FAQ - Filter 一問

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funoooo
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FAQ - Filter 一問

文章 funoooo » 週四 01 10月, 2009 11:02

希望總結所有 Filter 及比較功效. 請各位師兄指教.

1. H-alpha narrowband 7nm CCD filter
  • - is designed exclusively for imaging of emission nebulae, but it also works great for capturing luminance data of those nebulae with monochrome cameras.
    - This filter provides 90% transmission at the H-alpha line of 656.3nm and has a very narrow bandpass of 7nm.
    - Take high-contrast CCD images of emission nebulas in full moonlight or in extreme light pollution with the Hydrogen-Alpha Narrowband filter.
    - 7nm H-Alpha Filter reveals subtle nebular details usually washed out by contrast-killing broadband light.
    - Imagine capturing brilliant, high-contrast images of nebulas in full moonlight or in the thick of downtown light pollution.
2. H-beta narrowband 8.5nm CCD filter
  • - The H-beta is a filter for visual observation, in particular with instruments of larger aperture.
    - This filter should be used on telescopes with a diameter of 8 " (20cm) or larger.
    - Visual observation (dark skies): Very good, but only suitable for a few objects.
    - Visual observation (urban skies): Unsuitable
    - Film photography: It depends, very long exposure time
    - CCD photography: Good, when used with an additional IR-block-filter
    - DSLR photography (original): Verry good, but only suitable for a few objects
    - Webcam / Video (Planets): Unsuitable
    - Webcam / Video (Deep Sky): Unsuitable
    - This filter is so specialized, it is really only good for two objects; IC 434, the emission nebula surrounding the Horsehead Nebula in Orion, and the California Nebula.
    - The filter emphasizes the glow of both of these nebulae, turning them red.
    - In the case of IC 434, the red outline around the actual Horsehead itself (B33) is what makes it visible.
    - The California Nebula is similar in that it is just barely visible without the H Beta. With it, the California "shape" becomes clearly visible.
3. S-II narrowband 8nm CCD filter
4. O-III narrowband 8.5nm CCD filter
  • - Delivers the absolute highest contrast views of many diffuse and planetary nebula.
    - For many objects, the O-III reveals a level of intricate detail that rivals the best deep sky photographs.
    - The majority of nebula, especially The Veil, Lagoon (M8), Swan (M17), Ring (M57) and Dumbell (M29). With a large scope, you can even view the famous 'Pillars of Creation' in M16 (Eagle).
    - Combine with UV/IR filter for the ultimate in O-III deep sky imaging.
5. UHC-S filter
  • - excels at delivering a high-contrast and natural view of emission nebula - without excessive dimming and loss of background star fields.
    - The perfect filter for viewing emission nebula from light polluted skies, or for boosting the contrast of nebula from dark sky sites.
6. Fringe Killer filter
  • - A minus violet filter to reduce chromatic aberration in achromatic refractors.
    - Improves detail of almost all objects viewed through fast achromatic refractors
    - Very high optical quality allows filter to be used anywhere in the optical path
    - Additional blocking of infrared light makes this a good general filter for web cam and digital camera imaging
    - Achromatic refractors, especially fast focal ratio models, suffer from significant amounts of "chromatic aberration" - false color.This is generally experienced as a violet halo around bright objects, particularly planets, bright stars and the Moon. The halo is caused by the fact that not all the wavelengths of light come to focus at the same point.
    - Although it is possible to ignore the halo, the real problem is that the unfocused violet and blue light is spread all across the object you are viewing, robbing it of detail.
    - The most common "cure" for this is to simply block the violet and blue light waves. The problem is, most minus-violet filters introduce their own resolution-robbing aberrations and they typically block significant "good" light, further reducing detail.
    - Fringe-Killer filter - Almost all violet light (light below wavelengths of 450nm) is completely blocked, while blue light (450nm - 480nm) is attenuated by 50%.
    - all other light is transmitted at higher than 95% efficiency
    - suggests that the Fringe-Killer combined with Red filter is a good Hydrogen-Alpha filter. This combination has a 95% transmission rate at the H-Alpha wavelength and yet costs very little. H-Alpha filters are great for imaging emission nebula in great detail, though a smaller pass-band would produce better results.
    - Conclusion
    In short, this filter transformed the views through this f/5 achromatic refractor into what I would expect to see through an f/12 achromatic refractor. A fast achromatic refractor will never be the best choice for planetary views, but Fringe-Killer filter can certainly make it a good choice. If you've been looking for something to improve your achromatic refractor - this is it!
7. Moon & Skyglow (Neodymium) filter
  • - brings an entirely unique approach to planetary contrast enhancement and light pollution reduction.
    - filters out a few specific wavelengths from streetlights as well as skyglow from the Moon.
    - also boost color contrasts by isolating the Red, Green, and Blue regions (so-called RGB enhancement).
    - The result is a filter that leaves natural colors mostly intact, but significantly enhanced.
    - The filter's high efficiency preserves image brightness, and many Galaxies, Nebulae and Globular Star Clusters are improved due to the reduction of skyglow from artificial lighting.
8. RGB (3 pcs) filter set
9. UV/IR block (L filter)
  • - This filter is a must for all digital imaging with Digital Cameras, CCD Cameras, and modified WebCams.
    - Also useful for protecting valuable H-Alpha filters from heat stress and damaging IR (Daystar, etc).
10. Contrast-Booster
  • - Based on the popular Baader Moon&Skyglow filter, the Contrast-Booster takes contrast enhancement and skyglow reduction to the maximum level possible from a filter.
    - Not only does this filter deliver a significant enhancement of planetary and lunar contrasts, but it is designed to also totally eliminate the damaging blue defocused haze present in achromatic refractors.
    - Gone are the annoying blue halo and obscuring haze that surround and smear resolution and contrast. Subtle contrasts and colors are boosted, while displaying the full resolution your scope is capable of.
    - The special coatings enable over 95% transmission across key portions of the visible spectrum (including H-Alpha).
    - This results in a filter that retains the colors and color contrasts in tough subjects like Jupiter, without excessive image dimming. Cloud bands retain their ruddy red hues, but now become more striking in contrast. The Great Red Spot finally becomes easier.
    - Planets and Lunarscapes are set against a jet black sky. The Contrast-Booster is a must for Solar observing.
    - As many users have found, this is a great filter for Mars! The views of Mars with all telescopes (including reflectors and apochromats) benefit from the RGB filtration plus blue blocking.
    - Combined with the UV-IR-Cut Filter, the Contrast-Booster makes the ideal filter set for digital imaging.
11. LPS-P2 filters
  • - manufactured by Hutech Corporation
    - is a thin-layered interference based light pollution suppression filter.
    - is designed to suppress the various common emissions lines generated by artificial lighting, yet allow important nebula emission lines to pass.
    - in the enhanced contrast of astronomical objects, especially emission nebulae such as the California nebula.
    - are designed for balanced color transmission. This allows color photographs to be taken of broadband astronomical objects, such as stars, galaxies, and clusters, to be taken with minimal color cast.
如有其他 filter 漏了, 請代追加上去.
最後由 funoooo 於 週五 09 10月, 2009 15:52 編輯,總共編輯了 12 次。

