As an optical engineer, I spent a career designing, building, using, and delivering optical test equipment to customers all over the world. Consequently, I’ve wanted to interferometrically test the C14 that I use for imaging to get some good data on its optical quality for some time but gathering all of the equipment needed for such a test isn’t easy and I was hesitant to take the scope out of action when the skies were clear. Since it's usually cloudy for months on end in winter, December seemed like a good time to try to get it done. Fortunately I have a lot of friends who were willing to help with this project and I was able borrow all the equipment needed to do the test. This data was taken with a 4D Technology PhaseCam, which is a state of the art, vibration insensitive, digital, dynamic interferometer with a 1 Mpx phase sensor. Here are just some of the results.

Rev A

Figures showing the monochromatic PSF performance for the system. These diffraction patterns are computed from the measured wavefront data (at 632.8 nm.) A) The system Airy disk displayed as I=Log(1+PSF) along with image scale circles. Keep in mind that even with good seeing, the long exposure FWHM blur size is rarely smaller than 1.2 - 1.5 arc-seconds. B) Shows the system PSF displayed as I=log(1+k*PSF), where k =1000 to amplify the low intensity outer ring structure. C) Shows the same data as the middle figure with the aberration errors scaled to zero to demonstrate a “perfect” pupil identically sampled. The bright second ring and low intensity of the third diffraction ring is due to the size of the central obscuration. The slight irregularity in the rings is simply due to a sampling artifact. 80% of the total energy is contained within a 0.75 arc-sec aperture and 90% of the total energy fits within a 1.45 arc-sec aperture.

Rev B

A photo of the test setup showing the 4D Technology PhaseCam 6000 dynamic interferometer on the left, the C14 Edge and a 24" high accuracy return flat on the right. Because the flat was so heavy and the interferometer is quite insensitive to vibration, the test was set up on the floor--without any need for vibration isolation. The box under the telescope simply acts as a spacer to get everything to about the right height. Rotation of the flat indicates that it has an accuracy better than about 0.01 wave rms (~1/20 wave PV) over the 14" aperture. It might not look like much but this is probably nearly $300k worth of equipment! The telescope is the cheapest part of this test.

Rev C

The single pass wavefront data for the C14 Edge showing the on-axis wavefront irregularity of 0.065 waves and Strehl performance at 0.845 at a wavelength of 632.8 nm. This data demonstrates that the system exceeds the Maréchal criterion for diffraction-limited performance. After aligning the secondary on the interferometer, the fringes (in the lower plot) show that it may still be very slightly out of alignment; though the residual 3rd order coma term is only 0.048 waves. Perfect alignment might only improve the Strehl by +0.010, so it's not far off. This data is an average of 128 individual phase measurements to reduce the effects of air turbulence. Using 128 frame averaging, the measurement to measurement repeatability is about 3.5 milli-waves rms.

Rev D

A 9-panel image showing samples of a calibrated, stretched, unprocessed image using a 900 second exposure showing the field performance over a 36.8 x 36.8 mm KAF-16803 sensor. This data was taken under relatively poor seeing conditions after interferometric alignment. Each panel is a 500 x 500 px sub-sample demonstrating round, sharp stars over the entire 52 mm image circle. When it's properly aligned, the C14 Edge clearly does a good job of covering a larger field than the factory spec of 42 mm. I've seen similar levels of field performance using the same sensor on C11 Edge systems from others here on AB as well.

Rev E

The system wavefront was optimized by rotating the corrector plate and the secondary was aligned to minimize on-axis coma to within 0.010 waves. This is the final test report for the telescope showing an on-axis Strehl of 0.93. That's pretty good for mass produced optics!

- John

PS You can find the full report here: http://www.cloudynights.com/page/articles/cat/articles/interferometrically-testing-two-celestron-c14-edge-telescopes-r3095