Collimation tip

[Moonshane]

Every now and again I do a full collimation (other than focuser alignment and vane centralisation as these never really change once set) of my scopes and was reminded of something that many people may not appreciate.

The faster the scope (i.e. f5 is faster than f8) for the same primary size or the larger the primary, the larger the secondary mirror and the more unlikely this is to be seen fully down a standard Cheshire eyepiece (technically a cheshire - sight tube combination). This makes the first main step of collimation quite difficult and is one reason a collimation cap is often recommended. The first step is to ensure that the secondary mirror presents a round face (shiny surface) to the focuser and is central in the drawtube.

As mentioned above the faster the scope the less likely it is that you'll see the whole of the secondary with the focuser as it is.

The easy and not necessarily obvious solution to this is to add an extension tube to the focuser and if necessary pull the cheshire out until you can just see the secondary edge.this makes the centralisation process a whole lot easier and really negates the need for a collimation cap. I find this works well with my f4 scopes and the opposite is true (i.e. in my 6" f11 newtonian I have to remove everything and have the focuser in as far as possible to get as close to the secondary as possible and even then it's not a 'tight fit').

You can then follow the rest of the standard collimation procedure all over the web. Generally though the above centralisation is only necessary once when a scope is rebuilt but in general I always do the following before each session:

1) check the centralisation and rotation of the secondary - not usually out - no adjustment needed.

2) check that the secondary is tilted correctly (i.e. that the reflected primary donut is centralised with the out of focus cheshire cross hairs) - not usually out - no adjustment needed.

3) check that the primary donut is centralised on the central hole of the Cheshire - this usually needs a small tweak.

I hope this helps people out there understand that collimation all the way through can actually be done with just a Cheshire, at least to an accuracy where any difference does not matter.

[kirkster501]

A great post. I agree, a Cheshire is such a simple and superb piece of kit and costs little compered to many other astro pieces of gear. You can collimate Newts or RC scopes with one just fine. Lasers can overly complicate things I think. Lasers have their uses but the good old Cheshire and a little judgement is a winning formula.

[Moonshane]

one thing i don't think you can do with a laser is centre and correctly rotate the secondary?

[faulksy]

any tips on sct collimation shane, or have i got to wait untill we meet up

[Moonshane]

hi mate. I am pretty clueless to be honest. never had one nor will I ever get one probably. (not for any reason specifically, I just find newts do everything I ever need).

[umadog]

one thing i don't think you can do with a laser is centre and correctly rotate the secondary?

Not unless you have a "holographic" laser: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/1,2,3,4,5,8,10/Board/reflectors/Number/4494930/page/0/view/collapsed/sb/7/o/all/fpart/1

Frankly, looks like more trouble than it's worth.

[Jason D]

Not unless you have a "holographic" laser: http://www.cloudynig...7/o/all/fpart/1

Frankly, looks like more trouble than it's worth.

Rob, it depends on the individual. As someone who centered/rounded my secondary mirror using several methods, I found the holographic method to be least confusing because:

1- There are no confusing reflections

2- It is much more relaxing to work at the OTA/UTA opening without the need to look through the sigh-tube and reach out to the secondary mirror adjustment screws. There is less likelihood to drop tools down the OTA. Besides, for large scope it would be almost impossible to look through the sight-tube and reach the secondary mirror.

3- I found out figuring out what adjustments to make are more intuitive. The holographic laser pattern movement is easily correlated to adjustment screws.

4- Using the method I described in the above link, you can do all 3 alignments at once: Secondary mirror positioning, focuser axial alignment, primary axial alignment.

The overhead to the mentioned method is the link is come up with under-the-focuser template (see attachment). One idea I have been entertaining is convincing premium scope makers to etch a similar custom template at the bottom of the focuser (the inside wall of the OTA/UTA). I highly doubt mass produced scope vendors will be interested.

For those who mastered the art of using sight-tubes will most likely find the holographic method more complex. It all comes down to personal preference.

Jason

[umadog]

I agree, with an etched template this method would be very tempting.

[Moonshane]

here's another couple of tips that just popped into my head.

1) Umadog recently mentioned that the main issues with scopes going out of collimation during observing sessions relate either to the mirror cell, which I had appreciated and also the secondary spider which in truth I had not. I had forgotten about a minor issue with my big dob (16" f4) whereby the collimation does shift slightly at low elevations - this is seen as a little more coma at the eyepiece. I have tried to resolve it but visually there's such a small difference that I had effectively given up.

I should have realised that the combination of thin walls on the OOUK tubes, thin spider vanes and a somewhat heavy secondary could be the cause. Due to the thin walls I had never been keen to over tighten the spider vanes but a little tweak all round seems to have sorted the above issue. So if you have this sort of problem and your cell looks OK, check the tightness of the secondary nuts. In the future, I am considering making a new spider with curved vanes so this might actually provide diffraction spike free views and also a more robust secondary attachment.

2) If when trying to adjust the secondary tilt you feel that you are getting nowhere and it seems that no matter how far you go, you cannot get in position, it may be that your first adjustment (rotation and position in the drawtube) is slightly off. You may find that a slight rotation of the secondary will allow a much easier centralisation of the primary reflection. This part of collimation can lead to a lot of frustration and this may well be the cause in a lot of cases.