How to do light frames.

[PhoenixRising]

Hi guys,

I've been doing pics using bias and dark frames but have no idea how to create flat frames, can anyone explain in idiot terms any methods I can use to create these?

[blinky]

The easy way is to simply cover the front of your scope with a white T shirt then point the scope at a nice clear blue sky. Take loads of shots (minimum of 20 I would say - I often took around 70) and then add them into Deep sky stacker as you do your bias etc. As for exposure time - if using a DSLR then stick the camera on auto and it should be OK (Maybe back it of 1 or 2 stops) and if using a CCD imager you are looking for your exposure to be about halfway between full brightness.

[blinky]

There are other (better) ways to take flats though...... Do a search for flat box or light box on the forum and you will find what folks have made. In my observatory (when it gets built!) I will be mounting an old 15" monitor on a swing out arm and plan to get this over the front of the scope then display a nice white screenshot for taking my flats.

[Earl]

Blinky, just pop it on to auto and snap away with a light box also?

[teh_orph]

Do you have to ensure that the image isn't too bright, and doesn't saturate?

[ollypenrice]

Yes, opinion varies between one third and two thirds the way to saturation. Since it varies I conclude that it doesn't much matter! I go for one third because that's what I first read.

BTW, the T shirt method on the sky never works for me and I'm not dead sure that I know why. I think light leaks in somewhere, even on my OSC camera without filter wheel, and I also get gradients. However, the light is very intense here by day. I do them at night or indoors using panel or light box.

Olly

[AndyUK]

Just another quick question re: fliats... I understand that the scope orientation is supposed to be left totally alone when taking flats, but if you're using an EOS clip filter, should this be removed first... or do you take them as is with the filter then convert them to B&W and then add those to DSS (and whilst in there, maybe make another couple of sets with different exposure settings?)

[daz]

The light train needs to be exactly as it is when taking the lights. Same filter, same focus position, same orientation, etc. Every change in filter needs a new set of flats for that filter. Flats help to remove dust bunnies as well as things like vignetting.

Also, if you have to pier flip during your session, you would need to calibrate the images first BEFORE rotating them to align them - remember, it is the object that has rotated in the view!

[AndyUK]

Thanks Daz - I was bit concerned that my flats were coming turquoise!