GIST 24 MeetingGERB Mirror Pitting Update

14th December 2005

Presenter: S.Kellock (Imperial)

Mirror Pitting

• First observed on the GERB-3 Flight Spare Telescope mirrors• Could be seen with the unaided eye using bright light at grazing incidence• Pits appeared to be in clusters• Silver coated mirrors used extensively for ground based telescopes and

tarnishing is a well know problem, but this appears to be a different effect.• GERB mirrors all have a Hafnium Oxide protective coating• Coating details are proprietary (Denton FSS-99UV) so little information is

available • Pitting could be due to presence of holes in the protective coating

allowing exposure of the Silver coating to the atmosphere• Witness coating samples held at RAL were re-examined under high

magnification (X150) and pits were seen on all samples.

Investigations

High magnification images of the witness samples obtained using Nomarski microscopy (performed by Imperial College)

Conventional microscopy also used, but often it was hard to distinguish pits from surface contamination.

Pitting often follows along the line of a scratch (see next slide)

More Nomarski Images (courtesy Imperial College)

This pit appears to show blistering of the Silver coating around its edges.

Pits following the line of a scratch.

Pit Geometry

Some preliminary measurements have been done at RAL using a Zygo interferometer. The selected pit has a diameter of ~10µm and a depth ~0.8µm. Not sure yet if this is typical.

SEM Investigation

ESTEC have undertaken some investigations of pitted samples using

their SEM. Initial findings are;

• Older samples show more pits

• Many pits show white areas in their centres

• White areas show presence of Sulphur. Hf is degraded and Ni and Cr spikes are elevated

• Sulphur is not found in pits with no white area, but Ni and Cr is still elevated compared to non-pitted area

• Pits seem to follow scratches

• Pits on newer (younger) coating samples are generally smaller.

Reflectivity Measurements

ESTEC also made reflectivity measurements. Preliminary results suggest;

• Older sample shows lower reflectivity• Relative difference between old (10/1998) and not-so-old (11/1999) is

– < 1% in range 670 to 2500 nm

– < 2% in range 540 to 670 nm

– < 4% in range 470 to 540 nm

Notes

• Coating to coating variation has been found to be significant

• Surface contaminates will also affect reflectivity

• Not clear if pits are responsible for the changes listed above

Key questions about presence of pits?

• What effect do the pits have on the instrument performance?• Are the pits still forming?• What is the rate of pit growth?

Instrument performance.

• Pits could change the mirrors’ spectral response and, in the case of the Scan Mirror, introduce more stray light into the system.

Way forward– Compare recent reflectivity measurement with those done at NPL in

1999/2000.– Ask NPL to repeat the measurements on GERB 1 & 2 samples– Measure GERB-4 coating samples at regular intervals to assess degradation

with time

• Stray Light– Coating sample roughness (<5nm RMS) greater than DSM mirror roughness

(<2nm RMS) so stray light measurements on coating sample not meaningful– Measure spare DSM mirror in a location that has pits?

Are pits still forming and, if so, at what rate?

• Measure the entire surface area of coating samples for all GERBs and determine surface area of pits. If pits still forming then older samples should show larger cumulative area of pits.– RAL is currently talking to a company that can scan the surface of coating

sample at high magnification (X150) and, through the use of image analysis software, determine the cumulative pit area. The size distribution of pits will also come out of this analysis.

– Repeat measurements at regular intervals ( yearly?) until GERB-4 launch.

Note• GERB 3 & 4 will be stored in a dry Nitrogen environment, but expose to

the atmosphere cannot be avoided completely.

Some soothing words!

• The pitting is thought to affect much less than 1% of the mirrors’ surfaces• The coating samples at RAL do not show the visible pitting seen on the

GERB-3 spare telescope, but pits can be seen at high magnification.• The DSM mirror on GERB-1 was checked at Kourou and is still very

reflective (we could not get close enough to the mirror to see pits).• The rate of pit growth would appear to very slow, as oldest sample (Oct

1998) still looks in good condition. • Preliminary measurements suggest that the effect of pits on the mirrors’

spectral response may be of the same order as contamination.