Sunday, March 28, 2010


The Mighty C90

Sorry, Mike Weasner. I know the 90mm ETX, the MIGHTY ETX, beloved of many of us, including Unk, is the classic small MCT—even moreso than the Questar 3.5, since many more people have been able to experience the charms of the little Meade Mak. But the ETX ain’t the only small CAT that deserves our attention. The seldom-seen Parks Jovian Four 4-inch, for example, is one cool kitty (the old JSO-made models, anyhow). So is a real sleeper, the good, old Celestron C90.

As I recount in my Used CAT Buyer’s Guide, I got right excited when, in 1977, rumors began to circulate that Celestron was fixing to release a new scope. That excitement turned to dismay, and even disdain, one evening when I was On Alert at Titan II Missile Launch Complex 373-4. I often tried to save up Astronomy and Sky & Telescope to read on alert. Almost as soon as I pulled one of ‘em outa my B4 bag (Sky ‘n Scope was usually first) and began thumbing through it, I encountered an ad not unlike the one above, an ad for that very new telescope. Turned out the new CAT was called the "C90" and it was a 3.5-inch MCT, a Maksutov Cassegrain, not a Schmidt Cassegrain as I’d oh-so-blithely assumed it would be.

Yep, a small, very small, CAT that cost about 500 dollars for the “Astro” version, which came on a one-arm-bandit fork mount. Just about as much as an 8-inch Criterion Dynascope, fer Chrissakes. I was disappointed. Badly disappointed, doing enough under my breath muttering about the depredations of California telescope companies to make my fellow crew member begin looking at me funny. Admittedly, there was a bit of disconnect in my attitude; after all, I loved the idea of the (unobtainably expensive) 90mm Questar. Why was I so P.O.ed at the idea of a 90mm Celestron MCT that cost a lot, lot less than a Q?

Mostly just that disappointment, I reckon. What I was hoping for was a cheaper small SCT, maybe a nice “C4,” a 4-inch. At the time, Celestron telescopes were still tough nuts for me to crack. I I just couldn’t stomach having to miss lunches (and dinners) to accumulate the large amount of George Washingtons even a C5 commanded—not much less than the price of the original C8. Bottom line? Looked like I'd be sticking with my beloved if not quite perfect Tasco 11TE unless Celestron released a less expensive scope.

Despite my disdain for Celestron’s wee kitty, I did absorb the details. It wasn’t just one scope, but three: Astro, Spotter, and Telephoto. The optical design was the same on all of ‘em: a 90-mm aperture MCT of the Gregory design (aluminized spot on the corrector serving as the secondary) with the moderate focal ratio for a Mak of f/11. This basic tube was—unfortunately and to my further chagrin when I found out—set up to use only .965-inch Japanese Standard eyepieces and other accessories. In addition to the scope and a .965-inch diagonal and eyepiece, you got a smallish 5x24 finder on two of the three models. I had to admit this C90 was cute, no doubt about it, with a tube painted that famous Orange.

The most expensive of the three variations was—wouldn’t ya knowit?—the most interesting for us amateurs. The 500 buck Astro model cost that much, I suppose, because it was furnished with the single arm fork perched on a C5 drivebase. Despite the lack of dual tines, it was a sturdy little rig, maybe e’en a mite sturdier than the Questar 3.5’s mounting, despite that shiny polished aluminum thing’s twin fork arms. Because of the one-arm nature of the mount, a counterweight was attached to the tube opposite the fork arm to steady the OTA when tilted over on a wedge for astro-use. Naturally, the smallish drivebase included a clock drive, and also naturally, for the dark ages, it was an AC model. Plug it in, it ran, unplug it, it stopped.

What could you do with the puppy—err, “kitty”? 90mm is not a huge amount of aperture, but it is sufficient for casual viewing of the moon and planets, and will bring back most of the Messiers, even from compromised skies, if not in detail. To make much astronomical use practical, though, you’d need a wedge in order to polar align the little thang and a tripod to carry the whole shebang. Celestron offered what was basically the C8/5 tripod and wedge for that purpose, and that did indeed make for a very stable platform for Little Kitty. Unfortunately, adding those items jacked the already surprisingly high price up to a cool seven-hundred and ninety-five simoleons, equivalent to at least two-thousand 2010 dollars, depending on how you measure such things. You can see why I was put out.

