Whenever one installs encoders on a mount for the first time, it is
highly advisable to perform what we call the
Daytime Encoder Test.
If you are considering performing a TPAS run for the first time and have
never performed a Daytime Encoder Test, now is the time to consider doing it.
Power the Argo Navis unit OFF.
Using a high-powered eyepiece or reticle eyepiece, sight and center a distant
terrestrial object. For example, I often like to use power pole insulators.
Now power ON the unit.
DIAL up
MODE ENCODER and press ENTER.
Spin the DIAL until it shows
AZ/ALT ENC STEPS.
The two values shown in the lower line of the display should both be zero.
Now rotate the mount in Az (or if it is an equatorial, lock the Dec axis and
rotate the mount in RA) through 360 degrees and reacquire the target.
Note the left-hand displayed value. It should be ideally within a step
or two of the original zero reading. Keep in mind that if you have, say,
10,000 step encoders, when the value reaches 9999 it will wrap back to 0000.
Once satisfied, try rotating the mount again in Az but in the counter direction
to what you previously did. When you re-center the target, ensure that the value is
again within a couple of encoder steps.
Now try swinging the scope in Az back and forth a few times, to try and
induce an encoder slip if something mechanical is amiss, such as a loose
set screw. Again re-center the target and check that the Az encoder value
on the display is within a step or two of zero.
Now do the same procedure, but this time on the Alt (or Dec) axis.
If you have a Dob, obviously you will not be able to rotate the tube through
a full 360 degrees. Instead, rotate the tube all the way through its full travel
above and below the target before re-centering it, this time taking note
of the right-hand displayed value, which should be within a step or
two of zero.
Sometimes when we ask users if they performed a Daytime Encoder Test,
they respond, "Sure, I did it during the night".
The advantage of performing the test during the daytime is that it is
easier to find a fixed object. Stars won't do as they have apparent motion
due to the Earth's rotation. (Northern hemisphere observers could use Polaris
in a pinch, but ...) The other advantage of performing the test during the
daytime is that if something is amiss mechanically, you can literally get
down on hands and knees and inspect up close the encoder installation.
At night, it is dark, often cold, damp and sometimes you are tired.
During the day with the benefit of sufficient light and a clear head, it is
often easier to spot something that is going wrong.
If the mount does not pass the test, then you have some work to do
to find the cause. Often it is a loose set screw or an encoder not mounted
to coincide with the axis in question. Also make sure that the tangent arms,
which are the brackets that go from the encoder to some fixed part of the scope,
typically the side of the rocker box on a Dob, don't rotate around the axis
of rotation themselves. They don't need to be fixed rigid at the far end and
can be allowed to 'float', by cutting a well toleranced slot into the end of the
arm and sliding it onto a locating pin. Though they can be allowed to move
in or out radially from the axis or up and down away from the side of the
rocker box, they should never rotate or shift around the axis of rotation,
as in doing so they are providing for a positional uncertainty in the encoder system.
For portable scopes where removing the Alt encoder is required to transport the
telescope, it is worth repeating the Daytime Encoder Test now and then.
Once confident of the integrity and repeatability of the encoder installation,
one is then ready to perform one's first TPAS run.
The first part of a TPAS run is acquiring data and that will be covered in
the forthcoming Part 2 of this tutorial.
Best Regards
Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place
Mount Kuring-Gai NSW 2080
Australia
Phone +61-2-9457-9049
Fax +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au