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Is this Celestron C90 "Classic" or "Used?"

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#51 CharlieB

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Posted 27 November 2012 - 12:49 PM

I don't know what the 3 little screws are for. I think you are right though collimation.
If I remember right the meniscus lens up front just has 3 tiny little set screws holding it in the tube, I don't think any adjustments can be made there but a blow to the meniscus could knock it out of alignment very easily.
All these things will show up in a star test.

I went back and looked at your photos and I'm not sure what is holding the meniscus in the black tube. I don't see the set screws on the outside.

Robert


The three screws in the back are not for collimation - they are too short. I have one of the orange C-90 Astro models that I replaced those screws with longer nylon bolts and nuts that CAN be used to move the primary. I had my mirror recoated and while it was apart, I figured that weak spring that is supposed to center the primary might not be the best way to accomplish the task. With the new nylon bolts, you can nail the collimation and it stays in place (assuming you have no issues with the secondary). I also flocked the inside of the tube for good measure. I use .965 eyepieces, but I do have a LAR for it. I seem to recall reading there are two slightly different LARs and they don't always fit properly. With the wedge and motor drive, this makes an incredible little portable scope.

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#52 Pierre Stromberg

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Posted 29 November 2012 - 12:35 AM

Joe, I'm just coming into this conversation but wanted to let you know that I purchased used the exact same black C90 from someone on Craigslist for 100 bucks. I use it primarily as an autoguider scope so it doesn't get stringent use from me.

When I've used it for observations, I was pretty impressed. Very sharp images of planetary objects. The helical focuser was a bit annoying at first but now it doesn't bother me much at all and it retains focus quite well. I wouldn't recommend the C90 for DSLR imaging because the small baffle leads to severe vignetting. But overall, for 100 bucks, it's quite a sweet little scope.

Like others suggested, get a LAR (large adapter ring) so you can use other Celestron accessories.

Pierre

#53 Joe Cepleur

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Posted 03 December 2012 - 11:02 PM

Is there an official way to collimate these old C90s? I saw the retrofit way described in this thread, but would happily try something easier first. Images of stars are not sharp. Daytime images are partly clear, partly slightly fuzzy in different parts of the field (not due to objects being at different distances). Seems it should be able to be better than it is.

What a lesson in the importance of contrast! Images are bright, but not vivid, the compromise for having so small a scope.

Fun gizmo. Hate the finder. Even a tiny scope needs a right angle finder.

Needs a good tripod and mount. Any recommendations? Homebuilt is in the budget, but Vixen Porta Mini might be great. Other ideas? Something lightweight and portable, good for birding or light astronomy.

#54 rmollise

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Posted 04 December 2012 - 07:50 AM

Is there an official way to collimate these old C90s? I saw the retrofit way described in this thread, but would happily try something easier first. Images of stars are not sharp. Daytime images are partly clear, partly slightly fuzzy in different parts of the field (not due to objects being at different distances). Seems it should be able to be better than it is.

What a lesson in the importance of contrast! Images are bright, but not vivid, the compromise for having so small a scope.

Fun gizmo. Hate the finder. Even a tiny scope needs a right angle finder.

Needs a good tripod and mount. Any recommendations? Homebuilt is in the budget, but Vixen Porta Mini might be great. Other ideas? Something lightweight and portable, good for birding or light astronomy.


Believe me, collimating these little scopes is not for the faint of heart, and it is fairly rare for one to be out of collimation. You can't tell by how "sharp" stars are, you must observe the diffraction rings of a just slightly out of focus star under good seeing with said star centered.

#55 Joe Cepleur

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Posted 04 December 2012 - 11:43 AM

Believe me, collimating these little scopes is not for the faint of heart, and it is fairly rare for one to be out of collimation. 


Thanks for the tip, Uncle Rod! Sounds as though I don't need to worry about collimation, and would not want to learn it anyway -- at least not while testing a fellow club-member's scope. 

You can't tell by how "sharp" stars are, you must observe the diffraction rings of a just slightly out of focus star under good seeing with said star centered. 


I was thinking in simpler terms. Often when people bring their Newts to star parties, the images are quite blasé, with a quick collimation making them far better than their owners had ever seen or imagined. I wondered whether miscollimation contributed to the imperfections here. 

To reach focus, the scope requires the length that the diagonal adds between the end of the baffle tube and the eyepiece. At this stage, I should either find a known good diagonal, or perhaps a plumbing fitting that would allow straight-through viewing. Ruling out bad accessories would help. 

As soon as I can get to the club to borrow a suitable mount, I'll report on a star test. 

Uncle Rod, calling the C90 "Little Kitty" in your blog was memorable. I know that referenced the classic, orange-tube astro version, but it was still a C90. 

#56 orion61

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Posted 04 December 2012 - 12:03 PM

The 3 screws in the front of the tube are to HOLD THE MENISCUS(front glass) IN! don't mess with them, Those usually have pretty decent optics, did you let it cool off long enough?

