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Thread: Difference between Telescope 400mm vs Telephoto zoom 400mm

  1. #1

    Default Difference between Telescope 400mm vs Telephoto zoom 400mm

    what's the difference besides price and aperture/diaphragm?

    ie: eg: a mirror lens can get 600mm focal length at f8, how does that compare to a 600mm telescope

    also http://www.skywatchertelescope.com/ProductsSP.html#MC90

    mirror lenses have the donut bokeh thing, that I know.

    so why would someone choose one over the other for astronomy photos.

  2. #2
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    Quote Originally Posted by loupgarou
    what's the difference besides price and aperture/diaphragm?

    ie: eg: a mirror lens can get 600mm focal length at f8, how does that compare to a 600mm telescope

    also http://www.skywatchertelescope.com/ProductsSP.html#MC90

    mirror lenses have the donut bokeh thing, that I know.

    so why would someone choose one over the other for astronomy photos.
    600mm is very short for an astronomical telescope. With telescopes the most important factors are the optical qualities related to chromatic abberation, coma and the overall effeciency of light transmission. Of equal importance is the "Light Grasp" of a telescope, that is how much light a telescope can capture. Focal ratio is a less important issue.

    For example, a fairly typical serious amateur astronomy telescope would be in the range of 900 to 2500mm focal length with an objective of between 200 and 250mm diameter. This gives f ratios of f4.5 and f10 respectively.

    A typical 600/8 telescope is a 3" lens/mirror objective, which is pretty small.

    The 300 Maksutov you linked to is a pretty little unit that's not really any use in astronomy except as a piggy-backed lens on a camera atop a telescope. It's too short for anything but fairly wide field of view shots of the sky. It would have around a 3.8 degree field of view (approx 188 arc minutes). Given that the moon is 30 arc minutes in diameter when full and that there are only a handful of objects in the night sky that are larger than the moon in diameter.

    Now to the differences between a telescope and Camera lens.

    Telescopes unless specifically labelled as an astrograph don't have flat image planes at focus. This non flat field leads to residual coma as the image progresses from the central portion of the field of view. The amount of coma is largely dependant on the type of optical design used in the telescope and also it's focal ratio. The longer the focal ratio the smaller the curvature of the image plane at focus.

    It should be remembered that the primary purpose for a telescope is visual observing with the human eye and as such eyepieces (oculars) are used to vary the magnification to allow for closer inspection of detail via the use of a larger image scale. One byproduct of using eyepieces is that the apparent field of view narrows with an increase in magnification.

    Camera lenses on the other hand are optimised for as flat an image plane at possible at focus as well as minimal chromatic abberations with low coma etc.

    There are a number of ways you can photograph astronomical objects with a telescope, the main ones are:

    Piggy-back: A camera and camera lens is attached a top the telescope and the telescope is guide the image.

    Afocal projection: A camera and it's lens are focused at infinity and the image is projected from the telescope eyepiece directly on to the camera's film plane. This method works very well for the moon and planets for novices and can be used with camcorders etc. The camera and telescope do not have to be physically connected. This method is useable with non SLR cameras.

    Eyepiece projection: The telescope's eyepiece is used to project the image directly on to the camera's film or CCD. No camera lens is used. The camera must be physically attached to the telescope via mechanical means.

    Telescope focal plane photography: Often misguidely called PRIME FOCUS by the illeterate in the astronomy community. Prime focus by the way is the term used to refer to the first optical lens/mirrors point of focus. Since the only telescopes (apart from a few wierd designs) that have this point accessable are REFRACTORS then the term should only be used there. For other telescope optical designs the convention is that the optical name is used, so a Newtonian telescope uses the Newtonian focus, a Cassegrain the Cassegrainian focus and so on.

    With focal plane photography the camera is placed at the image plane of the telescope, a point which usually co-incides with the location of the eyepiece.

    Other methods of astrophotography include "compression" "barlow lens" and "teleconverter" all of which use additional optical lenses to produce an image on film.


    Telescope showing both a piggy-backed camera and lens plus a camera at the Schmidt-Cassegrainian focus (often incorrectly refered to as "Prime Focus").

    Ian
    The Ang Moh from Hell
    Professional Photography - many are called, few are chosen!

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