SBO 24" Spectrograph 1 9/28/98. Spectrograph

Transcription

1 SBO 24" Spectrograph 1 9/28/98 Spectrograph Spectrograph Controls... 2 Rotatable Turret...2 Slit Viewer...2 Lamps...2 Order-Blocking Filter Controls...4 Translation/ Mount...4 CCD Detector...5 Slit Viewer Assembly... 6 Slit and Decker Plates...6 Finder and Slit Viewer Optics...7 Boller & Chivens f/8 Spectrograph

2 SBO 24" Spectrograph 2 9/28/98 SPECTROGRAPH Rotatable Turret 1. The slit of the spectrograph assembly can be reoriented to a different sky angle by releasing the gold knob clamp, rotating the turret, and re-clamping. Normal north-south slit orientation is obtained when the gold knob is aligned with the fiducial mark on the telescope backplane. Slit Viewer Light from the 24 telescope comes to a focus at the slit plate, a front-surface mirror on which lines have been scribed to remove the reflective coating to form narrow transparent openings. Light passing through the slit produces the spectrum. Light falling to the side of the slit is reflected back to a pick-off mirror where it is re-directed through a pair of transfer lenses to the eyepiece viewer, which serves as a guider telescope for viewing the surrounding starfield. During an exposure, telescope pointing is continuously monitored so that light from the observed object falls directly onto the slit and disappears through the opening. The viewer has a second pick-off mirror and set of transfer lenses designed to intercept the light beam before reaching the slit. In this mode, the viewer provides a relatively wide-field view of the sky that serves as a finder telescope for the apparatus. 2. The slit/sky selector is a horizontal rod passing through the upper portion of the viewer. When moved fully to the left, the wide field finder optics are selected; when moved fully to the right, the high-magnification slit-viewer guider optics are in place. 3. The eyepiece rotation ring can be loosened slightly to rotate the eyepiece to a convenient angle for observing, and then re-clamped. 4. to accommodate individual eye characteristics is achieved by moving the eyepiece in or out of its tube. Obtain a sharp visual focus of the slit using this adjustment; then focus the starlight onto the slit using the telescope secondary mirror focus control. 5. Slit and decker controls are used to select the appropriate slit width and slit mask. See the later section on the slit viewer for slit/decker options. Lamps Wavelength calibration is achieved by comparing a stellar spectrum with that from a comparison lamp producing emission lines of known wavelength (leaving all other instrument parameters unchanged). 6. The teardrop-shaped comparison shutter knob selects whether the spectrograph views light from the sky (knob pointed up, towards the sky) or from the comparison sources (knob pointed horizontally, towards the lamp housing). 7. The comparison lamp selector/timer box houses a main power switch, two timer dial selectors, on/off buttons, and a lamp selector. The selector switch chooses between a helium-argon (He-Ar) or a neon (Ne) lamp comparison spectrum. See the Spectrograph Manual for wavelengths.

3 SBO 24" Spectrograph 3 9/28/98 The timer knobs determine the total time that the selected lamp remains lit after the on button is depressed. The timer is a holdover from the days of photographic plate spectroscopy, and is obsolete since the CCD shutter now determines the length of the comparison exposure. However, it still provides lamp protection by automatically shutting them off after the specified length of time. Hence, the timer should be preset for a duration somewhat longer than the intended exposure time. lamp exposures are made by switching on the box power, selecting the lamp source, depressing the on button to light the lamp, making the CCD comparison exposure, and then depressing the off button to extinguish the lamp INCH TELESCOPE Mirro Lamp Selector/Timer 4 Rotation Ring 5 3 Slit & Decker Controls TURRET Selector 2 Viewing Optics 8 Order-Blocking Filter 6 Slit/Decker Plates Rotation Release Knob 11 Lens 1 Lamps Tilt/Cover Controls on opposite side 12 Translation Micrometer TRANSLATE/ FOCUS MOUNT SHUTTER 9 13 TI CCD (LN2 Collimating Mirror Readout 10 ST-8 CCD (thermoelectric CCD DEWAR Liquid Nitrogen Connection

