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Help with secondary mirror on '75 C8?

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#1 scythe

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Posted 08 October 2019 - 12:13 AM

[Newbie here, working on a 1975 C8, second post ever!]

 

The secondary mirror on my C8 is very loose and moves easily from side-to-side.  I removed the corrector plate for cleaning and was able to tighten the secondary at that time by screwing opposite sides together.  I think it’s meant to be glued in position, though, and I’m not sure how to get it oriented correctly. 

 

I’ve seen a few posts here about how to adjust and orient loose secondary mirrors, but my secondary doesn’t have the serial number imprinted on it (photo attached), so that doesn’t help in my case.

 

Any advice on how to figure out the correct orientation of the secondary on this scope?

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  • C8 Secondary.jpg

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#2 Stevegeo

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Posted 08 October 2019 - 04:04 AM

First off, did you mark the position of the corrector in in relationship to the tube?  This is important. In some it makes no difference.  But I found on my old C8 it did.  

Then it will need to be re collimated..   there are instructions here on CN and you tube.. take your time and make minor adjustments. A good collimation will bring out great night sessions..

Let us know how you make out.... Good luck

Stevegeo


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#3 scythe

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Posted 08 October 2019 - 08:11 AM

I did mark the tube, retaining ring and the corrector (I think my pencil marks are visible in the photo above).  I'm just seeking guidance on how to orient the secondary, since I can't use the "make the serial number horizontal, pointing to 3pm" rule I have often seen cited here.  

 

If I take the secondary off, will I find indicators there?  Should one of the screws on the back of the secondary (visible in the photo) be pointed N, S, E or W, relative to the tube? 

 

Or do I just go with trial and error, potentially collimating and re-collimating the scope through 360 degrees until I chance upon the correct orientation?  How will I know for sure when I hit it?



#4 macdonjh

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Posted 08 October 2019 - 08:15 AM

scythe,

 

Welcome to Cloudy Nights.

 

The secondary mirror in your C8 is spherical, so it shouldn't make any difference what orientation it's in when your reassemble your scope.  That said, I've heard Celestron does some hand figuring to their mirrors, so there may be an orientation that is "more right" than other orientations.  If you didn't mark your secondary mirror for orientation before you disassembled your scope, you'll have to use trial and error to get it right.

 

As Stevegeo said, the orientation of the corrector IS important.  There should be a black pen mark on the edge of your corrector.  That mark should be at the 3:00 position when you reassemble your scope.  3:00 when you look into your scope from the sky end (the end opposite the focuser, where the corrector goes), with the dovetail bar down.  If there is no mark, and you didn't mark it during disassembly, you'll have to use trial and error to get it right.

 

As yo your original question, the secondary mirror housing threads together, with the corrector in the middle.  The secondary mirror should be centered within the corrector, Celestron used cork shims to center their secondaries.  Paper would work, though, if that's all you have.  Then put the secondary on the inside of the corrector, and the other half of the plastic housing on the outside and thread them together.  Just snug is enough.

 

The three screws on the outside of the secondary mirror housing are for collimation.  If they are very loose you should gently snug them down to prevent your secondary mirror from moving around.  When you get your scope reassembled you'll have to collimate it.  That's a topic for another thread.  Or for a search in the Cloudy Nights archives, since it's been discussed dozens of times.

 

If you have trouble with the orientation of your corrector, post again.  We can help with that, too.



#5 macdonjh

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Posted 08 October 2019 - 08:23 AM

 

Or do I just go with trial and error, potentially collimating and re-collimating the scope through 360 degrees until I chance upon the correct orientation?  How will I know for sure when I hit it?

Well, there you go, now you don't have to post again.  That's the procedure for figuring out the correct orientation of both the secondary mirror and the corrector plate, if you have to use trial and error.

 

If you have to, find the correct orientation for the corrector plate first.  That will most likely have way more impact on the sharpness of your view than the orientation of your secondary mirror.

 

What I would do is reassemble your scope and collimate it as well as you can.  If you are not satisfied with the sharpness of the image, rotate either the corrector or the secondary mirror (which ever you are adjusting) 90o, collimate again and see if you are satisfied with the image.  If you are, you're done.  If not, repeat the process.  

 

Once you are satisfied with the image by rotating one of the optical components by 90o you can continue to fine-tune your orientation by rotating by smaller and smaller increments, like 30o, then 10o, until you get it as perfect as you can.  When you're satisfied, I'd suggest marking the parts of your scope, even though this will be one more disassembly and collimation.  You'll be really good at collimation by this point, though.


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#6 scythe

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Posted 08 October 2019 - 10:11 AM

Thanks for your very helpful advice, Stevegeo and macdonjh!  I apologize if the answers to my questions seem obvious to you--I'm new to all this, and still trying to work things out.

 

Just to be clear, the secondary was rotating and moving freely side to side before I began working on the scope.  If there were cork shims between the secondary and the corrector plate, they were long gone before I got it.  No way to mark its original position.

 

I am more hopeful that the corrector plate is still oriented properly.  I guess I missed the threads discussing a black mark on the edge of the corrector (late-night speed reading).  But the retaining ring was stuck fast--it took some pounding with a wood wedge to get it free--which made me think this might be the first time the corrector had been removed (filthy inside, cork shims still in place, etc.).  So I marked the locations of the shims and serial number carefully, cleaned the corrector, and put it all back together as it had been.  While I had the corrector off, I tightened the center screw in the secondary back as much as I dared, but it began to loosen again immediately after reassembly.  Is the center screw all that holds the secondary in place?  I thought I read that Celestron used glue to help hold the secondary in place, but can't recall where. 

