Below are images of a 12th mag star (20sec exposures) observed through a Hartmann mask mounted in one of the UFTI filter slots. The angular size of each frame is the same, as are the contour levels. The fast guider fine-focus (FF) lens was set to different values in each case. In all data the optical guider was “auto-focusing” on the star (so the optical beam was in focus); in most data – depending on the FF setting – the infrared beam was out of focus, resulting in the four discrete images of the star. Note how the four images of the star converge as we approach UFTI’s nominal in-focus FF value of 1.05.

Click on the icon for a larger GIF image.

FF = -2.0 (out of focus)	FF = -1.5	FF = -0.8	FF = -0.4
FF = +0.5	FF = +1.05 (in focus)	FF = +1.8	FF = +2.3

The plots in the table below show the horizontal and vertical distances between peaks at the various Fine-Focus (FF) settings.

The four images of the star viewed through the mask when UFTI is out of focus are only sufficiently separated at large FF offsets ; for FF values in the range 0.0 – 2.0 (near focus) the four peaks blur together so that peak separations can’t be measured (see the example images above). At present, the fine-focus lens can only be moved between values of -2.3 and +2.3 (software limit?), so we don’t get useful data on one side of focus. If we could, then I would think that the intercept between two linear fits (made to the data on either side of focus) could be used to accurately measure the instruments in-focus FF setting, regardless of observing conditions. Click on the plot for a larger PS figure

foc_image	foc_image	foc_image	foc_image	foc_image
Horizontal Distance between top two peaks in defocused images	Horizontal Distance between bottom two peaks in defocused images	Vertical Distance between keft hand two peaks in defocused images	Vertical Distance between right hand two peaks in defocused images	Mean of four measured distances (left)

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