Collimating Advice - 130SLT

[DarrenG]

Hi all,

Invested in a laser collimator for my 130SLT and was surprised when I used it to find the alignment appears to be miles off.

Currently the laser is hitting the primary mirror about 3cm's from the outside edge!

The instructions supplied with the collimator don't make a great deal of sense to me but I think I need to be adjusting the secondary mirror initially, is that correct?

Any advice on how best to proceed, so I improve the alignment rather than make it worse, would be appreciated!

[AstroTux]

The first thing you should check is that the laser collimator is itself collimated. This may sound strange but quite often they are shipped with the beam of the laser not coming out in line with the body. There are many posts on this in this forum and a recent one showed how to make a cheap V-block with a piece of wood and some nails http://stargazerslou...hl__ collimator .

After you know that the collimator is collimated then you can try and collimate the 'scope. Again there are many threads on this subject and an excellent guide written by Astro-baby at http://www.astro-bab...ation guide.htm .

Hope these help..........

[DarrenG]

Thanks, first hurdle is the collimator I bought has no adjustment at all, so a lesson learnt there! I'll have a good read of the guide before I wade in with my size 10's

[DarrenG]

Strike that, reading another post here on lasers and noticed that mine does have adjustment screws, they were hidden under black silicon sealant which looked like part of the housing.

[umadog]

Hopefully this might help: http://www.physiol.ox.ac.uk/~raac/collimationLinks.shtml

[DarrenG]

Thank you. I made what was doubtless an idiotic decision to remove the secondary and the spider 'before' I realised how much trouble this would cause with re-alignment.

So I've jumped straight in at the deep end and spent yesterday re-aligning the secondary as best I could. The good news is I now have the laser collimated and then the secondary and spider re-fitted and aligned and all now seems a lot better. I don't appear to have caused any major damage as I was able to view Jupiter for the first time last night and could clearly make out the bands and it's moons using the supplied 9mm eyepiece.

I'm going to invest in a cheshire and a collimation cap in the near future and check everything is OK and I've learnt a lot about the whole alignment procedure so it's not all wasted!

Great forum btw, as the owner of a large GPS forum it's great to find one where there is such a wealth of help and information on a topic where I'm a complete Newbie!

[umadog]

Nothing wrong with removing your spider. Good learning process.

The "Cheshires" you buy are actually a combination tool comprised of a Cheshire and a sight-tube. If you buy one of those then you don't need a collimating cap, since it's just a simple Cheshire. In other words, the combination tool will do what the cap does and more: The sight-tube portion (cross-hairs) measures secondary tilt and aids in rounding the secondary. The Cheshire portion measures primary tilt. Those two tilts are what you need to correct in order to achieve axial alignment of the optics. Those are also the two tilts your laser measures:

1. sight tube = first pass of laser = secondary tilt.

2. cheshire = second pass of laser (or, better yet, barlowed laser) = primary tilt.

It's worth owning the combo tool since the laser won't help you round the secondary or centre it in the focuser. Also, the combo tool is more robust and doesn't require batteries. However, it's probably easier to read the laser when adjusting secondary tilt than to read the cross-hairs.

[DarrenG]

Excellent, I'll get one of those ordered pronto then. Should have done a little more research before diving in and buying the laser.

[DarrenG]

The Cheshire tool arrived this morning and after an hour of adjustment I'm now happy with the alignment of the secondary mirror. Getting that correctly aligned is a real pig! You need three hands to turn the adjusters, hold the mirror, slacken the central screw etc!

But it's all now aligned and I'm much happier with the result than I was with the laser.

All I need know is some clear skies!

[DarrenG]

Having read a ton on this subject, and seeing the tips on plastic washers on the secondary, I ordered a set of M4 Thumbscrews from 365Astronomy (these) a few days ago.

They arrived this morning so I set to work, disassembled the secondary, inserted two washers cut from an old milk bottle and re-assembled with the thumbscrews.

Aligning the secondary is now so much easier. The mirror can rotate smoothly without the grub screws catching, and the thumbscrews mean it can be aligned much more easily.