Author Topic: Confusion over Which Master Frames to Deduct from Flats  (Read 789 times)

Offline STAstro

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Confusion over Which Master Frames to Deduct from Flats
« on: 2018 February 12 21:25:01 »
Hi All

I am reading a lot of contradictory posts on here as well as the PixInsight Tutorials.

I am using the QHY183M Camera, and I am just building my Darks, Bias and Flats, I have created my SuperBIAS already.

Now I know I have to apply the Super BIAS to my Dark Frames before Integration to create my Master Darks, however for the flats, since my exposure for flats are all below 2 seconds, do I really need to subtract darks from my flats?  Surely I just subtract the SuperBIAS from my flats no?

Also should all my masters be saved in 32Bit Floating?

Many thanks all
Simon

Offline pfile

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #1 on: 2018 February 13 00:00:59 »
you might be able to get away with just a master bias for the flats. or you could take darks that match the duration of the flats and use those. you dont *have* to subtract the bias from the darks, but if you want to scale (or optimize in the PI parlance) your darks you do need to bias-subtract the darks one way or another. on the other hand, theres probably not too much reason to scale the dark flats as they will very likely be mostly bias signal anyway.

rob

Offline STAstro

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #2 on: 2018 February 13 22:39:32 »
Thanks Rob

Yes my Master Darks don't need to be scaled as they're all done at the same exposure length and temperature as the light frames.  So if I hear you

1. Subtract the SuperBIAS from Flats during Flats Calibration and then Combine Master Flat(s)
2. Subtract the SuperBIAS from Darks during Darks Calibration and then Combine to Master Dark(s)
3. Subtract the SuperBIAS, Master Darks and Master Flats during Light Calibration and then combine Light(s)

Correct?

Simon

Offline pfile

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #3 on: 2018 February 14 07:02:57 »
that is one way to do it. as bernd has pointed out in his thread (https://pixinsight.com/forum/index.php?topic=11968.0), there is a risk of clamping pixels to 0 in the dark when pre-calibrating the dark subs.

alternately you just integrate the uncalibrated dark subs into an uncalibrated master dark, then at the time you are calibrating lights, you tick "calibrate" in the master dark section.

oh, in #3 - you are dividing lights by the master flat. not that you need to specify that or anything; ImageCalibration does the right thing, but just so you know.

rob

Offline STAstro

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #4 on: 2018 February 14 20:27:31 »
Thanks Rob

I have been using this guide for my SuperBIAS and Master Darks, and it talks about creating the Calibrated Master Darks in there, so I do that part

However when it talks about generating the master flat, it is refering to calibrating them with a SuperBias as well as a Master Dark, which I find a little odd considring my flat frames are all below 3 seconds (Including Narrowband)

"As with the master dark, before we proceed with stacking the flat frames, we need to calibrate them individually with the master superbias and the master dark. To do this, we return to the ImageCalibration process. To avoid confusion, we can click the Reset button to reset the process back to defaults. "

Hence my post, this is where I am confused, I thought that if you do not take BIAS frames you need to do dark flats at the same exposure length of your flats, but if you take BIAS frames you should not need to do dark flats....now you see where the confusion comes from?

Offline STAstro

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #5 on: 2018 February 14 20:34:55 »
So the tip here is to use "Uncalibrated" Darks and Flats and just chose the option during the ImageCalibration

Pitty the Pre-Process script doesn't have the option of calibrating them :(

Offline STAstro

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #6 on: 2018 February 14 20:49:47 »
After reading that, I get

1. Do not Calibrate my Darks, just Integrate them as a MasterDark-Uncalibrated
2. Calibrate your Flats - With SuperBIAS (That post doesn't talk very much about flats), don't see the point in calibrating a dark with the Flats, correct me if I am wrong here, but the exposure is so small my lowest Dark Frame exposure is 90 seconds
3. During image integration do not select calibrate or optimise against the MasterDark and since I am using a Temperature Cooled Camera, I do not need to optimise the Darks, so therefore I do not need a SuperBIAS in the image calibration.  So I will just be calibrating against my Un-Calibrated Master Dark and my Calibrated Master Flat

« Last Edit: 2018 February 14 21:05:48 by STAstro »

Offline pfile

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #7 on: 2018 February 14 22:46:00 »
well you'd never calibrate a dark with a flat... you calibrate darks with bias/superbias only.

you can always make special dark subs that are the same length as the flats, but when your flats are very short, a bias frame usually suffices.

BPP always produces uncalibrated dark masters and calibrated flat masters, then i believe by default it calibrates the master dark and scales it during light calibration. the reason it makes calibrated flat masters is so that you can prepare a master flat separately (for instance, at a different gain or ISO) and still use it when you are processing a different set of lights.

rob

Offline STAstro

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #8 on: 2018 February 15 09:40:32 »
But if all my dark masters are the same exposure and temperature of my lights then the "Scaling" and "Optimizing" of the darks during Light integration is not neccesary right?  That's if I am reading correctly...lol

Offline pfile

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #9 on: 2018 February 15 16:44:20 »
well even if the darks are exactly matched, there could be some benefit in scaling them. pixinsight iteratively scales the darks until the noise in the calibrated image is minimized.

rob

Offline STAstro

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Re: Confusion over Which Master Frames to Deduct from Flats
« Reply #10 on: 2018 February 15 17:05:32 »
Just completed my BIAS to SuperBIAS

Went through the process of:

1. Creating 251 BIAS Frames
2. Integrating them to a MasterBIAS Frame
3. Creating a SuperBIAS based on Columns
4. Used PixelMath with the following MasterBIAS_251_Frames - superbias_h + 0.1
5. The resulting frame then had a SuperBIAS applied to it using Rows
6. A Pixelmath was then applied using superbias_v + superbias_h - 0.1

Resulting in a SuperBIAS Frame with horizonatal and vertical BIAS corrected, here's the results, left is the MasterBIAS of 251 frames, right is the SuperBIAS, both had auto STF applied