The Plaskett 1.85-m Telescope, Spectrograph and Imager

This telescope is known as the Plaskett Telescope, and since 1953 has held on its foundation pier a plaque commemorating John Stanley Plaskett. The official naming of the telescope took place on June 4, 1993.

Overview

The 1.85-m diameter parabolic primary mirror of the telescope feeds an f/5 light cone either onto a flat aluminum secondary to illuminate an imaging focus, or alternatively onto a 0.5-m hyperbolic convex secondary, to provide an f/18 Cassegrain focus where the spectrograph is mounted. This telescope has seen considerable evolution since it went into operation on May 6, 1918, and it is roughly true that only the tube and mounting remain from the original telescope. The all-reflection Cassegrain spectrograph dates from 1967, the Cervit primary mirror was installed in 1974, and the f/5 modified Newtonian secondary and imager in 1991. The telescope encoders and control system are the products of the last decade. Recent evolution of the telescope has been driven by “user-friendliness” and safety considerations: the original centrifugal-governor clock drive has been retired from operation, the dome shutters and windscreens are now activated by a remote control handset, and most structures protruding from the rotating dome have been removed. In late 1993, the telescope tube was enshrouded in a black canvas mantle to shield out stray light at the imaging focus.

The current style of operation and complement of instruments is comparable to equipment found at all major observatories, thus enabling students to gain experience directly relevant to the use of larger telescopes elsewhere. From the observer’s room on the north side of the observing floor, the Telescope Control System (TCS) monitors the position of the telescope, while a Unix workstation controls data acquisition with instrument control software (DICE) that is uniform for all detectors and telescopes at DAO. An adjacent Xterminal allows field verification from the HST guide Star Catalogue or the Digital Sky Survey, and permits immediate data transfer and processing if desired. Routine observing operations can be performed with ease by a single observer. Observing manuals are available on-line to guide the observer.

The Cassegrain spectrograph is particularly versatile: its various apertures, gratings and detectors allow spectroscopy of objects down
to 16th magnitude, at resolutions ranging from 0.4 to approximately 20 Å (lambda/Delta-lambda approximately 200-15000), with
signal-to-noise ranging up to 150 or better. The practical range for spectroscopy is 3600 Å to 9500 Å subject to the constraints of
atmospheric transmission. Long-slit spectroscopy and sky subtraction features are provided by using a conventional slit. Fe-Ar and Cd-Ne
hollow cathode sources provide wavelength calibrations, and a tungsten lamp source provides flat-field calibrations for the
spectrograph configurations.

Spectrograph Configurations & Exposure Times

The Cassegrain spectrograph is an all-reflective system, except for the refractive field flattener just before the detector. Its optical elements are a 1750-mm focal length off-axis (to avoid internal obscuration) collimator, a diffraction grating, and a 538.7-mm (21-in) focal length f/5 camera, a diagonal mirror, and the field flattener. The collimator and camera mirrors exist as two interchangeable optics sets, differing in their reflective coatings. The default set (designated “Al”) has standard aluminum coatings, and is useable over the entire range of the spectrograph, or approximately 3000-9500 Å. The other optics set is gold (Au) coated for better reflectivity in the red-near infrared region, and is available if requested.

The various configuations of the spectrograph are listed below. The designation “21181” refers to the combination 21-in focal length camera, 18-hundred grooves per mm grating, 1-st order, and so on. All these gratings are used in first order. The wavelength of peak “blaze” efficiency is so designated; B,G,R indicate peak efficencies in the blue, green, or red-IR regions. Peak blaze efficiencies are typically around 80%. The last column gives approximate exposure times in minutes for approx. m=12 magnitude star exposed to approx. S/N=30 at the wavelength of the blaze, with the SITe-5 CCD as detector. See the section on digital detectors for a comparison of detectors in their readout noise and spectral response characteristics. In practice, exposure times also vary widely according to detector binning, grating rotation, spectral type of the star, spectrograph aperture, and with the quality of the night (“seeing” and transparency). The best guide for exposure times is usually to check in the observing log book for an application similar to your own (particularly spectrograph configuration, wavelength and detector) and noting the exposure times and counts.

