Collimating a 8" sct

[Want a Be]

Just put some Bob's Knobs on my 8" Celestron CPC...Going to do a star collimation on it and it's my first time. Can anyone offer any advise?

[CELESCOPE]

No more than what it says really in your instruction manual , . you should have the star patterns there to view , . but the norm is to set the scope out an hour or so before collimation, run the star alignment for tracking , then use the last star of the alignment for collimation , as long as it high in the sky near to the zenith as possible, use a 9 mm eyepiece or as near to that as possible , de focus the star till you see the dark circle in the centre , if it looks set even ,ie not shewed to one side , then turn the focus knob to near focus and note that the dark section is still dead centre, if not ,then de focus again slightly . make sure the star is in the centre of the eyepiece, now run your finger around the outside of the front of the tube , you will see your finger shadow whilst looking through the eyepiece, continue till the shadow reaches the narrow part of the circle , then back of the screw nearest to that narrowing, only do very small adjustments. take your time and always make sure you centre the star after any adjustments are made.

This can take some time, but with Bobs Knobs it should make it easier as you can look through the eyepiece whilst making these adjustments.

the final test if you think its ok , is then to focus the star, it should be pin point , if it looks like a mini comet , for example if it flares to one side when focused , it means you are still not 100 %, so go through the routine again.

Good luck

Rog

OH AND DONT FORGET TO RE-ALIGN YOUR VIEWFINDER AS THIS COULD NOW BE OUT.

[ElijahB]

Being a newbie... this was more than helpful to me too. Only one question though.... what's Bob's Knobs?

Cheers Loads.

[CELESCOPE]

Bobs Knobs , are where you remove the phillips screws from the secondary mirror and replace it with Bobs knobs, these are thumbs screws , this enables you to avoid frumbling about in the dark , with a screw driver or Allen key ,trying to adjust the secondary, b/knobs are easy to get hold of and adjust whilst looking through the eyepiece to see which way you are effecting the offset.

Rog

[Albireo380]

Rog's replies are excellent advice. You might also want to look at this site:

http://legault.club.fr/collim.html

This site gives some excellent examples of what you see in the EP as you collimate - at least, it seems very realistic when I collimate my 8" SCT using Bob's Knobs.

Tom

[beamish]

Having only just checked my collimation this morning this was a fascinating link, cheers Tom

Karlo

[AstroTiger]

now on the subject of Bobs knobs, i bough these for my SCT.

As i replaced them though, i noticed they would not go all the way in. As i was paranoid about damaging the instrument i hesitated until they were finger tight, did i do right?

many thanks

Al