[ATM] ATM: Collimation by concentric circles, last part

["David Harbour" <scarab2@cox.net>]

To all:

After falling behind several days ago due to an excacerbation of one of my
illnesses, I am now re-doctored, and caught up on chores. So with pleasure I
sit here and write the last step in collimation for a reflector by means of
concentric appearances of components, without any additional accessory aids.
Also I will include Mike Byorick's excellent last paragraphs of his notes to
me on the same subject. Now we come to the last, and "trickiest" part of the
procedure: getting your diagonal well tweaked.

Assuming:

(1) Your primary is now perfectly SQUARED ON to its secondary (which we
ascertained was centered in your tube- I cannot help those who have offset
their diagonal mirror cells, holders), and:

(2) Your secondary is located "relatively" well centered below the focuser,
(more on this immediately),
we may make the last steps to be well collimated.

As it happens, one may collimate a Schmidt Cass with the method of
"Concentric Circles" easily and quickly, and very accurately (some
commercially made S. C.s have very minute errors of centering of one or more
components; especially they are prone to have a light baffle tube not
precisely aligned) because there are no problems peculiar to those
encountered with a diagonal as in the Newtonian. Let us then consider why
the Newtonian presents a slightly more challenging situation, in regard to
its secondary.

When we look at our well located Newt. diagonal, it appears, superficially,
to be a circle (an ellipse viewed at a 45 deg. angle to its surface appears
circular) but in reality, it cannot appear perfectly circular. It could, if
we were viewing its outline at infinity; but of course we cannot do this. In
actual fact, perspective effects make our relatively circular appearing flat
to be not perfectly circular, as the end of the diagonal that is farthest
from us will be reduced in scale, because it is farther away from the eye.
Nevertheless, it is possible to locate the flat reasonably well-centered
below the focuser drawtube. In fact, some leeway is allowed here, or the
practice of offsetting the secondary would necessarily be forbidden (it has
dubious value at best, and can complicate this method of collimation).

In daylight, aim your telescope to a blank piece of sky. Pull your draw tube
out as far as it can come and still not begin to fail to be extended
perfectly straight, i.e., do not pull it out so far that it can wobble
slightly). Now, using the appearance of the exposed walls of the drawtube,
with your eye relatively close to it, and keeping the eye centered on the
central axis of the drawtube by noting the appeance of the walls of the
drawtube, compare the distance between the edge of the primary, all the way
around, with the lower circular opening of the drawtube- IGNORE THE
PERIMITER OF THE DIAGONAL. This, because of the defect that perspective
causes in the apparently circular edge of the diagonal's surface. It is only
necessary that the perimeter of the diagonal be "reasonably" well centered;
in fact, when one adjusts it towards or away from the primary, the
perspective distortion caused by viewing it too closely (the perimeter of
the flat's surface) will generally cause you to slightly mislocate it;
again, leeway is allowed, or the practice of offsetting a diagonal would be
forbidden.

All that remains is to tweak the secondary's screws until the appearance of
the space between the draw tube's lower end, and the circular outline of the
primary, appear to be "concentric". You're done. Fine tune later on a star,
with someone holding a rod in front of the end of the scope, or again, by
your self, and by observing the out of focus expanded disc of a star, move
the rod until its image is approximately well centered in the part of the
expanded disc to indicate which direction the primary needs to be tweaked
in, and make the tweak, and you are as fine tuned as you can be. Many, if
not most of you, have probably had the experience of losing your collimation
frequently; with my first large fork mounted Newt (and my second mirror, for
Lunar and Planetary observing, f/8, 12.5", I lost collimation frequently
with a Novak cell; now, when I build a cell, I provide for more rim
retention; lateral slippage in the cell is, I believe, one of the more
common causes of loss of collimation in a Newt)

I was only kidding when I said that you needed a blindfold and a
straightjacket to do this. I have been trying to develop a sense of humor. I
think we can leave this (humor) for Richard (aka Dr. Morbius) to be master
of. In fact we are overdue for some light entertainment from him (hint,
Richard). Now, here is Mike Byorick's excellent conclusions in his email to
me. He has elocuted the entire process and methods very well in his notes to
me; I hope he will continue to write about atm-ing in depth, as he did with
his notes to me on the subject.

>Satisfying the above condition removes any dependency between the
>squareness / parallel-ness of the tube/truss and the relative alignment
>of the focuser.  The step can be implemented by running a string from
>the center of the mirror cell ('parallel' only to the edge of the tube
>on which the focuser is mounted) to the opposite end of the tube, where
>the string passes through the bolt hole in the center of the spider
>where the diagonal is normally attached. (Of course, the spider will
>likely have to be adjusted to align with the string!)  This technique
>should work regardless of whether the mirror is 'centered', and obviates
>any need to determine the 'center' of an irregularly-shaped tube.  The
>string/line will also establish the 'left/right' focuser alignment,
>leaving only the 'up/down' alignment to be solved.  The 'up/down' (with
>respect to the longitudinal direction of the OTA) focuser alignment can
>be carried out simply by putting a carpenter's square to the string and
>aligning the focuser accordingly.  No need to take measurements from the
>front end of the tube, the accuracy of which presupposes that, not only
>are the tube sides parallel, but that they are exactly the same length,
>and in addition, that the tube is perfectly rectangular as viewed from
>the side, and not just parallelogram-shaped.

>I now think that my scope previously suffered from hopeless misalignment
>due to the fact that the mirror was off-center, and, although the
>focuser was 'square' relative to the tube, the diagonal was 'slightly'
>tilted to compensate for the off-center condition.  After 'collimation',
>this situation provided what appeared to be aligned optics, but resulted
>in a not-exactly-90 degree bend at the diagonal, thus causing a type of
>insidious astigmatism that is (was) difficult to pin down.

>How does the above 'logic' sound to you??

>Regards,
>Mike Byorick

It sounds deeply thought out and perfectly correct, to me, Mike; well done.
Please post your entire short treatise for all of us in one installment; it
is good material and not really overly long.

Davey



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