Coma expectations from C8

[Mickconn77]

Finally collimated and tilt adjusted my C8 and the field curvature is quite substantial, even with the 0.63 corrector/reducer.
Have included the diagnosis from CCDI (please note the image was cropped unintentially) and the severity of the coma is quite apparent. Does this seem standard for this scope?
What correctors would be recommended? Starizona may be too expensive, Alan Gee (confused about its application).

[garymck]

What spacing are you using for the Celestron focal reducer? I've used one on a C8, C11 and currently on a C925 without any problems. The correct spacing is 105mm.
Gary

[multiweb]

Not as much coma as field curvature. First thing would be to find out the optimal separation between your mirrors then add spacers at the back to reach focus and take it from there.

[Dave882]

I’ve used a celestron 0.63x on a c8 and results are pretty decent (but I wouldn’t say an entirely 100% flat field). The spacing is critical for imaging and while i aim for about 105mm some report slightly different depending on the age of the reducer.

Also - shat size camera sensor are you using? I use a 4/3” asi294 which I feel is right on or just above the upper limit that it can handle. A smaller sensor will give a better flatter result.

[Mickconn77]

I’m currently at approx 110 but have had the issue from 90mm-120mm.
I’ve done away with my OAG for now to attempt get more range and take subs from 80mm. Have read that at about 85mm the reduction is approx 0.7 and many have found a flatter field there.
There is so much debate in countless forums about this reducer but I haven’t found any that have quantified the curvature using CCDI.

[Mickconn77]

Yeah I’m using the same sized sensor on a 1600mm which makes me think I’ve got something gravely wrong. My spacing is approx 110mm as the oag caused some issues. I’ve since done away with it and waiting on a clear night to do subs from 80mm to find the sweet spot.
Glad to hear the curvature isn’t meant to be this bad, user error is a best case scenario.

[JA]

Hi Mick,

Maybe you are doing nothing wrong. Perhaps look for uncropped Astrobin images using a 0.63x reducer on the C8 with a 1600mm sized sensor to be sure. The images may point you in one direction or another in terms of understanding the root cause of the image edge quality issue.

In any event, using a focal reducer, whilst making for a faster scope will mean you see more of the image circle than you may wish to, revealing all sorts of foibles at the edge of the image circle, even on the relatively modestly sized 4/3 sensor of the 1600mm. I'm not sure another brand of 0.6ish-X focal reducer would help that much, on that sized sensor (~22mm diagonal). Also, perhaps consider trying/researching a 0.8x reducer.

Best
JA

[Mickconn77]

I’ve scaled the spacing back to 85mm which lands the focal reduction at 0.7. It’s much better to the eye. Will run it through ccdi to make sure tilt isn’t an issue now the oag and visits back has been removed. Will respond once I have quantified the curvature😉

[Mickconn77]

Ok. Although the image looked a little better the curvature is just as severe. Could be a defective reducer/corrector. Could be just how it is.

[evltoy]

I have been battling this back focus on my C8 with the celestron x0.63 reducer since I ever got it in Jan 2021!

First thing. There is currently 2 celestron x0.63 reducers out there. There is no markings to tell you the difference.
- The old original one which needs 105mm back focus
- And a more recent unit (last 2 years) that needs 85mm back focus.

If you bought new stock in the last year or so it will be 85mm like mine is. Celestron and/or the paper that comes with the reducers doesn't tell you they have changed the back focus after changing glass manufactures. This is why there is so much confusion with users on the new item.

My current setup is now bang on 85mm (measured the extensions (30mm), T-Adaptor (8.5mm) with a vernier caliper) + DSLR (46.5mm). My Camera is a Nikon D600 in APS-C mode and using M42 extensions.

I did my focus with a Bahtinov mask with the star 1/2 way between edge of field and centre of FOV. Also did the recommended finishing the focusing in CCW.

The attached image is a 90 & 120 sub frame I took last night. Guiding through PHD2 was around 0.21, Polar align through SharpCap (0.3arc/sec). Star alignment was perfect.... the best I have ever gotten in a long time.

[Mickconn77]

Wow thanks for that! Great images.
My stars look identical to yours so I’m guessing it’s a just how it is. I did a quick stack and edit (30mins) this morning and I don’t hate the result. I think I’ll keep on with it and save for an edgehd when the time comes.

[Mickconn77]

Here’s the shot I took last night.

https://imgur.com/a/ROjy1jg

[evltoy]

Thank you for the positive comment.

Personally I dont think this is acceptable especially getting what I get mid field of view. I get better images without the use of the reducer.

I think we may have come across a faulty batch. When did you get yours? What it new from a store?

Cheers
Wayne

[evltoy]

I just saw your image, it that cropped?

[Mickconn77]

No, that’s uncropped.

I think the stack rounded out some of the stars on the edge.

I bought mine second hand and believe it to be around 10 years old. It’s from an old SE that I’ve remounted to my HEQ5.
In my subs some of my stars look triangular or V-shaped. I agree it’s not acceptable, perhaps there’s a faulty lot (or two)?
I’ve attached an image of what it looks like on the edges with zero tilt and almost perfect collimstion.

[evltoy]

Well knowing yours is the old version you can say you are needing 105mm.

Try starting at 100mm and work up by 1mm increments up to 110mm. I think by you doing this you will find the sweet spot.

The reducer helps flatten out the field, but you have to expect to get weird stuff on the edges especially when using a large sensor like 3/4 & APS-C. Centre to middle of FOV should be perfect round stars.


For me, I'll be doing the same. Starting at 80mm and working up to 90mm with 1mm increments. This takes time and is taking up good quality image time and $$$ on shims. I'll be very annoyed if I find out my reducer is faulty after all the time, effort and money spent on this or being mislead by the retail shop.

Let us know your results.
Wayne