If the Astro was too much, you could opt for li’l sister, the mountless Spotter version, which included the OTA, and, same as the Astro, the 5x24 finder, .965 diagonal, 18mm f/l . Kellner eyepiece, and 2.5x Barlow lens. Also like the Astro, the Spotter came with a beautiful little attaché style case made of plastic composite. At $395.00, it was at least approaching “reasonably priced.” Course, that was more than your buddy paid for his unarguably more capable RV-6 six-inch Dynascope, doggonit. Still, with the Gas Crunch of 1973 fresh in memory, and Pinto and Vega-size cars beginning to outnumber the Mercury Montereys (younguns: don’t ask), a lot of amateurs was beginning to put a premium on “portable,” and the C90 most assuredly was that.

If $395.00 was still too rich for your blood, you could bargain basement it down another hundred bucks with the final (for the moment) C90 configuration, the Telephoto. Initially, the OTA was exactly the same as the other two. Well, exactly the same except for the tube color; it was black, better to match your SLR, I reckon. While you got the same nice attaché case as with the Spotter and Astro, all you’d find in that case was the scope and a T-adapter; it was shipped sans finder/diagonal/eyepiece. I’ve been told Celestron sold more Telephotos than either Astros or Spotters. Maybe that was true in the beginning, but most of the C90s I’ve run across over the years have been Spotters. Sadly, but not unexpectedly, the Astro didn’t sell well, and was discontinued in fairly short order.

After a while, Celestron began to offer the Telephoto in a focal ratio of f/5.6 to make it more practical for photography (a fixed focal length f/11 1000mm lens not being much sought after), if not as useful for astronomy. The secondary obstruction was larger and the optics of the 5.6 were, in general, not as good as those of the f/11s.

The Spotter? In various guises, including many with protective rubber armoring, and occasionally with mounts, including the EQ-2 GEM, it continued on its merry way for decades and decades, and was the sole survivor of the original trio by the time the original C90 was put in the ground by Celestron.

As the 1980s faded into the 1990s, and both disco and Orange Tube telescopes became dim memories, the little C90 carried on in, as above, slightly different permutations. (Celestron even sold one with a brass tube/barrel for a while!) I didn’t pay much attention, though. I’d got it into my head that the C90 was optically BAD, BAD, BAD. What did I base that on? A few brief peeps through one or two over the years, and what some of my buddies said about the Wee One. That was it. Which wasn’t much. How little did I know about the C90? I didn’t even know how she worked.

TSP 1997.
Case in point? The misbegotten 1997 Texas Star Party, where I had my first close encounter with the C90 in years. There was plenty of time to wander around the observing fields admiring folks’ gear. In fact, that was about all there was to do. That was the year the famous TSP temporarily and infamously moved from Fort Davis and the Prude Ranch to the Texas Hill Country and the Alto Frio Baptist Encampment. It was a nice facility, but in the late spring, the time of TSP, Hill Country is pretty much all wet. It rained and rained and rained. It would sometimes clear in the morning, and folks would un-tarp their scopes to dry ‘em out and leave them that way after dark, hoping for clearing. That was when I ran across, yes, a C90.

This worthy, one of the rubber-armored versions from the late 80s - early 90s, I recall, was riding piggyback on a C11. Seeing me staring at the little thing, the rig’s obviously proud owner stepped up and asked how I liked his guide scope? “Cute,” says I, “but don’t you have trouble with its mirror flopping during exposures?” The owner quickly informed me the C90 didn’t have mirror flop problems, since the mirror in a C90 don’t flop. As he demonstrated for me, the C90 focuses more like a lens than a telescope. Unlike the Celestron SCTs, the primary mirror doesn’t move; the forward part of the tube is separate from the rear cell and turns in threads to focus. Embarrassment reigned supreme. I’d been roundly critical of the C90, and didn’t even know how to focus one.

