
Orange C90
#1
Posted 15 May 2014 - 12:14 PM
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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#2
Posted 15 May 2014 - 12:18 PM
Here's a nice view of the clouded corrector plate halfway through cleaning.
#4
Posted 15 May 2014 - 01:40 PM
Yes, buying old scopes on Craigslist is a problem of mine as well but I am trying to recover.

Bought and sold one of those a few months ago and they are fun to use.
Have fun with the new purchase.

#5
Posted 15 May 2014 - 02:34 PM
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..
#6
Posted 15 May 2014 - 04:19 PM
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.
#7
Posted 15 May 2014 - 04:48 PM
#8
Posted 15 May 2014 - 05:11 PM
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.
#9
Posted 15 May 2014 - 05:19 PM
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

#10
Posted 15 May 2014 - 05:25 PM
#11
Posted 15 May 2014 - 08:47 PM
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.
#12
Posted 15 May 2014 - 11:05 PM
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

#13
Posted 16 May 2014 - 06:47 AM
- Erik Bakker likes this
#14
Posted 21 May 2014 - 11:56 AM
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.
#15
Posted 21 May 2014 - 02:02 PM
https://www.astronom...ter_p11924.aspx
#16
Posted 21 May 2014 - 05:14 PM
#17
Posted 21 May 2014 - 07:15 PM
#19
Posted 21 May 2014 - 07:40 PM
#20
Posted 22 May 2014 - 10:02 AM
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.
#21
Posted 22 May 2014 - 06:15 PM
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).
#22
Posted 23 May 2014 - 04:44 AM
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.
#23
Posted 23 May 2014 - 05:10 AM
I just use BBQ lighter fluid! followed with a cleanup with 409. to remove the smell! then White Lithium grease..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.
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
#24
Posted 23 May 2014 - 05:17 AM
C90's are always a Maksutov, the Mirror lens designs are usually Meades. the 2045 series. stay away from the mirror lens versions.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.
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!
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#25
Posted 30 April 2016 - 09:41 AM
C90's are always a Maksutov, the Mirror lens designs are usually Meades. the 2045 series. stay away from the mirror lens versions.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.
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."
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