0.9-m Reflectrometry


Mirror washing. Iris reflectometry and BRDF data before/after wash was respectively:

  • 80.1/91.1% (470nm), 79.5/90.9% (530nm), 76.9/87.2% (650nm), 74.8/86.2% (800nm)
  • 7.4/0.5% (+20deg), 6.2/0.6% (-15deg), 1.1/0.05% (-45deg)

Thus a spectacular 11% net gain in reflectivity. Only about 1.5% was not recovered compared to the last washing results. It also shows a 2.2% reflectivity loss per month (averaged over the last 6 months), which is about 3.5 times higher than usual!



Mirror washing. Iris reflectometry and BRDF data before/after wash was respectively:

  • 87.2/92.6% (470nm), 87.5/93.1% (530nm), 83.4/89.9% (650nm), 83.1/89.0% (800nm)
  • 2.4/0.8% (+20deg), 2.5/1.1% (-15deg), 0.4/0.09% (-45deg)

It was very successful, bringing the mirror back close to a fresh aluminium performance.

We used isopropyl alcohol at the end of the rinsing but this did leave a few ugly whitish marks that we could not recover with another soap wash. Rancourt explains (Optical Thin Films, User's handbook, McGraw Hill 1987) that the 'evaporation of the solvent on the surface beeing cleaned will reduce the temperature locally and may cause the moisture in the air to condense in the vicinity of the droplet, leaving a mark when water dries' (p175). This might be what happened. Since then, we do not use alcohol anymore in our washings... We also identified thousands of pinholes in the coating by shining light from underneath the mirror, sign that the mirror was dusty when last put in the coating chamber.



We washed M1. The following graph show reflectivity and scatter as we found the mirror after 10 months (without cleaning it at all), after a CO2 cleaning, and finally after washing. Last aluminizing data is also plotted for reference. Final state of the mirror is actually better than after aluminizing, a surprise also noticed by Giordano at the NTT using Collodion peeling films (Wilson R, Reflecting telescope optics II, A&A library 1999, p 444). All measurements made with the Minolta.

M1 reflectivity and scatter

Between June 97 and June 99, M1 reflectivity as decreased by 7.8% per year at 400nm and 7.2 per year at 700nm, while scatter has increased by 3.8% per year at 400nm and 5.3% per year at 700nm. Between June 99 and April 00, M1 reflectivity and scatter have varied according to the same trend as the previous period, which is a monthly variation of: R-0.68% at 400nm and R-0.73% at 700nm, SC+0.36% at 400nm and SC+0.52% at 700nm. In theory, scattered light dependence on wavelength is given by TIS=(4.pi.d/l)^2 where TIS is the total integrated scatter measured in 2pi str, d is the roughness height and l is the wavelength. Therefore, scattered light should be lower in the nearIR than in the blue. Particle size might modify that law if they are about the size of the wavelength.


JUNE 99:

We aluminized both mirrors. Last time was: 6June97 for M1 and 9Dic97 for M2. See the following plots. In the text below, we give the average number (2 points sampled each time) at 550nm for a rapid evaluation of, respectively, the reflectivity and the scattered light measured with the Minolta.

    • M1: was very dirty visually, covered with dust (77.0% and 10.80%). We noted the presence of lots of particles longer than 0.5mm, even a few (10) >1mm. First we cleaned it with CO2 (86.7% and 4.40%). Then we washed it with Orvus soap (contact with natural sponge), rinsed with bi-distilled water and dried with dry nitrogen (91.0% and 1.10%). The result was spectacular, close to a fresh aluminium

      Finally, we aluminized (91.9% and 1.10%).

      M1 reflectivity and M1 scattering.

      From the reflectivity of fresh aluminizing (91.9%) and after CO2 cleaning (86.7%), we can estimate the monthly loss if the mirror had been regularly CO2 cleaned: (91.9-86.7) / 24 = 0.22% which is in agreement with the data measured at the 1.5m with regular cleaning. We conclude that CO2 cleaning slows the natural dust deposition process by a factor of about 3.

    • M2: was quite clean after removal from the telescope (88.1% and 1.80%). Cleaning it with C02 didn't make any difference (87.9% and 1.90%). Aluminizing improved the reflectivity but not the scatter (89.8% and 1.80%), sign that the mirror was probably not clean enough when coated.

      M2 reflectivity and M2 scattering



Reflectometry and BRDF at the Schmidt: measured in April 01

  • 81.3% (470nm), 81.4% (530nm), 83.0% (650nm), 82.6% (800nm
  • 10.0% (+20deg), 9.5% (-15deg), 9.3% (-45deg)


Last update: 16 April 01