MODIFIED HARTMANN SCREEN Last updated: 4 August 2006

Sent:	Wednesday, August 2, 2006 10:17:38
From:	Ted Rafferty (raff650@erols.com)
This is a modified version of the standard Hartmann Screen with a string
running across the holes to create diffraction spikes.



The holes in the Hartmann screen are one inch in diameter for my Meade
LXD75 8-inch SN. I tried various string/wire sizes until I found a
diameter that created nice looking diffraction spikes for my imager, a
Meade DSI I.

Pointing at a single bright star, when the telescope is slightly out of
focus the resulting image looks like this:



And when in focus, it will appear like this:



I find it easier setting the focus using the parallel diffraction spikes
than using just the round or triangle holes in the Hartmann screen.

I should point that that a very accurate focus can be determined using a
Hartmann screen if the focuser is equipped with a digital readout and
the separation of the two out-of-focus star images for each focus
setting can be determined, since there is a simple linear relationship
between the focus settings and image separation created by the Hartmann
screen. Taking two image frames at different focus settings on one side
of the focus and two frames at different focus settings on the other
side of the focus, and giving negative values for the measured
separations between the out-of-focus star images for each focus setting
on one side of the focus and positive values to the measured separations
on the other side of the focus, solving the equation:

x = a*y + b

with the x's being the focus readings and the y's being the separations
between the two out-of-focus star images for each x focus reading, the
intercept (b) is the ideal focus setting.
 
Ted Rafferty
Gaithersburg, Maryland

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