Lunar focus

[jambouk]

What features do others focus on when imaging a gibbous moon? Do others find the focus is slightly different for features near the centre of the illuminated surface compared to the limbs? Or is this just my imagination and a result of atmospherics and other aberrations?

I'm surprised such a relatively short change in distance of target, a few thousand km, at such a long way away requires a change in focus.

James

[NigeB]

Hi James,

That's an interesting one... I don't recall any difference in focus position between features on the limb and in the centre of the disc. 

A quick calculation: for your 2700 mm focal length 180 Pro, and assuming the Earth-Moon distance is 384,400 km, the image distances for a feature in the centre of the Moon's visible face, and one at the absolute limits of the limb, differ by something like 0.09 nanometers. That's about the size of an atom.

Whatever it is that you're seeing, it's not related to focal position changing across the disc. As to what it could be - I don't know, but I'm going to take particular notice next time I'm looking at the moon to see if I can detect anything similar...!

Nigel

[NigeB]

James - an afterthought. When you switch between observing the lunar centre and the limb, are you repointing your telescope, or is the field of view wide enough that you can see both regions in the same view, with no repointing required?

If you're not repointing, then what you are describing might due to the slightly curved focal "plane" generated by the optics (not implying your optics are poor - all optical systems do this to some extent). This might mean that there is a very small difference between the point of best focus for targets in the field centre, and those further out, towards the edge.

Nigel

[Steve Ward]

If you're not repointing, then what you are describing might due to the slightly curved focal "plane" generated by the optics (not implying your optics are poor - all optical systems do this to some extent). This might mean that there is a very small difference between the point of best focus for targets in the field centre, and those further out, towards the edge.

Nigel

I suspect that is what causes the small but definite difference in focus I can determine between details in the centre of the Sun's disc and those on the limb ... 

[jambouk]

Thanks both. Yes, An effect I think occurs even without moving the scope, so the curved field does sound like the likely cause, i'd not thought of that, it is something I have only paid attention to when imaging, though it may happen with visual too.

Interesting.

James