Chris--

I'm sorry--I was being facetious--of course you are right. When I made a similar
comment regarding spacing some weeks ago, Ric immediately jumped in and stated that
neither spacing nor number made any difference.

Clear skies, Andy

Chris Heapy wrote:

> I don't recall Ric claiming that. I came to my own conclusion based on
> experiments I performed using various sized holes and spacings, and the
> math is fairly basic too :)
>
> Almost any pair (or number) of holes WILL work, in as much as you would
> get 2 or more images that overlap towards critical focus (maybe that's
> what Ric said...). However, smaller holes will give smaller images when
> out of focus, so the apparent separation appears greater - i.e., very
> near focus you still get 2 small separate circles rather than 2 larger
> overlapped circles - the centers of either large or small circles will
> be separated by the same distance of course. If the holes are at the
> edge of the aperture the effective baseline is increased, thus for a
> given amount of de-focus the separation is at it's greatest - until
> focus is reached. So much is intuitive.
>
> You can get pretty damn close to focus fairly quickly without any mask
> at all, the mask's function is to indicate that last little tweak which
> visually would not appear to change the image much - but is so important
> for revealing fine planetary detail. With big holes and/or holes close
> together this last tweak becomes more difficult to assess. You want the
> sharpest cut-off you can get and that is obtained as above.
>
> You can do pretty much the same job just using the peak count readout on
> the PView focus screen (without a mask), and others have described on
> this list how it's done. Maybe it's my average poor seeing but I find
> that method requires mental averaging of many exposures before I'm
> convinced a particular focus position is giving the true peak value.
> It's possible to combine both methods to get close to focus because as
> the two small images begin to overlap the peak count will rise rapidly.
> But there's a danger here also - when using a mask diffraction effects
> can give rise to images consisting of a bright 'ring' with a dim center
> (look at the excellent pictures on the website quoted earlier), where 2
> bright rings intersect near critical focus you will get an abnormally
> high peak count. It will still increase slowly towards critical focus
> but the peak count can can be confusing when used in combination with a
> mask.
>
> Bottom line is that whatever works for you... use it.
>
> Chris
>
> Andy Wallace wrote:
> >
> > Heapy Chris CG wrote:
> >
> > > The most
> > > discriminating image is provided by 2 small holes (the smaller the
> > > better) set as far apart as aperture will allow.
> > >
> > > Chris.
> > >
> > > >----------
> >
> > Chris--
> >
> > Surely this is not true--Ric claims you can have any number of holes with any
> > spacing you desire??
> >
> > Clear skies, Andy
>
> --
> *Model Engineer Support Page*
> Engineering: http://easyweb.easynet.co.uk/~chrish/homepage.htm
> Astro Stuff: http://easyweb.easynet.co.uk/~chrish/lx200.htm

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