Most effective processing approaches for small astronomical targets

In my opinion you need at least 2000mm focal length to start with planetary nebula and then you’ll only get a handful. If you are imaging from under light polluted skies you will need a larger sensor to make plate solving work and at that focal length a crop sensor camera will almost be required.

Practically speaking F/10 or F/11 is the slowest optical system I would try to use and given my own experience with my edge 8” at F10, you are better off reducing it to F/7 for imaging work.

I used it at F10 for 1 image and it was not fun to throw out 50% of the frames. I use a reducer for the scope, operate at F7 at around 1500mm focal length where the performance is much better and you can easily live with it.

I recently captured the medusa and headphones nebula. I think there may be one or two more that I can yield a tennis ball sized image to work with.

Basic advice as I’ve some a bit of this lately. For shooting small PN and most galaxies you need focal length. Independent of aperture it’s practical and possible to image all the way up to F/12. You’ll have to fight a bit more for the needed integration time and pick your targets wisely but It’s not imposable to image at those ratios. The type of scope is less important than the quality of the optics. It has to be sharp because you’re going to be pushing it. Don’t be afraid to oversample and despite general advice, you can use a Barlow, just make sure it’s high quality and well color corrected.

The really big factor in how far you can push this is your local seeing. I’m in the central United States where the seeing is highly variable. When I’m imaging at 0.66” per pixel, I can still get useful data on rough nights, not so at 0.33”. At least not with a huge amount of culling which is the last thing you want to have to do with a slow system.

On the processing side, good sharping tools become really important. BlurX is the gold standard right now as it tightens up the images while preserving star profiles. Cosmic Clarity by Seti Astro is powerful but you’ll have to work hard to preserve your stars. The sharpening tools in Affinity are also excellent and can be layered in to get that final finish. This is an area were it pays to experiment with frequency separation, wavelets and so on. All of these tools are powerful and can ruin an image quicker than boiled asparagus so you’ll need a light hand and good judgement.

Interactive Sky · Mar 27, 2026, 09:00 PM

What setups have worked well for you in terms of:

• focal length and aperture

• camera choice (pixel size, sensor type)

• exposure strategy

Also curious what processing approaches you find most effective for bringing out detail in such small targets.

1) The longer the EFL, the better. I personally use 4,200 mm with a 600 mm aperture. If you want to see image detail you need to couple all of that focal length with good seeing. A big scope with a long EFL under 4” conditions isn’t going to work well. That’s why I operate in Chile. You don’t have to go all the way to S. America to find good conditions but putting your scope in a location with mostly clear skies and good seeing will make a huge difference.

2) Properly sampling to about 1/3 of the best possible seeing blur diameter for your location is a good balance between signal and sampling. (This is a simplification because the actual formula is a bit more complicated.) Here’s a chart based on MTF analysis for optimal sampling for various seeing conditions.

📷 image.pngAlternatively, here’s a “reverse” calculation showing optimum seeing conditions for any given combination of focal ration, aperture and pixel size.

📷 image.png

3) Exposure strategy depends on a lot of factors (including the brightness of the object) but in general, using the shortest possible exposure with the lowest read noise for each sub is your best bet. Take a lot of subs! The higher the SNR, the easier it becomes to extract image detail.

4) Process for detail. BXT is a must. BXT is the most accurate and easiest to use deconvolution algorithm in existence for astronomical image processing. Experiment to learn how to get the best results without overdoing it. Gentle application of detail contrast enhancement with tools like bandwidth sharpening and MultiScaleLinearTransform can help enhance small features. Again, the more SNR you start with, the easier it is to extract image detail.

Good luck with it!

John

Interactive Sky · Mar 27, 2026, 09:12 PM

Hi, Spacey

Thank you for your suggestion. I also wonder which type would work best. RC, SCT, or Mak?

Coming soon … SkyWatcher 8”/2000mm Mak

📷 image.png
https://skywatcheraustralia.com.au/shop/otas/maksutov/skymax-200dx-maksutov-cassegrain-telescope/