Blue & Red Channel Spectrographs

A Brief Description:
The MMT Spectrograph is actually composed of two spectrographs sharing a common focal plane assembly and two filter wheels. The spectrographs are denoted Blue Channel and Red Channel for the wavelength ranges for which they are optimized. The instrument mounts to the standard MMT f/9 top box which houses the acquisition and guide cameras, the comparison lamp system and a wavefront sensor.  For instructions on how to propose to use either Red or Blue Channel, please see here. Note: Remote observing is available for both instruments if you have previously observed with them in person at the MMT during good weather with the dome open for at least part of the night. For more information on remote observing, please see here.

 

Blue Channel (Available): a low-to-intermediate resolution instrument optimized for spectroscopy in the range from 3200-8000 Å. A number of gratings are available giving resolution as high as 30 km/sec. Three gratings can be resident in the spectrograph at any one time, thereby facilitating rapid reconfiguration of the spectrograph. The current detector is a highly-optimized ITL/STA 2688x512 CCD. Instrument reference.

 

Red Channel (Available*): a 3.75" beam spectrograph optimized for the region 5000 Å to 1.0 μm but usable down to about 3700 Å. A highly-optimized ITL/STA 520x1032 fully depleted CCD is used as the detector. The spectrograph has several operating modes including high-throughput long-slit modes at a variety of spectral resolutions from 20 Å to about 2 Å and prism cross-dispersed modes yielding complete coverage from 4500 Å to 1.0 μm at moderate spectral resolution. As in the case of the Blue Channel, three gratings can reside in the spectrograph at any one time. Instrument reference. *There is a glow present in the Red Channel detector. Please see the Red Channel webpage.

The Fundamental Capabilities:

Please follow the Blue and Red Channel links below to see tables of the fundamental capabilities.

Further Information:

For hints and tips on how to operate the instrument please see the Cheat Sheets.

Aperture Plates and Filters

The aperture wheel, the shutter, and the upper and lower filter wheels are used for both Blue and Red Channel.

Aperture Plates

The aperture plates are tilted 12.5° with respect to the optical axis to allow for reflection to a slit viewing acquisition and guide camera. The available aperture plates are listed in the table below.  The aperture wheel will accept up to seven aperture plates at a time. One position in the wheel has been modified to allow the aperture to be changed while the spectrograph is mounted on the telescope. Observers may request any aperture plate for this position. The other six positions are typically populated with the 180" long slits excepting only the 0.75" wide slit.

The 20" and 9" slits are generally used for the Blue and Red Channel echellettes, respectively, in cross dispersed mode. The last two columns provide the FWHM of a Gaussian fit to an unresolved comparison lamp emission line as a function of the width of the entrance slit. In both cases a low-resolution grating was used, so the anamorphic factor is near unity. The CCDs were not binned on readout. Selecting the links in the table entries will display measured profiles.

Note that the comparison lines are not well approximated by a Gaussian for wider slits so the quoted FWHM are not particularly accurate.

Standard Slits: (√ = available)

Slit Width (arcsec)180" Length20" Length9" LengthBlue FWHM (pixels)Red FWHM (pixels)

0.75

  2.6

1.00

 √2.83.2

1.25

3.13.8

1.50

3.64.1

2.00

4.55.3

3.50

7.910.0

3.75

   

5.00

11.516.0

Special Purpose Slits:

Comb - line of 1" diameter circular holes on 10" centers

3" diameter single circular hole

1.0", 1.4", and 5.0" diameter double circular holes

1.0" x 2.6" and 2.0" x 3.0" double slits

1.25" x 90" slit for use with the Blue Channel cross dispersing prism

90" circular hole for direct imaging

Slit Length

The finite size of the filter wheels vignettes light near the ends of the long slits. The unvignetted slit is about 150 arcsec long.  The following links show plots of the intensity along the slit. For this data, the CCD was binned by a factor of two in the spatial direction, giving a spatial image scale of 0.6"/pixel, twice that of the nominal unbinned scale of 0.3"/pixel.

Blue Channel Slit Intensity

Red Channel Slit Intensity

Shutter

A Uniblitz shutter is located below the slit assembly and above the filter wheels. The shutter is controlled by the data acquistion computer and the CCD controller.

