The main Goodman webpage can be found here.

Status:

• 300 l/mm Grating (360 - 915 nm)
• 600 l/mm Grating (355 - 630 nm)
• 600 l/mm Grating (450 - 725 nm)
• 600 l/mm Grating (640 - 915 nm)
• 1200 l/mm Grating (TBD)

Optics:

Imaging Mode:

In imaging mode the plate scale is 0.15 arcsec/pixel and the field of view is 7.2 arcmin in diameter (3096 x 3096 unbinned pixels). 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, 0.84, 1.03, 1.35, and 1.68 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.

NOTE: At present, only the 5 fixed long slits are in use for spectroscopic mode.

Gratings and Preset Observing Modes: 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 2400 l/mm grating being a high priority. The table below shows the dispersion and the wavelength coverage for observations in our set spectroscopic modes.

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. A set of fixed observing modes for each grating are given below. The only gratings available for 2008B are the 300 l/mm and the 600 l/mm gratings.

Calibration Lamps: For the calibration of Goodman spectroscopic data we have a quartz lamp to for spectral flats and HgAr, Ne, Ar, and a CuAr lamps for wavelength calibrabtion. Plots of these spectra with the lines identified in each of our standard spectroscopic modes will will be posted here, as they become available. Please contact spoints@ctio.noao.edu for the latest information about these data.

CCDs: The Goodman focal plane is imaged onto a Fairchild 4k x 4k CCD with the following properties:

Detector Type	Fairchild CCD 486 Backside
Image Size	4096 x 4096 @ 16bits/pixel plus overscan and header 32 Mb per image
Pixel Size	15 microns/pixel = 0.15 arcsec/pixel
DQE	Wavelength (nm)  QE %    300  45  350  70  400  90  500  88  600  85  700  80  800  65  900  42  950  27  1000  14
Dark Current	0.0003 e/pixel/sec
Single Pixel Full Well	139.8 ke
Linearity	0 to 80% Full Well
Cosmetics	Trap: Column 2706, Row 1435; Cluster Defect: Column 2048-2065, Row 129-145
Read Out	Read Rate	Analog ATTN	Gain (e/ADU)	Noise (e)	Read Out Time (sec) Imaging/Spectroscopic
	400 kHz	0	5.67	8.62	24/20
	200 kHz	0 2	1.40 2.67	4.74 5.12	48/40
	100 kHz	0 2 3	0.56 1.06 2.06	3.69 3.72 3.99	96/79
	50 kHz	0 2 3	0.25 0.47 0.91	3.33 3.35 3.41	192/157
Charge transfer efficiency	>99.999

The default image size for imaging mode (1x1 binning) is 3096 x 3096 pixels and the default image size for spectroscopic mode is 4142 x 1896 pixels with 1x1 binning. These values were used calculate the read out times given above using one amplifier readout. Users should also expect 1 or 2 seconds of overhead on every exposure.

Throughput:

The imaging mode throughput has been measured relative to the SOAR Optical Imager (SOI) for which we have good zero-points. As the table below shows, the throughput relative to SOI is better in the R, comparable in the V and lower in the B and U. The U band throughput should improve when the camera optics are rebuilt. The B band is somewhat more mysterious and we are investigating why it is only 80% of the SOI value.

We have also measured the spectroscopic throughput relative the imaging throughput by taking spectra and slit images of the continuum lamp with filters in place. In the 600 line grating set to optimize response on the blue end, the U and B throughputs are ~60% and ~65% of the imaging values respectively. These translate into ~40% overall spectroscopic throughput in the B band. These numbers are provisional and are subject to change once our analysis of spectra of spectrophotometric standards in the different observing modes is completed.

Figure 1 shows measurements of the overall system efficiency (telescope+instrument+detector) obtained with the wide (>10" slit), derived from observations of spectrophotometric standard stars. Slit losses will reduce the efficiency obtained, depending on the seeing, especially for the narrower slits.

Scattered (and Stray) Light:

Measured to be small in imaging mode by comparing imaging FOV and surroundings with a bright star illuminating the pupil. Additional scattering in spectroscopic mode (e.g., from gratings) TBD.

Calibration Issues:

Obtaining good quartz spectra over the entire wavelength range with the 300 l/mm grating is difficult because of the different spectral response at the red and blue ends. The most noticeable effect is that to obtain sufficient counts in the blue end, the red end becomes saturated. A blocking filter is on order so that composite quartz flats can be made. This effect is not as great with the other gratings because the wavelength range isn't as large.

There also exists contamination of the flats by scattering from the back of the second filter wheel. Use of the blocking filter mentioned above mitigates this contamination.

Finally, fringing appears in the spectra to the red of the Hα line. Data are being obtained to measure the fringing and data reduction schemes are being developed to remove this contamination.

Flexure:

The instrument has active flexure compensation based on the Nasmyth rotator angle. The corrections for the old camera were succesful at the fraction of a pixel level for the full range of rotator angles. Calibration needs to be tweaked for the new camera once the adjustment of the counterweight is completed. We also need to check for flexure driven by other effects, such as temperature.

This page was last updated on 04 April 2008 by SDP.
Please send comments or questions to spoints@ctio.noao.edu.