Collimating a Celestron 8SE

[Droogie 2001]

I have never collimated my SCT since I purchased it nearly three years ago. But as I have recently begun imaging with the scope again I suspected it needed checking. Stars in the images were ugly and off centre, this was even after a short 5 seconds exposure.

So last night I set up my Celestron 8SE SCT and had everything up and ready. I did a basic alignment and brought Vega into view with a 12mm reticule eye piece. Centred the star and unfocused the image.

Sure enough the secondary mirror appears to be out slightly.

I had watch various You Tube videos, read a number of articles so I was happy with the procedure.
I went to loosened one of the Philips screws off of the secondary mirror and it would not budge at all, tried all three and the same happened. Tried two other screw drivers and the same again.
At this point I had to abort because the screw driver was slipping and I did not want to round the screws off.
I have got hold of some better screw drivers if that makes sense and will look again tonight.

My question is has any one else had a tough time loosening them? I am rotating the screw anti clockwise so I am pretty screw that’s correct?

The scope had been outside for two hours cooling down so I surprised the screws were so tight I would have expected them to have contracted a little and therefore been easier?

Going forward I have seen Bobs Knobs maybe an option though I have read these can cause the collimation to go out of alignment more regularly as they do not tighten as well?

In any case I seriously hope I can loosen them off or else I’ll be stuck with an out of collimated scope! Sticking a sharp instrument close to the front plate is slightly nerve racking to say the least!

Thanks
 

[MarsG76]

Hello,

I remember that the screws were a little tight to begin with but nothing like what you describe... I'm thinking that the screw driver you're using is perhaps too big and doesn't "grab" the screw.

In collimation you only need to turn the screw about a quarter of the way, I found that clockwise (when looking toward it) it moves the shadow toward the direction of the screw driver and anti clock wise moves away.

I ended up getting bobs knobs and replacing the original screws made collimation much easier and faster, I definitely recommend doing it. With bobs knobs the collimation stays spot on for me and only every so often I need to slightly re-tweak the secondary and only mostly when planet imaging.

[Pan Euro]

It's possible the screws have thread lock on them making them difficult to loosen. Make sure the drive head is a perfect match and possibly put a cloth underneath to stop a chance of other damage. Good luck.

Ed

[Droogie 2001]

Thanks for the advice. Yes I wonder if it is the screw head on the screw driver. I hope the screw drivers I have borrowed will do the job properly.

Good idea about the cloth.

I know now why I have been reluctant to do the collimating

[MarsG76]

It's not so daunting once you get a bit of practice. Get a set of jewelers type of screw drivers, from memory the biggest one will be the right size.

[Northern Soul man]

Hi Ya Droogie, nice to see your getting it sorted now mate - just a point though - when your happy with the collimation, just try and increase the power your using to refine the procedure, try and choose a star nice and high in the sky as to move away from the effects of the atmosphere to do a final check - when I first started I became a little obsessed trying to get everything bang on - I think everyone's the same though.

Once collimated try to get each screw fairly tight - screw tension needs to be firm, as this will help to keep the scope collimated over time much better than a loose screw - try to tension equally over the 3 screws first, as well as checking the secondary is centred - this way, you can keep good tension as well as keeping the secondary central - then you only need a small fraction of a turn to adjust - this will hold your collimation and over time it shouldn't need adjusting - just checking every now and then.

Its a little difficult to hold tension as well as getting the secondary centred the first time - but once achieved the scope will hold collimation very well for much longer - I have checked mine over a few years and not had to adjust at all - the key is gettting good tension to hold everything in place.

Paul.

[Droogie 2001]

Ok so I have managed to loosen and then tighten the screws. They were extremely tight so I have tighten to 'firm' rather than how they were originally.

Now need to wait for some clear skies. Have to say Bobs Knobs are looking more appealing as I don't fancy the screw driver technique too often...

Thanks all for the advice.

[Grotemobile]

This might help U a bit. This is what I

done with mine.

Steve.

[Droogie 2001]

Thanks, yes I have watched that. Found it handy too.