THE OPTICAL COMPONENTS

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.


Front Slit Viewer

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.


Post-Slit Viewer

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.


Comparison Lens

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.


Decker Dimensions

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
            ----------------------
Length Length
----------------------
arcsecs mm
---------------------
1 12 1.85
2 24 3.71
3 49 7.42
4 98 14.86
5 205 30.99
Open 330 50.
---------------------

Slit

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.


Upper Filter Bolt - Order Separation Filters

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.
# A crystal-mosaic filter, too small for multi-slit panoramic work.

Additional Filters

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


Lower Filter Bolt - Neutral Density Attenuators

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.

Shutter

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.


Collimator Mirror

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

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.


Gratings

A list of gratings and their parameters.

Grating Change Policy

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.


deveny@noao.edu
Friday, August 4, 1995