看不见北极星，该如何将赤道仪对准极轴？

Agent001

原帖由 universe24 于 24-8-2007 11:00 PM 发表
焦比其实就是相机镜头上的光圈，所以如果你了解光圈的作用，那么对于了解为何观测星云会不利，会有一定的帮助。

那是因为进光量的不足？

光圈的功能就如同我们人类眼睛的虹蟆，是用来控制拍摄时，单位时间的进光量，一般以ｆ／5、Ｆ5或1：5来标示。以实际而言，较小的ｆ值表示较大的光圈。其中，f/1是进光量最大的光圈号数，光圈值的分母越大，进光量就越小。

光圈号数与进光量的关系

大光圈的镜头非常适合低光量的环境，因为它可以在微亮光的环境下，撷取更多的现场光，让我们可以用较快速的快门来拍照，以便保持拍摄时相机的稳定度。

除了考虑进光量之外，光圈的大小还跟景深有关。景深是物体成像后，在相片（图档）中清晰的程度。光圈越大景深会越浅（清晰的范围较小）、光圈越小景深就会越长（清晰的范围较大）。

左边是大光圈、右边是小光圈，可以明显的看出两张图的景深明显不同

那请问前辈，使用 Focal Reducer 的话，是否能改善这种情况呢？
以我的镜子为例：

Focal length = 1500mm
Aperture = 127mm
Focal ratio = 1500/127 = f/11.8

如果使用 0.5X Focal Reducer，
Focal length = 1500mm * 0.5 = 750mm
Focal ratio = 750/127 = f/5.9

Aperture 还是127mm，但 Focal ratio 从 f/11.8 减成了f/5.9。。。
Magnification 也会因此减半。。。



先感谢前辈在此耐心的指点。。。

[ 本帖最后由 Agent001 于 26-8-2007 11:08 AM 编辑 ]

Agent001

起了个大早，拍下弯月。。。请各位前辈多多指点。。





[ 本帖最后由 Agent001 于 10-9-2007 05:09 PM 编辑 ]

universe24

哦！不好意思，沒有看見你在#21的問題。

使用reducer把焦比減低，當然對進光亮有幫助，大大提高觀測星雲的可能性。但有一點要注意，不是每個reducer都用在不同的望遠鏡上，有些reducer只設計給折射望遠鏡用，有的設計給反射望遠鏡，所以在選擇reducer時要留意其設計目的。

Agent001

原来如此。。。 多谢！

Agent001

八月初七的月亮。。。

gradient

很好噢！继续加油！

Agent001

感谢前辈鼓励。。。
还得向前辈多多学习。。。

Agent001

向gradient前辈偷师。。。



Full size: 3072 x 2304

[ 本帖最后由 Agent001 于 23-9-2007 01:14 PM 编辑 ]

Agent001

Full Size

[ 本帖最后由 Agent001 于 25-9-2007 10:23 PM 编辑 ]

gradient

很好噢！比我拍得更好，继续加油！
真的很好噢！

Agent001

前辈过奖了。。。前辈的作品是我的学习目标。。。 我会继续加油！

opo25

拍得不錯哦，
看了我也有意思入門天文觀賞。
以后請多多指教。

Agent001

多谢鼓励。。 欢迎加入。。。 我们一起学习！

opo25

打算明年買進一個望遠鏡。
目前還在觀望當中，預算有限。

Agent001

由于小弟是新手，所以明白在挑选望遠鏡时所面对的疑惑。。
首先必须了解各种望远镜结构的利与弊。。。（主要有3大结构：refractor, relector & catadioptric)

REFLECTOR
--------------

  

Advantages:
- Reflector telescopes cost less to produce than other telescope types and offer the most telescope for the money. Reflector telescopes produce good images and are favored by astronomers who need the largest possible telescope to find distant, faint objects beyond our solar system (deep-sky objects). If you are thinking of giving a telescope that will allow room to grow in astronomy, a medium (four and a half inches or larger) reflector telescope is the least expensive way to go.
Disadvantages:
- Reflector telescopes are more fragile than other types of telescopes and need periodic readjustment of the telescope mirrors (a process called collimation) and, because they are open on the top (Newtonian reflector), the mirrors are exposed to dust and dirt and therefore need periodic cleaning. The reflector telescope is by no means fragile, but it is the least childproof telescope design. The telescope eyepiece, located at the top of the telescope, may also be more difficult for children to reach.




