Goodman High Throughput Spectrograph

The Goodman High Throughput Spectrograph was built in the Goodman Laboratory at the University of North Carolina under the leadership of Prof. J. Christopher Clemens. It employs all transmissive optics, and Volume Phase Holographic (VPH) Gratings to achieve the highest possible throughput for low resolution spectroscopy over the 320-850 nm wavelength range. In order to capitalize on the properties of VPH gratings, the instrument features an articulated camera. This allows the camera-collimator angle to be adjusted so as to tune the blaze wavelength thus maximizing the efficiency at any desired wavelength.

The Goodman Spectrograph Team (from left to right) Matt Bayliss, Chris Clemens and Adam Craine, pose in front of the Goodman Spectrograph following succesfull instalation at the Optical Side Nasmyth Focus of the SOAR telescope.

Status

The Goodman Spectrograph was delivered to Chile on 12th June 2004, and was quickly assembled and integrated with the Optical ISB. Following a period of laboratory testing it was installed on the SOAR telescope achieving First Engineering Light on August 8th. Unfortunately poor weather conditions prevented the collection of data which would allow measurement of the instruments throughput. However, the laboratory and on telescope testing confirm that the spectrograph meets its image quality specification, and that flexure is within the range of correction of the active adjustment built into the camera mount. A thorough shake down of the instrument control and observing software was also carried out during this period. These initial tests were carried out using an Apogee AP10 camera in place of the Fairchild CCD.

It is anticipated that the Goodman Goodman Spectrograph will be made available for "Early Science" during the 2008B Semester. Use will be restricted to imaging, and spectroscopy with a single long slit. The Fairchild CCD is currently under commissioning at the telescope.

Imaging Mode

In imaging mode the plate scale is 0.15 arcsec/pixel and the field of view is 7.2 arcmin in diameter. There are two independent six position filter wheels that hold 4 inch diameter filters. One of these will be used for imaging, the other for spectroscopic order sorting filters. The initial compliment of imaging filters includes U, B, V, and R on the Kron-Cousins system. The filters are in the collimated beam (tilted to avoid ghosts), so no refocus of the camera is required when the filters are changed. Installing different filters is straight forward, but should be considered a day-time operation.

Spectroscopic Mode

slits

In Spectroscopic mode the Goodman Spectrograph will be able to obtain spectra of multiple objects simultaneously over a field of 3.0 x 5.0 arcminutes using multi-slit masks. A carousel style mask changer, holding up to 36 masks will allow the slit plates to be accurately and reproducibly located at the instruments entrance aperture.

The instrument will initially be deployed with a compliment of 5 fixed long slits with widths of 0.45, 0.60, 0.85, 1.10 and 1.5 arcsec. These are each 3.9 arcminutes long, but can be fitted with optional decker plates to mask the upper and lower portions.

A cutting machine for the fabrication of custom multi-slit masks, has been purchased during 2007 using Brazilian funding, and will be shared between SOAR and Gemini South. We anticipate that the software to design masks from either images or coordinate lists, and to tweak the alignment of the masks at the telescope, will be developed on the same timescale.

Gratings

Up to three gratings can be installed in the spectrograph at a time, in a linear stage which allows the rapid interchange of gratings. Installing different gratings is straight forward, but should be considered a day time operation.

The initial grating complement includes 300, 600 and 1200 l/mm transmission VPH gratings. Each of these is sub-optimum in some way, and they will be replaced with Goodman Laboratory manufactured gratings within the first year of operation. A wider compliment of gratings may also be fabricated, depending on user demand, with a 2400l/mm grating being a high priority. The table below shows the dispersion and the wavelength coverage for observations centered at 550 nm.

Because VPH gratings operate via Bragg scattering, efficient operation requires Littrow or near-Littrow operation of the spectrograph. A grating rotation stage sets the incident angle to the desired value, which depends upon the line density of the grating and the wavelength of interest. A concentric camera rotation stage must then be set to nearly twice this angle to intercept the diffracted beam. An Observation Planning Tool, which calculates and displays approximate efficiency curves using rigorous coupled wave models, will be made available shortly, to allow users to establish the optimum choices for their observations.

Detectors

Final Science Array

The planned science array for the Goodman Spectrograph is a 4096 x 4096 Fairchild CCD. This CCD is mounted in a compact dewar cooled using a Cryotiger cryocooler.

At the present time testing of the CCD is underway. So far the results look promising, however, the most important parameter, the QE, remains to be measured. The resulting performance data will be posted here as soon as testing is complete. The full 4k x 4k science CCD covers the entire 7.2 arcmin diameter imaging field with 15 micron/pixel (0.15 arcsec/pixel). It will also allow for a longer dispersion axis in spectroscopic mode, and un-illuminated shift storage regions on the spatial axis, supporting use of "nod-and-shuffle" modes for better sky subtraction in multi-slit spectroscopy.

Additional Information

• J. Christopher Clemens, J. Adam Craine & R. Anderson "The Goodman Spectrograph" 2004 Proceedings of Glasgow SPIE meeting.