Astronomical Spectroscopy - Physics - University of Cincinnati

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gratings blazed at the red. One could do well with KPC-22B (1st order blaze at 8500Å) with
a reciprocal dispersion of 1.44Å per pixel. One would have to employ some sort of blocking
filter to block the blue 2nd order light, with the choice dictated by exactly how the Ca II
triplet was centered within the 2450Å wavelength coverage that the grating would provide.
The blue spectrum can then be obtained by just changing the blocking filter to block 1st
order red while allowing in 2nd order blue. The blue 1200Å coverage would be just right
for covering the MK classification region from 3800-5000Å. By just changing the blocking
filter, one would then obtain coverage in the red from 7600Å to 1µm, with the Ca II lines
relatively well centered. An OG-530 blocking filter would be a good choice for the 1st order
red observations. For the 2nd order blue, either the BG-39 or CuSO 4 blocking filters would
be a good choice as either would filter out light with a wavelength of >7600, as shown in
Figure 3 6 . The advantage to this set up would be that by just moving the filter bolt from
one position to another one could observe in either wavelength region.
2.3. Echelle Spectrographs
In the above sections the issue of order separation for conventional slit spectrographs
have been discussed extensively. Such spectrographs image a single order at a given time.
On a large two-dimensional array most of the area is “wasted” with the spectrum of the
night sky, unless one is observing an extended object, or unless the slit spectrograph is used
with a multi-object slit mask, as described below.
Echelle spectrographs use a second dispersing element (either a grating or a prism) to
cross disperse the various orders, spreading them across the detector. An example is shown
in Figure 5. The trade off with designing echelles and selecting a cross-dispersing grating
is to balance greater wavelength coverage, which would have adjacent orders crammed close
together, with the desire to have a “long” slit to assure good sky subtraction, which would
have adjacent orders more highly separated 7 .
Echelles are designed to work in higher orders (typically m ≥ 5) and both i and θ in the
6 The BG-38 also looks like it would do a good job, but careful inspection of the actual transmission
curve reveals that it has a significant red leak at wavelengths >9000Å. It’s a good idea to check the actual
numbers.
7 Note that some “conventional” near-IR spectrographs are cross dispersed in order to take advantage of
the fact that the JHK bands are coincidently centered one with the other in orders 5, 4, and 3 respectively
(i.e., 1.25µm, 1.65µm, and 2.2µm).