On Tuesday, December 30, 1997 11:59 AM, Chris Heapy wrote:

> Other ideas along similar lines exist which use pairs of rods across the
> front of the objective to produce 2 diffraction spikes on out of focus
> images.

Diffraction focussing is a little different that the Hartmann Mask approach
and only one obstruction (rod) is needed. Multiple obstructions increase
the intensity, but not the quality. What happens is the diffraction spike
produced by the obstruction is initially split at the ends near focus. It
then narrows, converges and sharpens as focus is approached.


> My basic mask has 2 holes
> of 2" diameter equivalent to approx 6.3 sq.in., or about 11-fold less
> light gathering. It works reasonably well visually with bright objects,
> and not too bad with the CCD - except that near critical focus it
> becomes difficult to judge whether the 2 images are exactly
> superimposed. This 'grey' area is sufficiently broad to prevent exact
> focussing.

You are correct about your observations, however, you can improve
results by combining shorter exposures with different star brightness
levels, though these methods are not straight-forward and difficult
to explain verbally.

>
> What I have in mind is using much smaller holes than commonly described,
> still positioned to provide the maximum baseline (separation). To this
> end I have equiped my mask with inserts having holes 2", 1.5", 1", 0.5"
> and 0.25" diameter. This equates to 6.3, 3.5, 1.6 0.4 and 0.1 sq.in.,
> respectively. The smaller holes would be useless for visual work as the
> image(s) would be far too dim - but for a CCD focussed on a bright star
> it should be usable.
>
> I can't test any of this theory because of continued awful weather, but
> would appreciate comments from anyone who may have gone down this route
> before.
>
> Chris Heapy

Smaller holes at the edge will produce a better defined image, but as
you point out, a bright star is needed. However, in the HPP mode, a
bright star is furnished automatically, so I like your idea for CCD,
especially in the HPP mode.

Please pursue your experiment and post results so I can learn the best,
most practical hole size.


--
Michael Hart
Husen Observatory

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