32 replies to this topic

[NoDarkSkies]

Hello All,

I know for Flats that you need to take both Gain 0 and Gain 100 that is if you are using both.

But do I also need to do this with Dark and Bias Calibration Files?

Thanks

[afd33]

Yes, you need calibration frames to match the gain of light frames.

[NoDarkSkies]

Yes, you need calibration frames to match the gain of light frames.

Thanks, that is what I have been doing but wanted to make sure it was correct being they take a Long time to create.
 

[afd33]

Thanks, that is what I have been doing but wanted to make sure it was correct being they take a Long time to create.
 

If you're using a cooled camera, you should make a dark library with all the master darks you'll need, and then make master bias frames at the different gains you use.

 

I only use one gain setting, so I took bias and dark frames last year when I got my camera, stacked them, and saved the master files. Been using the same ones since November. Yeah, it took a while to do the darks with all the different exposure lengths, but still well worth it to save time in the long run. You'll be saving time by not having to take the frames, but possibly  more so when stacking.

[bobzeq25]

Thanks, that is what I have been doing but wanted to make sure it was correct being they take a Long time to create.
 

So why not just use gain 100  as you start out in imaging?  It's perfectly good.  Using gain zero can be slightly better in some situations, but only slightly.  So, simplify.

[idclimber]

As someone who has imaged with both gain 0 and 100, I would strongly suggest staying with 100. The added complexity is simply not worth it especially for a novice. 

 

That said you absolutely should match gains, temperatures, offset on all four frames (lights, darks, flats and bias). You can do some mismatching but absolutely need to match Darks to Lights and Bias to Flats. The reason is simple each of those frames are paired together. 

 

But I caution you NOT TO DO THIS MISMATCHING. At least not until you fully comprehend the underlying concepts, the math and process of calibration so that you can do it manually in your sleep while you are hung over or still drunk. Just today I assisted someone who got it in their head you can mismatch these frames and turned the cooler off for his bias and flats. Don't do anything like this until you have the knowledge and skill to compare these alternate ways against the standard.

 

The standard is to use to do full calibration with gains, temperature and offset matched on all Lights, Darks, Flats and Bias frames. Exposures must be matched between the Lights and the Darks. Bias is set to the lowest time the software will allow (typically millisecond range). Use Dark flats instead of bias if you have one of the finicky cameras like the ASI294. 

[NoDarkSkies]

As someone who has imaged with both gain 0 and 100, I would strongly suggest staying with 100. The added complexity is simply not worth it especially for a novice. 

 

That said you absolutely should match gains, temperatures, offset on all four frames (lights, darks, flats and bias). You can do some mismatching but absolutely need to match Darks to Lights and Bias to Flats. The reason is simple each of those frames are paired together. 

 

But I caution you NOT TO DO THIS MISMATCHING. At least not until you fully comprehend the underlying concepts, the math and process of calibration so that you can do it manually in your sleep while you are hung over or still drunk. Just today I assisted someone who got it in their head you can mismatch these frames and turned the cooler off for his bias and flats. Don't do anything like this until you have the knowledge and skill to compare these alternate ways against the standard.

 

The standard is to use to do full calibration with gains, temperature and offset matched on all Lights, Darks, Flats and Bias frames. Exposures must be matched between the Lights and the Darks. Bias is set to the lowest time the software will allow (typically millisecond range). Use Dark flats instead of bias if you have one of the finicky cameras like the ASI294. 

So even in Bortle 1/2 and even Bortle 3/4 I should always use Gain 100 for both the 2600 and 6200?

When I do Pre-Process I do Calibration and save the files at the correct Gain and all and then I stack them and Post Process them. ASIAIR Files.

In MaxIm DL 6 I Calibrate as I am imaging with the correct Calibration files.

 

Thanks
 

Edited by NoDarkSkies, 24 September 2023 - 11:09 PM.

[idclimber]

So even in Bortle 1/2 and even Bortle 3/4 I should always use Gain 100 for both the 2600 and 6200?

When I do Pre-Process I do Calibration and save the files at the correct Gain and all and then I stack them and Post Process them. ASIAIR Files.

In MaxIm DL 6 I Calibrate as I am imaging with the correct Calibration files.

 

Thanks
 

I live on the Edge of Bortle 2. I do not think that issue matters. 

 

I have used gain 0 for my broadband imaging with my mono ASI2600mm camera. That is through LRGB filters. Lately I have switched to using gain 100 for those filters. There is little gain if any from an image quality standpoint. 

 

This would be the same for the 6200 as well. 

[NoDarkSkies]

Thanks everyone, I guess it is time to change my process to Gain 100 from now on.

