Dark flats option in Flats Capture

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zerolatitude
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Dark flats option in Flats Capture

#1

Post by zerolatitude »

Hi,

2 requests for flat capture

1. For bias, make the exposure the minimum time allowed by camera instead of a standard 1ms. I've tried with 35us versus 1ms, and the histogram does seem different (maybe just imagination?)

2. Option for dark flat as well. Flow would be - Capture Flat > Select Dark Flats Option box (instead of Bias) > Start flats > Popup reminder to cover scope (like in sensor analysis) > User accepts > Capture dark flats > Make master flat. The duo-band filters (and some cameras) do sometimes need a flat exposure of more than 500ms, or even 1s, and doing dark flats manually is painful compared to the nice automated flow that SC has.

Hopefully it will be easily doable.

Thanks
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admin
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Re: Dark flats option in Flats Capture

#2

Post by admin »

Hi,

I deliberately avoid using minimum exposure as some cameras misbehave (over bright images) at or near their minimum claimed exposure level, and that leads to rubbish flat correction. 1ms keeps well clear of the danger zone and does not affect the flat correction quality (you could get a perfectly good flat by taking 1s exposure and subtracting 0.25s ones as the 'dark/bias' as long as the 1s exposures are not saturated - effectively you are measuring two points on a line to give the response of each pixel and there is no need for one of the points to be particularly close to zero exposure as long as the two points are well separated).

I will have a think about the addition of a dark flat option - unless your flat exposures are long (probably significantly more than 1s) I don't think it brings much extra benefit.

cheers,

Robin
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Menno555
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Re: Dark flats option in Flats Capture

#3

Post by Menno555 »

Robin

For your info: more and more, flats of >3 seconds are made, especially with ASI cameras and/or when Optolong filters are used. This is also mentioned more and more on Cloudynights for example.
With my AS071MC Pro for example, making flats of 5 seconds is no exception. Advantage is that my flatfield generator then can be set to a minimum, which reduces the chance of internal reflections or rings/flares (especially with Optolong filters).
And it really works, in my case the >3 sec flats are giving better results than the <1 second ones.

Menno
zerolatitude
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Re: Dark flats option in Flats Capture

#4

Post by zerolatitude »

admin wrote: Sat Aug 07, 2021 2:33 pm Hi,

I deliberately avoid using minimum exposure as some cameras misbehave (over bright images) at or near their minimum claimed exposure level, and that leads to rubbish flat correction. 1ms keeps well clear of the danger zone and does not affect the flat correction quality (you could get a perfectly good flat by taking 1s exposure and subtracting 0.25s ones as the 'dark/bias' as long as the 1s exposures are not saturated - effectively you are measuring two points on a line to give the response of each pixel and there is no need for one of the points to be particularly close to zero exposure as long as the two points are well separated).

I will have a think about the addition of a dark flat option - unless your flat exposures are long (probably significantly more than 1s) I don't think it brings much extra benefit.

cheers,

Robin
Hi Robin,

Thanks for the explanation. I did some tests (which I should have done earlier, I suppose), and the difference for my 533MC between image average for a 35us bias and a 500ms dark is 4 ADU@16 bit (596 vs 600). With the sample flat mean ADU is 27.7k, normalising ratio in the flat shows a difference only at the 4th decimal place. So that's that :-)

Regarding the dark flats, looks like they are becoming much more "needed" now.

First due to some cameras like the 294MC - early users had major issues with calibration and it turned out that longer flats (3s) are the solution due to some sensor design parameters. From the post above could extend to 071MC also.

The more widespread reason is the new OSC duo-band filters from Optolong etc. The L-Extreme, for example, has a bandwidth of 14nm total and unless you use a very bright source, the exposure needed is several seconds. Suppose it would be the same or longer with the usual NB for mono filters.

So do hope it is possible to incorporate these.

Thanks
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Re: Dark flats option in Flats Capture

#5

Post by admin »

Hi,

thanks for the info on the long exposure flats needed for some ZWO cameras - that one has slipped past me as I don't tend to focus on the details of using one particular camera (I have too many others to think about!)

cheers,

Robin
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Re: Dark flats option in Flats Capture

#6

Post by admin »

Hi,

I started to write code for this and got stuck because the flat correction results were never quite as good as using bias frames. I came back to it about 3 times before I finally realised that I was inadvertently changing the focus of my test lens when putting the lens cap on/off, so that when I was testing the focus was a bit different and therefore the dust specks didn't get corrected out properly :(

Anyway, all seems to work now - this will be in next week's update.

cheers,

Robin
zerolatitude
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Re: Dark flats option in Flats Capture

#7

Post by zerolatitude »

Thanks very much
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