Bahtinov Masks for visual?

[Stonius]

Hi there,

Everything I've read about Bahtinov masks says they're great for nailing focus, but no-one really uses them for visual. In fact, many say 'don't bother' for visual.

This surprises me. Don't people want the very sharpest image they can get? It seems to me the circle of confusion would be limited by the atmospheric conditions anyway, so it can't be to do with cameras having potentially a higher resolution than the human eye.

I mean, think about it - typically the extra wobble induced by adjusting the focuser obscures any fine details, so you rock back and forth through focus a few times, before settling on what you think is 'pretty close'. But it's never exact, is it? Yes, I know, that's what battery powered autofocusers are for, but I'm trying to have a scope that doesn't have more wires and machines attached than a person undergoing triple-bypass surgery.

So why?

Markus

[Atmos]

I commonly use a bahtinov mask for visual focusing, I think they're great :)

[raymo]

Bahtinovs are great, as Colin said. It's a bit sad though that your scope is not stable enough for you to be able to focus manually. For about $90 the
electric focuser is the way to go; if you get one you'll wonder why you didn't get one from the outset. For visual focus, when I've got it right I remove the cable and controller, so no trailing cables.
raymo

[RickS]

Unlike a camera your eye can adjust to compensate for small differences in focus, so it's not as critical for visual observing. For the same reason some field curvature is OK in visual set ups but a flattener is needed for imaging.

Cheers,
Rick.

[Stonius]

Maybe my eyes don't deal as well with motion blur as others, but I find even gently touching the fine knob on the Crayford, or even the cooling fan running creates enough HF vibration that details are lost (I need to investigate a PWM speed controller to see if I get less blurring with lower speeds). The fan vibration is not a lot, but it's enough that the very finest details are obscured, and you wouldn't know because it's high frequency. Mars still looks like Mars, only with less fine detail that would probably be put down to atmospheric conditions unless a comparison was made, but I digress...

[Stonius]

I've heard that 'the eye can adjust' thing before, as you mention, when discussing field curvature. I've often wondered whether it is the eye that physically adjusts, or the brain? Does the cornea make microadjustments? Or does the brain just adjust its 'algorithm'? I fthe latter I would have thought feeding the brain more signal and less noise would still hold some advantages?

[glend]

Squinying changes the shape of your eyeball. The eye is surrounded by muscle bands that move it around, when you look at something and squint the eye shape can be changed slightly. This can alter the eye focal length.

[RickS]

It's done by varying the shape of the lens like for "normal" vision: https://en.wikipedia.org/wiki/Accommodation_(eye)

[Stonius]

I wonder then why it is that my eyes can focus from 20cm to infinity, while the in-focus range of a telescope is only very narrow?

[RickS]

The telescope and eyepiece are presenting the eye with a virtual image. The eye can accommodate minor imperfections in the virtual image but it can't change the focus of the telescope/eyepiece optical system. You have to do that by adjusting the spacing between the objective and the eyepiece.

[skysurfer]

Because a scope has a MUCH longer focal length than your eyes (17...20mm). Your eye is a similar optical device as a 20mm wide angle lens.