#57 Joe Cepleur

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Posted 04 December 2012 - 12:15 PM

I've used the scope both when it was surely cool and when it may not have been. I think I need a really clear night, a certain cool-down period, a solid mount, and a new diagonal or straight-through pipe to test this further. It also sounds as though it would be easy to re-lubricate the helical focuser, so I'll do that. Everyone says these scopes should be good, so whatever is off, this one likely is, too.

For all its imperfections, such a potent little kitty sure is seductive! Love the idea of a miniature, if limited, powerhouse.

#58 dbledsoe

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Posted 04 December 2012 - 06:56 PM

Joe,

You can get an LAR here:http://www.cncsupplyinc.com/index.htm?etxstuff.htm for $29.95 plus shipping.

I bought one for my black C90 earlier this year and it works fine.

#59 Joe Cepleur

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Posted 04 December 2012 - 07:15 PM

Joe,

You can get an LAR here:http://www.cncsupplyinc.com/index.htm?etxstuff.htm for $29.95 plus shipping.

I bought one for my black C90 earlier this year and it works fine.


Perfect, thank you! Had no luck Googling for that part, so your referral is truly important.

#60 Joe Cepleur

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Posted 07 December 2012 - 12:11 AM

Following the link to CNC Supply and emailing the owner (who is amazingly responsive and deserves our business), I found a likely better solution than even the LAR. The site does not allow direct links to specific items (bad programming; hope they fix it), so click the link in its left sidebar for this item:

Rear Port Adapters (Visual Backs) ETX/C90 (34mm)

It screws on to the same threads as the LAR, but accepts 1.25" accessories directly, without the added weight or excessive back focus of large accessories, such as the visual back from a C8. It is better than a 0.965"-to-1.25" adapter because it allows the inserted diagonal to sit about 1/4" shorter in the optical train, and it properly centers and squares the eyepiece. A lot of the problem I've had focusing is related to the floppy fit of 0.965" accessories in the baffle tube. So, this adapter should help the C90 to focus.

I have a sense this is an exceptional small manufacturer. I have a hunch he'd work with us to produce all the adapters we'd ever need, including those coveted gizmos that screw directly into classic 0.965" drawtubes to natively accept 1.25" eyepieces.

It also helps to better understand the odd illusion this scope creates. If I look at a fence or window frames or clapboards a mile away with the C90, they look pretty sharp. The bushes with leaves down for winter look hazy, because their many fine branches are too small to resolve.

Still need to lubricate the focuser and perform a star test. After a long cool-down tonight, the skies turned cloudy.

#61 CharlieB

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Posted 07 December 2012 - 08:16 AM

I used SuperLube on the focuser threads and it smoothed out the focusing and removed all of the focus shift I was experiencing in my older C-90.

#62 Ducky62

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Posted 08 December 2012 - 01:47 AM

I dragged mine out the other night for a few quick looks at Jupiter and M42. Fine focus is a PITA even though it feels perfectly smooth. I still want an Orange Astro (or better yet a brass Astro) but I'm thinking of getting rid of my black f/11. It has the nice Celestron International tackle box, a diagonal and .965 Kellners so I'm not in a real hurry. Anyone use theres as a telephoto? Early 80s Celestron marketing suggested it was a good lens for taking pictures of bikini babes on the beach :lol:

#63 Joe Cepleur

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Posted 17 February 2013 - 05:19 PM

Upon reading about "sky-flooding" in another thread, a quick experiment noticeably improved contrast under most daytime conditions. I rolled a newspaper into a tube, and stuck it over the tube like a long dew shield, to block stray light. The newspaper is quite reflective, yet still got results. I'm going to try something similar with something less reflective, such as black construction paper or black felt.

Even with just the newspaper, the effect can be dramatic. Looking across the lake, the forest a mile-and-a-half away can be a wash of ill-defined lines. Pop on the shade, and the individual trees appear.

#64 actionhac

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Posted 17 February 2013 - 07:15 PM

I'm experiencing "sky flooding" with my new Orion Mak 90mm.
I recognize the problem from my ATM projects, very similar to what happens if you build a refractor and not install baffles.
Its difficult to do anything internal on this new scope of mine but I'll try a lens shade for daytime like what you are doing Joe.
The baffle tube is quite far from the secondary baffle. I think your on target with the lens shade idea.
You can see how illuminated the inside of the baffle tube is in this photo from this angle:

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#65 Joe Cepleur

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Posted 17 February 2013 - 11:58 PM

An excellent photograph, ActionHac! Hard to capture the inside of these beasts. It shows the problem exactly!

Next, I'll try a better cheap material, such as black construction paper. If the idea works, I envision a good quality shade made from plastic sheeting lined with flocking, tightened around the tube with Velcro. This could be rolled for use or flattened for transport, and would add essentially no weight. The scope would be as portable with it as without it. The only problem comes from the necessary length of the shade. It catches wind, but no worries if it can't be used all the time. I may even make two, long for best shading, and short to catch less wind when necessary.

