Using the Offset Guider
Using the offset guider is very important to get the most out of your
observations. Remember that the telescope was never designed to track
and nod to a precision of better than 0.1'', which is necessary to
keep a stellar image centered on one of SpectroCam's 0.25'' pixels.
The offset guider system can provide this level of accuracy.
Spectrocam 10 can use either the Palomar Shepherd Autoguider system or the f/70
tip-tilt system to provide the autoguiding. This section
will assume that the Palomar Shepherd Autoguider system is being used. See
Tip-Tilt Autoguider Commands for more
information about the tip-tilt system.
The guider arm, which rotates about the r-theta plane, is the tool that
provides the video to the guider system. This guider arm is controlled
by the Mirella program and is run either on SAM (in the monitor adjacent to
the Spectrocam 10 workstation) or on the Spectrocam 10
workstation in the Mirella Xterm window.
The offset guider command sequences described below control the
guider's r-theta arm, so they are essentially the same for the two autoguider systems.
Before the offset guider can be used effectively, the center of
rotation of the guider coordinate system must be set on the center of
the SpectroCam field of view. This procedure is usually performed at
the beginning of a new setup and will not have to be done each night.
That is, if you are following another observer you will not likely
have to align the guider and you can skip this section. However, if
you find that the guider has not been aligned by an experienced
observer, follow these steps carefully to set up the guider.
Make sure that you have placed a bright focus star in the center of the
Spectrocam 10 detector, and you have created a fiducial mark on the chopped optical
counterpart image of the star in the centerfield video just below the Spectrocam 10
pickoff mirror. See the previous section Finding the First Star
if you haven't already done this.
- Locate the center of rotation of the guider coordinate system as
follows:
- Use the bright standard star that you have just finished focusing on,
or move the telescope to an 8th to 9th magnitude SAO star. Place the
lower of the two chopped images on the centerfield TV monitor
crosshair you defined in the previous section. If you retract the
SC-10 pickoff mirror, you should see the other (upper) image nearly in the
center of the screen.
If you haven't set up a crosshair you can put
the image at approximately the center of the screen, but the
coordinate system won't be rigorously tied to the center of the
infrared detector's field of view; center a 10 micron bright star on your
detector before you move on in this case, and then set up the fiducial crosshair.
- Move the guide head to its origin and reset any guider parameters from
earlier observing runs. This is performed with
three commands. The first command sets the differential refraction
angle to zero arcseconds. The second command sets the origin of the
offset guider polar coordinates to R=0'',
=0 deg, at the
center of rotation of the guide head. The third moves the guide head
to the just set guider origin.
M> ogs d 0
M> ogs o 0 0
M> og r 0 0
- Tell the telescope operator to zero the telescope offsets with Z.
- Using the hand paddle, move the telescope to put the star in the approximate center of the
offset guider TV fiber hexagon. Make a note of the telescope offsets that got you to this
position. Put a fiducial marker on the star using
the ADD button in the "Fiducial Marker" area of the Shepherd Autoguider.
If you are using the f/70 Tip-Tilt guider, you will need to turn the guider on
in such a way that the guide cursor (4 white dots in a square pattern) appears but the telescope doesn't move
(with GO 4). This cursor is the guide box; you will want to steer the star into this box.
See the Tip-Tilt Autoguider section for all you need to know.
- The next step is to verify your first guess of center of rotation and
adjust it if necessary. The correct position is located when you can
rotate the guide head by 180 deg and see no motion of the star in
the offset guider TV.
- Execute the command
M> og r - .01 0
to effectively rotate the guide head by 180 deg with negligible
change in R. Turn up the gain in the centerfield video to watch the guider arm move the
180 degrees; it may take 30-40 seconds. If the star moves in the TV from its original position,
move the cursor halfway between its current position and the new
position of the star, and then move the telescope so that the star
falls on the cursor.
If you are using the f/70 Tip-Tilt guider, you will need to move the guide cursor (4 white
dots in a square pattern). There is a way to do this; see the Tip-Tilt Autoguider section.
- Then rotate the guide head back to its original position via
M> og r 0 0
and check again for motion of the star and move the telescope as needed.
- once again rotate 180 deg via
M> og r - 0.01 0
and see if the star moves.
