Capturing darks when using ROI

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lampcord
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Joined: Sun Jul 19, 2020 9:12 pm

Capturing darks when using ROI

Post by lampcord »

My camera has a large resolution (5500 x 3600) and when I'm doing planetary, I use a much lower resolution so that I can get a higher frame rate. Usually something like 640x480. This of course is using only a small area of the sensor that happens to be around the planet. I use the capture to video feature and post process in Autostakkert.

My questions:

1) In the above situation, if I capture a dark, will it use the same ROI that I was just using for my capture or will it make a dark of the entire sensor?
2) If the entire sensor, then how do I line up the proper region of the dark with the ROI that was actually used in the capture?
3) If I create the dark first and set SharpCap to use it, will it apply the dark to each frame of the video as I process it?

Thanks in advance for any help!
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admin
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Re: Capturing darks when using ROI

Post by admin »

Hi,

darks need to be captured at the same resolution as you are imaging at, there isn't any feature to use part of a larger dark frame depending on the ROI settings.

This means that if you wanted to use dark frames for the kind of imaging that you are describing you would have to get the ROI setup to begin with, then capture a dark frame, then use that dark frame without moving the ROI (if you move the ROI then the dark frame will still be applied, but it will be doing the wrong thing).

All of this sidesteps the bigger question – is there actually a great deal of value to using dark frames when capturing planetary images? I would understand if you are using flat frames to correct for dust doughnuts, but I'm not sure that dark frames are widely used among planetary imagers.

Cheers, Robin
lampcord
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Re: Capturing darks when using ROI

Post by lampcord »

Thanks for the detailed response. I see your point about using darks with planetary. So I guess that really simplifies the problem LOL! Thanks again.
howardgrams
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Re: Capturing darks when using ROI

Post by howardgrams »

Hi Robin,

I've been meaning to ask about this subject, but for deep sky imaging

I use a ZWO ASI183MC for deep sky objects, part of the time full frame 5496x3672 and part of the time (near) half frame 2752x2754 pixels. It's been a constant annoyance to need to capture darks for both sizes at all of the exposure/gain/cooler settings I might use.

Wouldn't it be possible to build an on-the-fly dark frame for a smaller ROI out of an existing dark for the full frame at the time that a new Live Stack is started? It would be very nice to be able to use some of the other pre-canned ROIs that my camera provides depending on the object I am imaging - I don't do so now because of the time and bother of needing to capture yet more darks. (For some reason I'd rather spend time capturing images than capturing darks!)

Regards,
Howard
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Re: Capturing darks when using ROI

Post by admin »

Hi Howard,

yes, it's certainly possible to cut out the ROI region from a full frame dark (or flat for that matter). It's something that I will definitely bear in mind once the existing new features in SharpCap 3.3 are closer to completion.

Cheers, Robin
howardgrams
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Re: Capturing darks when using ROI

Post by howardgrams »

Hi Robin,

To add another wrinkle: would it also be possible to construct an on-the-fly dark from a full frame dark for use if binning is turned on?

Regards,
Howard
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Re: Capturing darks when using ROI

Post by admin »

Hi Howard,

I'm not really keen on that one – the thing is that the binning for the light frames would be done in the camera software but the binning for the dark frames will be done in SharpCap – although binning is supposed to be a fairly simple operation any discrepancies between the two would mean that the dark frames wouldn't work properly. I think the safest thing to do is to collect dark frames at the appropriate binning factor.

Cheers, Robin
howardgrams
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Re: Capturing darks when using ROI

Post by howardgrams »

Hi Robin,

I suspected that something like that would be the case for binning. Thanks for considering using ROI for darks at some point.

Regards,
Howard
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