WIYN Observatory

OVERVIEW OF SPECTROSCOPY AT WIYN

Introduction

Spectroscopy at WIYN is provided by a versatile bench-mounted spectrograph that accepts fiber input from either a multi-fiber positioner (Hydra), or one of three Integral Field Units (Sparspak, Hexpak, or Gradpak). The Bench Spectrograph (BS) is a stable instrument mounted on an air table in an insulated room below the telescope. It permits relatively easy changes of configuration between high and low dispersion and between various spectral regions. The figure above shows the BS and indicates the location of its elements including the fiber cables.

Hydra is useful for multiple targets within a 1 degree field diameter with separations ≥ 37". Two separate sets of fibers, red and blue, with 90 and 83 active fibers respectively, are positioned by an internal robot arm. The fiber positions are predetermined by the observer using her or his precise coordinates and a program called WHYDRA.

The Integral Field Units (IFUs) can also be used with the BS for two-dimensional spectroscopy of extended objects. Since the IFUs mount on the same Nasmyth port as Hydra, they cannot be used when Hydra is mounted and vice versa.

Determining a Bench Spectrograph Configuration

The prospective user will need to make several decisions to arrive at a suitable setup for his/her science objectives. WIYN staff may also be consulted for help in this regard. The items in a spectrograph configuration include:

  1. Fiber cable or IFU (wavelength region, resolution, field sampling).
  2. Grating (wavelength region, spectral coverage, resolution).
  3. Filter (interference filter - echelle, order separation - other gratings.
  4. CCD gain and binning (resolution, read noise, read time).

A. Choosing a Hydra Cable or IFU

The Hydra blue cable fibers (3.1" or 310 microns) transmit well from 3500 Å - 8500 Å with a 20% dip in transmission between ~7100 Å - 7300 Å. They have higher transmission than the red fibers ~3500 Å - 5500 Å.

The Hydra red cable fibers (2.0" or 200 microns) are useful between ~4000 Å and 11,000 Å, and must be used >8500 Å. The red fibers will generally yield higher resolution than the blue fibers due to their smaller diameter.

The Sparsepak IFU has 82 500 micron (4.7") fibers in a 72" x 71" grid with a densely packed core of 16 fibers. The 82 fiber total includes 7 sky fibers located ~25" away from the grid. All of the IFUs can be rotated to align to a particular angle on the sky.

The Hexpak IFU has a high spatial and spectral resolution core of 18 0.94" fibers subtending 6" in diameter, surrounded by a hexagonal array of 2.9" fibers subtending 40.9" in diameter.

The Gradpak IFU consists of an array of 5 different fiber sizes (1.87" - 5.62") in an approximately rectangular field of 38.89" x 54.97".

B. Choosing a grating

Nine gratings are available to provide a wide range of spectral coverage and resolution. This table lists the dispersion in Å/pix for each grating with 2x2 on-chip binning for a sample of BS configurations. The listed central wavelengths are near the gratings' peak efficiency, but of course the central wavelength will depend on the observers requirements. Values for spectral resolution assume typical projected fiber sizes for the red and blue cables.

C. Filters

Interference filters are provided for the various orders of the 316@63 echelle grating to insure only one order will be present in the spectra. Schott glass filters are used with the low order gratings to eliminate unwanted orders. These are typically needed when working in 1st order above 7000 Å, and in 2nd order below ~5500 Å.

D. CCD Gain and Binning

The CCD control software, MOP (Monsoon Operating Platform), provides gain and binning options for observers. With higher gain values, there are longer readout times but the readout noise will be lower. Five on-chip binning options are also available with larger binning affording shorter readout times but also lower resolution for some bench configurations.

Bench Spectrograph CCD Binning Options


Binning
(cols x rows)
Image Size
(#cols x #rows)
Readout Time
(sec)
Comments
Gain = 0.8 Gain = 0.4 Gain = 0.2
1x1 2600 x 4000 97 140 222 no binning
2x1 1300 x 4000 65 93 132 2 pix col, no row bin
2x2 1300 x 2000 34 45 71 2 pix col, 2 pix row,
most commonly used
4x2 650 x 2000 25 35 43 4 pix col, 2 pix row
4x3 650 x 1333? 18 22 29 4 pix col, 3 pix row
often used w/SPSK

Bench Spectrograph CCD Gain Options


Gain Setting
(e-/ADU)
Full Gain Value
(e-/ADU)
Readout Noise
(e-)
Comments
Low 0.8e-/ADU 0.876 electrons per ADU 4.3 low conversion gain
Med 0.4e-/ADU 0.438 electrons per ADU 3.7 medium conversion gain
High 0.2 e-/ADU 0.219 electrons per ADU 3.3 high conversion gain

Reducing Bench Spectrograph Data

IRAF contains an extensive library of software tasks to calibrate, extract, and wavelength scale BS data. Besides the short tutorial (Appen. F) in the WIYN Hydra and Bench Spectrograph Users Manual, a very comprehensive guide is forthcoming from Eric Hooper, Marsha Wolf, Paul Sell, et al.: "Guide to Reducing IFU data".

Last modified: 12-Oct-2015 11:19:21 MST