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Cheshire collimator

bambuko

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bambuko

bambuko

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I have done my first collimation using bog standard Cheshire (or to give it it's proper name? combination sight tube) and have wondered whether I would not be better off using 2" inch collimator (and I don't mean 1.25" collimator in 2"/1.25" adapter) and whether using 2" collimator would avoid having to use both Cheshire and sight tube (because using my Cheshire alone will not allow me to see outer edges of primary)?

and... if I were to make 2" combination sight tube, how do I get all the distances correct (to suit f/ratio of my scope)?

I understand that bought ones are "one size fits all" design?

Would there also be an advantage in using 2" over 1.25" (without any intermediate adapters) in that the whole thing would be more stable and less prone to build up of tolerances, easier to use etc.

BTW I have both lathe and milling machine, so as long as I know what I am supposed to be making, actually making it should not be a big deal.

Or... am I missing something and talking totally out of my rear orifice?

ps apologies if my ramblings are somewhat incoherent - I am still proper newb and only just barely grasping the subject :grin:

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bambuko

bambuko

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I can't see outer edges of primary (for whatever reason, and I am sure I am not the only one).

Also, the Cheshire is a sight tube and collimator.

that's why I have said "...to give it it's proper name? combination sight tube..."

and now (if I may :smiley: ) back to my questions about 2"

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NIGHTBOY

NIGHTBOY

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Haha sorry just trying to help :)

What scope are you using??? After I got my secondary mirror set using the centre screw in it, I then brought the edges of the primary into view on it by using the 3 allen head bolts to tilt the mirror. I had no trouble seeing the outer edges of my primary (although I could only just see them) an I didn't need a 2'' to do so.

I'm new too so I maybe barking up the wrong tree but its how it worked for me. :)

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bambuko

bambuko

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I am using Skywatcher 150p and Skywatcher "Cheshire Collimation Eyepiece"

I also use my own "pepehole cap"

I have no problems using either or both of them, but wanted to make my own to fit directly into 2" (instead of the more usual 1.25") hence all the questions.

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wookie1965

wookie1965

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My short Cheshire 1.25" inch collimator shows all three primary mirror clips in my 150p, If your secondary is centralised under your focuser and presenting a full circle the three collimation screws should tilt the mirror to bring the three clips into view. If you want to make a 2" one you would have to make sure that the reflective angle would be right i would think other than that just copy the 1.25" inch.

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bambuko

bambuko

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Perhaps I should have bought "short Cheshire" ?

My secondary is centralised under the focuser, but my "skywatcher Cheshire" seems to restrict field of view allowing me only to see central part.

I am not the only one - Astro Baby says in her tutorial:


...With my Sky-Watcher 200P I find a collimation cap is the better solution as a sight tube or my Cheshire do not allow me to see the very edges of the secondary mirror through them...

and as for:

...other than that just copy the 1.25"...

as I have said in my first post, bought Cheshire's are "one size fits all" design, so no point copying something that is not necessarily perfect for the job.

Also I would like to understand, rather than just copy.

Another question I have is about the differences between standard (on the right) and premium (on the left) Cheshire:

collimator.jpg

Standard one has a step on the body (red arrow) which only allows it to be inserted that far and further into 1.25" adapter.

Premium one has no steps and I presume can be inserted down to any point you care (as long as it is not too deep in to hit secondary on small dia scope)?

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wookie1965

wookie1965

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I have just read about the Premium Cheshire and two things may help you one you only insert as you would an eyepiece and secondly it has a narrow field of view, now if your skywatcher is the same i bet that is why you cant see your 3 mirror clips, have you no one you could borrow a different one and try that or go to astro society. The reflective plate has to be set at  45 degree angle by the way if your making one your self.

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bambuko

bambuko

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Yes, both "premium" and "standard" have narrow fov, hence inability to see primary edges/clips.

I know you only insert it as you would an eyepiece, but... both eyepieces and my standard Cheshire have a step in the body which determines how far you insert it.

