Restrictions imposed by the VLT

Any device to be installed between the Vlt and the Crires instrument interfaces with the telescope on the one side, and the instrument or its pre-optics (eg. the AO system) on the other side. The major constraints imposed by the telescope is the focal ratio (f/15), and simply space limits. As for the latter, the back-focal distance (clearance between the unobstructed end of the Nasmyth adapter structure and the focus position) is only 250 mm. This space is the principal location for any (short) calibration gas cell, and in fact the two Crires standard cells occupy this part. The availabe space is further diminished (see next paragraph) by a calibration mirror immediately after the Nasmyth adapter, leaving an effective 180 mm clearance between the calibration mirror unit and the focus. A mechanical sliding train fills this room to position eg. the standard cells (mounted on the slide) into the optical beam.

The telescope beam comes to focus on the very edge of this calibration train. The telescope focusing mechanism can accomodate the focal plane position within 215−255 mm behind the adapter’s rotator instrument mounting flange. Any mechanism to be installed here has to accept the f/15 ratio and to not physically extend beyond the nominal focus position (see below); the proximity to the focus also dictates the required dimensions for an unvignetted field (55 mm clear diameter at a back-focal distance of 144 mm). Fig. 11.5 illustrates the arrangement around the telescope focus.

Figure 11.4.: Schematic drawing of the Vlt unit telescope in its dome enclosure. The tele-scope structure (center) is indicated together with the path of incident light (dash-dotted lines) towards the Cassegrain and Nasmyth foci (labelled). The Coudé foci are redirected from inside each of the Nasmyth adapter rings (by means of a pick-up arm) via a five mirror train into the telescope basement (used for interferometry). Crires (not shown here) is mounted on the left Nasmyth platform. (adapted from ESO/ Rupprecht 2005)

11.2. CRIRES requirements 113

Figure 11.5.: Arrangement of the Crires spectrograph on the Nasmyth platform (section of drawing), seen in top view. The telescope adapter is at the top, below is the optical bench with the Crires warm optics (in oder of the optical chain:

calibration mirror unit, calibration slide, de-rotator unit, adaptive optics module and wavefront sensor unit). The cold part is located in a cryogenic vacuum tank (bottom), with the main optical components and the light path shown (dimensions omitted). (adapted from ESO/VLT-DWG-14500-0-300003).

Figure 11.6.: Crires optical layout (detail). The telescope adapter flange is located to the top (not shown), 250 mm before the Nasmyth focus. Clearance and a calibration mirror unit occupy 70 mm after the flange, leaving only 180 mm space until the focus for a sliding train (“calibration cells”) to host eg. gas cells. The instrument derotator unit is located immediately after the focus, followed by the AO module. The entrance window is a dichroic (reflecting the optical light to the AO wavefront sensor) to the vacuum tank and separates the cold instrument part in a cryogenic environment. After the instrument focus follows theCrires pre-slit optics and the spectrometer; cf. Fig. 11.5. (adapted from ESO/VLT-DWG-14500-0-300002).

11.2.4. Restrictions imposed by the CRIRES instrument and its pre-optics The Crires optical layout is on display in Fig. 11.6 (in parts). As reasoned in Sec. 11.1, a calibration gas cell device must be placed into the optical beam as early as possible in the optical train, in order to witness and thus make traceable instrumental effects from this location on until imprinted onto the detector in a spectrum’s signature. The Vlt-focus and the location of the Crires vessel must be assumed fixed and non-movable. Any re-location of the vessel or optical components therein must be considered too costly, and would imply a major redesign of the instrument and a change of the spectrograph properties. In this way, a simple re-design of the arrangement on the Nasmyth platform with the aim to increase the

11.2. CRIRES requirements 115 distance between telescope focus and instrument entrance window (ie. instrument focus) is out of scope. Such a solution would offer space to accomodate a collimated beamline, suitable for a long-path tube-like gas cell (cf. Sec. 10.3).

Thus, the instrument input parameters are required to remain unchanged. These con-straints translate into the following requirements:

1. The Crires fore-optics inside the cold instrument part serves to a) provide an image of the telescope pupil. This pupil image is used to place a cold stop and filters (cf.

