C8 with loose secondary housing

[mental4astro]

Hi folks,

I've got to do some TLC on a mate's C8. The other night looking through it for the first time, the image revealled a nasty case of miscollimation. When I fit the allen key in one of the three collimation screws I found that the entire secondary mirror housing was loose on the corrector plate.

Removing the corrector plate isn't a problem. I'll mark the corrector plate's current position relative to the tube, that's fine. But how does the secondary's housing actually attach to the corrector plate? I don't notice any other screws on the secondary other than the three collimation screws.

Cheers,

Alex.

[dannat]

my orange c8 has a central screw

i wonder if it underneath the plastic cap maybe

[mental4astro]

If I remember correctly (it's not infront of me right now) there is no central bolt. Just the three collimation hex-head screws.

The secondary housing is totally loose. I can jiggle it on the corrector, as it has not only lateral movement, but also vertical.

I'll look to post a pic or two later on tonight if that helps.

[Omaroo]

If I remember correctly, isn't the secondary's baffle tube threaded, holding the whole assembly tight?

[mental4astro]

I'll have a look. I'll be checking for the correct orientation of the corrector plate, so I'll be needing to remove the retention ring for that, so I can double check the baffle, and recentre the secondary.

Should there be any type of soft 'washer' between the corrector and the secondary housing, rubber, cork, or something else?

[GeoffW1]

Hi,

Some information here.

http://www.nexstarsite.com/OddsNEnds/SCTCorrectorRemoval.htm

http://www.wilmslowastro.com/tips/c14_optics_alignment.html

http://www.celestron.com/c3/support3/index.php?_m=knowledgebase&_a=viewarticle&kbarticleid=2224

http://www.astromart.com/articles/article.asp?article_id=594

The later model Celestron SCT don't have a central screw holding the secondary mirror back.

The secondary housing on mine was retained in the corrector by means like that described in the links. The retaining ring isn't done up ultra tight to avoid cracking the corrector, and I found that it was ideal to have tightness enough to stop it moving around by itself, but not so much that I could not adjust the centering of the secondary by a firm push.

Cheers

[multiweb]

The secondary mirror cell is threaded into a retaining ring which is on the other side of the corrector. Usually there are a couple of spacers on each side of the glass. Make sure you put the spacers back in the same side. The secondary should have a line at the back and that needs to be in line with the corrector (sometime an X mark in the glass). This mark needs to be at 3 o'clock looking at the scope from the front usually inline with the focusing knob on the other side.

Putting the corrector back is a no brainer. Just mark the edges. Make sure you know which side is in and out. Lateral position is not that critical. What is critical is the orientation of your secondary in relation to your corrector plate and the primary. Just 5 degrees out and you might not be able to reach focus with your scope. Also do not tighten the corrector outer ring. Just contact, no pressure. That's it. Hand tight, barely touching.

Other than that that 's pretty much it.

[GeoffW1]

Alex,

How did it go?

Cheers

[Poita]

By all reports it went extremely well, but I'll let Alex give out the details.

[Poita]

Alex was able to secure the secondary and get the collimation pretty much perfect, resulting in stunning images at both high and low powers.

[mental4astro]

Yep, as Peter said, it went very well!

Prior to making the adjustment, the poor Cat gave a very dodgey image. Low power stars showed a 'tail' and high power was impossible to use.

Post adjustment, holy cow! I first secured the secondary and then checked the collimation - Unless I was doing imaging with this C8, I really wouldn't attempt to collimate any more. We fluked a position on securing the secondary that gave a collimation result within a bee's d..k of perfect.

The Moon was extraoridanary, and M42 was brilliant for light polluted skies and a first quarter Moon to contend with, & with pin sharp stars at high magnification.

The process went smoothly. All the marking to look out for were there. It paid big time to ask and read the posted links!!!!

Many thanks everyone for your considered advice, :D.