關於 Sky Quality Meter 讀數的疑惑

[mwtse]

參考資料：
http://pintovalleyobservatory.com/PVOIm ... es_004.htm

http://en.wikipedia.org/wiki/Surface_brightness

[社長]

SQM 開發人最新回覆：

We don't subtract starlight. The measurement is of all sources in the field of view.

It would be possible to estimate the fraction of the signal in starlight after the fact since the stars are constant and the readings are usually taken at zenith. But we haven't bothered to do that yet and it would only make a difference in the very darkest skies (and when the center of the Milky Way is overhead).

[mwtse]

SQM 開發人最新回覆：

We don't subtract starlight. The measurement is of all sources in the field of view.

It would be possible to estimate the fraction of the signal in starlight after the fact since the stars are constant and the readings are usually taken at zenith. But we haven't bothered to do that yet and it would only make a difference in the very darkest skies (and when the center of the Milky Way is overhead).

From an engineer's point of view, it is impossible to subtract the contribution of star light without substantially increasing the cost. The most cost effective compromise will be to use a narrow band filter that is only sensitive to artificial light source (i.e. the inverse of light pollution filter). It could not eliminate star light, but the reading given out should be better.

[社長]

我反而覺得若設定一個常數，例如全天平均星光亮度，固定從SQM的讀數中減去會更好，最後讀數跟天空背景光總有些許分別，但貼近真實數字。

你說的濾鏡應不可行，因為背景光有人造或非人造，也分窄頻或寛頻的，但星光則肯定是寛頻，所以問題會更複雜。

[mwtse]

你說的濾鏡應不可行，因為背景光有人造或非人造，也分窄頻或寛頻的，但星光則肯定是寛頻，所以問題會更複雜。

You are correct!

[mwtse]

Cloudy nights 的 small wonders 中提到，M51 的表面亮度約 13 等，應該是合理的數值。所以，我應該要收回我的土法計算了。

http://www.cloudynights.com/item.php?item_id=1073

[社長]

今天收到一個全新的SQM，厰方負責人說給我帶到西藏去測試一下，照對比較我自己那個是否有問題。

今時今日這種服務態度真好

[Robin]

今天收到一個全新的SQM，厰方負責人說給我帶到西藏去測試一下，照對比較我自己那個是否有問題。

今時今日這種服務態度真好

如此顧客至上的服務，牛！