12.7 Considerations for Observing Planetary Targets

STIS' FUV solar-blind and NUV solar-insensitive MAMA detectors make it particularly well suited to slitted spectroscopic and imaging planetary observations. In addition, the 52X2 long slit (2 arcseconds wide in the dispersion direction) is particularly well suited to the "slitless" UV spectroscopic study of small planetary bodies (using a slit limits the background continuum contribution; see Section 6.5.2). STIS has been used successfully to perform long-slit and echelle spectroscopy and UV imaging of a number of solar system objects including comets, planetary satellites, and planets.

Planetary observers may wish to use the sequence PATTERN=STIS-PERP-TO-SLIT (see Section 11.3) to map out the surface of a planet by taking a series of long-slit observations, each one stepped by the slit width perpendicular to the slit's long dimension, relative to the previous exposure. Advice on performing target acquisitions for solar system targets is provided in Chapter 8.

12.7.1 Slit Orientation

Planetary observers requiring specific long-slit orientations will want to be aware of the tight scheduling constraints of specific orientations for observations of targets in the ecliptic plane (see discussion in the Phase II Proposal Instructions).

For example, say an observer wanted to orient the STIS long (52 arcsecond) slit in the north-south direction on Jupiter. For a Jupiter north-pole position angle of ~20° from celestial north, an ORIENT constraint of ~65–70° or ~270–275° would be required. These values take into account the ~45° rotation between the ORIENT frame and the STIS slit. See Section 11.4 for a more extended discussion about fixing the orientation of the detector and slit on the sky. The nominal roll angle (orientation) for Jupiter varies from ~250° before opposition to ~70° after opposition, and it goes through the intervening 180° range during a four-day period centered on opposition. Deviations from nominal roll are allowed as follows:

• 5° when the sun-target angle is 50–90°.
• Up to 15–30° when the sun-target angle is 90–178°.
• Unlimited when the sun-target angle is 178–180°.

Thus orientation of the long slit in a north–south direction on Jupiter is possible only during a single four-day period near Jovian opposition, since no allowable deviation from nominal roll is large enough at any other time to permit a north–south orientation of the slit on the planet. Although the situation for Saturn is not nearly as extreme, observers should be aware of the tight scheduling constraints on observations requiring specific slit orientations. Additional information on the placement of a long slit on extended sources can be found in Section 12.8.

Observers should also be aware that other operational constraints, such as the lack of suitably positioned guide stars, may place additional restrictions beyond those listed above on the range of orientations that are available for a particular observation.