[acrh2]

I live on the Edge of Bortle 2. I do not think that issue matters. 

 

I have used gain 0 for my broadband imaging with my mono ASI2600mm camera. That is through LRGB filters. Lately I have switched to using gain 100 for those filters. There is little gain if any from an image quality standpoint. 

 

This would be the same for the 6200 as well. 

How do you deal with saturated stars? 

With 60 sec exposures and L filter, a 72 mm scope (f/4.8) already saturates magnitude 8 stars at gain 0. 

[idclimber]

How do you deal with saturated stars? 

With 60 sec exposures and L filter, a 72 mm scope (f/4.8) already saturates magnitude 8 stars at gain 0. 

Most of my LRGB imaging the last year has been with my SCT at f/7 or the SVX152 at f/8. If anything I am probably underexposed on my subs as I typically have around 200-300 clipped pixels at gain 100 with 60" subs on RGB and 120" on Lum. 

[afd33]

How do you deal with saturated stars?
With 60 sec exposures and L filter, a 72 mm scope (f/4.8) already saturates magnitude 8 stars at gain 0.

Shorter exposures.

[Daniel Jouret]

Are bias still required when shooting mono ?
I just got delivered with the ASI 2600 MM pro and starting to build my library. I believe if I'm shooting dark flats, there's no need for bias frames right ?

Also, I prefer to ask before wasting too much time : do you shoot dark frames with the filters in (LRGBSHO)?

[NoDarkSkies]

Are bias still required when shooting mono ?
I just got delivered with the ASI 2600 MM pro and starting to build my library. I believe if I'm shooting dark flats, there's no need for bias frames right ?

Also, I prefer to ask before wasting too much time : do you shoot dark frames with the filters in (LRGBSHO)?

I do on both my Mono Cameras but I also use MaxIm DL 6 to take the images and Stack and somewhat Post Processes.

I get a Complete Calibration doing so, if not I miss the Dark Bias that somehow creates the Dark Flat or something like this with MaxIm DL 6.

 

HISTORY    Bias Subtraction (Bias 2, 9576 x 6388, Bin1 x 1, Temp -20C,           
HISTORY    Dark Subtraction (Dark 12, 9576 x 6388, Bin1 x 1, Temp -20C,          
HISTORY    Dark-Bias(Bias 2,9576 x 6388,Bin1 x 1,Temp -20C,Exp Time 0ms)         
HISTORY    Flat Field (Flat R 1, R, 9576 x 6388, Bin1 x 1, Temp -15C,            
HISTORY    Flat-Bias(Bias 2,9576 x 6388,Bin1 x 1,Temp -20C,Exp Time 0ms)         
HISTORY    Flat-Dark (Dark 12,9576 x 6388,Bin1 x 1,Temp -20C,Exp Time 300s)

Edited by NoDarkSkies, 25 September 2023 - 07:28 AM.

[FrostByte]

Are bias still required when shooting mono ?
I just got delivered with the ASI 2600 MM pro and starting to build my library. I believe if I'm shooting dark flats, there's no need for bias frames right ?

Also, I prefer to ask before wasting too much time : do you shoot dark frames with the filters in (LRGBSHO)?

You need either bias or dark flats for calibrating your flats, not both. Pick one or the other and stick with it.

 

No, you don't need filters in place when shooting darks. You can remove the camera entirely and stick it in a dark box to shoot your darks - optics have no impact on them whatsoever.

[Tapio]

Are bias still required when shooting mono ?
I just got delivered with the ASI 2600 MM pro and starting to build my library. I believe if I'm shooting dark flats, there's no need for bias frames right ?

Also, I prefer to ask before wasting too much time : do you shoot dark frames with the filters in (LRGBSHO)?

Only some cameras require dark flats and I believe asi2600 is not one of them.
Using them doesn't hurt but you could use just bias frames.

Dark frames - it doesn't matter if you have filters or not, because no light should go through.
Does this camera even need dark frames?
Maybe if you take longer NB exposures.

[mjanzou]

I have always used g100/0-degree temp with my 533 adjusting exposure time as needed.
I have modified siril scripts that use a master (bias) calibrated flat for each filter and master dark for exposure time(s), swapping them in as needed. Saves a lot of time and disk space during preprocessing.

Using both has always produced a better result.

Edited by mjanzou, 25 September 2023 - 08:29 AM.

[Daniel Jouret]

Only some cameras require dark flats and I believe asi2600 is not one of them.
Using them doesn't hurt but you could use just bias frames.

Dark frames - it doesn't matter if you have filters or not, because no light should go through.
Does this camera even need dark frames?
Maybe if you take longer NB exposures.

