360 replies to this topic

[SouthernOracle]

Hi! My name is Jonathan and I think I have a problem. I've just purchased yet another old scope off of Craigslist. This time it was a Celestron C90 spotting scope and tripod. It has the classic orange OTA and finderscope and everything is in excellent condition in original packaging. It included a padded carrying case, an 18mm Kellner eyepiece, a diagonal, and a 2.5x barlow. The tripod is pretty heavy duty and much better than the plastic one I have been using. That's one of the things that drew me to the scope in the first place. I've been looking for something for grab and go that collapses down and has a carry case so when I saw all this for $75 I called right away and told the lady that I would be there in an hour to pick it up. She said while I was there that she must've gotten half a dozen calls and emails after she hung up the phone with me.

Her husband Don bought it new for around $350 back in the mid 80s in Tuscon, AZ. Depending on the year purchased, that's equivalent to $700-900 in today's dollars. He said he used it quite a bit back then, but after they moved away it stayed in the attic for most of the next 20 years. They are planning on selling their house and it was time for some storage purging.

The eyepiece was a bit oily and corrector plate was very cloudy and had smudges from somebody probably trying to wipe it clean with a dry cloth. The cloudiness was the same on my Swift scope I got a few months ago. I cleaned both VERY carefully and they are much better now. The rotating tube focuser takes a bit of getting used to, but I've found that it focuses very smoothly and you just have to hold it for a second so it doesn't move after achieving focus. The finderscope is a 5x42 straight through which is a total pain. I'm hesitant to replace it because it looks so nice in orange matching the tube, but it really is pretty difficult to use.

First light was very promising. The view of the moon through the 18mm is incredibly sharp and there is no sign of color fringing even with the moon being full. Inserting the 2.5x barlow tells a different story, though. At that high magnification there is obvious fringing (not sure if it's chromatic or spherical aberration) around the edges of the moon and planets. Stars are funny colors when they are defocused, but sharpen up once focused. Using a non-barlowed 6mm ep from my Swift scope shows a bit less color fringing and the image is a bit brighter. When focused it's very sharp and I could see Saturn's rings very clearly. I couldn't see the Cassini division, but that may have been the seeing conditions. I definitely could make out the 2 main cloud bands on Jupiter, but not much else. After much searching with the finder I finally found M3 and it was pretty dim, but visible and cloudy. Higher magnification didn't resolve any stars and dimmed the view further.

All in all, I'm very happy with the purchase. It should make a great little quick-view or travel scope and I have upgraded my .965 eyepiece collection and tripod in the process. Here's a few pics from assembly and cleaning.

Anybody have a one arm fork mount for this thing that they'd love to get rid of? I'm sure it would make pointing it much easier.

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[SouthernOracle]

Serial number is 907177. Anybody know the exact year that was made? The eyepiece has a marking that looks like a K with a backwards C next to it as well if that helps. On the edge of the corrector plate is an etching or writing that appears to say P8P5 but the 5 is written backwards. It's kind of strange.

Here's a nice view of the clouded corrector plate halfway through cleaning.

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[SouthernOracle]

And the OTA after cleaning mounted on the tripod.

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[scopeboy42]

Hi Jonathan,

Yes, buying old scopes on Craigslist is a problem of mine as well but I am trying to recover. Saw that C90 on Craigslist as well and almost pounced on it.

Bought and sold one of those a few months ago and they are fun to use.

Have fun with the new purchase.

[orion61]

Very very nice!, that scope is from the early to mid 80's,
they changed to Black in about 1985.
the "Astro" single fork mounts are hard to find and fairly expensive.
If the focis is a bit loose it needs to be taken apart/cleaned, and trlubed with Super lube or lite Lithium
White grease.
To get it apart, take the tripod mount off the bottom. Between the decal s there is a hidden screw, take it out (it is the stop screw)and you can unscrew it! Those also had a bad tendency to have the secondary baffle fall off...
Clean the old threads off with a Petrol based cleaner..
Nice find. those had fantastic optics!
Good luck..

[Joe Cepleur]

Welcome, Jonathan! Great find.

Clean the old threads off with a Petrol based cleaner.

Duane, can you name a suitable cleaner? I have threads on a black Taiwanese C90 needing to be cleaned. Not nearly as good as this orange tube, but still in need of service.

[rolo]

An 18mm and 2.5x Barlow give you approximately 137x on this scope.I've had 7 of these older C90 and all had poor optics. None of them could handle more than 75-100x without without falling apart. These are low power scopes at best, and okay as telephoto lenses.

