Tom Droege took a set of images with the Mark IV at his home in Batavia, varying the focus setting. Here, I analyze the results of this focus test for the I-band camera. The bottom line:
Tom used the following settings for focus on the I-band camera:
I looked at three different portions of the frames, each sub-section a box 200 rows by 200 columns.
Below, I show pictures of each subsection at the different focus settings. I've used the same contrast settings in each image: full white is <= -20900 DN, and full black is >= -17900 DN.
First, the center of the image:
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Note that the out-of-focus donuts at the center of the chip are not uniformly bright around their edges. I believe that this means that the center of the chip is not at the optical axis of the lens. The donuts look most round to me around row = 870, col = 1350, about 0.7 degrees to the SSE on the chip.
To my eyes, a setting of 160 looks best. See the numbers in table below, after all the pictures.
Now, the "upper-left" corner, which I believe to be the North-West corner.
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There's a trade-off here between the width of the core, and the size of the outer halo.
Now, the "upper-right" corner, which I believe to be the South-West corner.
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In all cases, I examined raw images. I did not perform any dark subtraction or flatfielding, because the shape of the core of a star is largely unaffected by such niceties.
I used one tool to evaluate the PSF in each of these sub-sections.
I didn't use another method (fraction of light within a circular aperture) because it seemed to give little extra information in previous work.
For each subsection, I picked three stars and measured the quantity for each one (yes, I used the same three stars for each focus setting). I list below the values for each star, and a rough average.
FWHM values for stars in "center"
Star 1 Star 2 Star 3 Average
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140 FWHM 6.1 6.4 5.9 6.1
145 4.7 4.7 4.7 4.7
150 3.7 3.7 3.6 3.7
155 3.5* 3.3 3.2 3.3
160 3.0* 2.5 2.5 2.5
165 3.1* 2.6 2.8 2.7
170 3.2* 2.8 2.8 2.8
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Stars marked with a "*" were saturated. Oops.
FWHM for stars in "upper-left"
Star 1 Star 2 Star 3 Average
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135 FWHM 4.6 4.9 4.6 4.7
140 4.2 4.2 4.0 4.1
145 3.5 3.9 3.4 3.6
150 3.5 3.5 3.2 3.4
155 3.0 3.4 2.9 3.1
160 2.9 3.2 2.7 2.9
165 3.0 3.3 2.7 3.0
170 3.1 3.1 2.9 3.1
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FWHM for stars in "upper-right"
Star 1 Star 2 Star 3 Average
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140 FWHM 3.6 3.6 3.6 3.6
145 3.1 3.0 3.1 3.1
150 2.9 3.0 3.1 3.0
155 2.9 2.8 2.9 2.9
160 3.1 3.0 3.0 3.0
165 3.4 3.4 3.6 3.5
170 3.9 3.9 3.8 3.9
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It looks to me like position 160-165 yields the best stellar PSF at the center of the image, although the wings start to appear for positions greater than 155. This focus position does a fair job in the corners, although there appears to be a small difference between the best position in two corners.
The optical center is not at the center of the chip.
The smallest FWHM is about 2.5 pixels, which is much larger than the smallest FWHM in the V-band. It's possible that with a set of finer steps through focus positions, one might decrease this minimum FWHM.