Hubble Space Telescope (HST) Archive System

Hubble Presentations
Optical Component Degradation Assessment and On-orbit Implications
Lee Feinberg / NASA Goddard Space Flight Center

Energy Calculations

Calculations support concluding that UV-induced contamination could have occurred in HST in space:

A calculation based on an IUE-based spectrum estimated the amount of ultra-violet energy (less than 300 nm) that could have hit the pickoff mirror while WF/PC-I was in HST and in space was 0.05 J/cm squared.
The amount of energy required to break 2 bonds per molecule for 450 Angstroms of contaminant is approximately 0.005 J/cm squared.
- Assumes contaminant is a phthalate.
- This implies an approximate 10% efficiency of bond-breakage per UV photon.

Calculations also support concluding that UV-induced contamination could have occurred during the Servicing Mission:

The amount of energy seen by the pickoff mirror in its 3.5 years in space is approximately equal to the amount of energy from 2 orbits of Earth exposure during the Servicing Mission (because of the solid angle of the Earth).
- WF/PC-I was exposed to Earth albedo/Shuttle environment for just under 2 orbits.

Possible Outgassing Sources: Optical Telescope Assembly (OTA) and Fine Guidance Systems (FGS)

HST "hub" contamination would likely come from either the optical telescope assembly or the three fine guidance sensors that face the WF/PC-I Pickoff Mirror.
- Some FGS components were not baked out (including filter wheel assembly and optical subassemblies).
- FGS and OTA subsystems were baked out until QCM at 1Hz/hr at 50-55 degrees Fahrenheit for 36 hours (much less stringent than science instruments).
- FGSs had verification test run with QCM at hub facing aperture, QCM 18 degrees Fahrenheit below on-orbit temperature of 70 degrees Fahrenheit plus or minus 5 degree, 8 hours at 1 Hz/hr.
Though science instruments also faced the hub, they were certified at -20 degrees Celsius at 1 Hz/hr (much cleaner) and have only a small aperture facing the hub.
FGSs do have a few-inch gap running across the top of them in the hub.

On-Orbit Data Since Servicing Mission

COSTAR optics deployed into the hub.
GHRS is the most reliable on-orbit UV data for judging UV degradation to COSTAR.
FOS data qualitatively agrees with GHRS data, FOC not accurate in desired wavelength.
WF/PC-II data is expected December, 1994.
Small degradation, as seen, is expected since outgassing in HST has been going on for four years. Therefore, the number of molecules is depleted.

Conclusions/Summary

Based on results from well-controlled optical surfaces (pickoff mirror, aperture window, HSP filters) the contaminant appears to be a UV-deposited organic with soluble and non-soluble components.
- Source of UV is believed to be Earth albedo.
Results from non-optical surfaces (MLI, aluminum aperture plate) are inconclusive and additional samples are currently being analyzed.
- Team is addressing question of whether any surfaces that did not see UV Earth albedo were contaminated.
- Team is addressing issue of whether deposition occurred in HST bay during 3.5 years in space or during the Servicing Mission.
- Additional samples from the High Speed Photometer exterior are being collected and are expected to help resolve these issues.
Based on existing data, scientific instruments currently in HST appear to have minimal UV degradation.
- Work is underway to assure future instruments are also minimally affected.