449 replies to this topic

[Terra Nova]

I have read the same thing about colimating these, but I did a star test and mine was very slightly off and I was able to adjust to pretty close to spot on by making some minor adjustments with the three phillips screws while looking at a defocused image of Arcturus at high power. You can see the collimation change when you adjust those screws with a phillips screw driver. With mine, I loosened all three the screws, a varying amount for each. It doesn't move alot but it doesn't take much movement either; at least mine didn't. It could just be that the screws were tightened down too much, slightly pinching the optics, that sprang back to original good collimation when I lestened the pressure by unscrewing them a little. I don't know. The views are nice at 143X and I am not going to take the scope apart to find out, however a post in this thread (post #26) states that the poster (Charlie B) replaced the screws with longer nylon screws in an attempt to facilitate better and easier collimation.

Edited by terraclarke, 22 May 2016 - 09:36 PM.

[albert1]

I must have missed Charlies post discussing that, Terra. Gonna give that a try. Longer nylon screws to push a bit on the primary - no reason that wouldn't work for minor adjustments which is all it would usually need if anything. All three screws on mine are short of coming in contact with anything - just act as pretty plugs.

[schang]

Here is the link on collimating the oragne C90

 

http://www.cloudynig...W5nIHVwIGEgQzkw

 

It appears that without taking it apart and adjusting the internal mechanism, these visual back screws does little for collimation...

 

Albert, you have a nice setup with the Orange C90, and with that rubber dew shield as well...Mine is used primarily for stationary backyard spotter .  I have another C90 (recent model) for g-n-g astronomy purpose.  

[Terra Nova]

I must have missed Charlies post discussing that, Terra. Gonna give that a try. Longer nylon screws to push a bit on the primary - no reason that wouldn't work for minor adjustments which is all it would usually need if anything. All three screws on mine are short of coming in contact with anything - just act as pretty plugs.

You really have to wonder what the intent of those three screws was then? I can't imagine them being there if they were to do nothing. It just makes no sense! At any rate, I did notice very slight changes in the collimation pattern of out of focus Arcturus with mine when I adjusted them. I could visibly see the dark disk of the secondary incrementally moving from slightly off center to on center within the larger illuminated disk. And the high-power image improved, from good to better!

Edited by terraclarke, 23 May 2016 - 07:11 AM.

[clamchip]

Here is the manual for the C90:

http://downloads.cel...ge_tube_c90.pdf

The factory doesn't want us touching the 3 screws at the front or the 3 screws

at the rear.

Now I'm curious, I might take mine apart and see what makes it tick.

 

Robert

[Terra Nova]

If they say NO, it means YES!

[clamchip]

Celestron has always had detailed collimating instructions in their manuals

but the C90 manual has nothing.

theres a big snap ring holding the primary on the baffle tube, and it sounds like

its difficult to remove so I'm not sure I want to remove mine unnecessarily.

I think a great place it start if collimation is off is make sure the meniscus lens 

is properly seated and the 3 retainers are snug.

Clean the old focuser grease out of the threads and regrease with Mobil 1 or

Superlube just in case the hardened grease is preventing the front half of the

telescope from centering its self by the focuser threads.

 

Robert

[Terra Nova]

Good idea! No way am I taking mine apart!

[starman876]

i thought all SCT or Maks were a mirror lens?

[desert_woodworker]

I might buy a second one, used, like the first one I just rec'd, and determine which of the two has the better collimation on arrival.

 

Then, I'll use the "worse" one as "the Guinea Pig", take it all apart, see how I can improve it, see what tools I'll need to make for it, or scrounge or buy, and learn what I can about "what makes it tick", as Robert says so encouragingly above.  Then, apply the same procedure to the second one (if it needs it.  It prob. could use a re-lube at least, if anything).  If all goes well, I may then give one away.

 

Meanwhile, via all this exploratory and tinkering invasive surgery, I'll surely have become the "C90 Expert" on my block.  Quite a distinction!  And probably a vanishing breed... .  An inspiration to the younger set.  And we can perhaps collectively write an Addendum to the 16-page C90 Owner's/User's Manual, including wrinkles on disassembly, cleaning, re-lube, re-assembly, and collimation (and on brewing a proper cup of tea, after all that).

