Weird flats

[thing]

I had a go at M37 last night, decided to use ISO1600 for the first time just to see what happened. When I applied flats in DSS there was a strange streaking across the image, I've noticed this occasionally when shooting at ISO800 but it's never as bad as this. Two questions, is it ISO related and what exactly are the streaks? 10x2 mins lights and darks.

Image without flats

(click to enlarge)

With flats

(click to enlarge)

[Mel]

Hi Thing

Nice to catch up with you again - you were on the IRC channel last night, weren't you?

I think the answer here is in your dialogue - you say you noticed it before at ISO800, but it was never as bad - so I guess if this is showing up at ISO1600 and this is your first go at this ISO setting, it pretty much must be ISO related. I wonder if the distortions are in the flat - can you post it? Have you tried tweaking the levels of the flat(s) in ps or similar to see if the streaks show up?

See you on the channel again.

Cheers

Iain

[narrowbandpaul]

looks like FPN...retake your flats...at the same ISO as your lights...

it might help

[thing]

FPN? The flats are at the same ISO. Yes I was on the talking bit last night Mel!

[George]

Try applying some BIAS frames, I had the same problem which was cured with a redo of the flats as well.

[blinky]

I have had this one as well. I now dont use darks or flats!

[Sam]

I've had the same thing thing with my images from time to time and that's been with flats, bias and darks.

[narrowbandpaul]

yeah, it could be a bias thing george, but that would have been removed with the dark, since it contains the bias...

does the strength of the patterning grow with signal...ie for longer subs, the pattern is more and more obvious.

If it observes this then i would have to believe it was Fixed pattern noise, which grows linearly with signal

Try bias frames, and some new flats

Paul

[thing]

It's got the lot, bias, flats, dark flats and darks.

[Psychobilly]

have you tried building it up bit by bit to see where the problem comes from ?

I guess you probably have....

[thing]

Yeah it's definitely the flats. Next time out I'm going to try ISO400. I normally use 800 so I'll see what it's like.

[Tim]

I had that too, in fact, its the reason i got the qhy8. Drove me mad.

If you check, you will see that it is concentrated in the red channel. I think it may be related to tracking errors, does your target slowly move over the frame over the course of your subs?

I noticed that the streaks always followed the pattern of target drift.

Using careful blurring and slective colour altering on purely the red channel can help to alleviate them. If you find the definitive answer, please tell me, I have 4 hours of subs from Kelling under a black sky on M31 that is ruined by these streaks.

TJ

[thing]

I had that too, in fact, its the reason i got the qhy8. Drove me mad.

If you check, you will see that it is concentrated in the red channel. I think it may be related to tracking errors, does your target slowly move over the frame over the course of your subs?

I noticed that the streaks always followed the pattern of target drift.

Using careful blurring and slective colour altering on purely the red channel can help to alleviate them. If you find the definitive answer, please tell me, I have 4 hours of subs from Kelling under a black sky on M31 that is ruined by these streaks.

TJ

Yeah I get the target moving, I wonder if I get a really tight polar align it alleviates it? Some of my images, exactly the same set up, are fine, do the same thing on a different night and I get the streaky bacons.