User's Guide to the Hamilton Spectrograph

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Table of Contents

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Introduction
Quick Reference
Hardware
Software
More Info:

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Data Archive
Mt. Hamilton Homepage

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.