Two filter wheels, each with 10 positions are available inside the cryostat. An extensive set of filters is installed currently, but there is room for more in the current configuration. Changing filters requires the cryostat to be opened up, so ANYONE WHO WANTS TO USE A FILTER NOT LISTED IN TABLE 4 MUST GIVE SUFFICIENT NOTICE FOR THE FILTER TO BE OBTAINED AND INSTALLED (6 Months).
In general, broad band filters are placed in the front wheel, while pupil apertures and any narrow band filters that require the broad band filters as blockers are installed in the second wheel. The Filters and apertures currently installed in WHIRCAM are listed in Table 4.
The transmission profiles for the main filters are plotted in Figure 4.
The Ks filter ( ) is mounted with a PK-50 long wavelength blocker, which unfortunately also cuts transmission by a further 20%. Ks is a relatively new filter from Barr Optical, without a fully characterised transformation to K. However, as Pahre & Mould (1994 ApJ 433, 567) point out, its bandpass is intermediate between K and K' (2.10 0.18 m), and rough corrections of Ks to K can be obtained by taking half those for K' to K. This transformation is well defined by Wainscoat & Cowie (1992 AJ 103, 332):
, for 0 < H - K < 0.4
(For stars redder than H-K = 0.4, the relation flattens off to mag.)
The K and L' filters are tilted by about 5° to measure any ghosting effects, but no serious ghosting is apparent. This means there may be a slight shift in an object's position when you change to/from these filters.
A fixed 17 mm diameter stop is fixed at the pupil (cold stop) near the filter wheels. This is slightly oversized with respect to the pupil image to ensure that all the light from the telescope aperture is used at J and H. The open apertures produce a restricted cold stop (this is not strictly correct as the rear filter wheel is a few mm from the true cold stop, but is close enough to be effective), which should block most of the thermal emission from the perimeter of the secondary baffle, and should be automatically selected by WHIRCAM only with filters longward of 2 m, as the penalty for reduced thermal emission is slightly higher vignetting. The different sizes correspond to the different magnifications used with Martini (see §12). For direct imaging the calculated telescope pupil diameter is 16.35 mm. The apertures with spiders are designed to also block thermal emission from the central obstruction.
Each of the filter options available in the CONFIG files will automatically load the appropriate combination of wheel 1 and 2 options. Thus selecting Ks will move wheel 1 to position 5 (the filter), and wheel 2 to position 3 (the 16 mm aperture to block thermal emission). Note that the CONFIG filters under OTHERS (ie J2, H2, K2 etc) are mostly for MARTINI. In particular, don't confuse H2 (which is the H filter plus a 16 mm aperture) with the H filter, which is the narrow band filter centred on the v=1-0 S(1) transition, called s1 in the CONFIG file. The 2 m narrow band filters are used with a broad band filter to block out of band leaks. DARKs are taken with a blank + Br to block both reflected and thermal emission. The combinations associated with each filter definition are defined in UKIRT_SCT:IRCAM.SCT, and while they must be ammended if a filter is changed (along with the UKIRT_CONFIG:WHIRCAM.DATA file), this should only be done by your SA or one of the telescope software engineers!
Figure 4: Filter transmission profiles.