William Optics Adjustable 0.8x Reducer / Field Flattener 73R for Z73 Telescope

[ninjageezer]

If my scope is F5.9 what will the speed come down to by fitting the above  ?

I had a look but cannot find any info or formulae.

[symmetal]

The focal ratio with the reducer will be 5.9 x 0.8 = f4.7

The reducer actually reduces the focal length by 0.8 so giving a wider FOV at the same aperture so it's now 344mm focal length at 73mm aperture. As the aperture hasn't changed the number of photons from your target which fitted in the original 430mm scope FOV haven't changed, so the scope isn't actually faster with the focal reducer. The photons are just spread over a smaller number of pixels with a reducer so the wells will fill up quicker, at the expense of a smaller target image.

You get extra photons from the wider FOV but these don't add anything to the target which fitted in the FOV of the original scope. 🙂

Alan

Edited October 26, 2022 by symmetal