The lunar ‘Cheshire Cat’ revisited, and problems of contrast
November 3, 2014
Just a quick post before I head off to work. London has the telescope bug and he has a birthday coming up, so we were looking at various scopes on Amazon and other places. He wasn’t clear on the distinction between the three main telescope designs, so we hauled out the DK Universe book and looked at the ray diagrams the three kinds (refracting, reflecting, and catadioptric). He was familiar with refractors, like his 60mm Meade, and reflectors, like his Astroscan, but was less familiar with catadioptric scopes, which is not super-surprising since I’ve used my Maks hardly at all in the last year and a half, other than last year’s All-Arizona Star Party. The sky was clear and the moon was high, so we popped outside and set up my 90mm SkyWatcher Mak for a quick look at the moon. Astonishingly, I had not had this scope out in more than two years, since July of 2012.
Here’s my best iPhone photo of the moon from last night. Up near the top of the terminator you can see two glowing dots like eyes peering over the limb of the moon. If you click through to the full-size version, you’ll see that the eyes have a wide mouth below them and that one nostril is showing. Yep, that’s the lunar “Cheshire Cat”, which I first identified back in November, 2010. It was nice to see it again.
While I was processing that photo I noticed something alarming: a circle of glare around the moon that was bright enough to make the eyepiece field stop visible. It’s more apparent in this over-brightened version:
I was shooting through the Celestron 8-24mm zoom, just like Saturday night. Since I had a comparable shot with the same eyepiece through the C80ED from that evening, I dug out the raw photo and tried brightening it up to see how much glare would appear.
The answer is “almost none”. I used the same tool in GIMP (‘Curves’), and I brightened the image way beyond what I did with last night’s shot through the Mak, and the space around the moon is still pretty black in the C80ED shot. Not grey, as in the Mak shot. And this was only with tweaking the brights up, and not moving the darks down, which would be cheating since it would mask the problem.
It’s tempting to read this as a refractor-vs-Mak thing, but it might not be so straightforward. In this case the refractor has very good optics and coatings, so it’s near the upper end of what refractors are capable of in terms of control of stray light. But the Mak does not have fully multi-coated optics–this SkyWatcher version only has ‘coated’ optics, which means possibly as little as one coat of MgF2 on only the outer surface of the corrector. I have heard from someone (Doug or Terry, maybe?) that this particular model of SkyWatcher 90mm Mak has poorer contrast than the comparable but fully multi-coated Celestron C90–irritatingly I cannot find that post or comment at the moment, but I’ll post it if it turns up. Also, the C80ED has a long-ish dew shield which helps control stray light entering the objective, whereas the Mak does not; you can buy or fashion such things for Maks, but I haven’t taken either of those steps. Finally, I’ve seen some threads on CN about glare from the baffle tube in Maks and SCTs, so that’s another possible culprit here.
An informative test would be to pit the C102 against the Apex 127 on the moon, with a homemade foam or cardboard dew shield on the Apex to eliminate that variable. If I get time this evening or next, I may just try that.
10 Minute Astronomy 
How is a Mak baffled? Since the light path goes back and forth, I would assume there is more chance for stray light. I would also assume the C80ED is quite well baffled.
Also, you didn’t do this test on the same night, so you would have had more ambient light with the Mak. You might want to try both scopes on the same night.
I always get the greyish circle when I’m viewing from home. When we’ve been at Sultan Sea, I don’t see it…..only black. So black, in fact, that with higher powers I find it difficult to find the view in the EP. The grey is almost comforting, since I know that I am at least looking through the scope!
That was me. I listed the SW 90 on CL because I never found the optics very good, mainly on the Moon, just not very sharp or bright. I bought a C90 Mak on Amazon and had both for almost a month before the SW sold, so I set up both on a waxing gibbous moon and did a back and forth comparison, and the C90 was a landslide winner: superior resolution, brighter and more contrasty images, more detail in the smaller features, craterlets, etc. Noticeably different ‘scopes. The optics in the SW were almost dull compared with those of the C90.
Doug
I always get the greyish circle when I’m viewing from home. When we’ve been at Sultan Sea, I don’t see it…..only black. So black, in fact, that with higher powers I find it difficult to find the view in the EP. The grey is almost comforting, since I know that I am at least looking through the scope!
