Flat Darks or Bias for Calibration - 2600MM camera

starvoyager

I’ve been using Bias frames to calibrate my Flats on my 2600MM for the last 8 months with no issues , however folk on another forum recommend I shouldn’t be using Bias frames.
They advised I should be using Flat Darks instead
My Flats exposure times are generally as follows -
Broadband LRGB 0.1 sec to 0.5 sec
Narrowband SHO etc. 1sec to 5sec

My Bias frames have exposure time set to 0.000032sec which is the shortest exposure time for the 2600MM camera ( read from ZWO specs )

I would appreciate some technical advice on the subject of Bias or Flat Darks for the 2600MM camera ( IMX 571 sensor ) as my Calibration has been fine using Bias frames.

Thanks in Advance

Martin

w7ay

starvoyager They advised I should be using Flat Darks instead

Yes, use flat darks, not bias frames -- some call them dark flats, but they are esentially flat frames that are taken with the same exposure settings as the flats, but with no light entering the camera. I.e., the same temperature, exposure duration and gain as you take flats. Bias will be incorporated in the dark frames already, and you can create a Master Flat by incorporating these dark frames.

The temperature does not have to be the same as your Light frames. I do my ASI2600 light frames at -10ºC (already an overkill), but then to do my Flats and Flat Darks indoors at +10ºC.

Be sure to use the same temperature for Flats and Flat Darks -- that is what makes Flat Darks work so well. With some sensors, like the one in ZWO's ASI294, you must use Flat Darks since the sensor is so unpredictable with temperature. The IMX571 in the ASI2600 is much better behaved, but it still doesn't hurt to use Flat Darks.

Since the Flat Darks take the same exposure time as the Flat frames, this will slow you down compared to what you are doing today (taking bias frames).

But, just 16 to 32 flat and dark subexposures should be sufficient (32 subframes will reduce the noise by about 15 dB, and that should be sufficient to not affect the noise of the final image). If you use a single flat frame, for example, it would double the noise and reduce Signal to Noise (SNR) ratio by 3 dB. 16 flat frames should reduce the noise of the Master Flat already by about 12 dB, and the resultant image will only suffer SNR degradation of less than 0.1 dB (you won't even notice it :-).

Do not use regular dark frames (taken at the temperature of the light frames). Just use the same temperature for the Flat Darks that you use for the Flats.

Chen

starvoyager

w7ay

Hi Chen,
Thanks for your reply , you always find time to assist others
Just to clarify, I know how to take Flat Darks ( I should have mentioned that in my post )
My question is specifically targeted at the technical differences between Calibrating Flats with Bias vs Flat Darks
My Bias exposures are super quick but also my Broadband flat’s particularly with a Lum filter is around 0.11 sec ) not that much different to a Bias frame. I can understand that a Bias frame compared to a Flat Dark taken with say my Oiii filter ( 3 to 4 sec ) are worlds apart so I just want to get my head around the technical aspects of Bias vs Flat Dark
Also you mentioned not to use Darks at same exposure and temp as lights.
I’ve found my images are better with Darks even though there is almost zero amp glow on the 2600MM. What’s your reasoning behind not using Darks.
Appreciate your well respected advice and input

Thank you
Clear Skies
Martin

w7ay

starvoyager I’ve found my images are better with Darks even though there is almost zero amp glow on the 2600MM. What’s your reasoning behind not using Darks.

Of course we use dark frame, but that is to correct for light frames (i.e., dark frames are taken at the same temperature, exposure time, and gain.

What we are talking here is correcting for flats. Your flats are probably taken with an exposure of a second or two at most, while your dark and lights are probably 120 to 180 seconds. So, using the Master dark to correct for flat frames will incur error.

Chen

starvoyager

Thanks but I’m aware what Dark frames are and how they mitigate hot and cold pixels caused by thermal or dark noise in your cameras electronics. I use them all the time.
I’m more interested the technical aspects of why folk should use Flat Darks instead of Bias frames for calibrating Flats with the 2600MM or even MC.
ZWO just say you can use Bias for the 2600MM but many astrophotography sites advise not to use Bias , we should be using Flat darks
https://diffractionlimited.com/calibrating-cmos-images/

A very confusing topic I must say

Clear Skies
Martin

w7ay

starvoyager ZWO just say you can use Bias for the 2600MM but many astrophotography sites advise not to use Bias , we should be using Flat darks

ZWO is wrong. Again. They are absolute neophytes when it comes to DSO imaging. Just look at all the design errors in ASIAIR.

BTW, Diffraction Limited now owns SBIG (I still have my SBIG ST-8300 CCD camera from over 20 years ago :-). That web page was probably created by the SBIG guys.

Notice that they say:

Take dark frames matching your flat-field frames.
Calibrate your flats using matching master dark frames.

Which is the proper way. You have to calibrate data frames with the matching (same exposure time, same gain, same temperature) dark frames.

Me? I take the time to take flat darks, punkt.

Chen

starvoyager

Chen,
I found an excellent video from Adam Block on Calibration ( ignore the WBPP plugs )

https://www.youtube.com/watch?v=sZmHbxIxZeM&ab_channel=AdamBlock

Basically what he is saying if your cameras sensor exhibits almost zero dark current when taking very short flats then Bias calibration is fine. If you don’t know your cameras characteristics with respect to Dark current , then the safest way is to use Flat Darks matched to your Flats ( exposure time , Gain and temperature) no matter what exposure time you use for your Flats.

In my case using the 2600MM taking LRGB broadband Flats , my exposure times range from 0.11 to 0.17 sec which is very short.
My latest image of NGC 5139 in LRGB , I stacked 2 options using both Bias and then Flat Darks to calibrate my Flats.
The results in the stacked images were visually identical for brightness , uniformity of flat field and noise. The final
Processed images were identical as well using the same processing workflow
So in my case using the 2600MM with broadband targets using Bias is fine.The camera must not exhibit any noticeable dark current at short exposures below 1 sec.
My Narrowband Flats usually range from 1sec to 5 sec so therefore there maybe some dark current creeping into the calibration and therefore using Flat Darks is the safest option in lieu of Bias.
In the video Adam mentions about Some Flats maybe 10 sec or 20 sec or even 30 sec long ???
Long Flats are certainly not the case for my rig
This scenario with super long Flats would definitely introduce Dark current

Your thoughts on my experiences and what do you think of the video ?

Martin
PS I’m going to take Flat Darks and also use my Bias library ( 2 options for comparison) to experiment on both Broadband and Narrowband data sets

w7ay

starvoyager Your thoughts on my experiences

You can take short cuts if you want. My background leads me to do the proper thing with imperfect instrumentation (we are using cheap commercial sensors as astronomy cameras), so I use darks that match the flats.

32 dark frames at even 10 seconds each, only takes 5 minutes. For wideband, my flat panel (Gerd Neuman) typically takes no more than 0.5 second per frame with my ASI2600 and ASI6200.

As I said earlier, punkt. End of story. Why worry about whether a short cut can work when it takes 5 minutes to do the right thing?

Just like cameras, even the world's most expensive network analyzer uses calibrations (Doug Rytting's work) of imperfect measurements to achieve their accuracy. I had impemented this many years ago:

https://www.w7ay.net/site/Applications/AAplot/Contents/Calibration.html

If things are repeatable (e.g., same temperature, camera exposure and camera gain in our case), errors can be calibrated away. But you need to do the right thing.

Chen

starvoyager

Thanks Chen

Clear Skies
Martin