Collimating

[E_ri_k]

Hey guys. Again I have another question about collimating my 10" Newtonian....

I am using a Glatter laser and TuBlug which are working great.

I notice that when the scope moves, the collimation shifts, usually quite a bit. You can hear the primary mirror shift around sometimes and "clunk", when moving from on part of the sky to another.

Is this normal? Should I tighten things up a little. Obviously there has to be a small clearance between the mirror and the clips, but I'm not really confident to remove the mirror from the OTA to check, in case I stuff something up...

Thanks.

[bmitchell82]

The mirror shouldn't "Clunk", but in saying that they are a Floating mirror setup aka, its not stuck down!

The mirror shouldn't be touching the clips at all (you will induce astigmatisim if its touching) and when it is mounted it will sit down in the mirror cell so it isn't required. the clips are to stop the mirror falling out and down the tube.

Do not be affraid of the OTA it is simplistic in design with nothing to stuff up that you don't already move around! With the collimation tools it is a matter of once you put it back together align the dots/reflections

[E_ri_k]

Ok, thanks for the info and re-assurance. I might take it out and have a look at it then too see what is clunking then. Surly it shouldn't move that much?

Could you place something soft, like foam tape between the clips and mirror, to reduce movement, but at the same time not applying pressure to cause astigmatisim?

It seams like when imaging I should re-collimate between long subs because the OTA has moved, and thrown out the collimation? I don't do this because of the hassle, but surly it shouldn't have to be done?

[Barrykgerdes]

The small amount of distortion that can occur if the mirror clips are tight enough to stop the mirror moving around on its carrier are much easier to live with than a mirror that slips when you move the telescope. If the mirror moves you are never going to get it collimated.

Also the collimation is not complete when the laser beam returns properly this is only the start of collimation. The real collimation starts when you do the star testing part.

The aim of collimation is to get every point on the mirror surface to focus at the same place. This requires very careful movement of the primary mirror mainly in distances less than the wavelength of light

Theirry Legault did an excellent report on collimation. Everyone should read it before starting a collimation.

Barry

[bmitchell82]

This is my opinion and I will state it because personally I don't agree with what Barry has stated. If im wrong please let me know as its a fundamental error in my knowledge other wise and I have been sending alot of people up the bum track.

minor distortion in the mirror is absolutely detrimental to the proformance of the mirror in regards to photography! in visual it matters very little.

I have always had a fondess to hate laser collimators no matter how much you spend on them as you forget about what it is that your telescope is capturing.... light! Understand the light ray path and you will very quickly understand collimation!

I will pit my autocollimator and cheshire over a laser any day for accuracy in collimation and to point for newtonians even star testing! Why? well because seeing is rarely good enough to determine the airy rings, and its a long laborius process only to have the mirror move on you in transport. Sure it may have merrit in a observatory but not in the setup every night crowd.

I will how ever agree whole heartedly Barry that the point of collimation is to get every point of the mirror focusing on the imaging plane that is Square to the top of your focuser drawtube. But thats only 1 of 4 different errors. :) of which you need to correct 2 out of 4 for perfect collimation.

The laser Erik owns corrects two but not at high accuracy.

Brendan

[multiweb]

Any pressure even slightly applied on a mirror whether it's mirror clips on a primary or glue on the back of a secondary holder will introduce aberrations such as astigmatism and other deformations in star shapes. So I'm with Brendan on this one. Pack, shim all you want but don't tighten. Use cork or paper not aluminum. Things move with temperature and need a bit of room to expand and contract. Cork is good.

[E_ri_k]

Ok, thanks for the advice guys. I will pull the mirror out tomorrow and check the clearance between the clips and mirror, and perhaps pack the gap out with something if needed. I'll let you know how I go.

Erik

[Robh]

Also check how strong/tight your primary mirror cell springs are.
If they are weak or not compressed enough, your whole mirror assembly may be shifting when moving the scope from horizontal to vertical and back again.

I have a 12 inch Meade dob and ended up cutting heavier springs for the mirror cell as even replacement Bob's Knobs springs just weren't heavy enough.

Regards, Rob

[Barrykgerdes]

I dont think you read what I said. I did not say tighten the mirror down I said firm enough to keep the mirror in place. Yes any pressure on the mirror will make distotions an appreciable portion of a wave length but you must keep the mirror stable first and if you don't get concentric diffraction rings on a star test you can not say your scope is fully collimated. There is no near enough about it!

and make sure you read Thierry's paper. I have it in MSword format if you can't find it.

Barry

[bmitchell82]

Im going a little by reading a fair bit of physics back ground and more by actual doing and experience.

To stop the mirror moving you have to touch the mirror. With my 10" newt what actually happend was i used tissue between the mirror and the rubber mounts to make sure that they didn't touch the mirror.

While collimating it distorted the primary mirror cell enough to touch the rubber mirror clips found on SW newts. End result = Oblong stars that nobody knew was the cause for 6 months untill a 5 minute conversation with somebody who had seen it before!!!

Understanding the light ray paths is paramount if you want to know what is happening and diagnose problems swiftly.

my best advice would be to loose the clips all together and replace it with small dots of silicone at the 60% zone with match stick gap between the cell and the mirror back. This does 2 things, gets rid of the mirror clips and holds the primary in perfect position without movement. Oh and a bouns increases your resolution and diffracted light energy!

As for Springs, go to bunnings it will cost you about 4 dollars each they are slightly longer but far more stable!

Brendan

The diffraction rainbow in the major stars are all concentric, the angle in the differentiation between the rainbow colours has more to do with 100% illumination than collimation.

Here (http://brendanmitchell.net/wp-content/uploads/2011/05/NGC-6729-120_25_25_30.jpg) is the image

[GeoffW1]

Hi,

Would a mass market primary dob mirror be so well figured you would then start worrying about finger tightening mirror clips that had a cork insert in them (that's what was in my Bintel 12" dob), as thick as the mirror is? I don't know, just asking.

Cheers

[mill]

That is what i did years ago and it works perfectly.
My dob held collimation for very long.

[bmitchell82]

Geoff, my 10" mirror is approximately 30mm thick, and had rubber clips yes it will push the mirror out of figure. very subtle you cant see it happen but you can very well see it!

[E_ri_k]

Ok, so today I pulled the mirror out to see what was going on. Found that the mirror was free to move around the cell in a circular pattern, with about 1mm gap between the mirror and the cell. The rubber clips were also quite loose.

I put a bead of silicone around the cell and replaced the mirror, also tightened up the rubbers, but they are only resting on the mirror, not pushing down, they can still move freely when wiggled.

Did a rough collimation in the daylight, and tweaked the secondary mirror a touch.

So went out tonight and collimated properly using the Glatter Laser collimator.

I'm pretty happy with the silicone, now when I can move the scope in almost any direction and the collimation stays spot on, according to the laser, nothing moves. Before when I used to move the scope, you could see the laser spot shift.

Tried to get some photos of the star test, but they are not fantastic. Visually everything looked really good except for a bit of "wobblyness" around the edge of the star.

Attached are some photos:
Focused out on star
Loose rubber clip
Loose rubber clip
Tightened Rubber clip
Focused in
Focused in

What do you guys think?

Erik.