- Repeat this sequence as needed until the cursor has been positioned so
that the star, when positioned on the cursor, does not move with guide
head rotation. This is the position on the TV that the autoguider
will use for guiding. All subsequent placements of a star in the
guider TV will be at this cursor position. The Autoguider cursor
should not be moved from here on. Note the telescope offsets, if they
have changed.
- The next task is to center the origin of the offset guider co-ordinate
system on the center of the SpectroCam field of view.
You have two options:
- Ignore the effect; find the guider star, then do
[
] mtg to move the
telescope 
and
(and the guider the other way so as not to lose the star).
- The better choice is to offset the guider center to the SC-10 center:
- Tell the telescope operator to RET
- Move the telescope, if necessary, to put the lower stellar image on the Centerfield TV
monitor crosshair (it should be very close; you are re-setting the telescope
which may have drifted in the last 10 minutes or so since you started this
procedure).
- og o 0 0 (remove any old offsets; you should have done this above)
- og r 0 0 (move guider to its origin)
- Move the guider using
og i
RA
DEC to center the star
in the guider cursor (your RA and
DEC should be
the same as you noted above in 2-4 [or 3-5]). Use as many og i
RA
DEC as necessary to center the
star.
- ogc x y (Take x and y from guider display monitor.)
- og o r
(Take r and from monitor.)
March 1997: r = 6.41" = 96°
- og r 0 0 (We are now back to the SC-10 origin.)
- Apply the optical/ir differential refraction correction:
M> og d 0.8 (for 8 to 13 um.)
Assuming the offset guider has been set up as described above, use the
following steps to set up a guide star for your program object. We
will assume that the first object is a bright standard star. It is as
important to have a guide star for standards as it is for faint
objects if you want accurate measurements of sensitivity and the point
spread function. Note that one can go as far as 3' from the program
source for a guide star. The minimum distance for a guide star is
~ 30''. The unvignetted field radius is 180'', and the fully
vignetted field radius is 220''. We will assume you do not have
accurate offset positions for the guide star but that they have been
approximately measured from a finding chart.
- Move the telescope back to the standard star and place its lower image
on the Centerfield TV monitor crosshair. Make sure the star is in the right
place on the infrared image.
- Zero the offsets DRA and DDEC on the telescope control monitor
by telling the operator to ``X'', ``TX'', or ``Z''.
- Use the hand paddle to move the telescope to your selected guide star.
After centering the guide star on the finder TV cross hairs, note the
offsets from the program object, DRA and DDEC, on the telescope
control monitor. For this example, we will assume DRA=15.6'' (east) and
DDEC= -22.8'' (south). If you already have
accurate offsets, this step is not necessary, but it is a good way to
find your star if something doesn't seem right in a later step.
- Move the telescope back to the program object, either manually or by
telling the operator to ``RET''.
- Offset the guide probe to the guide star with the command
M> og r 15.6 -22.8
The guide star should now appear in the offset guider TV when the
object is back in the finder TV field center.
- Tweak the guider as necessary to get the star properly centered
in the autoguider cursor in the small guider TV. Move the probe
incrementally with commands such as
M> og i -2.0 0.5
- Once the guide star is set up in the center of the guide field, type
M> og
and read the value of HEAD given at the end of the output.
- Tell the telescope operator the value of HEAD; the operator will need to
enter this value at the TCS control window with the command ghead.
- Now set up the video autoguider. Place the guide box over the fiducial marker
such that the green guide box cross hairs match up over the yellow fiducial mark.
Size the guide box such that it provides good star signal versus good sky subtraction.
Tweak the "average" and "integration" sliders as you like. Have a guru explain the
other guider parameters; the default values are likely good enough. Click on "ENABLE"
in the Autoguider section of the Shepherd Guider (make sure the Guide button is pressed
in; don't use the Drift mode of guiding).
For more details on setting up the Shepherd Autoguider, consult the
Shepher Guider Web Page or other documentation available in the data room.
If you are using the f/70 Tip-Tilt Autoguider, you are ready to GO as long as
the guiding parameters are set up properly. See Tip-Tilt
Autoguider.
- With the guider operating, you can use the mtg command (Move
Telescope and Guider) to adjust the source's position on the infrared detector.
For example,
M> 0. 1. mtg
moves the telescope 1 arcsec north and repositions the guider head to
follow the star in the focal plane. Note that for such guider motions
to be accurate, the coordinate system center should be calibrated as
described in the preceeding section.