Collimation question

[04Stefan07]

My laser collimator arrived today and I was watching a tutorial on how to properly collimate the telescope. The red dot was just outside the circle on the primary mirror so I adjusted it to be in the dot then aligned the primary mirror according to the red dot in the collimator.

I found a picture online to explain the problem I am now having (pic below).

http://i25.tinypic.com/2ep340g.png

The black section on that diagram, that is exactly what mine looks like when I look into the focuser. It looked like this before collimation and after I collimated it.

Is my scope properly collimated or is there something wrong with it? :shrug:

Thanks!

[04Stefan07]

Here is an image of my own one.

Also to note according to the collimation I did, the laser says it's bang on centre.

[Allan_L]

Hi Stefan,
I have found this article to be of help in my collimation.
http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide .htm
Hope this helps.

Also note, some collimators themselves may not be perfectly collimated.
There are articles on IIS to identify and rectify this.
For example: If you hold the collimator steady (eg in eyepiece holder) and rotate it 360 degrees, does the red dot stay centred?

[04Stefan07]

Great article thanks.

Not sure mate will check later.

[barx1963]

Stefan
Are you talking about your 130mm newt in your sig? if so I think it is an f5?
The pic you mentioned shows "offset" when using a Cheshire collimator. Offset is needed aspecially with fast scopes to ensure the secondaty intercepts the light cone from the primary without losing any light. It is certainly a good idea in scopes faster than f5, but for a 5" f5, probably not worth worrying about.
The other question is how are you doing collimation up to the point of using your laser?

Malcolm

[04Stefan07]

Hey Malcolm.

Yeh it is the 130 SLT in my sig.

Steps I took:
1) Put collimator in the focuser and check where on the primary mirror it hits
2) Adjust secondary mirror's screws with an Allan key and make sure the red dot is in the centre on the circle on the primary mirror.
3) Check where the red dot is on the collimator and adjust the primary mirror until the red dot is bang on in the middle of the collimator.

Funny thing is when putting the collimator in the eyepiece and tightening the screws to hold it in place because of the tiny wiggle room depending how you tighten which screw first the red dot will change position. So for example if you put the collimator in and align the primary mirror, take the collimator out and put it in again it will be unaligned depending on how you tighten the focusers screws.

[Jason D]

That illustration in your link is mine (just check my avatar). The secondary mirror silhouette (shadow) will look shifted as the illustration shows for all reflectors though the shift will be more prominent for faster scopes.

If you are interested in knowing the reason, it is because you see the secondary from two difference perspectives. The first perspective is from the focuser side looking at the secondary mirror directly – the secondary will look concentric. The second perspective is from a virtual point behind the primary mirror – the secondary will look shifted. Check attachments.

You can never have the secondary mirror look concentric from both perspectives -- only from one.

I am also including a photo from my collimated scope.

Jason

[04Stefan07]

Third image is what I was after!!

Makes much more sense now why it looks not centered.

It was right all along, thanks!!!

[jsmoraes]

i find another issue after all : flexion

if you change the position of OTA in DEC and RA you will see the laser dot change the position over the center circle of primary mirror. At least it does with my GSO 12" on NEQ6.

i always check the better positon of primary with star test.

[SingleMalt]

A bit of a thread revival here, but the original post seemed somewhat relevant to me...

I used Astrobaby's guide and a cheshire to collimate my scope last night. Was a bit of a painstaking procedure, especially getting the secondary right, but finally I thought it was good to go. I threw my glatter laser in just as a cross check. In the tublug, the primary appeared to be perfectly aligned, but when I checked the alignment of the secondary by observing where the red dot fell, it was maybe half a cm from the center of the mark.

I ignored that and used the scope anyway, the collimation seemed pretty good (saw the Cassini division in Saturn's rings) although a few stars had a "cross" of light coming from them - Astrobaby's guide called this "astigmatism" and blamed it on bad collimation of the secondary (I spent ages on that #@&*ing thing!!!) or on poor quality optics (even worse).

Anyway, my question is, if you collimate a scope using a cheshire, should the collimation appear spot on when you check it with a laser, and vice versa?

Thanks guys!

[Jason D]

Check my diagrams on post #7 of this thread. That is expected. The laser should not hit the geometric center of the secondary mirror.



Are you sure you are not referring to the diffraction spikes? If your scope have spider vanes then bright stars will show "X" or "+" spikes coming out of the star. That is normal.

Jason

[SingleMalt]

Hi Jason,

Thanks very much for your reply. I saw those images and that's what I saw when collimating without the laser, I just didn't follow that thinking through to the logical conclusion that the laser dot would appear offset.

As for the diffraction spikes... to be honest, that's the first I've ever heard of them. It makes perfect sense, but that's just something I haven't come across in my reading. I'm only very new and I'm self taught (well, self and internet taught) so I guess I've missed a few things!

Thanks again for your time and patience. Cheers!

[killswitch]

If your focuser uses two screws to hold the EP/collimator in place, keep one screw completely static and only use the other to loosen and tighten. This should make it a bit more consistent.