Quick star drift alignment (was: Re: Some questions from a Losmandy user)


Pierre Henrotay
 

Yesterday night was the first time that I tried the GTO Quick star
drift method (meridian delay).
I selected:
Procyon for alt adjustments (E of meridian)
Regulus and Dubhe for azimuth adjustments.
Not convincing so far, but this is my very first try and the clouds
did roll in very early, unfortunately. I need to figure out if I
overlooked something, hence the present post.

Alt adjustment was fairly simple; I was just surprised that I had to
specify a meridian delay of more than 1 hour (1E) although the
difference btw RA and LST was under 1 hour; anyway, I ended up
switching between 6E and 0W to be sure.
The simple thing is that a N-S keypad move and a change in Alt do
result in (nearly) parallel directions of move.
So, a good start.

Not so with my azimut adjustment, of course.
OK, I understand that the alt axis must be left untouched, same for N-
S keypad buttons.
The manual says the azimuth adjuster should be used "to correct half
of the error and the E-W buttons to finish centering".
The E-W buttons trigger a move in directions which are not parallel
to the one from the alt axis (looks normal), so what is actually
to be done ? Use the alt adjuster to move halfway to the reticule
axis then use the E-W to bring the star at the middle of the reticled
eyepiece (if ever possible ?)
Thanks for clarification.

Pierre


--- In ap-gto@..., chris1011@... wrote:

In a message dated 2/4/2008 3:00:11 PM Central Standard Time,
Pierre.Henrotay@... writes:


The manual points out that the problem can be that the scope is
not
orthogonal; I have yet to verify (and I thought the 2 stars
method
would minimize this problem if any).
I am able to align within 1 arcmin using 2 or 3 iterations of
PoleAlignMax. But this requires a PC.
You can check orthogonality in about 30 seconds. Simply point the
scope to a
star overhead (put star on crosshair at low to medium power) on one
side of
the mount, then do a meridian delay, enter the same star again and
do a goto.
The scope will now swing around to the other side of the mount and
pick up the
same star. Any E-W difference is twice your scope's orthogonal
error. Any N-S
difference is twice your altitude error.

With this method you kill 2 birds with 1 stone. Not only do you
instantly
know your orthogonal error, but you also can fix very quickly your
polar
alignment error in the altitude axis. How? Simple, my dear. Just
move the star in the
N-S direction 1/2 way with the altitude adjuster. Go back and forth
between
east and west a few iterations until the star is placed on the
crosshair each
time, and you are done. Takes only a minute for each GoTo. No more
adjustment of
the altitude axis is needed - you have adjusted this axis without a
polar
scope and without knowing exactly where north is, and all without
any kind of
complicated software (by the way I doubt that one can get within 5
arc sec of the
pole with the Losmandy software, or at least I am very skeptical -
besides
one needs to offset from the pole in order to eliminate drift
anyway).

Once you have adjusted the altitude axis, all that is left to do is
to line
up the azimuth, and that is even simpler. In fact, I could probably
do it
blindfolded, well not really, but almost. Since altitude is now
fixed, you just
need to adjust the azimuth until you get no drift in Dec. Or you
can use two
stars, one in the north, and one in the south. Slew between them
and adjust the
azimuth until they fall on the crosshairs (don't touch the
altitude!!).

Normally when I set my scope up fresh at a remote site, I can get
it aligned
before the sun has set and do precision drift alignment before
twilight is
done. The whole process takes 15 - 20 minutes at most. I don't use
the polar
scope much, except for when I'm lazy and have people around who are
asking
questions and I can't concentrate on alignment the other way.

If you have a good understanding of how the sky works vis a vis an
equatorial
mount, you really can do polar alignment in an intelligent way and
quickly
too. Otherwise you will be a slave to your computer.

Rolando


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