A friend of mine who has the 2600 MC noticed that if using only flats for calibration, he gets a darker center of the image. Only dark flats solved this issue. 
I guess that they shouldn't be that much of a difference with the 2600 MM, and as I'm taking flats anyway, it's quite easy to dark dark flats by keeping the same exposure time and just put the cap on. 

Regarding filters for dark frames, it does make sense, cheers! 

[FrostByte]

A friend of mine who has the 2600 MC noticed that if using only flats for calibration, he gets a darker center of the image. Only dark flats solved this issue. 

Flats will overcorrect if you don't use either dark flats OR bias. I've used both on my 2600MC and they each work just fine, but no harm in using dark flats if that's your preference. I've switched to bias for calibrating the flats as I use sky flats and the exposure time varies to keep the ADU ~30K with changing sky conditions.

[idclimber]

Are bias still required when shooting mono ?
I just got delivered with the ASI 2600 MM pro and starting to build my library. I believe if I'm shooting dark flats, there's no need for bias frames right ?

Also, I prefer to ask before wasting too much time : do you shoot dark frames with the filters in (LRGBSHO)?

 

Bias and darks flats are interchangeable from how they are applied with current processing work flows (this was not always the case). Either are integrated into a master and that master is simply subtracted pixel by pixel from an individual flat frame. This sensor especially at gain 100 has extremely low read noise. As such there is really no difference between a millisecond exposure and one that is one or two seconds. As such using either is purely academic. 

 

A friend of mine who has the 2600 MC noticed that if using only flats for calibration, he gets a darker center of the image. Only dark flats solved this issue. 
I guess that they shouldn't be that much of a difference with the 2600 MM, and as I'm taking flats anyway, it's quite easy to dark dark flats by keeping the same exposure time and just put the cap on. 

Regarding filters for dark frames, it does make sense, cheers! 

The next step in calibration is to integrate those calibrate flats into a master flat. Integration is simply averaging each pixel and rejection of outliers. There are mathematical differences between settings and there are some other tricks that are employed when integration of lights are done, but this process is relatively basic math. 

 

Then the master flat is applied to each light frame . This is done through division, again pixel by pixel. That result is then normalized by multiplying by the mean ADU of the master. This is essentially a ratio. The dimmer areas in the corners are reduced and any areas that are brighter than the average are increased. Average areas are multiplied by one and are unchanged. 

 

Skipping bias subtraction changes this ratio, and changes the math. The result is exactly what your friend got. Over correction. 

 

Each and every frame has a component of fixed and random noise. We tackle random by taking more of these frames. We go after the fixed with this dark frame subtractions. In the case of bias or darks, people often suggest their low noise means that using them adds more noise (through subtraction) then they are removing. That can be the case if insufficient frames are integrated. 

 

Instead of elaborate libraries of darks at different gains, temperatures, and offsets. Simplify and take more of them. This is the exact same logic of taking more light frames and more total integration time. 

 

For my library I really only use 60", 120" and 300" exposures. I occasionally use 30" and very rarely 600". The is covers most of the 10s of thousand of subs the last couple years. 

 

I suggest taking at least 50 darks at each exposure. For bias take 200. This is not hard and you do not need to keep that data after it is integrated into the master.

 

I only use one temperature  -10c. I have a relatively cool climate, especially at night here in the mountains of Idaho. -5c or 0c is nearly indistinguishable and will work just fine in warmer climates. It needs to be lower than your expected lows. And not so low that it puts the cooler above 70-80%

 

Offset is simply a value that is added to each frame to prevent zeros. Zero measurements are like black holes in processing. The ASIAIR doesn't allow you to change this. I use 30, 50 is also fine. Just pick either and stay with it. On this camera and offset of 30 adds 300, and offset of 50 adds 500. Either is trivial to the 65,535 count that 16 bits provides. 

[idclimber]

 

I do on both my Mono Cameras but I also use MaxIm DL 6 to take the images and Stack and somewhat Post Processes.

I get a Complete Calibration doing so, if not I miss the Dark Bias that somehow creates the Dark Flat or something like this with MaxIm DL 6.

 

HISTORY    Bias Subtraction (Bias 2, 9576 x 6388, Bin1 x 1, Temp -20C,           
HISTORY    Dark Subtraction (Dark 12, 9576 x 6388, Bin1 x 1, Temp -20C,          
HISTORY    Dark-Bias(Bias 2,9576 x 6388,Bin1 x 1,Temp -20C,Exp Time 0ms)         
HISTORY    Flat Field (Flat R 1, R, 9576 x 6388, Bin1 x 1, Temp -15C,            
HISTORY    Flat-Bias(Bias 2,9576 x 6388,Bin1 x 1,Temp -20C,Exp Time 0ms)         
HISTORY    Flat-Dark (Dark 12,9576 x 6388,Bin1 x 1,Temp -20C,Exp Time 300s)

This looks like you are subtracting the bias from the lights??? This is the workflow in a graphic form taken from the DSS website. 

