OPERATING INSTRUCTIONS FOR THE SANTA BARBARA INSTRUMENT GROUP SELF GUIDED SPECTROGRAPH (SGS) AND SPECTRA ANALYSIS SOFTWARE Alan Holmes 3/30/2001 1 SBIG Self Guided Spectrograph (SGS) Operating Instructions Alan Holmes 3/30/01 1.0 - Overview: this document describes the operation of SBIG's self guided spectrograph and the installation and use of our analysis software. This instrument has been optimized to capture stellar spectra with high resolution, but has enough sensitivity and flexibility to allow its use on brighter galaxies and emission nebula. This unit is a scientific instrument: we expended considerable effort in making collection of the spectra easy, but you will find that a good spectrum of an object requires significant care and effort. Analysis of the data for astronomical meaning is beyond the scope of this document. SBIG's expertise is in the hardware, not the astronomy, so we will not be able to provide much help with data interpretation beyond the basics. To use the spectrograph you must first align it to your camera, and go through some initial calibration steps. This will help familiarize you with the product. Safety Warning: if you use calibration lamps such as a mercury PenRay that emits short wave UV (at around 2537 Angstroms, or 253.7 nM), be very careful with corneal and skin sunburn from the lamps. The little mercury PenRays (such as an Edmund H40759) with quartz envelopes, held one foot from your face for five minutes, will put you in the hospital. They do not appear that bright, but the UV emission is tremendous. Even one minute will give you a sunburn and scratchy, dry feeling eyes. I have personally suffered the effects of exposure to these sources twice, and have had two coworkers (separate incidents) requiring bandaging of their eyes after exposure. The SBIG spectrometer has an order sorter that blocks these wavelengths from the system, so long wave mercury sources are adequate. I have no experience with how safe these are, so take precautions. Fragility warning: All of the optics in the SGS can be cleaned with isopropyl alcohol and cotton swabs except for the gratings. The grating surfaces should never be touched. The groove structure is easily damaged. If they are dusty, blow them off with light air flow. A little dust will not bother your spectra at all, but cleaning can easily do much greater damage. 2.0 - SGS Description: the spectrograph is designed to operate with the ST-7/8/9. The object that is to be analyzed is viewed on the tracking CCD, simultaneously with the slit. The slit is backlit by an LED during object acquisition to render it clearly visible on the tracking CCD. The object is manually maneuvered onto the slit using the telescope controls, and is held there using our patented self guiding feature during a long exposure. The spectra is recorded by the imaging CCD, oriented long-ways so the spectra falls across 765 pixels, with a height of about 8 pixels for stellar sources. Two gratings are available. The standard grating, 150 rulings per mm, gives a dispersion of 4.3 angstroms per pixel, and allows the user to capture the entire interesting range from the calcium H and K lines to H-Alpha with a single exposure. The resolution is about 8 angstroms. A 2 high resolution grating on a carousel in the instrument can also be used that gives 1.07 angstrom per pixel dispersion, with a resolution of about 2.2 angstroms. The spectral range is smaller, being only about 750 angstroms. This resolution is adequate to detect the doppler shift due to the earth's motion around the sun when carefully calibrated, and detect spectroscopic binaries. Two slits are provided with the unit. The slit installed at SBIG is 25 microns wide, but it appears to be 18 microns wide to the spectrograph since it is tilted. A wider slit, 100 microns wide, is included with the spectrograph for use in capturing the spectra of dim extended objects, such as galaxies. It appears to be 72 microns wide. It is more effective on dim objects since more light makes it through the slit, but at the cost of spectral resolution. 3.0 SGS Specifications: Dispersion: 1.07 or 4.3 Angstroms per pixel Resolution: emission line is recorded with 2.2 or 8 Angstrom Full Width at Half Maximum Spectral coverage per frame: about 750 Angstroms with the high resolution grating, or 3200 Angstroms with the low resolution grating Center Wavelength Selection: Calibrated Micrometer Adjustment Wavelength Range: 3800 to 7500 Angstroms Sensitivity: Signal to noise ratio of 10:1 for a 10th Mag star, 20 minute exposure using an ST-7E and a 10 inch (25 cm) aperture in high resolution mode. An ABG ST-7 will reach magnitude 8. The low resolution mode with the wide slit will be 2 magnitudes more sensitive Entrance Slit: 18 micron (2.3 arcseconds wide with 63 inch (160 cm) focal length Telescope Acceptance Angle: F/6.3 by F/10. F/6.3 recommended for maximum signal. Dimensions: 4 x 5 x 8 inches (10 x 12 x 20 cm) Weight: spectrograph plus ST-7 weigh 5.2 pounds (2.4 kg) Uses: Stellar Classification Analysis of Nebular Lines Identification of spectroscopic binaries Measurement of Stellar proper motion to +/- 6 km/sec accuracy Measurement of Emission Nebula Proper Motions Spectra of Laboratory and field sources 3 Galactic Spectra and Red Shift Measurement of Brighter Quasars Galactic Red Shifts and Spectra: Difficult to obtain due to faintness, extended nature of source, and lack of high contrast emission lines. Only the brighter galaxies can be measured. Seyfert galaxies which have excess H-alpha emission are much easier to measure. 