Collimation hassles - Hotech Laser V Cheshire

[Spaced Out]

Hi All

After wrestling with a Cheshire for over a year trying to learn to collimate accurately I’ve just crumbled and bought a 2^nd hand Hotech laser for collimating my newts. First time I used it I couldn’t believe how easy it was, I barlowed it too and that seemed to work fine.

I was starting to put my trust in the laser until today when I decided to compare methods on a different telescope. I checked the secondary with a collimation cap in a sight tube. I got the secondary perfectly circular with the mirror clips equal on the edges, then I used a Cheshire to collimate the primary the best I could. Next I attached the laser expecting it to put a dot smack in the primary donut, but it was off by a couple of cm ? So I did it all over again but got the same result.

Next I tried using the laser to collimate the secondary instead (putting the dot in the middle of the primary donut) but when I checked the results with the collimation cap the secondary mirror was clearly out with only 2 primary mirror clips visible. I’m guessing the laser isn’t the best tool for that job then ?

This has left me scratching my head. Not wishing to start a “which is best Cheshire or laser” discussion, just wondering if anyone has any insight into what is happening here ?

I assumed the Hotech is well collimated, they seem to have a good rep and it seemed to do the job on my other scope reasonably well. However, I did notice that it wasn’t always consistent, I could take it out and then replace it and the dot would be in a slightly different place, I assumed this is just down to my rubbish sloppy focusers rather than the laser itself ? I have also slowly rotated the laser in the focuser and the dot doesn’t move on the primary, so I’m assuming it is straight ?

If I have my secondary collimated as well as possible with a collimation cap, shouldn’t the laser dot then sit inside the donut on the primary ?..... Or am I missing something ?.... Any advice welcome !   

[Tim]

A laser is a great quick check tool, for basic collimation. However it is entirely possible to have a near perfect reflection with the laser and yet have quite poor collimation, due to the secondary mirror being kind of rotated. It is easier to check and adjust the rotation of the secondary in a chesire, so always worth going back to that every so often and just making sure you haven't been gradually mis-aligning your mirrors with the laser.

 

[cloudsweeper]

I was tempted by a laser, but got a Cheshire and an Aline Cap instead.  Now, I don't even use the Cheshire - I do it by eye, then fine-tune with the cap.  The results speak for themselves - good sharp images/views.

Anyone want to buy a Premium Cheshire EP??

Doug.

[Ricochet]

Which scope is it? It could be that when you tighten the screw(s) on your focuser it tilts the laser or Cheshire slightly. When you remove and reinsert the laser it could be tilted by a different amount and hence the laser dot is in a different place. Lasers seem to be more sensitive to this sort of tilting. 

[Spaced Out]

14 minutes ago, Ricochet said:

Which scope is it? It could be that when you tighten the screw(s) on your focuser it tilts the laser or Cheshire slightly. When you remove and reinsert the laser it could be tilted by a different amount and hence the laser dot is in a different place. Lasers seem to be more sensitive to this sort of tilting. 

I've got skywatcher 130 PDS and 200 PDS, both have fairly floppy focusers to be honest ! However, the Hotech laser is self centering so you don't use the screws, while the cheshire uses the screws, so maybe that throws in some element of variation.

I'm concerned about the fact that when the secondary is looking perfect through the collimation cap, the laser looks about 1cm + out of the primary donut ?! Is that normal ?

 

 

[Ricochet]

14 minutes ago, Spaced Out said:

I'm concerned about the fact that when the secondary is looking perfect through the collimation cap, the laser looks about 1cm + out of the primary donut ?! Is that normal ?

I don't know if you could describe it as "normal" for those particular scopes but I would say that it isn't right. Both tools should agree. I know the laser is supposed to be self centring but if the laser changes position between reinsertions something is going wrong. If the eyepiece clamp is quite short that could stop the self centring mechanism registering properly and cause tilt. Perhaps try using the eyepiece clamp rather than the self centring and see if that makes a change.

Given the popularity of the scopes I'm sure someone here must have experience with the same setup and be able to offer some insight.

[spillage]

Have you removed the primary mirror and checked that the donut is centered correctly?

[Spaced Out]

9 hours ago, spillage said:

Have you removed the primary mirror and checked that the donut is centered correctly?

Very good point, I hadn't considered that ! No I haven't, I'll check it and report back.

[JOC]

I've tried a laser and in the finish bought a Cheshire and then follow Astrobaby's collimation guide absolutely to the letter.

[Spaced Out]

16 hours ago, spillage said:

Have you removed the primary mirror and checked that the donut is centered correctly?

Well the donut seems centred OK so put the primary back in and decided to loosen off the secondary and start again from scratch.

I collimated it really carefully using a cap and a cheshire, then tried the laser and it looks OK this time ?! I can only imagine my collimation skills need more practice. perhaps my secondary wasn't as well collimated as I thought originally !

Anyway, it seems sorted now and I am not touching the secondary again !

Thanks for all your advice.

[Jason D]

To OP,

Check this post

and the next post 

https://stargazerslounge.com/topic/186945-collimation-woes-laser-vs-cap/?do=findComment&comment=1946524

Jason

[Jason D]

23 hours ago, Tim said:

A laser is a great quick check tool, for basic collimation. However it is entirely possible to have a near perfect reflection with the laser and yet have quite poor collimation, due to the secondary mirror being kind of rotated. It is easier to check and adjust the rotation of the secondary in a chesire, so always worth going back to that every so often and just making sure you haven't been gradually mis-aligning your mirrors with the laser.

 

Hello Tim,

With proper knowledge, quality laser collimators are great and accurate collimation tools.  The rotational error you have described is hardly noticeable at the eyepiece for visual observation. It does not impact coma or focus but rather it has a "theoretical" impact (that is hardly noticeable visually) on the distribution of field illumination within the FOV.

It should be known to any laser collimator owner that laser collimators can only used to achieve axial alignment -- when the eyepiece axis points at the primary mirror center and the primary mirror axis points at the center of the eyepiece center. Unless the laser collimator is equipped with a holographic attachment, it can't be used to optimally center/round the secondary mirror under the focuser simply because a typical single beam laser collimator does not interact with the secondary edge.

Jason