Advice on Collimation

[Fish]

New to collimating Im checking my 200mm f6 Dob, I have read most things on the net including Astro Babys guide and as allways with

too much information I have ended up confused over some points.

To start off I made a collimation cap and checked it with the skywatcher user guide and all looks good with the alignment.

I did a star test (not been exacltly good seeing recently) and again all looks good , even compared with my Mak's.

Decided that I would make a laser collimation tool myself to help in setting up the primary on my own in the future.Made the tool

and set up the laser (collimated) then inserted in the scope, the fit is exact no movement in the focuser and as a final check

rotated it in the focuser and the laser dot on the primary does not move . I was expecting the laser on the primary to be ok but its

about 10mm away from the center of the mirror , the light does bounce back up the focuser and again is slightly off from the center

hole of the tool.

My problem is that I have read that lasers can lie, but I dont understand how reflected light that you know comes from the center of

your focuser and reflects off one mirror can be wrong.

I have only had chance at the moment to look at the moon because of work etc and would like to make sure the scope is at its best

for later on in the year.What do I do ? trust the laser and adjust it or trust my inexperienced eyes (my gut feeling is to go with

the laser ) Any advice on which way to proceed and more info on lasers lie needed.

Thanks.

[Peter Drew]

A common dilemma . I've collimated telescopes for over 50 years, long before lasers were available for the job, with nothing more than a 4mm eyepiece with the lenses removed. With the image of the primary central to the secondary and the spider and secondary image central I've always found this to be close enough to do any final tweaking on Polaris. I've also used a well adjusted laser to perform the same procedure but on double checking with the eyepiece I've often found that although looking good with the laser, the eyepiece view appeared to be incorrect, so, as you say, which do you believe?. My eye has always served me well for this task, I do find though, that the laser is a good visual help for starters.

[Fish]

Thanks for that. In that case I will set it up with the lazer then see if I can see anything diffo by eye .

[earth titan]

Have you looked at using a 'barlowed laser' for collimation? Works a treat and doesn't need a well collimated laser.

Also, from your post, you said the laser dot is not hitting the centre of the primary. In which case, this suggests secondary is not true. Adjust the secondary until the laser dot hits the centre of the pimary then adjust the primary until the dot falls back in the hole on the laser. From here I use my barlowed laser to get it spot on(ish).

Another thing to check, if using a laser, is the centrallity of the donut on the primary. On 2 out of the 3 newts I've owned, this has been offset straight out of the factory and in one case (my fast f4 newt) by as much as 3mm. This seriously throws it all out of alignment on a faster scope.

As Peter points out, a star test is the only real way to do it.

[Fish]

Thanks again for the help, with regards to the barlow if I read it correctly you actually use it for primary adjustment and you align the shaddow of the doughnut .

Decided to adjust it tonight, had another look through the collimation cap first, set up laser and adjusted the secondary so that the unbarlowed laser hit the center of the mirror. The laser light then went straight up the hole in the alignment tool. Then I introduced the barlow and it appeared that the shaddow of the doughnut is on the hole on the collimator. Nothing but cloud tonight so will have to wait to star test.

This is the first time I have ever done this and looking at the laser reflection it looks like its a long way out but in reality its nothing, I hardly moved the screws and it was spot on . Had another look afterwards through the collimation cap and to be fair I can not see any difference may be that is inexperience .

Time will tell.

[earth titan]

Sounding good. If you still have problems,check the centrallity of the donut. I know whipping the primary put sounds scary, but it isn't a hard job.

[Jason D]

My problem is that I have read that lasers can lie... Any advice on which way to proceed and more info on lasers lie needed.

Thanks.

There is a single source for the above claim and it comes from a misleading video on youtube presented by someone who does not fully understand collimation.

Lasers do not lie.

There are 3 collimation alignments:

1- Centering/rounding the secondary mirror under the focuser

2- Focuser axial alignment

3- Primary axial alignment

Lasers do well on the 2nd and 3rd alignment but poorly on the 1st alignment

Collimation caps do well on the 1st and 3rd alignments but poorly on the 2nd alignment.

In your case, you used the collimation cap to collimate your scope but the collimation cap did a poor job on the 2nd alignment. Then you inserted the laser and the laser actually caught the poor 2nd alignment. See attachment.

To solve the problem, you need to iterate between both tools until both agree.

You can read the first page of the following link

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/reflectors/Number/3532750/page/0/view/collapsed/sb/5/o/all/fpart/1

And the following post

http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/4242330/page/0/view/collapsed/sb/5/o/all/vc/1

Jason

[Fish]

Jason thank you for that info, those links will need to be read a few times to be fully understood. I'm now quite happy with the alignment for a first attempt and will spend the summer months reading and tinkering.

Im glad lasers reflected off mirrors dont lie

Thank you all for the help....