Collimation issues

[Craig20264]

Hi all, using Astrobaby's guide to collimation, Step 1 is to make sure the secondry mirror holder is central in the tube. Following the guide I have mrasured from all 4 points of the spider vanes to the central screw and it is spot on. However, when looking through the cheshire the crosshairs do not line up with the vanes. Which do I believe, the ruler or the cheshire?

Confused:icon_scratch:

[LukeSkywatcher]

A ruler can't lie.

[Tinker1947]

Is the secondary to far or not far enough down the tube...???

[Craig20264]

Trying to post a picture of my view through the Cheshire but my scanner is misbehaving

[Craig20264]

Here's my view through the Cheshire. Wonky drawing but you get the idea

[kniclander]

I suspect the x-hairs of the cheshire are not central. as I understand it, the cheshire x-hairs are not a key part of the process.

[Duns]

I've just sort of got the hang of this collimation game and as Astro Baby suggests don't use the Cheshire until you need to adjust the primary - it complicates things. I would use a collimation cap to get the secondary lined up and also use Astro Babys trick with the paper when working on the secondary - it greatly simplifies what you are looking at and what you should see. I found it helpful anyway - good luck.

[umadog]

No, no, no

First of all, a Cheshire and the collimation cap are basically the same thing. Some have a 45 degree window thing and others don't, but they all work in the same way. It all intents and purposes, a collimation cap with a silvered inner surface is a Cheshire. These tools are for adjusting the primary by looking at the reflection of the centre spot. A Cheshire is not the best the tool for adjusting the secondary. Neither, by extension, is the collimation cap. This is why the AB guide tells you ignore it until doing the primary--because you can only do the primary with it.

Cross hairs are part of a tool known as a sight-tube, which is often attached to a Cheshire. The cross-hairs are important and used to adjust the tilt of the secondary until they point at the primary centre spot. The extended tube is used as a guide to centre and round the secondary mirror as viewed from the focuser.

Your image suggests that you need to adjust the tilt of the secondary mirror right now as the primary centre spot isn't aligned with the cross hairs. The purpose isn't really to line up the cross hairs with the vanes. In fact, it's easier if you don't try to do that as it becomes harder to see the cross hairs.

Info: Rob Campbell's Home Page

Pay particular attention to the second and third links.

[LukeSkywatcher]

When i collimated my Heritage i used a colli-cap (very generously sent to me by a fellow member). Its just an old 35mm film canister cut to size with a small hole in the cap.

It was so simple to use. It aligned my mirrors to about 95-98% (as my eye can tell). Not sure i needed to do it as the scope was brand new but i did it anyway as a learning process.

[Jason D]

However, when looking through the cheshire the crosshairs do not line up with the vanes. Which do I believe, the ruler or the cheshire?

Confused:icon_scratch:

Ignore the spider vanes reflections when collimating. A simple and basic rule.

Most scopes will not have their spider vanes reflections line up with the sigh-tube cross-hair

Jason

[Craig20264]

Thanks for all your input guys, as usual all good advice. I think I've got my head around it now, (we'll soon know in an hour or so!). It's like everything I suppose, once you've done it a couple of times it becomes second nature.

I'll shout up again if it's not right

Thanks again