Cassegrain f/8.8 Ritchey-Chretien focus

VIRUS-P

Visible Integral-field Replicable Unit Spectrograph - Prototype


Memo: VIRUS-P and VIRUS-W finder chart software and IFU coordinate files

Phillip MacQueen - 5 June 2013

There are 4 parts to this memo:

  1. the current VIRUS-P and VIRUS-W astrometry
  2. the installation or update on Atlas of a user's copy of the finder chart generating software
  3. the basic use of the finder chart generating software
  4. generating the fiber coordinate files for each dither for earlier or updated astrometry solutions

Part 1) The current astrometry

The current astrometry solution for VIRUS-P was released on 2013 06 05, and replaces the 2013 04 13 solution. The key properties of this solution are:

  1. the solution, like the previous solution, is for the new VIRUS-P IFU that went into service on 2013 03 06. All users who installed this software before 2013 06 05, and new users, should do Part 2 of this memo
  2. the solution uses the measured positions of the fibers in the IFU, whereas the previous solution used the design/theoretical positions. The distribution of the differences between the measured and design positions has a 0.15 arcsecond r.m.s.
  3. the solution applies to all VIRUS-P data taken from 2013 03 06 onward
  4. the VIRUS-P astrometry solution has an r.m.s. error of about 0.2 arcseconds (1/20 of the fiber diameter)
  5. the VIRUS-W astrometry solution has not changed, and is currently the 2012 12 13 version
  6. all coordinates associated with the finder_gen software, either input or output, are equinox J2000.0 and epoch J2000.0. The coordinates are from the USNO NOMAD catalog, which is formally ICRF, but equivalent to equinox J2000.0 to precision much greater than that of the VIRUS-P astrometry

Part 2) Installing and updating the software

Typically, each VIRUS-P or VIRUS-W user will have the finder chart making software installed in their personal account on Atlas. Here's what to do to either install the finder code, or update a previous installation of the finder code:

Assuming the user will install the finder code in the user's home directory on Atlas, or that an earlier version of the finder code is already installed in the user's home directory on Atlas, then: (Note: it can install it in any directory in the user's Atlas account without issues)

cd ~
tar -xvf ~pjm/finder_code.tar

This install or update sets the astrometry used by the finder software to the VIRUS-P astrometry. See part 3 below for instructions about selecting either the VIRUS-P or VIRUS-W astrometry.

When updating an existing installation, only the system files are overwritten, and all the user finder files are left untouched.

That's all for the installation or update (famous last words).

Part 3) Using the finder chart software

To make finder charts, run the finder code in the standard way. For example:

In your account on Atlas:

cd ~/finder_code/scripts

If making finder charts for VIRUS-P, use the current VIRUS-P astrometry by issuing the command:

cp -p astrom_vp.now astrom.now

If making finder charts for VIRUS-W, use the current VIRUS-W astrometry by issuing the command:

cp -p astrom_vw.now astrom.now

Make the finder charts and IFU coordinate files for each dither with:

find_gen.csh <RA as hh:mm:ss.s> <Dec as sdd:mm:ss.s> <Field_name>

If necessary, move the IFU pattern around on the ds9 image to position it as required.

The 3 standard finder charts will be in directory:

~/finder_code/images

with the names: field_name_field, field_name_guider, and field_name_tot

The fiber coordinate files for each of the 6 dithers observed when using script vp2_subdither will be in:

~/finder_code/coords

with the names: field_name_D1_coords.txt, field_name_D2_coords.txt, ... field_name_D6_coords.txt

Note: should you see a number of "xpaset" errors, which is unlikely running on Atlas, typically it's easily fixed. Edit the file

~/finder_code/scripts/find_gen.csh

and search for the line 'sleep 10'. Change the 10 to a larger number, I think 12 or 15 would be fine. You should also edit the line before the sleep, too, changing the 10 to the new numerical value you selected for the sleep command.

Part 4) Using updated or old astrometry

The VIRUS-P and VIRUS-W astrometry solutions specify where the fibers in the IFU are pointing relative to the guider camera. This allows a target object to be centered on a particular IFU fiber by placing a particular guider star at specified pixel coordinates on the guide camera.

When the VIRUS-P guider camera is dismounted and remounted, for whatever reason, the alignment between the guide camera and IFU changes, and so the astrometry solution gets redetermined. Also, if the IFU is replaced, then the astrometry solution is also redetermined. Often the new astrometry solution is not determined until some time after the hardware change that required the new solution. Consequently, the sky coordinates of the IFU fibers will be poorly known until the new astrometry solution is used to determine where the fibers were actually pointing. The procedure for this will be described below. Sometimes fiber coordinate files get lost or need to be regenerated for older data sets taken with a previous IFU or astrometry solution. The procedure is also covered below.

To determine which astrometry solution to use for making fiber coordinate files and finder charts for existing data, follow this procedure on Atlas.

cd ~/finder_code/offsets cd ~/finder_code/scripts

Noting well the comments that follow below, now make the finder charts and IFU coordinate files for each dither in the normal way with the finder generating command: find_gen.csh

The 3 standard finder charts will be in directory:

~/finder_code/images

with the names: field_name_field, field_name_guider, and field_name_tot

The fiber coordinate files for each of the 6 dithers observed when using script vp2_subdither will be in:

~/finder_code/coords

with the names: field_name_D1_coords.txt, field_name_D2_coords.txt, ... field_name_D6_coords.txt

However, this is where the process can be tricky, depending upon details. For the coordinate files to be correct, and this is critical, the finder charts you make in the step above must end up with the guide stars at the same guider pixel coordinates as on the guider finder chart used at the telescope to take the data.

If you made finding charts with a wrong (out of date) astrometry solution, and took data with those charts, you will have to iterate on the coordinates used in the find_gen command above until the guide star pixel coordinates on the resulting guider finder chart match those of the guider finder chart you used at the telescope. The coordinates on your original field finder chart will be a good starting point. The guider pixels are 0.523 arcseconds square, so you should normally iterate until the guider pixel coordinates are correct to better than +/- 0.5 pixels to match the precision of the astrometry.

If you are remaking coordinate files for data taken using finder charts made with the correct astrometry solution, you will not have to iterate if you use the coordinates given on the field finder chart in the find_gen command.

Important: remember to reinstall the current astrometry solution, as described in Part 3 above, before making new finder charts.

2.7 m (107") Harlan J. Smith Telescope

Cassegrain f/8.8 Ritchey-Chretien focus

DIAFI

VIRUS-P

CCD WHT Camera

Imaging Grism Instrument (IGI)

Coudé f/32.5 focus

Robert G. Tull Coudé Spectrographs

Tull Spectrograph (TS1)

Cross-Dispersed Echelle Spectrograph (TS2)