Contents
- Detector Upgrade (October 2006)
- Optical Layout
- ITL/STA CCD Operating Parameters
- Red Channel Configurations
- Red Channel Gratings
- Theoretical Grating Blaze Functions
- Use of the Echellette Grating
- Aperture Plates
- Slit Length
- Variation of Line Width with Slit Width
- Order-Sorting Filters
- Spectrograph Throughput
Detector Upgrade
Click here for details on the upgrade to a deep depletion CCD.
Optical Layout
Click here for a picture of the optical layout
ITL/STA CCD Operating Parameters
Click here for detector information.
Red Channel Configurations
Red Channel mounted on Blue Channel:In this configuration, the Red Channel is bolted to the bottom of Blue Channel, utilizing the Blue Channel slit, filter assembly and exposure-control shutter. The unvignetted slit length is limited to about 150 arcsec. Only standard slit plates can be used, but both the slit plates and filters can be interchanged with others resident in the Blue Channel quickly by remote control.
Red Channel Gratings
| Grating (grooves/mm) | Blaze (order; wavelength) | Resolution (with 1" slit) | Coverage (Å) | Blaze Angle (deg) | Dispersion (Å/pix) |
| Echellette | 11th/5235 Å | 1.6 | 456 | 26.8 | 0.58 |
| 150 | 1st/4800 Å | 20.0 | 4800 | 2.2 | 6.38 |
| 270 | 1st/7300 Å | 10.8 | 2870 | 5.9 | 3.59 |
| 300 | 1st/4800 Å | 10.0 | 2500 | 4.3 | 3.21 |
| 600 | 1st/4800 Å | 4.8 | 1300 | 8.6 | 1.63 |
| 600 | 1st/6310 Å | 4.7 | 1310 | 11.4 | 1.64 |
| 1200 | 1st/5767 Å | 2.1 | 650 | 21.1 | 0.82 |
| 1200 | 1st/7700 Å | 1.9 | 640 | 28.7 | 0.80 |
| Lo-res. Ech. | 7th/5134 Å | 5.6 | 1390 | 8.6 | 1.74 |
| Hi-res. Ech. | 18th/5790 Å | 1.6 | 320 | 25.7 | 0.67 |
Notes:
- Spectral coverage is quoted for central 800 pixels only. Images and throughput degrade somewhat in first and last 200 pixels.
- Echellette is used in orders 6-13.
- The Hi-res and Lo-res Echellettes are superceded by the new Echellette.
Theoretical Grating Blaze Functions
Click here for a Postscript plot of the blaze.
Use of the Echellette Grating
The Red Channel Echellette grating gives complete coverage from 4300-8900 at a spectral resolution of a near-constant 90 km/sec when using a 1"x20" slit. Nominally it works in orders 7-13 but it can be used in order 6-13 with some sacrifice of coverage in the blue and some order overlap between orders 6 and 7.The echellette should be used near its blaze at a tilt of 4.675, corresponding to a central wavelength of about 5250 Å in order 11. The cross-disperser should be set at 480 to put order 7 at the top of the frame; to get order 6 at the top, set the cross-disperser at 430.
Sample Echellette flat-field image: Blue is at the bottom and to the left. Orders are marked. This image was taken at a cross-disperser setting of 480. By judicious placement of the camera in the cross-dispersion direction, it is possible to include order 6, which is off the top of the frame, at the expense of a few hundred A in order 13.
Sample Echellette HeNeAr image: Strong Helium comparison lines are marked.
