19
via the development and digitization processes. Uncertainty in the absolute cali-
bration can lead to uncertainties of up to 10
,
15 in the FUV ux. A detailed
discussion of the reduction of the UIT data to ux-calibrated arrays is given by
Stecher
et
al.
1992, 1997.
The FUV background level
25 mag arcsec
,2
for each of the images is
minimal. It is measured by taking the mean of 20, 20
20 pixel boxes in areas void
of any galaxy or stellar ux. The FUV background levels are 0.0
0
:
4, 0.53
0
:
1,
and 0.0
0
:
01 analog data units ADU's for the HoII, IC2574, and Sextans A
FUV images, respectively. The UIT has limited accuracy at the lowest light levels.
The errors in the background levels re ect this intrinsic uncertainty.
A wide, vertical stripe appeared in the center of a sizable percentage of the
UIT ASTRO-2 images, including the IC2574 image. The aw appeared on the
lm and was not a result of the digitization process; its origin is unknown. This
image aw has the e ect of raising the background level in an
216 pixel wide
stripe in the center of the IC2574 image, which contains part of the galaxy by an
average of 1.52
0.33 ADU's. To correct for the aw, the stripe is modeled in parts
of the image void of the galaxy. Since the stripe varies horizontally in intensity,
the best result is obtained by dividing the stripe in three parts, subtracting the
mean value in each region.
2.2
U,
B,
V,
and
R
Observ
ations
CCD observations of IC2574 and HoII were obtained in the Johnson U and
B bands, and the Kron-Cousins R and H bands with San Diego State Univer-
sity's 1.0 m telescope at the Mount Laguna Observatory MLO. Observations
in the same bandpasses were obtained for Sextans A at The Carnegie Institute's
Las Campanas Observatory LCO using the 1.0 m Swope Telescope. Table 2.2
