The GRISM Star Spectrograph is a medium resolution spectrograph suitable for Harvard and MK stellar classification at the university student level and basic professional level. GRISM is a transmission diffraction grating laid over the hypotenuse of a right angle prism. The grating diffracts light into a linear spectrum, while the prism serves to correctly angle the grating for straight-through viewing without refracting or changing the spectrum in any way.

The GRISM Star Spectrograph can be inserted on-axis into a telescope's light path with no need for additional collimating or projection optics which are required with all other types of stellar spectrographs. In fact, using the GRISM Star Spectrograph is no more complicated than prime focus astrophotography.

Unlike prism spectrographs, which disperse blue light much more than red, the GRISM produces a linear spectrum in which all colors are equally dispersed. This makes it much easier to analyze the spectrogram, compare it to other images, and determine wavelengths of spectral lines.

For decades, amateurs have been building simple spectrographs by inserting low dispersion transmission gratings into the light path of a telescope. There are several fairly expensive commercial products which are also based on this principle. Such products do work but have serious drawbacks. The image of the spectrum is deviated markedly off center, and has to be of a small size and low resolution to fit into the photographic frame. What is worse, the spectrum image is angled to the focal plane, and can not be focused for all colors at one time. See diagram 2 among the photographs.

The fact that the GRISM produces on-axis images completely solves both of these problems, and greatly enhances resolution. First, the spectrum image can be enlarged to fit most of the photographic frame. Second, the image of the spectrum is parallel to the focal plane, and all colors of the spectrum are in focus at the same point. See diagram 1 among the photos.

The GRISM Star Spectrograph can be used on all telescopes with 1.25 inch focusing tubes whose range of travel allows prime focus photography. Reflecting optical systems and apochromatic lenses produce the best results. Conventional achromats can also be used, but with the understanding that residual chromatic aberration will somewhat degrade the image.

Exposures with the GRISM Star Spectrograph range from a fraction of a second to about 20 seconds, depending on the size of the telescope and the brightness of the target star. Exposures need to be tracked with a clock drive, but do not need to be manually guided. A 5 inch telescope will take excellent spectrograms of stars down to magnitude 6. For each additional magnitude, the exposure, or the surface area of the objective, has to be increased by 2.5x.

Please note that this spectrograph is intended only for stars. It does not have a slit or a cylindrical lens, and is not suitable for extended objects or for visual use.

Included in the package are:
- GRISM Star Spectrograph inside a 1.25" to T adapter
- T2 to Canon EOS camera adapter, factory adjusted to image a perfectly horizontal spectrum. Write a note about adapting to other camera brands.
- One factory adjusted 20mm Plossl Parfocal eyepiece for focusing and centering target stars
- Celestron / Meade 1.25" right angle star diagonal OR a 1.25" extension tube for the Parfocal eyepiece
- Instruction CD which contains:
     1) A detailed Instruction Manual
     2) A lengthy article on the Basic Theory of Spectroscopy, written on the high school level
     3) An extensive article on the Introduction to Stellar Spectroscopy, written on the college level
     4) Numerous stellar spectrograms and links to excellent freeware image processing software, XnView

WHEN ORDERING, BE SURE TO WRITE A NOTE SPECIFYING THE RIGHT ANGLE OR STRAIGHT-THROUGH PARFOCAL EYEPIECE.

The camera is NOT included.

FREE USPS FIRST CLASS SHIPPING ON DOMESTIC ORDERS
FOREIGN ORDERS $35 FOR INSURED USPS FIRST CLASS SHIPPING
BUYER IS RESPONSIBLE FOR CUSTOMS DUTIES
PAYPAL ONLY

GRISM Specifications:
GRISM diameter 8.5 mm
Exit pupil (facing the camera) 6.5 mm
Grooves / mm 600
Groove angle 34 deg
First order blaze wavelength 540 nm
The PRISM part of the GRISM deflects the first order spectrum so that the green color is in the center of the photographic field when the target star is centered in the telescope.