Help with collimating my Newtonian Reflector

[grahamgoo]

Hi everyone.

I recently got my first telescope - a SkyWatcher 130 explorer - 130mm diameter, 900mm focal length.

http://www.wexphotographic.com/buy-sky-watcher-130-explorer-newtonian-reflector-telescope/p1017011

While I am very much a beginner with lots to learn and discover, I am enjoying using it very much indeed.

From reading up on the subject, I decided to invest in a Cheshire collimating eyepiece - one of these...

http://www.scopesnskies.com/prod/generalaccessories/skywatcher/cheshirecollimatingeyepiece.html

However, despite watching various videos and reading materials on the internet, I am still not sure exactly how to collimate properly.

My first problem is that even with the collimating eyepiece fully inserted, it wobbles about a little.  I can of course tighten up the outer screws to stop this, but depending upon how much I tighten each of the screws, the final fixed position of the eyepiece, and hence the cross-hairs, can be slightly different.  Should I be concerned about this and what can I do about it?

The other thing I have noticed is that several web pages have referenced having a centre marker ring or dot on the primary mirror.  As far as I can see, mine does not have this.  I am a bit nervous about taking the primary mirror off to do this - is it essential for effective collimation?  

Any responses will be gratefully read!

Graham.

[Floater]

You don't have to - and shouldn't - take off the primary to collimate the scope and not sure if the centre spot/ring is essential.

Cleverer people will be along to comment on the 'slop' effect.

[Cornelius Varley]

Hello and welcome to SGL. Before trying to collimate the telescope you should actually check that it needs collimating by taking the telescope outside, letting it cool down for about an hour and then performing a star test. Aim the telescope at a medium magnitude star using the 10mm eyepiece and then defocus the telescope. If the telescope is correctly collimated you should see a series of concentric circles. If you don't, then and only then attempt to collimate the telescope.

[grahamgoo]

Thanks - yes I will give a star test a go when we have some clear evenings, I have not tried that yet....  from what I have read, i got the impression that reflectors need collimation on a fairly regular basis, so i thought it would be useful to familiarize myself with how that is done, and why, and I wanted to check whether the precise fixed position of the collimating eyepiece and its crosshairs (once tightened up) would be a problem or not.

Thanks and regards.

Graham.

[Dr Hamzonian]

I had a similar issue with my collimator. I then realized that it actually wasn't fully inserted in. At least with mine, when it is inserted in fully, only about 30% of the total lenght is left outside. For some reason mine had a slight change in diameter in the early part of it, making it seem that It was fully inserted (about 25% of the lenght), then when I wiggle it a bit, it clears that step-up into the tigher fitting diameter and the is fully inserted with a nice fit with only about 30% of the lenght left outside, and no wobble issues. I can take pictures for you if needed. Hope this helps!

[Dave In Vermont]

Many of the 'How-To' papers out there fail to demystify collimation, and leave people scratching their heads in confusion. So I'll pass along the one from Astro-Baby - a member here - that is one of the best and most lucid:

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

This shoud help. Do tighten the set-screw after inserting the Cheshire into the drawtube of the focuser. If it's still loose, I'm at a loss.

Clear & Centered Skies,

Dave

[Astro_Baby]

A cheshire will almost always wibble a bit in the average focuser - a compression fit focuser is best BUT a cheap way of getting a good fit is wrap some PTFE tape round the Cheshire until its a snug fit.

You can also get a precision centreing ring which uses squishy rubber to get a centred fit.

There are three schools of thought on this.....

Orthodox - allow the chesire to be a bit wobbly because if the chesire is pushed a bit out of line so will the eyepieces be so the cheshire and eyepieces will both line up roughly to the same amount of not fitting exactly.

New Age Cheapo - Centre the cheshire using some tape - the cheshire wont be in the same position as an eyepiece as a result but at least the scope is accurately done.

Obsessive Compulsive - Get a precision centreing adapter which uses the scopes 2" fitting (assuming it has one) and allows any 1.25" EP (and Cheshire) to be fitted precisely.

Very Obsessive Compulsive - Upgrade the focuser to have a compression fitting system.

I found collimation was always a bit of a headache until I fitted a decent focuser which solved a lot of the collimation issues for me.

Mel

[scuffer]

Do the star test first , I only move mine gently from the garage to the garden and it is always in good collimation , if you chuck it in the car then you will probably have to do it more often.

[Coastliner]

Obsessive Compulsive - Get a precision centreing adapter which uses the scopes 2" fitting (assuming it has one) and allows any 1.25" EP (and Cheshire) to be fitted precisely.

This fits in nicely with a thread of mine, where I mentioned buying a HoTech self centering collimator til someone pointed out that with my scope at £120-odd pounds it would be like putting slicks on a Ford Focus. I've ended up buying a New Generation collimator for £17, and I see there is an Altair self centering ring for about £20. Ok, the combo probably won't be as good as the HoTech but  it may be pretty close.

Neil.