Collimation question

[slowflow (Barry)]

I have colluminated with a cheshire & every thing lines up exactly, however when a laser tool is used it shows the collumination to be incorrect. I have rolled the laser in vee blocks and the image does not move. Due to cloud there has not been an opportunity to use the scope. The telescope is a 12" skywatcher. Thanks in advance.

[barx1963 (Malcolm)]

Barry
Firstly, the correct spelling is "collimation".
Next, what is the error and how big?
Usually with a laser, you first check that the beam is hitting the centre spot on the primary. If the Cheshire is done correctly, it should be very close. Tweak the secondary to centre it. Then check the return beam against the target on the collimator and tweak the primary to centre.
Unless the error is huge, I wouldn't worry to much.

Malcolm

[Moon (James)]

The correct spelling is collimation

[Varangian (John)]

Personally, I would shelve the laser and go with what the Cheshire is indicating. This is what I did a number of months ago with my 12 Dob. I never could trust the laser. You don't need to he exact either!

[Allan_L (Allan)]

Hi Barry,
Yeah thats a tricky word isn't it. I had trouble with it myself for a while.

OK. Unless you have a very expensive laser collimator, they can tend to be a bit sloppy in the draw tube.
Then the way you secure it (with the two or three grub screws) can effect the collimators central position and/or way the laser is aimed.
Depending on the size of your "error", this could account for the difference.
So just try adjusting the relative tension on the drawtube screws.
BTW This can be further exacerbated on the skywatcher because the eyepiece holder itself is also held tight by grub screws as well. (Well mine is anyway - 10" flex-dob)

Just a thought.

[slowflow (Barry)]

Thanks for the help.
Copy paste from CNs, bugger.
One layer of tape makes the laser tight in the tube. The difference is almost the radius of the laser.
Allan, it is sloppy engineering all-round as screw tension will shift the laser. I keep the screws tight to minimise the error.
John, the old fashion way may be best.
Malcom. I will check the top mirror too.

[barx1963 (Malcolm)]

Barry
Have you checked the secondary's alignment with the Cheshire? I am assuming it is a combined Cheshire and Sight tube? If not checked make sure the secondary is centred under the focuser and it looks "circular" in sight tube.
The thing to remember is that spending lots of time getting collimation exactly right is often not time well spent. Unless you are imaging it is really hard to tell the difference in the view from a perfectly collimated scope to one requiring a slight tweak. You are usually better of using the scope than spending hours collimating it IMHO.
So if the cheshire look good, that should be OK unless you are unhappy with the views!
With regard to your comment regarding engineering sloppiness, tightening the screws will shift the laser, but you would have tightened them with the Cheshire and also on the eyepieces you use so effect should be the same.
Malcolm

[slowflow (Barry)]

Thanks Malcom,

Yes the secondary appears circular. Visual use only.
I had notices out of focus stars were not producing round images which must have been caused by rolling in & out of the shed. The eye pieces have less slop. As soon as cloud permits I will check it then adjust the laser to maintain consistency.

Barry

[erick (Eric)]

I use laser to check everything is not grossly out, then refine with cheshire and autocollimator.

On a different, but related matter, how are the primary mirror support springs? If they are a bit weak, the mirror can move/sag and collimation change as you elevate the tube. Either replace them with firmer springs (Bunnings) or screw all three collimation screws up tighter to put the springs under greater compression.

[Moon (James)]

Copy paste from CNs, bugger.

I noticed that too. I really hope this CN version of the spelling doesn't take off. Can you edit the post title to fix it up? Hopefully it will then drop out of the google index and won't become a 'thing'.

[GeoffW1 (Geoff)]

Originally Posted by Moon

I noticed that too. I really hope this CN version of the spelling doesn't take off. Can you edit the post title to fix it up? Hopefully it will then drop out of the google index and won't become a 'thing'.

What is it, a new CN term meaning "illuminated collimation" or something?

[Moon (James)]

No, just a simple misspelling.