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42.6 Orbital Period and Ramp

A science verification test to check the short term photometric stability of the HSP was run in August 1991. The rapidly oscillating Ap star HD60435 was chosen because of its position in a region of the sky that was continuously visible from HST and also its intrinsic variabilities were well known. Data was collected for 5.5 hours. Figure 42.2 is a plot of the data obtained from the test. The data shows two unexpected effects: a sinusoidal oscillation with a 94-minute period and a monotonic increase in flux. Both the zero-to-peak amplitude of the 94 minute oscillation and the total amplitude of the monotonic linear ramp are about one percent.

Figure 42.2: 1389 SCP Data

Nelson [5] states that these sinusoidal variations occurred in all HSP data. Several tests to understand these photometric changes were preformed with the HSP and other HST instruments. Onboard temperatures exhibited a sinusoidal variation with a period equal to the orbital period of the telescope but a causal relationship between the temperature of a particular location on the telescope and the photometry has not been established.

An orbital periodic axial motion of the focal plane known as breathing has been acknowledged[6]. Nelson has used models of the HST PSF at the HSP position in the focal plane to determine that the 2.5% variation (peak to peak) as seen in the 1389 data, would take 8 to 10 microns (peak to peak) of axial motion of the secondary mirror (despace).

A test run by STScI resulted in an empirical formula using OTA temperatures that corresponds to the orbital period and ramp of the HSP data [7], but a correction to the data has not been defined.

The HSP team has been able to fit these systematic variations with an equation that models the fluctuations well and can be used to correct the data to within photon statistics. However, this is done at the expense of eliminating any possible detection of intrinsic variability which occurs on the timescales of these systematic fluctuations.

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