Much was learned from the early near-infrared observations that could be applied to optical observations, given accurate data gathered over a sufficiently broad wavelength range. CCDs provided just that opportunity, bridging the development gap slowly being filled by the relatively small-area IR arrays. Given a wavelength sensitivity running from the B band (0.45 µm) to the I band (0.7 µm), CCDs afford the opportunity to gather seeing-limited, panoramic, digital data, which can subsequently be reduced using local sky subtraction and point-spread-function fitting techniques, to derive accurate magnitudes and colors. Crowding and confusion errors can be dealt with at the one square-arcsecond level. Use of CCDs and near-IR arrays decrease the areal confusion by about an order of magnitude over the near-infrared apertures.
CCDs also offer the advantage of a large wavelength coverage, thereby allowing an explicit determination of the reddening from the optical data itself. In this case, it is not necessary to rely purely on foreground estimates and/or on assuming that additional reddening is of negligible importance. Given a knowledge of the interstellar extinction law as a function of wavelength it is possible to fit all of the data simultaneously. Freedman (1988b) introduced this new approach to determining true moduli for extragalactic Cepheids using multiwavelength data, applying it first to single-epoch observations of Cepheids in IC 1613, and later refining it and expanding its application to data obtained for M31, M33, and NGC 300 as cited below. For a detailed discussion of the technique and its implementation the interested reader is referred to those papers. Briefly stated, one determines differential apparent moduli, scaled against the corresponding LMC PL relations. By assuming that all of the difference as a function of inverse wavelength can be attributed to selective absorption, fitting an interstellar extinction law to the data simultaneously estimates the total (foreground plus internal) absorption and the true distance modulus, relative to the LMC.
In the following, we briefly review the systematics of the PL relation as observed at wavelengths ranging from the blue to the near-infrared. To do this, we concentrate on self-consistent data sets assembled for this purpose by Madore & Freedman (1991) and plotted in Figure 5. The stars included in this compilation are Cepheids in the LMC and SMC for which there are time-averaged mean magnitudes at all seven wavelengths (excluding R for the LMC sample which largely lacks this bandpass in published Cepheid observations). Note that the slope and the dispersion of the PL relation change systematically as a function of the effective wavelength of the filter bandpass. These impressions are quantified in the equations presented at the end of Section 8. In any case, as the longer-wavelength data are considered, it is clear that both quantities (slope and dispersion) have already begun to converge on an asymptotic value (set by the period radius relation, which because of its geometrical nature is largely wavelength independent). From this point of view it makes no practical sense to observe Cepheids at wavelengths much beyond 2 µm if the aim is to decrease the dispersion in the observed PL relation. Fortunately, this regime is still accessible from the ground and is not far into the thermal IR where background effects become extremely large.
For comparison, we show the rate of fall-off in the monochromatic extinction as a function of wavelength scaled to the blue extinction. As is well appreciated, the extinction does continue to decrease with increasing wavelength, making it sensible to extend the observations as far into the infrared as is practical. Of course, a decrease in extinction by a factor of over 5 is realized at K in comparison to B, so for most purposes this too is a reasonable wavelength at which to stop the effort.
The impact of panoramic linear detectors, such as the CCD, on the study of extragalactic Cepheids has been significant. It is now possible to obtain from the ground, high-quality light curves of Cepheids in galaxies 2 Mpc away, ranging from blue and visual wavelengths, as well as reaching out to nearly one micron with CCD detectors, and then extending to 2.2 µm with the newly available HgCdTe and InSb infrared areal detectors. With HST, the distances reached have now exceeded 20 Mpc, and NICMOS is being applied to a number of these galaxies in follow-up studies in the near infrared.