Why would I choose the 1800 l/mm grating for my Lhires III instead of the standard 2400 l/mm?
You would choose the 1800 l/mm grating when your primary goal is to capture a wider range of wavelengths in a single exposure. The standard 2400 l/mm grating offers higher resolution but sees a smaller piece of the spectrum. The 1800 l/mm module suits bright stars and bright novae when a wider window around a line matters more than the finest detail.
What astronomical targets are best for the Lhires III with the 1800 l/mm grating?
The 1800 l/mm grating suits bright targets where a wider window around one line helps. Good examples include:
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Bright novae: To follow a broad emission line such as H-alpha as the outburst evolves.
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Be Stars: To observe the profile of a hydrogen Balmer line such as H-alpha.
Is the 1800 l/mm grating a direct replacement for the Lhires III's original grating?
Yes, this is a fully compatible, interchangeable module designed by Shelyak specifically for the Lhires III spectroscope. It is intended to be swapped with the standard 2400 l/mm grating to change the instrument's optical characteristics based on your observing target.
Will the Lhires III 1800 l/mm grating let me capture the spectrum of a star like Vega (Alpha Lyrae) in one shot?
No. Shelyak's Lhires III figures give a field of about 8.5 nm per image with the standard 2400 l/mm grating and about 25 nm with the 1200 l/mm grating, so the 1800 l/mm grating sits between the two. The Balmer lines of Vega run from H-gamma (434 nm) to H-alpha (656 nm), so each line needs its own exposure.
How does the 1800 l/mm grating affect the brightness of the spectrum from my Lhires III?
By dispersing starlight over a smaller area (lower dispersion), the 1800 l/mm grating can produce a brighter continuum in the resulting spectrum compared to the 2400 l/mm grating, assuming equal exposure time and conditions. This can be advantageous for obtaining a better signal-to-noise ratio on the overall spectrum of fainter objects.
Does installing the 1800 l/mm grating on the Lhires III require recalibration?
Yes, absolutely. Any time you change the diffraction grating in a spectroscope, you change the instrument's dispersion (how many angstroms per pixel). You will need to perform a new spectral calibration using a calibration lamp to accurately measure wavelengths with the 1800 l/mm grating installed.
