(2023/6/25 updated 分季節製圖表)
Further statistical chart analysis of my visual observations and sketches of 300 deep-sky objects at Anbu.
(Part 1). Variation characteristics of the sky surface brightness at Anbu.
註：當天空暗度超過20.2當日清晨通常伴隨不同程度的雲海覆蓋台北盆地，合理推論因為這些雲海阻擋了來自城市的部份光害使得天空更暗。目前記錄到的這些超暗時刻幾乎都在凌晨三點之後。從凌晨四點後分歧較大是因為在四月底至八月間約四點左右便已進入天文曙光，天空已受曙光影響漸亮。
一、
鞍部的天空表面亮度的變化特性（不分季節）
由於這觀測資料庫的測量數據是在進行眼視觀測時隨觀測的天體指向而測量，且因為數據夠多，大致均勻分佈於各方位，雖有極少數幾個數據因天候關係而使得數值偏離正常，並不影響統計結果 的判讀，結果大致與直接定向測量以及長期的觀測經驗相符，可以明顯看出鞍部或郊山的天空亮度隨方位與仰角、時間的變化：
1.鞍部南方的天空暗度隨仰角增加而變化的幅度很大，北方仰角的差異較小（從西北至東，即方位角315至90之間這135度的範圍內無論仰角SQM都在19以上）(圖3, 4)。
2.不分方位，仰角約在35度以上天空暗度皆能達到19以上。東方至西北是鞍部天空最暗的方向(圖2, 4)。排除天候的影響，最暗與最亮（南南西低仰角，也就是在城市的光害影響最大方向），亮度可差0.5至一個星等。（北方平均19.9-20，南方約19.0-19.4左右）
3.市郊因城市作息的影響天空亮度隨時間而有明顯變化，入夜後天空亮度隨時間而緩緩下降，約在十點以後平均暗度才會達到19以上，而到達最暗時刻已過午夜（圖1）。
4.台北市的光害特性在六年後有明顯變化，從光害光譜可見LED的盛行使得其光譜強度幾乎完全淹沒了高壓鈉燈，而金屬鹵化物燈的型態沒有變化。LED光譜強度的增加將使得輕光害濾鏡與星景濾鏡的效果降低許多，對窄頻觀測的影響稍小。
5.並非深空天體都需要在很暗的天空亮度下才看得到，實際觀測記錄顯示，素描天體時天空亮度從17.5到20.5都有，也就是觀測不同天體有不同的門檻，詳細請参考記錄表上倍率及濾鏡等其他觀測參數（視深空天體的特性與亮度，通常觀看星團與小行星狀星雲不需要很暗的天空）(圖5)。
光害分析亦可作為觀測計劃擬定時的參考，選擇受光害影響較低的觀測窗口，譬如在鞍部利用天體過子午線前觀測會比過子線後較佳，而十一點後觀測的效率會好很多。
The SQM data in this observation database was measured during my visual observations, aimed in the direction of the observed deep sky objects. With sufficient data roughly distributed in all directions, the results match those of direct directional measurements and my long-term observation experience.
It is apparent that the sky brightness of the Anbu (Datun saddle) or suburban mountains varies with direction, elevation, and time:
1. The southern sky's darkness of the Anbu changes significantly with the increase of elevation, while the difference in elevation in the north is smaller (pic. 3, 4).
Regardless of direction, the sky darkness can reach above 19 at an elevation of about 35 degrees or higher.
2. The darkest direction of the saddle sky is from east to northwest (pic.2). SQM is above 19 regardless of elevation within the range of 135 degrees between the northwest and east, i.e., between azimuth 315 and 90 degrees.
Excluding the influence of weather, the difference between the darkest and the brightest (south-southwest low elevation, that is, the direction with the greatest influence of urban light pollution) can be as much as 0.5 to one magnitude (northern average of 19.9-20, southern about 19.2-19.5).
3. The sky brightness in suburban areas changes significantly with time due to the influence of urban daily activities (pic. 1).
After nightfall, the sky brightness gradually decreases with time, and the average darkness will reach above 19 after about 10 pm.
4. The light pollution characteristics of Taipei City have changed significantly after six years. From the light pollution spectrum, it can be seen that LED lights have almost completely replaced high-pressure sodium lamps, while the form of metal halide lamps has not changed.
The increase in LED spectral intensity will greatly reduce the efficiency of light pollution filters and star-landscape filters, with a slightly smaller impact on narrowband observations.
5. Not all deep sky objects require very dark sky brightness. My observation records show that the sky brightness when sketching celestial objects ranges from 17.5 to 20.5 mpsas (pic. 5). That is, different celestial objects have different observation thresholds. For details, refer to the spreadsheet for other observation parameters such as magnification and filters. Depending on the deep sky object's characteristics and brightness, viewing star clusters and little planetary nebulae does not require very dark skies.
Light pollution analysis can also be used as a reference when planning observation projects, choosing observation windows with less light pollution, e.g., observing in the Anbu before the object passes the meridian is better, and the efficiency of observation after 11 pm is much better.
