[ATM] ATM: Collimation by concentric circles, pt. 2 (oops)

[scarab2 at cox.net (David Harbour)]

OOPS! I forgot to include Mike Byorick's second paragraph from his email to
me, his excellent treatment of the difficulties in collimating when not
having a well made tube. Here is his second paragraph, along with my second
installment on collimation. Mike's second paragraph:

>What I 'discovered' was that the initial phase of the collimation process,
as implemented by many (most?) scope builders, is FUNDAMENTALLY premised on
the assumption that the >OTA (tube or truss) has uniformly parallel sides,
the ends of which are exactly orthogonal to the sides. In breaking down the
collimation process into steps (not the ones necessarily >called 'steps'), I
found that the critical 'step', which is key to the entire rest of the
collimation procedure, is aligning the LONGITUDINAL CENTER OF THE DIAGONAL
>HOLDER/SPIDER with the optical center line of the mirror.



If we may assume several things pertaining to the construction of the OTA,
collimation is simple and can be done in daylight. Assumptions;

(1) Tube is well made, and has an accurate geometric form (is not crooked or
warped in any other way)

(2) The primary and secondary mirror are both accurately CENTERED in the
tube (this method will not work for those who offset their secondaries;
offsetting a secondary away from the eyepiece holder and closer to the
primary is a practice of questionable utility, and it impairs easy
collimation).

Proceed as follows: The FIRST step is to square on the primary with the
secondary. Do this by levelling the tube so that you can look down into it
while backing up away from it. A friend, to assist you, would be helpful.
One must make sure that he or she is looking down the centerline of the
tube; look at the tube's inside walls and move your head until you see an
equal amout of the tube's walls visible all around. Then, back up far enough
so that the reflected virtual image of the secondary holder in the primary
is larger than the secondary holder; i.e., you can see its reflection in the
primary slightly larger than the secondary holder occulting it. Then have
your assistant, at the collimating screws, adjust the tilt of the primary,
as you instruct him, until the reflected image of the secondary, and the rim
of the primary, and the area of the tube's inner walls, all appear to be
perfectly concentric. The important thing here is that you end up with the
secondary mirror's reflection, and the secondary holder, appearing to be
centered in the primary mirror- an equal amount of primary mirror will be
showing all the way around the reflection of the secondary. Everything
should appear "concentric". Next installment in a day or so.

Sorry, this easy method will not work for those who have offset their
secondary holders. I recommend against offsetting the secondary. Offsetting
secondaries might be the reason that all the high tech stuff for collimating
had to be developed, perhaps.

Davey

I will include Mike's excellent third paragraph with my last installment on
collimation by concentric appearances.