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some useful web-site

文章 鄧登凳 » 週四 01 10月, 2009 12:39


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funoooo
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文章 funoooo » 週五 09 10月, 2009 16:13

最後由 funoooo 於 週一 14 12月, 2009 21:33 編輯,總共編輯了 1 次。

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MANDII
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文章 MANDII » 週五 09 10月, 2009 16:18

哇,几时发表左呢个文章都5知咖,真系一揽无违,值得收藏 。

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funoooo
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文章 funoooo » 週四 22 10月, 2009 13:16

Usually the filter is screwed onto the bottom of the eyepiece.

see www.galacticfool.com/astronomy-filters/

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文章 funoooo » 週五 27 11月, 2009 15:10

资料整理-浅谈光害滤镜 - from: 牧夫 northwolfwu

http://www.astronomy.com.cn/bbs/viewthr ... tid=124473

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文章 funoooo » 週一 14 12月, 2009 21:12

Very good review of the best filters to use (UHC/Ultrablock, OIII or H-Beta) for some common nebulas:

http://www.cloudynights.com/item.php?item_id=1520

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文章 鄧登凳 » 週一 14 12月, 2009 22:48

funoooo 寫:Very good review of the best filters to use (UHC/Ultrablock, OIII or H-Beta) for some common nebulas:

http://www.cloudynights.com/item.php?item_id=1520
That link discusses filters for visual astronomy. For the eyes, colour balance is not an issue as colour vision is significantly limited during deep sky observation.