於鞍部長期觀測下目標背景天空亮度的樣本呈現常見的偏態分佈，中位數為19.63。長期穩定、安全的觀測環境，暗度足夠（光害較低處），是選擇郊山觀測地最重要的考量。但過分強調天空最暗值反而無法反應實際觀測時會遇上的平均天空狀況，因為一旦觀測數夠多，觀測目標很少會剛好在最暗的時刻與位置，目標有高有低遍佈四方，樣本會呈現偏態或常態分佈。相較下中位數更能反應平均狀況。
(2023/6/25 updated)
二、季節別 seasonal
請注意以下圖表為觀測深空天體時該天體的背景天空表面亮度，故測量值涵蓋各方位仰角以及時段。此外，各季節資料間刻意間隔一個月，即各季皆僅取兩個月的資料統計。
從統計圖表也可大略看出三百個深空天體的觀測在不同季節的分佈情況（天球南北以及觀測時的仰角分佈）。
在夏、冬兩季觀測南天、低仰角的天體明顯多出許多，所以觀測值的天空亮度差距也較大。
冬季適合的觀測時間（以東北至西北一帶達到19.5以上暗度為依據）比夏季約長了2~2.5小時，夏季北方入夜後約過了十點後達到19.5以上。冬季北方有時入夜至8:30後暗度即可達到19.5。
所有季節達到19.9至20暗度的時間沒有明顯差異，約是在午夜零點左右可達20。換言之，在暮光結束之後主宰天空暗度的是來自城市的光害。
西南方是鞍部光害最大的方向，南方次之，這一帶也就是台北盆地的方向，尤其在仰角 20度以下受盆地光害形成的明顯的球狀光暈影響嚴重，天空表面亮度在18.5~19之間。夏季南方低仰角的透明度明顯較差， 30度以上則好很多。
冬季北方的平均暗度是全年中最暗的，有時午夜一點半至兩點後可以達到20.2~20.4間，並且一直維持到曙光來臨前。
春季約 9點半東北方暗度可達19.6，10:50至11點間達到 19.8至19.9。3、4月需要在低仰角觀看的目標不多，大部份星系的目標都可以在接近中天的附近觀測，所以在午夜有很多南天高仰角快接近天頂的觀測值，穩定的數值在19.8，午夜1點正南方45度仰角暗度約19.2。當星系群到了西南、西方在午夜2:30之前，只要仰角不低於50度觀測，天空都能維持在19.8左右。北方則在4:50分時仍有19.8左右。
而秋季9, 10月與春季類似的需要在低仰角觀測的南半球天體不多，西北方仰角較高處暗度約從9:50的19.5緩升到11:30的19.9左右，北方約在午夜達到20，東南方45度在4:00前平均仍有19.7。
以上僅是簡單說明，参照觀測記錄會更為準確。
Please note that the following charts represent the surface brightness of the background sky when observing deep space objects. Therefore, the measured values cover various azimuth angles and time periods. In addition, there is a one-month interval between each season, and only two month data are taken for each season for statistical analysis.
From the statistical charts, we can roughly observe the distribution of 300 deep space objects in different seasons (celestial sphere north-south distribution and elevation distribution during observations).
During the summer and winter seasons, there are noticeably more objects observable in the southern sky and at low elevations. Consequently, there is a larger difference in sky brightness between the observed values.
The suitable observation time in winter (based on reaching a dark level of 19.5 or higher from the northeast to northwest) is approximately 2 to 2.5 hours longer than in summer. In the summer, darkness level 19.5 or higher is reached after around 10 PM once the northern part of the sky becomes dark. In the winter, darkness level 19.5 can sometimes be reached after sunset around 8:30 PM.
There is no significant difference in the time it takes to reach darkness levels of 19.9 to 20 in all seasons, which is around midnight. In other words, after twilight ends, the dominant factor affecting the darkness of the sky is light pollution from urban areas.
The southwest direction has the highest light pollution at Anbu, followed by the south, which corresponds to the Taipei Basin. Especially at elevations below 20 degrees, there is a noticeable spherical halo formed by light pollution from the basin, resulting in a sky surface brightness between 18.5 and 19. In the summer, transparency is significantly poorer at low elevations in the south but improves considerably above 30 degrees.
The average darkness level in the northern sky during winter is the darkest throughout the year. Sometimes, it can reach levels between 20.2 and 20.4 after 1:30 AM to 2:00 AM and remains that way until dawn.
In the spring, darkness levels of around 19.6 can be reached in the northeast around 9:30 PM, and between 10:50 PM and 11:00 PM, they can reach 19.8 to 19.9. In March and April, there aren't many targets that need to be observed at low elevations. Most of the target galaxies can be observed near the zenith, close to the meridian, around midnight. Therefore, there are many observed values in the southern sky with high elevations approaching the zenith, with stable values at around 19.8. At 1:00 AM, the darkness level at a 45-degree elevation in the south is approximately 19.2. When galaxy clusters are in the southwest or west, as long as the elevation is not below 50 degrees, the sky can maintain a darkness level of around 19.8 before 2:30 AM. In the northern direction, darkness levels of around 19.8 can still be observed around 4:50 AM.
In autumn, similar to the spring months of September and October, there are not many objects in the southern hemisphere that need to be observed at low elevations. Darkness levels in the northwest at higher elevations gradually rise from around 19.5 at 9:50 PM to around 19.9 at 11:30 PM. In the northern direction, darkness levels reach around 20 around midnight, and at 45 degrees in the southeast, an average darkness level of 19.7 can still be observed before 4:00 AM.
The above description is just a brief overview, and referring to the observation records would provide more accurate information.
(一)包含所有方位仰角 Includes all azimuth and elevation angles.
（二）僅統計45度（含）以上仰角
Only statistics for elevation angles of 45 degrees or above
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