The three filter/slit wheels require some additional discussion. The three wheels, starting with the one closest to the telescope focal plane, are referred to as the "upper", "middle" and "lower" wheels. Only the upper wheel can be brought into focus by the ESI collimator and this is where the slits for echelle or low-D mode reside. The middle wheel contains an aperture selection plate ("Decker") and can also be used for user-supplied filters. The lower wheel contains the three of the four standard ESI broadband filters, and a slot available for a "subfield" blocker. A subfield blocker is used for undersized user filters such as 2 x 2 inch or 4 x 4 inch filters, for which standard holders are available.
In each wheel there are five openings into which slit plates, filters or aperture plates can be loaded. Standard holders slide into these "slots". Each holder can have one or more "positions." For example the slitmask containing the echelle slits has nine positions (although only a single mask): the size echelle slits, two pinholes, and a line of pinholes. Other holders are available that can hold three 2 x 2 inch filters, and still others that can hold two 4 x 4 inch filters. There are separate subfield blockers for these two types of user holders.
The wheel positions can be configured in a variety of ways. Generally, because the positions are named, it is not necessary to know the slot number and position that is in the beam. It is possible to configure them in a way that produces undesireable results!. Typical combinations are:
Table 2. Typical configurations for the triple wheel
| Mode | Wheel | ||
|---|---|---|---|
| Top | Middle | Lower | |
| Echelle | (a) | Decker | Clear_S |
| Low-D | (b) | Clear_S | Clear_S |
| Imaging | Clear_I | (c) | (c) |
(a) One of the echellette slits.(b) One of the long slits.(c) A filter in one wheel, Clear_I in the other.
When Echelle Mode is selected, the middle and lower wheels move to Decker and Clear_S respectively. When low-D mode is chosen, the middle and lower wheels automatically go to Clear_S.
With the GUI you can create and name "setups" to simplify nighttime observing (see §6.5). It is in general a good idea to use the facility setup files to choose echelle, low-D, or imaging mode. These will not only set all three wheels to the appropriate positions, but also focus the collimator properly and possibly window the CCD. Note that when we determine that a standard filter or observing mode has changed focus, the setup scripts are the first things to get changed. They are hence more up-to-date than this manual, for example. You can create your own setups for variations such as binning the CCD by editing the copies of the facility setup files in your setups/ subdirectory on kanaha.