Help with Flats please ??

[Skipper Billy]

I have made a light panel to take flats.

A sample of the Fit file converted to a Tiff is attached - no stretches or anything - taken at set point cooling of minus 10º for 0.02 seconds with an Atik 314+ on an ED80.

I read somewhere that the 'ADU' (???) should be a certain value but I have no idea what that means - if I bring up the histogram window on PS the peak is just below half way from the left - about 40% - 45%

Does this sound about right ???  Does it look OK ????

Thanks in advance.

lumflat 0.02 minus10.tiff

[Davey-T]

Hi Billy, downloaded your flat and measured it in Maxim and has average pixel value of around 27500 so should be OK, I use an average value of around 22000 for my Atik 314L.

Just a case of trying it to see, main issue I found is removing vignetting, most values in the right area seem to cope with dust bunnies.

Dave

[Skipper Billy]

Oooh - thanks for that - I have Maxim - how do I measure it ???

[Davey-T]

Open the flat and the info window, either view / information window or click the icon that looks life gun sight. then roll the cursor over the image and it shows the ADU value.

Dave

PS: don't worry if the flat looks a bit strange in Maxim, not sure what it does seems to default to showing either white or black values in the image

[Skipper Billy]

Fantastic - thanks very much.

[ollypenrice]

If you capture in Artemis Capture with your Atik (as I do with ours) all you need to do is look at the Display Window. Check Auto Stretch and look at the Black and White values. The black will show your darkest values in ADU and the White will show your brightest. It's the white that should reach about 23000. SImple as that.

Don't forget to shoot a set of bias which you can then use as darks to calibrate your flats. This is important or they'll over correct.

When shooting darks and bias with an Atik it's best to do it off-scope with the metal chip cover screwed on. That is really and truly lightproof.

Olly

[Skipper Billy]

Thanks Olly - some great advice there.

Should the exp. for the bias frames be the same as the flats or just as short as they can be - 0.004 or 0.008 ?????

[ollypenrice]

Bias is as short as can be done. It won't do any harm, or make any difference, if you do dedicated darks for flats (at just the same exposure as the flats themselves) but it's not necessary and is simply a waste of time. 

A good flat looks brighter in the middle and darker round the edges and particularly the corners. It will also have dust bunnies unless you're lucky. If it doesn't look like this - eg if it has an obvious gradient top to bottom or side to side, then be suspicious.

Olly