WFMOS-A: Guide Star Allocation Efficiency

Introduction

Results of allocating guide stars from the SDSS catalog to a WFMOS-type instrument. The tests focus mostly on how the allocation efficiency varies as the spine patrol radius is changed. A field with a low density (high galactic latitude) is used as this is likely to be representative of the worst-case scenario.

Method

  1. A guide-star target-list was prepared from the SDSS-DRv5 catalog using the VizieR web interface. A 4° diameter field with a high galactic latitude was chosen. Only good and acceptable stellar objects with a G, R & I magnitude < 20 were selected. This resulted in a preliminary target-list of 6867 objects.
  2. A color-cut was performed on the target-list to select appropriate guide stars. (ie. avoid galaxies/asteroids/extended objects/etc in favour of well isolated stellar objects.) See diagram below:

  3. A G-band magnitude cut was also performed to permit only a narrow ½-magnitude range of targets to be allocated (so the A&G camera will not saturate).

  4. An appropriate Instrument Description File for a 28x28 12mm-separated grid of guide-spines was created for use with the Echidna spine-to-object allocation software. The layout of the spines, along with the 2° FOV and the entire color-cut catalog, can be seen in the diagram below:


    All 5839 color-cut targets extracted from the SDSS catalog. The full target density (no additional magnitude cut) is ~464.7/°2.

    In total, there are 784 guide spines. This large number is not meant to be representative of the likely number of guide spines in a WFMOS instrument, but rather to give an accurate statistical indication of the efficiency.

  5. Technical details about how to extract data from VizieR, cut it & prepare a .fld file (for input into the spine-to-object allocation software) are at: $e/wfmos-a/doc/guide.star.catalog.txt.

    6. Results

    Target Density

    The allocation efficiency is highly-dependant on the target density.

    The diagram below shows the target density (over the full 4° field) in narrow ½-magnitude bands of our color-cut catalog:

    Note: labels on X & Y axis are the wrong way around!

    We have used the 15.0-15.5 magnitude band with the patrol-radius tests.

    Allocation efficiency vs patrol radius

    The data/diagram below shows how the allocation efficiency varies as a function of the patrol radius of the guide spines.

    The 15.0-15.5 magnitude cut gives a target density of: 164/4π = 13.1/°2

    Spine patrol radius
    (mm)    		 Number of
    allocated targets    		 Percentage of
    guide spines allocated
    1.010.1
    2.050.6
    3.0131.6
    4.0172.1
    5.0283.5
    6.0425.3
    7.0496.2
    8.0536.7
    9.0536.7
    10.0557.0

    An example allocation:


    6mm patrol radius with a magnitude cut of: 15.0 - 15.5