Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

8.4 A "Road Map" for Optimizing Observations

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An outline summarizing how to prepare and submit a Phase I proposal for HST time is provided at:

 http://apst.stsci.edu/apt/external/help/roadmap.html

If you have APT running, this web page will appear if you click "Roadmap" under "Help." Although the roadmap is detailed, it can be paraphrased and reduced to eight steps:

1. Learn about the tools to use and the rules governing HST proposals.
2. Prepare your proposal's first draft.
3. Choose the instruments and configurations you will use.
4. Check for potential problems.
5. Estimate your orbit needs.
6. Finish the proposal.
7. Edit all the needed information into APT and submit the proposal.
8. Talk to us so we can improve the process.

Most of these steps apply to any HST proposal and so are adequately described on the web page noted. Here we emphasize any items specific to COS.

8.4.1 Get the tools and rules

As we described in Chapter 1, there are two essential software tools you will need:

• APT, the Astronomer's Proposal Tool, and
• The COS Exposure Time Calculator (ETC).

For this first cycle of COS usage, there are no previously executed programs whose data you can examine, but the ETC includes a number of examples of many different kinds of celestial objects as reference points, or you can use an existing spectrum of your own or from the HST archive as a starting point.

The rules and policies that pertain to applying for HST time are described on the web and in the Call for Proposals and the HST Primer. In particular, the HST Primer contains a brief description of all HST's scientific instruments, which should provide what you need to decide which instruments to use. A template is needed for the text portions of the proposal and it may be found on the HST web pages.

We urge proposers to use APT in planning their observations, even for Phase I, for these reasons:

• APT includes detailed and accurate knowledge of an instrument's operation that can be difficult to describe. In particular, using APT will ensure that your estimates of the available exposure time in an orbit are accurate.
• Entering accurate and complete target information right at the start saves you from doing it later.
• A proposal with detailed determinations of the potential observations is a more credible one.

We also urge you to use the ETC to determine accurate exposure times for both acquisitions and science exposures.

8.4.2 Choose instrument configurations

Determine your science requirements

   

List your targets

You will probably want to start with more candidate targets than end up in the final proposal so that you can balance factors once you need how long the exposures will be.

   

Note your spectroscopic data requirements

What features at what wavelengths are needed for your program? What resolving power is needed? What COS gratings and settings are necessary to get those wavelengths? What level of signal-to-noise is needed for the science?

   

Are there other observing requirements?

Does a particular target need an unusual acquisition, perhaps because of nearby objects? Is the object variable and need to be observed at a particular time or phase?

   

Determine instrument configurations

The above information should suffice to create a list of your targets and the COS instrument configurations for each.

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 need to 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.

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

Use the COS acquisitions ETC to determine the exposure time needed, and then APT to get the full time required, including overheads. Special acquisitions will take longer, and you may wish to consult with a COS Instrument Scientist.

Step 4: 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 R2k mode a single exposure should suffice to record all the useful spectrum that can be obtained, but in R20k mode the bandpass recorded can be limited, especially in the near-UV.

   

Are predicted count rates safe?

See Section 7.2, "Safety First: Bright Object Protection," on page 84.