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[ATM] Re: ATM Digest, Vol 3, Issue 21
--- You wrote:
------------------------------
Message: 10
Date: Mon, 15 Mar 2004 18:08:15 -0600
From: "David Harbour" <scarab2@cox.net>
Subject: Re: ATM [ATM] Collimating, the eccentric way.
To: "ATM LIST" <atm@atmlist.net>
Message-ID: <000f01c40aea$c73d87e0$a41d0c44@xyloryctes>
Content-Type: text/plain; charset="iso-8859-1"
Nils, and all:
Yes, yes, by all means, have it so, if necessary. But I have always
preferred to make a job simple, keep it simple. "KISS".
It helps to "get in the ball park first", and no, one does not actually need
a straightjacket to get in the ball park first; but then again, neither does
anyone need a laser just to get in the ball park, either.
Squaring on the primary mirror first, and making sure that the diagonal is
not too far from centered in the axis of the focuser (some leeway is
allowed), and most assuredly (try it if you don't think it is true) the
focuser must be mounted at 90 degrees to the optical path of the primary,
unless the secondary is RELOCATED and TILTED to COMPENSATE for it, all help
you to GET IN THE BALL PARK, without lasers or straightjackets. Then,
instead of a straightjacket and blindfold, OR a LASER, all one needs is a
second or third magnitude star, and about 750X, to make a nice little nest
of CONCENTRIC diffraction rings by using the diagonal's adjusting screws.
The oddest thing is, sometimes when you have just made all components look
concentric (not necessarily including the perimeter of the front surface of
the diagonal) and then go to a second magnitude star with 750X,
miraculously, at least sometimes, the little nest of out of focus rings will
surprisingly appear perfectly concentric, requiring no more tweaking. Maybe
I have magic in my fingertips. My formula, summed up, in "25 words or less"?
By all means:
Concentricy,
Not ellipticity,
Nor electricity,
Perhaps only simplicity.
Not bad, huh? Somebody tromp on me tonight. I feel good for a change. Help
me, Dr. Morbius. "Help, Professor Schwartz- I've fallen and I can't get up!"
(Have actually built a few scopes, actually finished a few mirrors).
>Mike Byorick wrote to Dave Harbour:
--- end of quote ---
Message: 4
Date: Mon, 15 Mar 2004 20:27:32 +0100
From: "Nils Olof Carlin" <nilsolof.carlin@telia.com>
Subject: [ATM] Collimating, the eccentric way
To: "ATM LIST" <atm@atmlist.net>
Message-ID: <008301c40ac5$fb37cdf0$4fc443c3@HUVUD>
Content-Type: text/plain; charset="iso-8859-1"
Mike Byorick wrote to Dave Harbour:
>I revisited the basics of collimation, and noted that the fundamental
>issue is one of ensuring that the focuser is aligned relative to the
>OPTICAL PATH, of course, and not necessarily relative to the tube/truss,
>since, in many cases, the tube/truss itself cannot be used as a reliable
>frame of reference (except in a case like your yoke-mounted scope, where
>the truss is very carefully constructed!). So the problem definition
>now becomes something like 'given the <optical axis> of the primary
>mirror, how does one then align the other components in the optical path
>with respect thereto?'.
You could look at the other side of the coin - and rephrase the problem as:
'given the focuser, how do you align the secondary and primary to it?' Might
look much the same, but there is a world of difference in practice. For one
thing, the focuser is usually not adjustable (if it is, so much the
better) - for another, the mirrors and the (future) optical axis (and
reflections) are readily visible through the focuser.
But first a word of tolerances. To get good performance out of a Newtonian,
you should know for sure that your method will make your collimation meet
the tolerances required - no guesswork allowed. And just relying on
everything looking concentric won't do the job - not if you have a
reasonably fast and large telescope. No way - you really need some tool for
the job. A simple, home-made one from a piece of plastic drain tubing, and a
piece of cardboard is great, but you could buy one for less than a simple
eyepiece - and it will give you orders of magnitude more precise results
than trying eye-balling to center things inside the fanciest of optical
tubes. But if you like a laser, by all means get one.
So how do you do it? You don't need three instalments, and you don't need a
strait-jacket before (or after) you're through - and the tube can be as
crooked as you please as long as it does its job of holding things in place.
Just put your sight-tube combination tool in the focuser, and make sure the
secondary appears in line with the sight-tube (it will appear as a circle or
as near as makes no difference - and you get the offset right, too, with no
extra effort).
