b) Photographic Procedures
Fifty of the galaxies were observed with the 1.2 m Schmidt telescope at Palomar Observatory. The remaining four galaxies were observed at the Cerro Tololo Inter-American Observatory: NGC 628, 7424, and 7793 using the 4 m telescope, and NGC 253 using the 0.6/0.9 m Curtis-Schmidt telescope. The plate scales are ~ 67" mm-1 for the Palomar Schmidt, 19" mm-1 for the CTIO 4 m, and 96" mm-1 for the CTIO Schmidt.
The near-infrared survey was made with IV-N emulsions. These plates were
hypersensitized by soaking for
4m in a 2% solution of AgNO3, followed by
30s in a one-third strength photo-fib solution. Techniques
are discussed by
Jenkins and Farnell (1976),
Hoessel (1978),
Schoening (1978),
and Pinto (1979).
The plates were
exposed behind a Wratten 88A filter which transmits wavelengths longer
than 7300 Å, so that H
light is
avoided. This plate-filter combination provides an effective wavelength
of 8250 Å for a flat spectrum. Exposure
times were 2h on the Palomar Schmidt (1h on the
Curtis-Schmidt and 4 m), and the plates are sky limited.
Seeing was typically 2" during the exposures, as estimated visually at
the telescopes.
Blue plates were also taken of each galaxy to provide a comparison with the near-infrared plates. For this purpose, 103a-0 emulsions were baked for 2h45m in forming gas at 65° C, and exposed behind a GG385 filter for l2m at the Palomar Schmidt (25m on the Curtis-Schmidt and 30m on the 4 m) to obtain sky-limited plates. The gray spectrum effective wavelength for this plate-filter combination is 4360 Å. All plates were developed for 5m in D-19 in a rotating rocker tray.
Blue or visual photographs of many of these galaxies appear elsewhere at higher resolution (e.g., Sandage 1961). The Schmidt B photographs are included in this atlas in order to compare the B and I images from the same telescope. Gross differences between the structures seen in the two passbands then may be interpreted as a wavelength-dependent effect rather than a manifestation of different plate scales and resolutions. The granularity of the IV-N emulsions is a factor of 2 smaller than the 103a-0 emulsions, so finer detail is discernible on the I plates. While a blue filter used with IIIa-J (or even IV-N) plates would have provided a better comparison of the features in the B and I passbands, the increase by a factor of 2 in total telescope time needed would have been unwarranted for the comparatively small benefits.
Most of the I plates were exposed with considerable moonlight present; typically the galaxies were observed when they were more than 30° away from the moon at its 0.5 to 1.0 phase. The B plates were exposed with a dark or nearly dark sky. The sky densities of the plates are, on the average, 0.7 to 1.5 above plate fog on the I plates, and 0.3 to 0.6 above plate fog on the B plates. The sky backgrounds of the B and I plates have been matched in the reproductions in Figures 1 through 19 in order to simplify comparisons.
Calibration wedges were placed on an unexposed end of each plate with a
Gunn sensitometer using the
appropriate filter immediately after each galaxy exposure. The exposure
times for the wedges were within 30% of
the galaxy exposure times. The characteristic curves of several
representative plates were measured by scanning
the calibration wedges with a densitometer. The slope,
, on a density
versus log (intensity) plot was determined
to be about 2.5 times larger for the I plates than for the
B plates, so that the I plates have a higher contrast.
The photographs in Figures 1 through 19 were printed with polycontrast paper, and the inherent contrasts on the plates have been reproduced as accurately as possible. An inspection of the plates confirms that the photographs were not reproduced at different contrasts. Because of the more limited dynamical range of emulsions on paper compared with plates, the faintest and brightest features cannot be reproduced in a single photograph. The central regions of many or most of the photographs are saturated, since it was more desirable to show the fainter features than the brighter ones. The original plates reveal more details about the innermost regions, which do not affect the conclusions about the blue and near-infrared structure described in this paper.