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Testing Cat spotters…

Posted by Ed Moreno   05/15/2009 07:00AM

This is a test of 3 catadioptric spotting scope: Celestron C90, Meade ETX, and the Celestron Mini Mak.

First, why go to a Mak as opposed to a small APO. Well, there are a hundred reasons, at least for me. I buy most equipment on the used market, and the price differential between a good ED scope for color free viewing and a catadioptric is about $125 to $150 for the Cat and about $250 to $300 for a decent ED spotter. This also translates over to the new market I think because I initially considered buying a small refractor spotter, and in the end, I discovered that they were all pretty pricy.

There are some other reasons, and I will touch on them in the review.

Now this telescope for me was going to be a DEDICATED spotter. It will live 100% of its life on my patio and will be engaged in bird watching and other close up activities (it is AMAZING to watch a bee on a flower at 50 ft… Totally amazing…)

First, in order of ownership, is the C90… This was a Craigslist find. It was an older C90 that included a case, a .965 diagonal, and a couple of .965 eyepieces.

The old C90s used a camera style focuser. In the end, this was what drove me away from the scope, but to be fair, the assistant astronomer said she preferred this kind of focusing because it was more consistent with her 35mm and digital SLR camera focusing.

Me? I didn’t like it because on a photo tripod, it was very difficult to reach focus. I have a truly outstanding C5 and I was sooo spoiled by the almost infinitesimal amount of pressure it takes to focus that scope. By comparison, the C90 was difficult to focus without shaking it. This was even at LOW power.

The other thing about this kind of focusing is that it makes the scope bulge out. The focusing ring makes the scope short, but stout. It also raised the center of gravity some, and the consequence is that when I tilted the scope back on a photo tripod, the cascade effect was more pronounced.

Then there was the .965 eyepiece issue.. I had to buy a diagonal to convert it from .965 to 1.25 so I could use modern Plossls, which are far superior to the eyepieces that these scopes were shipped with.

And finally, the C90 allows you to use the scope in different orientations. If you have a modern Alt-az mount, you can mount the scope to a dovetail and rotate it 90 degrees so that it can be used in one of these new, light, modern Alt-az mounts.. Just rotate the diagonal to get the eyepiece to the right orientation. The use of a 45 degree prism also provided correct orientation, but this is not that big a deal to me because I am an astronomer. I am totally oblivious to mirror reversal.

The finder on this scope (original) was rather stupidly a standard astronomical finder with an upside down image. Silly, since the scope was sold as a spotter.

The down-side to the 45% erecting diagonal is that it makes you have to put it on a slightly taller and perhaps less steady mount. There were many times when I was using it that I think I would have preferred a standard right angle diagonal so I could bend down over it regardless of the scope height or how high it was pointing (Ok, I DID look at the sky once or twice).

Optically, the C90 was actually a mixed bag. The optics were actually quite good I think. Star testing indicated nice Spherical Abberation correction, and the optics were fairly smooth.. There was no sign of a turned edge or any zones. In fact, I would say the optics were very high QUALITY. The scope vignetted noticeably to me with an eyepiece over about 20mm, though in daytime use, this was difficult to see, and even a 32mm Plossl was acceptable in bright lighting. At night, I found the vignetting with anything more than a 26mm to be pretty noticeable. That being said, the scope was very good optically. The focus was quite sharp.

Ah, but they were also maybe 20 years old. I could not see very much in the way of coatings. Also, the small Cat was f/10, and that made the central obstruction quite large. The lack of modern coatings and the largish central obstruction took a small toll.
The result of this was that the C90 was not a “Great” spotter. While it was fairly sharp in terms of focus, I could see that the view was somewhat “washed out.” Since most daytime targets tend to be high contrast, this wasn’t really a big factor to me, but at the same time, I always felt like I was looking through an extremely thin haze.

Ultimately, it was the focuser that drove me away. I just found it hard to focus even at low power without shaking when using a photo tropod.. I insist on the photo-tripod as a mount because they are CHEAP and they are LIGHT.

So, bye-bye C90, and enter Meade ETX.

The make at first blush looks longer than than the stocky C90, but I think this is an optical illusion. Because the diagonal is built in, when you look at the scope, the proportion seems long and slender. I did not measure them or compare them side by side, but my bet is that the 90 ETX is not really any longer, or if it is, only maybe an inch.

It is slimmer and maybe a bit lighter. The Celestron had a cast mirror housing while the ETX has a lot of plastic… Still, I thought the ETX was a very attractive scope, and the assistant astronomer thought it was very pretty.

The diagonal is built in, but of course it is mirror reversed.. You can add a standard 45degree erecting prism to the back, but to do so, you have to buy an adapter ring. These are not all that cheap ($30??) and now you also greatly increase the lengthy of the scope. I was actually quite pleased to use it in this configuration. Again, I am an astronomer, and totally oblivious to mirror reversal, but if it counts to you or your assistant, or perhaps some of your observatory urchins, you might find the C90 with a 45 degree diagonal a better choice.

The only serious problem with the ETX configuration is that if you want to mount it on a modern alt-az mount, the eyepiece can’t be rotated as with the C90. You are now FORCED to buy the accessory back. For this reason alone, I would say for dual use on a modern alt-az mount, this is NOT the scope to own.

The ETX also doesn’t like longer focal length eyepieces. Again, for daytime viewing, the vignetting with a 32mm is not obvious, but at night, it is pronounced. Still, I don’t intend to use this telescope at night, so it doesn’t bother me.

