Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

8.1 Designing a COS Observing Proposal

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Here are the steps to follow when designing a COS Phase I observing proposal. The process is likely to be iterative.

• Identify your science requirements and select the basic COS configuration to satisfy those requirements.
• Estimate exposure time to achieve required signal-to-noise ratio and check the feasibility, including count-rate, data volume, counter rollover, and bright-object limits.
• Identify any additional non-science (target acquisition, peakup, and calibration) exposures required.
• Determine the total number of orbits required, taking into account all overheads.

This Handbook provides the information needed to estimate exposures and timing, but proposers are urged to use APT to achieve the most accurate results.

8.1.1 Identify the Science Requirements and COS Configuration

Identify the science you wish to perform with COS. Some basic choices you will need to make are:

• FUV or NUV channel;
• TIME-TAG or ACCUM, considering time resolution and background minimization;
• Spectral resolution and spectral coverage;
• The need for multiple grating settings (especially for NUV);
• The need for imaging;
• Signal-to-noise requirements;
• Wavelength and photometric accuracy required; and
• Safety of the object to be observed (i.e., that it does not produce excessive count rates).

Spectroscopy

For spectroscopic observations, the base configuration needed is detector (configuration = FUV or NUV), operating mode (TIME-TAG or ACCUM), aperture, grating (spectral element), central wavelength, and wavelength dither offset (FP-POS). See Chapter 5 for detailed information about these quantities.

Imaging

For imaging observations, the base configuration is NUV detector (configuration = COS/NUV), operating mode (TIME-TAG or ACCUM), aperture (PSA or BOA), and mirror choice (spectral element = MIRRORA or MIRRORB).

8.1.2 Use of Available-but-Unsupported Capabilities

There are no Available-but-Unsupported modes for COS.

8.1.3 Calculate Exposure Time and Assess Feasibility

You can determine the expected count rate and the recommended BUFFER-TIME value (for TIME-TAG mode) for your targets with the COS ETC. Determine acquisition exposure times with the COS Target Acquisition ETC. Count rates and exposure times from the ETC will help you to determine the feasibility of using TIME-TAG and NUV ACQ/IMAGE. Determine the number of exposures needed to cover your desired spectral range.

Once you've selected your basic COS configuration, the next steps are:

• Estimate the exposure time needed to achieve your required signal-to-noise ratio, given your source brightness. (You can use the COS ETC for this.)
• Ensure that your observations do not exceed brightness (count rate) limits.
• For observations using ACCUM mode, ensure that for pixels of interest, your observations do not exceed the limit of 65,535 accumulated counts per pixel per exposure imposed by the COS 16 bit buffer.

To determine your exposure-time requirements, consult Chapter 10, where an explanation of how to calculate a signal-to-noise ratio and a description of the sky backgrounds is provided. To assess whether you are close to the brightness, signal-to-noise, and dynamic-range limitations of the detectors, refer to Section 8.2 below.

8.1.4 Identify the Need for Additional Exposures

Having identified a sequence of science exposures, you next need to determine what additional exposures you may require to achieve your scientific goals. Specifically:

• If early acquisition images in support of bright object checking are necessary, they must be included in the Phase 1 orbit request.
• If the success of your science program requires calibration to a higher level of precision than is provided by routine STScI calibration data, and if you are able to justify your ability to reach this level of calibration accuracy yourself, you will need to include the necessary calibration exposures in your program, including the orbits required for calibration in your total orbit request.

8.1.5 Estimating Data Volume

For TIME-TAG observations: each photon recorded requires 4 bytes. Each buffer dump nominally contains 2.35 × 10⁶ photons (~9 Mbytes). Data volume may be approximately estimated as: (exposure time / buffer-time) × 9 Mbytes. Observers are strongly urged to use TIME-TAG mode whenever possible.

For ACCUM observations: NUV ACCUM exposures require 2 Mbytes of on-board storage. FUV ACCUM exposures require 4 Mbytes per segment.

For acquisitions: NUV ACQ/IMAGE exposures require 4 Mbytes of on-board memory. All other acquisition types require insignificantly small amounts of storage.

If COS data are taken at the highest possible data rate for more than a few orbits or in the Continuous Viewing Zone (CVZ), it is possible to accumulate data faster than it can be transmitted to the ground. High data volume proposals will be reviewed and, on some occasions, users may be requested to break the proposal into multiple visits.

8.1.6 Determine Total Orbit Request

In this step, you place all of your exposures (science and non-science, alike) into orbits, including tabulated overheads, and determine the total number of orbits required. Refer to Chapter 9 when performing this step.

At this point, if you are satisfied with the total number of orbits required, you're done! If you are not satisfied with the total number of orbits required, you can adjust your instrument configuration, lessen your acquisition requirements, or change your target signal-to-noise or wavelength requirements, until you find a combination which allows you to achieve your science goals.