Funny you should bring that up–one thing I’ve noticed with the C80ED is that it is very difficult to spot that grey circle visually, even from my driveway. I have to actively hunt for the field stop. I think it just has much better control of stray light than any of my other scopes. I’d like to see how much improvement I could get just by making long-ish dewshields for my newts and cats.
the C90 was a landslide winner: superior resolution, brighter and more contrasty images, more detail in the smaller features, craterlets, etc. Noticeably different ‘scopes.
Yep, now I remember. Astonishingly, the C90 is still available at around $160; that is a heck of a lot of scope for not much dough.
I honestly had forgotten just how small 90mm Maks are until I saw the one on the observing field at the Griffith Observatory. For a second I wondered if it was an 80mm Mak. Such things do exist–at one point, Lomo had Maks in almost every 10mm increment between 30mm and 100mm (the 30mm and 40mm instruments were supposed to be hand-held; I don’t think there was a 50mm version). But no, it was an Orion Apex or StarMax 90.
Although the contrast in my little SW Mak is unimpressive, it is still a fun and compact little scope. And other than my 50mm refractors, it’s the only one that could go as airline carry-on. But I think the C80ED will take over its niche pretty handily.
Hi – wonderful articles on your site. Thought I would ask a quick question. I notice a greenish blue fringe around the moon in your posted C80 ED picture that does not appear in the Mak shots. I was looking at the moon this week with my own C80ED and noticed the same thing. A very bright moon this week but the color was expected. Do you see this often with this scope? Thoughts on why the Mak would not show it?
Hi Steve, thanks for the kind words. As for the green fringe on the moon in the second shot, there are several factors to consider. One is that I boosted the contrast to seriously insane levels in that shot. I didn’t notice the green fringe at the eyepiece. Another is that CA can be introduced by any refractive element in the optical train, including eyepieces. That probably is not a factor in comparing my two photos since they were shot through the same eyepiece, but it might–might–explain why you’ve noticed the green fringe visually and I haven’t (if we’re using different EPs, although I’d expect the Celestron zoom to have more CA than most). Another totally plausible explanation is that I just haven’t looked for it carefully enough. Or perhaps my eyes are less sensitive to that color than yours–all kinds of color perception in astronomical objects are highly variable among observers (for example, perceived colors in double stars).
As for why the Mak would have less CA than even an ED refractor–I think it could be that although the Mak has a refractive element up front (the corrector plate) that element is not bending the light very much, just enough to correct the spherical aberration introduced by the very fast primary mirror. Whereas in the refractor, the objective lens is doing all of the heavy lifting of forming parallel rays into a converging light cone. The ED glass knocks the resulting CA down some but does not entirely remove it. Even triplet APOs have some detectable CA, although for many of the best ones that CA is only detectable by machines like photometers, and they really do look color-free to visual observers. My understanding is that CA in refracting systems can be knocked down to arbitrarily low levels, but never completely eliminated.
Once when reviewing eyepieces I wrote that I get a kick in the brainpan going from 50-degree EPs like Plossl to widefields, but that I can go back and forth between 68-degree and 82-degree EPs without really noticing the change in apparent field. When I go up to 100 degrees, then the jump is noticeable again. Different observers probably have different “bins” of sensitivity. CA probably works similarly. Personally I’d break down CA into at least four categories:
1. CA so bad that it actually degrades the view in a way that is impossible to ignore (a short-tube achro on a bright object)
2. CA that is noticeable but doesn’t bother me (the C102)
3. CA that I might be able to detect if I go look for it, but which fails to rise to the level of my attention otherwise (the C80ED [I assume])
4. No detectable CA even if I go hunting for it (reflectors)
For me, the improvement in going from an achro to an ED scope was totally worth it, but that’s a pretty subjective thing that every observer would have to weigh for themselves.
Clear skies!
Matt – thanks for the reply. I will compare with different eyepieces when next out. I have some ED eyepieces and some standard plossls in similar FLs. Never thought to compare CA before. Overall I love the 80 ED. Bought it to compliment my 6SE SCT and maybe do some AP, but have ended up using it for visual as often as the Cat.
The C90 needs something to kill the glare in the baffle tube. You can try coarse sandpaper with a light coat of flat black paint.
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