Collimating without a tool

[MintSauce]

Many of you don't know me, but I bought a scope because I had 70x15 bino's that I loved using; I spent 15+ nights learning the scope and eyepieces and practicing focusing with barlows etc (this was all before christmas as I've now had a kid who needs alot of attention - I'm now planning to get a night a week or so set aside just to use the scope).

I have a 8" GSO Dob from andrews and a couple eyepieces etc.
But I read quite alot about the mirror in the GSO dobs being held too tight - so I'm thinking of losening the screws that hold the mirror and making sure they not too tight - but will I wreck the Collimation on the scope? I've read a couple things about Collimating without a tool - just looking into the eye piece and seeing your reflection to test - But is this accurate? should I buy a Collimating tool before I loosen screws?

Sorry for the mass block of text post :) I hope someone replys :P

[Starkler]

If you remove the mirror cell to get at the mirror clamp screws your scope WILL need recollimating.

If your mirror is already centre spotted you could use a film cannister to make a collimating tool.

[Starkler]

Heres an exerpt from my upcomming how-to you might find useful.
You may not have to do anything. Check first.


Star testing for collimation and astigmatism

We can check for correct collimation and detect astigmatism in our scopes optics by performing a “star test”.
Make sure you put your scope outdoors with covers off for at least 60 minutes before starting because to successfully star test requires a properly cooled scope and also steady atmospheric conditions.

Take your highest power eyepiece (shortest focal length) eyepiece, find a nice medium bright star and focus in the center of the field of view.
Now the adjust the focuser outwards slowly until the star defocuses and you see a series of rings like in fig 1 below.

(insert startest.gif)

If you see nice concentric round rings like in A. your scope is well collimated (assuming your secondary mirror is correctly placed).
Most likely it will look more like B, indicating some tweaking is required. If it looks like C, your scope is way out of alignment!

If you are getting nice round circles all is well. But what if the rings are not perfect round circles?

Often the outermost ring will appear a bit “hairy” and waver about a bit. This is due to either atmospheric conditions, or a scope that hasn’t cooled fully down to ambient temperature.
If the image is reasonably steady, take note of the general shape and rack the focuser in and out on both sides of focus.

Does the image look a bit like a three/four leaf clover or triangular/squarish?
This usually indicates astigmatism in the primary mirror caused by the three/four rubber holding clips being clamped down too hard.

Does the image appear oval or some other shape and shift plane by 90 degrees either side of focus?
This could indicate a pinched secondary mirror caused by its plastic holder being too tight. Try moving your head to rotate your eye relative to the eyepiece. Does the plane of the image follow the movement of your eye or stay fixed in relation to the scope?
If it moves with your eye I’m sorry to inform you that’s where the astigmatism lies. If its in the scope there’s still hope ! J

[ving]

hey minty! good to hear from ya :)

hows the goat going then? (get it? kid!).
read the above :)
and welcome back. we have grown heaps since your last visit :)

[RAJAH235]

MintSauce, You can safely loosen the lock screws without effecting the collimation. These should NOT BE OVERLY TIGHT anyway, just nipped. Check article in S&S.
*http://skyandspace.com/printable/howto/scopes/article_790.asp*
HTH.
Just the facts m'am.
ps. similar/same way as Geoff's.
pps. A Cheshire is an easy, cheap aid.

[iceman]

I think he's talking about the 3 clamps holding the mirror in, not the 3 lock screws on the back of the mirror cell.

Nice to hear from you again minty!

I used a film cannister for a while before I got a cheshire.. Cheshire's are only about $50-$60 if you get them from overseas, or about $69 locally.

[RAJAH235]

Sorry, don't know exact set-up of t'scope. My Meade 10" doesn't have lock screws, thought the GS did. :o
Just the facts m'am.

[MintSauce]

I've not been away, only posting less - I've just not been using the scope much at all (still don't know many targets, I look at the same things each time with amazement).

Thanks for the tips, so if I centre spot the mirror then I can use a film can? (I've read about a pin hole in film can collaminating)

if I get a chesire do I still need to centre spot the mirror?

[MintSauce]

oh and the kid is fine, it's just the broken sleep thing I cannot cope with very well - should not last too much longer, he's 6 weeks old already

[janoskiss]

If you find a film canister that fits a 1.25" focuser, could you post the brand of film? I can't find any that fit.

