OPO  -  Monitoring the reflectivity and scattering of telescope's coatings


CT7a: absolute reflectance and scattering

CT7a Reflectometer Scatterometer: absolute measure of the reflectance and scattering of telescope's mirror coatings
The absolute version of the CT7


The CT7a reflectometer / scatterometer is the new absolute version of the CT7 , which is already in use on the most advanced telescopes worldwide and is as of today the best available measuring instrument for the reflectivity and scattering of giant telescope’s coatings.

The new feature of the CT7a is an absolute calibration system. For every set of measure, you can make a new calibration in your actual working environment and thereby avoid any alteration of a reference gauge.

Due to the mechanical system that provides the on-site absolute calibration, the precision of the CT7a is as a matter of fact a little lower than that of the CT7 . Its technology relies for the rest on the same unmatched algorithm devised by Dr Daniel Malaise for the CT7 . It provides a far better accuracy (0.5%) and repeatability (0.02%) than any other available measuring apparatus besides the CT7 itself.

Like its relative counterpart, the CT7a is handheld operated. The instrument is 95mm long, 85mm wide and 135mm high, and weights less than 800 gram! It’s designed to measure any surface safely and easily, in any environment.

The CT7a reflectometer / scatterometer works on batteries and can thus easily be taken with on the surface to be measured. The CT7a is positioned by a three point soft contact. Curved mirrors can be measured without correction down to a radius of + or – 10m. Due to the longer beam path, the CT7a is more sensitive than the CT7 on misalignment and mirror's curvature. You can save up to 10.000 measures and set the instrument to take a measure in loop at chosen intervals in console operation.

To read the measures, connect the reflectometer / scatterometer to your computing station, and open the included software CT7 Console . You get all the results in an easy to read table, you can configure your results sheets, copy-paste to excell or export them easily in a .csv file to save or share.

The instrument comes with an in-depth documentation. We are also providing a constant customer service and are happy to answer all your questions and help you getting started or manage unexpected events.

The CT7a was first released in 2020 and is still benefiting from constant development.

Reflectivity monitor

The CT7a is primarly a reflectometer especially designed to mesure the specular reflectivity of telescope's coating in 7 wavelength bands. It has an acceptance angle of 2.0° (half angle) and works at 8° incidence. The 7 Bands can be choosen in the UV-Visible-NIR region; current choice is the following :

Wavelength (nm) Bandpass (nm)
365 10
411 15
528 30
630 30
750 50
850 50
940 40

The sources are modulated at a frequency around 2kHz and detected by the technique of digital lock in amplifier based on a DSP board. This makes the instrument immune to stray light.

Two identical detectors are used; one to measure the reference signal of the source, the other to measure the reflected signal. Both signals are digitaly detected and the ratio of the source signal to the reference signal is recorded and processed to yield the reflectivity value. Each color is lit and measured sequentially for one second.

The two detectors are matched for temperature sensitivity variations. This ensures a long term absolute stability and immunity against temperature variations even when the sources degrade or vary in intensity.

The absolute accuracy (+/-0.5%) of the CT7a is not yet as good as the one of CT7 + gauge , but we aim at a final figure of the same order.

Cherenkov version

The "Cherenkov version" of our instrument, working with dectectors reaching the vUV wavelengths and specially developed for institutes studying the Cherenkov radiations , is only available on the regular CT7 . Due to the longer beam path of the CT7a , it is currently not possible to reach an accuracy of measurement and repeability that meets our quality standards on the absolute version. It works perfectly on the regular CT7 with gauge .

Dust detector

The CT7a is also a scatterometer , which operates as dust detector. The scatterometer part is identical to that of the CT7 . The latter measures the scattered light in the seven chanels. It measures the integrated scattered light between 9° and 19°. This measurement yields a fair approximation for the value of the TIS. The dust index indicates the quantity of dust (in relative surface) covering the mirror.

Technical specifications

Global absolute accuracy:

+/- 0.5% for R > 50%


+/- 0.02% for R > 50%


The CT7a is small (L 95 mm, l 85 mm, h 135 mm) and light (<800 gr) and in one piece (no wire; one hand operation).


The CT7a is positioned by a three point soft contact. Curved mirrors can be measured without correction down to a radius of + or – 10m. Due to the longer beam path, the CT7a is more sensitive than the CT7 on misalignement and mirror's curvature.

Other specifications:

Internal memory keeps up to 10.000 measurements including date, time and temperature, sequential number and individual identification. One can also operate CT7a via a dedicated console on a PC through an USB2 connection. Batteries are Li-ion charged while connected to the USB port of the PC and the CT7a is off.

Price and quotation request

Starting price for the CT7a reflectometer / scatterometer is 22.500€ + VAT and shipping costs. Please contact us directly by email to ask for an offer, and we will discuss all the details with you.

Delivery time

We can under normal circumstances assure a shipping 8 weeks following your order. Orders of more than 5 instruments could have a longer delivery time.


The CT7a reflectometer / scatterometer comes with a 5 year full warranty. Any dysfunction will be fully solved in a 6 weeks delay. Only the shipping costs are borne by the customer.


We also provide upgrades to all major new versions, under the same shipping costs policy.

More information

A more detailed description is presented in the Application Note #11 .

« Accurate perception of reality is the condition for rational and successful action. » - Daniel Malaise