many photometric studies do not require absolute accuracy but can be accomplished in a differential sense with high precision. Unless the point-spread function is highly oversampled, photometry does require a camera without “gaps,” such as the masked alternate rows in the fine-tracking camera’s line-transfer CCD, in order to collect all the light from a source. The inclusion of an optical framing camera would circumvent this problem.
An excellent example of the necessary level of precision is seen in the field of stellar seismology. In these observations, astronomers search for radial and nonradial pulsational modes in stars by looking at the variation of the light with time. The amplitude of the variations and the relevant time scales are functions of the properties of the star. The Sun, for example, has oscillations with periods between 3 minutes and 27 days, but relatively small amplitudes.
Observations of much more distant solar-type stars will require rapid sampling at high precision, an approach that probably will not be possible to high enough precision with an imaging detector. However, if a star is mostly degenerate and has a large effective gravity (e.g., white dwarfs and neutron stars), the amplitude of the pulsations will be as large as a few hundredths to tenths of a magnitude. CCDs are capable of accuracy of this order even sampling at the 20- to 60-Hz rate at which white dwarfs and neutron stars pulse (though this requirement may compromise the low-read-noise performance of the CCD system, and so some trade-offs may be needed).
Observations of this type are typically made in a differential sense, that is, by comparing the target star with nearby field stars so that drifts in instrumental sensitivity with time are not important. A critical problem with ground-based studies of white-dwarf pulsations is the aliasing introduced by observing only at night. With sufficient control of the photometry, the ATD/NTOT’s Molniya orbit could minimize or eliminate much of the aliasing, allowing determination of pulsation modes more accurately and more quickly than would be possible from the ground.
A program to test the photometric stability over time scales from milliseconds to months is an excellent test of the ATD/NTOT’s technological capability for astronomical applications. If the photometry is found to be precise and stable, surveys of oscillating stars may become a feasible science project to consider for an extended mission.
1. Morrison, David (ed.), The Spaceguard Survey: Report of the NASA International Near-Earth-Object Detection Workshop , NASA, Office of Space Science and Applications, Washington, D.C., 1992.