Reduced sharpness around sides of subs - does this look right?

[JP-S]

I get quite noticeably reduced sharpness away from the center of my subs and while I know this is to be expected to varying degrees with most optical systems, I'm just wondering if this looks about right for my setup and if there's anyway I can improve it.

My imaging train is an Orion ED80T-CF (apo triplet) -> Orion 0.8x Reducer -> Orion TOAG -> Thin t-ring -> Modded (LPF2 removed) Canon 600D w/ cooling mod.

I've attached a few random subs to illustrate. They're unprocessed apart from resizing and whatever opening them in irfanview does which I think is just a stretch. Theyre all 180s guided exposures except the seagull which is 300s.

I was wondering if it could be something to do with my reducer? I bought it for the flattening effect as my stars were extremely eggy originally. It's helped tremendously in that regard but you can still see some eggyness in the corners. It was also labelled only as a reducer rather than a reducer/flattener. Could this also be related to the camera being modded and different wavelengths focusing at different points? I dont use any cut filter but I figure even if that was the case the effect would still be consistent across the frame. Seeing probably isn't so good where I am as it's sea level and not far from the coast but again I'd except this effect to be consistent across the frame.

Otherwise I'm at a loss unless this is simply a limitation of the scope itself. If that's the case then that's slightly disappointing but will be good to know. I'm already considering moving on to a new scope so that I have something with a bit more reach anyway. Thanks in advance.

[CraigT82]

Certainly looks like field curvature.Is the reducer also a flattener? 

[jambouk]

Flat field and maybe some field rotation, how good if your polar alignment? 

[JP-S]

25 minutes ago, CraigT82 said:

Certainly looks like field curvature.Is the reducer also a flattener? 

This is the reducer. It's labelled only as a reducer not a reducer/flattener. It certainly produces a much flatter image than I had without it but maybe it's still not specced to do the job completely.

10 minutes ago, jambouk said:

Flat field and maybe some field rotation, how good if your polar alignment? 

Under 1' last time I checked and I'm on a pier. Next time I'm out I'll check to make sure though. How can field rotation be identified? Is the shape of the stars in the corner the giveaway? I thought that was due to the flatness but now that I think about it those stars would be stretched across the diagonal of the image if that was the case wouldn't they? Instead they appear to be stretched perpendicular to the diagonal.

Well thanks for the advice, seems like the reducer could be an issue. I'll look into getting an actual flattener and see how it goes. Is it possible this could be due to spacing of the reducer (I'm sure I have the correct spacing but I know this can be finicky), or even due to a slight tilt of the camera? The thin t-ring I use does have a heavy load on it due to the dslr+cooler combo. It feels tight but I wouldnt be surprised if there's a slight sag.

[jambouk]

JP-S, invariably these things are not just the result of one issue.

I think sorting out the flattener and its spacing would be a good place to start. I suspect if your PA is <1' then it is less likely to be field rotation.

James

 

[ollypenrice]

This is consistent with a number of things but certainly with reducer-to-chip distance.  The diagram below, from an old SGL thread, suggests your chip is too far from the flattener.

Olly

[JP-S]

Interesting. It is consistent with being too far away from the sensor then. Would anyone care to guess roughly how far out I am based on the images? Are we talking 1mm or something more like 0.1mm?

If the spacing is too far then I may be in a bit of a bind as I'm not using any spacers currently and don't think I could reduce it.. The reducer requires 55mm of backfocus and with the 44mm of the 600d + 10.5mm of the OAG + canon ring adapter (not exactly sure how thick it is, maybe 0.5mm-1mm), I'm not sure how to make any reduction without turfing my OAG. What do people normally do in this situation? I'll check it tomorrow during the day just to make sure there isnt any slop or slippage in the train somewhere as well.

[JP-S]

Follow up:

So I've resolved this issue and had one of those 'I'm such an idiot' moments. The problem is that I've been relying on a bahtinov mask to focus and somehow I've been using it incorrectly without it ever occurring to me that that process could be going wrong.

I recently switched to NINA from APT and after getting frustrated by it's bahtinov aid, which never seemed to be able to identify the spikes properly, I decided to drop the bahtinov and try focusing manually just going off the HFR measurement it's image analysis provides. As soon as I did this I very quickly saw the HFR dropping a lot with large progressive turns of my focuser knobs and eventually halved it. Compare the carina sub below to the one I put in the first post..

I still have no idea what I was doing wrong with the baht though.. It always looked good by eye then I would fine tune with the bahtinov aid tool in APT without any issue. Is it possible that the mask just isn't a match with the scope or am I likely to be doing something wrong?

 

[fireballxl5]

I can't help with your B-mask query as I use them with no such issue,  but it seems to me that your spacing problem remains.  

I tried fitting the TS 9mm OAG in my mobile OSC setup but couldn't get the necessary spacing and so changed to a guide scope config. As I use this rig to image at a undemanding image scale I can use a 50mm finder scope, keeping the rig light weight.

Still a very nice image btw🙂

[ollypenrice]

11 minutes ago, fireballxl5 said:

I can't help with your B-mask query as I use them with no such issue,  but it seems to me that your spacing problem remains.  

I tried fitting the TS 9mm OAG in my mobile OSC setup but couldn't get the necessary spacing and so changed to a guide scope config. As I use this rig to image at a undemanding image scale I can use a 50mm finder scope, keeping the rig light weight.

Still a very nice image btw🙂

I agree, the distortions are still present even thought the focus is much improved. I can see no pressing reason to use an OAG with a refractor, especially a small one. I use guidescopes at 0.9"PP on a large refractor (140mm). It depends on how fussy you are about getting the corners right.

This is the first time I've heard of a malfunctioning Bahtinov mask! Maybe it is incorrectly configured for your FL or incorrect in some other way. However, I use a B mask made for a Tak Baby Q on a Tak 106 with perfectly good results.

And, yes, a nice image!

Olly

Edited May 6, 2021 by ollypenrice
typo

[JP-S]

Good point! I haven't actually resolved the original issue but have instead identified and resolved a different problem. Still though I am very happy as I feel like my setup is actually usable now and @ 384mm FL, with most objects I can afford to crop the outer regions of the frame where the aberration is most noticeable.

Unfortunately I'm not sure how to proceed on the spacing issue short of going to a guide scope. I did check closely for slop in the imaging train around the reducer and everything is tight. I'd be fine going to a guide scope if I had one but I started with an OAG so that means spending more $$$. That said I did just buy an 8" Quattro as my second scope so maybe I can now just mount this ED80T on it and swap the guiding and imaging cameras between them depending on which one I want to image with.