Subject: Altitude limits on the 3.5m

From: Russet McMillan

Submitted: Sat, 23 Jun 2001 19:01:00 -0600 (MDT)

Message number: 513 (previous: 512, next: 514 up: Index)

Hello, folks,

Here are some pointers to help observers avoid a problem we've been
seeing too often lately.

A lot of observers have been slewing the telescope to high altitudes,
which is not good for the primary mirror.  Although the telescope is
mechanically capable of tracking objects up to 88 or 89 degrees, the
rapid azimuth motion sets up oscillations in the primary mirror which
can be unhealthy for the glass, as well as degrading image quality
significantly.  Anyone who tries to track an object this high should
expect the observing specialist to move the telescope to a lower
altitude immediately.

The current high-altitude limit is approximately 80 degrees.  This is
approximate because it's all right for the telescope to be a little
higher as long as it's tracking downward rather than moving rapidly in
azimuth -- that means it's okay to be a little above 80 degrees in the
west, but NOT okay to be higher than 80 degrees for an object
transiting in the south or north.

How can you know before slewing if your object will be too high?
Remark can tell you, or you can use a simple rule of thumb for
choosing targets.  

1. When you enter slew coordinates in Remark, once you've finished
   typing in the coordinates but BEFORE you press the Slew button,
   click on empty gray space somewhere in Remark.  The top window
   should show you where you're about to slew.  The right-hand column
   will be labeled "source" and it will give the El(evation) of the
   target you just typed in.  Don't slew if this target is higher than
   83 degrees; ask the observing specialist for help if the target is
   between 80 and 83 degrees.

   [Note: if you entered the target coordinates a while ago, the
   elevation won't update unless you make a change.  Change one of the
   coordinates a little, then change it back and click on empty gray
   space to update the elevation display.]

2.  The simple rule of thumb is this: the zenith is at a Right
    Ascension equal to the current sideral time (displayed in your top
    Remark window as LST), and a declination equal to +33 degrees.
    For any target with a declination between about 24 to 42 degrees,
    avoid observing the target when it is within one hour of transit.
    That means you should wait until the LST is more than an hour
    different from the target's right ascension for targets in this
    range of declinations.

What if the target is just a little above 80 degrees?  Ask the
observing specialist for advice.  If the target is going through
transit exactly now at an altitude of 80 degrees, waiting just fifteen
minutes will reduce the azimuth tracking rate without increasing the
airmass of your observations.  If the target is in the east at 80
degrees, you might be able to observe it for a few minutes, but it
will soon be too high.  A target in the west at 80 degrees is safe.

Warn the observing specialist before you slew to a target near 80
degrees of altitude.  You may be permitted to make the slew while the
observing specialist watches the readouts showing the state of the
primary mirror.  If everything looks all right, you should be allowed
to observe.  But slewing to such objects without a warning will only
annoy the observing specialist.

What about lower limits on altitude?  The telescope is mechanically
capable of pointing to targets below 10 degrees altitude, already a
scientifically undesirable airmass.  However, we have recently started
seeing some primary mirror oscillations at low altitude as well. Let
the observing specialist know if you'll be going to a target lower
than 30 degrees altitude; we can check the primary mirror readouts and
find out if there's a problem.  If oscillations do show up, the
observing specialist can eliminate them and improve your image quality
in just a few minutes, and you will still be able to observe your

We understand that these altitude limits are more restrictive than
those used in the past, but it's important for observers to recognize
that this is not an arbitrary rule change.  It's based on observed
unsafe behavior of the primary mirror -- a very important component of
the telescope!  Please keep altitude in mind when you choose your
targets, and avoid slewing objects that are too close to the zenith.
Low altitude targets are usually fine, but you should let the
observing specialist know to check for possible trouble when you slew
at low altitude.  Know your target's altitude BEFORE you slew to avoid
unpleasant surprises.

				  Russet McMillan.

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