Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

10.6 Examples

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In this section we present a few examples of the way in which the COS ETCs may be used. They illustrate the information that is returned by the ETCs, and how they can be used to plan your observations.

10.6.1 A Flat-spectrum source

One often does not know that exact spectrum shape of the object to be observed, so the answer to a simple question is desired: How long will it take to achieve a given signal-to-noise ratio at a given wavelength if the flux at that wavelength is specified? The easiest way to determine this is to input a flat spectrum.

The question then is: how long will it take to achieve S/N=10 per resolution element at 1320 Å with a source flux of 1 CFU, using a medium resolution mode?

Only the G130M grating covers the desired wavelength at medium resolution, but several choices of central wavelength are available. We select a setting of 1309 Å. We enter these values into the spectroscopic ETC, select the Primary Science Aperture (PSA), select "Exposure time needed to obtain a S/N ratio of 10.0," and enter the specified wavelength of 1320 Å. For the spectrum distribution, choose a flat continuum in F. Make sure the reddening, E(B-V), is set to 0. Normalize the target to 1.0 × 10^-15. The Zodiacal light and Earthshine were not specified, so we choose average values.

When this case is submitted, we find the required time is 9917 sec; the total count rates are 39 and 104 counts sec^-1 in detector segments A and B, respectively, well below the safety limit; the count rate in the brightest pixel is 15 counts sec^-1, also well within the safe range; and the buffer time is 16,500 sec.

What if somewhat higher S/N were desired and one were willing to devote 5 HST orbits to the observation? Assuming each orbit allows 50 minutes of observing time (ignoring the acquisition time here), we find that in 15000 sec we will get S/N = 12.3 per resel. Note that (15000/9917)^1/2 = (12.3/10.0). That is, the S/N ratio scales as t^1/2, as stated in section 5.2xx.

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 1678 sec, considerably shorter than the medium resolution case required. Note that only segment A is used for the G140L observations.

These cases also illustrate that the Earthshine and Zodiacal light are completely negligible in the FUV unless the target flux is much lower than that considered here. This is also true of the airglow if the wavelength of interest is away from the airglow lines. Of course, the airglow cannot be ignored in terms of the total count rate of the detector, or the local count rate if the source contributes at the same wavelengths as the airglow lines.

10.6.2 An early-type star

We wish to observe an O5 star at medium spectral resolution at a of wavelength of 1650 Å. We know that the star has a magnitude of V = 16. How long will it take to obtain S/N = 30?

We select the G160M grating set to 1623 Å. We select a Kurucz O5 stellar model, and set the normalization to be Johnson V = 16. All other settings remain the same as in the previous example. We find that the required exposure time is 1510 sec.

Suppose this star is reddened, with E(B-V) = 0.2. We select the Average Galactic extinction law, which is shown in Figure 5.3xx. 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 = 30 can be obtained in 3544 sec.

10.6.3 A solar-type star with an emission line

We want to observe a solar-type star with a narrow emission line. Consider the Si ii 1810 Å line, with the following parameters: FWHM = 30 km sec^-1 or 0.18 Å at 1810 Å, and integrated emission line flux = 1 × 10^-14 erg cm^-2 sec^-1. The measured magnitude of the star is V = 12. The exposure time is 1000 sec.

In the ETC we select an NUV grating, G185M, set to 1817 Å. Select a 1000 sec exposure, with the S/N specified at 1810 Å. We add an emission line with the line center at 1810, FWHM=0.18, and integrated flux of 1 × 10^-14. We specify the normalization as Johnson V = 12. We set the Zodiacal and Earthshine to be average.

The ETC returns S/N = 16.9 per resel. The local and global count rates are within safe limits. The buffer time is 1752 seconds.