3.1 Imaging Spectrograph
The imaging spectrograph or imaging spectrophotometer (ISP) consists of 3 main optical elements.  The 
first of these is a diffraction grating which is described in section 3.1.2.  The collimated light from all of 
the stars in the field of view of the instrument is dispersed by the diffraction grating onto the second of the 
major optical elements, the objective lens.  This lens is the element which places the most stringent 
constraint on the aperture diameter of the system (see section 3.1.3).  The objective lens will focus the 
incident light from the grating onto a fiber optic taper.  A description of the third optical element, the 
fiber optic taper, which will minify the image produced by the objective lens is given in section 3.1.4.  
There are two other important parts to the ISP, the environment surrounding the optical elements which 
includes the entrance window, and all baffling and stops used to minimize contributions due to stray light, 
and the CCD detector which collects the intensity spectra produced by each star.  These two portions of 
the instrument are explained in sections 3.1.1 and 3.1.5.
3.1.1 Window / Baffling and Stops
The opening allowing light into the ISP will be a simple opening in the “skin” of the payload which will 
be covered during take off.  The cover will be ejected at or slightly after the payload achieves apogee.  
The opening will be as small as possible to acieve the necessary field of view given the position of the 
diffraction grating and objective lens.  Between the window and the diffraction grating a series of knife 
edge baffles will be inserted in order to reject light outside the field of view and to decrease the amount of 
light reflected onto the grating from within the instrument.  Finally, between the lens and the fiber optic 
taper a stop will be inserted in order to eliminate any reflections from the sides of the instrument before 
they contaminate the image at the fiber optic taper.
3.1.2 Transmission Grating
A transmission grating with a ruled area of x and a ruling frequency of y will be used to disperse the 
incident starlight.  This results in a resolving power of z.  Therefore, features of b nm or larger can be 
resolved over the entire frequency range of interest.  The trasmission grating will allow a total of 80% of 
the incident radiation to pass through.  By blazing the grating at c   the amount of light in the first order 
maximum at 570 nm will be maximized with as much as d% of the trasmitted light appearing  at this 
point in the spectrum.  The incoming light is assumed to collimated, thus the spectrum from each star will 
be dispersed over the entire aperture of the lens.
3.1.3 Objective Lens
3.1.4 Fiber Optic
A fiber optic taper is necessary in order to take the image produced by the objective lens and make it a size 
suitable for readout by the CCD detector.  The transmission of the fiber optic taper given a length of x cm 
and a minification factor of y will be z.  The image produced by the objective lens will be too large to fit 
on the CCD due to the low ¦/# and large aperture required to attain a transmission high enough to achieve 
the desired SNR.  While the minification process will degrade the SNR the advantages of having a large 
aperture and low ¦/# were considered to outweight the disadvantages introduced by the taper.
3.1.5 CCD