For astrophotography, it is a completely different story.

See http://www.astropix.com/HTML/I_ASTROP/FILTERS.HTM

I have both the Hutech IDAS LPS P2 filter and the Celestron LPR filter. For visual use, Celestron LPR gives a darker sky in light polluted area. For photographic use, LPS P2 performance in terms of colour is better.

The following is the the Hutech own web site
圖檔

The two graphs explain the kind of difference in terms of the multipass technology by Hutech.
圖檔
as compared with Astronomik
圖檔

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文章 鄧登凳 » 週二 09 3月, 2010 09:58

Just come across this post in http://photography-on-the.net/forum/sho ... p?t=826938. I consider it well written.
Location: Georgia, USA
Posts: 113
IMAGE EDITING OK Re: Which filter for light pollution ?

--------------------------------------------------------------------------------

Hi Karlo,

There are a handful of broad band light pollution filters that are popular today and they fall into two categories - the Neodymium filters and, for lack of a better term, the "multi band pass" filters.

Neodymium filters are sold with the Baader, Astronomik and Orion names. Multi band pass filters are sold by Hutech and Orion. The primary differences you will notice are in the color balance of images made through the filters, and to a lesser extent the difference in the amount of sky glow that the two types remove.

Neodymium filters (Baader UHC, Atronomik CLS, and the original Orion Sky Glow filter, for example), are quite effective at blocking the common emission lines associated with light pollution, but they tend to produce images with a strong blue-green cast when used with unmodified DSLR's. If your DSLR is modified to remove the IR blocking filter over the sensor, the color balance with a neodymium filter will be more natural. These filters commonly are sold in 1.25" and 2" sizes to thread onto the prime focus adapter that connects the camera to the telescope. Astronomik also sells the CLS filter in a clip-in mount that installs into the body of the DSLR between the mirror and the shutter. With this arrangement it is possible to use the filter with normal camera lenses as well as telescopes, but the Canon EF-S lenses will not work with a clip-in filter because of the length of the rear section of the lens cell.

The Hutech IDAS-LPS and the newer Orion SKyglow Imaging filter fall into the multi band pass category. They pass more wavelengths across the spectrum while still blocking the common light pollution wavelengths, resulting in a more natural color balance when used with an unmodified DSLR. I have and Orion Skyglow Imaging filter and I also have a neodymium filter. I find that the Skyglow Imaging filter allows more light to pass than the neodymium filter, resulting in shorter exposure times, while still eliminating the ugly orange cast caused by light pollution, but that it cuts more from the visible red wavelengths of emission nebulae than the neodymium filter. I have never used the Hutech light pollution filters. The Hutech IDAS LPS costs substantially more than the Orion filter and is available in a clip-in style, while the Orion filter is only available in 1.25" and 2" sizes.

Lumicon and Celestron also sell light pollution filters. I suspect that they are neodymium filters though I don't know that for sure.

Sorry if you wanted me to tell you which one to buy - they all have their advantages and disadvantages. I think if I was going to buy another LP filter now I would get the Hutech IDAS LPS with a clip-in mount, because 1) I dont have one yet, 2) they get good reviews from people who use them, and 3) the clip-in concept is interesting. Besides performing as an LP reduction filter, it protects the sensor from dust, insects, etc. during those long exposures.

Don

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Poor man's light pollution filters for cameras

文章 鄧登凳 » 週二 09 3月, 2010 10:55

釹鐠化合物濾鏡 didymium filters 一般稱為紅光強化濾鏡, 如 KenKo Red Didymium Enhancing filter或Hoya red enhencing filter, 是有減輕光害影響的作用。見圖:
圖檔

詳細內容: http://theastronomybum.blogspot.com/200 ... e-rol.html

相比同大小的相機鏡頭前濾鏡, didymium filter的價錢是LPS-P2的四分一至七分一。

[註: 釹 neodymium也用於天文濾鏡(見上一回覆), 但找不到鏡頭前用的釹濾鏡, 也找不到1.25"或2"接口的釹鐠濾鏡]

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文章 roh » 週二 09 3月, 2010 12:24

Hello Deng sir, Is it the one (didymium filter) like this ...

