Документ взят из кэша поисковой машины. Адрес оригинального документа : http://www.stsci.edu/ftp/instrument_news/FOC/Foc_handbook/v5/handbook_v5_ch09.ps Дата изменения: Fri Dec 9 19:44:33 1994 Дата индексирования: Sun Dec 23 20:15:21 2007 Кодировка: Поисковые слова: zodiacal light

FOC Instrument Handbook Version 5.0 85
9.0 LIMITING MAGNITUDES
FOCSIM can be used to predict the limiting magnitude of any observing configuration.
An example of this type of calculation is shown in Figures 41 and 43 for scenarios matching
the observed average in­flight conditions. Studies of calibration images have been used to
determine an average zodiacal light intensity of S10 = 191 units and a detector background
for the new F/96 relay of B p = 6 \Theta 10 \Gamma4 and for the new F/48 relay B p = 2 \Theta 10 \Gamma3 counts
s \Gamma1 pixel \Gamma1 . Figure 41 shows the predicted exposure time in seconds needed to reach a S/N
= 10 for a specified visual magnitude of a B5V star through the F342W (U) filter for the
two cameras. With average in­flight conditions, we should expect to detect a B5V star of
V = 28 with the U filter in about 10 hours of exposure time with the new F/96 relay. The
limiting magnitude is V = 27:5 if a S/N=5 in a 5 hour exposure is deemed sufficient. Figure
42 illustrates the results of calculations using extended sources. For this case, the specific
intensity of the source is expressed in terms of visual magnitudes per arcseconds squared and
the spectrum is assumed to have the shape of a B5V star. The spatial resolution in this case
is chosen to be 0.112 arcseconds, which corresponds to binned regions of 4 and 8 pixels on a
side for the new F/48 or new F/96 relays respectively. One should be able to detect a source
of intensity = 23:5V magnitudes per arcseconds squared at S/N = 10 and 0.112 arcsecond
resolution in 10 hours of exposure with the U filter under the average in­flight conditions
described earlier.

86 FOC Instrument Handbook Version 5.0
22 24 26 28
1
10
100
1000
Figure 41. Exposure time required to reach a S/N = 10 on a B5V star with the U filter in
an average observing condition with B p = 6 \Theta 10 \Gamma4 counts sec \Gamma1 pixel \Gamma1 and a
zodiacal light intensity of 191 S10.
18 20 22 24
10
100
1000
Figure 42. Exposure time required to reach a S/N = 10 on a B5V spectrum extended source
with 0.1 arcsecond resolution with the U filter in the observing condition listed
in Figure 41.