Jump to content


Is this light pollution?

Recommended Posts

Here's my latest attempt. It's 1hr 20mins of data (5 min exposures) from a ASI533MC Pro (no filter). I've stretched as best as I can and applied some Astronomy Tools actions but I can't get rid of the massive gradient from the top left to the bottom right. Is the light pollution? Any other criticism is most welcome. I'm looking to improve.

Is this as good as I'm likely to get without more integration time or do my processing skills need work?



Link to comment
Share on other sites

Looks like it as the bottom of the picture will be closer towards the ground imaging at that orientation.

Although it costs, gradient XTerminator has always worked well for me to flatten the image and get a rough colour balance as well. A free option (but more basic) is to use the colour range tool in photoshop and then use it to select only the lighter part of the image. You could then use levels/curves to balance out the brightness difference.

The image as well would always benefit from more integration time. 1 1/2hrs really isnt too much and imaging with a dual pass narrowband filter would help bring out the contrast as well with the nebula. 5 mins is a really long exposure time to be using as well. Even with a narrowband filter I was using 2min exposures to stop blowing out Alnitak.

Link to comment
Share on other sites

Gradients like this will be in every shot not taken in perfect darkness. I dont know if some pristine bortle 1 location is different since have not imaged from one, but at least from bortle 4 upwards its not something you can really avoid.

There are a number of tools available for the job, some free some not. In SiriL: https://siril.org/ There is a background extraction tool that is meant to fix gradients, although for images like yours that have signal from corner to corner it is sometimes tricky to get working.

Another option would be the linear gradient removal tool in ASTAP: https://www.hnsky.org/astap.htm that i find is sometimes easier to use than the SiriL background extractor.

GradientXterminator i have found to be almost foolproof. It can remove pretty much any gradient (with some practice), but costs 60 USD so a bit pricey for a plugin with a single use. Astro pixel processor and Pixinsight have gradient removal tools too, but both are quite expensive.


Link to comment
Share on other sites

Noticed that you had included the stacked file in the post so i dont have to guess if SiriL or ASTAP gradient tools work on the file but can test them. Both of the files processed the same, in 4 simple steps. First BIN2x2 in ASTAP, then gradient removal in either ASTAP or SiriL, photometric colour calibration in SiriL, then SiriL autostretch with 1% blacks clipped. Both tools removed the gradient well, but i think the SiriL version worked better and without leaving the emptier half of the image green.





If you're looking to improve, i would recommend trying to learn a dedicated astronomy processing software for the early part of processing which for me consists of: Background extraction (gradients), colour calibration and a stretch. These are easy to do with SiriL, a free app that is somewhat easy to use for an astronomy software. Especially the photometric colour calibration and stretching in SiriL is really painless. For stretching these images i just clicked the autostretch button and pushed the blackpoint a little bit further afterwards, so 2 clicks whereas with Photoshop manual stretching its a bit more involved. The photometric colour calibration tool checks which stars are in your image using platesolving and then tries to match your captured colours to the measured colours of the stars in some photometric star catalogue to produce the best (realest) possible colour balance in the image. After these steps you have the groundwork of the image laid out, and can then adjust every little thing how you like in photoshop. I find that i do this early processing in a few minutes with SiriL and then spend who knows how long tweaking things with masks in Photoshop.

  • Like 1
Link to comment
Share on other sites

More hours will present more photons to stack together though there's a practical limit to how long will keep improving the result. Your image from such a short time is a good start.

To me the focus looks ever so slightly off as the stars look soft edged, either that or as has been suggested the exposure length might be too high making them over saturated. Ive been shooting this in mono and usually have to do 3-5 minute subs due to shooting through a narrowband filter.

Also check each sub/light frame, if any are blurred or have tracking errors and are included in the stack they can also affect the perceived sharpness/overall form of objects.

Link to comment
Share on other sites

  • 3 weeks later...

Thanks everyone for the responses. I've taken some time to digest everything and attached is my latest attempt. It includes the original data as well as some more integration time (2 min subs this time). I now have just over 5 hours of data and the result is a lot better. This is stretched and processed in Photoshop using the  Arcsinh curves and, although not perfect, I think it's a massive improvement.


Siril: I didn't realise this was an astro processing application, I always thought it was a free alternative to Photoshop (Think I was getting it mixed up with GIMP). I'm currently installing so will try to learn about Siril and then reprocess.

Dual narrowband filters: I've been thinking about investing in an Optolong L-Extreme for a while now. Do we think that the attached image is as good as I'm likely to get without using filters?


Link to comment
Share on other sites

  • 1 month later...

As above, where is this gradient? This is my result from a very dark site.


The top left of your image is in a region of obscuring dust with little hydrogen emission while the bottom right is in a strong Ha region.

What I'd concentrate on would be improving focus at capture and keeping a sharp eye on colour balance during the processing. The slightly magenta fringe around Alnitak tells you that the green is too low (green-magenta being one of the key colour axes.) Also the colour is not distinguishing between the reds of the Ha and the very unusual orange-yellow of the Flame nebula, possibly, again, because the greens are not in play.




Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.