Cheshire collimation: are there safety precautions?

[trickybilly]

I have picked up a cheshire collimator and I'm up to try collimation for the first time (I will have to use the internet and Youtube to learn how to do it). One of the reasons I went for the cheshire is because I have heard it is safer for the eye than laser collimation (my one eye is damaged and I really need to spare my sight on the other for professional reasons). Are there any safety precautions at cheshires? Should I collimate at day or at night (like using the moon or a flashlight)? Any advice for personal/eye or equipment safety? The telescope which I'll collimate is a 200/1200 Skywatcher Flextube, dobsonian.

[Renato1]

You just put a flashlight into the side of the Cheshire while looking into it and then make adjustments to move the dark dot over either a dot or annulus marked into the centre of the main mirror.

I use a low powered flashlight that I also use for reading star charts - I replaced its incandescent bulb with one of twice the voltage. With newer LED flashlights, just put insulation tape over the light, either red or lots of white strips, to lower the amount of light going out.

It doesn't matter whether you use the Cheshire, day or night.
But if you have a truss tube dob, you pretty much have to use it every time you assemble the dob.
Regards,
Renato

[raymo]

You don't have to do it each time with a Skywatcher Flex Tube , they hold
collimation very well indeed; my 8 and 10" ones are far better in that
regard than my friend's Lightbridge 10".
The advantage being that there is no assembly to do, they just slide on very
solid tubes that are accurately adjusted.
raymo

[barx1963]

The only safety precaution is if using it outdoors in daylight never point the scope anywhere near the sun. I usually find the Cheshire easiest to use around sundown so no fiddling around with lights, just point the telescope at the sky.

One small safety point with any truss or open tube style scope is be aware that the primary can still focus the sun even when the scope as a whole is pointed away from it. . Probably not a major issue with the SW Flextube as the mirror is quite deep within the bottom tube, just be aware of it.

Cheers
Malcolm

[trickybilly]

One more question: is collimation with a cheshire the same with a collimation cap like seen here? https://www.youtube.com/watch?v=YAVGcGEBmCE

[barx1963]

A Collimation cap is a good idea for "slower" scopes eg f6 and above. All it is really doing is centering your eye in the focuser. In Theory you can collimate a scope with no tools if you could hold your eye dead centre above the focuser tube, but that is nearly impossible.
A cheshire allows a finer level of adjustment as against a cap and is more suitable for faster scopes (f5 and faster)
Given yours is f6, a cap would be OK but if you have a cheshire, you are better of learning to use that.

Malcolm

[trickybilly]

I've tried collimation and have 2 questions.

http://i.imgur.com/TerhJqQ.jpg
1) I collimate with the cheshire being as close to the focus as it can get (turning the focus so that the cheshire gets as near to the telescope as possible). The closer I get a black square (marked by red arrow) is getting bigger. On images representing good collimation there is no such square.

2) How to look into the cheshire "the right way"? I try to look into it without much angle, but when I move my head a little the "crosshairs" move quite significantly (green lines illustrate this)

Note that this image is not from my telescope, but I edited a picture from Astrobaby's guide to represent my view.

[Jason D]

That square is the reflection of the focuser's drawtube. If you rack in the drawtube all the way in, it will protrude into the OTA.



You need to do your best to keep your eye's axis coincident with the cheshire's axis. That is, you need to do your best to look straight down the cheshire tube. You can use the cheshire lower edge is a guide by trying to keep it centered in the view.

Jason

[Renato1]

I don't have your problem because my Cheshire doesn't have the cross hairs. With mine I just get the dark spot into he annulus in the middle of my telescope. I have a separate cross hair gadget which I used once to make sure the secondary was in line with the primary.

Quite frankly, I don't think I've ever adjusted the focuser when using the Cheshire - because it doesn't make any difference.
Regards,
Renato
Regards