REFRACTOR
--------------


Advantages:
- Refractor telescopes are virtually maintenance free since the telescope tube is closed (not open as in a Newtonian reflector) and there is no telescope mirror to adjust. The refractor telescope is the most rugged telescope design and in the most expensive versions, a refractor will produce the brightest and sharpest image per inch of telescope. The refractor telescope is a good choice for children, not only because it is rugged, but also because the telescope eyepiece is located at the bottom of the telescope where it easier for small people to reach. Refractor telescopes are also easier to use as a daytime telescopes (with an image erector) than reflector telescopes.
Disadvantages:
- The refractor telescope is the most expensive telescopes per inch of telescope size; beyond three inches, the refractor goes up very quickly in price. If you buy a beginner a small refractor, be advised that the beginner may quickly outgrow it if they stay with astronomy for long.


CATADIOPTRIC
-----------------

  
Advantages:
- A Schmidt Cassegrain telescope offers some of the advantages of a refractor - durability and less maintenance - but at price that is lower. The main advantage of an Schmidt Cassegrain telescope is portability - even a five inch Schmidt Cassegrain is small enough to carry on the front seat of your car. A Schmidt Cassegrain is regarded as the best all-purpose telescope for an astronomer who will be mixing visual observing with photography.
Disadvantages:
- A Schmidt Cassegrain telescope has a mirror in the front that partially obstructs the field of view and causes some light to be lost. For this reason, you may notice a slight darkening at the center of the field of view, though the effect on performance is negligible. Schmidt Cassegrain telescopes do not produce images quite as bright as the best reflector telescopes or an image quite as sharp as the best refractor telescopes, but it's versatility is better than either.


Question:
- What are Maksutov-Cassegrain telescopes?
Answer:
- Maksutov-Cassegrain telescopes, like the Schmidt-Cassegrain telescopes, are catadioptric telescopes (a telescope with lenses and mirrors). The difference between the Scmidt Cassegrain and the Maksutov-Cassegrain is in the shape and size of the corrector plate (lens) at the top of the telescope. The Maksutov-Cassegrain uses a heavier, more complex corrector plate.


Question:
- What are the advantages and disadvantages of a Maksutov-Cassegrain telescope?
Answer (Advantages):
- The Maksutov-Cassegrain telescope corrector plate allows for very fine correction of its optics. The Mak is capable of superb image quality, second only, perhaps, to the finest refractor. The Maksutov-Cassegrain is also a great design for smaller telescopes and is very popular for spotting scopes.
Answer (Disadvantages):
- The corrector plate on a Mak is heavier and more costly to produce. Maksutov-Cassegrain larger than 7 inches are, therefore, rare.



明白自己所要达到的目的（观测行星或深空天体，天文摄影等。。。）及预算，再考滤它的便携性。。
需要了解的是我们应该选择一个最适合自己的望远镜， 而不是最好的。。。

希望这能让各位和我一样想入门的新手有一个大概的概念。。。 也请各位前辈分享经验。。

opo25

很好的分享，
从中国的网站看到，很多人建议新手用refractor 比较好。
但是refractor 的价格很贵。

Agent001

第一次尝试拍摄木星。。。


Full size: 1581x1189

lonely2

请问楼主的望远镜多少钱

Agent001

是二手的镜子.. 大约3k..
包括10mm,25mm和32mm目镜及一些配件..

Agent001