Calibration frame question

[Larry 1969]

My camera (Altair 26C) produces very little noise. So little that I don't bother taking dark frames. My question is regarding dark flats (or flat darks). If the purpose of dark flats is to remove the noise from the flat frame and I'm not getting any noise in my light frames, should I even bother with them? I can turn off the calibration warnings in APP and just stack lights and flats right?

Thanks!

Larry

[bobharmony]

You can try just stacking lights and flats and see if it makes a difference, Larry. Nothing compares to your experience when it comes to making this kind of decision. Each camera has different characteristics so the mix of calibration frames may be different depending on those characteristics.

For me when it comes to calibration frames, I will use anything that might improve my images. As it doesn't take much to create a master dark flat, I use one for each of my images. Since my flats are 5 seconds each time, reusing a master dark flat is very simple for me. Mabula has documented why he likes to see all the frame types used in APP calibration and in that list he also includes Bias frames. The short of it is that in addition to "noise" there is also unwanted signal in each of our frames that will be handled by a specific type of calibration frame. Here is a link to the article:
https://www.astropixelprocessor.com/com ... must-read/

APP also has the capability of creating a bad pixel map from the calibration frames which is unique among the stacking programs I have used. I am in the beginning stage of learning APP so can't speak to how effective it might be, but it sounds very good in theory.

Bob

[sdbodin]

I'm with you, ditched my cal file stuff a couple years ago and haven't looked back. However, I sometimes still use a BPM, but not often, Photoshop clone stamp just as good.

Remember cal files contain NO data, just noise to cancel noise, to some this makes sense, to me not.
Steve

[Larry 1969]

I'm going to give it a try and see how it turns out. I do believe I need flats as I have significant vignetting, but I'm not so sure about darks.
Thanks for the link Bob. I'll give that a read this weekend.

Larry

[KathyNS]

If you don't need darks, then you shouldn't need dark-flats. But if you don't use dark-flats, you must include bias frames.

You can process lights without removing the read noise and you probably won't notice. But applying flats without removing read noise messes up the math. You can easily have your flats over-correct if you don't use bias frames. It is a common problem.

Note that read noise is not the same as thermal noise. Modern cameras, including yours it seems, produce so little thermal noise that you don't need darks to correct it. But all cameras produce read noise, and the bias frames are there to correct it.

[Larry 1969]

If you don't need darks, then you shouldn't need dark-flats. But if you don't use dark-flats, you must include bias frames.

You can process lights without removing the read noise and you probably won't notice. But applying flats without removing read noise messes up the math. You can easily have your flats over-correct if you don't use bias frames. It is a common problem.

Note that read noise is not the same as thermal noise. Modern cameras, including yours it seems, produce so little thermal noise that you don't need darks to correct it. But all cameras produce read noise, and the bias frames are there to correct it.

Thanks Kathy! That makes sense


Larry

[Larry 1969]

Kathy,
I have a friend who doesn't use any calibration frames at all and now he's wondering if he should use bias frames to correct read noise out of his light frames... Any thoughts? :Think:

Larry

[SkyHiker]

My camera (Altair 26C) produces very little noise. So little that I don't bother taking dark frames. My question is regarding dark flats (or flat darks). If the purpose of dark flats is to remove the noise from the flat frame and I'm not getting any noise in my light frames, should I even bother with them? I can turn off the calibration warnings in APP and just stack lights and flats right?

Thanks!

Larry

I used to skip bias frames with my DSLR, then when I switched to ASI2600s I had horrible stacks. Asking at the DSS IO group what could cause it they pointed out that bias frames are used to measure the offset. That explained it, after adding bias frames back in the problem was solved. So, if you don't use bias frames and it still works, that's pure luck. I would include bias frames, they take little time to get.

[KathyNS]

Henk is right. You might get away with it, but it is better to use them. Definitely essential if you use flats.

[Juno16]

The only time that I don’t use calibration frames is when I am shooting a two-nighter, and can’t wait to peek at the data.

It’s just too easy to shoot a set of darks once or twice a year and flats/bias each session once you develop a routine.

I’m like Bob@bobharmony in that I will do what I need to do to get the best results. I really doesn’t require much.

I have to say though, the “snapshot” image sometimes rivals the two nighter!

[STEVE333]

Just a little clarification:
1) None of the calibrations frames remove noise. In fact, they actually add a slight amount of noise. That is why it is important to average many calibration frames to reduce the noise in the "Masters" so that the noise they add is negligible.

2) All frames (Light, Dark, Bias, Flat and Dark Flat) contain offsets by design to ensure linearity and to prevent clipping any signal.

3) Flat frames require the offset to be removed to work properly. For CCD's subtracting a Master Bias frame is usually used for this purpose. For CMOS subtracting a Master Dark Flat is usually used for this purpose. Flat frames help remove any effects from vignetting or dust motes.

4) Master Dark frames are used to remove bad pixels and any Amp Glow for CCD's and CMOS.

With CCD cameras the Master Bias is often subtracted from the Light frames as well as from the Dark frames before the Master Dark is created. Then the Master Dark is subtracted from each Light frame.
With CMOS the Master Dark often still contains the offset, and, the Master Dark is subtracted from each Light frame (that still contains an offset).
Both approaches work and properly remove the unwanted offsets.

Hope this makes sense.

Steve

[Juno16]

Just a little clarification:
1) None of the calibrations frames remove noise. In fact, they actually add a slight amount of noise. That is why it is important to average many calibration frames to reduce the noise in the "Masters" so that the noise they add is negligible.

2) All frames (Light, Dark, Bias, Flat and Dark Flat) contain offsets by design to ensure linearity and to prevent clipping any signal.

3) Flat frames require the offset to be removed to work properly. For CCD's subtracting a Master Bias frame is usually used for this purpose. For CMOS subtracting a Master Dark Flat is usually used for this purpose. Flat frames help remove any effects from vignetting or dust motes.

4) Master Dark frames are used to remove bad pixels and any Amp Glow for CCD's and CMOS.

With CCD cameras the Master Bias is often subtracted from the Light frames as well as from the Dark frames before the Master Dark is created. Then the Master Dark is subtracted from each Light frame.
With CMOS the Master Dark often still contains the offset, and, the Master Dark is subtracted from each Light frame (that still contains an offset).
Both approaches work and properly remove the unwanted offsets.

Hope this makes sense.

Steve

Very nice explanation Steve!