Re: [APML] needed- an excellent sense of direction

["Bobby Middleton" <bobm@koyote.com>]

OK. I've always assumed that focuser alignment was critical to optical
alignment but what you're saying is logical. What you say is that we've got
an optical light path that's true (presuming a true collimating tool) but
the focuser is simply not square.
But I've got something besides flexure Dave. The long stars are only at
opposite corners of the image. If I had flexure it would be across the whole
frame, right?
I've done the laser rotation thing. It looks good. But here's a nasty quirk
I found this afternoon. The laser spot will move IF I tighten down the 3
thumbscrews in different order. I guess that means too much play in the
focuser tube which allows the collimator to slightly shift. Maybe i could
use something to increase the diameter of the collimator like several
adhesive labels or tape around it to keep away any play.
Bobby


----- Original Message -----
From: <Aplanatic@aol.com>

> Hi Bobby:
>
> Precise alignment of the focuser with the tube is not necessary.  It only
impacts vignetting if the tube is just large enough to accept the marginal
off-axis rays.  It is still a good idea to get the focuser as close to
square as possible for this reason, but a non-square focuser will not cause
the focal plane tilt that you think you are seeing.
>
> An easy way to see this is to image that all you have is a focuser, a
secondary mirror and a primary mirror in perfect alignment.  Then mentally
add a tube that is not quite aligned with the optical axis.  The misaligned
tube has no effect on the optical performance (collimation), but could cause
some vignetting if it starts to cut into the marginal off-axis rays on one
side or the other.
>
> If you do have a focal plane alignment problem, it's likely caused by the
laser collimator itself.  I have found that these bloody things can change
their alignment a lot over time.  The laser diode is encased in epoxy and it
cold flows over the years causing the beam to be considerably off-axis with
respect to the barrel.  To check this, rotate the collimator in the focuser,
locking it down each time, and note how large a circle is made by the beam.
>
> In the photos that you presented, I don't really see a strong case for a
misaligned focuser/focal plane.  If this were the case I'd expect to see
small donuts in the corners, not elongated stars. (Although with coma, I
guess one could make some case for a misaligned focal plane.)  Instead, it
looks a bit like you have differential fexture between the guider and the
film.
>
> Anyway, it's necessary to check the collimator alignment as a first step.
>
> Hope this helps,
> Dave Rowe
>
>
>
>
>
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