M97 owl processing help, and strange background noise

[Jannis]

Hi,

I'm kind of struggeling very much with the background noise on this one. I have taken both dark, flat and bias, but the background noise i got now seems less like random like i'm used to. It's simply RGB lines of a type.

Can anyone take a look and see if i might have done something wrong eitehr with stacking or with taking teh calibration frames to cause this?

The full TIF from DSS is here. Was stacked with kappa-sigma clipping:

https://www.dropbox.... flat, bias.TIF

Also, i'm struggeling to get any good details out of the whole picture, and absolutely no red what so ever. I know the red is faint on this target, and especially with an unmodded DSLR, but i would have thought 4 hours of data would at least be able to show signs of it...?

I'd be very glad If anyone would have a go at processing it to see if it's possible to; 1, do something about the straneg background and 2, see if there really isn't possible to get out any red at all from this.

Also of course, everyone feel free to download and play with it as you wish, lol

Attaches is more or less how far i've gotten so far, but only been working on it for like 30 min or so. But unsure of how i shuold contineue in this case on getting rid of the background noise of this type, and i can't get any red out of teh target.

[Cloudwatcher]

Don't know if this will be acceptable........a bit drastic perhaps. Isolated and cut background,colour balanced foreground and really pushed saturation.

[andrewluck]

Definately some red in the nebula. It's in the right place.

The first place I'd start looking for the noise problems is in the calibration frames. Re-stack without using them and see if it looks any better. If it does, then take a very critical look, starting with the flats.

Cheers

Andrew

[Jannis]

Thanks guys.

Cloudwatcher, that's a really huge inprovement and it's looking much more like how i was hoping to have it looking like. It shows that it's quite a lot of data still in the nebula, and that it's only my processing skills that are failing, lol.

Will try again and see what i can do about it.

andrewluck, thanks for the tip, i will give a closer look at the calibration frames. I've noticed that my flats for this run was underexposed, but I do have alternative flats though, so i will give those a try instead and see if it's any difference.

[Tim]

Jannis,

Try stacking it again, only this time draw a box around the Owl in DSS and crop to just the nebula, and stack with 2x or 3x drizzle.

There isn't a huge amount of red in the nebula, most of the Ha glowing around the outside, with a brighter peak in one point, so dont be too disappointed with not much red from an unmodded dslr

[Tim]

Just had a look at the tiff. There isn't much red in the nebula apart from a little bit on the brightest ha part, so no point trying to put in what isnt there in the data.

However you have bucketloads of little fuzzies and background galaxies, I'd be really pleased with this one

[Jannis]

Thanks for the tip, will try drizzle

Also going to try with more exposed flats. I took flats right before sunrise, so the flats are slightly unevenly exposed though, but i'm guessing this would be averaged out anyway and won't make much of a difference?

[Jannis]

I'm currently restacking now, but probably won't be able to process it before the weekend. I'm using 3x drizzle now.

I've changed out my flat frames now, using the alternative flats i took even though the exposure from the first and last frame is quite different.

I've attached the first and last frame in the 111 flat frames list. Would this exposure difference cause any problem?

Also attached an example of a flat frame used in the first stacking - as you can see, it's underexposed by ~2EV, and i'm asuming this could be the reason for the strange background noise?

My guiding was all over the place though, so unfortunatly i see a lot of star movements back and forth in the light frames. In my previous stack kappa sigma clipping sorted that out quite well, but i'm worried my poor guiding and focus that was slightly off might be less forgiving at 3x drizzle, but i guess i'll just have to see.

RAW file examples, in case someone shuold be able to see something wrong with them causing the strange background noise:

This is a single light frame:

https://www.dropbox....m9/IMG_1828.CR2

I guess that could easily have been exposed longer, or put the ISO up one step. I'll try to remember that for next time, hehe.

This is a sample of the new flat frames i'm using now, if anyone would want to look into them and see if they're any good:

https://www.dropbox....to/IMG_2236.CR2

And the old flat frames:

https://www.dropbox....a7/IMG_2295.CR2

[Jannis]

After stretching teh new stacked master flat to the moon and back this is how it looks. Seems acceptable, and not very noisy.

However, do i have some serious alignment issued with my mirrors, as it's much darker on the left side, and especially on the top left corner?

[Jannis]

Well, as an update, after a quick look in DSS, the strange background noise is not there anymore, so it seems like the new flats did the trick. As i asume the drizzle wouldn't have anything to do with that?

[andrewluck]

Pleased you located the source of the problem. The humble flat field is such an easy concept, it's hard to appreciate the problems it causes in real life!

Andrew

[Jannis]

Hehe, yeah. But at least now i know one more thing; never underexpose flats, and never take only a few of them if you're using high ISO. ~100 frames at ISO1600 seems rather ok to begin with..

Still wonder why i have such a dark top right cormer though.

[andrewluck]

I also subtract bias frames from the flats before stacking.

If you rotate the camera, does the dark patch move as well? That'll indicate if the problem is in the optics or the camera.

Does the camera have a shutter? You need to make the flat a reasonable length to reduce the possibility that the shutter is shading the sensor.

Andrew

[Jannis]

i think DSS automatically does that, doesn't it?

I will give it a try later to rotate the camera and see if it remains in the same spot.

I have a shutter box in the camera, yes, i use the canon 550d. You mean that in some cases the camera starts to capture the image slightly before it have lifted up the mirror completly, and/or that it flips down the mirror slightly before it's done capturing the image, and this can cause it? I guess i could try mirror lockup for flats too, but i can't remember seeing that my flats from my 50mm lens was ever this uneven either with roughtly the same or faster shutter speed.

[andrewluck]

It does look like uneven field illumination. As you say, mirror misalignment could do that. Do you collimate regularly?

Cheers

Andrew

[Jannis]

I actually haven't been able to collimate yet - at all.. So, bought the scope 3 years ago or so now, but have never been collimated.

Will try to buy a collimator this summer though, as i've had a bad feeling all season that something must be wrong with the mirror alignments from my pics. I just took a test pic of a star here the other day, just to have o9t confirmed that it does indeed look quite a bit off.

But at the moment i can barly aford food on the table, so a collimator isn't first priority... >_<

[andrewluck]

No need to buy anything you can make a sighting tube very easily. Once you've spent some time looking down the empty eyepiece holder and figured out what everything is in the view then it's pretty simple to get yourself into the right ball park. Do your initial adjustments in the daytime until you get used to the process.

http://www.skyandtelescope.com/howto/diy/3306876.html

Cheers

Andrew

[Jannis]

Thanks for the tip. But as i'm struggeling with my eyes and use glasses, that option isn't something i trust too much as my glasses shifts and curves what i'm seeing quite a bit. So I think i'll just wait for now untill i get a laser collimator.