Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

11.4 Specify Instrument Usage Particulars

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11.4.1 Gather essential target information

Get target coordinates and fluxes

Depending on the type of source, you should be able to obtain target coordinates, magnitudes, and fluxes from on-line databases. For COS, target coordinates should be accurate to one arcsec or better. If that is not possible, you may wish to consider acquiring a nearby object with well-determined coordinates and then offsetting to your target.

Ideally, you want to base your exposure estimates on measured UV fluxes at or near the wavelengths of interest. Much of the time, however, you will need to make an estimate based on much less complete information. For much of the sky, observations from the Galex mission provide accurate UV fluxes for almost any object bright enough to observe with COS. In other areas, rougher estimates must be made by comparing the source to an analogous object for which better data exist.

You will also need at least rough estimates of line fluxes and the breadth of lines if there are emission lines in your object's spectrum. This is so you can check to ensure local rate counts will not be excessive.

Are there neighboring objects?

Are there other objects near your targets? First, you want to avoid having more than one source within the COS 2.5 arcsec aperture, otherwise the recorded spectrum will be a blend. Second, other objects that lie within the COS acquisition radius will have to be checked to ensure they are not too bright. Galex data work for much of the sky, but in other areas the available information is much rougher.

Within APT, the Aladin tool allows you to display the Digital Sky Survey in the vicinity of a target and to overplot Galex sources if they are available.

11.4.2 Assess target acquisition strategies

Acquisition strategy is not ordinarily a concern in Phase I, but you may wish to check that an ordinary acquisition will work for your targets because sophisticated acquisition strategies will use some time in the first orbit that would otherwise be available to use for the spectrum. Some considerations include:

Check for nearby objects

As noted above, other UV-bright objects near your source could cause confusion during the acquisition, so extra care needs to be taken in crowded fields.

Check target brightness

Some targets may be permissible to observe with COS to obtain a spectrum because the light is dispersed, but may be too bright for a safe acquisition. The ETC provides a means of checking this.

It is very unlikely that a source could be too faint to acquire if a spectrum can be obtained of it. Again, the ETC will provide guidance.

Estimate acquisition times

Ordinarily, a COS acquisition uses several minutes at the beginning of the first orbit of a visit; see Chapter 7, "Target Acquisitions" on page 83 and Chapter 9, "Overheads and Orbit Usage Determination" on page 111. Special acquisitions will take longer, and you may wish to consult with a COS Instrument Scientist.

11.4.3 Determine the science exposure needs

Is the target flux safe?

The COS ETC should warn you if a source will produce a count rate too high for COS. If you expect emission lines be sure to check that at their peaks there is no violation of the COS local count rate maximum.

TIME-TAG or ACCUM?

We strongly recommend use of TIME-TAG mode with the default parameters as a means of ensuring a well-calibrated, high-quality spectrum. However, some sources produce counts at too high a rate for TIME-TAG mode, in which case ACCUM should be used.

Are there special needs?

Parallels? Variable objects? Observing at airglow wavelengths?

How many grating settings?

In low-resolution mode, a single exposure should suffice to record all the useful spectrum that can be obtained, but in medium-resolution mode the bandpass recorded can be limited, especially in the near-UV.

Are predicted count rates safe?

See the count rate limits in "Safety First: Bright Object Protection" on page 84.