Since I have left Cloudy Nights, I am going to share the story of testing my new scope here. It's a little different than my normal "new image" post but hopefully it's interesting and at least a little informative.

Last August I made the decision to buy a new telescope. My current C14 based system works quite well so I wanted something that would provide more signal and better resolution, which means a system with a larger aperture. Cost and location were the two key factors in making the decision. Last year at NEAIC I met some of the folks involved with ObsTech in Chile and after long discussions, that looked like a pretty viable option. They said that it was not uncommon to have over 300 clear nights per year (320 was the number they tossed out) and more important, the median seeing is claimed to be around 0.5 arc-seconds. Finding a site with sub-1" seeing addressed my concerns over where to put the scope. (I also uncovered the possibility of putting the new scope on Mt. Lemmon outside of Tucson where the seeing is around 1"; but, that's my second choice--for now.)

Then I started looking at scopes. Everything gets exponentially harder and more expensive as the diameter goes up so I quickly ruled out a 24" system. For me, the sweet spot fell dead-center on the Planewave 20" system. At F/6.8, the effective focal length is actually 395 mm shorter than my current C14 system, which increases the field of view to about 36.7 x 36.7 arc-minutes using my FLI-16803 camera. Under 0.5" conditions, the seeing blur diameter would be just slightly over 8 microns so the 16803 will be a bit under-sampled under those conditions. Under the very best conditions, switching to a CMOS camera with smaller pixels might be better; but, to start off with, I'm willing to try dithering to improve resolution a bit. The real advantage is that if you take into account all of the factors (including obscuration, diameter, pixel size, throughput, etc.), the Planewave will accumulate signal at almost 2.5x the speed of my C14 (on extended objects.) Of course, it will go MUCH deeper on stars.

I spent my career working in the field of optical metrology as an engineer and entrepreneur so I couldn't simply submit a purchase order without a requirement for test data. Rick Hedrick kindly agreed to let me test my new scope at the factory with a 4D Technology PhaseCam. The PhaseCam is a dynamic interferometer that uses a high-speed, single frame phase measurement method to minimize the effects of mechanical vibration. Most digital phase-measuring interferometer systems are so sensitive to vibration that they could not be used to make this kind of measurement without a very large (and expensive) vibration isolation system. The PhaseCam makes ultra-high precision interferometric testing very easy!

I want to publicly thank the folks at 4D Technology who kindly let me use their demo instrument to make this measurement. It's a ~$150,000 instrument so it's not something that most amateurs would be able to use to do this kind of test. By the way, PhaseCams have been used to test most of the worlds largest telescope components and space borne systems--including the GMT mirrors, the Gemini telescope, the LSST primary, HIRISE (now orbiting Mars) and the JWST mirror segments. It is a state-of-the-art interferometer system.

The first image (rev H) shows a photo of the double pass set up. The return mirror is a 45" diameter flat supplied by Planewave. It was made by Kodak and spec'd to a flatness of 1/20 wave. The engineers at Planewae qualified it to be "pretty good" but there wasn't any way to rotate it in order to place error bars on its effect on the results. We used fans to stir the air and averaged 512, 2,000 x 2,000 phase images for each result. This approach greatly reduces the effects of air-turbulence on the final result.

The results show that over 98% of clear aperture, the system has a Strehl ratio of 0.85. PVq(95%) shows that 95% of the data fall within a range of 0.25 waves. The encircled energy shows that the system will put 80% of the energy from a star into a 9 micron circle. Ultimately, the system should work quite well for my imaging application.

It should arrive by freight next week so the next thing will be to get it commissioned and shipped to Chile! At the moment, the world seems to be falling apart so it could be a while before I get it down there.

Let me know if you have questions and I'll do my best to answer them.

John