This page documents infomation which is specific to blind offseting with the LRS/MOS. Blind offsetting can be used on almost any target with V > 21 to speed setup times and should be considered required for targets with V > 22.
There are two methods of blind offseting
Using 2-N offset stars
This method requires that the PI submit an .xy file with the relative positions of the target star and the offset stars (2 minimium). The format of the file is two column ascii with each column being a decimal number. The first entry is always the target and all lines there after are the setup stars. If you have 3 setup stars (A,B,C) then the .xy file will have 4 lines: the target x,y; A's x,y; B's x,y; and C's x,y. The relative positions need not be in RA and DEC but if they are then they must be decimal RA and DEC (e.g. 14:30:00.0 should be written 14.50000). If you are not using RA and DEC please be sure that the coordinate system you are using is flat on the sky. If you have a flattened x,y pixel coordinates then those will work just fine. NOTE: LRS pre images and DSS images are not flat on the sky! It is wise to use a unique name for you .xy file since all of the xy files go into a single directory.
You may use as many setup stars as you wish but we have found the using more than 4 can lead to slowed setup times. Three seems to be the ideal number of setup stars. Try to pick point sources cleanly seperated from other point sources as poor image quality or very good image quality might lead to different centering than you intend. The setup stars should be brighter than V=20.5. Try to pick setup stars as close to the target as you can and the setup stars should roughly surround the target. The offset stars should be no more than 1.5 arc minutes from the target star.
We have found that the most common sources of failure for this mode of observing are
One the finding chart you should CLEARLY mark the target star and the setup stars. The setup stars should be clearly marked AND LABELED as A,B,C or 1,2,3. The name of the .xy file should be included in the comment line on the phase II associated with that targets as is done with the mos files.
Here is an example:
My target Phase II
Program = UT05-3-001
Target = Draco269
RA = 14:14:14.13
DEC = +12:12:12.0
EPOCH = 2000.0
PA = par
Exp = 1800
CRsplit = 2
Visits = 5
Comment = "blind offset with Draco269.xy"
Here is what I would put in my .xy file (I am using 3 setup stars) 1,2,3. In my flattened image that I got from some other source the target is at 256.3 and 100.2 while the 3 setup stars are at 1=(200.2,101.2), 2=(300.3,200.3) and 3=(350.3,150.3).
My Draco269.xy file would look like:
256.3 100.2
200.2 101.2
300.3 200.3
350.3 150.3
My finding chart will have my target marked with a cross hair and then 3 circles with labels next to them marking them 1,2,3.
Using a single offset star
We have found that single offset star observations require very precise positioning of the PA and should only be used in the most extreeme cases of need.
This should only be used if there is only one V<20.5 star within the 1.5 arcminute field of the target. In this case the .xy file should contain the RA and DEC of the target star and the RA and DEC of the offset star. The RA and DEC can be in decimal notation or colon seperated. The PA for the observation must be specified. The offset object should be a point source and as close to the target as possible to minimize errors.
The .xy file should be noted in the comment field of the Phase II and uniquely labeled. The target position and offset object should be labeled on the finding chart.
Here is an example:
My target Phase II
Program = UT05-3-001
Target = Draco269
PA = 230.0
RA = 14:14:14.13
DEC = +12:12:12.0
EPOCH = 2000.0
Exp = 1800
CRsplit = 2
Visits = 5
Comment = "blind offset with Draco269.xy"
Here is what I would put in my .xy file (I am using 1 offset stars) A which is located at 14:14:14.22 and +12:12:15.2 . My Draco269.xy file would look like:
14:14:14.13 +12:12:12.0
14:14:14.22 +12:12:15.2
or like
14.23725833 12.203333
14.23728333 12.204222
My finding chart will have the position of my truely invisible target marked with a cross hair and then my offset point source labeled with a circle.