WORKING WITH LDSS
revised 03/08/99
LDSS is a wide-field multiaperture spectrograph mounted at the CASS focus
of the WHT. The multiapertures are obtained by using special designed
masks
STARTUP PROCEDURE
TCS:
- USER> STATION CASS
- USER> INSTRUMENT LDSS
- USER> AGSELECT CASS
- Zeroset AZ and ALT
to zeroset on the targets switch to computer control:
- USER> AZ 298
- USER> ALT 89
- USER> ENG
- USER> ZERO AZ TAR
- USER> ZERO ALT TAR
Drive AZ and ALT slowly in + direction over the zeroset target points,
then switch to computer control again
to zeroset absolute:
- USER> ZERO AZ ABS
- USER> ZERO ALT ABS
Zeroset absolute will go wrong in case an absolute encoder bulb is broken!
When slewing to the first object check azimuth and altitude incremental and
absolute encoders are in close agreement and updating.
- USER> ZERO ROT ABS
- USER> WAVELENGTH 0.6
- USER> TEMP ...
- USER> PRESS ...
- USER> HUMI ...
- USER> CAL LAST
AUTOGUIDER:
Autoguiding is done using the CASS autoguider. GSS for finding
guidestars, ICL>prag x y for moving the guideprobe to the required position
TV:
The LDSS masks are not tilted with respect the the incident light beam (as
ISIS), but slit(mask) viewing is still possible as the TV can pick up the
reflected diffuse light. A 5th magnitude star is well seen with full gain
in direct mode, for a 10th magnitude star you need at least 40 frames of
integration.
PREPARATION PROCEDURE
Telescope focus:
Usually around 97.2 mm.
Removing any Apertures and Grisms in the LDSS lightpath allows for
direct imaging. Running the standard ICL>focus_image is the best way
of dermining the telescope focus, although the large pixel size
(0.6"/pixel) often results in undersampling.
Calibrate procedure:
The only option for dermining the rotator center and performing the
calibrate procedure is using the TV Direct Viewing mode
(acquisition mirror in by typing ICL>AGCOMP). Proceed as usual.
OBSERVING PROCEDURE
Tweaking:
You may center up bright objects using the slit viewing TV as you do
for ISIS. This works fine for objects brighter than 10th magnitude.
For fainter objects, you'll need to compute the required offsets.
The easiest way to get a standard star under a mask hole or mask
slit is the following:
- take an image without the mask, and note down the x and y coords of
the standard star: Xstar, Ystar
- then take an image of the mask, and mark x and y coords of the slit or
fiducial hole: Xmask, Ymask
- for SITe1 compute the following:
Xtweak=0.591*(Xmask-Xstar)
Ytweak=0.591*(Ystar-Ymask)
This should work for any PA!
Tweaking the telescope is done with the TWEAK command, which
needs three arguments: X (arcsec), Y (arcsec), rotation (degrees).
The LDSS manual says that large tweaks should be cut in smaller parts,
in order not to upset the autoguider. However, what the manual doesnt say
is that the X and Y tweaks should be executed before the rotation tweak.
This is because a rotation followed by a translation is not the same as
a translation followed by a rotation.
Correct example:
a total tweak of 5 5 3 should be broken up as
- tweak 2 2 0
- tweak 2 2 0
- tweak 1 1 1
- tweak 0 0 1
- tweak 0 0 1
Wrong example:
so not this way (nb the LDSS manual is wrong!)
- tweak 2 2 1
- tweak 2 2 1
- tweak 1 1 1
MISC LDSS PROCEDURES
Changing masks:
- ICL> ldoor 0 (unlocking)
- Open the door and remove old mask
- Push new mask left until 2 point contact.
- Rotate clockwise until third contact
- Verify contacts with paper (paper not falling through)
- Lock postion (anyone) before closing door
- ICL> ldoor 1 (locking)