WFPC2 Instrument Handbook for Cycle 11

Observing Faint Targets

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For broad band filters the sky background will limit the detection of faint targets. For example, an 8-orbit observation in F555W gives a ~5 detection limit at Johnson V=28.6 for an average sky level of 23 mag arcsec^-2 in V. Note that the sky background is a strong function of position, especially for targets near the ecliptic; the sky level can vary from V=23.3 mag arcsec^-2 at the ecliptic pole to about V=20.9 mag arcsec^-2 on the ecliptic near the solar avoidance limit. (See Table 6.3 for sky level as function of ecliptic coordinates.)

If these higher sky levels would severely impact the science data, observers should consider specifying the special requirement LOW-SKY on the Phase II proposal. This parameter forces the observation to be made when the sky background is within 30% of the minimum value for the target. Note, however, that this will also reduce the number of HST calendar windows available to the observation, and so could result in scheduling delays or may even make the observation infeasible if there are other constraints such as ORIENTs. A minor decrease in the per-orbit visibility period also results from LOW-SKY, but for background limited programs this is a minor price to pay for the guarantee of a much lower background. In summary, LOW-SKY will reduce the sky background, but should only be used if the science goals require it.

Note that LOW-SKY cannot be used for CVZ targets, as they imply mutually exclusive pointing constraints.

 

Scattering of bright Earth light in the OTA can produce non-uniformities in the background which may hamper analysis of faint target images. Most often these take the form of diagonal bars of suppressed background light in several of the CCDs. These effects tend to occur for broad band filters when the OTA axis is about 25° from the bright Earth. This effect is most often seen in observations of targets in the CVZ (continuous viewing zone), since the Earth limb is never very far from the OTA axis when observing in the CVZ. Figure 7.1 shows a typical case. LOW-SKY will eliminate this effect for non-CVZ targets, as it places the OTA axis more than 40° from the bright Earth. Alternatively, one can place the target away from the CCD center to avoid these artifacts.

Figure 7.1: Example of Scattered Earth Light. Scattered light contributes ~100 e^- of background throughout this image. The camera spiders block some of this scattered light along CCD diagonals, hence forming "X" patterns and bars where the background is reduced by ~40% in this image.
 

Another option for reducing the sky brightness, is the special requirement SHADOW, which forces the observation to be made when HST is in the Earth's shadow. This usually has a large negative impact on the observing efficiency, and is recommended only when observing far-UV emission lines (e.g. Ly and OI 1304е). Its primary goal is only to reduce geocoronal emission lines. Moreover, it does not attempt to minimize zodiacal emission, which dominates at visible wavelengths.