The MagE Spectrograph User Manual
The MagE (Magellan Echellette) Spectrograph is a moderate-resolution optical echellette mounted on the center folded port (FP2) of the Clay Magellan II telescope.
The wavelength coverage of MagE is approximately 3100 Å to 1 micron. MagE has been designed to have exceptional throughput in the blue; throughput measurements are found in Table 2 below.
MagE has eight available slit widths: 0.5, 0.7, 0.85, 1.0, 1.2, 1.5, 2.0, and 5.0 arcsec. There is also a mask position with three 0.35 arcsec pinholes for focusing the spectrograph. All slits are 10 arcsec long with a plate scale of 0.3 arcsec per pixel on the detector. There is currently one grating available that gives a resolution of R = 4100 for the 1 arcsec slit.
MagE is a very simple spectrograph with only four moving parts. The control of these mechanisms is described below.
The figure below shows the optical layout of MagE. The input beam is collimated by an off-axis parabolic mirror, dispersed by the reflection grating, and is separated into echelle orders by double-pass and single-pass prisms. The camera is a prime focus Schmidt with a doublet corrector. The CCD is mounted on the inside of the second element of the corrector which is also the dewar window.
The figure below shows a solar spectrum taken with MagE, with the order number and central wavelength of each order indicated.
MagE currently has only one available grating with 175 lines/mm blazed at 6.2 micron The approximate central wavelenght for any order is equal to 6.2 micron/(order number). This grating gives a resolution of R=4100 for a 1 arcsec slit.
MagE has eight available slit widths to give various resolutions: 0.5, 0.7, 0.85, 1.0, 1.2, 1.5, 2.0, and 5.0″. All slits are 10″ long.
The MagE CCD is an E2V 42-20, which has a 2048×1024 format with 13.5 micron pixels. The plate scale at the detector is 0.3 arcsec/pixel. The CCD may be read out at three speeds: Slow, Fast, and Turbo. The characteristics of these speeds are described in the table below.
|Read speed||Read time (s)||Gain (e-/DN)||RMS noise (e-)|
The measured dark current is 1.0 electrons/pixel/hour. The CCD has no column defects. The response of the
CCD is linear to better than 0.5 per cent up to digital saturation (65,536 DN including bias) in the Fast readout mode.
The dewar vacuum is maintained with a Varian ion pump which remains on during normal operations.
Instrument Control and Data Acquisition
The MagE control GUI should be familiar to Magellan observers: it is similar to the MIKE and B&C GUIs.
The MagE control program is started by typing “mage” in a terminal on the observer computer. The MagE configuration GUI will appear. For regular observations most of the default settings should be accepted (CCD, instrument hardware Programmable Logic Control (PLC), and telescope all “online”). The observer can select the number of overscan rows and columns in the CCD readout, the defaults of 128 pixels are recommended. Clicking on “OK” will then start the MagE data acquisition GUI.
Data Acquisition GUI
The acquisition GUI contains several sections:
At the top of the GUI are the File, Module, and Options pull-down menus. The File menu allows the user to reload the DSP program, and to exit the GUI. The Module menu is used to start the small or large format Quicklook tool. This tool displays the image in real time as the data are read from the CCD, as is shown in the following three figures. The Quicklook tool is similar to that used with LDSS; see the LDSS manual for a more detailed description. One difference is that the MagE Quicklook tool includes an order and wavelength readout feature. When the cursor is moved in the Overview window, the order and approximate wavelength is updated on the Wavelength line of the QL-Tool window. If the count level in a pixel is higher than 60,000 DN then this pixel is displayed in red in the Overview and Magnifier windows of the QL-Tool. This level can be adjusted by the observer by clicking on the Options menu of the QL-Tool and setting the SatLev variable.
The Options menu allows the observer to change the data path (the location on the disk where the raw data frames are written), and to launch a dewar status window which monitors the CCD temperature and the dewar vacuum level. This window is shown below:
The CCD temperature is displayed along with the temperature of the cryotiger cooling head. The internal temperature of the instrument is also displayed. The second section contains estimates of the dewar vacuum (in mbar) from a convection gauge and from the ion pump controller. Note that the convection gauge has a limited dynamic range, and in normal operation reaches a lower limit of approximately 9.5 x 10e-5 mbar. The ion pump pressure normally reads approximately 5.0 x 10e-7 mbar. This ion pump value is the correct dewar pressure. The ion pump can be controlled manually; this function is for engineering purposes only.
The Options menu also contains commands to adjust the CCD voltages, pixel read speeds, and slit positions–these are password-protected engineering tasks.
The second section contains fields in which the observer may enter the exposure time, loops (number of consecutive exposures), exposure type, binning, and subrastering.
