Sbig SGS User Manual

OPERATING INSTRUCTIONS

FOR THE

SANTA BARBARA INSTRUMENT GROUP

SELF GUIDED SPECTROGRAPH

(SGS)

AND

SPECTRA ANALYSIS SOFTWARE

Alan Holmes

3/30/2001

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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

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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

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Galactic Spectra and Red Shift

Spectrograph

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

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

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on its back when retightening the clamp to insure that the camera is fully seated. The

Micrometer

Grating

Grating

Second Fold

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

Toggle

Lever

Focus Achromat

Spherical Mirror

Mirror

Camera Clamp

Screw

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

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