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Stars for Measuring PWV

Stars for Measuring Precipitable Water Vapor with MIKE

An important objective of the GMT Site Testing program at Las Campanas is to provide information on the Precipitable Water Vapor (PWV) characteristics of the site. To help with the absolute calibration of the measurements that will be taken, we are requesting that MIKE observers contribute a small amount of their observing time on each clear night to take a high signal-to-noise spectrum of one or more of the bright stars listed in the table below. The specific requirements for these spectra are:

  • Photometric conditions
  • Red Camera
  • Slit width ≤ 0.7 arcsec
  • Two exposures
  • Exposure time ~ 5 sec, or whatever is required to get a minimum of 10,000 counts at the peak of the blaze in order 50 (See the MIKE ThAr atlas).
  • Calibrations including ThAr arcs, Quartz flats from a previous or subsequent observation close in time and Milky flats from any time during your run all at the same binning and resolution (slit width) as the target observations.

The list of bright stars is given below; an observing catalog file named telluric_stds.cat has been provided to the Telescope Operators so that the telescope can be quickly pointed to the selected star. Note that two of the stars are wide binaries; magnitude differences and separations of the secondary stars are given to for these avoid any confusion with respect to the identification.

If you are going to make an observation of a PWV calibration star, please let your operator know so that he can contact the GMT site testing operator (Cesar Muena or Gabriel Prieto, ext 655) to ensure that IRMA will be taking data concurrently (as opposed to its own internal calibrations that occur every two hours, 0, 2, 4, etc... with a duration of 30 min).

If you are able to take one or more spectra of these stars during your observing run, please list in the nightly report the names and location of the pwv calibration observations and the appropriate calibration files (arcs, flats, and biases).

HR RA(2000) Dec(2000) V Sp. Class ΔV Separation(")
845 02 49 54.2 -27 56 30 5.39 A0 V    
1621 05 01 25.6 -20 03 07 4.91 B9.5 Vn    
2595 06 55 46.8 -22 56 29 5.30 B3 II-III    
3090 07 53 18.2 -48 06 11 4.24 B0.5 Ib    
3476 08 43 40.3 -49 49 22 5.16 B0 IIIn    
4172 10 38 50.4 -12 26 37 6.04 A0 Vn    
4748 12 28 22.5 -39 02 29 5.44 B8 V    
5174 13 46 56.4 -36 15 07 5.15 A0 V 7.3 26.3
5517 14 47 57.5 -26 38 47 5.77 B9 V    
5987 16 06 35.5 -36 48 08 4.23 B2.5Vn    
6141 16 30 12.2 -25 06 54 4.79 B2 V    
6930 18 29 11.9 -14 33 57 4.70 A3 Vn    
7830 20 29 53.9 -18 35 00 5.94 A3 Vn 0.8 21.9
8431 22 08 23.0 -32 59 19 4.50 A2 V    
8998 23 44 12.1 -18 16 37 5.24 B9 Vn    
9098 00 03 44.4 -17 20 30 4.55 B9.5 Vn    

 

Many thanks to those who have previously contributed data!   These observations are now more important than ever with the installation of IRMA (a 20 m radiometer to measure PWV) in mid Sept 2007.  We successfully used the 2005 data to calibrate the University of Arizona 225 Ghz Tipping Radiometer (Thomas-Osip et al 2007, PASP, 119, 687).

For those who are interested, the spectra that are taken will be used to measure the equivalent width of an H2O line at 6943.79 Å which has been shown to be effective for measuring precise values of the PWV (Brault et al. 1975, J. Quant. Spectrosc. Radiat. Transfer, 15, 549). A plot of the MIKE echelle order with this line can be seen by clicking here. For a plot showing a blow-up of this order centered on the line, click here.


Updated  Oct. 15, 2007 jet AT  lco DOT cl
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