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Old 18-09-2020, 06:24 AM
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Slawomir (Suavi)
Going visual

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Image brightness and focal ratios

Hi all,

I'm starting to explore visual astronomy and due to a lack of experience I'm wondering whether in visual astronomy slower focal ratios mean dimmer images, given the same aperture and the same magnification (and the same skies).

For example a 10" f/4 and a 10" f/15 at the same location, as long as eyepieces are matched so magnification remains the same in both telescopes, my gut feeling tells me that image brightness should remain the same
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Old 18-09-2020, 06:35 AM
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mental4astro (Alexander)
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Same aperture, same magnification, same image brightness despite different f/ratios

Where things begin to change, say with reflectors (of whichever flavour you care for) is the condition of the reflective surfaces. Namely reflectivity drops over time. With refractors, needs similar/same glass types and coatings.
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Old 19-09-2020, 06:26 AM
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Slawomir (Suavi)
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Thank you Alex
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Old 19-09-2020, 06:45 AM
glend (Glen)
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Yeah, plus 1, on Alex's comment. My 8" Classical Cass at f12 has the same apparent brightness as my 8" f5 Newt.
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Old 21-09-2020, 05:51 PM
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Can't say I have done a specific test but I don't think that its exactly the same.

Telescopes don't focus all their light onto an eyepiece or a sensor. A lot goes past it hence the ability of a reducer to focus more of that missed light onto the sensor giving a wider field of view.

Eyepieces complicate the scene though. How much of the total light collected is focused into that eyepiece? Not all, how much though I don't know but some is missing the eyepiece or the sensor for sure. Perhaps not enough to make a big brightness difference but noticeable in imaging.

Greg.
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