom" nature, I'd like to dive in with a series of questions about bias frames.
                > >
                > > I have been a DSLR imager for a couple of years, and have used DSS. I diligently took my lights, darks, dark-flats, flats, and flat frames and plugged them into the program.
                > >
                > > But I'm upgrading to a QSI 683, I've started using CCDstack, and I'm dipping my toe into Pixinsight. I'm realizing that I need to understand these calibration frames much more.
                > >
                > > I've studied these sections in Ron's book, and I understand the concepts behind his bias chapter. From my understanding of what Ron wrote, if you take darks that are the same time as your lights, you don't need bias frames. Is this the current thinking?
                > >
                > > Taking CCDstack as an example.
                > > CCDstack asks you to make a master bias, and a master dark. But in the creation of the master dark, it doesn't ask you to subtract the bias.
                > >
                > > Where it asks you to subtract the bias is in the creation of the master flat. And it actually asks you twice--once in the creation of the master flat, and the second in the routine to calibrate the light frames.
                > >
                > > Pixinsight seems to have a similar routine (although frankly learning Pixinsight to me is about as fun as sticking a hot poker in my eye so I haven't explored it very deeply and am missing something for sure).
                > >
                > > Can someone please help me deystify this?
                > >
                > > Regards,
                > > Stuart Forman
                > > TEC 140
                > > A-P Mach1 GTO
                > >
                > >
                >
                >
                >
                > [Non-text portions of this message have been removed]
                >
              • rwright@starstonesoftware.com
                ... I actually take darks for my flatsà so again, no bias needed there either. Somebody remarked about not wasting starlight time on calibration frames. Heck
                Message 7 of 12 , Jul 19, 2013
                • 0 Attachment
                  > However: if you are making flats, then you need bias frames, because the flat is always scaled when it is applied.


                  I actually take darks for my flats… so again, no bias needed there either.

                  Somebody remarked about not wasting starlight time on calibration frames. Heck no! I use a Spike-a-flat flat field panel to take flats whenever I need to. With a cooled CCD you can also create a nice dark library from your den. I refresh my dark library every 6 months, as chip characteristics can erode somewhat over time. Whenever I archive my raw data, I also archive the calibration frames as well, as I've found as I learn more PixInsight, etc. I can sometimes revisit year old data and get a much better result that I did originally. Something to do during the rainy season in Florida.

                  I too will be looking forward to Ron's new book.

                  Richard

                  On Jul 18, 2013, at 6:24 PM, Ron Wodaski <yahoo@...> wrote:

                  > Yes, if you take darks of the same duration as your lights, you do not need bias frames. The purpose of a bias frame is to remove the initial count of electrons from a dark frame in order to scale only the dark current.
                  >
                  > However: if you are making flats, then you need bias frames, because the flat is always scaled when it is applied.
                  >
                  > (So: anything that is scaled needs a bias.)
                  >
                  > It's not unusual that you are being asked to include a bias in your flats, therefore. That is normal. (A flat is not like other types of calibration frames, in that it is always scaled.)
                  >
                  > I'm writing a book on PixInsight. It will help you get that poker out of your eye. :-)
                  >
                  > Ron Wodaski
                  >
                  >
                  >
                  > On Jul 18, 2013, at 1:59 PM, Stuart Forman <s24man@...> wrote:
                  >
                  >> Hi all,
                  >>
                  >> I am a new member to this group, and in the spirit of the "classroom" nature, I'd like to dive in with a series of questions about bias frames.
                  >>
                  >> I have been a DSLR imager for a couple of years, and have used DSS. I diligently took my lights, darks, dark-flats, flats, and flat frames and plugged them into the program.
                  >>
                  >> But I'm upgrading to a QSI 683, I've started using CCDstack, and I'm dipping my toe into Pixinsight. I'm realizing that I need to understand these calibration frames much more.
                  >>
                  >> I've studied these sections in Ron's book, and I understand the concepts behind his bias chapter. From my understanding of what Ron wrote, if you take darks that are the same time as your lights, you don't need bias frames. Is this the current thinking?
                  >>
                  >> Taking CCDstack as an example.
                  >> CCDstack asks you to make a master bias, and a master dark. But in the creation of the master dark, it doesn't ask you to subtract the bias.
                  >>
                  >> Where it asks you to subtract the bias is in the creation of the master flat. And it actually asks you twice--once in the creation of the master flat, and the second in the routine to calibrate the light frames.
                  >>
                  >> Pixinsight seems to have a similar routine (although frankly learning Pixinsight to me is about as fun as sticking a hot poker in my eye so I haven't explored it very deeply and am missing something for sure).
                  >>
                  >> Can someone please help me deystify this?
                  >>
                  >> Regards,
                  >> Stuart Forman
                  >> TEC 140
                  >> A-P Mach1 GTO
                  >>
                  >>
                  >
                  >
                  >
                  > [Non-text portions of this message have been removed]
                  >
                  >
                  >
                  > ------------------------------------
                  >
                  >
                  >
                  >
                • Ron Wodaski
                  I don t know how I could do that. :-) It will be an ebook (though probably I will set it up on CreateSpace so that those who want a hard copy can get one). I
                  Message 8 of 12 , Jul 19, 2013
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                    I don't know how I could do that. :-)

