Here we select the 1309 Å setting. We enter the grating and cenwave into the spectroscopic ETC, select the Primary Science Aperture (PSA), select "Exposure time needed to obtain a S/N ratio of 10.0," enter the specified wavelength of 1320 Å, and select "Point Source" as the source type. For the spectrum distribution, choose a flat continuum in Fλ
. Make sure the reddening, E
(B – V
), is set to 0. Normalize the target to 10–15
at 1320 Å. The zodiacal light, earthshine, and airglow were not specified, so we choose average values.
When this case is computed with the ETC, we find the required time is 15,884 s; the total count rate is 43 counts s−1
in detector Segment A, well below the safety limit; the count rate in the brightest pixel has 1.5 ×
, also well within the safe range (but see below); and the buffer time indicated by the ETC is 8,307 seconds (COS.sp.1033055). The results for Segment B can be ignored, since it will be off.
What if somewhat higher S/N
were desired and one were willing to devote 10 HST
orbits to the observation? Assuming that each orbit allows 50 minutes of observing time (ignoring the acquisition time here), we find that in 30,000 seconds we will get S/N
= 13.7 per resel. Note that (30,000/15,884)1/2
= (13.7/10.0). That is, the S/N
ratio scales as t1/2
, as stated in Section 7.3
If a low-resolution observation is acceptable, then one could switch to the G140L grating. With a grating setting of 1105 Å and S/N
= 10 per resel, we find the required exposure time is 2732 s, considerably less than the medium-resolution case required. Since this cenwave has always been offered in single-segment mode, the ETC returns the correct buffer time for Segment A only.
For other science goals, the user may wish to retain the use of both segments by switching to a different cenwave, such as 1222. For this example, we use the same inputs as before, except the cenwave is set to 1222. When this case is computed with the ETC, we find the required time is 16,014 s; the total count rates are 63 and 38 counts s−1
in detector Segments A and B, respectively, well below the safety limit; the count rate in the brightest pixel has 0.008 counts s−1
, also well within the safe range; and the buffer time indicated by the ETC is 23,434 seconds (COS.sp.1033327).
Suppose this star is reddened, with E
(B − V
) = 0.2. We select the Milky Way Diffuse (RV = 3.1)
extinction law, which is shown in Figure 7.3
. We must now decide if this extinction is to be applied before or after the normalization. Since the star has a measured magnitude, we want to apply the reddening before normalization. Otherwise, the extinction would change the V
magnitude of the stellar model. Making this selection, we find that S/N
= 15 can be obtained in 2762 s (COS.sp.1033334). The ETC returns a BUFFER-TIME
of 3043 s. To be conservative, we scale it by 2/3 to get 2028 s.
We want to observe a solar-type star with a narrow emission line. Consider the Si II λ
1810 feature with the following parameters: FWHM = 30 km s−1
or 0.18 Å at 1810 Å, and integrated emission line flux of 1 ×
. The measured magnitude of the star is V
= 12 mag. The desired exposure time is 1000 s.
In the ETC we select a Kurucz G2V star and an NUV grating, G185M, set to a central wavelength of 1817 Å. We request an exposure time of 1000 s and specify that the S/N
be evaluated at 1810 Å. We add an emission line with the line center at 1810 Å, FWHM = 0.18 Å, and an integrated flux of 10−14
. We specify the normalization as Johnson V
= 12 mag. We set the zodiacal light, air glow, and earthshine to be average
The ETC returns S/N
= 16.3 per resel (COS.sp.1033356). The local and global count rates are within safe limits. The recommended buffer time is 2730 s. This BUFFER-TIME
exceeds the exposure time of 1000 s, so, following the procedure outlined in Section 5.4
we set the BUFFER-TIME
to 2/3 of the BUFFER-TIME
value returned by the ETC, which is 1820 s.
An important science goal for the design of COS was to obtain moderate S/N spectra of faint QSOs in the FUV. In the ETC, use the FOS-based QSO spectrum (in the Non-Stellar Objects menu) and choose G130M at 1291 Å, S/N = 20, and a continuum flux of 10−15
at 1320 Å. (Note that only FP-POS 3 and 4 are available at this cenwave, but these are sufficient to attain the S/N goal; see Table 5.6
.) The indicated exposure time is 63,154 s, or about 21.1 orbits (COS.sp.1033359). The source count rate is 0.001 count/s, with a background rate of 6.230 ×
count/s, 16 times less than that of the source. The background is completely dominated by the dark current of the detector. The count rate over the entire detector is 316 count/s, well below any safety limits, and the maximum BUFFER-TIME
is 7459 s. Scaling by 2/3 yields 4972 s for the BUFFER-TIME