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Basic Specifications for the 0.9 m Coude Feed
Telescope and Spectrograph


NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation


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Basic Specifications for the 0.9 m Coude Feed Telescope and Spectrograph

The coude feed telescope/spectrograph consists of a 0.9 m alt-azimuth three-mirror telescope with a dedicated spectrograph system. With suitable grating tilts several cameras of different focal lengths can be accessed easily. In practice, only cameras 5 and 6 (see below) have been used for several years, being the only two where the focus is accessible for CCD dewars.

A description of the telescope and spectrograph follows. For more details the Coude Spectrograph Instrumentation Operation Manual may be consulted (http://www.noao.edu/kpno/manuals/coude/coude.html).

I. Telescope

#1 mirror: (alt-az flat) 1.5 m aperture
#2 mirror: (fixed tilt, but focusable off-axis f/31.2 parabola) 0.9 m aperture
#3 mirror: (fixed folding-flat) 11.5 cm aperture (2 Al coated and 1 Ag coated)

The configuration of the mounting causes vignetting at declinations north of +70 degrees.

Image quality - While not easy to measure, seeing of 1 arc-sec or slightly better can be obtained. Image jitter of a few tenths of an arc-sec is noticeable in good seeing due to the digital drive. Wind buffeting can induce considerable image motion.

Pointing - Approximately 10 arc-sec RMS after pointing map.

Field of view - Variable with TV zoom lens, up to 3 arc-min.

Acquisition and Guiding - Acquisition and guiding are presently done with an intensified CCD TV camera viewing the slit at all times. A zoom lens on the TV enables a variable slit magnification and field of view. Stars down to about V = 14 are visible with the TV. This description is provided for performance information only. The zoom lens but not the TV camera itself is offered with the telescope.

II. Spectrograph

Slit - The slit width is continously variable up to 1.25 mm, and length is set by decker plates with many lengths from 0.6 arc-sec up to 186 arc-sec. The image scale at the slit is 7.23 arc-sec/mm.

Collimators - Two f/31.2 collimators are present with diameters of 221 mm and 325 mm. The choice of collimator is usually determined by the grating size.

Gratings - The five available gratings are described in the table below. Note that the echelle grating must be either cross-dispersed or used with narrow band order sorting filters to be of use. Cross dispersion is presently accomplished with a set of grisms borrowed from the 4m Cryocam. These grisms will be available to coude users except when needed by the 4m Cryocam.

The grating wavelength setting is done with the telescope control program.

                   Coude Spectrograph Gratings
------------------------------------------------------------------------
grating               A         B         C         D         Ech
gr/mm                632       316       600      1200       31.6
blaze (A),1st ord. 12,000    12,000     8000      8000      562,560
resolution*        28,500    13,500    17,100    35,100     116,000
integ. time (s)**  140(2)     99(3)    125(2)    258(2)     5200(125)
collimator         large     large      small     small      small
-------------------------------------------------------------------------
*for 2 x 15 micron pixels
**for camera #5, B=10, s/n=25, in order (n) at 4500A

These integration times assume a quantum efficiency of 80% at 4500A and a slit width of 1.8 arc-sec. For camera 6, the resolutions are about 3 times higher, but integration times could be 6 - 8 times longer as the slit width would need to be reduced to realize the higher resolution.

Cameras - Cameras 5 and 6 are described in the table below. Camera 5 is a folded Schmidt design with the CCD dewar mounted behind the spherical mirror. It has interchangeable correctors for red or blue wavelengths.

Camera 6 is a classical lensless Schmidt with the CCD dewar mounted below the focus and perpendicular to the camera axis. A small pickoff mirror diverts the beam into the dewar.


                   Coude Spectrograph Cameras
---------------------------------------------------------------
camera                  camera 5           camera 6
type                 folded Schmidt     lensless Schmidt
f-ratio                   3.6                  16
focal length (cm)        108.0                365.3
demagnification
   -small col.            6.3                  1.9
   -large col.            9.4                  3.0

III. Telescope Control Software

The control software resides on a Heurikon processor under a VxWorks operating system. It is interfaced to the user with a Sun IPX computer. The control hardware will be provided with the telescope. The feed control software is largely accessed through Graphical User Interfaces (GUIs): windows with buttons controlled by mouse clicks. Facillities are also present for entering object coordinates into caches for later retrieval via the the Star Cache Window. By clicking on a cache entry, the coordinates are sent to the Telescope control Window for pointing to the object and activating guiding. Many caches can be stored including catalogues such as the HR stars, SAO and others. One other important function of the telescope control software is to set the grating to the desired wavelength for the camera in use. All of the grating parameters are stored and a GUI exists for quick selection of grating, camera, order, and wavelength.

The control system has provided consistently reliable and low-maintenance operation for this telescope.

For further technical information about the telescope and spectrograph, please contact Daryl Willmarth, dwillmarth@noao.edu.


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Posted: 05Apr2000
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