Overview

Figure 1: The spectrograph with the grating box to the right.

TRIPPEL is situated in the spectrograph room which is adjacent to the imaging ("URSIES") room. The AO platform is rotated 180 degrees to feed the spectrograph with light. (General observers do not have to do this beam rotation.) The light beam goes through a hole in the wall. Observers can access the spectrograph room from the URSIES room via a somewhat larger hole. Watch your head! And be careful not to bump into the two Dalsa cameras and their cables.

NOW: The light beam encounters a beamsplitter cube which currently and nominally transmits 90% of the light and sends the rest to the wavefront sensor and correlation tracker camera. The current beamsplitter performance degrades below 500 nm - it is not well fitted for Ca H&K observations.

TRIPPEL is built on two optical tables which are connected but have different heights. The slitbox is on the first higher table where also the WFS and the correlation tracker setups are built, as well as any slit-jaw setup or other imaging cameras.

TRIPPEL is a Littrow spectrograph using a 79 grooves/mm echelle grating with a blaze of 63.43 degrees.

The slit has a width of 25 $\mu$m and is etched on a glassplate with a reflecting layer of coated silver. The slit plate is mounted on the turnable slit holder which consists one wall of the slit box. The slit plate is interchangeable. The slit box also contains a corrector lens and mirrors that divert the outgoing beam to the exit ports.

The grating is mounted on a rotational stage in the grating box. The Littrow lens - a doublet with 1500 mm focal length - is on a translation stage in front of it. The lens should always be at a fixed position determined by the optical design. It is not used for focusing the spectrograph - that is done by moving the spectrum cameras. The lens is only to be moved while checking the focus position of a spectrum camera.

The grating is slightly tilted to the vertical so that the image of the slit - the spectrum - falls on a plane slightly above the slit. The three exit ports are called A (to the left when facing the sun), B (to the right when facing the sun), and C (central port on the top of the slit box). Currently only ports A and B are operated. Spectropolarimetry will be possible via port A in the future.

Outside the exit ports the observer can mount cameras of the same sort and on the same kind of mounts as when doing imaging at the SST. (With the exception for port C for which there is a special camera mount that fits only Megaplus II cameras.) For ports A and B, the only difference is that the beam is higher so the track has to be placed on an aluminium block to get to the proper height.

Since there is no predisperser, all spectral orders fall on top of each other. To separate orders filters are used.

The spectrograph has an off-plane design which causes keystone and smile deformation of the spectra. (There is also some image distortion from the corrector lens.) Thus the spectral lines and the dispersion direction will be slightly curved and not exactly orthogonal.