Image brightness and focal ratios

[Slawomir]

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 :question:

[mental4astro]

Same aperture, same magnification, same image brightness despite different f/ratios :thumbsup:

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.

[Slawomir]

Thank you Alex :thumbsup:

[glend]

Yeah, plus 1, on Alex's comment. My 8" Classical Cass at f12 has the same apparent brightness as my 8" f5 Newt.

[gregbradley]

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.