The P60 Filter Wheel

The command SWITCHTAB enables you to switch between the filter table corresponding to the direct imaging filters and that corresponding to the echelle slit wheel at the 60-inch telescope. If the table that is typed out is the correct one (whichever of ECHELLE or DIRECT you want), even if you wish to change individual entries, do not change the table. (If you do, the echelle table could end up in both tables or vice versa.)

The P60 filter wheel and the P60 echelle slit wheel are controlled by similar electronics. These words are used to initialize the filter wheel's rotational position and to change individual entries in the filter tables. The filter wheel has 12 positions for 3x3 inch filters. The 12 slots are labeled in the wheel. (The echelle slit wheel has six positions.) There is a home position (corresponding to filter position 1) which is sensed by turning a LED on and then rotating the wheel until the LED is directly under a small hole cut in the filter wheel and the transmitted light is sensed by a photodiode. There is no feedback on actual position of the the filter wheel at any time, except for the home command. Thus, one should type FZERO (or ECHZERO) at the beginning of each night to properly initialize the filter wheel position. This command turns on the led, and the wheel rotates until the led is sensed. The home electronics then turns off the led and stops the wheel from rotating, but doesn't tell the Microvax that the operation has been completed successfully. Thus there is a 20 second wait in FZERO, with beeps every 10 seconds. Listen to the continuous rotation of the wheel over the intercom. If it stops rotating before the first 10 seconds are up, the operation has been completed successfully. If it is still rotating when the 20 seconds are up, the operation failed. When the operation has been completed successfully, the filter position visible through the removable viewing port is number 7. (That is the position 180 $^{\circ}$ from filter position 1 in the wheel.) (The slit visible in the guider camera for the echelle is the largest one, the 5 arc-sec slit.) At the end of 20 seconds, the Microvax assumes that the operation has been successful and that the filter wheel is in position 1.

There are at present 8 publicly available 2x2 inch filters for direct imaging. They are the g, r, i, and z filters of the Thuan-Gunn system, the B and V filters of the Johnson system, a filter centered at 6562 A with a FWHM of 100 A, and one centered at the forbidden line at 5007 A of OIII. (The OIII filter is quite narrow, and if you use it in the converging beam which passes through the filter wheel, you should calculate the change in effective wavelength. The filter was designed to be used in a parallel beam.) FZERO next asks you to identify (with a word not more than seven letters long) what filters are in each of the 12 positions of the wheel. The default values are given, but if you have your own filters, they can be inserted and the default identifiers altered. Thus, if you are using the echelle slit wheel, you must identify six of the filters as the appropriate slits, and the other six as something you needn't remember. We list below the default values for both cases.

Position	Filter (P60 Direct)	Echelle Slit
1	g	aslit5
2	empty2	empty2
3	z	bslit4
4	visual	empty4
5	empty5	cslit3
6	red	empty6
7	i	dslit2
8	empty8	empty8
9	oxygen	eslit1
10	halpha	empty10
11	empty11	fslit0
12	B	empty12

Note that the first letter of the filter identifier is used to identify the filter in the names of the frames (i.e., A123F.dst). Do not have two filter identifiers that are in use have the same first letter, otherwise you will be unable to use the batch flat fielding words as the program cannot identify from the header the filter with which the frame was taken. For the echelle, if you want to use a single flat field for all slit sizes, then follow the above name convention, i.e., begin all the slit identifiers with the same letter (above it is ``S''), so that the flattening words will recognize all echelle exposures as having a single filter. If you want to use separate flat fields for each slit size, change the identifiers above to ``ASLIT1, BSLIT2, CSLIT3'' etc.

The command FMOVE will move the filter wheel to a position specified by any of the 12 identifiers. After you type FMOVE, you are prompted for the identifier. If you type something which is not one of the 12 identifiers, an error message will appear. Otherwise, a wait ensues while the filter wheel is moving to the desired position. The length of time for the wait depends on how far apart the initial and desired final filter positions are.

BFMOVE is identical to FMOVE but is coded to allow use in command procedures, so it interacts with the user in a slightly different way than FMOVE. See Section XV for more details.

ECHMOVE will do the same thing for the echelle filter wheel. (The motors are slightly different, so we need 2 separate words.)

To find out where the filter wheel (or echelle slit wheel) thinks it is at any given time, type FSTAT.