The Ritchey-Chretien Spectrograph for the 4-meter telescope is the standard low resolution spectrograph in use at this telescope. It is a versatile instrument which has accommodated several detector systems over the years. All use at present is with CCDs.
Most of the components on the spectrograph can be moved by
computer controlled stepping motors. A GUI software package
fully integrates control of the spectrograph with the telescope and the
Rotator and Guider.
This optical system is designed for viewing the slit at all times using a ICCD TV acquisition system. The optical design uses the light reflecting off the spectrograph slit for acquisition of the object and also allows monitoring during an exposure. A three position filter bolt is available with a blue (BG38) and an orange (OG550) filter, and a clear position.
An integrating video memory, "the leaky memory" is available for acquisition of faint objects. Stellar objects of V~19.5 can be seen under excellent seeing conditions with 8 seconds of integration (or "leak"). Measured offsets are generally recommended for setting on stellar objects fainter than about V=18.5 under average seeing conditions.
The post-slit viewer can be equipped with an ICCD TV camera allowing the observer to see what is coming through the slit. A motorized pick-off mirror gathers all of the light coming through the slit, and, therefore, prohibits its use during an integration. By manually inserting a special negative lens in front of the camera, knife-edge focusing of the telescope is possible. The post-slit camera is normally mounted only for multi-slit work. This camera cannot be used during integrations since a auxiliary on-axis mirror must be in the beam. No filters are available.
This lens located immediately above the spectrograph slit is used in conjunction with the comparison calibration sources. The lens is necessary to produce a f/8 beam from the comparison sources and also significantly improves the uniformity of illumination.
Observers should be sure that this lens is NOT in position over the slit during focusing or an incorrect telescope focus will result.
The decker is a movable mask located immediately above the slit to control slit length. Its dimensions are given in the following table. It is generally advantageous to reformat the chip to the size decker needed for the observations. Be sure to allow sufficient slit length for adequate sky subtraction.
No. Stellar Window ----------------------
1 12 1.85
2 24 3.71
3 49 7.42
4 98 14.86
5 205 30.99
Open 330 50.
The slit is bi-parting with a length of 50mm and a maximum width of 50mm. The front surface is aluminized and tilted for use with the front slit viewing system acquisition TV. Typical slit widths are 150-400 microns or 1 to ~2.5 arcsecs. The scale of the 4-meter telescope is 6.624 arcsecs per mm (150 microns/arcsec). On the chip the spatial scale is 0.69 arcsecs/pixel for a 24-micron pixel TEK chip.
The slit width is readable via the encoder and spectrograph control software or via the mechanical Durant counter on the spectrograph itself. Each unit on the Durant counter is 20 microns. Note that the slit encoder has 15 bits and, consequently, can only be set to 32,767 microns via computer [for multi-slit programs]. Hand cranking is required to reach the full 50mm width.
Great care must be taken when setting the slit width, as damage can result if the mechanism accidentally attempts to set widths less than about 12 microns.
The upper filter location on the spectrograph can be occupied by any one of four interchangeable bolts each carrying four filters plus a clear position. These filters are 3.5-inches square and are not interchangeable with the filters in the lower filter bolt. The bolt must be manually installed in the spectrograph. The current configuration for the run is kept in a TCS configuration file which will be updated by the Instrument Support person assigned to the observing run. One filter bolt contains a standard filter configuration which we try to keep intact. It is listed below.
--- Upper Filter Bolt Std. Configuration --- Slot Filter,Thick. Posn.Readout ------------------------------------ 1 Clear 734 2 GG-495, 3mm 567 3 GG-420, 3mm 381 4 RG-610, 3mm 197 5* CuSO4 #, 8mm 011* Located at the handle end of the bolt.
Here is a complete list of the filters available for the upper bolt.
WG-345 1mm BG-39 2mm WG-360 2mm OG-530 3mm GG-375 1mm OG-570 3mm GG-385 3mm RG-610 3mm GG-420 3mm RG-645 3mm GG-455 3mm RG-695 3mm GG-475 3mm RG-830 3mm GG-495 3mm KG-2 2mm CuSO4 8mm# KG-3 2mm BG-38 2mm CS 4-96
# Note: The CuSO4 filter is a crystal mosaic - very fragile. PLEASE DO NOT REMOVE IT FROM THE STANDARD FILTER BOLT. Also, due to its retangular shape, it will vignette the field and it should not be used for multi-slit panoramic work.
