Filter Wheel

Filter Wheel

4.1 Before Starting

  It is vital that the filter manifest matches the filters actually loaded in the wheel. Be sure to update this page whenever new filters are installed.            The filters are the most fragile and most difficult to replace components after the detector, so be sure that you have all equipment and parts prior to starting work on the filter wheel.   The filters are cooled to 28K and tilted by 3.1 degrees to the optical axis. 

4.2 The Filter Manifest

Wheel A/1

Filter Wheel IndexElement 1Element 2InstalledNotes
1F105B53 (OCLI W10527-8)F105B53 (OCLI W10527-8)30/7/99
2F22B15 (K)F22B15 (K)6/11/98Chipped within beam diameter
3F34B9 (L)F34B9 (L)6/11/98
4F47B5 (nbM)F47B5 (nbM)6/11/98
5F198B27 (FK Q Band)F198B27 (FK Q Band)6/11/98
6F209B42 (1NL)13/5/99
8F103B10 (OCLI-Q)6/11/98
10F116B9 (OCLI-R)BaF26/11/98
11F125B9 (OCLI-S)BaF26/11/98
14F79B10 (OCLI-N)CaF26/11/98
15F88B10 (OCLI-O)CaF26/11/98
16F97B10 (OCLI-P)F97B10 (OCLI-P)6/11/98
17F107B425/7/01CWL 10688 HW 379
18F95B225/7/01CWL 9516 HW 241
19F128B2 (FK nb)F128B2 (FK nb)6/11/98Appears banded across centre
20F66L (A)20/9/00Cut-on filter (OCLI L06460-8/A)
21F185B9 (1NB)F185B9 (1NB)10/3/99
23F86B2F86B226/6/01CWL 846 HW 213
24F122B3 found broken after 1 cool down (25/7/01

Wheel B/2

Filter Wheel IndexElement 1 Element 2 InstalledNotes
1F14S (A)F14S (A)25/7/01BaF2 short pass element
2F14S (B)F14S (B)25/7/01BaF2 short pass element
6F209B42 (1NS)13/5/99Chipped outside beam. Moved here on 20/9/00
9Polarising Grid T18/8/99Polarisation axis tangential to wheel
10Polarising Grid R26/6/01Polarisation axis radial to wheel
17Etalon NEtalon N6/11/98Moved here from A21 on 30/7/99
18PolystyrenePolystyrene10/3/99Moved here from A22 on 30/7/99
20F66L (B)20/9/00Cut-on filter (OCLI L06460-8/B)
21F185B9 (1NC)F185B9 (1NC)10/3/99Moved here from A24 on 20/9/00

You can compare filter installation dates with the Cooldown Record to see how many times a filter has been cooled from room temperature to <100K.

4.3 Disassembling the Filter Wheels

4.3.1  Cover the aperture in the filter cover plate and remove the eleven screws which hold it down to the filter wheel assembly.

4.3.2  To remove the motor drive, undo the four screws holding the worm gear support bracket onto the main structure, rotate the Oldham coupling to allow the gear to be removed and remove the gear and bracket together.

4.3.3   To remove the filter wheel, remove its cover plate, (leave the cover plate on the other filter wheel), and undo the four sprung Allen screws which hold down the brass coloured bearing cover.

4.3.4   Underneath is a brass coloured block held in place by two hex screws. Slacken these slightly (you will need to hold the filter wheel to stop it rotating), then lay the assembly flat, with the block uppermost and the wheel supported on its central cover plate. As you undo the two screws the filter wheel will drop down onto its support. The rest of the assembly can then be lifted clear.

4.3.5   Remove the block and the four Belleville washers as well as the two bearings from either side of the filter wheel. Note that the bearings should have the two markings facing each other, on the inner side of the bearing pair. Also, it is the bearing with the smaller chamfer in its inner diameter which mounts first on the filter wheel.

4.3.6   Repeat the procedure for the other filter wheel, remembering to to keep the parts marked and separate.

4.4 Installing a Filter

Remember to update the filter manifest at the same time as installing them in the wheel.

4.4.1 Support the filter wheel so that it is raised up a few inches, with the retaining clips on top as shown in Figure 4.4.1.

Figure – Mounting filters in filter wheel A1 using the special tool.

4.4.2   Mount a melinex film washer in the empty filter wheel socket to protect the filter from being scratched by the socket.

4.4.3   Insert the special filter mounting tool through the socket from underneath, place the filter on it and retract the tool to leave the filter in the socket, as shown in Figure 4.4.1. If the filter comprises two elements (i.e if it has a blocking component), separate them using a melinex washer.

4.4.4    Place a melinex washer on top of the filter stack to protect it from the locking filter spacer.

4.4.5    Place the locking filter spacer (shown at the right in Figure 4.4.5) carefully in the socket, using the tongue and groove feature to seat it properly. Some filters will have cylindrical aluminium spacers (2.75mm thick for the OCLI silicate set) which must be mounted in the socket before the locking spacer.

Figure – The special filter insertion tool (white) and an aluminum filter spacer (right).

The distance that the upper surface of the locking filter spacer projects sets the force that will be exerted on the filter. If the limit of 3mm is exceeded the filter may break.

4.4.6    Check that the upper surface of the filter spacer projects no more than 3mm above the surface of the filter wheel and is parallel to it. Carefully lift, twist and lower the filter retaining clips into the indents in the locking filter spacer.