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ESI TV Guider Focus Analysis | |
A series of guider frames was taken at different secondary focus positions (TELFOCUS). The globular cluster M3 was observed to provide a scattering of stars across the field. The echelle slitmask was installed, centered on the 0.75 arcsec slit. Note that the seeing was poor (as high as 1.5 arcsec FWHM according to AUTOFOC). Poor seeing is also accompanied by variable seeing. The individual files can be found here.
Thirty-three stars were identified and their (x,y) positions measured using VISTA. The stars were scattered over most of the field, with the exception of the right hand side of the echelle slitmask. This area was farthest from the core of M3, and is affected by vignetting from the flat slitmask.
An algorithm, courtesy of Greg Wirth, that calculates a width parameter was first tried. For each star (each field position) a hyperbola was fitted to the width parameter vs. TELFOCUS. Unfortunately the algorithm did not produce good results.
Instead, a box was defined around each star, and that region of each image sequentially displayed. The best focus was determined by eye. Astigmatism in the images made the position of best focus reasonably well-determined, even given slightly different seeing between frames.
The plot below shows four sets of data. Points in red are from the offset mirror, while points in blue are from the echelle slitmask. The abscissa represents the X-positions of the stars for the open red squares and the blue ``Xs''. The abscissa represents the Y-positions for the red diamonds and the blue crosses. The ordinate is always the optimum TELFOCUS value. Note that each star produces two points in this plot: one for its X-position and one for its Y-position.
Clearly the blue data points consistently lie below the red points, indicating that the optimum TELFOCUS is lower for stars on the slit. Looking at only the red open squares, it does not look like there is a trend on the offset mirror for the focus to change with Y. This would indicate that there is no Y-tilt on the offset mirror. There is also little evidence from the blue crosses for a Y-tilt across the slit, although the lever arm is shorter and the data noisier.
The red diamonds show a small slope, consistent with an X-tilt across the offset mirror. The smaller number of blue ``Xs'' is consistent with this slope, although the lever arm is again shorter, due to the lack of suitable stars on the right side of the slit.
The largest affect is the shift on focus from the offset mirror to the slit mask. A gradient in X is not as much a problem as a gradient in Y, since the best images are generally required at the REF and ESI pointing origins, both near the middle of the X range.
The mean offset between the blue crosses and the open red squares is 0.28 mm. This is consistent with the value of 0.24 mm determined by comparing AUTOFOC results from April engineering. A value of 0.28 mm corresponds to 20 mm of shift in the focal plane. In turn this implies that the slit mask is 10 mm lower than the offset mirror (since the optical path doubles back on itself after the reflection).
Plotted below is the same data with the blue points (from the slit) shifted higher by 0.28 mm. The agreement with the data points from the offset mirror is much improved.
The X-tilt amounts to about 0.2 mm from left to right. Since it is not a high priority to correct this tilt, I have not calculated what this means in terms of tilting the CCD camera.
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ESI Master
9 May 2000