[Starkler]

Yes you need to have a centre spot to use any collimation tools really, but if a star test looks ok you'll be alright for now.

[wavelandscott]

Yes, you will need to mark the centre of your mirror to collimate it with a collimation cap or cheshire...most folks do a "spot" (some already come that way).

I read somewhere on the net about someone who used a triangle (instead of a circle) aligned to the collimation screws...they claim it helped them speed up the process (interesting idea).

There are several opinions about the best way to mark the centre of your mirror, mine came marked so I don't have an opinion on this.

I did get a chesire collimating eyepiece (Orion). I highly recommend it as it works a treat for me...

Some people will recommend a laser collimation tool but, for me a simple solution is better. It does not get much more simple than no moving parts and no batteries.

The final check is a star test...

[MintSauce]

I'm the sort of person who wants to take it apart and figure how it works tho - even tho it's not broken. Same goes for the focuser - I want to dismantle and clean out that grease that I hear about, it is a little sticky.

So I may centre spot the mirror and try a film can, I really might just order a cheshire actually - anyone got and links to good sellers?

[silvinator]

My scope already came centre spotted as well but I did read somewhere about how to mark the centre of your mirror by folding a circular piece of paper the size of you mirror into quarters. You cut out a hole in its centre and then overlay it on the mirror. Place some sort of marker there (like a cirular sticker or something) and there you have it, centre spotted. But how to get the paper onto the mirror? :shrug:

[Starkler]

Centre spot

http://www.telescope-collimation.com/collimation-articles-mirrordot.htm

[MintSauce]

that is a point that I had thought of, what if my centre spotting is not very accurate :)

[MintSauce]

that link makes it look so easy !

[ving]

hey minty. if you want a cheshire try $45 from andrews com. I got one and it works a treat :)

[MintSauce]

"hi lee, I'm from ice in space, I keep recommending you - can I get a cheap tool please"

\o/

[MiG]

BTW, your title should be "newbie with APERTURE" not apature

[Thiink]

Sorry about bringing this old thread back, but I have a question. :P

I've just been using a film canister for the moment until I get a chesire, and have found that the only way to get a good look at where the centre spot is, is to point the telescope outside during the day. If I try against a lit wall at night, or by shining a touch onto the roof it doesn't work as well. Anyone have any tips?

Also after pointing it outside (through the window), checking colimation and then looking at the primary through the top, I noticed that the sun was pretty damn close to where I had the scope pointed (could almost see it around the edges) it left me wondering how long you could do something like that before the secondary ends up damaged from the heat?

I really need that chesire!

[slice of heaven]

Thiink. I use a 'clear' film cannister . I leave half protruding from the focusser .The scope I place under the light and aim it at the ceiling near the light, not at it. The light shining on the cannister gives you better contrast on the hole and the white ceiling gives a nice background. Check your collimation at vertical, horizontal and at 45 degs. Take note of any change.

ps : To check how central your hole is,rotate the canister after coll. and see if the hole stays central or if it has an orbit.

Slice

[gaa_ian]

Great article on centre spotting a primary mirror.
Sometimes the simplest things are the best !
It's been one of those questions in my mind too :confused:
I was thinking of measuring, etc ... So easy this way :)

[ballaratdragons]

Simon,

Pointing at or near the sun will not damage your secondary at all.

It WILL blind YOU however!!!!!!!! Permanently!!!!!!!

Avoid, at all costs, pointing anywhere near the Sun. When I do outside collimating before sunset I point in the opposite direction of the sun just in case I move the scope slightly while checking collimation.

You will only ever make this mistake once. Blindness is a reminder.

[Roger Davis]

It doens't matter if your centre dot is out by a mm. It's only there to guide you! Once you have performed the collimation you will soon figure out that the dot "ain't quite right". Make a mask out of a manilla folder, give it three tabs that can sit on the retaining clips so that it doesn't touch the mirror surface. It will look like a trefoil so you can match the circumference of both the mask and the mirror. NOTE: the human eye is very good at centralising, you can collimate a telescope quite well without any tools, close enough that you can finish off with a star test. Other tools are great, but not always necessary. Longer focal ratio telescopes are quite forgiving!