Low-Resolution Spectrograph

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Overview

• PI is Gary Hill, McDonald Observatory, UT Austin
• CCD PI is Phillip MacQueen, McDonald Observatory, UT Austin
• Rides on HET tracker at prime focus
• Grism-based with three modes: imaging, long slit, and multi-object
• R = 600 - 3,000
• 400 to 1000 nm wavelength coverage in two exposures at R ~ 1,000
• Also provides broad- and narrow-band imaging over four arcmin HET FoV
• 30% throughput on the sky for spectroscopy (absent slit losses)
• CCD is Ford Aerospace 3072 x 1024 device with 15-micron pixels
• Optics partly constructed in Mexico at IAUNAM
• Funded primarily by Stanford, Ludwig-Maximilians-Universität München, and UT Austin
• Commissioned in Spring 1999

The LRS is a long-slit imaging spectrograph, located at the prime focus of the HET. It is described in Hill et al. (SPIE Conf. 3355, p 424, Kona, March 1998). Note that the position angle of the Prime Focus Instrument Package (PFIP) can be rotated to set up on arbitrary position angles, allowing exposures of more than one object simultaneously.

Basic Instrument Parameters

• Plate scale: 0.235 arcsec/pixel; 4 arcmin square field of view, vignetted severely outside 4 arcmin diameter by HET optics. LRS is used in 2 x 2 binning as a default, but other binning is available.
  
  
• Slits: 1.0, 1.5, 2.0, 3.0, 10.0 arcsec wide long slits, all 4 arcmin long.
  
  
• Filters: BK7, B, V, C, R, I, GG385, GG475, OG515, OG550, OG590. BK7 is clear glass, rarely used, and should only be selected if no filter is needed. GG385 should be the default blocking filter. For those unfamiliar with Schott Glass blocking filters, the number in the name is the approximate half-power point of the transmission cut-on in nm. C is Phillip MacQueen's B+V filter (3mm GG385 + 2mm BG39 ).
  
• Grisms:
  The resolving power decreases linearly with increasing slit width.
  • G1: 300 l/mm, 4150 - 10100 Å, at 4.5 Å/pix (after 2x2 binning), R=550 at 5100 Å with 1.0 arcsec slit
  • G2: 600 l/mm, 4270 - 7280 Å, at 2 Å/pix (after 2x2 binning), R=1300 at 5950 Å with 1.0 arcsec slit
  • G3: 700 l/mm, 6250 - 9100 Å, at 1.88 Å/pix (after 2x2 binning), R=1900 at 8000 Å with 1.0 arcsec slit

  The instrument has two grism holders. The G1 grism is permanentlky mounted in one of the two holders, while the he other holder sometimes holds the G2 and sometimes the G3 grism.

Configuration names are constructed in line with the following convention: <grism>_<slit width>_<filter name>. Examples are found in the following table:

Configuration	Grism	Slit (arcsec)	Filter
lrs_g1_1.0_GG385	g1	1.0	GG385
lrs_g2_3.0_BK7	g2	3.0	BK7
lrs_no_no_B	none	none	B

LRS Performance

We do not report on the imaging performance because the depth depends so much on image quality and sky brightness. This is not a useful telescope for deep imaging yet.

Use the following table to make rough estimates of exposure times for your spectroscopic observations. These are sky-noise dominated observations, so remember to scale by the sqrt(time):

Configuration	Type of Object	Mag	Time (sec)	S/N @ Wavelength
lrs_g1_2.0_GG385	stellar/QSO	20	1800	15-20 @ 6500 Å
lrs_g1_2.0_GG385	stellar/QSO	21	1800	3-8 @ 6500 Å
lrs_g1_2.0_GG385	distant galaxy	21	1800	9-14 @ 6500 Å
lrs_g2_2.0_GG385	distant galaxy	21	1800	4-9 @ 6500 Å
lrs_g3_2.0_OG515	stellar	20	1800	14-17 @ 8000 Å

In your proposals, you should state an integration time per object, rather than a required S/N ratio, since there are too many variables to scale. Delivered S/N ratio is determined primarily by the average image size during a track. The size depends on the temperature profile during the night, which determines the rate at which the primary mirror segments de-stack. On stable nights, 1.5 arcsec FWHM average is typical, but on poor nights it is worse. Hence, detailed S/N estimates are difficult, and the uncertainty is reflected in Table 3 above.

More detailed throughput informaiton is available at the HET Resident Astronomer Information Page. This link takes you directly to the LRS Throughput Page