[Joe Cepleur]

An 18mm and 2.5x Barlow give you approximately 137x on this scope. I've had 7 of these older C90 and all had poor optics. None of them could handle more than 75-100x without without falling apart. These are low power scopes at best, and okay as telephoto lenses.

At 35x per inch, the 3.5" C90 would manage just a hair over 120x. Still, 137x is only 39x per inch. One would hope the scope would handle that!

My only first-hand experience with a C90 is with an inferior black tube made in Taiwan, but from what I've read, the picture is complex. Anyone who knows better, please confirm or correct what I have to say:

First, in the C90, there are "telescopes" and "mirror lenses." "Telescopes" have Schmidt correctors; "mirror lenses" have window panes holding the secondaries, and corrector lenses somewhere inside the baffles. By design, the telescopes are superior to the mirror lenses.

Second, the C90 has been made in many locations. If the orange tube in this thread was made in California, it could have fine optics. Those that were farmed out off-shore are hit-or-miss, mostly "miss."

Quite bad luck to have had seven bad C90s, but possible. Were any of them classic orange tubes from California? If they are not as good as I used to believe, I could knock one more scope off my "covet" list, and stop hoping to find one! I know ActionHac loves to kick back and scan the skies with his, but that sounds like something for low powers.

It's possible to buy a new 1.25" visual back for the classic C90. I have wondered how much that would help, by better centering the ocular in the light path, compared with just fitting a 0.965" diagonal into the scope, and knocking it slightly off-center with the set screw.

[SouthernOracle]

The collimation seems spot on with the 18mm and images are truly sharp and color free. A star test reveals a perfectly circular ring with the secondary shadow exactly in the middle. The problem may be the barlow not fitting the tube exactly and introducing a slight misalignment of the optics and introducing aberrations. I'm very interested to see what a 9mm eyepiece similar to my 18mm would be able to do. The other .965 eyepieces I have are all Huygenian and not to the same standard as the Celestron Kellner. I put in a Wanted request in the classifieds to see if I can find one.

I believe my scope was at least assembled in the US since it says Made in USA by the serial number. From many posts and articles I've read on these scopes, the optics are usually very good. However, I have found a small following that say they're total garbage. Further testing is required

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[rolo]

Similar to the ETX, the newer C90's are much better optically...

[Larry Geary]

Similar to the ETX, the newer C90's are much better optically...

I've seen orange, black, and white C90's for sale. How does one identify the newest ones? And are some labeled "telescope" and others not?

To the OP: A diagonal can make a big difference. I have an old Celestron S80 80mm f/5 refractor that came with a 45 degree erect image diagonal. With that, I couldn't even resolve Saturn's rings. One day I put in a spare 90 degree diagonal and holy cow, what a difference! I recently found that a TeleVue Everbright diagonal makes an additional, significant improvement.

[core]

First, in the C90, there are "telescopes" and "mirror lenses." "Telescopes" have Schmidt correctors; "mirror lenses" have window panes holding the secondaries, and corrector lenses somewhere inside the baffles. By design, the telescopes are superior to the mirror lenses.

From what I can recall through the past 30+ years, a Celestron C90 means a Maksutov Cassegrain design, with a meniscus lens in the front. iirc mid 80's Celestron had a 500mm and 300mm mirror SLR camera lens, not sure of the optical design but probably an optical window in the front. the C5 came in 750mm and 1250mm schmidt cassegrain designs for SLR lens.

I'd presume when we're talking about 'Old' C90's, it's the 1000mm fl f/11 design. 'New' C90's are of the 1250mm f/13.9 design (and there are 2 variations there, one with the internal flip mirror and rear port).

I had a newer C90 a year+ ago and wasn't too impressed with powers over ~110x, and on top of that daytime contrast wasn't too good - don't have it since. I picked up an orange-tube C90 a couple of months ago which required quite a bit of cleaning the inside of the optics from previously done sloppy job, and re-greased the helical. I'm surprised that the contrast is much better, and it does seem a tad sharper (no side-by-side unfortunately) than the new one I had. Images top out using a Meade 9mm RG ortho, but side-by-side, a current day 90mm f/6.7 achromat outperforms it on planetary details (easy polar cap on Mars, better belt details on Jupiter). The compactness of the orange c90 is really hard to beat imo, and I keep it around for that reason (and mainly nostalgia ). I'd like to give the old C90 with special coating a go at it though.

[Bomber Bob]

A member of the B'ham Club had an orange C90 Astro in his collection, along with a C5 & C8 (both orange tubes), and he would line them up at star parties. His C90 performed well against my 80mm Towa, but the refractor could handle higher magnification. That was around 1980, so I assume the optics were American. At the time, it was the Budget Questar.