Next, adjust the tilt of the secondary to make the primary appear centered
in the sight-tube (its periphery inside the opening, or its center spot
behind the crosshairs). This done, adjust the primary's tilt to make its
center spot appear centered in the bright circle of the combination tool -
and you're ready. And what's more - you have full control of tolerances at
each stage - you can see how far off the secondary is from the line of
sight, you can see how far the center of the primary is, and you can see how
far the center spot appears displaced from the center of the collimation
tool (typically, this should be less than 2 mm for a fast Newt - easily
checked!)
As simple as that. Forget about the tube/truss - as long as it doesn't
intrude into the optical path (but if it does, you will notice) and supports
adequately what it should support. And don't worry if the optical axis is
reflected *exactly* 90 deg or a bit more or less - the secondary is flat and
this won't affect the image the slightest. But unless you have some kind of
suitable tool, you'll never know when you're close enough.
Nils Olof
for some helpful details, you can try
http://skyandtelescope.com/howto/scopes/article_787_1.asp
and
http://w1.411.telia.com/~u41105032/kolli/kolli.html
Message: 4
Date: Mon, 15 Mar 2004 20:27:32 +0100
From: "Nils Olof Carlin" <nilsolof.carlin@telia.com>
Subject: [ATM] Collimating, the eccentric way
To: "ATM LIST" <atm@atmlist.net>
Message-ID: <008301c40ac5$fb37cdf0$4fc443c3@HUVUD>
Content-Type: text/plain; charset="iso-8859-1"
Mike Byorick wrote to Dave Harbour:
>I revisited the basics of collimation, and noted that the fundamental
>issue is one of ensuring that the focuser is aligned relative to the
>OPTICAL PATH, of course, and not necessarily relative to the tube/truss,
>since, in many cases, the tube/truss itself cannot be used as a reliable
>frame of reference (except in a case like your yoke-mounted scope, where
>the truss is very carefully constructed!). So the problem definition
>now becomes something like 'given the <optical axis> of the primary
>mirror, how does one then align the other components in the optical path
>with respect thereto?'.
You could look at the other side of the coin - and rephrase the problem as:
'given the focuser, how do you align the secondary and primary to it?' Might
look much the same, but there is a world of difference in practice. For one
thing, the focuser is usually not adjustable (if it is, so much the
better) - for another, the mirrors and the (future) optical axis (and
reflections) are readily visible through the focuser.
But first a word of tolerances. To get good performance out of a Newtonian,
you should know for sure that your method will make your collimation meet
the tolerances required - no guesswork allowed. And just relying on
everything looking concentric won't do the job - not if you have a
reasonably fast and large telescope. No way - you really need some tool for
the job. A simple, home-made one from a piece of plastic drain tubing, and a
piece of cardboard is great, but you could buy one for less than a simple
eyepiece - and it will give you orders of magnitude more precise results
than trying eye-balling to center things inside the fanciest of optical
tubes. But if you like a laser, by all means get one.
So how do you do it? You don't need three instalments, and you don't need a
strait-jacket before (or after) you're through - and the tube can be as
crooked as you please as long as it does its job of holding things in place.
Just put your sight-tube combination tool in the focuser, and make sure the
secondary appears in line with the sight-tube (it will appear as a circle or
as near as makes no difference - and you get the offset right, too, with no
extra effort).
Next, adjust the tilt of the secondary to make the primary appear centered
in the sight-tube (its periphery inside the opening, or its center spot
behind the crosshairs). This done, adjust the primary's tilt to make its
center spot appear centered in the bright circle of the combination tool -
and you're ready. And what's more - you have full control of tolerances at
each stage - you can see how far off the secondary is from the line of
sight, you can see how far the center of the primary is, and you can see how
far the center spot appears displaced from the center of the collimation
tool (typically, this should be less than 2 mm for a fast Newt - easily
checked!)
As simple as that. Forget about the tube/truss - as long as it doesn't
intrude into the optical path (but if it does, you will notice) and supports
adequately what it should support. And don't worry if the optical axis is
reflected *exactly* 90 deg or a bit more or less - the secondary is flat and
this won't affect the image the slightest. But unless you have some kind of
suitable tool, you'll never know when you're close enough.
Nils Olof
for some helpful details, you can try
http://skyandtelescope.com/howto/scopes/article_787_1.asp
and
http://w1.411.telia.com/~u41105032/kolli/kolli.html
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