II. Observations and Data Reduction

a) Equipment

All observations presented in this paper were made with an InSb detector system for which further details are given in Appendix B. The filters used are listed in Table 1, where the effective wavelengths and bandwidths shown were determined from filter scans provided by the manufacturer. The [2.36 µm] - [2.19 µm] color is used to measure the strength of the first overtone absorption bands of CO (bandhead at 2.29 µm). Similarly, the [2.01 µm] - [2.19 µm] color measures the strength of the H₂O absorption feature centered at 1.9 µm.

Table 1. Filters

Name	0(µ)*	(µ)
J	1.24	0.28
H	1.65	0.30
K	2.22	0.41
H2O	2.01	0.07
Continuum	2.19	0.14
CO	2.36	0.08
* Referred to 77° K.

In the course of this work three silicon field lenses optimized for telescope focal ratios of 7.5, 9.6, and 13.5 were employed. An important feature of the InSb dewar was the existence of an external focus mechanism which allowed focusing of the very steep field optics (f / ~ 1 in the most extreme case) with the dewar cold directly at the telescope. A typical example of the relatively flat beam profiles obtained is shown in Figure 1. (Experience proved that a focus change of only ~ 0.01 inch could produce significant degradation of the profiles.)

Figure 1. Typical beam profile scans of a star across the aperture are shown for the filters used in this study.

The beam size used was defined by an aperture wheel in the focal plane. The field lens, aperture wheel, filters, and detector were all cooled to pumped nitrogen temperatures (T ~ 60 K). The photometer employed had an offset guider and star-sky chopper consisting of a rotating sector wheel mirror near the focal plane. The telescopes, field lenses, chopper throws, and apertures used, along with the dates of the observations, are listed in Table 2. Chopping was always in the north-south direction. The physical aperture sizes employed were 4.0, 3.0, 2.0, 1.5, and 1.0 mm. However, due to spherical aberration of the plano-convex field lenses, the effective aperture sizes were typically 3-6% smaller. Only the effective aperture sizes, determined from the half power points of stellar scans (cf. Figure 1), are listed in Table 2.

b) Galaxy Selection

The galaxies observed in this study were selected so as to fulfill one or more of the following criteria: 1) They were bright and could thus be observed in a reasonable amount of integration time - the sensitivity of the InSb detector being such that an object of 9th magnitude at K observed with a 60 inch telescope required typically one hour of integration time to achieve a statistical accuracy of ~ 0.02 mag with the CO filter. 2) They would span a range of morphological type extending from early spiral through Magellanic irregular. 3) They would include objects for which broad band UBV photometry, and/or narrow band optical line indices, and/or detailed synthesis models were available from the literature.

The galaxies selected are listed in column 1 of Table 3. The morphological types, on the modified Hubble system of de Vaucouleurs, are also listed in column l, and were obtained from the Second Reference Catalogue of Bright Galaxies (de Vaucouleurs, de Vaucouleurs, and Corwin 1976, hereafter RC2), with but two exceptions: The type for NGC 7316 (not classified in the RC2) is adopted from Huchra (1977a), and the type for NGC 5253 is taken from the original Reference Catalogue of Bright Galaxies (de Vaucouleurs and de Vaucouleurs 1964, hereafter RC1), where the galaxy is classified as Im, rather than from the RC2, where it is classified as I0. The former classification is considerably more consistent with this galaxy's broad-band colors.