Table 1: DAO 1.8-m telescope Cassegrain spectrograph configurations.

Slits & Image Slicers

The slit or one of several image slicers is mounted by wing clamps at the Cassegrain focal plane, to form the entrance aperture to the spectrograph.

The slit is adjustable from 0.001 inch (1 thou) to wide open; the width should be matched (in microns) to the resolution of the detector employed. During conditions of poor seeing, wider settings may be used to conserve speed, although resolution will be sacrificed. Typically, a 1.5″ slit is used, which allows a comfortable oversampling with all the CCDs. Note that the 0.8″ slit (with projected slitwidth of approximately 37 microns (FWHM)) results in undersampling for the larger CCDs, such as the SITe-5 with 24 micron pixels, 48 micron resolution.

Table 2: DAO 1.8-m telescope slit adjustments.

For long-slit spectroscopy, the scale is 6.2″ per mm at the slit, 20.1″ per mm at the spectrograph focal plane. The slit length is adjustable with a decker to 20″, 40″ or 60″, corresponding to widths of 1, 2 or 3 mm on the detector. There is, however, a perceptible deterioration of resolution at the tips of the lines with the longest (60″) slit.

For well-guided stellar spectra, the sky spectrum will naturally be exposed alongside the stellar spectrum and recorded on the CCD detector. Because of the proximity of the city of Victoria, mercury and sodium emission features from the sky will usually be recorded on the spectra of stars around 12th magnitude and fainter, depending on the quality of the night, the direction of the star, and the wavelength region under observation. At about the same magnitude, the solar spectrum can be a contaminant at full moon under hazy skies. In these situations, it is quite important to detect and subtract out the sky contribution to your spectra.

Richardson-type superpositioning image slicers may be used to optimize slit efficiency at the higher resolutions, where sky subtraction is not required. Four image slicers are available for the 21-in camera: they have either blue (B) or red (R) coated optics for maximum throughput, and come in two levels of resolution. The resolution of the “very short image slicers” (VSIS21) is approximately that of the 0.8 arcsec slit given in the above table. Resolution of the regular (IS21) slicers is usually limited by the pixel size of the detector.

Table 3: DAO 1.8-m telescope image slicers.

Newtonian Focus Imaging

The modified Newtonian system on the telescope allows direct imaging at f/5 with the E2V CCD detector. A flat secondary mirror is installed below the Cassegrain secondary to direct the axis of the light cone at a 30 degree angle to the primary optical axis, to form a focus about half way up the telescope tube. The scale at this focus is 22.5” per mm (0.31” per pixel), which provides a well oversampled 23.9′ × 10.6′ field with the E2V CCD. The detector is permanently aligned to witihn 0.8 degrees of celestial north with the long axis in the east-west direction. . The E2V CCD is generally binned by a factor of two to decrease the read out time. The scale on the detector is then 0.62” per pixel.

The TV guider for the Newtonian focus is operated from the observer’s room, and displays a field 7.5′ × 6.5′ (alpha × delta) on the monitors. As controlled by the `point’ command of the TCS, the field by the TV guider can be either the co-ordinate position entered on the TCS (for field verification), or else a guide field offset from the detector field, for guiding during an exposure. Since the pointing errors of the telescope are small compared to the detector field sizes, field verification is usually unnecessary; hence the main use of the TV guider is indeed for guiding! The offset from the center of the CCD field is fixed: the guider position cannot be rotated or translated to aquire suitable guide stars. The offset of center of the guider field on the sky is 18.2′ west and 3.2′ south of the center of the CCD detector. The autoguider is effective with guide stars as faint as m~15, but occasionally at high galactic latitude no suitable guide stars will be found within the guider field. For long exposures near the Galactic poles, it is advisable to verify that guide stars are available before you come to the telescope. See the `Before You Come’ section below for how to do this.

