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


Autoguiding is done using the CASS autoguider. GSS for finding guidestars, ICL>prag x y for moving the guideprobe to the required position

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.


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.


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: 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


Changing masks: