2.2 Essential questions for preparing COS observations

Each of HST’s science instruments has its own unique qualities, and COS particularly does for reasons closely related to its design.  Here are the key factors that the COS team looks at when reviewing accepted programs to ensure science goals are achieved, enhancing the archival value of the observations, and extending the useful lifetime of COS.

Is COS well-suited to the stated science goals?

Considerations include:

  • Signal-to-noise: COS is limited to a maximum S/N of 40-50 (Section 5.8) due to fixed-pattern noise, so STIS would be better in some cases.
  • Target brightness: STIS is better suited to very bright targets, but COS does include some features such as the Bright Object Aperture (Section 3.1).
  • Wavelength coverage and resolution needed: see Section 5.1.2. COS is unique in being able to record spectra below 1150 A.
  • Preferring STIS when it is adequate: this is done to conserve COS’ unique capabilities as long as possible.
  • Extended sources: COS’ small 2.5-arcsec apertures can limit total flux, and STIS includes long slit apertures.

How good are the relevant available target data?

Accurately estimating source brightness and variability are critical for assessing the safety of observations and for calculating the BUFFER-TIME for TIME-TAG observations.  For example, many sources have existing ultraviolet data in the Hubble Legacy Archive.

Good-quality astrometry also helps to ensure a good acquisition and to determine ORIENT ranges if a bright, nearby source must be kept out of COS’ apertures.

Is the acquisition strategy optimal?

A specific acquisition exposure is almost always required.  To get reliable flux and wavelength calibrations an ACQ exposure is needed to center the source in COS’ small (2.5 arcsec) apertures. There are several ways to acquire a target (see Chapter 8: Target Acquisitions).

Is the exposure safe for COS? (Ch. 10)

Most acquisitions are done in the NUV in imaging mode, for efficiency reasons.  This concentration of the source’s light can risk the very sensitive photon-counting detectors on COS. Spectroscopy can also raise brightness concerns. In either case, the COS Exposure Time Calculator (see Chapter 7) will warn of potential over-bright conditions, and these need to be addressed.  As noted, observers should provide information on the quality of the data used to estimate count rates, including sources of information, and estimates of (potential) variability.

COS can be used to observe objects with brightnesses that are not easily predicted, but only under carefully controlled circumstances (see Chapter 10).

Do the observations use TIME-TAG when possible, and not ACCUM?

Nearly all COS exposures are taken in TIME-TAG mode, as that leads to better data quality and flexibility.  ACCUM mode may be used for objects too bright for TIME-TAG, but in those cases STIS may be a better choice (see 5.2 TIME-TAG vs. ACCUM Mode).

Is the BUFFER-TIME calculated properly?

In TIME-TAG mode, individual photon detections are recorded in a buffer of finite capacity.  Calculating the time in which the buffer will fill is important to avoid losing data while minimizing buffer read times.  Reliable estimates of count rates help ensure BUFFER-TIME is calculated accurately (see 5.4 Estimating the BUFFER-TIME in TIME-TAG Mode).

Are the chosen FP-POS positions correct and reasonable?

In the FUV, the inherent nature of the COS detectors limits the top S/N to ~40-50.  A key factor is fixed pattern noise (see section 5.8 Fixed-Pattern Noise), and so to improve data quality observers are ordinarily required to use all the FP-POS positions available for a given grating+wavelength setting.  Deviations must be justified.

Are scheduling factors taken into account?

Some visits are more easily scheduled than others.  Use of the ORIENT Special Requirement constrains when visits can occur, even if sometimes ORIENT must be used to avoid having a nearby bright star fall in a COS aperture (See Target Orientation Visit-Level Special Requirements in the Phase II Proposal Instructions).

The length of a visit matters, with short 1- and 2- orbit visits especially easy to schedule, and 4-orbit visits particularly difficult (see Section 5.2 TIME-TAG vs. ACCUM Mode).

Is it advantageous to use a non-default Lifetime Position?

Other factors may need to be considered in constructing a program.  The overall useful life of COS is being extended by commissioning new Lifetime Positions (LPs; see 5.12 FUV Detector Lifetime Positions) as older ones get depleted.  Each grating+wavelength setting has a default LP, but in some circumstances it may be advantageous to use a different LP.

All of these topics are discussed in depth in the referenced sections of this Handbook.