There are six exposure types, selected by the ExpType pulldown menu: Object, Bias, Dark, Flat, ThAr, and Xe-Flash. A Bias exposure automatically sets the exposure time to zero. The shutter is not opened when the exposure type is set to Dark. For ThAr and Xe-Flash exposures the lamp is turned on and a flipper mirror is inserted into the beam, directing the light from the lamp into the spectrograph. The flipper mirror is automatically removed from the beam and the lamp is turned off at the end of the calibration exposure. Exposure types Object and Flat are for header information only.
The Binning pulldown menu allows the observer to select pixel binning factors of 1, 2, 3, or 4 in the X and Y dimension independently.
The Subrastering pulldown menu provides a way to set up subrasters (small regions of the CCD that are read out rather than the entire CCD). Subrastering may considerably shorten the readout time. When “Full” is selected, the entire CCD is read out. If “Subraster” is selected, a second window is launched (Fig. XXX) that allows the appropriate pixel values to be entered. As many as 8 subrasters can be selected; the user fills in a table of X0 and Y0 coordinates and the size of each sub-array. The cursor on the Quick-Look display can be used to select the center of the subraster: type “a” to transfer the position and set the subraster size. Taking a picture with the subraster feature will save time, since the CCD is clocked without reading the data between the readout of subrasters (at any binning factor). However, as the number of subrasters increases, this advantage will diminish. Data for subrasters can be stored separately by choosing SaveMode = minimal or embedded in a full frame, with zeros filling the non-subrastered areas, by choosing SaveMode = full in the Subraster dialog box. The SaveMode = full is the default.Be careful if you are reading the subraster values from a file that you input the correct file name. If the filename is misspelled, the code will accept it, but not apply a subraster.
The next section contains commands to control the slit position and collimator focus as well as status flags for the lamps and flipper mirror position. The SlitMask pulldown menu lists the available slits by their width in arcseconds (all slits are 10 arcseconds long). There is also a selection for the pinholes that used to help focus the spectrograph. Note that the encoder that reads the slit position is NOT accurate enough to reposition the slit to better than a pixel. Separate wavelength calibrations must be obtained for every new slit setting. To change the collimator focus, click on the Focus field, enter a new focus value, and type carriage return to move the focus to the new value. The Lamp and Mirror fields indicate whether either the ThAr lamp or the Xe-Flash flat field lamp is on and whether the flipper mirror is in the beam.
The next section contains Object and Comment fields that are stored in the image header.
Below this are the Start, Snap, Pause, and Abort buttons, the File Number indicator, and the Shutter status indicator.
The Start button starts an exposure as defined by the current configuration in the GUI. The Snap button takes an exposure with 4×4 binning in the turbo readout mode. The file number is not incremented. Clicking on the Pause button will pause an exposure (the shutter is closed and the countdown timing is halted). The exposure may be restarted by clicking on the Pause button again. The Abort button XXX.
The File Number field shows the file number of the current exposure. The number is incremented at the end of an exposure. The file number may be changed by the observer, but note that it is possible to overwrite a saved exposure with the same file number. File names have he format “mageNNNN.fits;quot;
The Shutter indicator field displays whether the shutter is currently open or closed.
Finally, the observer may select the readout speed using the Speed pulldown menu. Three speeds are available; Table XXX lists the readout time, CCD gain, and read noise for the three available options. The Disk indicator bar indicates the remaining capacity of the data disk. The bar turns red when the available disk space drops to less than 10%.
At the bottom of the GUI is a system message window.
Wavelength comparison lamp frames are enabled with the “ThAr” field in the ExpType pulldown menu. The comparison source is a ThAr hollow cathode tube, which provides suitable lines over the entire wavelength range. Typical wavelength solutions give an RMS of 0.06Å for 500 arc lines.
Flat Field Calibration
The broad wavelength coverage of MagE means that it is difficult to obtain good flat field observations. There are two flat field lamp sources that can be used. One is an internal Xe-flash lamp and the other is an array of incandescent lamps mounted in the telescope secondary cage.
Xe-flash frames are enabled with the “Xe-Flash” field in the ExpType pulldown menu. The Xe-flash lamp flashes pulses of light instead of a constant source. The lamp flashes at a constant frequency, the user selects the total exposure time and hence the number of flashes. The lamp is particularly good for obtaining flat field observations in the blue region of the spectrum.
Unlike in the MIKE echelle spectrograph, there is no diffuser for the flat field lamp. The spectrum of the Xe-flash lamp contains broad emission lines, but these lines can be filtered out in the preparation of the final flat field. To make the filtering more effective, the emission lines can be broadened by observing the lamp with the 5.0 arcsec slit. The emission lines can be further smoothed by observing the Xe-flash lamp with the collimator taken out of focus.
Pixel-to-pixel flat field observations for the red end of the spectrum are obtained by taking exposures of the flat field screen which can be deployed in front of the secondary mirror. The screen is illuminated with the incandescent lamps installed in the secondary cage. The lamp and screen control GUI is launched by typing “ffs” in an xterm window. An additional set of incandescent lamps is controlled with a power supply mounted underneath the east nasmyth platform if more light is desired. The MagE CCD shows fringing in the red region of the spectrum, starting at about 7000Å, with peak fringing amplitudes reaching about 10 per cent. The incandescent lamp flats can also be used to correct for this fringing.
