Filter:
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.
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.
Calibration:
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.
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?
