Another Question on Flats Sorry !!!

[spiezzy]

Hello to all
right I have sorted my Gain and off set issues and I think the guiding is under control my next big venture is FLATS yes Flats ooooh noooo !!!
I have a ASI1600mc and all through my astro imaging I have not had any luck with flats so I never bothered taking them .I have used synthetic Flats in Nebulosity 4 but I guess they would not be as good as the real thing .
Soooo I have a Illuminated Panel which works good for even illumination and is variable brightness .I have been playing around with it and can get the ADU down to about 35000 my question is if I take lights at Unity Gain 139 and 21 off set do the flats have to been taken at unity Gain as well which the best ADU I can get is approx 40000 the other thing is I have 3 peaks showing on the Histogram would this be due to the camera being a OSC ?
I have only tried this during the day and I have a truss Newtioian with a shroud cover but the light still gets in so my exposures are 0.06 sec is this a bit fast is will be a little longer at night but what type of exposure times should I be looking at sorry for all the questions but this is the last this I need to conquer and would love to do so
many thanks for your time and help
cheers Pete :thanx::thumbsup:

[Slawomir]

Hi Pete,

I cannot answer most of your questions, but I remember reading on Cloudy Nights forums that people were aiming for under 20k ADU for flats with your camera, otherwise you get into non-linear region for this sensor and flats won't work properly. To my knowledge gain value affects bias, so ideally I would take flats with the same camera settings as lights, since the same bias will be present in both lights and flats so it should make calibration more effective, but I guess any differences would be negligible anyway? Binning will need to remain the same for both flats and light frames and I would definitely take flats in the dark when using a panel to avoid additional light leaking into the camera that would normally not be there at night.

[RickS]

You can probably get away with doing the flats at a different gain setting to the lights but you'd want to check. Here's how to do that: take a couple of sets of flats at different gain settings, make master flats out of each set and then use one master flat to calibrate the other. If the result is a completely even and featureless image then you're good. If not, then the gain setting is having some odd non-linear effect and you'll need to take flats at the same gain setting as your lights.

The three peaks will be because it is an OSC camera and the brightness of the panel and the sensitivity of the sensor will be different in R, G & B.

Very short exposures are fine so long as you don't have a mechanical shutter to leave a shadow.

Cheers,
Rick.

[spiezzy]

Thanks Suavi and Rick for that information I will check out cloudy Nights and see what is there .
Rick Yeh I thought that would be the case for the 3 peaks but thanks for confirming the ASI 1600 does not have a mechanical shutter so the shot exposures should be fine then .
I could get the ADU down to 20k I will try that experiment Rick and follow it up many thanks
cheers Pete :thumbsup:

[Atmos]

Considering the short amount of time it takes to take the flats, I would definitely suggest taking flats at whatever gain setting you're using for the lights. As you're not only trying to correct for vignetting but also seniors pixel to pixel variations which may change with gain settings PLUS as the gain changes it may change thing that are unexpected. Testing is the best way though.
Suavi and Rick seem to have covered the rest pretty well :)