This is an update to my prior post on camera detilting. My setup suffers from a tilted focal plane problem that causes stars to not be in perfect focus across the whole field of view. This post describes how I removed the tilt.
I purchased and installed a Gerd Neumann Camera Tilting Unit (CTU) XL [gerdneumann.net], shown in the photograph below. Rather than the usual jack screw design for tilt adjustment, the unit uses three radially positioned threaded cones to separate the two parts of the unit by a small amount. The threaded cones mate against an internal conical flange that is held in place by three strong internal springs. The unit seems well designed and strong. I have found it easy to make fine adjustments and lock them in place using the three pairs of locking screws. During slewing tests across the sky as the night cools I have not seen signs of flexure.
The photograph below shows the installation. The CTU is the disc shaped device positioned between the FLI Atlas focuser and the QSI camera. To avoid a potential light leak between the two halves of the CTU, I wrapped the device with a strip of light-proof fabric [thorlabs.com] and taped it in place. The fabric and tape cover the ends of the threaded cones, so they are not visible.
The AP2.7" thread depth on one side of the CTU is relatively shallow. Gerd recommends a 3mm adapter thread depth, however on my adapter I used a depth of 4.5mm to provide a more secure attachment. This increase in depth caused no interference problems.
The adapters were built by PreciseParts.com. I oriented the CTU with the locking screws facing the camera. Had the device been installed in the reverse orientation access to the locking screws on the face of the unit would have been partially blocked by the adapter that connects it to the focuser.
The impact of the tilt on star full width at half maximum (FWHM) measurements prior to installation of the CTU is shown below. The chart shows FWHM measurements in pixels across a 40 minute subframe exposed through a 3nm H-alpha filter. The tilt runs from the upper-left corner to the lower-right corner, with the upper-left corner inside of focus, the center portion in focus, and the lower-right corner outside of focus.
I used a Bahtinov mask [spike-a.com] to adjust the CTU. Results are shown below. The two photographs below each show four images of the Bahtinov diffraction patterns of stars located near the four corners of two successively exposed 40 second subframes. The subframes were exposed through a 3nm H-alpha filter [astrodon.com] and focused at frame center. Each of the patterns show a nearly perfect focus condition. A 10 micron change in focus position is visible with the Bahtinov mask. Hence the CTU adjustment has achieved a focal surface position parallel to the CCD sensor surface within +/- 10 microns at the center and each corner.
The diagrams shows the FWHM measurements on two successive 40 minute subframes exposed through a 3nm H-alpha filter after installation and adjustment of the CTU. Measurements of FWHM are nearly equal across the subframes. The tilt appears to be removed nearly completely. Measurement variations are due to software measurement accuracy limitations and noise.
After adjustment I measured the thickness of the CTU at several positions along the outer radius of the device with a micrometer. Based on these measurements I estimate that the amount of tilt removed by the CTU equals about 4 +/- 0.5 microns per millimeter, or equivalently about 3 to 4 mils when measured across the CCD sensor.
My setup is mobile and so I need a couple of months of additional testing to see if the CTU remains robust and stable when transported.
November 2013 update: I transported my setup to an Arizona observing site, 650 highway miles with an additional 12 miles of gravel roads. The CTU alignment seemed stable with no noticeable change. I exposed 28 subframes, each 40 minutes in duration. Star FWHM measurements are less than 1 pixel across the entire area of all of the frames. The tilt remains nearly completely eliminated. This is the first sub 1 pixel FWHM subframe set I have achieved for a target. The result is my November 2013 photograph, Sh 2-202, Stock 23, Camelopardalis.