I came back to this particular 90 during the course of one of TSP 97’s few clear evening stretches, and found its owner had set it up for visual use. High winds and poor seeing meant nobody was gonna be taking pictures or doing serious visual observing despite the clearing. He offered me a look through the little scope, and I accepted—rather eagerly, actually. Surprise. The C90, this C90 anyhow, was not bad optically; it was actually pretty good. Especially given the less than optimum conditions. To be honest, its images of deep sky objects didn’t look much different from those a buddy of mine’s Questar 3.5 produced. In other words, pretty danged good for a three-and-a-half inch scope.

How did the C90 acquire a less than stellar reputation? Frankly, most people who actually own the telescope like it. A lot. Most of the bad press comes from those who haven’t used one much. I suspect the uninitiated’s views on the C90 are mostly the result of two things: focusing and mounts.

The C90’s focusing, via the rotating forward section of its OTA, is smooth when properly lubricated, but not overly easy. Place the scope on a light camera tripod, as many Spotter owners tend to do, and there’s Big Trouble in River City. Once you develop the proper touch, focusing the scope, e’en on a too-puny tripod, is reasonably easy. Someone not used to the 90, however, will probably never get the telescope focused properly. It’s focus-jiggle, focus-jiggle, focus-jiggle until the person trying to sharpen up Jupiter gives up and walks away from the 90 shaking their head at how “bad” the optics was. And even when properly focused, an f/11 telescope, even of this small aperture, cries out for decent support.

Following my C90 epiphany, I started thinking casually and informally about getting me one. I’d picked up a Short Tube 80 refractor to satisfy my grab ‘n go needs, but it would have been nice to have something that would handle magnification a little better than the achromat, which pooped out at about 100x. The C90, based on what I’d seen at TSP, clearly would allow you to pump up the power a little. The new G3 model, in particular, caught my eye in the late 1990s. This was a C90 mounted on a decent Synta EQ-2 German equatorial mount that sported an R.A. drive as standard equipment. Purty sweet. But I dawdled. The C90 had been around for twenty years; surely it would be around for a few more?

Alas, no. After the turn of the century, during the course of the various changes and hiccups that wracked Celestron, the C90 disappeared. Oh, they still sell a “C90” MCT. Actually they have sold a couple of "C90s" in recent years, but they are all Chinese imports, none of which has had anything at all in common with the real C90, and none of which has had optics or build quality as good as the original Little Kitty.

I considered hunting up a used specimen, but shortly after the C90 was discontinued, I acquired what has proven to be my favorite grab ‘n go rig of all time, the Orion (Synta) StarBlast 4.5-inch f/4. So, I didn’t feel motivated to go seriously looking for a 90. I figgered that if I were destined to own one, it would just fall into my hands somehow, someday. Which was exactly what happened, believe it or no.

I hadn’t thought much about the 90 in quite a while when, one afternoon a few weeks back, Miss Dorothy returned from the University with a little plastic attaché case with the word “Celestron” emblazoned on its side. I wasn’t surprised; I knew she’d be bringing a Celestron telescope of some kind home, I just didn’t know which one. One of her colleagues was helping her elderly Daddy sell off his camera gear, and knowing Unk Rod’s penchant for telescopes, and especially the telescopes of a certain Torrance, California maker, she wondered to Miss D. if I might be interested in a BIG Celestron. I knew “big” is relative when it comes to lay people and telescopes, so I didn’t think this would be no C14. I figgered it might be a C5, which would have been cool…but in the back of my mind was the whisper, “C90…C90…C90.”

After I’d greeted Miss Dorothy, I took the little case from her hands with what I hope was not unseemly haste. Snapping the attaché open revealed, as I’d expected, a sweet little Orange Tube C90, apparently the spotter version (more on that in a moment). The scope was in pretty good condition, looked to be, with a minimum of scuffs and dings to the tube, and a good looking corrector and primary. Unlike some older C90s, the baffle that protects the secondary from stray light was still glued firmly in place.

Down-checks? Firstly, the finder. It did not seem to be the original orange finder scope that shipped with the Spotter and Astro versions. Instead, it was a gloss-black painted 5x24, though apparently of the same vintage. It was also rattling around in its mount. Like many Celestron finder ring-mounts, then or now, only one ring had finder-alignment adjustment screws. The other should have been equipped with a rubber o-ring to hold the finder steady. At any rate, I later learned this probably was the original finder. Celestron had begun using black-tube finders at some point in the original C90's run for some reason known only to them.