The photometric capability of the system has been demonstrated to m=20, and may extend fainter if great care is exercised in sky subtraction. A detailed description of the DAO photometric characteristics may be found in Clem, Vanden Berg and Stetson (2007). When configured for CCD imaging, a filter wheel holding five 75 × 75 mm filters (up to 7 mm thick) is mounted in front of the detector. Alternatively, four filters and one clear glass aperture (so that the focus is not shifted) may be used. Johnson-Cousins UBVRI and Sloan Digital Sky Survey ugriz filters are available. Transformation coefficents to standard UBVRI and SDSS ugriz values for our filter set depend on the choice of detector. Approximate transformation coefficent values for the SITe-5 CCD detector and the DAO ugriz filter set are as follows (from Clem, Venden Berg and Stetson 2007):

Table 4: DAO 1.8-m transformation coefficients for the SDSS ugriz filter set.

Average extinction coefficients for SITe-5 and the DAO ugriz filter set, from Clem et al. (2007) on 7 nights are as follows:

Table 5: DAO 1.8-m extinction coefficients for the SDSS ugriz filter set.

Zero points (i.e., the magnitude of an object at the zenith that produces one ADU/second) for the E2V CCD and the DAO ugriz filter
set, obtained by Dave Balam in 2010 February, are as follows:

Table 5: DAO 1.8-m zero points for the SDSS ugriz filter set.

Telescope Control System

The TCS will automatically position the telescope and dome to star coordinates of any designated epoch, as entered on the keyboard.
Alternatively, the TCS can set by star number from a list on the UNIX netwark. See the section on Before You Come … for the requirements for such a target or “get” list. The TCS can also accept offsets from a reference star, which can be
useful for setting on very faint objects. The positional encoders are accurate to approximately 1″, but residual errors in the system
limit the whole-sky setting accuracy to something like 20″.

For Cassegrain (spectroscopic) observing, the TCS terminal also controls the light feed into the spectrograph and/or TV
viewer/guider. By choice of diagonal mirror above the Cass focus, the monitor for the TV guider will display either:

• view solid: a full 7′ × 6′ (alpha x delta) field to magnitude 17, to allow target identification and centering, or

• view holey: the outer ring of the 7′ × 6′ field, to allow offset guiding while the slit is illuminated by the target object, or

• view slit: direct viewing of the slit or image slicer aperture, to allow guiding on the target object itself. The field of view is determined by the dekker length adjustment.

• view solid or view holey: 1 sec exposure allows detection to 17th magnitude; a 10 second exposure, to 18.5 magnitude.

• view slit : 1 second exposure allows detection to 15th magnitude.

Manual guiding of the telescope (either directly on the slit/image slicer or offset on a guide star, as seen on the TV guider screen) is accomplished with the keypad buttons on the TCS keyboard. These buttons move the telescope stepwise by preset amounts (usually 0.5″ when guiding) in the north-south or east-west directions for each keystroke. Alternatively, an autoguider may be invoked at the TCS terminal for automatic guiding on longer exposures.

The TCS will not allow you to set the telescope outside safe limits: the permitted range of hour angle and declination is plotted on the safe zone map. There are a number of potential danger areas within the physical range of motion of the telescope: collisions between the spectrograph and either pier, the telescope tube and the north pier, the detector and the hour angle drive housing and collision between the telescope tube and the remaining portion of the Newtonian elevator access platform on the dome. If the TCS accepts any given target without returning a limit error to the user, it will automatically negotiate the danger areas to set on that target. This often results in an apparent slew to a “wrong” position which the TCS has deemed to be a safe intermediate position; final slew will begin immediately after the intermediate position is achieved. In particular, on the safe zone map the telescope will move from zone 7 to zone 2 through zone 1, from zones 2 to 6 through zone 3, and so forth. As presently cabled, the telescope is used with the tube east of the piers and must not be reversed, so it is not possible to circumvent the limits imposed in the northeast and southeast directions.

