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  1. #1
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    Default Fast Focus Short Tube Newtonian Telescopes



    I would just like to inform everyone that if you want a scope for planetary viewing it's best to avoid the Fast Focus Short Tube Newtonians.But for deep space object viewing the short tubes do a fine job, I was at a star party recently and people kept wanting to view thru my 6 inch Short tube Chinese Newt that I bought off E-Bay, and there were a lot of much larger and more expensive scopes around me, so I got curious and went and viewed thru some of the larger scopes and to my surprise my 6 inch Newt was doing as good as some of the larger and much more expensive scopes on some of the deep space objects and I was only using a 25mm low power EP.

    let me know if i am misguided.
    cheers

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  3. #2
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    Any focal ratio reflector will work for viewing either planets or deep sky objects. The large focal ratio reflectors, say around F/8, have a smaller central obstruction and hence provide a little more contrast than a low focal ratio reflector, say around F/4.5. Also they can use fairly inexpensive eyepieces to provide a sharp view. But to get a very wide field of view, required for many deep sky objects, one has to use a reflector with a low focal ratio. This is because of limitations on commonly available eyepiece focal lengths and also eyepiece barrel diameter. Eyepieces that work well with low focal ratio reflectors will cost much more than the ones that work with large focal ratio telescopes. One should use whatever he or she has available and not worry about focal ratios too much.
    As the aperture increases the size of a large focal length reflector becomes cumbersome. Hence the popularity of the low focal ratio dobsonians. Also most people prefer to observe with their feet on the ground, not up on a ladder.
    Assuming that all telescopes had acclimated to the temperature of their surroundings at the star party you had participated in. And assuming that the air temperature was not dropping rapidly the similar views that you noticed between the larger telescopes and yours is entirely due to atmospheric turbulence. As the aperature increases a telescope becomes increasingly sensitive to the seeing. On a night where the stars scintillate, or twinkle, slightly a ten inch telescope might present a pleasing view at 200x. That same night a four inch might get by with using 175x and still show an image of the same steadiness. The image in the four inch will be much dimmer and not have the fine detail hidden within but it will look similar. Perhaps many of the large telescope users were using a higher magnification in the hope of getting split second detailed views if the seeing steadied periodically. I have found that when the seeing is poor and one compares a four, thirteen, and twenty five inch scope at their lowest power that the largest scope will still provide more detail and a brighter image than the smaller telescopes. All the above applies to either dark or light polluted skies. Bigger always reveals more.

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  5. #3
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    You've ignored basis of photography - for a given film speed your shutter speed is based on ~aperture~, not just the diameter of the lens. Aperture is the ratio of focal length over lens diameter. IOW, just because you have a huge lens/mirror doesn't mean more light is coming through the eyepiece. The only way to get more light through the eyepiece is with a numerically smaller aperture, i.e. a focal length closer to the lens diameter.

    A shorter focal length at low magnification will give a steadier image because it takes more angular deviation to slew the image from one side of the lens to another. So a short fast reflector can be easier for casual observing because there is less blur from minor vibrations.

    A 4" f/4 with a 24mm EP will always deliver a brighter image to the eye than a 10" f/11. Especially when you take full advantage of that 10" with a high mag eyepiece.

    People don't build fast Dobsonians because it's awkward to build slow ones. They do it because it lets them see dimmer objects. The Newtonian design is well-suited to fast apertures as the mirror diameter gets larger. There are plenty of other designs well-suited to long focal lengths if that's your design goal.

    In closing, a 25" f/5 is not going to give a brighter image at the eyepiece than a 6" f/5. But it can show more detail and most likely has a much nicer, much more expensive eyepiece attached with deeper eye relief. It still isn't a brighter image, and especially not when the owner uses a Paracorr or Barlow.


    MODERATOR'S NOTE:
    Just so we don't confuse people starting out.