 

I have a "ProOptic" 90 mm f/5.6 (much larger secondary spot on its Mak corrector than the C90 has) that I want to do all this to, too, in preparation for doing the same to a new Orion 180 mm Mak (esp. to remove the horribly stinky outgassing grease on the foc. mech., and regrease with Superlube, I think, which to me has no detectable odor at all).

 

I'll start with either the ProOptic or the C90, and then progress to the 180 mm Mak using what I've learned from the 90 mm jobbies.

 

I'll see how it goes.  Anyway, a long time ago, a supervisor of mine (in experimental Science) once said to me:

 

"Joe, experience is directly proportional to the amount of equipment you've destroyed".

 

I hope I've developed a lighter touch since then, and learned some skills besides, but there may yet be time to learn s'more! 

 

No more "destroying", I hope.

 

--Joe

[clamchip]

If I was to make a wild guess about the 3 holes in the rear cell I would say

Celestron installed longer collimation screws in those holes and collimated

the telescope while the RTV holding the primary set. Once the RTV hardened

they removed the long screws and plugged the holes with short ones.

 

Robert

[desert_woodworker]

Robert,

 

Not a wild-sounding guess to me!, but an educated one, instead. 

 

I found the threads of the three screws in the back-plane to be actuating or bearing on NOTHING else but the metal of the casting of the backplane (rear cell, I mean) that they are tapped into.

 

Sorry!, Terra!, I don't mean to cast aspersions on your collimating results (maybe your 'scope's screws are in fact longer, or the mirror cell surface is closer to the rear, in your earlier serial-numbered unit).  In my C90, s/n 909542, the three Phillips screws are just dust-plugs I've become assured, by snooping.  

 

--Joe

Edited by desert_woodworker, 23 May 2016 - 06:42 PM.

[Terra Nova]

Or maybe somebody fiddled with it?

[desert_woodworker]

Terra,

 

> Or maybe somebody fiddled with it?

 

Not Zero Mostel.  He was the singer, not the fiddler. 

 

--Joe

[kansas skies]

If I was to make a wild guess about the 3 holes in the rear cell I would say

Celestron installed longer collimation screws in those holes and collimated

the telescope while the RTV holding the primary set. Once the RTV hardened

they removed the long screws and plugged the holes with short ones.

 

Robert

 

This is my thought as well.

 

My C90 was ever so slightly out of collimation when I got it, so I removed the snap ring in front of the primary, then removed the plastic washer that is between the snap ring and the primary. The primary was apparently glued in place and did not budge. I found that the plastic washer's surface was rough (casting flash), so I smoothed it with sandpaper, then reinstalled the washer and snap ring. After reinstalling the corrector assembly, I found the collimation settled in nicely and was very close to spot on. It's stayed that way ever since.

 

As for installing nylon screws to press on the primary for collimation, I tried that and it did work to a degree. After leaving it that way for awhile, I felt like it really wasn't worth the trouble as it didn't seem to stay put as I would have liked, so I removed them and reinstalled the original screws, which really are nothing more that plugs.

 

Bill

[Terra Nova]

Terra,
 
> Or maybe somebody fiddled with it?
 
Not Zero Mostel.  He was the singer, not the fiddler. 
 
--Joe

And certainly not when Rome is burning!

[Terra Nova]

Are the plug screws about a 1/2" long?

[clamchip]

Terra's C90 may be built differently its very early.

I own 2 C90's and Celestron must have changed the serial number system

on the C90 because I have 911440 and 913908 and Terra has 9429.

The C90 was introduced in 1978.

913908 I own is a Astro model and the drive motor date is 11-80.

 

Robert

[schang]

Are the plug screws about a 1/2" long?

Mine is about 3/8" long, or about 1/4" threaded length...They sit on raised bed of the face plate so the screws are more like plugs.   I think that Celestron does not want users to do anything there...  

 

About checking collimation of C90...I'd suggest that doing it without any diagonal, just straight thru.  I found that helps to eliminate introduced alignment errors by the diagonal.  I'd think that the factory collimation is good to begin with, and still is, at least on mine via straight thru test... 

