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Cheshire Eyepiece = Colli cap +


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Been going through the Cheshire or laser collimation tool debate with myself, and after quite a bit of thought have decided to buy a Cheshire eyepiece ;) (thanks in most part to some posts by Astrobaby)

But I have a dumb question : The cheshire eyepiece incorporates the colli cap peep hole thing, Yes??? So I wouldn't have to buy a colli cap?

cheers

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I find the cheshire is so narrow you can't see your mirror clips hence I have both.

Is that because of the length of the cheshire, or the peep hole is to small?

Sorry I have re-read it : to narrow! so its because of the length & diameter of the cheshire itself not the hole!

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If you have a Cheshire ep, you don't need a collimation cap. I can just barely see the clips by moving my eye around the peephole in the Cheshire. If in doubt, I sometimes look into the focuser tube with my naked eye and see if it looks pretty good. You can drive yourself crazy getting the last little bit of perfect out of the system, with increasingly diminishing returns.

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If you can find an old 35mm film canister, preferably white/translucent, you can drill a small hole in the centre ( about 1-2mm and it makes a perfect colli cap..

Allan

I have aquired a white semi transparent 35mm film canister ;)

So do I drill a 2mm hole in the cap? and what size do I drill in the base? I think I read 8-9mm somewhere, would that be correct?

Dave

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That would work.

I used the base only and drilled a 1-2mm hole dead centre.

I then wrapped electrical tape round the canister till it fits perfectly in the 1.25 focus tube to make it more accurately centred.

Allan

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A cheshire is also a peep-tube, ie colli cap. But strictly speaking, the cheshire should match the f-ratio of the scope, i.e. its length divided by width should roughly equal the scope's f-ratio. The standard available one is really matched to something around f/6 or longer, I think (to be honest I haven't measured).

Users of fast scopes (such as myself and previous posters) can find that when we look through the cheshire we can't see the whole of the primary mirror. The solution would be to buy a shorter cheshire, or get out a hacksaw (not my preferred solution!). Or use a film cannister when adjusting the secondary. Or (the method I use) use a laser to get the secondary sorted, then the cheshire for adjusting the primary.

In any case I found that by loosening all the collimation bolts and getting the mirror nearly all the way down the tube (while still leaving some room for collimating) I could see the whole of the primary, including clips, when looking through the cheshire. My scope is a flextube 12-inch f/4.9.

I still prefer the laser for secondary adjustment, though, as I find this a lot easier in the dark when setting up my scope for observing.

I've heard it suggested that the secondary mirror is undersized on some scopes (this was rumoured of the 10-inch Skywatcher) in which case this would make it difficult to see the whole of the primary. (There is a potential advantage in an undersized secondary because although you lose light you also lose some contrast-debilitating central obstruction.)

But the upshot is that a short-tube cheshire is possibly advisable for an f/5 or faster scope, otherwise you may need one of the suggested workarounds. For f/6 upwards there should be nothing to worry about. For slower scopes it is indeed possible to do the whole thing well enough with just a film cannister and a star for testing, since those scopes allow for a considerably wider margin of error in collimation.

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Thanks for all the input, I think I know what does what and how now! ;)

acey : Very good post, gave me I good grasp of whats going on with the cheshire etc. Thank you.

My scope is a 200mm Dob F/5.9 so a standard cheshire should be fine.

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Bit late to this - I made my own colicap out of a blank cap for the 1.25 focuser. Its fits very snug. I just drilled a 2mm hole in and already stated - yes I use a colicap to set up the secondary as the Cheshire is a bit too long and I cant get a good view of the mirror clips just using the Cheshire.

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Bit late to this - I made my own colicap out of a blank cap for the 1.25 focuser. Its fits very snug. I just drilled a 2mm hole in and already stated - yes I use a colicap to set up the secondary as the Cheshire is a bit too long and I cant get a good view of the mirror clips just using the Cheshire.

AB,

Just to clarify : you drilled a 2mm hole in the top cap, what did you do to the bottom of the canister?

cheers

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I bought an Astroengineering Supreme APO barlow last week. The metal dust cap has a hole in the centre and makes a perfect colli cap. Film canister now redundant!

Allan

Think I'd have a bit of a job swinging that one passed the missus at the mo. ;)

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I reckon as others will agree here, its imprtant to know if a purchased laser is aligned, but if it is, i think it can show very slight missalignments that may not be obviouse with a cheshire.

I think a cheshire to see the edge of the secondary under the focusser,

a colli cap to see the prmary inside the secondary.

a laser to get the beam dead centre on the primarys centre stcker

and a glow in the dark colli cap to get the primary exactly centred.

if all this lines up your laughing. if it does not its headach time..As others know i have had this problem, But im still learning.

Even so, i think its a great technique that could promise virtually perfect collimation. as every area is coverd with the easiest to see tool.

I still belive a good laser might show tiny errors, the eye wouldnt possibly see with just a cheshire and colli cap alone. I am still learning but thats what ive found so far. I could be wrong.

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I reckon as others will agree here, its imprtant to know if a purchased laser is aligned, but if it is, i think it can show very slight missalignments that may not be obviouse with a cheshire.

I think a cheshire to see the edge of the secondary under the focusser,

a colli cap to see the prmary inside the secondary.

a laser to get the beam dead centre on the primarys centre stcker

and a glow in the dark colli cap to get the primary exactly centred.

if all this lines up your laughing. if it does not its headach time..As others know i have had this problem, But im still learning.

Even so, i think its a great technique that could promise virtually perfect collimation. as every area is coverd with the easiest to see tool.

I still belive a good laser might show tiny errors, the eye wouldnt possibly see with just a cheshire and colli cap alone. I am still learning but thats what ive found so far. I could be wrong.

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