I have heard many great things about the optical quality of the ETX line. I am sorry to say this, but frankly, I was somewhat disappointed. Star testing showed a fair amount of spherical aberration. Now I thought that these machine ground spherical mirror systems were supposed to be easy to make at a very high level of correction, but just because something is EASY to do doesn’t mean that it will always be done right. I would estimate the SA to have been on the order of one quarter wave. Not horrible, but not great. Otherwise though, the optics were very smooth and free of any zones or turned edge. Also, the ETX has the benefit of having modern coatings and a smaller CO than the C90

Now there has been some controversy on the validity of Suiter style star testing of “Exotic” scopes and I was concerned that maybe the testing was wrong, but several comparisons to other scopes left me feeling that my assessment is correct.

See, there is another “Simple” way to determine Spherical Abberation. It is one of the simplest tests I know in terms of just getting a “Feel” for the amount of SA that is available.

When the light cone of a scope with no SA reaches focus, it reaches focus at an EXACT point. What this means is that if you turn the focuser, there will be ONE POINT and ONLY one point where the focus looks EXACT. Even the SLIGHTEST movement from this one point in either direction will soften the view.

If the scope has any meaningful SA, the result will be that the cone does not have one EXACT focus point. Some of the rays reach focus before the point of “Best” focus, and others reach focus AFTER the point of “Best” focus. In reality what this means is that there is not a “Un-ambiguous” point of focus. And this was the case with the 90 ETX.
At higher powers, it seemed like there was tiny range of movement where the focus didn’t get better or worse, but the image was not really “Sharp.”

To be fair to the ETX, I pulled out some other scopes.. Out came the Televue 101 and Tiny Tiger, my absolutely outstanding C5.

My target was a small fruit fly about 100 feet away. I was fortunate that he was lingering on a vent pipe on my neighbor’s house. I use this pipe because it has a UPC code on it and that is my “Usual” target.

The TV 101 was of course “Perfect”. The focus point was EXACT. When I had exact focus, I could EASILY see the “Veins” in the fruit fly’s wings, and the joints on its abdomen. It even had little strips on the abdomen. This is an OUTSTANDING telescope, and I got a magnificent image.

Next came the C5. Once again, the focus point was EXACT. And in spite of all that is said bad about SCTs, if the optics are superb, my own experience is that they can be pretty formidable challangers to high quality refractors if you bump up the aperture a bit.

And sure enough, I could see almost all of the same fine detail in the C5. It was SURPRISIGLY close to the TV 101, and I am not surprised by that. SCTs can be a crap shoot from a quality standpoint, but when they are really great, well, you can see it at the eyepiece.

Then the 90 ETX.. YES, it is a smaller scope. Even taking that into account, the problem was that with the other scopes, there was a SINGLE point where focus was EXACT; where even the slightest movement with the focuser would degrade the image.
With the 90ETX though, this was not the case. There was a small range of movement were I could not really see a single point of exact focus… And THIS is what happens with Spherical Aberration. My take here is that in spite of the reputation these scopes have for having really great optics, I am not convinced that this is the case. I think they are probably variable as are most mass produced catadioptric systems.

In spite of this, I felt that at low power, the image in the ETX was very difficult to differentiate from the C90. In fact, it actually looked maybe a bit BETTER than the C90. The image did not seem to have the “Haze” that was present in the C90. Color saturation seemed a bit better. So, at lower powers, at the kind of powers a spotter on a photo tripod are typically used, the 90ETX performed quite well. And I preferred the way it focuses (though I wish the knob were longer and stuck back passed the flip mirror housing.

So, neither of these scopes is “Perfect”, but honestly? As a spotter, each I think are more than capable of providing excellent 30x to 60x viewing (maybe even 90x with the C90) in birding and other daytime nature activities. I think packaging here is perhaps more important than optical quality and I will go out on a limb here and say that based on my own experience, BOTH can vary optically in terms of quality with some being better or worse than average.. Again, for daytime 30x to 60x use, I give thumbs up on either (though to be fair, my C5 would be the “Spotter” I would bring on a nature trip, but I don’t want to leave this scope outside 24/7).

Final scope in this test is the Celestron 65mm Mini Mak. I bought this NEW (Man, I NEVER do that!) for around $60.

Ok.. Bottom line? This scope is not worth writing much about because I only took a brief look at it. It simply was not worth anything CLOSE at an in depth assessment. My first look through it told me immediately that I wouldn’t hit a pooch in the booty with it. Focus was impossible to find. This did NOT look like cat problems to me. The scope has a built in zoom, and a lot of what I saw looked like it was in the realm of chromatic aberration. There just seemed to be a softness that was way beyond what you get from things like under-correction or zones or things of that nature. I FELT like I was looking through a toy REFRACTOR with PLASTIC LENSES. This was WITHOUT DOUBT the WORST QUALITY OPTICAL PRODUCT I HAVE PURCHASED IN 30 YEARS. SHAME on Celestron for putting their brand name on this piece of trash! It was THAT bad. It receives the ABSOLUTE LOWSEST RATING that I have EVER given to ANYTHING in astronomy. Aviod at all costs!

So, this concludes my longish review. I spent a lot of time on the ETX because I did not want to attack their reputation lightly. At the end of the day, I think it is quite passable for spotting and NOT A BAD SCOPE AT ALL… It was simply not a “Great” as I had been led to believe it would be. I would recommend it with the caveat that the optics probably vary in quality from sample to sample, but I would say this about most mass produced catadiptric systems sold today.

C90 or Meade ETX as choices for a reasonable quality spottier vs a much more expensive ED spotter? Yes.. Two thumbs up. Either will give you quite good low to medium power performance, so packaging would be the key differentiator between them.

C65 Mini Mak? Where did I leave my Mini-trash can…