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  1. #1
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    Default Focal Reducer Questions



    I understand the main use of a focal reducer is to provide telescopes with longer focal lengths a short focal length allowing them to view objects requiring a larger field of view.
    I currently own a 127mm with a focal length of 1540. I have been looking at possibly purchasing a focal reducer, which reduces my focal length by 0.63. With the focal reducer, my telescope would have a focal length of 970.2.

    I have heard that a focal reducer will give me brighter objects. I personally don't understand this concept and was wanting some clarification of this.

    Consider, the use of my telescope with two different eyepieces (both FOV of 68 degrees) that give me the same magnification. One uses with the focal reducer and one without it.

    Case #1
    1540mm Focal Length without the reducer / 13mm EP = Magnification of 118.5
    Case #2
    970.2 Focal Length with the reducer / 8mm EP = Magnification of 121.2

    If I'm looking at a DSO, will it be brighter in case #2 or case #1, or will it have the same brightness?

    Some people stated that with the short focal length the light is in focus closer to the front lense. In case #2, when I am adjusting the focuser will it be closer to the front lense than in case #1?

    Any technical information on the focal reducer and how it works would be apppreciated.
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  2. #2
    KathyNS's Avatar
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    Default

    Focal reducers are primarily used in imaging. A prime focus photographic image will be brighter because of the lower magnification.

    However, you are talking about changing the eyepiece to maintain the same magnification. If you maintain the same magnification, then you get the same brightness. For visual use, brightness is determined by aperture and magnification only. In your example, the two views would have the same brightness.

    If you kept the same eyepiece and added a focal reducer,then the magnification change would increase the brightness.

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  4. #3
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    Default

    Quote Originally Posted by KeithBC View Post
    Focal reducers are primarily used in imaging. A prime focus photographic image will be brighter because of the lower magnification.

    However, you are talking about changing the eyepiece to maintain the same magnification. If you maintain the same magnification, then you get the same brightness. For visual use, brightness is determined by aperture and magnification only. In your example, the two views would have the same brightness.

    If you kept the same eyepiece and added a focal reducer,then the magnification change would increase the brightness.
    That's what I thought. I just wanted someone with more experience than me to verify it.

    Thank you.
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  5. #4
    Phil Leigh's Avatar
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    the whole and only real point of an FR is to give you wider and brighter views... As Keith said this is primarily (but not exclusively) for imaging. there's simply no point in installing an FR and then putting in an eyepiece that gets you back to where you started... Because it will actually be worse because now you have the additional glass elements of the FR that Are now doing nothing to help you and can only add distortion.

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