Celestron C8-N Collimation Problems

[Philter]

Hi guys.

I’ve got a Celestron C8-N (200mm F/5) Newtonian and I’m having a hard time with collimation.  I’ve had the scope for about 8 years but it hasn’t seen that much use with the weather, work, etc.  I want to start using it again particularly for planetary imaging but I just can’t seem to collimate it.  I never really got the hang of collimation but I want to do it properly now.

I’ve followed Astro Baby’s collimation guide and after a lot of effort I think I’ve got the secondary sorted but I’m having trouble with the primary.

The scope has three Phillips head primary adjustment screws with three Allen head locking screws. 

The only way I can align the primary is to tighten one screw until it can’t go any further and loosen the other two so that the mirror wobbles until the locking screws are tightened. 

I’ve tried moving all the screws to a more neutral position and tried to adjust only two screws but I always end up in the same situation.

There are rubber O-rings around the adjustment screws between the mirror holder and the back of the cell. They are about 2.75mm thick.

 

 

Because they are thin they don’t touch the mirror holder when the adjustment screws are loosened to move the mirror towards the front of the tube, this means the mirror has room to wobble.  I’m no expert but it seems to me that these O-rings should be thicker to allow for more movement without the mirror being loose.  Does that sound right or is the something else I should look at?

Thanks.

Edited March 8, 2016 by Philter
Changed picture size.

[Peter Drew]

Hello Phil, welcome to SGL. And the "joys" of collimation.  I agree, there is not much room for adjustment of the primary due to the thin O rings. I'm not familiar with the method used for attaching the mirror cell to the main tube but if it is by means of screws round the outer diameter it might be possible to loosen these enough to adjust the cell to a more neutral  position before collimating with the intended system.

[Philter]

6 hours ago, Peter Drew said:

Hello Phil, welcome to SGL. And the "joys" of collimation.  I agree, there is not much room for adjustment of the primary due to the thin O rings. I'm not familiar with the method used for attaching the mirror cell to the main tube but if it is by means of screws round the outer diameter it might be possible to loosen these enough to adjust the cell to a more neutral  position before collimating with the intended system.

There are six small screws around the tube that secure the mirror cell but I don't think there be enough play to adjust the position of the whole cell.

[Peter Drew]

In that case I would either slot the holes or replace the O rings with different ones or something more suitable.

[michael8554]

Hi

The rubber donut suspension will not allow much adjustment without the mirror becoming loose, as you have found out.

If the philips screws are long enough, slip suitable diameter nylon or steel washers under the donut that becomes loose when the mirror is tilted.

You should then have enough tilt and have the donut slightly compressed.

But are you sure the secondary isn't the problem?

Tilt your primary as much as you can with the donuts still slightly compressed.

Then go back to your secondary and adjust that to align with the primary.

Then when you sight down the focuser, is what you see now badly out, or still pretty good?

Just a thought.....

Michael

[Philter]

9 hours ago, michael8554 said:

If the philips screws are long enough, slip suitable diameter nylon or steel washers under the donut that becomes loose when the mirror is tilted.

I didn't think of doing that, I'll get some washers and give it a go. 

 

8 hours ago, michael8554 said:

But are you sure the secondary isn't the problem?

I'll have to have look again tonight if I get the chance but I think the secondary is ok.  I made sure the secondary holder was centred in the telescope tube by adjusting the spider vanes, I then made sure the secondary appeared circular and centered under the focuser using a collimation cap and Cheshire eyepiece. As far as I could tell everything looked right with the whole primary visible and concentric with the secondary, at least to my eyes.

 

8 hours ago, michael8554 said:

Tilt your primary as much as you can with the donuts still slightly compressed.

I've tried to start the whole collimation process (before adjusting the secondary) with all three primary adjustment screws in a neutral position allowing some movement of the mirror in either direction without the donuts becoming loose or too tight. That still doesn't seem to give much room for adjustment of the primary though, maybe 1/2 a turn of a screw in either direction before it becomes too tight or the donut becomes loose.  I know the adjustment should be carried out using small fraction of a turn at a time but I can't seem to get anywhere without having to make larger adjustments.

Thanks.