What else? Experimentally twisting the orange-painted section of tube to focus resulted in sticking and grinding rather than the smooth motion these telescopes usually display. While the focus-action on C90s is not, as I mentioned earlier, ever what I’d call “easy,” the proper amount of damping grease on the threads should at least yield “smooth.” That and the finder would have to be dealt with before the scope would be useable.

In the course of evaluating the little refugee so I could come up with a reasonable monetary offer, I checked the still visible (if slightly worn) serial number of the little feller against what I knew about Celestron’s numbering scheme. This one appeared to have been produced in 1982.

One thing was sure: the original eyepieces, diagonal, and Barlow was gone. The star diagonal in the case was an uber cheap (and very dirty) one made completely of plastic, and was not labeled “Celestron” anywhere. The eyepieces, likewise, were clearly not Celestron. In the box were seven (!) very cheesy .965s. The barrels were metal, but the tops plastic. There was another real blast from the past: an 80s vintage Meade .965 zoom eyepiece. Finally, there was an old Minolta SLR body, a T-adapter and a Minolta T-ring.

Before even considering the purchase of this retro kitten, I needed to get an idea of what the optics were like. I mounted the scope on my Manfrotto tripod, inserted the puny little diagonal, and trotted the scope and a handful of the nasty oculars into the front yard. The verdict? Despite the punk eyepieces and diagonal and the difficulty of focusing due to the lack of grease on the threads, gibbous Luna was good and sharp. Mars, a much tougher target, was a nice, hard B-B that even hinted at a trace of detail once I’d cleaned the eye and field lenses of the Meade zoom (believe it or not, the best-performing eyepiece of the lot). I even, in true sidewalk astronomy fashion, showed the Moon off to a couple strolling Selma Street in the dusk, who oohed and ahhed appropriately.

Now, I had to come up with a fair price. I didn’t want to lowball the daddy of one of Dorothy’s colleagues, but, while I’d thought about acquiring a C90 off and on over the last several years, it was not something I was desperate to have. Yes, the seller had included that manual SLR body, a good condition Minolta SRT-201, in the deal, but I figgered the chance of me running any film through a camera in these latter days was slim to none at best. I gave Dorothy a price quote to relay to her friend, though I wasn’t sure it would be accepted.

The reason being was the “Must Be” factor. As I said when I initially mentioned this C90 a couple-weeks back, laypersons tend to have an exaggerated idea of what REAL TELESCOPES are worth. Remember, this had been described to Dorothy as a BIG Celestron. As I’d expected, the daughter was dubious: “But Daddy paid almost 300 dollars for the scope alone when he bought it; all this stuff MUST BE worth at least that much now.” She did say she’d ask her father. I promptly forgot the cute little scope.

Imagine my surprise, then, when Dorothy returned a week or so later once again bearing the C90. Turned out Daddy and Daughter decided they were very happy with my offer after all. OK! Now, I’d just have to do a little work on the scope. First task was greasing up the focuser. How? With what? Gotta hand it to Celestron; not many companies would have any information to offer about a discontinued thirty-year-old telescope. They did. A little Googling found a Celestron webpage with instructions for lubricating the focuser, including a suggestion as to which sort of grease to use (Mobil 1).

Step one is getting the tripod footsie off.
I procured said grease, or something close to it, and followed Celestron’s instructions for disassembling the OTA. Very simple, it turned out. Remove the screws holding the ¼-20 mounting foot in place, partially back out two screws hiding under there, and then just unscrew the fore part of the tube. Simple. Should have been, anyhow. Alas, when I removed the two machine screws holding the foot in place, it still refused to budge. What the—? Grabbed my Sherlock Holmes-style Zeiss High Power Lens (magnifying glass) and examined the sitchy-ation. Ah-ha! What I saw was the tell-tale signs of GLUE. A little prying with a flat-blade screwdriver and the mounting plate popped right off. I’m not sure what the glue was for; the two screws hold the ¼-20 mount in place quite securely. Oh well.

