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  1. #1
    Planck's Avatar
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    Question Planetary observation- focal ratio- (dobsonian VS refractors)



    Hello I am new in this passion and I am in the process of buying a telescope mainly for planetary observation. I read a lot of posts but still have a doubt. It is usually said for planetary observation you need magnification. So buy dobsonian for deep sky (because of aperture) and refractors for planetary because of magnification. Ok but when I look to a dobson 12" for example they can reach very high magnification and high resolutions so I don't understand why they would not be adequate or even better than refractors. From what I understand the preference is given thanks to the focal ratio where long focus (example f/5 of dobsonian) is worst than short focus (f/10) of refractors for planetary observation. So to summarize I don't understand this focal ratio preference as I say: with dobsonian you have higher magnification, better light gathering and higher resolution so they should be better also for planetary observation.
    Let's take a real example

    dobson 12"
    max useful magnification 600x
    resolution 0,38"
    focal ratio 5


    refractor 4"
    max useful magnification 200x
    resolution 1,15
    focal ratio 9

    Roughly same range price
    Orion Apochromatic refractor AP 100/900 ED SkyViewPro
    Meade Dobson telescope N 305/1524 12'' LightBridge

    I would say that even for planetary observation (let's take a difficlut planet to see details: mars) the above indicated dobsonian should be better than the refractor but I fear that my considerations are forgetting some aspects and are not correct. Can somebody clarify that to me ? thanks
    Last edited by Planck; 02-25-2012 at 01:54 PM.

  2. #2
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    You rarely can or need to go higher than 200x, so mag isn't really a factor. As far as which scope, i'd go with the refractor. Refractors are known for their contrast and sharpness and for planetary viewing that'd be optimal. The dob can give you some good views also, but all that aperture means that Jupiter will be really bright, and either you would have to put in 1 or 2 moon filters, or stop down the aperture.
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  3. #3
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    Where the refractor wins is "scatter". Scatter makes the area surrounding bright objects seem to have some illumination. Lack of scatter makes the edge of the planet be absolute completely black.

    When looking at object contrast right at the edge of being seen, this lack of scatter gives the refractor an edge.

    The 'effective' refractive index of a mirror is -2.0 whereas the 'effective' refractive index of a lens is +0.5. Thus a bump on the surface of a refractor has to be 4X as big as a bump on the surface of a mirror before it "damages" the converging wavefront. Thus, it is easier to polish a lens to a given surface criterior than it is a mirror. Add to this that lenses are invariably spherical whhile mirrors are almost always aspherical. The asphericalization of the mirror adds more of those bumps creating scatter.

    On top of the bumpy surface of the mirror, you apply a bumpy thin layer of aluminum so the light reflects off towards focus.

    It is by this analysis that refractors give better views of detail right at the edge of observability.

    However, reflectors are invariably bigger, resolve finer, and most fo the time can see detail a refractor cannot, simply because A) there is more light, and B) it is focused to a smaller spot, making the contrsting feature that much more visible (than in the smaller refractor).

    Refractors also do not have the central obstruction that reflectors have. When comparing resolution you should take the linear size (minor diameter) of the secondary and subtract it from the diameter fo the primary mirror and use that resulting number as the size of the refractor that would equal the reflectors performance when looking at contrast at the edge of visibility.

    Focal length does not play a part in this other than allowing the secondary mirror to be smaller.

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  5. #4
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    Thank you both for your clarifying answers. There are enough reasons to go in the refractors direction then ;-)

  6. #5
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    Yes, the basic rule is: aperture for visual observing, and focal ratio for photography.

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  7. #6
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    Thanks Keith but does that mean that in the above given example between the 2 types iof telescopes (and if I am not interested in photography) I would see more details with the dobsonian even for planets? From Celestron/Mitchalsup answer I understand that I would see more but actually because of scattering, contrast and light excess even for visual observing the aperture would not allow me to have good planetary observation.

  8. #7
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    Personally, for visual use, I will take a 12" anything over a 4" anything any day of the week. The difference in magnification is sufficient to warrant the bigger scope for planetary observing, and the difference in light-gathering power is sufficient to justify it for DSOs.

    While I don't have any information that is contrary to what Mitch was saying about scattering, I have never observed that effect. I suspect it is minor compared to the difference in resolution.

    But, some audiophiles like CDs and some still prefer LPs. You have to make up your own mind. Go to a star party and compare scopes.

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  9. #8
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    I will second Keith's suggestion to go to a star party and see for yourself. For example, I am a refractor guy who got rid of his reflector to purchase two refractors. The views are different, the operation is different, so a lot of it comes down to personal preference.

    Then again, I am mainly an AP guy now so...... :-)

    Allan

    PS. Keith, you left out my SACDs and DVD-As!!!

  10. #9
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    Three aspects of a telescope to consider:

    1. Aperture is a measure of light gathering and resolution ability .... the bigger the aperture the greater the light gathering and resolution abilities.

    2. Focal length is a measure of image size ... the longer the focal length, the larger the image sizes produced

    3. Focal ratio is a measure of image brightness .... the lower the focal ratio, the brighter the image.

    You mentioned a 12 inch f/5 DOB and were wondering if it is as good for viewing planets as a smaller 4 inch f/9 refractor.

    A f/5 12 inch DOB has a focal length of 60 inches (1524mm). A telescope with a focal ratio of f/5 will have a bright image. A telescope with a focal length of 1524 mm will have an fairly good image size. Thus, a f/5 12 inch telescope will have both a bright image and a decent image size... great for viewing planets Compared to a f/9, 4 inch telescope....

    Aperture: the 12 inch telescope will have 9 times the light gathering capability of the 4 inch telescope and far greater resolution power

    Focal length: the 12 inch telescope has a focal length of 1524mm and the 4 inch has a focal length of 914 mm.... thus for any given eyepiece the 12 inch scope will have the larger image

    Focal ratio. The f/4 telescope will have a brighter image than the f/9 telescope for any given eyepiece

    You can vary image size by using the appropriate eyepiece. The fact of the matter is that between the two telescopes you mentioned, the f/5 12 inch DOB will always have a larger and brighter image.... in other words it simply outclasses the smaller telescope and will be great for viewing planets as well as deep space objects... that's one reason people buy large DOBs.


    Now if you want a real planet buster.... go for a f/4, 16 inch DOB..
    SXINIAS

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  12. #10
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    Thanks a lot for your answers! more clear now.

 

 
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