Creating Flats

[Anthonyexmouth]

So, I've read that the ASI294mc Pro likes to have slightly longer flats. I've got a little EL panel and darkened it a little with a few sheets of paper as it was too bright. 

At the moment to achieve 25000adu im getting 4.5 secs exposures, should I remove a few sheets or will this be ok/preferable?

heres the histogram from Gimp, debayered

Edited March 28, 2020 by Anthonyexmouth

[Rocket Stars]

Im not sure. But could be to long exporsure. I use Sharpcap to mersure this. It usally gives me very short exporsures. I can highly recommend sharpcap for sensorananlysis ect! Cost nothing to use. Pro version is 15quid, If i dont remember wrong. 

[Anthonyexmouth]

Just now, Rocket Stars said:

Im not sure. But could be to long exporsure. I use Sharpcap to mersure this. It usally gives me very short exporsures. I can highly recommend sharpcap for sensorananlysis ect! Cost nothing to use. Pro version is 15quid, If i dont remember wrong. 

Yeah i got sharpcap pro, use it for my PA, issue is that a lot of people have said the 294 needs longer exposure flats, just not sure how long is too long or too short. 

[Rocket Stars]

1 minute ago, Anthonyexmouth said:

Yeah i got sharpcap pro, use it for my PA, issue is that a lot of people have said the 294 needs longer exposure flats, just not sure how long is too long or too short. 

Intresting. What would the reason be, deep welldepth? High readnoise? 

What would Sharpcap give you? 

[Anthonyexmouth]

2 minutes ago, Rocket Stars said:

Intresting. What would the reason be, deep welldepth? High readnoise? 

What would Sharpcap give you? 

I did read the reason a while ago but cant remember now, someone found that using short flats was causing issues when stacking and found longer ones in the seconds rather than milliseconds was producing much better results. 

[symmetal]

It seems that many CMOS cameras have different modes of operation between short (less than around 1 second or so), and long exposures as detailed in this thread here CMOS Bias Stability where short exposures have internal bias calibration applied, while long exposures don't. So flats and flat darks of short duration won't accurately match your longer lights exposures when calibrating. Longer exposure darks and dark flats avoids this issue hence the recommendations to use flats of a few seconds duration at least.

CCD cameras dont have this issue so short duration flats and using bias as flat darks is fine for them.

Alan

[Rocket Stars]

8 minutes ago, symmetal said:

It seems that many CMOS cameras have different modes of operation between short (less than around 1 second or so), and long exposures as detailed in this thread here CMOS Bias Stability where short exposures have internal bias calibration applied, while long exposures don't. So flats and flat darks of short duration won't accurately match your longer lights exposures when calibrating. Longer exposure darks and dark flats avoids this issue hence the recommendations to use flats of a few seconds duration at least.

CCD cameras dont have this issue so short duration flats and using bias as flat darks is fine for them.

Alan

Exellent info! 👌

I assume that is a reason to not use bias frames to calibrate cmos lights? 

[vlaiv]

Just some info - I've used very short flats - a few milliseconds long exposures with my ASI1600 and did not have issues with flats. Just made sure I was using matching darks (also few ms long - exactly the same settings as flats).

[Anthonyexmouth]

1 minute ago, vlaiv said:

Just some info - I've used very short flats - a few milliseconds long exposures with my ASI1600 and did not have issues with flats. Just made sure I was using matching darks (also few ms long - exactly the same settings as flats).

from what i've read it's a specific thing with the 294. 

[symmetal]

21 minutes ago, Rocket Stars said:

I assume that is a reason to not use bias frames to calibrate cmos lights? 

Yes, that's correct. You normally need to use darks with CMOS cameras due to so called 'amp glow' anyway and the darks include the bias signal so that takes care of it.

16 minutes ago, vlaiv said:

Just some info - I've used very short flats - a few milliseconds long exposures with my ASI1600 and did not have issues with flats. Just made sure I was using matching darks (also few ms long - exactly the same settings as flats).

That's interesting vlaiv. I've just used around 2 second or so exposures for flats, and flat darks for all CMOS cameras to save having to be concerned with the issue. 😀

Alan

Edited March 28, 2020 by symmetal