That done, removing the front tube section was easy once I partially backed out the two aforementioned screws. I just “focused” clockwise until the orange section of the tube screwed off. That left me with two tube sections with the primary in one half and the corrector plate and secondary (spot) in the other, all of which looked good and needed no attention of any sort. All I had to do was clean the threads with a little alcohol to remove the last vestiges of the old, semi-dried grease, and apply an appropriate amount of Mobil 1. Threaded the tube back together, focused in and out a few times, replaced the ¼ -20 mount, and that was that.

I also needed to secure the finder. A check around the Old Manse and a visit to Pep Boys didn’t turn-up an appropriate O-ring for the ring mount, unfortunately. Hmm… What’s right after duct tape in the amateur astronomer’s bag of tricks? VELCRO! A small strip of adhesive Velcro in the forward ring worked just as well—or maybe better—than the missing O-ring, and we were ready to roll.

Disassembly complete.
Or we would have been if I’d had a way to use 1.25-inch eyepieces in the scope. Clearly there wasn’t no point in fooling with the .965 junk that came with my C90. There were two ways to proceed: with an L.A.R., a “Large Accessory Ring,” an adapter designed to allow the C90 to use standard accessories, OR a “hybrid diagonal,” a star diagonal with a 1.25-inch eyepiece-side barrel and a .965 scope-side barrel. Since L.A.R.s have apparently become increasingly rare and expensive, I opted for a hybrid diagonal, ordering one from Jim Henson’s

Jim is normally a real speed-demon, so I hoped to have the diagonal before the clear skies evaporated and the Moon made it to Full. Yep, Scopestuff is amazing. I’ve sometimes ordered something on a Monday and got it on a Wednesday. Not this time. The clear sky came and went; International Sidewalk Astronomy Night was clouded out for us and with it my chance to use the C90 to watch a crescent Luna drift through the Pleiades. The rain passed, the sky cleared, and the Moon fattened, but still no diagonal did I find on my front porch.

I emailed Mr. Jim this morning, and he responded quickly that my diagonal is out of stock, and that despite his best efforts it will likely be anudder week or so. Not that it makes much diff at the moment, as the clouds have now rolled back in and we will soon be at Full Moon. But that’s just as well, I suppose, since our time and space for this week have done disappeared faster than a backward-stepping tachyon. I’ll update you on the C90 next week, I hope. Meantime, a new issue of the excellent Astronomy Technology Today has just dropped through Chaos Manor South’s mail slot. If’n you don’t mind, I’m a-gonna grab me a… “sarsaparilla” …and read for a spell.

Thank you for this very interesting post; it provides quite a bit of insight into my C90 which you know alot more about than I do.

I have a question which you might k now the answer to--I have lost the darned calbe for the prehistoric a/c clockdrive (and it's not a standard kind on the end that plugs into the drive). Any idea what that cable is/was? It's frustrating having the whole thing not work due to my having lost that.
Try Telescope Warehouse and Teletrader...
The oval Celestron grey power cords are available, (new) and they are OEM. These cost $25.00 and are available From Gary Hand (Hands on optics)
Perhaps this will keep people from keep retaining the power cords, and selling them separate on popular web auction sites to make an extra buck (up to $38.00)
that I have personally witnessed.
Love the post and my C90 Astro
Larry Beach
I recently puchased curious c90 package on ebay. The tube is white but it says celestron c90 1000mm/f11 on it so it is a c90 and not a 500

It was listed as a photography package that celestron offered. It comes in the square plastic case with the prism and eye peices like a normal scope but also comes with the t-mount and a tripod. The hood is longer than a normal c90

It came from an estate sale and looks to never have been used, the scope is in the original plactic bag and everything else in its original box. It looks brand new

Just wondering if you have ever heard of this model\package. I couldn't pass it up for the price

I wish to use my Celestron C90 Mak Spotting Scope as a telephoto lenses to my Sony Alpha NEX-5 camera.

What adapters do I need? Will I need eyepieces OR can I attach NEX5 to the back of C90 using this "T Mount Lens to Sony NEX Camera Adapter" (

Please can I have some suggestions!


That depends, Nyan...if it is a classic C90 like in my blog article, all you need is a T-adapter ring for your brand of camera. Newer C90s may require a prime focus adapter. The older C90s had t-threads on the visual back/rear port of the scope.
Greetings. I know this blog entry is really old ... but the Celestron C90 clean/lube instructions are gone from the link location, and don't show up in a search of the Celestron site. Any suggestions? Anyone have a copy, or remember the instrucctions? Also, I've seen Mobil 1 here, and another place someone used and was happy with white lithium grease. My main questions are: how much, and recommended method to distribute the lube. Thanks! (Mike)
Frankly, there's really not much to it beyond what's in this entry...