 

 

This is not needed for these current CMOS sensors and very easy to double subtract the bias. What you want to use and what PixInsight does in WBPP is this. Bias is substituted for dark flats. 

 

 

 

Also -20c, are you sure you are able to maintain that low? I would not unless I was imaging in the winter. 

Edited by idclimber, 25 September 2023 - 09:30 AM.

[mjanzou]

 

 

This looks like you are subtracting the bias from the lights??? This is the workflow in a graphic form taken from the DSS website. 

 

 

 

This is not needed for these current CMOS sensors and very easy to double subtract the bias. What you want to use and what PixInsight does in WBPP is this. Bias is substituted for dark flats. 

 

 

+1. I always thought the bias/dark-flat was always applied to the flat. Perhaps the DSS graphic is a graph-o???

Nice thing about AsiStudio deepstack is that it will not stack your flats without providing a set of bias/dark-flats (whichever you prefer) as well.

The end result is that it spits out a calibrated 'master flat'...

[idclimber]

+1. I always thought the bias/dark-flat was always applied to the flat. Perhaps the DSS graphic is a graph-o???

Nice thing about AsiStudio deepstack is that it will not stack your flats without providing a set of bias/dark-flats (whichever you prefer) as well.

The end result is that it spits out a calibrated 'master flat'...

Subtracting the bias from the darks allowed you to scale them to different exposures. This was common with CCD sensors. It does not work with CMOS and as such went away. The math is equivalent if done correctly. 

[bobzeq25]

A friend of mine who has the 2600 MC noticed that if using only flats for calibration, he gets a darker center of the image. Only dark flats solved this issue. 
I guess that they shouldn't be that much of a difference with the 2600 MM, and as I'm taking flats anyway, it's quite easy to dark dark flats by keeping the same exposure time and just put the cap on. 

Regarding filters for dark frames, it does make sense, cheers! 

You need EITHER bias or dark flats to calibrate your flats.  If you don't do either the flats won't work.  Math.

 

With a 2600 there is no significant difference between a bias frame and a dark flat frame.  They're the same thing, the same data.  I own both 2600s, have run the numbers, even done the experiment.  The reason is that the only difference between bias and dark flats is thermal noise, and the 2600 doesn't have enough thermal noise to worry about it.

 

Bias are easier/faster so I use bias.  It doesn't matter.  An advantage is, if I change flat exposure, my old bias frames are still good.  Dark flats were necessary with some early CMOS cameras (the 1600s and 294s) that didn't handle short exposures well.  Modern CMOS cameras have no need for them.

 

Doing both has no value, and it makes getting the processing right harder.

 

I use PixInsight.  Here's what I (it) does.

 

Stack the bias to get a master bias.  Stack the darks to get a master dark.  Calibrate the flats with the master bias.  Stack the calibrated flats to get a master flat.

 

Calibrate the lights with the three masters; bias, dark, flat.  Stack the calibrated lights.

 

I have not used the ancient DSS in many years.  It does nothing better than a modern processing program, and it causes many people headaches.  If PI is too complex for you (as it can be), I recommend Astro Pixel Processor.  Does almost as good a job as PI, much more easily/intuitively.

Edited by bobzeq25, 25 September 2023 - 10:40 AM.

[NoDarkSkies]

 

This looks like you are subtracting the bias from the lights??? This is the workflow in a graphic form taken from the DSS website. 

 

This is not needed for these current CMOS sensors and very easy to double subtract the bias. What you want to use and what PixInsight does in WBPP is this. Bias is substituted for dark flats. 

 

Also -20c, are you sure you are able to maintain that low? I would not unless I was imaging in the winter. 

 

In MaxIm DL 6 the Calibration Process is automatic as that the software completes all the Calibration Process.

https://cdn.diffract...OC_SETCALIB.htm

Yep! that was in the winter and it was 37°F (2.77°C) outside. The rest of the time it is set between -10°C to -15°C

I am creating a my Bias and Darks at -15°C and now I am going to also run a -10°C set for the warm Fall Nights that we sometimes have here in Colorado.

I also Calibrate all my images before using ZWO ASIDeepStack, but this also helps me later when I have added more data months later with new Calibration files. I do them each year.

As far as how ZWO ASIDeepStack process works I have do not think it Calibrates each file only the final stacked image being that each image show no data in the FITs Header.

Edited by NoDarkSkies, 25 September 2023 - 01:18 PM.