Re: [Utah-astronomy] Non-Laser Collimation
Yes, I use a Chesire sometimes. It is usually used to check on the accuracy of the laser. It is also a great way to check the centering of the secondary. On some telescopes this is hard to do with the laser and grid. The grid is sometimes too small to cover the seondary or with all the lines coming at you while you are checking, you can't see the lines on the secondary. My first collimator was a 35mm film canister with a hole drilled in it. OAS still uses the film canister on BOB. Dave ----- Original Message ----- From: utah-astronomy-bounces@mailman.xmission.com <utah-astronomy-bounces@mailman.xmission.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Tue Apr 22 18:44:53 2008 Subject: Re: [Utah-astronomy] Non-Laser Collimation
Anyone use the "old fashioned" Chesire Eyepiece
Erik
I've set up and collimated my truss tube dobsonian over 150 times. When you do the same thing many times you start to find short cuts. Somewhere I began collimating by just studying the view through the focuser. It's not as not as simple minded as it sounds, there is a skill to finding the null point and there is a major pitfall to avoid.
First an overview of the picture you see. Start by racking the focuser out and placing your eye near the top. This places you close to the focal plain and shows you the view that the eyepiece sees.
1. The first ring in is the bottom of the focuser tube. (the top is too close to be seen) 2. The second ring is the secondary. 3. The third ring in is the edge of the primary mirror. 4. The fourth ring way inside of the other three is the image of the secondary in the primary mirror. 5. The fifth ring is the image of the top of the focuser tube reflected twice in the secondary and once in the primary. 6. Lastly, floating inside is the image of the centering ring on the surface of the primary.
You make the first three rings concentric(1,2,3) in order to collimate the secondary. But to do this accurately you need to be sure your eye is centered in the focuser tube. You can check this by looking at your eye and centering it in ring number 5. Nulling the primary mirror requires just bringing the centering ring (6) over you eye while both are centered in the image of the focus tube (5). It's not hard with practice to achieve millimeter accuracy at this nulling point.
You notice I didn't try to center 4 inside of 3 or 5 inside of 4, That's because the Newtonian offset resides here and it is impossible to get them all centered due to the geometry of the bent optical path. I've known people who've tried and failed to get "everything centered". You don't need to and you can't even if you tried.
At some point I heard about the collimation cap that Orion ships with their Newtonians. I made one and it's just a two inch white disk with a 2 mm hole in the center. This device simplifies the nulling procedure by automatically centering your eye in the top of the focuser tube. Place the cap in the top of the focuser and look through the 2mm hole, null the primary by moving the centering disk(6) over the image of the 2mm hole. This makes it easier to do in the dark, just shine a red led light up the focuser tube so the 2mm hole can be seen against the white disk. The final trick is learning to center your eye in the 2mm hole; watching the diffraction effects around the edge of the hole does this.
I've used the cap method on a mirror as fast as f/4 and my results matched that of a nulling laser. I still use the eyeball only method whenever I can just because it's so convenient. Both these methods fair well against the star test which I'll cover in another post.
DT
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Daniel makes me proud with his non-laser collimation process. Russell Porter is smiling down on him tonight from the heavens. He wrote that he's going to address the star-test so I won't go into that, other than to say that for years, I've star-tested in daylight using a real star- the sun. Some of you probably know exactly what I do- it's somewhat of a trick statement. Can you guess? A Cheshire is a good tool for rough alignment. I've made many on my mini-lathe from PVC & clear plexiglass. People need to remember that there is often such a thing as "good enough" in collimation. It depends on the f-ratio of your telescope, the atmospheric seeing, and your observing habits. A fast system (say, f/4) of long effective focal length (i.e., large aperture), used for imaging at high power or planetary viewing or very close double star splitting, should be collimated very precisely. But if you are mainly observing deep-sky objects at low to medium powers with wide-field eyepieces, collimation can be off by a surprising amount with no noticeable effects on the imagery. By a surprising amount I mean on the order of 3 millimeters instead of say, 0.5 millimeters.
Quoting Chuck Hards <chuck.hards@gmail.com>:
I've star-tested in daylight using a real star- the sun. Some of you probably know exactly what I do- it's somewhat of a trick statement. Can you guess?
