View Full Version here: : Is astigmatism normal?

17-11-2017, 08:00 PM
Ok, dumb question on the face of it, but I have to ask. I've seen mention that *off-axis* astigmatism is inherent in newtonians, but it's usually overwhelmed by coma. When one introduces a coma corrector then, is it expected to see off-axis astigmatism in a "normal" set of mirrors that are well-mounted?

I understand that on-axis astigmatism is definitely abnormal, but I'm confused about off-axis.

The reason I ask is that I can see what I believe is off-axis astigmatism. In the attached example I've taken a section of an image showing the same stars from the top left corner of the frame. One was outside focus, one at focus (roughly) and the other inside focus. The in/out show perpendicular elongation and the "at focus" image looks a bit misshapen. On-axis stars look fine, I only see this increasingly towards the edge of the frame.

17-11-2017, 08:15 PM
I found that rotating the primary in its cell helped with that.

17-11-2017, 08:28 PM
Couple of quotes from http://www.telescope-optics.net/index.htm#TABLE_OF_CONTENTS

"The dominant off-axis aberration in the Newtonian is coma; astigmatism is low in comparison."

"For complete correction of a paraboloid, more complex correctors are required. In general, they need to have three (or more) single lenses, or two or more achromatized doublets. The lenses are more widely separated, creating more degrees of freedom, so that combined aberrations can be brought to a negligible minimum (for instance, the Paracorr-like corrector corrects coma with the front achromat without inducing spherical aberration, but it does come at a cost of inducing enormous astigmatism, then corrected with the rear achromat - something that cannot be done without widening lens separation). Examples of this advanced corrector type are TeleVue's Paracorr and Wynne triplet."

Am no expert, but as I read it, astigmatism like that could come from either the primary (there will be some off axis astigmatism) or from incomplete internal correction of the astigmatism introduced by the front section of the CC. Might be worth playing with the corrector spacing to make sure it is right in the sweet spot for your particular setup. In any event, if the astigmatism right on focus is submerged under the seeing blur, it may not be a major issue.

17-11-2017, 08:45 PM
Thanks Marc :-)

Thanks Ray :-) Interesting post! I used the Paracorr in these images, though I believe I've seen the same thing with a GPU corrector. The Paracorr is currently spaced at its documented 55mm... I can go from 55mm to 59mm but nothing in between with my current adapters.

I'm suspicious of the secondary mirror/holder. I can see a big smear of some kind of adhesive and I suspect I might need to separate that and re-glue it. I'll start with easier things though, but just wanted to first confirm that this is indeed not normal.

17-11-2017, 08:52 PM
if the secondary is a bit bent, it would also produce astigmatism on axis - have you seen any? If it is only off axis, would look more closely at the CC than the Newtonian itself.

Slight complicating problem is that astigmatism can come from the primary mounting, the primary mirror figure, the secondary mounting, the secondary figure, the CC fabrication quality and the CC spacing - but I guess that isn't helpful :lol:

Agree though, secondary mounting is a big issue.

17-11-2017, 08:55 PM
Ah, I did not know that, cool, thanks! No, I've not seen any on-axis astigmatism, only off-axis. So that speaks to either a collimation issue, a spacing issue, or an issue with the correctors? The latter seems less likely since I've seen it with two correctors.

17-11-2017, 09:03 PM
well, maybe give Marc's idea a try - there will probably be a primary orientation that minimises the problem.

another question, does the best focus off axis correspond to the sharpest image in the field centre (ie is there much field curvature)? If you can get a sharp central region and round stars across the field at focus, maybe just accept that there is a little astigmatism at the field edge.

18-11-2017, 10:35 AM
Forgot that one. The slightest pull on the secondary glass (glue/silicon dabs, etc...) will create astigmatism so something to check first is the mounting. If you haven't fiddled with the secondary then rotate the primary in increment of let's say 10 degrees and check for the star shape intra/extra focus until you find a sweet spot. You usually don't have to recollimate so it's fairly easy to do. Not very scientific I know but it worked for me.

18-11-2017, 08:37 PM
I'm not sure yet; I've just got this corrector and there was definitely tilt such that the best focus in the top left corner was *not* the best focus in the center, but it wasn't the best on the right either... but there could be curvature in there as well, I can't say just yet.

Thanks mate. So just to confirm, as per Ray's message above, a poorly mounted secondary (and presumably primary) will result in *on-axis* astigmatism? If that's correct then I can rule that out, because on-axis is fine.

Looks like at least a week's worth of clouds now... and I'm probably going to organise a Lite Crawler tomorrow (pending the sale of the Avalon), which will require the return of the Moonlite CRL and thus prevent me from doing any further testing for the foreseeable future.