[kansas skies]

I guess it possible that Terra's C90 might well be different, given its age. From the focusing mechanism, I would guess that Celestron originally designed it and the 500mm model to be prime focus camera lenses, with the spotting scope and astro models being afterthoughts. If this really is the case, then perfect collimation was not really a necessity. I do know that when I attach the C90 to my Canon T3, it sure makes a great telephoto.

 

Bill

[Terra Nova]

My screws are about 1/2 inch long, maybe a tad bit more and threaded all the way down to the end. I took one out but was afraid to take the other two. They are threaded deep down into something behind the metal base plate and when you turn then they cause the primary to tilt. It lost,collimation when I took the screw out and I had to recollimate it again tonight on Arcturus. Turning the screws produce enough movement to cause the black central disk of the secondary shadow to drift from the edge of the bigger illuminated circle to the center. Mine does seem to be different. It says 'mirror lens' rather than 'Maksutov telescope', it is not drilled and tapped for a finder, and the box that it came with is made for just it. It also came with some sort of T-adapter ring that can be threaded onto the visual back, but it is not the 'large accessory ring'? When collimated, it produces nice sharp images with a 7mm Meade R.G. Ortho in a prism hybrid star diagonal. I'm pretty happy with it. It's fun. I played with it some tonight alson with my 4" F11 refractor looking at Jupiter and Mars.

Edited by terraclarke, 24 May 2016 - 12:15 AM.

[desert_woodworker]

I've meas'd the lengths of the three screws at the back of the mirror cell on C90 s/n no. 909542 (which calls itself "Maksutov Telescope"), and find them all to be within a few thousandths of 0.250 inch long.

 

To me, it seems these screws match the Metric Standard, the screw M4 0.75 (4 mm, 0.75 threads per mm).   But I may be too sleepy, or ham-handed tonight.   Someone, please check me on this?  If this is so, then maybe these 'scopes were not made in USA, if Metric hardware was used?

 

--Joe

Edited by desert_woodworker, 24 May 2016 - 12:26 AM.

[kansas skies]

I have to think back a ways, as it's been quite a while since I did all of this. I do remember that I tried the nylon set-screw adjustment mod prior to removing and leveling the plastic washer as I described in my previous post. I also remember looking through the screw holes and seeing what was obviously the back of the glass mirror. It was this that was being pressed on by the screws. I also know that this idea for collimating a C90 has been around for quite awhile, so it's possible that the screws on the back of Terra's C90 are not original. I don't think there's much, if any, harm that can be done from this since the washer that the mirror is being forced against is made from a fairly soft plastic. Still, although it did seem to work fairly well in my scope, I was afraid of introducing astigmatism, so I abandoned the idea in favor of leveling the washer.

 

Bill

[Terra Nova]

I didn't measure the lengths of the screws, just eyeballing them, they looked to me to be around 1/2 inch, perhaps a tad longer. Having spent my college summers working as a draftsperson when it was all still done on a board tho, I have to say that I have a pretty good eye for fairly accurate 'eyeball' measurements. They were definitely longer than 3/8". They looked old and consistent with the rest of the OTA. As for thread pitch, whether they were metric, etc. I did not put a thread gauge on them. I just took the one out to see what it seemed to do which was to put it back out of collimation when I put it back in. The holes are tiny. I didn't see anything inside. Perhaps I should have poked a needle in and routed around, but the screw does seem to catch on something inside like a threaded metal back plate? Each screw makes little incremental movements in the dark secondary shadow just like my Lomo 95mm Mak did. I assumed that it was made in a similar way as the screws seemed to have a similar action. I think Robert is right, this is a different beast from the others I have seen. It doesn't say Maksutov telescope (though it certainly has the same thick corrector plate in front with the silvered secondary), it says 'Mirror Lens' and the SN is totally different that the others as well. It is quite heavy for it's size. I think there is more in the back than plastic(?) behind the mirror.

[schang]

Here it goes...

 

My threaded screw length is 0.25", the space between the base of the screw hole to something solid inside is 0.7".  You can measure it to see if those screws do anything at all.  There may have several design changes over the years, so the best we can do is to measure and confirm...