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Hi guys

Help please. Last night I purchased an old celestron c8 sct, 2000mm fl, f10 on a fork mount. The mount has no power cable so is pretty useless, but i brought it to defork the ota for planetary imaging and hopefully some small faint dso's. It was a reasonable price. Ive got a heq5 pro mount that i will be mounting it onto. The guy said it needs collimation. I know i need to mount it on a vixen dovetail but the bolt holes dont line up anywhere. Its as if someone has rotated the corrector plate housing around 120 degrees. Would it make a huge difference if i removed the housing screws and put it back so that the bolts line up, keeping the mirror and plate in the same spot on the housing bracket,  or would i have to try put everything back separately in different positions and collimate everything from scratch. The picture at the minute doesn't quite get crisp so whatever has been done needs rectifying. Can this even be done by us regular folk? Any thoughts or ideas, instructions? I feel the plate could do with a clean and the primary mirror itself. I just want it as optimum as possible. And can a hyperstar be fitted to these older versions? Im literally starting from scratch with an sct setup. Also what type of camera would suit a scope with this focal length for faint dso's, i have a gp290c for the planetary side of imaging. Sorry for the bombardment of questions. 

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I have had a look at my more modern C8 SE.  The dovetail bar screw holes may not line up in the way you think - mine has two screws at one end of the bar, (spaced around the cast rim) and one at the other.   Looking from the back, the focus knob is at the bottom and the dovetail bar on the left.

At the other end, it does look as though the front casting could be unscrewed and put back at 120 deg rotation.

Opinions differ on the effect of rotating the secondary mirror or correcting plate, suggesting also that it may or may not matter depending on how old the SCT is.  The collimation though is something you can check with a star test.

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Hi Mark 2020

I also own a very old (early seventies) C8 SCT and am using it on a HEQ5 Pro mount. As soon as everybody wakes up here I will go up in the observatory and  have a look at the screws (my C8 is already mounted on a vixen bar) and I will try and take a pic for you to check, if this can help

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On 22/10/2020 at 21:44, Cosmic Geoff said:

I have had a look at my more modern C8 SE.  The dovetail bar screw holes may not line up in the way you think - mine has two screws at one end of the bar, (spaced around the cast rim) and one at the other.   Looking from the back, the focus knob is at the bottom and the dovetail bar on the left.

At the other end, it does look as though the front casting could be unscrewed and put back at 120 deg rotation.

Opinions differ on the effect of rotating the secondary mirror or correcting plate, suggesting also that it may or may not matter depending on how old the SCT is.  The collimation though is something you can check with a star test.

Hi Geoff 

I just realised aswell the secondary mirror housing turns freely on the plate, the numbers on the corrector plate are nowhere near the 3 oclock position like ive read, also this seems to be glued together not screwed together so as to trap the glass. And.... i did a star test last night and collimation is out alot but the collimation screws are tight down and im not able to pull in the concentric circles anymore. How or where can i start to get this all sorted. I found mars last night but was just a fireball on the one side. And pleides all stars were badly hazed and stretched the one side. Physically cannot get the seconday mirror to move any more the right way. 🤦‍♂️

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The secondary mirror housing should not be rotating in the plate, and the three collimation screws should slacken off.  I hope that somebody who has worked on one of these will come along and advise.  Seems that you will need to dismantle the front end, sort it and reassemble it. Not that I'd want to try it myself except as a last resort.

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While I have often read advice that you should only ever tighten the collimation screws, I’m not convinced by it myself. The mirror sits on a central pivot. If you keep tightening all of the screws it will eventually deform the pivot or backplate. There are a few examples of this happening. Obviously, if you keep slackening all three screws, the mirror will eventually detach from the holder and drop into the tube, and I think that’s where the only tighten rule comes from.

I’ve not had to collimate an SCT many times, as despite being carried out every time I use them, mine have kept their collimation after I’ve done an initial collimation. I start by tightening all the screws up. When I then need to make a collimation adjustment, if the screw I need to tighten is already tight, I slightly loosen one or both of the opposite screws and then tighten the desired screw. Afterwards I nip the other two back tight. I only turn the screws a fraction of a turn at a time and at no point do I loosen all three screws. After each minor adjustment, all three screws are tight before I recheck the collimation. Because of this, I think the risk of completely loosening the mirror is minimal.

In terms of the rotating secondary, and I’m not familiar with your generation of C8, so double check before you remove anything, but i think you need to tighten that up from the back of the corrector plate i.e. from the inside of the scope. I think the secondary shroud on the inside of the scope screws onto the secondary holder, sandwiching the corrector plate between them.

Modern C8’s have a nut on the inside of the bolts that hold the front corrector plate housing on (which will fall into the tube if you loosen them), so you would need to remove the corrector from the housing to get to the inside. However, it’s worth checking as I know some of the Celestron SCT’s don’t have nuts on the inside so in theory you can remove the entire housing, tighten up the secondary holder, and refit without having to remove the collector itself. You would just need to make sure you mark the orientation of the housing with the tube so that you don’t rotate it when refitting.

 

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I recently had the corrector plate out of my 8SE, while it may not be exactly the same as the older C8 it shouldn't be that different a procedure to put it back and collimate.

I discovered small triangular markings on the edge of the corrector plate glass, possibly made during assembly to find the best position / rotation in the tube, you could note / photograph these (if present) and where they lined up when you remove the corrector plate.  Before you remove anything though, get some fresh masking tape (I discovered that old stuff leaves sticky residue - test it somewhere else beforehand) and stick a piece all the way from the outer metal casing right onto the glass of the corrector plate, then, using a sharp knife, very carefully cut the tape so that you can separate the corrector plate from the holder, and the holder from the metal casing.  This way you will know exactly how the glass should be aligned when you put it back.  It'll probably be worth cleaning both sides of the corrector plate (there are youtube videos on this, basically treat it like a Faberge egg), I wouldn't touch the mirrors unless they were actually dirty, use an air blower bulb to blast away any loose dust or particles.  If the primary needs cleaning then do it with the OTA mounted horizontally, and use just enough water to wet the mirror surface (use soft cotton wool).

As AstroTim said, it's the same process as aligning a finder scope - slacken one collimation screw very slightly before tightening another.  I found the collimation of my 8SE pretty easy on the whole, I just needed to point the telescope at a bright star (I don't have enough distance for an artificial star in the house).  Bob's Knobs / knurled screws really help.

I wonder if you might be able to fit tube rings to that thing though, maybe just behind the front cell and in front of the rear band that the finder fits to.

Edited by jonathan
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