Nasmyth Port Optical Bench

Overview

The Auxiliary Telescope at LSST has two instrument ports, fed from a rotating tertiary mirror. The folded Cassegrain focus configuration is called a Nasmyth system, and the ports are called Nasmyth ports. Each is equipped with a 'rotator' that compensates for field rotation as the alt-az telescope tracks the sky. 

One of the two instrument ports is occupied by the primary instrument, the dispersed imaging spectrograph that is used for atmospheric transmission determination. We're making a unit to bolt onto the other port, that will provide an optical bench for experiments. This entails a flange that mounts onto the rotator, and using Thor Labs 90 degree brackets to hold an optical bench. Rigidity is provided by four wire cables with turnbuckles. The concept is shown in Figure 1. 

Figure 1. Nasmyth optical bench concept. 

The mechanical drawing for the flange, showing a back focal length of 10.50 inches = 266.8mm is here (LTS-496.pdf).

That means beam size at mounting flange is 10.50/18 = 0.58 inches in diameter. 

The drawing for the flange is NasmythMountFlange.pdf

The hollowed-out side of the flange should be mounted to the rotator. 

A cable spool is placed at the end of the optical bench, to allows for a draped cable wrap. It mounts to a bar piece that is in turn bolted onto the top of the optical bench. 

Uses

This gives us the opportunity to displace a sensor at the focal surface while keeping the position of the secondary fixed. That allows us to make distinct changes to the primary-secondary distance and the primary-instrument distance. The RC telescope needs a specific separation between the primary and the secondary to correct properly for optical aberrations. 

We can also put a fast-framing camera on the telescope to look at scintillation effects. 

Finally, we can use a pair of apertures as a pupil mask (with wedges) far upstream in the optical system, and turn the Aux Tel into a differential image motion monitor (DIMM) for seeing studies.