DIMM Seeing Monitor

WHAT IS THE EFFECT OF NO PRISM AND PRISM OPERATION ? DEFOCUS ISSUE !

A. Tokovinin said in his note of September 14, 2004 "Influence of defocus on DIMM"

"It is shown that DIMM measurements can be severely biased by defocus. A simple and efficient method to control this bias is by computing the Strehl ratio. Data with Strehl below 0.5 should be rejected. We verified that without aberrations, Strehl stays above 0.8 even for 200 seeing, thus if data with low Strehls are rejected the bias in a seeing statistics for a given observatory should not be a problem."

The next image shows a star (Alpha And) with a full aperture of 180 mm (INTES telescope) and 6560 mm focal length. recorded with a DFK camera embeding a 5.6 µm pixel sensor
The sampling is 0.17 arcsec per pixel. This instrument is able to resolve 0.78" (4.6 pixel).

Image 1

When installing the two hole mask with the prism the image looks like this :

Image 2

The star image is duplicated, and since the aperture is 64 mm the airy disk has increased by 2.8x, resolution cannot better than 2.2 arcsec.

If the prism is removed, a defocus has to be achieved to get two stars from a signle. Here is the defocus with no mask, this can be compared to image #1

Image 3

When the two hole mask is installed (64mm hole diameter with 122mm center to center separation) a dual image is obtained. The star separation is shorter compared to image #2. Image separation can be increased if defocus is increased, BUT the star image tends to increase also, leading to loss of accuracy and loss of light... so a trade of has be found, and I think this is a good one. If one compare the star strehl of the next image (#4) and the star strehl of image #2 (with prism), there's visually few difference and it does not seem to have fallen by a factor of two.

Image 4

Some results :

TBC