http://www.hkastroforum.net/viewtopic.php?t=16621
funoooo 寫: ...
十五、紅色增強鏡:該鏡為一種高檔濾鏡,由溶入昂貴的稀土元素製成的進口光學材料製造。它能使光譜中570-600nm之間渾濁的黃光和橙色光完全吸收,同時使紅光順利通過。因此它可以顯著提高景物中紅色、淺紅、棕紅物體的色彩飽和度,使之更加鮮明、艷麗、濃烈。例如使秋天的楓葉更紅,草莓和西紅柿更加誘人垂誕;日落的氣氛更為濃郁;婚禮或聖誕更添喜慶……。該鏡與偏振鏡合用時可使藍天色彩更濃、更美麗。使用該鏡需增大1/2-1級曝光量。
...
http://item.taobao.com/auction/item_det ... l?cm_cat=0

Thanks.

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文章 鄧登凳 » 週二 09 3月, 2010 12:54

roh兄, 那應是釹鐠玻璃濾鏡了。

另想多說一點兩種濾鏡的分別:

(一) 吸光式濾鏡: 如釹濾鏡(Neodymium filter)及釹鐠濾鏡(Didymium filter)是以化學物質來吸收某些光譜的頻率。由於吸光是物質的化學特性, 是不可以調整的, 所以只能選用不同的物質, 但不可能按要求生產容許多頻譜通過(multiband)的濾鏡。但由於是物質吸光作用, 所以吸光特性不受光線角度的影響, 因此用於相機鏡頭, 可以由廣角至遠攝鏡都可用。

(二) 光波干擾式濾鏡: 如IDAS LPS-P2, 是以不同折射率的電介質(dielectric material), 使入射光和反射光產生干擾而互相抵消, 由於可用不同厚薄的多層電介質, 因此可以選擇的設計讓多個光譜通過(multipass)。但由於干擾是受入射光角度的影響, 所以通過的光受入射角影響。(參考:
http://en.wikipedia.org/wiki/Interference_filter)

因此, 這類濾鏡不能用於廣角鏡。下圖的星流跡說明:
圖檔
(from: http://www.nightscapes.net/techniques/L ... ilter.html)

而IDAS 廠方也指他們的LPS-P2濾鏡, 不適用於短於100mm焦距的鏡。(見http://www.sciencecenter.net/hutech/idas/lps3.htm)

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文章 Wah!! » 週二 09 3月, 2010 13:00

"Interference" 在光學上一般會翻譯為 "干涉" :D

吸光濾鏡雖然在吸收頻譜不會受光的入射角度改變而改變, 但是卻會因為大入射角度的光經過較厚的濾鏡而被濾掉更多, 會形成(比沒有濾鏡時)較明顯的周邊減光現象.

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文章 鄧登凳 » 週二 09 3月, 2010 14:20

Wah!! 寫:吸光濾鏡雖然在吸收頻譜不會受光的入射角度改變而改變, 但是卻會因為大入射角度的光經過較厚的濾鏡而被濾掉更多, 會形成(比沒有濾鏡時)較明顯的周邊減光現象.
小弟的腦袋不靈光, 所以較喜歡用實例去考慮:

APS-C相機, 18mm廣角鏡, 相角至相角對角線的視角為73.8度, 即光入射角最大為36.9度。

在玻璃內的出射角度, 可用司乃耳定律(Snell's Law)算出, 約為23.4度。那最大角度的光束, 所行的距離為直射光束的1.09倍[1/cos(23.4)]。

那如濾鏡的在1倍距離可吸90%不想見的光, 也吸10%想要的光, 那在1.09倍距離下會吸98.1%不想見的光, 又吸了10.9%想要的光, 這個差別是愚見認為不是「明顯」, 但我也不打算干涉Wah!!兄認為是明顯的看法 :D

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文章 Wah!! » 週二 09 3月, 2010 14:58

參考你俾既圖:
圖檔

根據網頁所述, 視場有 150度, 即是最大入射角為 75度.
假設玻璃折射率為 1.5
則最大入射角的光束在玻璃內的出射角為 40度, 透過玻璃的距離將會是直射的 1.3倍, 即是最多可以達到30%厚度差異.
若濾鏡本身是接近100%通透, 這個減光現象的確是不明顯.
若濾鏡本身的透光率不是極高, 這個減光現象便很容易被察覺.

即使用18mm廣角鏡的情況, 不想見的光也可以有 90% 與 98% 吸光率的不同, 這個變化也應該可以在相片中找到的.

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