4 SBO 24" Spectrograph 4 9/28/98 Order-Blocking Filter 8. The order-blocking filter holder holds a small filter in the optical path just below the slit. The filter blocks contamination from overlapping grating orders, so that only the intended wavelength order is seen by the spectrograph. Filters are changed by loosening the two clamping screws and pulling the housing straight out of the spectrograph. The filter is simply taped in place over the opening at the end of the housing. See the Spectrograph Manual for the list of available filters. Note: The empty envelope should be taped to the spectrograph beside the filter holder, enabling the telescope operator to verify which filter is currently installed. 9. The collimator shutter was used as the exposure shutter for photographic plate spectroscopy. It is no longer useful in CCD work, and should be left in the open position (teardrop-shaped knob pointed upwards, towards the sky). If inadvertently turned horizontal, no light will reach the collimator, and no spectra will be acquired. 10. The collimator focus knob moves the collimating mirror forward or backward so as to convert the diverging rays of light from the slit into parallel rays falling onto the grating. A positional readout dial is on the side of the collimator tube. See the Spectrograph Manual for recommended collimator settings. Controls 11. The grating housing (south side of the spectrograph) provides the means for tilting and covering/uncovering the wavelength-dispersive plane grating of the spectrograph. The grating is tilted by loosening the two silver clamping knobs, then rotating the large knurled ring until the appropriate tilt angle is properly aligned with the vernier scale, and then re-clamping the small knobs. Note that grating angles may be either positive or negative! Relationships between grating angle and wavelength are shown in the Spectrograph Manual. The grating has a cover sleeve that is removed to expose the grating by pulling outward on the knob at the center of the tilting ring, and then swiveling the rod to latch it into one of two holder detents along the perimeter of the ring. The grating should be re-covered when not in use. Forgetting to uncover the grating before an exposure is the most common mistake made with this spectrograph! A grating tag (marked #1, #2, or #3) remains attached to the cover knob to indicate which grating is currently installed in the spectrograph. Translation/ Mount 12. The camera mounting assembly includes the imaging lens for the CCD camera, and translation, rotation, and tilt controls. These have been preset to properly focus the spectrum onto the detector, and should not be disturbed. If difficulty is encountered, contact the SBO staff for assistance.

5 SBO 24" Spectrograph 5 9/28/98 CCD Detector 13. The CCD detector may be either the SBIG ST-8 theromoelectrically cooled CCD, or the Texas Instruments (TI) liquid-nitrogen cooled unit. The ST-8 CCD includes a built-in shutter assembly and thermoelectric cooler, and is interfaced to the PC computer "selene" via the CCDSoft software package. The TI CCD includes a separate mechanical iris shutter attached to the translate/focus assembly. The cryogenic dewar houses the TI CCD, and holds 1 liter of liquid nitrogen in a vertical orientation for cooling the chip, and 1/2 liter in a horizontal orientation. The hose connection reattaches the CCD dewar supply to the 25-liter LN2 supply when the telescope is not in use. Warning: excess LN2 may flow out of the overflow fitting at the rear of the dewar when it becomes tipped. The original plate holder used for photographic glass plates is no longer supported by the Observatory INCH TELESCOPE Mirro Lamp Selector/Timer 4 Rotation Ring 5 3 Slit & Decker Controls TURRET Selector 2 Viewing Optics 8 Order-Blocking Filter 6 Slit/Decker Plates Rotation Release Knob 11 Lens 1 Lamps Tilt/Cover Controls on opposite side 12 Translation Micrometer TRANSLATE/ FOCUS MOUNT SHUTTER 9 13 TI CCD (LN2 Collimating Mirror Readout 10 ST-8 CCD (thermoelectric CCD DEWAR Liquid Nitrogen Connection

6 SBO 24" Spectrograph 6 9/28/98 Slit Viewer Assembly Slit and Decker Plates The slit plate is a front-surface mirror on which is scribed five slits ranging from 20 to 80 microns in width. In addition, a 0.5 mm wide hole known as the "blob" has been added for lowresolution spectrophotometry. Starlight from the telescope is brought to a focus at the slit plate. A stellar image is positioned (by moving the telescope) until its light falls onto the chosen slit. The light passes through the slit to the spectrograph and ultimately to the detector. The remainder of the light is reflected from the mirror to the slit viewer assembly so that the observer may monitor the field under observation. The decker plate is a mask that overlies the slit plate and limits the length of the slit that "sees" the sky. Five slit lengths ranging from 0.7 to 25 mm are possible. In the good ole days of photographic spectroscopy, the stellar image was translated along the slit in order to average the spectrum over many photographic grains; the comparison windows were used to acquire comparison spectra on the same photographic plate, placed on either side of the stellar spectrum. With the CCD detector, neither of these operations are necessary, so that the decker's principle use is to mask out unwanted stars that might lie on the slit. Using a plate scale of 42.3 arcseconds per millimeter, the following table shows the projected angle in the sky subtended by the slit widths and decker lengths. SLIT ARCSECS DECKER ARCSECS 20 µ mm µ mm µ mm µ mm µ mm 30 Blob 20 ` Blob 40 SLIT PLATE Aluminized Front-Surface Mirror DECKER PLATE Opaque Die-Cut Mask Mask Heights (Slit Lengths) (millimeters) Slit Width (Microns) "Blob" Stellar Slit Masks Windows for Indicated Stellar Slit Mask

7 SBO 24" Spectrograph 7 9/28/98 Finder and Slit Viewer Optics The observer chooses between a "wide-field" (approximately 15 arc-minute) finder view of the sky, and a somewhat magnified view of the starfield and slit (approximately 10 arc-minutes of sky along the slit). Selection is made with the push-pull rod on the slit/decker control assembly. When the rod is pushed fully to the left (or "in", as seen from the north side of the plate), the finder optics are brought in-line with the optical path. Light from the telescope is reflected by a diagonal mirror to the eyepiece and does not reach the slit. When the rod is pulled fully to the right (or "out", as seen from the north side of the plate), the slit viewer optics are positioned in the optical path. The view is of the slit/decker assembly, plus the starfield reflected off of the exposed slit plate mirror. This enables the observer to simultaneously view the field and its relationship to the slit.