Question of understanding about flats - complete field of vision and ROI

[artofpix_astro]

Hello!
I have a question about flats in planetary and lunar images.
Is it sufficient if I create a master flat of the complete camera sensor, i.e. the entire image field - and is this flat then also applied correctly if I use a smaller ROI section window for the image? Or do I always have to record the flats separately for each ROI?

Example: I take pictures of the moon with the SC11EHD and the QHY678, full disk, at 3856 x 2180 - then I set an ROI of 1024 x 1024 - do I then have to take new flats, or does SharpCap then convert this to the appropriate ROI section?

And, another question - the help says you don't need dark frames, only bias and flats - are the same settings to be used with bias as for the flats, only with the shortest possible exposure time, or should you use the same settings as for dark flats, with the exposure times of the flats?

Thanks for the info, Thomas

[admin]

Hi Thomas,

yes, SharpCap will deal with ROI selection if you have a flat frame at full camera resolution, cutting the correct ROI out of the full resolution flat and using it as needed. This will also work if you move the ROI around, although there may be a few frames lag while the ROI is actually changing position - once it has been in the new position for a few frames then all will be fine.

For the sort of short exposures that you use in lunar/solar/planetary, there is no need for dark frames - in fact there's not really much need for bias except to measure the average bias level so that level can be subtracted from both the flat frame and the light frame before correction.

The recommendation is to not change things that would affect the bias level between capturing the flat and the lights (that includes white balance, offset/brightness/black level settings). Changing exposure and gain should be fine. There's more in the documentation here :https://docs.sharpcap.co.uk/4.1/#Gettin ... Correction

cheers,

Robin

[artofpix_astro]

Hello Robin!
Thank you very much for the detailed information! I'm really glad that you only have to make a flat of the whole sensor and then continue working with ROI - that makes the shooting process much easier So far I have always made the flats separately for ROIs