Let's see, you use a welding hood with a number 14 lens? ;) Either that or you use it's reflection.
Guy gets partial credit. I make an artificial star using the specular reflection of the sun on a polished ball-bearing (sphere), attached to a tree or pole several hundred feet away. You poke a small hole in a black posterboard and position it just in front of the sphere in line with the telescope view. This gives a sufficiently dark background when examining the "star" at high magnification. Once I stuck the ball on a piece of electrical conduit with a magnet, on the side of a building a couple of blocks away. Richard Berry also published plans for making an electrical artificial star-in-a-box some years ago, for the same purpose of optical testing and collimation in daylight. On Wed, Apr 23, 2008 at 4:54 AM, <diveboss@xmission.com> wrote:
Quoting Chuck Hards <chuck.hards@gmail.com>:
I've star-tested in daylight using a real star- the sun. Some of you probably know exactly what I do- it's somewhat of a trick statement. Can you guess?
Let's see, you use a welding hood with a number 14 lens? ;) Either that or you use it's reflection.
Do you do this on location prior to night fall? Quoting Chuck Hards <chuck.hards@gmail.com>:
Guy gets partial credit.
I make an artificial star using the specular reflection of the sun on a polished ball-bearing (sphere), attached to a tree or pole several hundred feet away. You poke a small hole in a black posterboard and position it just in front of the sphere in line with the telescope view. This gives a sufficiently dark background when examining the "star" at high magnification. Once I stuck the ball on a piece of electrical conduit with a magnet, on the side of a building a couple of blocks away.
Richard Berry also published plans for making an electrical artificial star-in-a-box some years ago, for the same purpose of optical testing and collimation in daylight.
On Wed, Apr 23, 2008 at 4:54 AM, <diveboss@xmission.com> wrote:
Quoting Chuck Hards <chuck.hards@gmail.com>:
I've star-tested in daylight using a real star- the sun. Some of you probably know exactly what I do- it's somewhat of a trick statement. Can you guess?
Let's see, you use a welding hood with a number 14 lens? ;) Either that or you use it's reflection.
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I've only ever done it once at a star-party, as a demonstration. Usually at home while working on a scope. But it could easily be done anywhere. The nice thing about an artificial star is that the scope need not be tracking to observe it- a nice feature for most Dobs. An electrical pin-hole-type star could be used at a star-party after dark, set up some distance away. With a decent battery pack, could be left on all night. On Wed, Apr 23, 2008 at 8:55 AM, <diveboss@xmission.com> wrote:
Do you do this on location prior to night fall?
A good place to get non-laser collimation tools and an informative book.
http://www.shop-cabinets.com/collimation/tools.html I've only ever done it once at a star-party, as a demonstration. Usually
at home while working on a scope. But it could easily be done anywhere.
The nice thing about an artificial star is that the scope need not be tracking to observe it- a nice feature for most Dobs. An electrical pin-hole-type star could be used at a star-party after dark, set up some distance away. With a decent battery pack, could be left on all night.
On Wed, Apr 23, 2008 at 8:55 AM, <diveboss@xmission.com> wrote:
Do you do this on location prior to night fall?
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There is also an AutoCollimation Eyepeice that consists of a mirror inside the eyepiece with a hole in it for daylight collimation. It works a lot like the StarTest.
Erik Guy gets partial credit.
I make an artificial star using the specular reflection of the sun on a polished ball-bearing (sphere), attached to a tree or pole several hundred feet away. You poke a small hole in a black posterboard and position it just in front of the sphere in line with the telescope view. This gives a sufficiently dark background when examining the "star" at high magnification. Once I stuck the ball on a piece of electrical conduit with a magnet, on the side of a building a couple of blocks away.
Richard Berry also published plans for making an electrical artificial star-in-a-box some years ago, for the same purpose of optical testing and collimation in daylight.
On Wed, Apr 23, 2008 at 4:54 AM, <diveboss@xmission.com> wrote:
Quoting Chuck Hards <chuck.hards@gmail.com>:
I've star-tested in daylight using a real star- the sun. Some of you probably know exactly what I do- it's somewhat of a trick statement. Can you guess?
Let's see, you use a welding hood with a number 14 lens? ;) Either that or you use it's reflection.
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