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

CT7: handheld reflectometer scatterometer

CT7 / CT7a / CT3 / Other
CT7 Reflectometer Scatterometer: handheld portable instrument monitoring the reflectivity and scattering of telescope's mirror coatings
Fourth version (2020)
As of january 2021, the
Cherenkov (vUV) version
of the CT7
is available


The CT7 reflectometer / scatterometer is as of today the reference instrument to monitor the reflectivity and scattering of telescope's mirror coatings. This instrument is specially thought for accurate long term assessment of the mirror coatings and is in use on the most advanced telescopes worldwide.
The CT7 is the greatly improved successor of the renowned Iris instrument developed by DMO. Its technology relies on the unmatched algorithm devised by Dr Daniel Malaise. It provides a far better accuracy (0.2%) and repeatability (0.01%) than any other available measuring apparatus. And all of this in the palm of your hand: the instrument is 88mm long, 65mm wide and 130mm high, and weighs less than 600 gram! It’s designed to measure any surface safely and easily, in any environment.
The CT7 reflectometer / scatterometer works on batteries and can thus easily be taken with on the surface to be measured. The CT7 is positioned by a three-point soft contact. Curved mirrors can be measured without correction down to a radius of + or – 1m. 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 .cvs 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 first version of the CT7 reflectometer / scatterometer was released in 2010, the second in 2012, the third in 2016 and the fourth version is available since the last major upgrade from 2020. As of january 2021, a new version of the CT7 is available, capable of measuring the reflectivy with high accuracy at wavelength bands down to 308nm, especially developed for the institutes studying the Cherenkov radiations.

Reflectivity monitor

The CT7 is primarly a reflectometer especially designed to measure the specular reflectivity of telescope's coating in 7 wavelength bands. It has an acceptance angle of 1.5° (half angle) and works at 8° incidence. The 7 Bands can be chosen in the UV-Visible-NIR region; current choice is the following:
Wavelength (nm) Bandpass (nm)
365 10
405 15
470 30
590 30
730 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 digitally 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 mounted on the same heat sink. This ensures a long-term absolute stability and immunity against temperature variations even when the sources degrade or vary in intensity. Since 2016, the detectors are selected for showing identical temperature sensitivity curves. This instrument is delivered with a factory calibrated gauge stable for at least one year (within 0.1%) when kept in its protective box.

Cherenkov version

As of january 2021, the "Cherenkov version" of the CT7 is available, working with dectectors reaching the vUV wavelengths and specially developed for institutes studying the Cherenkov radiations. The 7 Bands are as following:
Wavelength (nm) Bandpass (nm)
308 13
325 13
365 10
430 25
528 30
590 20
625 18
We were able to achieve an accuracy of measurement of the reflectivity and repeatability similar to the standard version of the CT7 on each wavelength down to 308nm and are very proud to present our new instrument to the astrophysical community.
NB: the two vUV channels (308nm and 325nm) do not detect the scattering (dust index) due to the weakness of the input signal.

Dust detector

The CT7 is also a scatterometer, which operates as dust detector. The latter measures the scattered light in the seven channels. 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.
CT7 Reflectometer Scatterometer: handheld portable instrument monitoring the reflectivity and scattering of telescope's mirror coatings

Technical specifications

Global absolute accuracy:

+/- 0.2%     for R > 50%


+/- 0.01%   for R > 50%


The CT7 is small (L 85 mm, l 65 mm, h 130 mm) and light (<600 gr) and in one piece (no wire; one hand operation).


The CT7 is positioned by a three point-soft contact. Curved mirrors can be measured without correction down to a radius of + or – 1m.

Other specifications:

Internal memory keeps up to 10.000 measurements including date, time and temperature, sequential number and individual identification. One can also operate CT7 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 CT7 is off.

Price and quotation request

Starting price for the CT7 reflectometer / scatterometer is 18.500€ + 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 4 weeks following your order. Orders of more than 5 instruments could have a longer delivery time.


The CT7 reflectometer / scatterometer comes with a 5 year warranty on the instrument and 3 year warranty on the 7 wavelength source. Any dysfunction will be fully solved in a 4 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 #5.