Re: ATM Not the poor Man's collimator

["Philip J. Tramdack" <tramdack@TCNJ.EDU>]

I agree that the star test is the ultimate test. After building in 1992
a 10" f/6 from Berry's design in his 1989 book, and being lazy about
collimating in general, I got a laser collimator this past May. At the
time I had logged about 400 observing sessions on the 10" and was aware
of a chronic bias in the collimation where is seemed where I could never
quite get the diffraction rings all the way around a star, and then the
collimation never seemed (mysteriously) to hold, even through one
session.

First I bought the laser collimator. This revealed instantly that the
diagonal mirror was not quite adjusted right, but even more critically
that the 1992 Novak helical focuser caused the point of focus to spiral
around the base of the eyepiece, due to eccentricity (slight) of the
focuser. 

Also on close inspection I suspected the focuser was not square to the
tube, but the collimator really doesn't help too much with that
adjustment on the Novak.

Next I bought a JMI focuser which is very steady, and the focus travel
allows more close inspection of the squaring problem. The design of the
JMI also allows squaring adjustments.

Now I regularly check the collimation, which is most affected by
temperature changes than anything else. If the holographic pattern is
perfectly centered on the mirror, the shadow of the diagonal is
centered, and the reflected spot shines on the hole in the base of the
collimator viewed from the front of the tube, I can be confident that
the telescope will be *very close*.

Strangely, on a steady night, the diffraction rings around a bright star
will tend to be slightly biased in the same direction as usual, and
adustments to the primary do not correct this.

No doubt I still can touch up the diagonal adjustment, or perhaps the
focuser is still not perfectly square. Possibly my astigmatism accounts
for some of this.

Notwithstanding this visible but (at my low energy level) uncorrectable
flaw, viewing is quite good. For instance this past weekend I easily
elongated and separated momentarily 4 or 5 times gamma Andromedae b-c,
which must be less then .45 by now. This was at 500x with a 4.8 Nagler
and TV 1.8 Barlow.

PJT

Clive Milne wrote:
> 
>     I must confess that I am quite seduced with the
> ease of collimation afforded by holographic lasers.
> It's an interesting exercise to walk around the field
> of a star party (in daylight hours) assessing the
> collimation of different people's telescopes.  The
> vast majority of amateurs I come across, maintain that
> their optics are perfectly aligned.... right up to the
> point where I turn the laser on.   I guess that is the
> real beauty of a holographic projection reticule. The
> errors of alignment & the centering of  components
> within the optical system are obvious, not only to the
> person collimating, but also to any bystander.
>     As with anything though,  it is important to
> understand the limitations of collimating with lasers.
> And for that, I find the obvious nature of a projected
> pattern (from a holographic laser) to be an invaluable
> teaching tool.
>     Ultimately of course, the star test has the final
> word.  But judging from the number of people that have
> come up (later in the night) to thank me for their
> scope's new found optical performance, (and their
> re-assessment of the true nature of collimation), Well
> I guess you can infer that (being mindful of their
> limitations) a projection style laser collimator is
> not a bad piece of kit to own.  (imho)
>      Clive.
> 
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-- 
Philip J. Tramdack
Associate Dean for the Library
Roscoe L. West Library
The College of New Jersey
Box 7718
Ewing  NJ 08628-0718
vox 609-771-2343
fax 609-637-5177