Cheshire Collimator

[Naemeth]

Long tube or short tube, which is better?

[ObscuredbyClouds]

Maybe im wrong but cant see if it will make any difference tbh, im probably going to get corrected though by someone. What you waisting money on them for when you could make one out of a film cannister?

[andrew s]

I don't think the length of a Chishire makes any difference as long as it fits well in the focuser. A sight tube is better to be long so there is some distance between the eye hole and the cross wire.

Andrew

[umadog]

Maybe im wrong but cant see if it will make any difference tbh, im probably going to get corrected though by someone. What you waisting money on them for when you could make one out of a film cannister?

You will It's not a waste of money because it will do a better job than the film cannister: http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/5052617

Briefly, if it's silvered on the inside then the film cannister will serve as a Cheshire and can be used to adjust the primary tilt. It can't be accurately used to adjust the secondary tilt. For that you need a sight-tube or a laser. The combination tool the OP is referring to contains a sight-tube and a Cheshire. The length of the sight-tube serves to create a "frame" with which you can more accurate round and centre the secondary in the focuser. I don't know exactly what length is suitable. The one I have is adjustable. Maybe you can get more information on what's right from this page: http://www.catseyecollimation.com/ So long the length is vaguely correct you'll be ok.

[earth titan]

I understand correctly, which I probably haven't, I think shorter site tubes are better for scopes with bigger secondaries as you can see more of it........

Someone else will perhaps put me right.

[umadog]

Yeah, that's about right, Tom. It'll be the focal ratio that you use to select the sight-tube: http://www.catseyecollimation.com/sxl-out-640.jpg

That said I've used the longer combo-tool tube at f/5 and found it to work ok.

[Naemeth]

So, at F/5, should I use long tube or short tube?

[Saleratus]

There seems to be a bit of confusion here. Or maybe I'm the one who is confused. Anyway, this is my take--

When you have a long tube on a Cheshire, it's a combination item. It's combining a sight tube and a Cheshire collimator in the same instrument. I would rather have these separate, because the sight tube is used once in a blue moon to align your secondary mirror, while the Cheshire eyepiece is used to collimate your scope maybe every time you take it out. There is no reason to burden yourself with a huge tube when this is something you will be using so often.

So I'd get a small, nicely made Cheshire without the tube, and if you are a real perfectionist and feel the need to align your secondary (which probably doesn't need aligning), you can get a sight tube as well.

[Saleratus]

One more thing, if you are a serious perfectionist, you can look into the Catseye collimation tube. It is precisely adjustable according to the f-ratio of your scope.

[umadog]

There seems to be a bit of confusion here. Or maybe I'm the one who is confused. Anyway, this is my take--

Yes, there is always confusion because, as you say, everyone calls the combination tool a "Cheshire"

I would rather have these separate, because the sight tube is used once in a blue moon to align your secondary mirror, while the Cheshire eyepiece is used to collimate your scope maybe every time you take it out.

The sight-tube portion has two uses.

1. Rounding and centring the secondary in the focuser. Hopefully that is done rarely. For this you frame the secondary in the round opening of the tube. The OP could use the long-tube quite happily, I would think.

2. Secondary tilt adjustment. This is an axial alignment (just like primary tilt) and is distinct from the above rounding of the secondary. It's achieved by centring the primary centre spot under the sight-tube cross hairs. This requires tweaking probably every time you use the scope (at least that's been my experience). The secondary tilt and primary tilt adjustments need to both be correct in order for the telescope to be "collimated." For this reason you need both a cheshire and a sight-tube to be aligned and this is why the combo tools are popular. If you're a perfectionist, I'd recommend dedicated cheshire and an auto-collimator. Much, more accurate and easy to read than the sight-tube and can be used to align both secondary and primary tilts.

[Suryaputha]

@umadog, May I know what is meant by dedicated cheshire.  Thanks in advance.