[SouthernOracle]

Since the original Large Accessory Ring adapters are almost impossible to find does anybody have experience with something newer that will actually work with this so I can use SCT threaded accessories on it? The one currently on CNC Supply specifically states that it may not work on older orange tube C90s,

Has anybody here tried photography through one of these? I'm tempted to get a focal reducer and try some photography through it, but there's no way I'm going to get any detail at f/11. Will a standard Celestron .63 SCT focal reducer work on this scope? The reducer/corrector says that it is designed to correct for SCT scopes, but will it introduce errors into the C90's MCT optics?

I know I can get a T-adapter for this scope that doesn't require the LAR. Is there a focal reducer out there that is T-threaded? I've seen SCT threads, 1.25" versions that go in the focuser like an eyepiece, etc but can't seem to find a T-thread focal reducer.

Sorry for all the questions, but I'm new to all this and can use all the help you can give.

[scopeboy42]

The Meade ETX to SCT Adapter works with the Orange C90. I had found one local and it threaded on with no issues. Sold it with my C90. Can be hard to find as a lot of dealers list these as backordered.

https://www.astronom...ter_p11924.aspx

[SouthernOracle]

Thanks a ton! I ordered one off Agena Astro (free shipping) and grabbed the Celestron SCT T-Adapter as well. I'm holding off on getting a focal reducer for now until I can measure everything and see if the Anteres 0.63 will work at its native focal length or if I need to get the actual Celestron one (which has a longer FL for 0.63 reduction). I have a full set of T-extension tubes to increase the distance if needed, but I can't make the T-adapter any shorter than it already is.

[Joe Cepleur]

There is a 1.25" visual back made for the old-style C90, allowing native use of 1.25" accessories. It's aftermarket, made by a machine shop (not the defunct and much-missed Crawmach) that stocks all sorts of adapters. I'm trying to find the address.

[core]

Joe,

It's TelescopeAdapters.com

[Joe Cepleur]

Yes; that's it! Here's the direct link:

http://www.cncsupply...tm?etxstuff.htm

[SouthernOracle]

Oddly enough I just tested the 1.25" 9mm Plossl that came with my AD8 last night in the original .965" diagonal. I have a .965 to 1.25 adapter from telescope warehouse and it came into focus with quite a bit of room to spare on the focusing ring. I was definitely surprised considering how much talk there has been on these and other forums about having to have a hybrid diagonal or 1.25" visual back in order to use 1.25" eyepieces. The view was incredibly sharp on Mars and Saturn, but still a little small. I think I'm comparing it too much to my 8" dob's view, though.

I'm very likely going to do the contrast mods from a thread I found on here. I did notice the background sky being rather bright even without the moon up last night. Stellar magnitude was limited to around 11 from my searching for M51 and the stars I could actually see around it. I should be able to see at least a magnitude and a half more than that. Transparency wasn't great last night, but I couldn't even see M51 either so something is wrong.

[Joe Cepleur]

I just tested the 1.25" 9mm Plossl that came with my AD8 last night in the original .965" diagonal... I was definitely surprised considering how much talk there has been on these and other forums about having to have a hybrid diagonal or 1.25" visual back in order to use 1.25" eyepieces.

Shorter focal length eyepieces have a narrow true field of view. When the eyepiece is magnifying just a small patch at the center of the focal plane, there is no benefit to a larger diameter tube. Compare a long focal length eyepiece with its wider true field of view. At some point, the eyepiece has the potential to see more of the focal plane than the tube will allow. So, the longer eyepieces benefit from 1.25" format (or even larger, in newer telescopes).

[chickenplucker]

If you get yours setup with a T-adapter don't be afraid to take it outside and play around with it as just a big ol stumpy camera lens. Not only do they make nice little scopes, but they can do borderline macro photography as well. They are a little soft, but they also can halo and saturate images like crazy which makes them a hoot to use.

Here are some random pictures of bugs, flowers, etc. as they came out of my Canon T2i:

Beetle on Queen Anne's lace
Bean field
Burdock
Random abstract grass

Its a great little grab and go scope, but by far my most favorite lens for just having fun.

[orion61]

Welcome, Jonathan! Great find.

Clean the old threads off with a Petrol based cleaner.

Duane, can you name a suitable cleaner? I have threads on a black Taiwanese C90 needing to be cleaned. Not nearly as good as this orange tube, but still in need of service.

I just use BBQ lighter fluid! followed with a cleanup with 409. to remove the smell! then White Lithium grease..
good for another 25 years, gets rid of the wobble!
If you live in a warmer climate even red axle grease.
But up here in the Winter where I regularly observe in sub zero temps, it is way too thick. Super lube is great too.
good luck
Duane

[orion61]

An 18mm and 2.5x Barlow give you approximately 137x on this scope. I've had 7 of these older C90 and all had poor optics. None of them could handle more than 75-100x without without falling apart. These are low power scopes at best, and okay as telephoto lenses.

