What to correct first

[Nicola Hannah Butterfield]

Coma or draw tube incursion.

We all know coma, this should be uniform across the image, tube incursion more subtle as it only affects one side of the star like this, yes I know a little soft, but you will notice the light bleed to the 2 o'clock position,

Is it better to fix this first or coma, I use a SW 200P with currently a Canon 40D I know any fix will involve the complete optical train. Will the https://www.rothervalleyoptics.co.uk/ovl-dual-speed-2-low-profile-crayford-focuser-for-newtonian-reflectors.html give an image without intrusion and also be closer to the tube for say a coma reducer or a camera rotator without cutting an inch off the bottom of the tube.

[SoftwareDeveloper]

The 4 spikes on the stars is likely due to the spider at the front of the Newtonian OTA. The 2 oclock blur is odd, its not symmetrical with another one at 7 oclock, so this is a reflection off of something, look at your setup, its most likely something very simple once you see the source. If you are new to imaging, I would call this a very satisfactory starting point!

Improvements are slow and tedious and sometimes they happen in the "wrong direction" and the improvements have to be undone.. such is life I suppose!

As to the spider, one with curved arms can reduce that diffraction pattern and fix that. The 2 oclock issue is not so obvious, let us know what you find.

[malc-c]

Not sure if this is similar to the issue I had as detailed in this thread mainly as you have a different focuser which could be intruding into the optical path where I used the stock focuser

[Nicola Hannah Butterfield]

@SoftwareDeveloperThanks for your input, however I know what is causing the light bleed at the NE position, it is the focus tube into the main tube.

I am not worried about the diffraction spikes bar the image was soft, it was merely meant to point out the problem, I have much sharper images, it was just the first that came to hand, I don't tend to keep test shots on stars.

@malc-c I am still on the stock focuser, the link was to ask if it would improve things without modding the tube by an inch or so.

It isn't so much the secondary spike, but merely what do I fix first comma or a shorter low profile focuser, to fix the light bleed caused by refraction around the focuser tube.

This is from the central stars in M44  yes I know should have shot it 90 degrees.

Just a single 2 min exposure from the raw file, it is shot on a Canon 40D and a SW 200P. I think I have answered my own question here. Given this single image coma, I think first.

Edited November 9, 2022 by Nicola Hannah Butterfield

[malc-c]

Personally I would correct the coma first, should improve the star shape, especially round the edges

[ONIKKINEN]

The coma first, definitely. Coma will degrade the image anywhere but the central 2,78mm of your F/5 newtonian whereas the focuser obstruction and other aberrations will be just minor cosmetic things mostly around bright stars.

On the obstructions, your star shapes show what most Skywatcher newtonians do and that is mirror clip artifacts around stars. Below is an example from your image, its a bit difficult to see but when you actually process an image fully and stretch heavily these will be very apparent. The second image is from someone elses scope, i dont remember which scope it was (probably a 130PDS) or where i got it, but it shows it better.

The yellow lines are where there is a "shadow" in the halow of a star. These are the mirror clips blocking light reaching all the way to the edge of the mirror.

There are 2 ways to interpret this issue and solve it, one is to remove the mirror clips and attach the primary mirror to its cell with some silicone sealant and then remove the clips as they are no longer needed. This method will make all of the star be surrounded by the dim halo and will get away with the shadows in it. The other way is to hide the rest of the mirror edge with a primary mirror mask placed in front of the mirror. This will reduce your aperture a tiny little bit, but will make the shadows, and the halo, mostly go away. I would go for option 2 from these. Some people do both however and get rid of the clips, silicone the mirror to its cell and still install the mirror mask.

Mirror mask sold at FLO: https://www.firstlightoptics.com/misc/wega_mc_sw_200.html

For coma correctors you have many options, but here i think its better to spend a little more than the cheapest options. I would avoid the baader correctors, the cheap skywatcher one, and the various 0.95x ones sold under TS and sharpstar (and others). The Skywatcher aplanatic or the TS GPU 1.0x would probably be the best options for fixing coma and not introducing other aberrations in the mix (the baader ones are astigmatic).

[Nicola Hannah Butterfield]

@ONIKKINENI know about the mirror clips issue. I have to take the mirror out for a clean, so might do something at the same time. Thanks for the input.

[malc-c]

One of the things I found out back in 2011 when I started using my 200P for imaging was that stars had unusual diffraction spikes because the 200P is primary designed for visual use.  In addition to mirror clips, unsilvered flats on the secondary mirror's minor axis where it had been clamped in the process also introduced a large fifth spike.  I ended up purchasing an over sized mirror from Orion Optics that matched the dimensions of the secondary on the 200PDS, which was fully silvered and resolved the problem.  This resulted in a hybrid as it still had the same distance between main and secondary mirrors (the PDS is shorter) but a larger secondary than the stock 200P.

I tried to find pictures from the time, but as they have been removed from the hosting websites they are no longer available.  Your images don't seem to show this artefact (it's slightly noticeable in the left hand image in Onikkinen's post, just after the bottom spike), but for me it was very prominent