Upper and Lower Filter Wheels

Each filter wheel has seven filter positions and one clear position. Each position can accommodate a 2-inch square filter with a thickness of up to ~6 mm. The available filters and their transmission curves are provided below. Filters cannot be changed while the spectrograph is mounted the telescope so please contact MMTO staff if you require a non-standard filter configuration. The two filter wheels are currently populated as follows:

Upper Wheel:

  • Clear
  • 2.5 Mag
  • R-63
  • L-38
  • ND 0.5
  • 1.25 Mag
  • UV-36
  • L-42

Lower Wheel:

  • Dark
  • CM500
  • CuSO4
  • LP-530
  • U-330
  • LP-495
  • 14th Order
  • Clear
Filters

The filter transmission curves provided below were measured in 1989 by Sally Oey using the monochromator at NOAO.


Blue Blocking Filters:

The following filters are available to block light from higher orders.  A plot of the filter transmission curves is available in pdf or png format.

  • 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.  A plot of the filter transmission curves for the CuSO4 and U330 filters is available in pdf and png format and for the C-500 and CM-500 filters is available in jpg format and gif format.  Or see the Hoya Color Compensating Filters Page.

  • CuSO4
  • U330
  • C-500
  • CM-500


Larger Filters:

We also have a few larger 6-in x 6-in filters. A plot of the filter transmission curves for the larger filters is available in pdf and png format.


Blue Channel Echellette Order Sorting Filters:

PDF plots of the echellette order-sorting filters are available below. Note that these are not usually resident in the spectrograph, so prospective users should contact the Instrument Specialist well in advance of their run.


Hoya Filters:

The following standard Hoya Glass 2-inch square filters 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 should contact the Instrument Specialist well in advance of their run.

  • ND 0.3
  • ND 0.5
  • 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

Blue Channel Details

Optical Layout

Optical Layout

Aperture Plates and Filters

Aperture Plates and Filters

CCD Parameters

ITL/STA Detector Information

 

Rotator Angles

 

Information on the instrument rotator

 

Blue Channel Gratings

Grating (lines/mm)Blaze (order/λ Å)Blaze Angle(°)Anamorphic MagR (@ Blaze, 1" slit)Resolution (Å)Dispersion (Å/pix)Coveragea (Å)

300

1st/48004.300.967406.471.965268b

500

1st/54108.050.9314303.791.193198

800

1st/40509.670.9117302.340.752016

600

2nd/480017.40.8533101.450.501344

1st/963017.40.8533302.901.002688

832

2nd/390019.70.8338301.020.36968

1st/779019.70.8338302.040.721935

1200

1st/483017.50.8433401.450.501344

Echellette

12th/448042.00.61114800.390.19360c

a Coverage is for the full 2688 columns. In practice the image quality may degrade slightly at the ends of the chip.

b May be limited by the blue cutoff or the free spectral range.

c Value of the free spectral range.

Theoretical Blaze Functions

Blue Channel Theoretical Blaze FunctionsBlue Channel Theoretical Blaze Functions

Or view a postscript plot or a pdf plot of the theoretical blaze functions.

 

Spectrograph Throughput

An illustration of the expected count rate (for a flat-spectrum (in F_nu) source of 1 micro-Jansky) per Angstrom is shown below.

 

The same data can be found in PDF form and in tabulated form (Blue Channel, Red Channel).

 

Cross Dispersed Echellette Mode

NOTE: If you are submitting a proposal to use this configuration, please specify very clearly that this is the case in order to alert the telescope schedulers.

The Blue Channel Echellette grating is a 6" x 10", custom-ruled, 240 g/mm grating which can be used in orders 8-17 simultaneously, giving coverage from 3100-7000 Å with a 10 arcsecond long slit. Cross dispersion is provided by a large quartz prism which is inserted into the beam between the grating and the camera.

In a perfect spectrograph, the spectral resolution of this configuration is about 10,000 with a 1 arcsec wide slit (i.e. 30 km/sec). In practice, the focus degrades at the ends of the orders. During initial tests, we picked a focus value that gave a decent compromise focus over much of the frame. The resulting resolution varied from about 30 to 60 km/sec.

 

Sample Images:

Several images which summarize the salient operating characteristics of this configuration are available as jpgs. Note that in these images, the 17th order is difficult to see or underexposed; while the order does not appear in these sample images, the data from the order is usable and calibratable. It is worth noting that the 8th order currently falls slightly off the detector near the atmospheric B-band.  The percentage of the trace lost in this manner is small and will likely not strongly affect point source spectroscopy.