Aperture Plates
| 0.75 arcsec slit, 180 arcsec length |
| 1.00 arcsec slit, 180 arcsec and 20 arcsec lengths |
| 1.25 arcsec slit, 180 arcsec and 20 arcsec lengths |
| 1.50 arcsec slit, 180 arcsec and 20 arcsec lengths |
| 2.00 arcsec slit, 180 arcsec and 20 arcsec lengths |
| 3.50 arcsec slit, 180 arcsec and 20 arcsec lengths |
| 3.75 arcsec slit, 180 arcsec and 20 arcsec lengths |
| 5.00 arcsec slit, 180 arcsec and 20 arcsec lengths |
| "Comb" -- line of 1 arcsec circular holes on 10 arcsec centers |
| 3 arcsec diameter single circular hole |
| 1.0, 1.4, and 5.0 arcsec diameter double circular holes |
| 1.0x2.6 and 2.0x3.0 arcsec double slits |
| 1.25x90 arcsec slit for use with cross-dispersing prism |
Slit Length
The finite size of the filter wheels in Blue Channel vignettes light near the ends of the long slits when the Red Channel is mounted on the Blue Channel. The unvignetted slit in this configuration is about 150 arcsec long, as shown in this plot of the intensity along the slit.
In this example, the CCD was binned by a factor of two in the spatial direction, giving a spatial image scale of 0.6 arcsec/pixel, twice that of the nominal unbinned scale of 0.3 arcsec/pixel.
Variation of Line Width with Slit Width
The following table gives the FWHM of a Gaussian fit to an unresolved comparison lamp emission line as a function of the width of the entrance slit. A low-resolution grating was used, so the anamorphic factor is near unity. The CCD was not binned on readout.Click on the individual table entries to see the line profiles.
Note that the comparison lines are not well approximated by a Gaussian for wider slits so the quoted FWHM are not particularly accurate.
| Slit Width (arcsec) | FWHM (pixels) |
| 0.75 | 2.6 |
| 1.0 | 3.2 |
| 1.25 | 3.8 |
| 1.5 | 4.1 |
| 2.0 | 5.3 |
| 3.5 | 10.0 |
| 5.0 | 16.0 |
Order-Sorting Filters
Blue-Blocking Filters:The following filters are available to block light from higher orders. Those marked with asterisks are normally mounted in the spectrograph. Click here for transmission curves and here for a Postscript plot.
| UV36 | L-38* | L-42* |
| LP-495* (Y-50) | LP-530 | R-63 |
Red-Blocking Filters:
The following filters are available to block light from lower orders. Those marked with asterisks are normally mounted in the spectrograph. Click here for transmission curves and here for a Postscript plot.
| CuSO4 | U330 |
| C-500 |
Click here for transmission curves and here for a Postscript plot.
Hoya Filters:
The following filters standard Hoya Glass 2"x2" filters that are kept on hand at the MMT. Consult a Hoya catalog for transmission curves. Note that these are not usually resident in the spectrograph, so prospective users must alert the Instrument Specialist well in advance of their run.
| ND 0.3 | ND 1.3 | ND 2.5 |
| ND 4.0 | ND 5.0 | ND 7.0 |
| U 360 | UV 28 | O 54 |
| O 56 | B 390 | R 60 |
| R 68 | IR 85 | LB 120 |
| LB 145 | LB 165 | LB 200 |
| LA 120 | RM 90 | Y 48 |
| V 10 | G 533 |
Spectrograph Throughput
The data referred to below are spectra of standard stars observed through a 5 arcsec slit in photometric, good-seeing conditions. We do not have data for all gratings at all possible wavelength settings.No corrections have been made to the data except to divide them by the exposure time, multiply by the CCD gain and divide by the average pixel width (in Å). Therefore, the spectra show measured number of photons per second per Angstrom. No corrections for extinction, telescope reflectance, slit losses, etc. have been applied. We list the mean pixel size and the airmass and plot the AB magnitude versus wavelength in the lower panel.
In the cases of high resolution gratings, data from several grating settings are presented on a single plot.
Note: The plots below were constructed with data taken on the 4.5-m MMT. Given that the 6.5-m telescope has two fewer reflections and more than twice the light gathering power of the 4.5-m telescope, the numbers in the plot should be multiplied by a factor of 2.5 or the effective AB magnitude should be increased by one magnitude when estimating exposure times for the new telescope.
Click on the appropriate configuration for a Postscript plot.