Flats what should they look like ???

[spiezzy]

Hello to all I am sorry for the dumb question but I am only just starting into flats I have avoided them due to the lack of understanding on actually taking them .
I have a illuminated panel and I use a 10" Truss Newtonian Astrograph camera used is a Sbig 8300c colour version .
I understand how to make master flats and apply them in a image along with darks but there is no real info on actually taking them and what they look like say in Nebulosity 4 which is what I use for image and darks acquisition
does any one else use Neb 4 for flat acquisition and is it possible to do a screen shot of what they are supposed to look like and the value readings on the histogram many many thanks to anyone that can assist
cheers Pete

[Bart]

Google flat frames, you should get what you need, including a write up at Stargazers Lounge on how to take them, its the first search result.

[rogerco (Roger)]

In an ideal system they should look like the panel, that is an evenly exposed "flat" tone. Any imperfection in the optical path should show as a deviation from that. a common one is vignetting where the corners will appear darker than the centre. Hope this helps

[spiezzy]

hi there Rodger and Bart
thank you so much for the reply it sounds easy enough so I will give it another go because I am using a ccd camera with a mechanical shutter I will have to keep the exposures long enough to get rid of the shutter shadow would this mean I am restricted to taking them at night not sure of the exposure time I should be using with this camera the Sbig 8300c I do collect my lights in RAW and my darks as well so this should be the same with flats and debayer in the processing is that correct many thanks again
cheers Pete

[Atmos (Colin)]

With a mechanical shutter, keeping them at 5s or longer removes all of the artefacts from it

[Bart]

Ideally, if you have a shutter on your camera, you would want some way of adjusting the brightness of your panel to give you a long enough exposure to not see the shutter blades and give you the ADU level you are looking for.

In any image capture program(Neb, MaxIm, SGP), set an exposure of your camera to say 2 seconds(ignore any automated process the program may offer at this stage). Take the shot and use the information or statistics box to find the min, max and average ADU of the image you have just taken. You will need to use the average as min and max only give you an idea of how even your field illumination is, vignetting, etc. I usually aimed for about half the level in my flats(been a while since I imaged) so around 25 to 35,000 ADU. If you are using any 8300 chip camera, it should have a maximum ADU range of around about 65,000.

Once you have got the level you want and they look like the ones on the 'net you could set up an automated run to collect the flat files, say if you want 20 or something and take them in the same format as the light frames, eg FITS so stacking software can do what it needs to integrate them.

You may need to experiment a bit with the flats and processing to make sure they are not under or over compensating, but from here you are right to go.

Hope that helps.