                    It will be an ebook (though probably I will set it up on CreateSpace so that those who want a hard copy can get one).

                    I will need a few beta readers; will announce when that's ready to go.

                    Ron Wodaski



                    On Jul 19, 2013, at 6:17 AM, larryl <k3fit@...> wrote:

                    > Can we sign up for the book now? ;-)
                    > Larry Leitch
                    >
                    > --- In ccd-newastro@yahoogroups.com, Ron Wodaski <yahoo@...> wrote:
                    > >
                    > > Yes, if you take darks of the same duration as your lights, you do not need bias frames. The purpose of a bias frame is to remove the initial count of electrons from a dark frame in order to scale only the dark current.
                    > >
                    > > However: if you are making flats, then you need bias frames, because the flat is always scaled when it is applied.
                    > >
                    > > (So: anything that is scaled needs a bias.)
                    > >
                    > > It's not unusual that you are being asked to include a bias in your flats, therefore. That is normal. (A flat is not like other types of calibration frames, in that it is always scaled.)
                    > >
                    > > I'm writing a book on PixInsight. It will help you get that poker out of your eye. :-)
                    > >
                    > > Ron Wodaski
                    > >
                    > >
                    > >
                    > > On Jul 18, 2013, at 1:59 PM, Stuart Forman <s24man@...> wrote:
                    > >
                    > > > Hi all,
                    > > >
                    > > > I am a new member to this group, and in the spirit of the "classroom" nature, I'd like to dive in with a series of questions about bias frames.
                    > > >
                    > > > I have been a DSLR imager for a couple of years, and have used DSS. I diligently took my lights, darks, dark-flats, flats, and flat frames and plugged them into the program.
                    > > >
                    > > > But I'm upgrading to a QSI 683, I've started using CCDstack, and I'm dipping my toe into Pixinsight. I'm realizing that I need to understand these calibration frames much more.
                    > > >
                    > > > I've studied these sections in Ron's book, and I understand the concepts behind his bias chapter. From my understanding of what Ron wrote, if you take darks that are the same time as your lights, you don't need bias frames. Is this the current thinking?
                    > > >
                    > > > Taking CCDstack as an example.
                    > > > CCDstack asks you to make a master bias, and a master dark. But in the creation of the master dark, it doesn't ask you to subtract the bias.
                    > > >
                    > > > Where it asks you to subtract the bias is in the creation of the master flat. And it actually asks you twice--once in the creation of the master flat, and the second in the routine to calibrate the light frames.
                    > > >
                    > > > Pixinsight seems to have a similar routine (although frankly learning Pixinsight to me is about as fun as sticking a hot poker in my eye so I haven't explored it very deeply and am missing something for sure).
                    > > >
                    > > > Can someone please help me deystify this?
                    > > >
                    > > > Regards,
                    > > > Stuart Forman
                    > > > TEC 140
                    > > > A-P Mach1 GTO
                    > > >
                    > > >
                    > >
                    > >
                    > >
                    > > [Non-text portions of this message have been removed]
                    > >
                    >
                    >



                    [Non-text portions of this message have been removed]
                  • Ron Wodaski
                    A dark contains the bias, so that statement of mine still stands. ;-) I agree that archiving your cal frames is a wise thing to do. Ron Wodaski
                    Message 9 of 12 , Jul 19, 2013
                    • 0 Attachment
                      A dark contains the bias, so that statement of mine still stands. ;-)

                      I agree that archiving your cal frames is a wise thing to do.

