The DAO 1.2m Telescope and McKellar Spectrograph

The McKellar Spectrograph

The 1.2-m telescope, with a primary mirror made of low-expansion borosilicate glass, is used at the coudé focus, with high-reflectance coatings for the second, third, fourth and fifth mirrors to optimize throughput.  The three mirror sets are Super Blue (3500-5300 Å), Aluminum (4500 Å-near IR) and Silver (6500 Å-IR) optics and are self-aligning so that it is possible to change from one set to another in about two minutes.  The coudé beam is f/145 after reflection from the hyperbolic secondary (the third to fifth surfaces are flat).  A lens located immediately following the fifth mirror, converts the beam from f/145 to f/30 as required by the spectrograph. This lens is is used to provide tip/tilt corrections.

The coudé spectrograph is named after Dr. Andrew McKellar (1910 – 1960) and is described in detail by E.H. Richardson.  There are actually two separate spectrographs in the coudé room, so one of two optical paths must be selected:

• A 32-inch focal length, on-axis f/5 spectrograph camera can be combined with one of two 4500-mm focal length collimators and one of six available diffraction gratings. The collimators differ only in the aspheric correction for spherical aberration and the choice is dictated by the grating in use.  These options provide many possible reciprocal dispersions, of which 10 Å per mm and 41 Å per mm are the most commonly used.
• Alternatively, the beam from a 7600-mm focal length collimator can illuminate a mosaiced 830 groove per mm grating and a 96-inch focal length camera on-axis camera at f/8 to give a spectrum in the red (1st order) at 4.8 Å per mm or in the blue (2nd order) at 2.4 Å per mm.

The resolutions of the various spectrograph configurations range from 0.07 Å to 1.2 Å and are summarized in Table 1.  For example, the 9682M designation is for the 96-in camera, 830 g/mm Mosaic, 2nd order, and so on.  The last column of the Table gives the approximate exposure time for a sixth magnitude star exposed to a signal-to-noise ratio of approximately 30 at the wavelength of the blaze, with the thinned 2K × 4K SITe-4 CCD.  Since currently this single detector is used with both cameras, to reduce the risk arising from frequent detector movement, the telescope is not scheduled for less than 3- or 4-night blocks for either camera.

There are three Richardson image slicers available for use at the coudé focus and described in Table 2.  These enhance the efficiency of coudé observing, as they effectively increase the slit width fourfold without loss of spectral resolution.  Since the projected slit width should match the resolution of the detector (i.e., two pixels or 30 microns in the case of SITe-4), the IS32 slicers are normally adequate for either camera, and the IS32R should be used for all red configurations.  An ordinary slit is available but very rarely used.

Wavelength calibrations for the spectrographs are provided by hollow cathode discharge lamps; three are available: Fe-Ar, Th-Ar, and Cd-Ar.  Various configurations of lamp illuminations can be used for detector flat fielding, depending on the spectral region and precision required.

Table 1:  1.2-m Telescope Spectrograph Grating Configurations and Exposure Times

Table 2:  1.2-m Telescope Image Slicers

Telescope Control System and DAO Instrument Control Environment

The telescope and dome operate under a Telescope Control System (TCS).  Commands and tools to control them, as well as the CCD and spectrograph hardware, are normally handled by the DAO Instrument Control Environment (DICE) software that runs on a Linux desktop in the control room.   Target coordinates are accepted for any specified epoch and the pointing accuracy is currently about 15″ over the whole sky.  The telescope pointing range in hour angle are -7 hours (east) to +8 hours (west).  There is also a limit switch in altitude which will cut off the telescope drive at about 12 degrees above the horizon, or at a declination of about -30° on the meridian.

DICE includes an acquisition and guiding camera graphical user interface (GUI) with a field of view of about  2 arcmin as well as a second camera that displays the entrance of the image slicer.  Additional GUIs are available to acquire data with the CCD, display the CCD image and spectrum, enter coordinates and acquire targets, adjust the telescope focus, move the dome and telescope, and control an exposure meter.  Command line scripts can be used to control the comparison arc lamp, select the desired coudé mirror train, and open or close the dome.

Note: there is no remote control of the grating rotation or flat field lamp.

Robotic Operation

For well over a decade, the 1.2m telescope has been operated in an unattended robotic manner on about 70% of the scheduled nights.  Point sources with V magnitudes as faint as 11 can be acquired, although close visual binaries and crowded fields can result in the wrong target being selected if another object happens to lie in the acquisition camera’s field of view.

Nightly observing programs are submitted via the Robotic Observing web page.  These are then used as the input to a robotic script that is executed by support staff on the DICE computer.  Such robotic programs are executed on a shared risk, best effort basis.  For example, if the weather is not conducive to observing until late in the evening the program might not be executed.  Intermittent clouds can also hamper the execution of a program if a target can’t be acquired or is lost during an exposure.

Interested users are encouraged to discuss their data requirements with telescope support staff before applying for observing time for robotic observations.