Throughput:

Throughput measurements are also not available for the cross-dispersed configuration.  There is however historical evidence that on a photons/Å/second basis, this configuration is clearly not competitive with the other gratings -- we estimate that it may be a factor of 2 or more slower when compared to the other gratings at their blazed wavelengths. Some of this is attributable to losses from the prism; the rest (and probably the lion's share) is due to the grating. However, some of this factor is compensated for by the fact that one is atop the blaze in each of the echellette's orders as opposed to the normal gratings where the blaze can often compromise performance.

This grating could be very useful for projects requiring moderately-high resolution over a wide wavelength range. The data are relatively easy to reduce (up to the point where adjacent orders are combined) using the "echelle" package in IRAF.

 

Scheduling Constraints:

Configuration of the spectrograph for cross-dispersed work requires that the spectrograph be taken off the telescope. In practice, this means that the spectrograph is scheduled for cross-dispersed "runs". It is possible to leave the prism in and use other gratings, but there is significant throughput loss and the spectra are quite curved. It is also possible to mount the Red Channel on the Blue Channel and use it for backup programs in case of inclement conditions.

Prospective users of the Echellette should contact Joannah Hinz.

Red Channel Details

Optical Layout

Optical Layout

Aperture Plates and Filters

Aperture Plates and Filters

CCD Parameters

ITL/STA Detector Information

The Red Channel detector has a glow. Please see below for details.

Red Channel Gratings

Grating (lines/mm)Blaze (order/λ Å)Blaze Angle(°)Anamorphic MagR (@ Blaze, 1" slit)Resolution (Å or Å/order)Dispersion (Å/pix)Coveragea (Å)

Echellette

11th/523526.8 32701.60.58599

150

1st/48002.20.9823021.06.376574b

270

1st/73005.90.9464011.43.593705b

300

1st/48004.30.9646010.33.213313b

600

1st/48008.60.929605.01.631682

600

1st/631011.40.8912904.901.641692

1200

1st/576721.10.8026202.200.82846

1200

1st/770028.70.7339301.960.80826

1200

1st/900032.70.6750971.770.78805

Lo-res Ech.

7th/51348.6 9205.61.741796

Hi-res Ech.

18th/579025.7 36201.60.67691

a Coverage is for the full 1032 columns.

b May be limited by the blue or red cutoffs or the free spectral range.

 

Theorectical Blaze Functions

 

Red Channel Theoretical Blaze Functions: Red Channel Theoretical Blaze FunctionsRed Channel Theoretical Blaze Functions: Red Channel Theoretical Blaze Functions

 

Or view a postscript plot or a pdf plot of the theoretical blaze functions.

 

Spectrograph Throughput

An illustration of the expected count rate (for a flat-spectrum (in F_nu) source of 1 micro-Jansky) per Angstrom is shown below.

 

The same data can be found in PDF form and in tabulated form (Blue ChannelRed Channel).

 

Red Channel Detector Glow

 

In Fall 2014, the Red Channel detector developed a "glow" on the blue edge of the detector.  This signal is well localized to the corner of the detector, so point source observations and observations focused on the red portion of the detector will likely be minimally effected.  Observations of extended sources and programs that utilize the bluest portion of the observed spectrum may be impacted by the signal and its associated noise.

A two dimensional image of the bluest portion of the detector is shown below (the spectral direction of the detector runs in the Y-direction, so blue is up in the figure).  The image is a bias-corrected dark image. The counts in the "glow" in the upper right depends on exposure time, so observers using long integrations are encouraged to take dark frames.   On the right, we show the median counts in this 600s bias-subtracted dark frame in the last 3 columns; again, the bluest portion of the red channel image is at higher Y-pixel values.  

Red Channel Glow ImageRed Channel Glow Counts

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cross Dispersed Echellette Mode

The Red Channel Echellette grating gives complete coverage from 4300-8900 Å 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 orders 6-13 with some sacrifice of coverage in the blue and some order overlap between orders 6 and 7. Due to the Red Channel detector glow, we do not recommend the use of Echellette at this time.

The echellette should be used near its blaze at a tilt of 4.555 V (confirm, old value = 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.