Apply a good coating. Not too thick, just enough to give smooth operation (clean the old lubricant off first). To distribute it, I just used my fingers. :-)
Thanks so much for this article. I bought my first DSLR and really wanted to connect to my C90. All of the adapters I bought online didn't fit. After reading your article I found my problem was my 20 year old C90 used .965 accessories. You also gave the solution and where to buy it. Great Job and Thanks again.
Thanks so much for this post. It provided a lot of great information on the "Orange Tube" C90 Spotting Scope.

I inherited my Fathers C90. (c1980 w/ Orange Tube) that my Mom gave him for his 50th birthday. I fondly remember looking at the Moon, Jupiter, Saturn and other celestial objects with him as a kid. Now I would love to do the same with my kids.

I have all of the original parts that it came with and they worked great tonight as I set it up to show my 5-year old the moon...once I found it in the eyepiece.

Unfortunately, the gray bracket that holds the orange finder scope is broken and missing the end with the adjusting screws. The finder scope itself is fine, but I obviously could not use it tonight.

I was hoping you might know where I could find a replacement part, or reccomend a new scope and mount that I could buy that might attached to the bracket mount screw holes?

Best Andy
This site has all kinds of stuff even dual spotter mounts. You might have to drill a couple of new holes but hey,

Does anyone know how to disassemble a C 90 WITHOUT the 2 screws under the tripod socket!!

I have some grease streaks on the primary mirror I'd like to clean.

This must be an older model of the C 90.

Recently I was very lucky to have purchased 2 vintage C90 Maks. One is the original Orange model and the other is a slick looking gloss black model with built-on (by the looks of it) dew/glare shield. They are both "made in U.S.A." and the S/N of the black model is 920433. Just wondering two things....if the "dew shield" can be removed (very tight if that is the case) and how date of manufacture is determined by the S/N numbering system. BTW, the orange C90's views are wonderful for a small mak! equal, if not better than the Orion and Meade longer focal units (in my opinion). I noticed that the AR coatings, if any, are not obvious on either Mak. Did Celestron bother to coat the meniscus lens on these Maks? Mirrors look first rate. Thanks for any help on my inquiries. George S. NY.
I'm trying to remember what the dew shield looked like. Can you email me a picture at The serial numbers are completely unreliable when it comes to figuring out how old a scope is. You might find my free used buyer's guide useful, though...I have a whole section on the C90.
Hi, Is it possible to collimate the old C90? Of course the secondary is fixed but do the 3 screws on the rear cell allow any adjustment? Thanks.
It's possible if not easy, I recall. There are some instructions on the web I believe.
I am in need of a C-90 orange tube owners manual, can't find it anywhere on the web
Dear Uncle Rod,
Thanks for your great blog-article on the C90; very helpful information.
Please help me!!! My father bought an old C90 from a flea market a few months back and I have been trying to help him get it usable.
Can you tell me what kind of adapter I will need to connect a 1.25” diagonal and accessories?
I have tried searching for the “Large Accessory Ring” you mentioned, but cannot find anything specific.
Thank you

What is the diameter of the scope?
The C90 makes a great “eclipse scope”!
The C90 makes a great “eclipse scope”!
Uncle Rod,
I math proud owner of a vintage C90 Mak! I have been Googling the Celestron webpage for the instructions for lubricating the focuser, with no luck. I also have been unable to find the website with the disassembly & cleaning instructions.

I would LOVE to be able to use this scope for astrophotography. Is there any chance that you can help be locate the information?
Warm Regards, Leslie
I've had one of these (spotter version) in storage for 20+ years; camera tripod hopeless for astro, but never quite able to part with it. Finally, got it out and a got an AZ3 mount; mirror still looks excellent, and objective lens, but diagonal and eyepieces need cleaning. Search for a "how to" led here, with exactly the info I need. Re-grease coming up. Will be a very handy for quick looks. Thanks for the info.
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