Sec. 11.2.1), and must be maintained. Also, b) the fore-optics adapt the focal ratio at the slit to match the spectrograph camera. This means that an input f/15 (through the entrance window) is strictly required (Delabre et al. 2000). The full Crires field of view is 50×50^′′ (projected onto the slit). A reduced field is feasible, but also results in a reduced slit-length. The goal is thus defined to maintain the full FoV, whereas the requirement is a FoV no smaller than 5×5^′′ (H. U. Käufl, priv. comm).

2. The MACAO AO system (Paufique et al. 2006) is located on the warm bench just in front of the cryostat entrance window. The four mirror module operates at unit magnification (Delabre et al. 2000), and provides the f/15 input to the cold part optics.

The input beam is thus also of f/15 (from the telescope), and any mechanism to be inserted before or after the AO needs to preserve this focal ratio. The AO system cannot be re-located without re-designing it as the instrument focus and the telescope focus are at defined positions. An important requirement concerns the AO deformable membrane mirror (”AO mirror“ in Fig.11.6). In order for the AO to function, an image of the telescope pupil needs to be placed onto the AO mirror surface. This is achieved by AOM1 and AOM2 (and the according distances). As the telescope pupil sits at the secondary (tip-tilt) mirror, the M2 must be imaged onto the AO mirror to ensure AO correction. If this condition is not preserved with a gas cell mechanism, the AO functionality forCrires goes astray.

The AO wavefront sensor also requires that optical wavelengths (450−950 nm) be transmitted up to the cryostat entrance window.

3. The instrument de-rotator is located just after the telescope focus (before the AO).

Although it only consists of three flat mirror surfaces, and thus is not optically active, its dimensions (clear field of view) prohibit moving it towards the AO system (this would enlarge the accessible space around the telescope focus, to accomodate larger/longer calibration units).

4. Likewise, the calibration mirror system and the sliding train with calibration units are tied to their locations immediately after the telescope adaptor flange and before the derotator (ie. effectively between adapter and Nasmyth focus).

Hence, there exist only two potential locations to insert a calibration device into the optical train. This is, first, the existing calibration train slide immediately before the telescope Nasmyth focus. The space provided by the slide is 180 mm along the optical path (the slide carriage is 862 mm long, perpendicular to the optical axis; other gas cells and calibration units such as a pinhole and a polarimeter need to be accomodated on this carriage).

Second, there is some 577 mm nominal space between the last derotator optical surface and the first AO mirror. Given the mirror housings and the extended derotator mount, some 300 mm clearance remain. However, the AO deformable mirror is closely adjacent to the

Table 11.1.: Vlt optical design data for the Nasmyth configuration (f/15, effective focal length 12.000 m). For details, refer to ESO technical report VLT-TRE-ESO-10000-0526 (Dierickx et al. 1995).

Surface RoC^a Distance^b Diameter Tilt Conic Comment

[mm] [mm] [mm] [deg] constant

OBJ Infinity Infinity - 0 WFM

1 Infinity 89134 8000 0 Entrance Pupil^c

2 -28800 -12396.43 8185.9 -1.00469 TEL M1^d STO -4553.571 9896.429 1116.043 -1.66926 TEL M2

4 Infinity -6550 752.1417 45 0 TEL M3^e

5 Infinity -250 60.05316 0 Adaptor/Rotator flange edge

IMA 2089.6 41.14407 0 Nasmyth focus^f

Notes.

(a) Radius of Curvature.

(b)Axial distance to next surface.

(c)The entrance pupil is located behind (ie. just below) the primary mirror.

(d)The Nasmyth configuration is maintained by the shape of M1, and controlled by the active optics.

In the Cassegrain configuration, RoC and conic change for M1.

(e) M3 is flat, tilted, and elliptical with 866×1242 mm (M3 is rotated off its tower parallel to the altitude axis during Cassegrain configuration, to liberate the Cassegrain hole in M1 through the M3 tower. This is depicted in Fig.11.2).

(f)The image plane/focal plane diameter is given for a 50^′′×50^′′field of view.

first AO mirror, so that vignetting of it would occur (see Fig. 11.5). Accounting for the structure mount of the deformable mirror, approximately 200 mm usable space remain (exact dimensions are unknown due to a lack of available documentation).

The second option thus does not offer considerably more physical space, but additionally suffers from a larger diverging beam diameter, and obstructions from other close-by elements.

Effectively, the calibration carriage remains as the only acessible location for an absorption gas cell mechanism.