The "premium" as shown on the photo has no step and there seems to be nothing preventing you pushing you all the way in.

If necessary I will sketch to explain what I mean.

45deg is obvious, so no problem with that - my design questions were to do with distance of peephole from the front end of sight tube and how does it affect suitability for a given telescope (of different focal length etc).

I think I may have found the answers (thank you google :kiss: )

If you look here it starts by saying :

for sight tube...the length of the tube should be the inner diameter times the focal ratio: I have a tube with 28 mm inner diameter for my f/5.6 telescope, so I cut it to 28*5.6 = 157 mm length...

(btw my "standard" shop bought version has 28 mm inner dia and is 145mm long overall or 110mm to the locating step - which seems just right for f/5 scope if it is overall length that matters?)

So if I were to use this formula for my 2" sight tube with about 45mm inner dia, it should be 225mm long for my f/5 scope.

So perhaps the way to do it, is to increase the diameter but only as much as needed to get required fov, to avoid having too long sight tube?

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bambuko

bambuko

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Thanks wookie1965,

It is this sentence:

...The sight tube is slid into the focuser until the outline of the end of the tube is just slightly larger than the outline of the diagonal mirror...

well... it is not possible with my "Skywatcher Cheshire" and I guess that's why premium version hasn't got any steps on the outside?

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wookie1965

wookie1965

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yes i did some more research sorry i didn't find about the peep hole for you, i was looking for that but you found that, i was wondering why you couldn't see your mirror clips then saw the narrow view, thought there must be a reason hence the sight tube has to be inserted far enough to be slightly wider. When you make one post a picture please.

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bambuko

bambuko

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OK, so I got rid of the step and my cheapo collimator contraption now looks the same as the so called "premium" one

IMG_7411.jpg

Now, I can slide it in or out to my heart's content and secure it in any position:

IMG_7413.jpg

It doesn't make slightest bit of difference to the cheshire bit of it.

It doesn't make any difference to fov - the only way to achieve this would be to shorten the tube I guess?

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bambuko

bambuko

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Thanks, sound like this would have been better choice...

Frustrating thing is that for all the multitude of info available, most (if not all of it) is incomplete, contradictory and confusing :grin:

I am still not totally sure how long should the tube be and why?

I don't understand why majority is as long as the one I bought and only few are like yours?

I am not absolutely sure why some have a locating step on the body and other do not?

and few other questions :laugh:

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wookie1965

wookie1965

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I think its a case of finding what works for you, the sight tube lines your secondary up and that's it the Cheshire is a combination of sight tube and a means of lining up your primary. The length of the tube must be something to do with view so as to facilitate the centering and tilting of secondary mirror,  you would think there would be one definitive site with all the information in one place. Im out at the moment but i think i have a link to a pdf file that may have more information i will check and post later. Just found this 

http://www.docstoc.com/docs/80610306/Telescope-Collimation

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bambuko

bambuko

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It must be my analytical (read [removed word]) mind :grin:

Most of the people just use the thing and don't wonder why? :tongue:

and... btw the answer from Sky's the Limit about your short collimator is:

...We recommend these short ones for people who have a telescope with a rack an pinion focuser. The long ones are heavier and can pull the focuser out of alignment if there is any play in the focuser...We recommend the longer one if you have a Crayford focuser which will not suffer any effects of the extra weight.

That makes sense (and the guy is to be recommended for very fast response), but doesn't address at issue of fov etc

ps funny thing is that forum software decide to protect minors from a totally innocent and perfectly legitimate word I have used to describe my mind :police:

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wookie1965

wookie1965

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After reading this it seems that the sight tube has a narrow field of view for the purpose of centering the secondary and the Cheshire with a wider field of view collimating the primary that's why you can see the 3 mirror clips in the  Cheshire.

http://www.buytelescopes.com/images/file/kendrick/cheshire_collimation.pdf

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