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George and Cynthia Mitchell Spectrograph (GCMS)

Visible Integral-field Replicable Unit Spectrograph - Prototype (VIRUS-P)



Dr. Phillip MacQueen
(512) 471-1470

pdfInstrument paper

GCMS is a high-efficiency, low- to moderate-resolution integral field spectrograph with a 2.82 square arcminute field of view. Poisson noise limited, sky subtracted spectra are routinely achieved. The reflective optical coatings set the bandpass of the instrument as 340 - 685 nm, and there are approximately 4.6 pixels per spectral resolution element. The instrument is extremely stable due to the use of invar for thermal stability, and a gimbaled mount for stability against gravitational flexure. The instrument went into service in 2007.

Integral Field Unit (IFU): The IFU consists of 246 fibers arranged in a fixed pattern with a 1/3 fill factor. Each fiber has a 4.16 arcsec diameter on the sky. Observations taken in a 3-position dither pattern provide essentially complete coverage within a 100x102 arcsecond field of view. The fibers are separated by 8 pixels on the CCD detector.

Gratings: there are 4 volume phase holographic gratings (VPHGs) available for use:

Grating Resolution Spectral Coverage Range of coverage
VP1 5.3 Å ~2240 Å   3400 -   6850 Å
VP2 1.6 Å ~609 Å ~3400 - ~6000 Å
VP3 1.0 Å ~420 Å   3400 -   4395 Å
VP4 1.5 Å ~650 Å ~5500 -   6850 Å

A program exists on the observing computer for determining the spectrograph settings for a user specified grating and desired spectral region. The McDonald Observatory Observing Support group will normally change gratings and configure the spectrograph for observers. If a special need exists, such as a grating change in the middle of the night, observers may ask to be trained to do this.

For observations including wavelengths longer than ~650 nm, an order block filter is used in front of the IFU to suppress second order light from the grating. The filter is 99% efficient, transmits the 400-700 nm spectral range, and cuts on and off very sharply. When the order blocking filter is used, the filter on the guide camera is changed so that the guider and IFU stay parfocal. The Observing Support group will install the filters.

Detector: the detector is a Fairchild 2048x2048 backside illuminated, anti-reflection coated CCD. It is typically binned either 1x1 or 2x1, with the binning in the dispersion dimension. The readout rate is typically either 50 or 100 kilopixels per second, with a readout noise of 3.7 electrons with a gain of 1.0 electrons per data unit.

Acquisition and guiding: a 512x512 pixel Apogee Alta camera views a field 540 arcsec north of the science field. The pixels are 0.51 arcsec square and the guider field size is 260x260 arcsec. The guide camera is feed by a dioptric, f/3 focal reducer. A 400-600 nm filter is standard and required in front of the guider.

Interactive finder chart software is available at the telescope and elsewhere. It allows the IFU to be positioned on a digital sky survey image of the science field, and produces both a science field chart showing where the fibers are located, and an off-axis guider finder chart with the pixel coordinates of the suitable pointing/guide stars. Targets can be placed on IFU fibers with ~0.5 arcsec accuracy by placing a pointing/guide star at the given pixel coordinates on the guider.

The finder chart software also makes files that give the sky coordinates of each fiber in each dither for the standard 6-dither pattern used while observing.

Since late 2012, all users have a personal account on the new computer atlas at the 107" telescope, and make their finder charts in their personal account. To learn how to install the finder chart software in your atlas account, to learn about the GCMS and VIRUS-W astrometry, and to learn how to make coordinate files and finder charts with an old or newly updated astrometry solution, read the information at this astrometry link.

Focus: the telescope is focused by minimizing the image size on the guide camera. The GCMS guider and IFU are parfocal, so once the telescope is in focus on the guider, it is also in focus on the IFU. The instrument focus is not adjustable by the observer. The instrument holds focus over the -10 to +30 degree Celsius operating temperature range of the telescope. Please also see the note about order blocking filters in the section on gratings.

Calibrations: wavelength calibration spectra are typically taken as dome arc lamp spectra. Six arc lamps are available, and lamps are typically used in pairs such as HgCd, HgNe, and HgHe. Flat fielding is done with twilight flat field frames taken at dusk and/or dawn. An observing script takes the data and maintains a uniform exposure level across the set of frames as the twilight sky brightness changes.

Observing: GCMS can be used efficiently by a single observer. New observers must receive training by either sitting in for 2 nights on an experienced observer's observing run, or by having an experienced observer present for the first two nights of their own observing run. Observing scripts are used to control the telescope and instrument to take sequences of data in precise dither patterns.

Data reduction software: a software reduction package called vaccine has been written for reducing the raw data to extracted spectra. Vaccine is widely used, but, it is not supported currently for general use. Vaccine runs on a Linux-based computer.

Instrument Reference (paper linked above): Hill, G.J., MacQueen, P.J., Smith, M.P., Tufts, J.R., Roth, M.M., Kelz, A., Adams, J.J., Drory, N., Grupp, F., Barnes, S.I., Blanc, G.A., Murphy, J.D., Altmann, W., Wesley, G.L., Segura, P.R., Good, J.M., Booth, J.A., Bauer, S-M., Popow, E., Goertz, J.A., Edmonston, R.D., & Wilkinson, C.P., 2008, “Design, construction, and performance of VIRUS-P: the prototype of a highly replicated integral field spectrograph for the HET”, Proc. SPIE, 7014, 257

Acknowledgments: the GCMS (VIRUS-P) project team was Michael Smith, Pedro Segura, Verner Altmann, Gordon Welsey, John Good, Stuart Barnes, Joe Tufts, Doug Edmonston, Martin Roth, Andreas Kelz, Jeremy Murphy, Chris Wilkinson, Josh Adams, Karl Gebhardt, Guillermo Blanc, Jimmy Welborne, George Barzack, Johnny Goertz, and Sherry Guthrie. The project was lead by Phillip MacQueen and Gary Hill.

We thank the Cynthia and George Mitchell Foundation for funding the GCMS instrument.