Tiling VIRUS observations for mapping contiguous areas

Layout of 78 VIRUS IFUs at HET focus (left) and example on-sky data (right). Each IFU is 51 x 51 sq arcsec. in size and are mounted on a grid with 100 arcsec pitch. The central region is used for the other HET instruments. On-sky data are presented with the gaps removed. Inset at top left shows 448 fibers in a single IFU back-lit. Note the 1/3 fill factor of the fibers that necessitates dithering to fill in the area of the IFUs. The inset at bottom left shows how the fibers map onto the different amplifiers of the CCDs in the two spectrograph channels per VIRUS unit. The array of IFUs is fixed with respect to the parallactic angle of the observation (telescope Y axis) which is up in this figure.

Notes on mapping:

  • VIRUS IFUs are 51"x51" on 100" grid

  • To fill in the 1/4 fill and map an area, need 4 observations (each with 3 dithers)

  • Example of well-known interacting galaxy pair the "Mice" (NGC4676):

  • The 4 positions are indicated in the color key

  • Dominated by continuum emission rather than emission lines

  • Scale of object is larger than one IFU, so mapping essential

  • 4x3 individual exposures each 120 seconds

  • Total observation time ~40 min

  • Choreography with guide stars may require care in preparation

Sample TSL file with commentary

#VIRUS ENG Observations of Mice interacting galaxies, to test mapping and field reconstruction
# blind, accurate coordinate setup; 4 pointings to fill in area
# 3 dithered exposures each pointing
  PROGRAM      ENG21-1-004
  SEEING       3.5
  CRSPLIT      3
  DITHER       Y
  OCD          N
  VISITS       1
  SKYCALS      Y
  COMMENT      "4 pointing offset sequence to fill area"
Mice_1   12:46:10.6  +30:43:36.0  seq_m1  SEQ    360  15    2    034   0        0
Mice_2   12:46:10.6  +30:43:36.0  seq_m1  SEQ    360  15    2    034   0        50
Mice_3   12:46:10.6  +30:43:36.0  seq_m1  SEQ    360  15    2    034   50       50
Mice_4   12:46:10.6  +30:43:36.0  seq_m1  SEQ    360  15    2    034   50       0

  • This TSL executes a map in the Mice interacting galaxy that consists of 4 dithered observations with the target centered on IFU034 offset on a 50 arcsec grid to fill in the IFU gaps; this is the simplest way to specify a map.

  • The map consists of four grouped observations each with a unique name and a common group name

  • Note that maps cannot be run in OCD even if the HETDEX exposure time is desired

  • Note also that this approach attempts to observe all 4 dithered observations on a single trajectory; for longer exposure times this approach will not be possible and the GNAME/GTYPE keywords should be omitted so that these 4 observations can be observed on separate nights/trajectories.

In case it is useful, you may consider using this tool to calculate the position angle of a target on sky given its declination.

Caveats when picking/planning mapping observations:

  • Consult the current status of VIRUS units here to decide which unit (or units) you want to use for your mapping. Some units still have problematic amplifiers which produce bad data and could create unintentional gaps in your maps.

  • Verify that your target's track time is adequate to contain the mapping sequence you are requesting, using our Observability and Feasibility Tool. Plan on the following overheads, conservatively estimated at about 35 minutes (may be less in reality):
    • 8min setup on first target
    • 1min readout after each of three dithers (3min total)
    • 5min setup on second target
    • 1min readout after each of three dithers
    • 5min setup on third target
    • 1min readout after each of three dithers
    • 5min setup on fourth target
    • 1min readout after each of three dithers
    For example, if your target is at declination +30deg and is 78 minutes long, allowing 35 minutes for overhead means you could have at most 43 minutes of exposure (divided among 12 integrations). However, targets which completely fill their tracks are the most difficult to schedule in the HET queue and generally require Priority 0 or 1 time to be successful.

  • If you are submitting multiple mapping targets be sure the GNAMEs are all unique - any targets with the same group name in your program will be grouped together, even if you submit them in separate TSL files.

  • We recommend GTYPE SEQ to observe your four mapping positions in sequential order. This keeps the sky conditions as similar as possible throughout your mapping observation. If you require deeper observations than can fit on a single track then it may be useful to un-group your targets and allow them to be observed on separate nights. Ask us for help if you're thinking about this.

When in doubt, please contact your friendly Resident Astronomers! We'll be happy to help answer any questions and to verify that your target submission is correct: astronomer(at)het.as.utexas.edu

Last updated: Sat, 30 Dec 2023 03:39:06 +0000 stevenj


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