Sky Quality Meter

[Raymond TSE]

Dear all,

Just setup and is now testing Ethernt version of Sky Quality Meter and its paid version SQM Reader pro software to record continuous data of sky darkness.

In comparison with other meters measuring relative sky brightness, this meter can convert the sky darkness in terms of sky magnitude/sq arcsec (or near eyesight limiting magnitude).
This may be useful to compare darkness in different observation sites with absolute data rather than making relative comparisons.

A sample graph taken recently is provided here.

Raymond Tse

[社長]

woooow.... 貼近21mag/sqarcsec

你的觀測點是在哪裡?

我手上有兩個SQM，曾帶到西藏好幾個點上測試，包括後藏阿里5000海拔的山上，量得是21等多，，去不到22等的。我跟SQM的開發人談過，在納米比亞量得史上最暗22、23等數據本身有問題，當時可能天氣不好星光暗淡，所以SQM量到大既21.8等以上便不再可靠。

viewtopic.php?f=13&t=1178#p10301

弓起，Raymond有個很好的Dark Site

[Raymond TSE]

woooow.... 貼近21mag/sqarcsec 你的觀測點是在哪裡?

Sai Kung.

我手上有兩個SQM，曾帶到西藏好幾個點上測試，包括後藏阿里5000海拔的山上，量得是21等多，，去不到22等的。我跟SQM的開發人談過，在納米比亞量得史上最暗22、23等數據本身有問題，當時可能天氣不好星光暗淡，所以SQM量到大既21.8等以上便不再可靠。 viewtopic.php?f=13&t=1178#p10301 弓起，Raymond有個很好的Dark Site

The graph was just a test but not scientific. The main purpose is to check the cut off point where imaging is worth trying and no longer necessary. For this month, the beginning imaging time should be 7.15 pm whereas imaging after 5.15 am is ineffective due to increasing sky brightness. This may be useful for remote imaging where you cannot assess the sky darkness by eyes at all.

[社長]

Sai Kung.

那按HKU的調查，你的地點是香港最暗的區域之一了：

http://nightsky.physics.hku.hk/

以我的經驗，若某天區達到20mag/sqarcsec，而銀河出現在那區域，銀心結構，或夏季大三銀河分裂位置都肉眼清楚可見，用雙筒鏡看深空天體已很精采。

The graph was just a test but not scientific. The main purpose is to check the cut off point where imaging is worth trying and no longer necessary. For this month, the beginning imaging time should be 7.15 pm whereas imaging after 5.15 am is ineffective due to increasing sky brightness. This may be useful for remote imaging where you cannot assess the sky darkness by eyes at all.

以香港的大氣透明度受不同風向影響甚大的情況下，我很同意加裝SQM會很有用。

西藏沒這問題，主要是受雲層影響，就算拍攝期間有雲掠過，曝光20分鐘的L通道，天空背景ADU也是2000水平。除西南方有丁點燈光外，整個天空背景非常均勻，實際上就在我最惡劣的西南方，日落後沒多久，也能在仰角10度曝光5分鐘拍下一顆暗於13等的彗星：

viewtopic.php?f=16&t=2124&start=20#p21144

所以我會用全天魚眼鏡配合Boltwood Cloud Sensor，有數據及直觀影像便可以。

[Raymond TSE]

The site is reasonbly dark but there are still some village lamps around.

I said the test is not scientific because the offset factor has not been deducted.

For remote recording of data, the meter must be stored inside a plastic/glass dome. I have used
the plastic dome for narrow FOV sky cam and thus the meter is temporary stored inside near the clear window (full of dust)
at an elevation angle of about 70-80 degree to point at the sky facing east.

Due to the reflection of the outside glass surface, some sky light would be reflected away due to the obtuse angle and due to non-coating. Some sky light might be absorbed by the glass materials.

My previous experience with platic aryclic dome is that some light would be absorbed by the plastic arcylic materials and some light reflected away. Thus the actual SQM reading should be lower than the recorded figures.

Would find a night to compare the actual reading with and without the glass window. There may be 0.5 or bigger difference. Let's see the delta amount later.

Raymond Tse

[社長]

why not pointing the SQM to zenith? but 70-80 degree instead?

I found there can be 0.5 mag or more difference, when point to or away from the Milkyway, in a dark site.

[Raymond TSE]

why not pointing the SQM to zenith? but 70-80 degree instead?

The meter is placed temporary inside a fibre glass encloure with side window only. Later, I shall place it under an arcylic dome.

Savio,

Where do you buy the arcylic dome with base (ie to put your DIY skycam) ?

Raymond

[社長]

Savio,

Where do you buy the arcylic dome with base (ie to put your DIY skycam) ?

Raymond

Raymond，大概鴨寮街153號附近的一間賣閉路電視器材小店，好像是300多元。

我現有一枝全新的Fujinon魚眼鏡閒置，日後有用的話可以找我。

[Raymond TSE]

Just buy a so-called "water proof" box enclousure for security camera with glass window at HK$128.
The long metal box is small but just large enough to place the little SQM inside.

Will test its ability to guard against rain and to find out the offset of such window glass.

Fortunately, SQM Reader Pro allows for offset to be deducted before plotting the data graph.

Another unscientific graph last night of eyesight limiting magnitude before applying any offset.

Raymond

[社長]

有機會可以上張實物圖片看看?

[Raymond TSE]

Just checked the offset and the real sky magnitude on Saturday night/Sunday after midnight.

The actual sky magnitude at zenith without any enclosure at Moon Age 1 was around 20.10 - 20.16 (or equivalent to Near Eye Limiting Magniture of 5.5 - 5.6).

Raymond

[社長]

看來情況不錯。

不過要留意一點，天空透明度也是另一因素，正如納米比亞在有雲影響下，因為掩蓋了星光而量出史上最暗的SQM數值，如何對應香港的多變而透明度時好時壞的天氣，還是一個疑問。

[社長]

21.8等是極限，亦即西藏量到的近似值，但也要避開銀河或亮星區域才成。


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Hello Savio,

Thanks for the update and links to your discussions. I also liked looking through your impressive photos!

We think that about 21.8 on the SQM is the darkest to be seen naturally at a dark site. Darker readings likely come from unusual aerosol conditions that are due to the sky being less transparent.

Please keep the second SQM for reference and let me know what results you get.

Best regards,

Anthony Tekatch
Unihedron

[Raymond TSE]

Even the SQM is designed to give objective absolute value of sky brightness (or darkness), there are still many factors which would affect the actual reading.

The lens is just 10 degree wide, but if you get Jupiter or Milky Way at zenith covered in the FOV, the reading may also be slightly different.

I therefore take the same SQM as as a tool to measure relative conditions for the same site, or to compare whether it is worthwhile to go to particular sites (eg Gaomeigu, HeHuan Shan, Yinna Hill) for imaging.

[社長]

or to compare whether it is worthwhile to go to particular sites (eg Gaomeigu, HeHuan Shan, Yinna Hill) for imaging.

"There is no substitute of dark sky"，我認為若純從數據上看，基本上的格局是 高米古 > 合歡山 > 陰那山 > 香港

至於去與不去，更大程度是旅費、個人時間等的取決。

我只跟自己比較，當在西藏20分鐘的L通道或30分鐘的Ha通道拍到這樣的訊噪比，減Dark後的天空背景ADU只有數百，我真的動不起勁再去其他地方拍了：

viewtopic.php?f=17&t=3254#p21027

viewtopic.php?f=17&t=3271