help with flats

[Gerhard]

Hi all,

Last week I made some photos using my canon 500d on fixed tripod, with also the darks and bias.

The results were not bad, but no milky way really visible, and I've been trying a little bit to fiddle with DSS parameters, or ACDSee, but no luck.

I thought maybe the lack of flats was partially the cause of this, so I made some, yesterday. But probably I haven't understood flats at all, because instead of cancelling the vignetting, it added bright vignetting......

So my question is, if my lights are 30 20sec subs, 1600 ISO, f/4.5 at 18 mm, what should my flats be like? I did 30 subs at 1600 ISO, f/4.5 18 mm, but left the exposure automatic, which meant 1/320sec... (I used a tablet screen using a flashlight app).

The flats show vignetting. If I had kept the 20sec exposure, they would obviously be completely white, overexposed...

I attach two photos, one with, and one without the flats... NB: the one with bright vignetting is the one WITH the flats... :-/

thanks for any help!!

Gerhard.

[Stargazer33]

Hi Gerhard, you want your flats histogram peak to be about a third of the way in from the left hand side. Set your camera to AV and adjust the setting until the peak is about a third of the way in from the left.

Don't know what has happened with your flat - normally they are darker towards the corners, not lighter! 

[Gerhard]

Hi Bryan,

In fact the flats themselves are dark in the corners, they show the "normal" vignetting... I also didn't get why they gave this result... Maybe because it came from a screen? Maybe screens give strange readings that confuse DSS?

The histogram of the flats was quite broad, with a not so distinct peak towards 2/3 from the left (I don't have the flats at hand right now, so cannot post one). I'll try to play around with the AV this evening...

Just a confirmation: so it's correct that flats do not have the same exposure time as lights and darks?

thanks!