|
7/5/2010 If you are into
astrophotography, as I am, having the GEM mount properly aligned to
the North Celestial Pole (NCP) is a MUST. There are numerous ways to
accomplish this. Some of the processes vary with the make
of mount you have.
There are a number of polar alignment processes on the
LINKS page at the left.
Using Polar Scope on the mount:
If your mount is equipped with a Polar Alignment Scope (PAS) it
can be used to set the NCP of your mount. The technique varies with
the scope manufacturer and you should consult the manual or get
help from members of various tech groups on the web. One thing that
is very important is that the PAS optical axis must be
aligned with the R.A. axis of the mount This process also varies
with the mount manufacturer. Drift alignment:
With drift alignment
you position the telescope close to the Meridian and
at 0°
elevation. You then watch to see if the star is drifting North or South and
adjust the AZ knobs to stop the drift.
While watching declination (DEC) drift only, if the guide star
moves UP, rotate the mount's AZIMUTH so that the guide star moves to the RIGHT in
your eyepiece. If the guide star moves DOWN, rotate the azimuth so that the star
moves LEFT. The reason you make these moves is described in the TUTORIAL. DO NOT
use the slew buttons on the hand controller to do this. Use the AZIMUTH
adjustment screws on the telescope mount.
The faster the guide star
drifts up or down, the more azimuth corrections you must make.
If the guide star drifts quickly, your mount is much further away
from the celestial pole, and will need to be moved a lot
further.. so far in fact that the guide star will move right out
of the field of view! No worry.. you can always grab another
star to finish the alignment process.
An excellent simulator that allows you to practice
this technique can be found at
Polar alignment (How to do Drift)
TO properly do the drift alignment you
should have an eyepiece with an illuminated reticule. If that's
not available you can find programs on the web that will
allow you to put a cross hair on your screen. One such is
STARTARG an can be found at
Andy's Shot Glass for a nominal fee. CCD
Drift Method: Another method that I have used is the
CCD drift method. Here you use you imaging camera to
get a picture of the drift angle. Basically you set up just
as in the Drift method above and take a 125 second image of a star in
the FOV. Hold that position for the first 5 seconds to get a
reference point then move the scope with the controller West for one
minute. Immediately after move the scope EAST for the remaining minute.
IF you scope is aligned properly the trace created by the East movement
should overlap the trace created by the WEST movement. If it's out of
alignment the image will create a horizontal "V" where the amount
of correction is shown by the angle created by the intersecting
lines. More details on this process can be found at
CCD
Drift alignment .
Drift Alignment Using PHD Guiding Software:
Just recently an acquaintance ( Ted Rafferty) put me on to doing
the drift alignment using the popular auto guiding program (PHD)
available for free at
Stark-Labs.
I would recommend that you align your camera to the RA/DEC
axis of the telescope. I usually shoot images with the DEC axis being up
and down on the CCD chip. This also helps me orient the image later as I
prefer to show them with North being up.
The process begins as all the other drift alignments. To check the RA
alignment choose a star at 0° elevation close to
the meridian. To set the DEC alignment choose a star as close to
the E or W horizon as possible.
- Turn on the guiding program (PHD) , select the mount and camera
and run thru your calibration a usual.
- To Check the RA select a star near the Meridian at 0°
elevation.
- Start Guiding on the selected star.
- Turn on the GRAPH and select DX/DY
instead of RA/DEC.
- Turn off the DEC guiding
- If your mount is perfectly aligned the DY
( red line) should track across the graph near the center line. You
may see some wiggle above and below the center line. If
it drift up or down you need to make a very small
adjustment to your AZIMUTH
screws to compensate for that. You will see the change
immediately. No need to wait 5 or 10 minutes for the visual
drift.
- Now set up the elevation.
- Stop the guiding and look for a star near the
western or eastern horizon at whatever elevation allows you to clear
any obstruction. The lower the better.
- Recalibrate the guiding on the new star.
- As in step 3,4 & 5 above start guiding and make sure that
the DEC guiding is off.
- Once again if your alignment is on then the
DY trace should not drift from the
horizontal graph line. If it does, carefully mar small adjusts to
the ELEVATION screws to
compensate. Again you should see an immediate change in the graph.
This process is simple enough to allow you to check you
polar alignment whenever you feel it necessary. The only
requirement is that you get a copy of PHD and have a
camera that is supported by the program ( there are many). |
