Last updated: 4 August 2006
Sent: Wednesday, August 2, 2006 10:17:38 From: Ted Rafferty (firstname.lastname@example.org) 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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