Dealing with light pollution from a Bortle 7/8 location

[Steve143]

Hi,

I recently captured some images of M31 and M42 using a DSLR and Samyang 135mm lens. I took 15s subs of M31, 30s subs of M42 as well as 60s subs of M42, all at ISO 800 at f2.8. I stacked all the subs in Affinity Photo and did some simple tone stretching. The results from M31 weren't too bad, but the two stacks of M42 had a really strong beige cast across them, which I'm struggling to remove without losing any detail of the target. 

M31 was pretty close to the zenith when I captured it and obviously M42 was closer to the horizon, and in the west, which I'm assuming accounts for the increased light pollution in my stacked images. 

Do you think I would be better off taking shorter exposures (and increasing the number of exposures) to try and counteract some of the effects of light pollution in my location? 

Would it be helpful to invest in a filter, like the Optolong L-eXtreme, to protect against light pollution while increasing light captured from nebula?

Could I just simply process out the strong light pollution in Affinity Photo while maintaining the target? If so, are there any tips on how to do that? 

I'm still relatively new to imaging but have just acquired an Altair Starwave 80ED-R refractor that I want to use as my imaging scope, initially with my DSLR and then with a dedicated astro camera.

Any help dealing with light pollution would be great!    

Thanks,

Steve

[raadoo]

You're bang on with a lot of your assumptions. I'd say this is a case of less more is more and suggest you approach light pollution reduction from multiple angles:

• I wouldn't go over 60s subs in your Bortle 7/8 with an astro camera. That number would be 30s for me, with a non-modified DSLR.
• The L-Extreme is good for strong Ha targets, but you may find that it's a bit too strong as a general / always-on filter. I'm quite happy with the L-Enhance, having been somewhat disappointed by the L-Pro.
• Proper calibration frames are always going to yield better results than synthetic solutions in software. Flats are going to have the biggest impact, but darks, biases and, in my particular case, dark flats are also important and looking back now, I'd never shoot without them.

Affinity is a fine piece of software but it's not what I'd call dedicated and it's likely that tools like Astro Pixel Processor or PixInsight with their Light Pollution Removal and Dynamic Background Extraction tools, respectively, will help you more with removing those pesky gradients.

[ONIKKINEN]

If its just a uniform colour of light pollution over your images it goes away with just colourbalancing. Have not used Affinity but in Photoshop you can do this with either levels, curves, the colour balance tool or all 3. Or the auto-colour and auto-tone options.

Try to bring your histogram peaks together like this:

Now this isnt "done" yet because its far too blue but that's the gist of it, to be honest i never learned to do this properly manually as there are tools that are infinitely better than this available for free. Dont be afraid of the beige cast on images, its very much normal unless you're imaging from a very dark site and its something that just goes away with colour balancing.

But it would be better to start using dedicated astronomy software as soon as possible as this kind of manual fiddling with sliders will just not get the same result as a tool meant to do that. There are some really simple and free software to choose from that do this kind of thing much better.

For stacking: Deep sky stacker and SiriL. Deep sky stacker preprocesses and stacks your images and is very simple to use for even beginners. SiriL is maybe a bit less simple to use but you can preprocess, stack and process the image using it. You could also stack in DSS and then process the stack in Siril, and then do final touchups in Affinity or PS.

SiriL has a gradient removal tool that attempts to fix gradients if your image has them. From B7 or 8 i would say its almost guaranteed you do have them so this step is quite useful or in many cases absolutely necessary to get a presentable image in the end. Once the gradient is fixed you can run the Photometric colour calibration tool which looks at your image, detects which stars are in your image and then looks up their colours from a photometric catalogue and attempts to match your image colours to the catalogue colours. It works extremely well for most types of images. There is also a manual colour balance tool where you just select a box that represents background and select a box which represents a white spot, like a white/yellow star or in the case of a galaxy you could select the entire galaxy.

The filters you are thinking of are situation dependent. For emission nebulae, like M42 you will benefit from narrowband filters such as the L-extreme. For broadband targets, like galaxies you do not benefit from these kinds of filters as there is no specific wavelength you can pick and choose to get the best result. For galaxy imaging the best choice is to just deal with the light pollution and capture through it, with no specific filter. If the conditions are really bad -  like inner city bad you might want to use a broad-ish light pollution filter like an L-pro, or some other similar filter. But you should know that using this kind of filter removes the possibility of getting a true colour galaxy image in the end as you have cut off a big chunk of the actual signal along with the unwanted light pollution signal. I would advice against such a filter for galaxy imaging, but for nebulae the L-extreme or similar would make a big difference.

[rickwayne]

And more integration time. Call me a curmudgeon but I find the combo of adding integration time and using dedicated astro s/w for gradient removal to work best. I get better results with APP than I did with SiRiL but your mileage may vary. 

[Stickey]

The chimneys about there being better astro-imaging software packages than Affinity Photo are no doubt true,  but it has one huge advantage over many of the other specialist image editing tools - it's much,  much cheaper and it's permanent licence covering all your computers. No having to pay a subscription or a separate licence for each computer. 

It also comes with a Gradient Removal tool, I think it's under the Filters menu. 

James Ritson did several You Tube videos on using it for astro work and has produced a suite of macro to perform common astro techniques and there are a number of 3rd party astro macros for Photoshop. Most of these will work on Affinity Photo to. 

It's quite memory hungry, so you'll need 8 MB or more.

[Stickey]

1 minute ago, Stickey said:

The chimneys 

God, I hate smart text 😳

[johninderby]

31 minutes ago, Stickey said:

God, I hate smart text 😳

First thing I do with a new phone is turn off predictive text  / auto correction. 😁

Edited February 13, 2022 by johninderby