Observers and instrument support personnel should check that the proper filters are in the spectrograph and that the GUI list is current. Don't make assumptions!
Copper Sulfate Crystal Filter Transmission Curve
The lower filter bolt contains Inconel on quartz substrate filters which are approximately neutral from ~3300 to 9000A. The filters should be calibrated with a standard star for accurate spectrophotometry.
These filters are 3.875-inches square and ARE NOT interchangeable with the upper filters.
Lower Filter Bolt Slot Filter Counter Setting 1 Clear 735 2 0.0 mag. 567 3 2.5 mag. 381 4 5.0 mag. 197 5* 7.5 mag. 011* Located at the handle end of the bolt.
A "guillotine-type" shutter has been installed in the spectrograph replacing the original four-position spectrograph shutter. The new mechanism, operated via the CCD controller only, is fast and accurate. One second exposures are reproducible to within \(+-3%. The shutter motion produces equal exposure across all parts of the optical beam, unlike iris-type shutters.
The mirror is an off-axis f/7.6 paraboloid with a focal length of 1161mm and a diameter of 241mm. The angle between the optical axis of the telescope and the center of the grating is 11 degrees. The included angle from the collimator to the camera is 46 degrees. The total focus travel is 38.1mm. The least significant digit on the mechanical readout equals 0.050 mm. The nominal center of travel is 500 on the readout. The auto-collimation position (without any filters in the beam) is 510 units. If the multi-slit assembly is used in place of the slit, the collimator position must be changed to ~290 for best focus. position.
Collimator Positioning Mirror Encoder Position Position Readout for GUI -------------------------------- UP LIMIT 0 -25mm NOMINAL 500 0 mm DOWN LIMIT 1000 +25mm mm = (encoder - 500) * 0.05
The UV Fast Camera is a catadioptric design by Jorge Simmons (NOAO). The camera was fabricated early in 1986 and went into full service in May 1986. It has a thin meniscus corrector, a solid body with one internal surface acting as a secondary and a rear element attached to the main body followed by a single lens. There are six air-to-glass intervals and two aluminized surfaces. The back working distance is sufficient to reach into CCD dewars. The quartz material used is of the highest quality and the overall camera transmission is thought to be >85% for the region 0.3-1.1 microns. The image quality is typically 25 microns FWHM at the center of the field and ~35 microns at the edge. The MTF is ~64% at 30 lp and ~30% at 50 lp, a noticeable improvement over the old Singer camera. The field vignetting is also noticeably improved over the Singer and is only 5% at a field angle of 3 degrees. The focal length is 265mm, producing a slit reduction factor (demagnification) of ~4.4x.
In order to achieve optimum focus, with the collimator near its auto-collimation position (510), the camera itself may have to be moved. If the collimator focus falls outside the range 425<= collimator focus <=575, the camera should be adjusted. If collimator focus falls below 425, move the camera to HIGHER numbers on the engraved scale on the camera body. Moving the camera longitudinally by ~.25mm will cause the collimator to move by ~100 units. This adjustment should only be attempted by someone intimately familiar with the instrument. The camera position is readout in volts from a linear transducer.
UV Fast Camera Focus Control ---------------------------- Turn Clockwise To Remove Backlash and Make Final Setting (toward more positive voltages) ---------------------------- Typical Focus Interval: 0.1 volts 0.012 inches / volt 0.311 mm / volt CCW moves camera away from chip
In this camera design, a ray must make a triple-pass through many inches of quartz. Even though the optics are of the best grade quartz obtainable, scattering within the quartz may cause difficulties with flat-fielding for projects requiring very high signal-to-noise ratios.
The current collection of twelve gratings represents a sizeable investment of capital and are a tremendous scientific resource. To protect this resource, we would like grating changes to be done either by your telescope operator, a technical assistant or your instrument assistant. Please let them take the responsibility as the gratings are quite heavy and difficult to handle. Grating changes at night can be done with only a loss of 10-15 minutes of observing time if tilts, filters, and focus are predetermined. Generally, however, it is most efficient to plan your observing program with grating changes taking place in the afternoon.