New light sensor for the CloudWatcher

From Lunaticoastro we’ve been improving the CloudWatcher since its launch more than 15 years ago; these upgrades (high-quality anemometer, humidity sensor, improved rain sensor, atmospheric pressure…) have usually been relatively straightforward – well, with some bumps along the road, but quite a smooth experience in general 🙂
This time, however, adding the new “sky quality capable” sensor to the CloudWatcher has been a long process, with some difficult decisions to take, as none of them had a “clear-cut” best approach.


We could not use our previous “clear” methacrylate window for the older brightness sensor, as we imperatively need to reject wavelengths out of the visible spectrum now.

A standard astrophotography dichroic UV/IR filter has been chosen, as it will more closely reflect the amount of light that our CCD/CMOS camera will be receiving. These filters have been custom-made for us, full size, so they double as filters and as weather protection windows.

We also considered a photometry filter (expensive, and not as useful in our opinion). The fact that broad spectrum LED lighting is becoming increasingly common has influenced this decision.

Field of view:

We have decided to go for a wide field of view (100% sensitivity directly on top of the sensor, dropping to about 20% at nearly 120º); taking into account the CloudWatcher is in a static position, and it seems more interesting to monitor the majority of the sky – again, the contribution of the near zenith will be the most important.

 This, of course, may not be the case for everyone, but we expect it to be a good idea for most instances.

Our aim to keep the CloudWatcher as safe as possible also influenced this decision, its main mission still being to be a safety device. Any light collecting device, concentrating light into the sensor, meant a (slight) safety issue under certain extreme conditions.


Regarding calibration, there were two decisions to be made: how, and where.
Again, in the spirit of keeping the costs reasonable—something increasingly difficult in the post-COVID era, with component shortages and crazy pricing — we have decided to perform a basic calibration at the factory, and leave up to the user the option to go for a more accurate one.
Expect a step-by-step guide for this calibration in the future – it will involve actual CCD/CMOS imaging to aim for “the real thing”.
We really hope this upgrade will be useful for the vast majority of our users. It will:
— enable you to measure how the darkness evolves over time, night to night and also over extended periods of time
— assist in cloud detection (under study right now)
— enrich the metadata in the images
— provide a much more accurate detection of twilight (for example, to take sky flats)
… and… who of us doesn’t love having more meaningful data?