Focus Sweeps 14-Sept-2010

D. Burke

15-September-2010

Summary

The reinstallation of the old secondary mirror solved one major problem with the Curtis-Schmidt setup for PreCAM. PreCAM was reinstalled on the telescope and a series of focus scans have been taken. At this time it is not possible to obtain a good focus across the full field of view. The problem appears to be that there is a tilt between the plane of the CCD pair and the optic axis at the CCD location. A first attempt to reduce this was done with the adjustment on the tilt-focus plate at the camera. This was partially successful, but is not sufficient for continued PreCAM work. Reported here is one analysis of focus scans taken after this adjustment.

Data Sets

The raw data taken on September 14-15 with image numbers used in this report can be found at

http://sdssdp62.fnal.gov/precam/Raw

Image Number Focus

2025 - 2035 5500 to 8000 in steps of 250 microns

2057 - 2067 6500 to 7000 in steps of 50 microns

2068 - 2078 7500 to 8000 in steps of 50 microns

NOTE: In these multi extension images the right-hand side of the CCD shown in the second extension abutts the left-hand side of the CCD shown in the first extenstion. The coordinate system is North upward and East to the left.

Analysis

Shown in the attached photograph is a set of sketches of the images of stars seen on the two CCD chips for images 2057 through 2068. These are crude but attempt to show what is visually seen on DS9. As the position of the camera (focus in microns) increases from 6500 to 7000 and then to 7500 away from the telescope (further along the optic path), a swath of focused stellar images running at 45 degrees from upper left to lower right is seen to progress through the field of view. At focus setting approximately 6850 microns (image 2064) this swath runs approximately through the center of the leftmost CCD. By 7500 microns (image 2068) the focused swath crosses the upper right corner of the leftmost CCD and the lower left corner of the rightmost CCD.

Examination of the full set of images listed above shows that, as the CCD plane is moved further along the optical beam (away from the telescope), the center of the leftmost CCD passes through the focus of the telescope approximately 0.850 mm before the center of the rightmost CCD passes through the focus. These centers are approximately 20mm apart. So the plane of the CCDs is not perpendicular to the optic axis (as it should be) by atan(0.850/20) or about 2.5 degrees. This is considerably more than can be corrected with the tip/tilt adjustment on the camera mount. The most probable cause of the problem is the alignment of the old secondary that was reinstalled; other contributions could be the alignment of the CCDs in the dewar and perhaps something from the primary mirror that was removed and reinstalled.