Is this normal?

[bjarnit]

Hi

Just had my 3rd imaging session and i still find my results a bit strange.

The following image is the results of 88x50 sek subs stacked in DSS with 10 dark frames at 400 iso taken with a canon DSLR and a skywatcher 150PDS. I chose my exposure length and ISO so that it would peak at the  1/3 point of the histogram. This is taken inside a small city with light pollution.  I have imported the stacked image into pixinsight and the only thing i have done is to crop the image slightly and remove the light pollution with automatic backround extraction. I still find my image strange. Some areas are apearing much whiter than others, e.g. the bottom half a little to the left.  The upper right corner is also completly dark. Is this image looking normal after just using automatic backround extraction in pixinsight or am i doing something wrong? Any advice and/or input is higly appreciated.

Regards Bjarni

[PhotoGav]

I am presuming that you didn't shoot Flat frames for this project as you don't mention them and the image definitely looks like you haven't used any. I would highly recommend their use, in fact, I think it is right to say that Flat frames are essential... They will even out all the inconsistencies within the optical system (well, nearly all!).

[billyharris72]

Hi Bjarni:

Something looks wrong, but it is hard to say exactly what it is - likely a combination. The main things I'm noticing are:

1) Dark corners. Could be vignetting (at least to some extent) but looks severe. Are you using flat frames (you will definitely need these, but also be aware that flats, if not correct, can cause lots of problems)? Also, is the DSLR full frame or cropped? This could be partly a collimation issue. Is the secondary collimation absolutely bang on so that is illuminating the centre of your sensor? I think both of those are worth checking. The star shapes suggest there might be a bit of collimation error (but I always struggle to tell the difference between that and drift).

2) Differences in brightness. Light pollution can cause all sorts of problems - if it's bad LP then it can be really difficult to deal with. However, I wonder if this could be due to something else, such as internal reflections. Is your setup well shielded from local light pollution (neighbouring lights etc?).

3) There seems to be a pattern in the noise of the image (bands running from lower left to upper right). I don't know what could be causing that but again, I'd start my making sure all local light pollution was well screened out to reduce the risk of reflections. Maybe amp glow of some kind (have you used dark frames)?

Hopefully someone more experienced that me will chip in, but that's where I'd start (darks, flats, collimation & light screening).

Billy.

[bjarnit]

I did not use flat frames this time. I used dark, flat and bias frames on the session before this one. Took 30 of each but i had the same problem with the image even though i used dark flat and bias frames.

[billyharris72]

Forgot an obvious question - how have the other sessions gone? Is this a recurrent problem or a one off?

[bjarnit]

This is a recuring problem. I find it strange because if i open up a random photo of the 88 i took, no stackin and remove the light pollution, the image looks fine.

[billyharris72]

1 hour ago, bjarnit said:

This is a recuring problem. I find it strange because if i open up a random photo of the 88 i took, no stackin and remove the light pollution, the image looks fine.

Interesting - I get something like that with narrowband (H alpha) filters. The subs look okay, but when I stack them I have this uneven illumination issue. I thnk it has (in my case) to do with stretching the images). Did you stretch these much, and if so how did they look before?

[bjarnit]

the only thing i did with the image after stacking was dynamic crop and automatic backround extraction

[bjarnit]

Can this be due to dirt on either the primary mirror or secondary? I have checked both and they seem fine