VIRUS IFU Layout, Target Placement, and Dithering/Tiling
Here we describe:
- VIRUS IFU layout on the sky (configuration is evolving)
- How to place your targets on VIRUS IFUs
- Dithering vs Tiling and sky coverage
VIRUS IFU layout
The image below show the layout of the VIRUS fiber bundles among the VIRUS units above the corrector.
Light blue squares are the VIRUS fiber bundles. Other bundles (LRS2B, HPF, etc) are located within the red square in the center. Not all VIRUS fiber bundles are connected to IFUs yet.
As new IFUs are brought to the telescope, the distribution of active fiber bundles is changing. Shown below is a (modified) view of the IFU distributions as of different dates. Note that the IFUs are NOT contiguous on the sky! The individual IFUs are magnified 2x here. If you need contiguous sky coverage, see the following information about tiling VIRUS observations. (Magenta areas show bad amplifiers or IFUs without spectrographs attached.)
How to place your target on a VIRUS IFU
The on-sky IFU positions are named using 3 digit code (2-digit COLUMN and 1-digit ROW), as indicated on the images above. For example the IFU in column 10 and row 3 would be IFU 103 and the IFU in column 2 and row 1 would be IFU 021. The center of the focal plane (referred to as the "IHMP") is referred to as IFU 000, and is the default position for VIRUS targets unless otherwise specified.
When you submit a TSL file in Phase II, you may specify the IFU which should contain your target. This is done using the TSL keyword VIFU, e.g., VIFU 046. You may want to consider the current status/quality of various IFUs before requesting observations with a specific unit -- see this page for more up-to-date details: http://www.as.utexas.edu/~gebhardt/virususe/virus_layout.html
The CCD for each VIRUS IFU is read out by four separate amplifiers, which are arranged internally in quadrants (right-upper, right-lower, left-upper, left-lower), but correspond to rectangular regions on the sky, as shown below:
The x,y coordinate system (shown in blue) is in units of arcseconds, with its origin at the center of that IFU. When you request a given IFU, the default position is (0,7), or centered within the RL amplifier.
You can request a different offset position with the TSL keywords offsetx, offsety, which are given in arcseconds. For example, if you wanted to observe a star on the LL amplifier of IFU in slot 046, you would include these settings in your TSL file:
VIFU=046 OFFSETX=0 OFFSETY=7
Dithering and Tiling VIRUS observations
By default, VIRUS observations are taken in a 3-point dither pattern, moving approximately ~1.5" between each position. This dithering fills in the gaps between fibers in each IFU, but does not fill in the gaps between IFUs. If you want complete sky coverage, you must tile multiple dithered observations together to fill in the gaps between IFUs.
Tiling VIRUS observations is not difficult, but first requires determining the parallactic angle(s) for the observations of your target. The focal plane is rotated to be aligned with the parallactic angle for each target, so the rotation relative to North will be different for each target. This angle will determine what offsets are required to tile multiple observations to fill in the gaps between IFUs (note that the parallactic angle will be different for a target observed on its Eastern track compared to its Western track).
If you have access to TACC and a modicum of bravery, you can run the "shuffle" code yourself to determine the parallactic angle(s) at which your target will be observed. Here are the basic steps (borrowed from one of Karl's webpages about VIRUS):
Once you have the Parallactic Angle, you can generate the positions of the 4 observations required to fully tile the sky. This style of tiling will suffice:
- Log in to TACC
- In an empty directory, run shuffle_config to generate config files
- Copy over the latest fplane file (ask if you don't know where to find it)
- run: do_shuffle -v RA DEC rad_off east/west IFU ra_offset dec_offset Name
- RA in decimal hours
- Dec in decimal degrees
- rad_off is not used, so use "0"
- east/west is 0 for east and 1 for west
- IFU is the position on the sky, like 046 or 000
- ra_offset,dec_offset are in arcsecond, relative to specified RA,Dec (probably both 0 for most applications)
- Name is a target name, used for output
- Locate and view the output file shout.result to get the RA,Dec of focal plane center and the Parallactic Angle (5th column in that file).
- nominal pointing (first position)
- offset +50" in the direction of the parallactic angle (second)
- offset +50" perpendicular to the parallactic angle (third)
- offset -50" in the direction of the parallactic angle (fourth)
When generating tiled VIRUS observations remember well that the East and West tracks will have different PAs and will require different positions to fully tile the sky. When you submit your TSL files for these observations you will want to specify an azimuth restriction of AZRES=E for East-only observations (or AZRES=W for West-only observations).
Please contact your friendly Resident Astronomer if you have any questions and to verify that your target submission is correct: astronomer(at)het.as.utexas.edu
Last updated: Thu, 01 Apr 2021 15:15:59 -0500 stevenj