The Whipple Mission: Design and development of the focal plane

Abstract:

Whipple is a proposed space borne mission intended to detect and characterize the
size and spatial distribution of Trans Neptunian Objects (TNOs) using the ``blind''
occultation technique. This technique measures the size of, and distance to, a TNO
by discerning features of the Fresnel diffraction pattern that is produced when a
TNO intercepts the light path between a distant star and the observatory. As the
observatory transects the diffraction pattern, it resolves that pattern as a light curve
using a differential photometer.
The light curve decrement is relatively large (few percent) and the
temporal duration is short. For a TNO in the Kuiper Belt the duration is a fraction
of a second. For objects in the Oort cloud the duration is ~ a few seconds. Since
a blind occultation event is rare, tens of thousands of stars need to be observed
simultaneously over several years to accumulate sufficient statistics. Stars need
to be observed at cadences up to 40 Hz with a read noise <20e rms (post CDS)
Though this is beyond the capability of CCDs, such a high speed, low noise, multi-
object differential photometer instrument can be implemented with CMOS imaging
technology.

The proposed focal plane for the Whipple photometer consists of nine Teledyne
HyVISI Silicon hybrid CMOS detectors behind a 77cm F1.34 optic. The detectors
consist of 1k by 1k 36 micron pitch pixels and each detector is connected to its
own SIDECAR ASIC. Due to the high cadence required, the detectors are operated
in window readout mode. Approximately 700 stars per detector, each in a 2x2 pixel
window, will be read out at 40Hz. Progressively more stars can be observed as the
cadence decreases, until the limit of the SIDECAR memory is reached at about 4,000
windows The lack of atmospheric turbulence combined with the large field of view
and high, speed low noise performance of the focal plane will provide the Whipple
mission with unprecedented capability in exploring our Solar System.