Spectroscopy

Stellar Spectroscopy is actually a method of classifying stars based on their spectra. The spectra, which reveals the chemical composition of the star, helps to understand where the star is on its evolutionary journey on the Hertzsprung-Russell [H-R] Diagram. All stars start their normal life as Main Sequence stars. The Main Sequence is a line on the H-R diagram running roughly from the lower right to the upper left on the diagram. Only stars that are fusing hydrogen to helium are located in the main sequence. As stars exhaust their supply of hydrogen, they leave the main sequence for one of the branched areas on the diagram.

There are also several subtypes for star classification which are too involved to mention here. More information can be found here.

I am currently using an grating type spectrometer, the SGS [Self Guiding Spectrometer] , which is manufactured by SBIG. This is a single beam spectrometer which uses a CCD camera as the detector. I have made several modifications to the equipment which are listed below. I wrote a paper on the Classification of the Nova V475 Sct 04 that makes good use of the SGS. The paper can be downloaded here.

Regulus is a class B7V star [Strong Helium lines and a longer toe into the UV spectrum].

Regulus Calibrated Spectra.gif.jpg (68227 bytes)

Arcturus is a K2IIIp [More to the red end of the spectrum and with a spectrum dominated by metallic lines]

Arcturus Calibrated Spectra.gif.gif (5146 bytes)

The Nova V475 Sct on October 1, 2003
The Hydrogen alpha line is clearly visible at 6562 Å as are the Fe lines at 4900 to 5400 Å

Nova Sct 03 Cumulative Spectra_25695_image001.gif (7796 bytes)

For those of you using the ST-10XME camera along with the SGS, here is a third order polynomial correction factor that I have worked up for the QE [Quantum Efficiency] of the KAF-3200XME chip:

QE= ((-6E-06 * A²) + (0.0761 * A) - (155.25)) * 0.01

Where QE = Quantum Efficiency as a fraction [e.g. 50% = 0.50]

If you divide the flux value for each pixel by the QE for the central wavelength for that pixel, the calculation will normalize the spectra for the QE response of the chip.

Those Darn Cover Screws!: Removing the cover screws something that is done a lot and is very time consuming. To make the process easier, I purchased some 4-40 x 1/4" plastic headed thumb screws from McMaster-Carr. www.mcmaster.com

The thumb screws are Part Number: 1185A221 and were about US$8.00 per 25 screws when purchased back in in 2002. These screws have knurled plastic heads and can be removed in the field with no tools.

Putting the object on the chip: One big problem with the SGS spectrometer is that

there is no provision to use an eyepiece for centering the program object on the guider chip. The optical inlet to the spectrometer is a rather elaborate threaded ring which attaches to the visual back of the typical SCT scope. I removed this ring and inserted a T-adapter [the SGS chassis is already threaded for a T-adapter] which is mated to a 2" Meade flip-mirror. Because the actual inlet aperture of the SGS is so small [~1/2"] there is no problem with vignetting of the light cone as it enters the instrument. The image to the right shows the flip mirror between the scope back and the SGS.

Grating Lock-Down: Another problem with the SGS is the rotary carousel which is used for the low resolution and high resolution gratings. The carousel rotates to provide the particular grating which is needed for the program application. This is a handy option for the SGS, but the method used to lock the carousel in place [two spring loaded detents] is not sufficient to keep the carousel from rotating while in use. Even

a change of a few arc seconds in the rotation of the carousel causes the calibration to drift. Slewing the scope from one position to the next moves the carousel sufficiently to require a new calibration spectra to be taken. This is a problem mentioned by Alan Holmes in the operating manual for the instrument. I solved this problem by replacing the outboard spring loaded detent set screw with a custom sharp pointed set screw which is accessible from outside of the SGS case. See image above. The set screw positively locks the carousel in place and is loosened when moving from one grating to the other. The inboard spring loaded detent remains and locates the grating before it is locked down.

Camera Attachment: The attachment of the camera to the SGS and the

subsequent alignment of the camera to the SGS is a rather tedious procedure. I made the attachment easier by machining a new clamping ring screw so that it extends outside of the SGS case and does not necessitate removing the instrument cover from the case to access the camera clamp.

As supplied, the SGS requires that the normal camera adapter plate be removed and replaced with a special SGS adapter plate each time that the camera is to be used with the spectrometer. This is also a tedious procedure. It also requires that the camera be realigned to the SGS before each use. I machined a new nosepiece for the camera which on one end has a ring to fit in the normal camera adapter plate and on the other end has a short tube to fit into the SGS clamp ring. This adapter stays permanently attached to the SGS and attaches to the standard camera adapter plate via the normal three set screws supplied with the camera. I put a mark on the machined adapter and the camera adapter plate so that they may be aligned during the assembly process. This alignment is close enough that a final tweaking can take place on the scope via the extended clamp ring screw mentioned earlier.

Calibration Light Sources: The SGS has an opal diffuser on the outside of the

case where a calibration light source can be placed for the purposes of taking a calibration frame. There is, however, no provision for attaching the light source to the diffuser. [It is mentioned in the operating manual to "tape" the calibration source to the case] I have machined a small metal adapter which will hold two - three neon bulb sized calibration lamp sources in close contact with the opal diffuser and also block out any extraneous light. It attaches to the case by means of the screw which was originally used to hold the light shield over the diffuser.

Remote Control of Slit Illumination: The SGS uses an LED to illuminate the

internal slit so that the program object may be centered on the slit. The on-off switch and intensity control for this LED are mounted on the body of the SGS. This is very inconvenient in that touching the controls makes the unit vibrate. It also necessitates that the operator move from the computer console over to the telescope [which may be several feet away or in a different room altogether]. To help solve this problem, I removed the 9 volt battery, the Power Indicator LED, the Power Switch and the Intensity control from the SGS, plugged the holes with nylon bolts and remounted the controls in a small plastic 'Radio Shack' project box. Only two wires are required to re-connect these controls to the LED inside of the SGS. These are attached back to the SGS body via a 1/8" phone jack. [You can also see the heads of those plastic headed 4-40 thumb screws in these pictures]