 

Anyway, I think I'll take it apart again, and look for any evidence of shims or markings on the secondary that would let me know how it should go.  Then I'll try collimating the scope as you suggest and mark my best results.  Hope not to have to bug you all again!



#7 bremms

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Posted 08 October 2019 - 11:01 AM

Centering of the secondary is important for proper collimation. Carefully center it with the entrance aperture. This assumes the primary is coincident and aligned with the center of the tube. You will get visible astigmatism if it is off center.IIRC.



#8 macdonjh

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Posted 08 October 2019 - 01:35 PM

Thanks for your very helpful advice, Stevegeo and macdonjh!  I apologize if the answers to my questions seem obvious to you--I'm new to all this, and still trying to work things out.

No worries at all.  It makes us feel smart...

 

Just to be clear, the secondary was rotating and moving freely side to side before I began working on the scope.  If there were cork shims between the secondary and the corrector plate, they were long gone before I got it.  No way to mark its original position.

I don't think it's that important for the secondary mirror, though I agree with bremms, centering is.  Orientation is important for the corrector plate.

 

I am more hopeful that the corrector plate is still oriented properly.  I guess I missed the threads discussing a black mark on the edge of the corrector (late-night speed reading).  But the retaining ring was stuck fast--it took some pounding with a wood wedge to get it free--which made me think this might be the first time the corrector had been removed (filthy inside, cork shims still in place, etc.).  So I marked the locations of the shims and serial number carefully, cleaned the corrector, and put it all back together as it had been.  While I had the corrector off, I tightened the center screw in the secondary back as much as I dared, but it began to loosen again immediately after reassembly.  Is the center screw all that holds the secondary in place?  I thought I read that Celestron used glue to help hold the secondary in place, but can't recall where. 

You shouldn't have to again now that you've gotten it free, but I don't recommend "pounding" on any part of a scope near any of the glass.  In the future, a couple drops of denatured alcohol or acetone has been known to dissolve whatever it is that sticks those retaining rings and correctors.  A few drops around the circumference and capillary action will wick it where it needs to go.  Let it soak for five minutes or so and try again.  Denatured alcohol is likely kinder to the retaining ring if it's plastic.  

 

It's been a long time, and your scope is older than the C11 I had, but: the secondary mirror in my C11 was glued to a metal plate.  The metal plate tip-tilted on a little plastic cone on the inside of the plastic housing, and was secured by the three colllimation screws.  Loosening the collimation screws too much resulted in the secondary mirror falling out of the housing and into the OTA.  That is not recommended.  I noticed in your photo (post #1) your secondary housing has a center screw.  I guess that's for securing the secondary mirror, but I've never had a Celestron scope as old as yours.

 

Anyway, I think I'll take it apart again, and look for any evidence of shims or markings on the secondary that would let me know how it should go.  Then I'll try collimating the scope as you suggest and mark my best results.  Hope not to have to bug you all again!

For initial collimation you can get surprisingly close during the daytime.  Set your mount and scope up in a location you can step ten or fifteen feet in front of the scope.  Tip the scope so it's pretty close to horizontal, so you can see into it while you're standing ten or fifteen feet away.  Step away and look into the front of your scope.  You want to be back far enough that the primary baffle tube is just hidden by the secondary mirror housing.  Notice all the reflections of telescope parts you can see in your primary mirror: the secondary housing, secondary mirror, reflection of the primary baffle tube from the secondary mirror, reflection of the primary mirror from the secondary mirror...  All of those reflections will be off to one "side", too.  Now step back to your scope and adjust one of the collimation screws by a little bit and step back to see what effect it has.  Repeating this a few time, tedious as it is, will give you a feel for which screw tips the image which direction.  Keep at it until all the reflections are concentric.  Really, if you stand back far enough that the primary baffle tube is hidden by the secondary mirror housing, you shouldn't see any reflections, just the secondary mirror housing and the primary mirror.  The closer you step, the more reflections you can see and the more sensitive this method is (up to a point).

 

That should get you close enough to good collimation that your scope at least forms a decent image of a star so you can do "real" collimation with a defocused star image.  It's best to do final collimation near the elevation where you do most of your observing, so I'd guess at least 45o up.

Comments in red...



#9 davidmcgo

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Posted 08 October 2019 - 05:40 PM

Macdonjh has a good description of how the secondary is attached in the slightly later style with the plastic housing and "Celestron" logo molded into it.  These used button head collimation screws that retained and allowed tilt, but don't tension too hard or you can crack the plastic housing at the center where the "nub" is the secondary pivots on.  This type has the 3 collimation screws pulling and the molded plastic nub inside at the center provides the push.

 

For the older aluminum secondary holders, the backing plate the mirror is glued to has one threaded hole at the center.  The center button head **** see retains the secondary as a "pull".  The three collimation set screws just "push" against the backing plate and induce a tilt while the center screw holds the secondary in place .  Under all circumstances the center screw must stay in and engaged or the secondary and backing plate will drop out of the holder.  On these older ones, you can remove all three of the little collimation set screws without the mirror dropping out, but it is free to rotate then and will lose rotational alignment if it mattered.  

 

Dave




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