When using pickoff mirrors,
the location of the guide star depends on the design software used
to define a slitmask. There are two software packages used to design slitmasks for LRIS:
Because the Pointing Origins are updated more frequently than the design software, the coordinates
of the guide star location specified by the design software is out of date. To determine the guide
star location please review the information below.
Autoslit
The Autoslit software allows you to offset the target field and guide star relative to the
center position as specified in the coordinate input file. Offsets are applied using the "C"
option.
- If no offsets were applied, the guide star should be placed at position
X=505, Y=616 (last updated 21
Oct 2009).
- If offsets were applied, you need to calculate the new X and Y location for the guide star
- xn = 505 + (NS* cos(theta) - EW* sin(theta))/0.137
- yn = 616 + (NS* sin(theta) + EW* cos(theta))/0.137
where:
- theta = -(29.58+PA)
- NS North offset
- EW East offset
- PA PA of mask
This has been tested once. Use at your
own risk.
In the output from autoslit, a guide star identiy is provided along with an "X" and "Y"
position. The X and Y location are in autoslit coordinates and should not be used to
specify the location of the guide star for any of your masks. See below for the typical output:
# =========================================== #
# Guide Star Identity #
# ------------------------------------------- #
# Guide star is STAR # 1 #
# 966 #
# At X = 258.7 Y = -16.0 #
# =========================================== #
UCSCLRIS
The UCSCLRIS software package also allows you to specify the location of a guide star
on the pickoff mirror.During a recent engineering night we re-measured the offset between
the predicted and actual location of stars on the guider. Until further notice, observers
should apply the following offset to their pickoff star coordinates:
- current offsets are ΔX=+9 , ΔY=-17 (2008-Apr-27)
- These represent only a small change from the last measurement made 3 years ago.