At 35x per inch, the 3.5" C90 would manage just a hair over 120x. Still, 137x is only 39x per inch. One would hope the scope would handle that!

My only first-hand experience with a C90 is with an inferior black tube made in Taiwan, but from what I've read, the picture is complex. Anyone who knows better, please confirm or correct what I have to say:

First, in the C90, there are "telescopes" and "mirror lenses." "Telescopes" have Schmidt correctors; "mirror lenses" have window panes holding the secondaries, and corrector lenses somewhere inside the baffles. By design, the telescopes are superior to the mirror lenses.

Second, the C90 has been made in many locations. If the orange tube in this thread was made in California, it could have fine optics. Those that were farmed out off-shore are hit-or-miss, mostly "miss."

Quite bad luck to have had seven bad C90s, but possible. Were any of them classic orange tubes from California? If they are not as good as I used to believe, I could knock one more scope off my "covet" list, and stop hoping to find one! I know ActionHac loves to kick back and scan the skies with his, but that sounds like something for low powers.

It's possible to buy a new 1.25" visual back for the classic C90. I have wondered how much that would help, by better centering the ocular in the light path, compared with just fitting a 0.965" diagonal into the scope, and knocking it slightly off-center with the set screw.

C90's are always a Maksutov, the Mirror lens designs are usually Meades. the 2045 series. stay away from the mirror lens versions.
have never seen a C90 with anything other than the deeply curved meniscus. I have also never looked through anything but superbly corrected C90's.
The only knock I have ever had was with collimation on 2 of the 20 or so that have come through my hands!

[terraclarke]

An 18mm and 2.5x Barlow give you approximately 137x on this scope. I've had 7 of these older C90 and all had poor optics. None of them could handle more than 75-100x without without falling apart. These are low power scopes at best, and okay as telephoto lenses.

At 35x per inch, the 3.5" C90 would manage just a hair over 120x. Still, 137x is only 39x per inch. One would hope the scope would handle that!

My only first-hand experience with a C90 is with an inferior black tube made in Taiwan, but from what I've read, the picture is complex. Anyone who knows better, please confirm or correct what I have to say:

First, in the C90, there are "telescopes" and "mirror lenses." "Telescopes" have Schmidt correctors; "mirror lenses" have window panes holding the secondaries, and corrector lenses somewhere inside the baffles. By design, the telescopes are superior to the mirror lenses.

Second, the C90 has been made in many locations. If the orange tube in this thread was made in California, it could have fine optics. Those that were farmed out off-shore are hit-or-miss, mostly "miss."

Quite bad luck to have had seven bad C90s, but possible. Were any of them classic orange tubes from California? If they are not as good as I used to believe, I could knock one more scope off my "covet" list, and stop hoping to find one! I know ActionHac loves to kick back and scan the skies with his, but that sounds like something for low powers.

It's possible to buy a new 1.25" visual back for the classic C90. I have wondered how much that would help, by better centering the ocular in the light path, compared with just fitting a 0.965" diagonal into the scope, and knocking it slightly off-center with the set screw.

C90's are always a Maksutov, the Mirror lens designs are usually Meades. the 2045 series. stay away from the mirror lens versions.
have never seen a C90 with anything other than the deeply curved meniscus. I have also never looked through anything but superbly corrected C90's.
The only knock I have ever had was with collimation on 2 of the 20 or so that have come through my hands!

True Du'! The discussion of optical windows vs. corrector plates with regards to the classic orangetube C90 is completely erroneous. It comes from misquoting what is known to be true in the context of the classic 4" Meade 2045 which came in two 'flavors'- one being the true SCT and the other being a shortfocal length Cassegrain with its secondary affixed to an optically flat front 'window'- which is known to yield less than stellar performance. The classic C90 is always a Maksutov design, F/11, 90mm x 1000mm. (Actually, the primary is larger than 90mm as is necessary in the design to yield an effective aperture of 90mm- the aperture of the thick meniscus corrector.) However, in the early days (low serial numbers), some were marketed as telephoto lenses, in smaller cases without room for an array of accessories. They were not drilled and tapped for a finder bracket and no finder was included, nor was any sort of diagonal or eyepiece(s); however, they did include the 'rare' large accessory ring. The base was set up for a photo tripod, with a drilled and tapped 1/4-20 socket. And, most germane to this discussion, this little Maksutov was labeled "Mirror Lens."