Improving RGB imaging quality under Bortle 8 light pollution

Rick Veregin · Mar 4, 2026, 07:44 PM

Hi Tony

I’m in B8, so I struggle as you are struggling. First and obviously, if you want the best images I would suggest focussing on what your conditions best allow, mostly NB and/or bright targets. It is the same as if you would love to image something at 10 degrees altitude, it will never that great, accept the limitations. So one images things higher in the sky, as that is what your site gives you.

I do understand wanting to do other RGB targets, so here are my suggestions based on 9 years of experience with OSC cameras in B8.

· Image when the target is overhead as the LP and LP gradients are the least there. The closer to the horizon the worse the LP will be.

· Image in the middle of the night, when it may be darkest in the city and when the sky itself is at its darkest.

· If you can set up a light blocking shelter, open at the top, or at least block out any nearby lights with some sort of shade so they don’t shine directly on your telescope.

· Use a long dew shield. I just ordered what looks like an amazing dew shield with baffles from Rouz Astro, which knocks out stray light. His demo with a laser pointer is amazing. I will report on this on my pages once I get a chance to test it. https://rouzastro.com/product-category/dew-shields/

· The smaller the field of view the less gradients—either in capture or in processing. I’m at 1x0.6 degrees for my FOV.

· Since sky background is higher, background noise is higher, you need to increase your total exposure to compensate, compared to those in lower Bortle. Not longer subs, with high LP in RGB with no filter, or even a standard LP filter, subs can be a minute or less, longer subs just risks saturation.

· I have been most successful using the L-Enhance as a LP filter, for galaxies, I get good results. I even add in the L-eXtreme if there are H2 regions. See for example: https://www.astrobin.com/wjmz2n/

· The Optolong UHC filter provides some benefits, but I think the L-Enhance is better in my conditions. The L-Pro doesn’t work for me, worse than nothing. https://www.astrobin.com/gijsiv/C/?nc=&nce=

· I do recommend to anyone who is choosing a filter to look up Thompson’s work here where he does a very good job of experimentally comparing LP filters, and showing the benefits or not of particular filters on NB and BB targets.

· Note any filter will require work to get the colour correct, so in your processing software learn how to do colour correction.

· Note as well, reflection nebula in B8 are extremely challenging, galaxies are a more favorable target for sure.

For processing:

· I use Startools, which has a background wipe feature with many options. With experience and a lot of trial and error I can usually get a reasonably flat background. In PI there is something similar, learn all its tricks. Or GradientXterminator in Photoshop, not sure what you use. This is the key, without good background correction—no luminance or colour gradients—you will not get reasonable colour across the image, there is no way to colour correct. I usually push the background extraction as far as possible, without eating into the main image. Spend lots of time on this step.

· In processing, you will likely have to give up on the very faintest parts of the image, it is difficult to keep them and wipe the background properly.

Hope this helps

Rick

Thanks for the details Rick. I did forget to mention that I’m shooting mono and usually running at 900mm F/6 or 1580mm at F/10.4.

I have just recently enforced the rule of never shooting much below 60 degrees of altitude for just the reason you mentioned. It’s not actually that much of a restriction for me as lower altitudes are blocked by trees to the south and west anyway. To accommodate that I’ve set up my sequences so that I’ll do multiple objects in the night, switching from one to the other as they enter the “good zone”. This has helped a lot in general quality and sharpness of the data but has really increased the time needed to get a complete data set to process out.

Other than a dew shield I haven’t done anything about light shielding but I probably should. Not sure if the best solution are carefully placed small blocking panels far from the scope or a portable encloser as you’ve mentioned. Both are certainly doable.

On RGB subs, I’ve pretty much settled on 60 sec. for most subjects although I suspect I can go longer when running at F/10.4

I have found that running the F/10.4 setup really helps in all areas, especially with gradients. With the darked background they are not as much of a problem compared to what seems to be a general pollution of the colors that can sometimes seem imposable to balance out.

Anyway, thanks for the input. I’ll take a look at the links you’ve provided.