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Flat Darks and channel assignment, bonus BPM question.  

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(@mikecmp)
White Dwarf Customer
Joined: 1 year ago
Posts: 19
July 17, 2020 19:26  

Hello,

Tried to do some searching but I was unable to find the answers to my questions. 

 

Just got my ASI1600 pro, doing some calibration files.

With the understanding that bias is generally not the best way to calibrate this camera, I shot my flats, covered the scope, and shot my flat-darks and darks. My flat darks are I shot for each filter, at the same time as each filter, just in cases that was necessary. 

 

When I load the files into APP, it defaults the selection of the flats to "filter" which is good as the filter tag is present (This is an awesome time saver BTW ) and the darks get set by default to "All channels" 

 

When I import my dark-flats, they are also defaulted to "All channels". Should I change this to per-channel, like my flats, or leave it be. I've read some folks say that adding them all together is preferable but for some reason that seems incorrect. I have 64 dark-flats of each channel so I have plenty of data.

 

Edit to add - the other thing I should note is the dark flats all have different exposure times as they are LRGB. And I have the setting checked to keep darks separate by exposure time - So there are still 4 master dark flats being created, just seperated by exposure time. 

 

Side note - my bad pixel map has about 5% bad pixels in it with kappa 2.5 and 50%..... I'm not sure if this is "good" or not 🙂 Is there a particular value I should be aiming for? And what is a good way to test a BPM to know its good?

 

Thanks in advance,

Mike


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(@wvreeven)
Quasar Admin
Joined: 3 years ago
Posts: 1113
July 17, 2020 20:08  

Very good questions and here is an attempt at at least the start of an answer: when taking (flat)darks, no light passes through the telescope so in principles a darkflat for a certain filter should be applicable to all other filters as well. Provided the exposure times of the flats are the same. In case of the ASI1600 there is (a bit of) amp glow at play so it may be possible that a, say, 1 sec darkflat cannot be applied properly to a, say again, 5 sec flat. But you can try that and see if you get good flat correction.

I usually try to take flats of the same time length for LRGB and, out of necessity, longer for SHO but I aim to have them at the same length as well. That way, I can take two sets of flatdarks and create two master frames and use them. Period. It works well for me.

I hope this gets you on the way a bit. Let's see if other people here chime in as well.

 

Clear skies, Wouter


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(@vincent-mod)
Universe Admin
Joined: 4 years ago
Posts: 3694
July 17, 2020 22:02  

Yes indeed (again 😉 ), darkflats and darks are applicable to all data. Regarding flats I would always keep them at the same ADU, using e.g. SGP I can set it to about 30.000 and do this for all filters.

BPM; I think between 1-5% is normal, 3% is a nice value to target and in the latest version APP tries to do this automatically for you.


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(@mikecmp)
White Dwarf Customer
Joined: 1 year ago
Posts: 19
July 17, 2020 22:29  

That's an interesting take on it - In my case, I was taking the flats to a certain ADU count, in this case about one third of the max. (About 16,000 in SGP) This means all of the exposure lengths will be different as the different filters will clearly pass differing amounts of light.

So I "think" that with the flats, and the dar flats, that we are using the dark flats to subtract out the inherent bias and amp glow from the actual flats, to calibrate them. So I would think, that it might be best to align them to the filters and thus the flats. I'm probably wrong on how the dark flat is used though. 

 

I am curious to other opinions on this too -


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(@vincent-mod)
Universe Admin
Joined: 4 years ago
Posts: 3694
July 17, 2020 22:40  

Ah yes, so it's then best to align them to the different exposed flats. At least in the case when a darkflat is long enough to see more signal then just bias.

I don't think amp-glow is going to be a thing at these low exposures though and a flat is so bright that I don't think it has a big effect if at all. So my gut-feeling is that one darkflat of say a second should work on all of them. I'll tag Mabula for his opinion as well. 😉 @mabula-admin


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(@mikecmp)
White Dwarf Customer
Joined: 1 year ago
Posts: 19
July 20, 2020 14:56  

Well, I am calibrating them per channel - I am bringing in my flat-darks based on the filter they were shot with. Yes, I know it's irrelevant (To shoot the flat with the filter) , but it makes the procedure less manual - in SGP if if I run the flat calibration wizard, if i pickl "Flats" as that exposure type and the filter it automatically populates the exposure time for me. I can then add a suffix like "Flat_Dark" and append to the file name. 

In APP i load the darks for all channels, then the flats per channel as you would expect, then the flat darks per channel. Seems to work out ok.

 

If there is a better workflow, I am willing to try it 🙂

 

Mike


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