    Astronomy Definitions:

    1. Aperture is the diameter of the primary mirror of a telescope or lens in the case of a refractor


    2. Focal ratio is the focal length of a telescope divided by its aperture
    Last edited by sxinias; 01-03-2012 at 03:57 PM.

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  7. #4
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    Default

    learned something here....thanks guys
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    Quote Originally Posted by OneGear View Post
    You've ignored basis of photography - for a given film speed your shutter speed is based on ~aperture~, not just the diameter of the lens. Aperture is the ratio of focal length over lens diameter.
    When talking with photography people the word apeture means focal/ratio and represents the geometrical convergence of the light cone towards focus. Since they won't or can't change, I use the word-pair relative-apeture to alkw ith thses people.

    Because I come from the world where apeture means diameter of the entrance pupil (i.e. telescopes). So, in this context the word-pair linear-apeture is appropriate.

    By using these pairs of words, both photography people and telescope people can understand what each other is tryinig to convey.

    Thus, I agree with the moderators correction. But attempting to get photographers to use the word-pair focal/ratio is fruitless at best.

  9. #6
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    With all due respect, the word is appropriately used in both circles.

    A telescope generally doesn't have an adjustable aperture, so the "aperture" as an opening and "aperture" as focal ratio will not change. Therefore, in that context "What aperture are you using?" refers to the diameter of the lens/mirror.

    Camera lenses typically have changeable apertures, so the question "what aperture are you using?" by convention refers to the focal ratio rather than the diameter of the lens itself.

    The issue that comes up is when people with zero knowledge of the science of photography refer to the aperture as an opening as the sole determining factor of how much light you see at the eyepiece with no regard to focal length. Having spent much time as a photographer, I have practical experience that contradicts that assumption.

    In reality, it's the focal ratio that determines the brightness of the image, the light-gathering ability of the system.

    I'm not sure it's "photographers" that need to change. I read complaints that magnification is a spec that is often used for telescope marketing but gives no real indication of light gathering ability. But do not ever read complaints that the term "aperture" referring to the lens/mirror size is not an accurate indication of light gathering ability. When in reality, the focal ratio is a more reliable indicator. Mirror/lens size merely indicates the highest practical magnification. Which is why many people will mention the focal ratio as well as the lens/mirror size when discussing their scope.

    I'm not here to argue. But physics doesn't somehow change just because the aperture is fixed.

  10. #7
    MitchAlsup's Avatar
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    Quote Originally Posted by OneGear View Post
    With all due respect, the word is appropriately used in both circles.
    But what we need is a single set of words that retain their meanings in both domains to avoid confusion.

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    Quote Originally Posted by MitchAlsup View Post
    But what we need is a single set of words that retain their meanings in both domains to avoid confusion.
    Why? Because you own a telescope and don't know anything about photography? I would submit that you are the one mis-using terms without understanding why. But I know that many astronomers and telescope users use focal ratio with lens/mirror diameter when talking about different scopes. It's because they understand that both data points are relevant - either one alone tells you very little about the capabilities of the system.

    Rather than complain that those who have more practical experience in the science of optics are "improperly using terms," consider a knowledge exchange. Astronomy and photography are two optical sciences that have long been allied. Telescopes are *very* simple lens systems compared to what the amateur photographer with a manual film camera uses off-hand.

    And IMHO, it's not the photographer/astronomers who are lacking in knowledge.

  12. #9
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    Have to agree, being a photgrapher. Its been debated a long time . Terminology is good but understanding it is better, way better.

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    Quote Originally Posted by OneGear View Post
    Why? Because you own a telescope and don't know anything about photography?
    No, because I own both astronomy gear and photography gear and talk to both kinds of people. Clearing up the confusioin between the groups is enhanced when there is a common vocabulary in use.

    I often have to translate between an astronomer on my right trying to get a point across to the photographer on my left. If a common set of terms wer in use, the two could talk directly without me as a translator.

 

 
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