#66 GlennLeDrew

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Posted 18 February 2013 - 12:12 AM

The photo actionhac posted only shows the upper end of the primary baffle's inner wall being illuminated. This is not 'sky flooding.'

The latter condition results when light from off axis can pass through the corrector, just outside the secondary baffle, into the primary baffle and thence directly out the back end. In other words, from the focal surface, you could peer at an angle into the rear opening and see a crescent of *direct* illumination framed by the inner edges of the two baffles.

One could think of this condition as resulting from one or both the baffles being not quite long enough.

Actionhac should have aimed the camera so that the line of sight was as close as possible to the secondary baffle's inner edge, so as to reveal a clear line of sight all the way through the scope. If this is not noted, or if a view up the rear opening from well off axis does also, sky flooding is not an issue.

Sky flooding is more likely when the focus is set so as to be near the rear opening. The farther rearward the focus must be set (as when longer light-path accessories are attached), the less likely the problem.

#67 Joe Cepleur

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Posted 18 February 2013 - 12:33 AM

Thanks, Glenn; I always like to learn to be correct in my knowledge and terms. Still, even if the problem isn't "sky-flooding," a long light shade helps, so there seems to be some sort of problem with stray light.

#68 actionhac

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Posted 18 February 2013 - 12:50 AM

I will look in the rear port tomorrow Glenn in the daylight and see if I can see any light coming straight though between the baffles.
I'll try looking at all angles and see if I can spot anything.
My C90 is fine but its a older orange tube Joe. The black tube like yours may have slightly different baffles.
My Orion has the problem. Very badly washed out with light, like over-exposed film.
Its exactly like a refractor with no baffles and a shinny unpainted tube interior. I've also seen the same problem with a shinny focuser tube.
I also want to make a lens shade, it could help and I need it for dew also.

Robert

#69 actionhac

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Posted 18 February 2013 - 01:28 PM

I can see around the secondary baffle when my eye is close to the rear port.
If I move my eye away from the port to the point of focus I can no longer see around the secondary baffle.
One problem I'm seeing is lots of reflected glare from the inside walls of the baffle tube. Check yours Joe. We need to kill that reflection. Some dead flat paint is in order I think, no room for Protostar light trap I can barely get my little finger in the baffle tube.
My drawing is inside focus, the focal plane is approx 4" away from the port.

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#70 GlennLeDrew

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Posted 18 February 2013 - 06:42 PM

You got it, actionhac!

Indeed, even in the absence of sky flooding, an illuminated baffle wall is a source of scattered light which reduces contrast. A C90 I owned about 20 years ago had a thread-on metal lens shade, which helped.

#71 Joe Cepleur

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Posted 18 February 2013 - 07:16 PM

ActionHac, you are a magnificent artist. Well done!

When I looked at mine earlier today, I did not see directly past the secondary baffle, so I'll look again. But, I did see a blaze of reflected light in the shiny, black baffle tube, which the long sun shade dramatically reduced. Best performance may come from both a long sun shade and blackening of the baffle tube.

How does one get the right amount of paint into so small a tube without making a gummy mess?

Why did Celestron not paint them better at the factory?

I *really* want to like this scope. It's so portable. Perfect to keep in the car for birding, hiking, and whatever impromptu astronomy it can manage.

#72 actionhac

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Posted 18 February 2013 - 08:18 PM

I've used this:
http://www.krylon.co...ion_technology/
Its about as flat as I could find.
I spray it on a plastic lid off of a margarine tub or similar and use Q-tips to apply, the sloppier the better.
I've done this on Edmund focuser tubes with great success. I did find a water base artist paint that was actually flater but it didn't adhear as well as the Krylon.

Robert

#73 Datapanic

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Posted 18 February 2013 - 09:25 PM

Over in the ATM Forum, there's a thread about Flockboard vs. Krylon Ultra Flat Black and a link was provided to the original non-"Camouflage" Ultra Flat Black 1602 which is said to be more flatter than the Camouflage mix. I've used both and haven't noticed a difference. Apparently, the 1602 mix was replaced with the Camouflage mix but still available, just not off the shelf...

#74 actionhac

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Posted 18 February 2013 - 10:19 PM

Wow I didn't know you can still buy the 1602. I used the 1602 and when I couldn't find it I reluctantly started using the camo version and I'm OK with it. It is non-reflective and doesn't flake off those brass Edmund focuser tubes.
I really should look for a brush on can rather than spray.
One problem with the Q-tips I forgot about is the paint can pull the cotton fibers off the tip and you might have a few in the optical path. You can push em into the paint as it dries or pull them out.

Robert

#75 Datapanic

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Posted 19 February 2013 - 12:37 AM

Try a sponge brush instead of Q-tips. With a sponge, you can also dab it on and get a little texture going to help dissipate the light reflections.

I haven't really noticed a difference between 1602 and the newer Ultra Flat Black, but I haven't done any real comparisons either.






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