Cheshire collimation

[Andrew]

I wonder if anyone could help me with some basic collimation issues?

I have not been out with my scope for a long time for various reasons, but have a new enthusiasm for it and decided I ought to collimate my telescope since I have not done this for a while. I had previously used a laser but was never really convinced by this method due to uncertainty about whether the laser was properly aligned. Consequently, I decided to have a go with a Cheshire, and bought a very nice one from FLO.

I read Astrobaby’s guide to collimation, which was very helpful since I haven't done this for several years, and progressed through the various stages of adjusting the secondary, which only required some minor tweaking. I’m a bit puzzled about the adjustment of the primary, which I think should be simple. I have done this plenty of times before with a laser, but never with a Cheshire, which leads me to my question… I am trying to align the center ring on the primary with the cross produced by the Cheshire. However, when I look through the Cheshire I can see two sets of crosshairs. There is the obvious set which is just the wires at the end of the sight tube. However, I find it pretty difficult to focus my eye on these and on the ring on the primary at the same time, presumably because they are a long distance apart. After puzzling over this for a while I realised that I can also see a much fainter but sharper set of crosshairs on the reflection of the light from the lateral opening of the sight tube (I’m not sure if this is a reflection of the crosshairs at the end of the sight tube or from the cross that is cut into the oblique surface in the side of the tube where the light enters). These two sets of crosshairs don’t sit on top of each other, hence the ring on the primary can’t be aligned with both, and there is no mention of two sets of crosshairs in Astrobaby’s guide (unless I have missed it). I have the ring nicely aligned with the faint set of cross-hairs, but I’m not sure if that’s correct. I’m sorry to ask such a simple question, but having never used a Cheshire before I’m a bit puzzled about whether this is right and why I see two sets of cross hairs.

Thanks in advance for any feedback

Andrew

[Charic]

Andrew.......Hi, If your happy with your secondary mirror set up and you have a Barlow, use that with your laser? makes no difference if your laser is pointing left?????? it will still work if you barlow it. What happens is this. When the laser  beam passes through the Barlow, it spreads out all over the place. Good thing here is that is illuminates the whole of the mirror. This causes the centre spot/ring to be illuminated too, but in effect it creates a shadow, which is so clearly visible in/onthe face of the  angled Laser target face . Your task is to centre that 'SHADOW' around the hole in the lasers target face. Once completed, your collimated. The final check is then on a Star when the telescope is sufficiently cooled.

If you can get to this stage, the art of using the Cheshire may  become clearer, if the telescope is already collimated.?  sort of working backwards. Barlowed laser is a sound way for collimating the primary mirror.  If this does'nt help, maybe an image of your alignment may help us to see which way things need to move? And your right, there's alot going on- inside when it comes to collimating.

[Andrew]

Unfortunately, I don't have a laser (I last used it years ago and it died). I was going to buy a new one but decided to opt for the Cheshire instead!

[jetstream]

If you rotate the cheshire while looking through it, the stationary ones will be the spider vanes. Then the primary is adjusted to put the dot in the donut.

[Moonshane]

if the two sets of cross hairs don't line up then your collimation is out.

the steps are

get your secondary round and central in the focuser

centre the reflected donut centred on the fuzzy cross hairs (this aligns the secondary to optical axis)

centre the dot within the donut (this aligns the primary to the optical axis)

as you say, the dot is the hole you look down. the donut in point 2 is the primary central ring reflected in the secondary.

if you don't do it in this order you'll just get frustrated.

[Moonshane]

yes, indeed, there are also the vanes (to ignore) but it sounded like the OP had identified the two Cheshire cross hairs. my guide might help  http://stargazerslounge.com/topic/197640-collimation-and-star-hopping/

[Charic]

Moonshane.......You beat me too it.....was just checking for syzmons thread ( http://stargazerslounge.com/topic/224540-this-needs-collimation-right/ )  and a link to yours, to see if anything can help visually?

[Andrew]

Thanks for your help.

I have looked at your guide Moonshane. I see where I have been going wrong (maybe). Astrobaby's guide suggests adjusting the secondary so that the reflection of the primary is centered in the secondary, as judged using the mirror clips on the primary and getting these aligned equally around the edge of the secondary. There is no mention of adjusting the secondary to move the donut in relation to the Cheshire cross hairs. When I centre my secondary to fit Astrobaby's criteria the donut does not sit perfectly under the cross hairs (as recommended in Moonshane's guide). It does seem a bit odd that I get my primary centred using her criteria yet the donut does not line up. I'm not sure if that is because the mirror clip criteria is not a good one, or because something else is amiss with my setup. But I appear to have the secondary in the centreof the focuser.

[jetstream]

If there were a separate sight tube you just make the sec round,under the focuser and put the crosshairs on the dot ( with the 3 sec screws).

A separate cheshire shows a "dot" and a "donut", turning the mirror cell screws puts one in the other. Collimated then.

Your combination tool shows you the exact same thing, but has it all in there at once, so identifying the tools features is necessary before collimating IMHO. To avoid this confusion I bought a Catseye sight tube and cheshire, expensive, simple and works. FLO has them. The tool is the hang up for you, not your ability to collimate, the combo tools DO work though.

[wookie1965]

http://www.schlatter.org/Dad/Astronomy/collimate.htm

I find this guide in conjunction with Astro Baby`s help`s it did me, If Moonshane could either post his collimation notes or search them they are quite easy to follow.

[wookie1965]

This is the one

Collimation of Newtonian Telescopes.pdf

[Moonshane]

In essence ceentering the primary reflection in the secondary and centering the donut with the cross hairs is the same thing. It is the centering of the primary that usually align the two sets of crosshairs

With collimation just get it as good as you can and enjoy the views!