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Equipment Discussions >> Classic Telescopes

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actionhac
Post Laureate
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Reged: 08/09/08

Loc: Seattle
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: greju]
      #5541189 - 11/26/12 10:27 PM

Yes anything .965" is a sloppy fit in there.
I like using the .965" stuff with mine because it fits the small scale of the optical tube.
One thing I've been meaning to do is star test the little bugger! I can't believe I haven't, I guess everytime I use it I'm having too much fun.
Well I mention star test because Celestron probably has a optimum mirror spacing figured out for the C90. Or does it matter with the Maksutov? I suppose it would? The C8's optimum is when you use the supplied star diagonal with the supplied 25mm eyepiece, and infinite focus will set the mirror spacing to the distance with the lease aberrations.
When we start putting visual backs and 1.25" star diagonal etc. on the C90 we may be upsetting the apple cart.

Robert


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5541227 - 11/26/12 10:47 PM

Maybe it would help to wrap the diagonal's barrel in tape, to tighten the fit and center it better. I want to get rid of that cat's-eye shaped field stop and improve the focus.

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actionhac
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Reged: 08/09/08

Loc: Seattle
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541364 - 11/27/12 12:01 AM Attachment (43 downloads)

Is the cat's-eye shaped field stop in sharp focus or blurry?
There is definitely a problem somewhere. Can you see anything out of place looking through the tube without diagonal and eyepiece?
My baffle tube is melted, probably from solar viewing somehow I guess. Not bad, I marked it with a B in this picture. But check yours for any damage:


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5541707 - 11/27/12 09:20 AM

Quote:

Is the cat's-eye shaped field stop in sharp focus or blurry?




Sharp focus, but variable. Seems to be related to the diagonal being mis-aligned in that big hole.

Another problem is an odd darkening of the edge of the field at low powers, extending maybe 20% from one edge, with a dead straight, sharp line cutting a cord across the outside of the field. This may be a weirdness of the diagonal or eyepiece, or an aberration due to moonlight (unsure about that last).

Quote:

There is definitely a problem somewhere.




Yes; I think so.

Quote:

Can you see anything out of place looking through the tube without diagonal and eyepiece?




No. All looks perfect.

Quote:

My baffle tube is melted, probably from solar viewing somehow I guess. Not bad, I marked it with a B in this picture. But check yours for any damage:




Again, all looks perfect.


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H.S.
member


Reged: 06/26/07

Loc: Terre Du Lac, MO
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541745 - 11/27/12 09:46 AM Attachment (45 downloads)

The Hybrid diagonal works for me for both visual and I just slide my SLR with 1-1/4 nosepiece into the diagolal to take a photo.
Harry


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H.S.
member


Reged: 06/26/07

Loc: Terre Du Lac, MO
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: H.S.]
      #5541747 - 11/27/12 09:48 AM

Forgot, the above photo is a Coyote at about 200 feet.
Harry


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: H.S.]
      #5541765 - 11/27/12 10:05 AM

Quote:

...the above photo is a Coyote at about 200 feet.




Amazing! Whatever its compromises, the C90 is a astonishing beast. Weighs nothing, occupies almost no space, yet allows counting the hairs on a coyote's nose from far away.


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actionhac
Post Laureate
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Reged: 08/09/08

Loc: Seattle
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541856 - 11/27/12 10:56 AM

Joe a star test may show a problem.
Your not concerned with optical quality right now just the optical alignment.
Try it without the diagonal first. Defocus a star and see if the diffraction pattern is round in the center of the FOV. Start with the pattern in the center of the Field and then check the pattern from positions around the edge like 12:00 6, 3, and 9.
Look for any deformity in the pattern when at the positions around the edge.
You can also use the sun reflection off a distant chrome bumper or power line insulator.

Robert


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5541941 - 11/27/12 11:48 AM

Quote:

Joe a star test may show a problem... Try it without the diagonal first.




Yes, a star test would tell plenty, when I can get one. So far, only specific combinations of my diagonal, 0.965"-to-1.25" adapter, and eyepieces have worked. I may need to borrow parts to get a star test.

Quote:

Your not concerned with optical quality right now just the optical alignment.




Yes; I think that's so. I suspect the optics are good. Just a hunch, based on the kinds of problems I see.

Those three screws on the back of the primary mirror cell... do they affect collimation? I *promise* not to mess with that unless it is absolutely clearly necessary!


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actionhac
Post Laureate
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Reged: 08/09/08

Loc: Seattle
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5541986 - 11/27/12 12:08 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

Edited by actionhac (11/27/12 12:17 PM)


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CharlieB
Carpal Tunnel
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Reged: 12/11/07

Loc: Southern NH
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: actionhac]
      #5542061 - 11/27/12 12:49 PM Attachment (32 downloads)

Quote:

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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Pierre Stromberg
sage
*****

Reged: 02/07/08

Loc: Woodinville, WA
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5544878 - 11/29/12 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


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Pierre Stromberg]
      #5553056 - 12/03/12 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.


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rmollise
Postmaster
*****

Reged: 07/06/07

Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5553488 - 12/04/12 07:50 AM

Quote:

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.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: rmollise]
      #5553822 - 12/04/12 11:43 AM

Quote:

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. 

Quote:

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. 


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orion61

*****

Reged: 10/20/07

Loc: Birthplace James T Kirk
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5553848 - 12/04/12 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?

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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: orion61]
      #5553866 - 12/04/12 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.


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dbledsoe
Pooh-Bah
*****

Reged: 06/09/06

Loc: Boise, Idaho USA
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5554452 - 12/04/12 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.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: dbledsoe]
      #5554475 - 12/04/12 07:15 PM

Quote:

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.


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is this Celestron C90 "Classic" or "Used?" new [Re: Joe Cepleur]
      #5558535 - 12/07/12 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.


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