Low-Resolution Spectrograph 2
For a deeper look into the LRS2 instrument please see the SPIE paper by
Taylor Chonis et al. 2014 and Taylor Chonis et al. 2016
Explanation of LRS2 Design – Spectrograph units
The second generation Low Resolution Spectrograph (LRS2) is a new facility instrument for the Hobby-Eberly Telescope (HET). Based on the design of the Visible Integral-Field Replicable Unit Spectrograph (VIRUS). LRS2 provides integral field spectroscopy with two units each with seeing-limited fields of 12"x6". The two IFUs are fed by 0.6" lenslets
The image below shows LRS2-B about to be installed in its slot in the VIRUS enclosure:
The image below shows a rendering of the LRS2 unit with two spectrograph channels; major components are noted:
The two grisms for the LRS2-B unit are shown below. The grisms consist of a volume phase holographic grating sandwiched between two large prisms. This allows LRS2 to work at higher resolving power than VIRUS but still use the same physical structure and angle between collimator and camera.
Note that the lower spectral resolution of the orange channel is reflected in its smaller wedge angle.
Explanation of LRS2 Design – Fiber feeds
The input for each LRS2 unit has a dichroic beamsplitter that divides the light into two spectral channels with different wavelength ranges. The anatomy of the beamsplitting feeds is shown above.
The input feeds use hexagonal microlens arrays to create a miniature focal-reducer feeding each fiber. Thereby the 6”x12” field of view is fully-filled in. Images of the assembly of the feeds are shown below.
There is a small offset in alignment between the two channels in each unit, amounting to about half a lenslet diameter or 0.3 arcsec.
The image below shows the registration between the far-red and red lenslet arrays in LRS2-R on sky. The alignment is reasonable but not perfect, but can be corrected in the data reduction pipeline.
Additional supplemental information
Shown in b) is a detailed view of the microfocalreducer, which consists of a field lens and two lenslet arrays. The optical path of a single spacial element is detailed in c) above.
And the spacial elements arrayed into a block 12"x6". The coverage is complete within an IFU so no dithering is required.
For a discussion of the LRS2 throughput please see LRS2 throughput.
Last updated: Wed, 26 May 2021 09:11:41 -0500 stevenj