                      Ron Wodaski



                      On Jul 19, 2013, at 7:05 AM, "rwright@..." <rwright@...> wrote:

                      >> However: if you are making flats, then you need bias frames, because the flat is always scaled when it is applied.
                      >
                      >
                      > I actually take darks for my flats… so again, no bias needed there either.
                      >
                      > Somebody remarked about not wasting starlight time on calibration frames. Heck no! I use a Spike-a-flat flat field panel to take flats whenever I need to. With a cooled CCD you can also create a nice dark library from your den. I refresh my dark library every 6 months, as chip characteristics can erode somewhat over time. Whenever I archive my raw data, I also archive the calibration frames as well, as I've found as I learn more PixInsight, etc. I can sometimes revisit year old data and get a much better result that I did originally. Something to do during the rainy season in Florida.
                      >
                      > I too will be looking forward to Ron's new book.
                      >
                      > Richard
                      >
                      > On Jul 18, 2013, at 6:24 PM, Ron Wodaski <yahoo@...> wrote:
                      >
                      >> Yes, if you take darks of the same duration as your lights, you do not need bias frames. The purpose of a bias frame is to remove the initial count of electrons from a dark frame in order to scale only the dark current.
                      >>
                      >> However: if you are making flats, then you need bias frames, because the flat is always scaled when it is applied.
                      >>
                      >> (So: anything that is scaled needs a bias.)
                      >>
                      >> It's not unusual that you are being asked to include a bias in your flats, therefore. That is normal. (A flat is not like other types of calibration frames, in that it is always scaled.)
                      >>
                      >> I'm writing a book on PixInsight. It will help you get that poker out of your eye. :-)
                      >>
                      >> Ron Wodaski
                      >>
                      >>
                      >>
                      >> On Jul 18, 2013, at 1:59 PM, Stuart Forman <s24man@...> wrote:
                      >>
                      >>> Hi all,
                      >>>
                      >>> I am a new member to this group, and in the spirit of the "classroom" nature, I'd like to dive in with a series of questions about bias frames.
                      >>>
                      >>> I have been a DSLR imager for a couple of years, and have used DSS. I diligently took my lights, darks, dark-flats, flats, and flat frames and plugged them into the program.
                      >>>
                      >>> But I'm upgrading to a QSI 683, I've started using CCDstack, and I'm dipping my toe into Pixinsight. I'm realizing that I need to understand these calibration frames much more.
                      >>>
                      >>> I've studied these sections in Ron's book, and I understand the concepts behind his bias chapter. From my understanding of what Ron wrote, if you take darks that are the same time as your lights, you don't need bias frames. Is this the current thinking?
                      >>>
                      >>> Taking CCDstack as an example.
                      >>> CCDstack asks you to make a master bias, and a master dark. But in the creation of the master dark, it doesn't ask you to subtract the bias.
                      >>>
                      >>> Where it asks you to subtract the bias is in the creation of the master flat. And it actually asks you twice--once in the creation of the master flat, and the second in the routine to calibrate the light frames.
                      >>>
                      >>> Pixinsight seems to have a similar routine (although frankly learning Pixinsight to me is about as fun as sticking a hot poker in my eye so I haven't explored it very deeply and am missing something for sure).
                      >>>
                      >>> Can someone please help me deystify this?
                      >>>
                      >>> Regards,
                      >>> Stuart Forman
                      >>> TEC 140
                      >>> A-P Mach1 GTO
                      >>>
                      >>>
                      >>
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