It is possible to move either dome or telescope under manual control with the handset, in which case the TCS will not impose the plotted limits: instead the operator must use extreme caution to ensure they are respected, and that the spectrograph does not approach either pier. Tracking cannot be engaged under manual control, so the TCS must be returned to auto control for observing.

Before You Come …

In preparing your observing program, the following points should be kept in mind:

• In addition to your base program, plan for a reduced or expanded program: weather conditions (bad or good) will often modify your hopes of what is achievable during your observing run.

• Have accurate coordinates; the telescope control system will accept coordinates of any epoch.

• Prepare adequate finding charts for faint objects or crowded fields; these are mandatory for service observing projects.

For ease of observing, particularly for projects with many objects, it is advantageous to write a file containing your program star list. The format for such a list is straightforward: for each object, enter on one line the starname, right ascension, declination, epoch and comments, with spaces or tabs as delimitors between the fields. If there is a space in the object name, enclose the name in double quotes; use colons and leading zeroes in the coordinate fields. For example:

HD47129 06:32:02.4 +06:13:10 1900 V=6.1, Plaskett's star "Epsilon-2 Lyrae A" 18:44:22.8 +39:36:46 2000 B comp. 2.3" away; quad system

Lines should not exceed 80 characters. This file will permit acquisiton of the star by selecting within a Graphical User Interface (GUI). In order to transfer your file into the DAO computer network, please contact an observing assistant.

In preparing for a run, it may be wise to prepare charts for field verification, or to verify that suitable guide stars are located in the TV guider field (located 18.2 arcmin west, 3.2 arcmin south of the detector field for the guider at Newtonian focus), using either the HST Guide Star Catalogue (GSC) or the Digital Sky Survey (DSS). Both these resources are online at DAO, and may be summoned at the telescope as needed if you do not have time to check them before your run. The Digital Sky Survey may be accessed directly, or from the CADC pages, and shows stars to much fainter limits than the TV guider will see, without direct photometric information. The stars of the GSC, on the other hand, are all sufficiently bright to be suitable for guiding with the Newtonian guider.

Guide for creating an observing program

The following lines are included in the program:

We usually open the dome ~10 min after sunset;

WAIT today 17:40

Take 5 sky flats through filter #4 (Johnson R), #3 (V), and #2 (B):

SKY_FLATS 5 4,3,2

Take 10 exposures 60 sec each through filter R, V, B:

OBJECT  C14  02:18:59.23  +57:07:52.7  2000.0  60  10 4,3,2

Note that the telescope gets re-pointed between individual “OBJECT” instructions. So the target might be in a slightly different place on the detector during the second “OBJECT” instruction on C14.

And here are some overheads:

An example observing program for imaging on the 1.8m:

#
# DAO 1.2-m robotic program for 14 Feb 2025 PST
# Contact email:
#
WAIT today 17:40
OPEN_DOME
FILTER 4
SKY_LEVEL 35000 DN
SKY_FLATS 5 4,3,2
BIAS 16
SKY_LEVEL 300 DN
AUTOFOCUS
#
WAIT 2025-02-14 18:40
OBJECT C14 02:18:59.23 +57:07:52.7 2000.0 60 10 4,3,2
OBJECT C14 02:18:59.23 +57:07:52.7 2000.0 60 15 3,2
WAIT 2025-02-14 20:30
AUTOFOCUS
OBJECT M44 08:40:12.96 +19:37:15.6 2000.0 60 10 2,3,3,2
OBJECT M67 08:51:23.04 +11:48:50.4 2000.0 60 15 2,3,4,4,3,2
#
PARK
CLOSE_DOME