Collimation Question

[Tribal-Wolf]

I have a Skyliner 200p Flextube and when looking into collimating it I read the advice on astro baby's which mentioned that certain scopes have an offset on the secondary mirror. Is this scope one of those as it certainly looks to be offset a little? Also I want to make a collimation cap but can't seem to get an old 35mm case from anywhere or find a spare focuser cover to drill, anyone got a spare they could donate???

[Mike73]

I was watching this earlier

I don't know how relevant it is to the Skyliner though?

[LeeB]

Hi its a common mistake that only short reflectors have an offset, they all do, but at f6 its v small.( as f number increases offset reduces). To understand offset the best thing to do is draw an F2 newtonian light path on a piece of paper. then draw an f6 one. Try it ,a picture is worth a 1000 words.

[umadog]

There are different ways of doing the offset: you can offset away from the focuser or you can offset toward the primary mirror. Don't worry about it, though. Basically, you can centre the secondary in the tube and the offset will sort itself out as you collimate. You can read more about it here: Collimation And The Newtonian Telescope V.4 - How To

Also, you should buy a Cheshire/sight-tube combination tool. The film canister isn't sufficient on its own for accurate collimation. Here's a discussion on why: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5052617

[Tribal-Wolf]

Thanks for the replies. One other thing, should collimation be done on a cold (cooled) scope?

[Tinker1947]

Thanks for the replies. One other thing, should collimation be done on a cold (cooled) scope?

I don't think it would matter (i might be wrong), but the point of focus moves as the tube/mirror cool down this is off-set by moving the focuser to gain focus...

[Sam]

If you want to get your collimating as perfect as possible then you should do it with a cold scope. You can roughly collimate when it isn't cold but if you want to do a star test to do the final bit then it'll need a very settled scope, temperature wise.

[LeeB]

collimate first where its comfy in the house you will be more relaxed and do it better. Then when outside in the dark use the barlowed laser method,(it removes the repeatabilty error and any slop) its easy and from experience having checked results with star test and cheshire it seems to be bang on.

I always check colli prior to observing as a knock or bump when moving to dark site can upset my 10" a little its only ever the primary that needs a tweak.

The reward for 5 minutes at the start pays big dividends particularly on planetary stuff.

[en51nm]

when outside in the dark use the barlowed laser method

Please elaborate on this method!

[LeeB]

To use it you will need a laser , a barlow and the mirror needs to be centre spotted. once colli of secondary is correct place barlow and then laser into focuser (the barlow will spread out the laser beam. the screen on the laser will show a shadow of the centre spot ( move the primary abit to get your bearings ) centre the shadow on the screen and roberts your mothers brother. to check how the shadow in unaffected by laser insertion just losen laser and wiggle a bit the shadow remains still if you used an un barlowed laser the beam would jump all over the place. hope that helps if you need me to talk you through it PM me and I'll give you a call.

[Tribal-Wolf]

I have collimated it with a cheshire however getting the secondary mirror right was a nightmare. It was like the spring wasn't strong enough, the mirror holder moved all over the place. I actually took the mirror off to check, there are three marks where the allen screws touch the holder and it looks like someone has tried to collimate without loosening the centre screw because the marks are quite deep. Took me a while but I think I have it right. Might have to modify that mirror set up though lol.