4.0 Initial Alignment - All SGS units that are shipped from SBIG have been aligned to an ST-7 camera here, so they should be pretty close. If you find during one of the following steps that the system appears to be seriously misaligned, make sure no optical elements have come loose and that you are following the procedure correctly. Step 1 ­ Attach Coupling: remove the D-block from the front of your ST-7 or ST-8. Using the four screws provided, attach the spectrograph coupling to the ST-7 as illustrated in Figure One. It is important to orient the coupling as shown, with the thick part to the left. Figure One:ST-7 with Attached Coupling ­ Note Orientation Spectrograph Coupling Step 2 ­ Attach camera to Spectrograph: remove the cover on the spectrograph by removing the four Phillips Head screws around the periphery of the baseplate. Loosen the clamp where the camera attaches, and insert the tube on the coupling. Lay the camera 4 on its back when retightening the clamp to insure that the camera is fully seated. The camera should be oriented so the exiting cables point away from the end of the spectrograph with the toggle switch. Figure Two labels the important SGS alignment points. Note: if you are using an ST-8 you will need to reposition the clamp plate to the other set of holes in the baseplate. This will maintain the tracking CCD at the original position, but move the imaging CCD over. Due to the larger size of the imaging CCD, the spectra will still fall upon it. Figure Two: Important Alignment Points Telescope Coupling Focus Achromat Spherical Mirror Grating Toggle Second Fold Mirror Grating Lever Micrometer Screw Camera Clamp Step 3 ­ Adjust slit focus: slip the cover back on, without the screws (you get to do this a lot), and connect the camera to your computer. Power it up, and run CCDOPS. With the assembly just sitting on the table, select FOCUS mode with the tracking CCD and adjust the room light so you can see the slit with a 1 second exposure. Power up the internal LED by flipping on the toggle switch. The slit should be approximately vertical, and in the center third of the tracking CCD. In this step, you should adjust the slit focus to be sharp ­ do not worry about the orientation to the CCD grid. Make sure the flag on top of the cover is oriented so as to let the light go by. When you can see the slit image, adjust the focus by removing the cover, loosening the focus achromat, moving it slightly, retightening it, and reinstalling the cover. Continue with this process until the slit image is sharp (1 to 2 pixels wide). This 5 is tedious, but need only be done once. The image file on the software disk, M57TRACK.ST7, shows what an acceptable focus looks like. Step 4 ­ Adjust slit image position: to center the slit on the tracking CCD, loosen the second fold mirror, rotate it slightly, retighten it, replace the cover and view the image. The adjustment is coarse. Once again, this is a tedious process. If the slit image is on the center third of the tracking CCD you are done. This completes tracking CCD adjustments. Step 5 ­ Find spectral lines: set the assembly on the table, resting on the handles. Tape a piece of paper over the telescope coupling, and illuminate it with a fluorescent lamp or a neon lamp. Choose the low resolution grating by rotating the toggle to point up (away from the handles) until it snaps into the detents. Set the micrometer screw to 5.44 mm (which should put the mercury 5461 angstrom line near the center). Orient the flag on top to block the tracking path since the light flood-illuminates the slit. In CCDOPS set the CCD to imaging, the resolution mode to 1xN, and the vertical binning to 4 pixels. Use focus mode with the imaging CCD, 1 second exposures, and you should see spectral lines. The cameras are shipped with the low resolution grating in position. Refer to Figure Three for identification of the most prominent spectral lines for mercury and neon. Figure Three: SPECTRA program Illustration of Spectral Lines (Low Dispersion) 6680 6403 5851 Neon Mercury 5770 & 5791 5461 4358 4046 Note: the camera can have very significant stray light when used this way, since very little light goes through the slit: most bounces off the reflective surface and ends up on the tracking CCD, where it can scatter to the imaging CCD. Astronomically this is only a problem when doing the moon or sun ­ all other sources are fairly small. Fortunately, the sun and moon do not need self guiding! This is the purpose for the flag on top of the cover. If self-guiding is not needed (generally for a spatially large source) block the tracking path. Step 6 ­ Rotate camera: Once you have spectra, switch to the high resolution grating and center a bright line. In the high resolution mode the spectral line should be nearly centered when the dial reads the wavelength. For example, a dial reading of 5.46 mm should position the 5461 angstrom line on the CCD. The dial reading is not perfect in the high resolution mode, but its good enough to find isolated spectral lines. The vertical binning really accentuates the slant of the spectral lines, but the slant is easily removed. Remove the cover, loosen the camera coupling and rotate the camera slightly, retighten the coupling, and slip the cover back on. We recommend you lay the 6 camera on its back while doing this to ensure the camera is seated. Repeat this process until the spectral lines are vertical to within a pixel, which is not that hard. Don't worry about momentarily blocking the fan while tightening the clamp ­ no harm will be done. Note: we align the spectrograph at SBIG such that when the spectral lines are vertical in the high resolution mode, a stellar spectra will be horizontal to an accuracy of a few pixels. This is only true in the high resolution mode. If you switch to the low resolution mode the stellar spectra will still be horizontal, but the calibration lines will tilt to the right about 15 pixels, top to bottom. At this point you have a choice. If you wish to work in low resolution mode you can live with the tilt, but calibrate on exactly the same strip that the star falls upon. This is recommended in practice. Or, you can rotate the camera to make the calibration lines vertical, and live with the tilt of the stellar spectra which, if you are binning 4:1, isn't too bad. If the source is an extended object this is not much of a problem. The inability to perfectly square up stellar spectra with calibration lines is a consequence of the optical design. We have rotated the slit slightly off horizontal to minimize this effect for the high resolution grating, assuming that the most critical wavelength position determination will be done with this grating. Step 7 ­ Focus the Spectrograph: Reach underneath the spectrograph and loosen the screw that clamps the spherical mirror assembly down. While viewing a bright, centered line using CCDOPS, focus the spectrograph using the focus screw, which translates the mirror toward or away from the grating. When satisfied with the focus retighten the mirror clamp screw. Check again. Clamping the screw shifts the focus slightly so, when you get close, you may need to clamp each time you move the mirror. Both gratings focus at the same point. Since the design uses only mirrors, all lines are in focus. Step 8 ­ Calibrate the micrometer: ...