Pushing past min magnification

[mental4astro]

Hi all,

I'm aware of the theoretical minimum magnification rule for a given diameter.

What happens if one goes below that minimum?

Do refractors and reflectors have this same lower limit?

Mental.

[Wavytone]

What happens is that the aperture stop becomes the pupil of your eye, and the effective aperture of your scope is reduced to the eye pupil x the magnification. It's much the same as putting a mask on the front of the scope that reduces the clear aperture.

On reflectors the obstruction ratio of the secondary increases (in percentage terms) and you may tend to see a large out of focus black blob swimming in the field of view, which is your secondary. It is particularly bad with short focal ratio Newtonians (f/5 or less) and also SCT's where the central obstructions can be 30-33% of the original aperture. The "black blob" effect is also made much worse if the eyepiece has a lot of spherical aberration at the exit pupil - which some Naglers do suffer from, mainly the early ones. Also known as the "kidney bean" effect.

Which is another reason to opt for a reflector with a smaller secondary, even if this means f/7 or more.

With a refractor you're just losing effective aperture, there should be no "black blob" effect.

[Chippy]

Great explanation again Wavy.

[mental4astro]

Thanks Wavytone. You've actually made sense. I had asked someone else, and they only confused me even more.

Mental