雲量監察器有多準確

[鄧登凳]

很多天文台, 特別是遙控天文台, 都會有雲量監察器(cloud detector), 那基本上是用紅外線探測上空的溫度, 沒有雲, 上空和地面的溫差很大, 有雲就會溫差縮小。

這種方法有多準確, 以下是一個事例, 上面是雲量曲線(或溫差曲線), 下面是最後的全天攝影機(AllSky camera)圖像。

按溫差測出的雲量是全晚都算好天, 但實時的全天攝影機所見就全晚都全天堂雲

[lawrencemok]

我覺得紅外線只係用黎測低雲~ 但高雲的溫度太低, 可能會睇唔到, 但同樣高雲也會影響觀星.

[鄧登凳]

由下圖看, 那些不似是高雲

[HK.Raymond.Tse]

Most cloud sensors have to be calibrated on the range of sky temperate difference to distinguish between clear sky, slight cloud, full cloud coverage & rain.

These parameters can be manually set. The most practical method is real site testing especially for partial cloud coverage.

Find a man/site attendant to view the sky camera or the observatory site. If you see some clouds, adjust the parameters, so that the cloud sensor report partial cloud coverage.

If the sky is fully covered, adjust the parameters so that the cloud sensor should report full coverage.

Every cloud sensor must be fine-tuned from time to time.

Raymond Tse

[anguslau]

Most cloud sensors have to be calibrated on the range of sky temperate difference to distinguish between clear sky, slight cloud, full cloud coverage & rain.

These parameters can be manually set. The most practical method is real site testing especially for partial cloud coverage.

Find a man/site attendant to view the sky camera or the observatory site. If you see some clouds, adjust the parameters, so that the cloud sensor report partial cloud coverage.

If the sky is fully covered, adjust the parameters so that the cloud sensor should report full coverage.

Every cloud sensor must be fine-tuned from time to time.

Raymond Tse

That's exactly right! In fact, I have to adjust these thresholds quite often (sometimes even through the night) in order to get accurate results.

These cloud sensors are merely IR sensors. We only know that when it is cloudy, it will give a higher reading than when it is clear. The exact point between cloudy and clear varies a lot given different conditions: ambient temp, humidity, transparency etc etc. In practise, I do not find this to be a big problem. Just fine tune it every now and then.