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Critical Focus

Posted by Don Clement   07/21/2005 12:00AM

Critical Focus
Clement Bellerophon Focuser
Review written by Dave Erickson

Critical focus is an essential element of astrophotography. Having put a lot of effort into a fast lens assembly, and considerable expense in a CCD camera, it seemed foolish to compromise on the focuser. I have had good luck with the Crayford style focuser and considered using one here. The Crayford style focusers available were only two-inch aperture, and while reasonably low profile consume more than three inches along the F/4 cone. The resulting vignetting would seriously degrade image illumination at the edge of the 36mm CCD and a good number of edge pixels would be rendered useless.

Fortunately Clement Focuser offers a standard product with 3” aperture and extremely low profile. The Clement focuser design is elegant, functional, and performance is excellent. I chose the new Bellerophon model for this camera project.

This Bellerophon focuser is part of a modest camera setup of 12” focal length at F/4 pictured below. The short focal length provides a nice wide 6.7°X4.7° field when used with an STL-11000M camera. At F/4 the camera is fast enough that the tilt and decenter from conventional focusers will cause unacceptable image degradation. Here the depth of focus is about 0.0015” and pixels are 0.00035” (9microns). To compound the problem the camera ensemble is not athermal and some focus adjustment is required to compensate variations in temperature over a nights use. The system focus drift is reasonably good measuring about 0.0003”/ °C. I’ve found that focus needs adjustment several times during an evening. I make these adjustments between exposures, or after a filter change. This makes stability over long exposures with absolute minimal image tilt essential.

I measured tilt, decenter, and the precision of travel of the new Clement Bellerophon over most of the focus range, using an electronic indicator and precision airbearing table. The setup for these measurements is the second image.

This setup consists of a 10” airbearing with a precision tip-tilt stage. The Clement focuser, not pictured, was placed on the tip-tilt stage in a secure non-constraining manner. The gauge head was placed on contact with the two shoulder surfaces of the eyepiece Bellerophon interface. This shoulder is precision machined by Clement and quite good. The tilt and decenter of these surfaces were measured and recorded along the focus travel at intervals of 0.050”. Measurement for tilt and decenter were made for each focus position by measuring a change in deflection as the airbearing-focuser ensemble was rotated. Air bearings have micro-inch level errors and are negligible for this class of measurement.

Performance of this focuser is outstanding, with better performance than any other focuser I have used in the past.

The small image displacement from focus adjustments is not noticed by the autoguider, a fact obvious from the graphed performance above. The Clement Bellerophon focuser could easily handle a system with several orders of magnitude more focus shift, and is ideal for systems requiring active focus control during exposure and-or a very fast optical system.

The nebula images included here were taken using the astrograph pictured, with the Clement Bellerophon for focus. Each nebula is a mosaic of (1) hour exposures in h-alpha.

I have been making optical systems in some capacity for more than thirty years. One of my early amateur systems was an RTMC award winner in 1975. As a professional in optics I have played part in the management, engineering, and construction of many thousands of optical systems. From test optics for the Hubble WFPC first servicing mission, to optical networking devices. My current position is Senior Scientist with the world leader in interferometric test equipment.

Recently, I have taken once again to making instruments for myself, a throwback to my roots as an amateur astronomer. I am nearing completion of another astrograph, an 8.2” aperture F/2.6. For this very sensitive instrument I am trusting focus to another Clement Bellerophon.

With more instruments in the planning stages, the only thing common to each will be the Clement focuser.