Spectral Format
see also
Table of Orders
The Hamilton spectrograph is optimized for high resolution, broad
spectral coverage, and high throughput. Its detector can simultaneously
record more than 100 spectral orders to the ends of their free spectral
ranges, enabling the instrument's full wavelength coverage (3400A-9000A)
to be captured in a single exposure. (In practice, CCD response and
blue absorption in the cross-dispersing prisms makes observations
blueward of about 3800A challenging.)
The spectral format consists of many, nearly parallel
spectral orders (see
Table of Orders).
Longer wavelength orders are at
the top, and wavelength in a given order increases to the right.
(N.B. The illustration is drawn as it appears if the image origin
(pixel (0,0)) is displayed in the upper left-hand corner of the image.
This orientation is a Lick convention not shared by all display tools,
many of which put the origin at lower left, in which case
the spectral format illustrated here would appear flipped top to bottom.)
The length of each order, as drawn, represents its free spectral range
(FSR), i.e., the portion of a given order not repeated in the order above
or below. The FSR increases towards the red, while the angular dispersion
and order separation increase towards the blue. At the red extreme,
order 57 has a FSR of about 176 Angstroms at a dispersion of 5.61 A/mm;
at the blue end order 163 covers 21.5 Angstroms at 1.96 A/mm.
The FSR's, dipersions, blaze (i.e., central) wavelength, and order
separation of all orders are given in the
Table of Orders.
In the illustration below, order numbers are given
along the left edge and the corresponding central wavelengths on the right.
The length of the line representing each order is its free spectral range
(FSR), i.e., the portion of a given order not repeated in the order above
or below. Though not shown in the illustration, orders are actually somewhat
curved, and not quite parallel to one another.
