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Light Pollution or Bad Processing?

szymon

By szymon
in Getting Started With Imaging

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szymon

szymon

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Stack of one minute guided exposures on M27, darks and flats...and I have this wonderful halo exactly around the object I want to image.  It's happened the same with multiple targets.  So my question -- is this Light Pollution or am I doing something wrong with the stacking process?

post-38149-0-85502100-1410262715_thumb.j

Raw TIF available here: https://dl.dropboxusercontent.com/u/3614054/bad.TIF if anyone wants to play with it.

-simon

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szymon

szymon

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Are you using an eyepiece cover? If not, it's possible some stray light is getting in through the eyepiece. There is usually a cover on the Canon camera strap.

Oh!!  A photographer for many years going back to 35mm Film SLR, and I didn't think of this!!

Hi Szymon

Yeah, looks mainly like a flats problem to me, especially if you're getting the problem with multiple targets. Can you provide more details of lights and calibration frames? And some info about your local lp?

Louise

The local LP is me living in Sudbury, north-west London.  The lights were taken on Canon 6D ISO800 for 1 minute with a nearly full moon (I got excited by the fact that I got my guider working and probably used too long for the subs).  The darks were accidentally set to ISO400 (I did some pics of the moon in between the lights and darks), but I've had the same issue with "correct" darks.  The lights were a macbook screen held up to the end of the scope, taken at 1/160 to get a nice histogram.

I'll try again tonight and cover up the damned eyepiece ;-)

-simon

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szymon

szymon

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Ok so it looks like it was both LP *and* processing.  I did it with some new flats, and here's the result:

post-38149-0-51003000-1410288488_thumb.p

The raw tiff is here:

https://dl.dropboxusercontent.com/u/3614054/better.TIF

If anyone wants to show me how to process it better it would be much appreciated :-).  You can still clearly see the light ingress from the viewfinder, but it's more reasonable and I can do something with it.

-simon

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szymon

szymon

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Sorry, the image is a crop yes; in DSS only a small part of the centre of the image is processed.  The results are similar if I do the whole image, but take longer and become huge to upload.  It's green probably because of my processing in DSS, I tried to line the colours up correctly.

In terms of changing the black point, yes I did -- but with the previous image doing so gets rid of the nebula entirely.  Here at least it can be seen!

-simon

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m37

m37

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Maybe try some of the recommended stacking parameters. Often they work very well.

Probably a fairly default set would be

Light = Average

Dark = Median

Flat & Bias = Median Kappa Sigma

Per channel background Calibration set to Linear / None

Auto Alignment

Then post stacking, try to just align the three colour graphs, maybe around the middle of the three to start with, and add saturation of around 15.

This should give you a fairly reliable starting point.

Other curves etc are better handled by PS or Gimp.

cheers

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szymon

szymon

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Ok, so I'm beginning to think it might be more than just my bad processing.  I took some more images last night -- with the cover over the viewfinder!!

I deliberately chose two separate targets; the Dumbbell nebula as per my earlier photos on this thread, and the Ring nebula since it's an easy target in a similar part of the sky.  Here are (quickly) processed images from both of them, using the same darks and flats.  As you can see, the problem is still clearly visible on the dumbbell, but not on the ring.

post-38149-0-46875100-1410334182_thumb.p

post-38149-0-70438700-1410334195_thumb.p

I have uploaded the master dark and flat here:

https://dl.dropboxusercontent.com/u/3614054/darkflat.zip

and all of the subs here (warning: big files):

https://dl.dropboxusercontent.com/u/3614054/ring.zip

https://dl.dropboxusercontent.com/u/3614054/dumbbbell.zip

There are a few more subs of the ring than the dumbbell because a few clouds popped up, but I've tried using less of the ring subs and haven't had the same problem.  All the subs are taken at the same iso800 and exposure 1.5 minutes (guided with my homebrew autoguider).

I'd really appreciate it if someone could run through the processing and find out what I'm doing wrong :-)

Many thanks,

-simon

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szymon

szymon

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Think I'll try a few with the Barlow + LP filter today if I can get it to focus (can't use the LP filter with prime focus yet, waiting for an adaptor to be delivered).

That is, if the clouds ever clear up!

Would still be interested if anyone wanted to look at the raw files I uploaded :-)

-simon

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Thalestris24

Thalestris24

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Hi

I had a quick look at your single sub. The main problem with it is lp and vignetting, but the latter should be removed with flats (making processing easier).

It's only a single sub but a wipe in Startools can remove the gradient (see below). A stack should allow you to get some sort of image. I've not looked at the files in your link as I don't like downloading files.

Hth

Louise

post-33532-0-04703000-1410389002_thumb.j

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szymon

szymon

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Oh sorry I only just noticed you asked for the flat as JPEG, can do that later - the computer is out taking photos at the moment.

I'm trying with an LP filter, using a 2x barlow (it's the only way today I have of getting the filter in the chain and focussing).

Sent from my iPhone using Tapatalk

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Knight of Clear Skies

Knight of Clear Skies

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Ok, so I'm beginning to think it might be more than just my bad processing.  I took some more images last night -- with the cover over the viewfinder!!

I deliberately chose two separate targets; the Dumbbell nebula as per my earlier photos on this thread, and the Ring nebula since it's an easy target in a similar part of the sky.  Here are (quickly) processed images from both of them, using the same darks and flats.  As you can see, the problem is still clearly visible on the dumbbell, but not on the ring.

Just a thought. The Ring Nebula is closer to the zenith, so could you be getting some kind of reflection off the scope tube when it is pointed lower to image M27?

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szymon

szymon

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What a difference a Light Pollution Filter makes!

This image was taken last night with an inexpensive Skywatcher 2" LP filter.  Please gnore the very obvious Coma which is expected, M27 was in the corner of my field as I was having trouble finding a guide star for my homebrew autoguider and when I finally found one I was too tired to move the guider and recenter.  (Can Coma be easily fixed in post processing?  I plan to get a Coma corrector when I can afford it but not yet!).  However I'm very happy with the difference to the actual image that the LP filter makes.

post-38149-0-26048100-1410768412_thumb.p

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