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Realistic expectations for image sharpness?

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When I look at my subs they seem to lack sharpness, is there something I can do to improve this or I have I reached the limit of my equipment or seeing conditions?

I know that my C8 SCT isn’t a great choice for imaging, but it’s fine until I can afford something that’s going to be significantly better (I’m not sure what that something is yet, or how much better it could be). My usual FWHM focus values are around 4 +/- 0.1 using an ASI1600MM for the imaging camera, unbinned with a 0.63 reducer.

The mount and guiding setup seems good, typically total guiding RMS values are around 0.3 px using a ST80 with an ASI120MM.  I built an Ardufocuser and control everything through NINA, including the autofocus, manual focusing doesn’t improve the focus and an out of focus collimation check looks fine.

I’ve attached a single auto-stretched sub as an example. And also the semi-finished image, I'm attempting to make best use of a Pixinsight trial.

NGC 281 - Pacman-SHO.jpg

2020-10-11_23-51-55_Ha_300.00s_0006_Pacman Nebula_1x1_200.jpg

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Absolute sharpness is related to seeing conditions, your mount/guiding and aperture used.

Relative sharpness is related to your sampling rate with respect to FWHM.

You say that your FWHM is about 4 (pixels I suppose and not arc seconds?) - this makes you over sampled by quite a bit. Ideally, you want your FWHM to be about 1.6 pixels.

This means that you should "bin at about x2.5" to get to proper sampling rate.

This image looks blurry when viewed at full size:


But this is resized down to 40% (2.5 times) - look how stars look pin point and detail no longer looks blurred out.


Key is using the proper sampling rate for what you scope and skies can deliver.

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Thanks for the suggestion vlaiv, I hadn't thought about resizing, I've previously binned the camera to 2x2 at capture but I wasn't happy with the results so I thought I try unbinned and compare against previous data,

The focus is measurement should of been HFD in pixels, not FWHM.

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