Collimating 8" Celestron SCT

[Pharian]

Hi all,

Total noob question here (I really have virtually no idea what I'm doing).

Does anyone know of a clear and simple guide to basic SCT collimation? Something similar to Trevor's excellent "Setting up a GEM" guide?

Basically I'm going bush shortly to a site down a bumpy dirt track with no internet at the end so I'm trying to get everything I might need prepped before I go.

Thanks!

[iborg]

Hi

I don't know how good this link will be, but, have a look.

http://www.iceinspace.com.au/forum/showthread.php?t=144423

I am yet to collimate! But, I do have Bob's knobs to change to when I do.

Philip

[miker]

Christopher, you will find a lot of good info links here on CN..

http://www.cloudynights.com/topic/111773-sct-collimation-howto-links/

Good luck.

Michael.

[The_bluester]

They are fairly simple to collimate, pretty much just centre and slightly defocus a star until you see diffraction rings at a reasonably high magnification and if the rings are not concentric, you want to move the centre dot of the star towards the side where the diffraction rings are spread out rather than bunched up.

To figure out which screw/s to adjust, while looking through the eyepiece, place your finger in front of the corrector plate, pointed from the outside towards the centre (so radially) it will take a little bit of contortion to look through the EP and put your finger there at the same time but is doable with the short tube of the SCT, don't touch the corrector plate as you will put finger prints on it. Move your finger around the outside of the corrector plate, always pointing at the middle. You will see the shadow of your finger in the image at the eyepiece.

Move your finger around so you can work out which collimation screws to adjust. the one you are closest to pointing either at or on the opposite side of the secondary when the shadow of your finger lines up with (Or is opposite) where the diffraction rings are bunched up is the one needing the most adjustment.

Re centre the star using the mount controls to check it after moving a screw. If the image improved when you moved the screw, keep going, if it got worse, back it out the other way. Do not move more than about a quarter of a turn at a time at most and by the time you are finishing up you will be moving the screws a tiny amount. Make sure you do not back a screw out far enough for it to become loose or it will move all the time, if one gets close to going that far, instead of backing that one out, adjust the other two inwards instead. It is a little more tricky if you need to move more than one screw due to the diffraction rings not bunching up neatly on or opposite one of the three screws but it is not that hard.

All that said, if it is OK now it will probably be OK by the time you get where you are going as they hold collimation surprisingly well. Nowadays mine gets transported as much as used and I tweak it maybe once a year. Plus, unless it is a fair way out it has to be a good night before collimation will make as much difference as seeing.

If it has or you can get Bobs Knobs collimation screws fitted to it, they make the job very easy and tool free. I always cringed at using a pointed screwdriver around the corrector plate in the dark.