Three sets of flat field exposures are therefore recommended. The first is a series of Xe-flash lamp flats used to define the individual orders in the spectrum. The second is a set of Xe-flash lamp exposures taken with the collimator out of focus. These are used to define the pixel-to-pixel flat field in the blue end of the spectrum. The third set of flat field observations is of the incandescent lamps illuminating the flat field screen. These exposures are used to construct a pixel-to-pixel flat for the red end of the spectrum, and to correct the fringing that is seen in the red.
The following is a suggested sequence of flat field exposures that will enable you to flat field MagE observations using Dan Kelson’s reduction pipeline:
1. Order Definition: Xe-flash lamp flats that will be used to define the location of the individual orders should be taken with each slit used for science observations. Saturated pixels will not affect the order definition. Averaging a few of these observations will suffice, a minimum of three exposures at each slit setting is recommended.
2a. Blue Flats: For the pixel-to-pixel Xe-flash lamp flats try 15 second exposures with the 5-arcsec slit with the collimator focus set to 1100 units. You can use the “Turbo” readout mode to increase the dynamic range for these flats. The bluest 6 orders (20 – 15) will be unsaturated. The orders to the red will all have saturated pixels. To get good statistics in the blue it is recommended that a sequence of at least 15 flats be taken.
2b. Very Blue Flats: If you require good flat-fielding in the bluest two orders (3100 – 3500 Ang), it is recommended that you also take very long (about 100 second) exposures. All orders but the two bluest will saturate. Note that if you are going to use Dan Kelson’s pipeline to reduce your observations then it is best to take your blue flat fields at the same readout speed as your science frames.
3. Red Flats: For incandescent lamp flats turn on the Qf lamp in the flat field screen GUI and turn on the power supply at the base of the east nasmyth platform. Set this power supply to 5.0 volts. Exposures of 45 seconds are appropriate when using the 1.0-arcsec slit (remember that when correcting for fringing it is important to take the flat field observations with the same slit as for the science observations). These frames will give good signal down to order 14. To get good statistics in the blue orders a minimum of 15 exposures is recommended.
This combination can be used to flatten the entire MagE wavelength region. The Xe-flash lamp flats are used to define the orders and to make a pixel-to-pixel flat for the six bluest orders (these flats are saturated further to the red), and the incandescent lamp flats are used for the remainder of the orders into the red and to correct the fringing in the reddest orders.
The throughput of MagE was measured on the night of 23/24 November, 2007. A total of 10 spectrophotometric standards were observed over an airmass range of 1.04 to 2.63. In the following table, the order number is followed by the central wavelength of the order and the dispersion at that wavelength. This is followed by the zero point magnitude at that wavelength for one air mass, the extra-order flux in magnitudes (to be added to the zero point magnitude), and the measured extinction. Finally the table lists the overall efficiency in per cent for the telescope plus instrument, and for the instrument alone (assuming three telescope mirror reflections of 0.85 per cent at all wavelengths).
We have measured the flexure of the MagE spectrograph at a range of instrument rotations and telescope elevations. The instrument flexes (i.e., a pinhole moves in a series of images) by about 0.65 pixels from 0 rotation to +/-175 degrees in rotation, and by about 0.1 pixels when the telescope is moved from 90 degrees elevation (zenith-pointing) to 20 degrees (nearly horizon-pointing).
Because MagE is mounted on the central folded port, this amount of flexure should be of little concern unless the telescope is tracking near the zenith. In particular, while the slit is aligned with the parallactic angle and the telescope tracks over 30 degrees in elevation the maximum flexure is 0.1 pixel.
Focusing the Spectrograph
The spectrograph is focused by adjusting the position of the collimator mirror from the MagE GUI. The focus is fairly uniform over the surface of the CCD, and the best focus in the x (dispersion) and y (spatial) dimensions agrees to within 100 collimator units. The best focus is about 2900 collimator units at an instrument internal temperature of 15.5C. The instrument focus is not expected to be a strong function of temperature.
It is expected that a script to focus MagE will be installed at the end of January, 2008. Until then MagE can best be focused using the fmike.cl script that is used to focus the MIKE spectrograph. This script can be loaded by typing “mike” in an IRAF window on the observer’s data acquisition computers (MAC MINIs). Observations of the ThAr arc should be taken with the 0.35 arcsec focusing holes, a good exposure duration is 4 seconds. The best focus values for the 0.35 arcsec aperture is about 1.8 unbinned pixels.
For an increase in the internal temperature of MagE of 4 C the echellogram moves one pixel to smaller y values (spatial direction).
There is a SlitView Camera and the TO is responsible of putting the target in the slit, once it has been selected by the observer using the MagE GUI. Targets brighter than 20th magnitude are easily visible under normal sky conditions. For fainter targets please prepare reference stars for executing blind offsets.