Optical Design Talk & Tests.....
Ritchey- Chretien Optical Systems
With the growing use of Ritchey Chretien telescope designs for CCD imaging it is useful to know the design features and limitations as we consider the growing size of CCD detectors coming available. In addition the effects of practical collimation is also important so our scopes are optimized for the type of imaging we intend to use them for. A good friend of mine Scott Milligan (email@example.com) with many years of professional optical system design experience, a long time optical judge at Stellafane and a master optician took on the task of identifying the performance characteristics of common RC optical configurations with respect to optical performance of field diameters to 40mm. He produced reports on 3 areas that should be of great interest to all RC users:
1. Field flatteners for RC Scopes from 12.5" to 20" apertures (2.3Mb pdf file)
2. Focal reducers for RC scopes from 12.5" to 20" apertures ( 1.2Mb pdf file)
3. Mechanical collimation tolerances for RC scopes to achieve various levels of imaging performance (47Kb pdf file)
Newtonian Field Corrector Test Images - May 2006
These first 4 images are taken with a 12.5" f/4 homemade Newtonian with a 5" ma secondary mirror and a 3" Wynne optical design field corrector manufactured by AstroOptik ( Philipp Keller) in Germany. These 20 second exposure test images are of the star field around and including 1.36 mag. Regulus. This is a harsh test for ghost reflections but it demonstrates what the reflections will be like from a 5th magnitude star with a 9 minute luminance exposure. You can scale from there for fainter field stars. The camera used for all images is the SBIG STL 6303E, AR coated cover slip, with non IR blocked AR coated clear filter.
(Update....June 2006 - further tests by Cord Scholz in Germany using an IR blocked clear filter has shown greatly reduced ghost reflections compared to using a non IR blocked clear filter. This would make sense if the AR coatings on the corrector were optimized for a bandpass of 400- 700 nm. This is a good finding and makes understandable the good performance reports of other imagers also using this corrector on fast Newtonians)
The corrector has superb correction and aperture but a reflection problem occurs limiting wide field imaging work especially with brighter field stars (5 - 9th mag) with luminance exposures in the >5 minute range. This has been reported by other users too however some users report no reflections. The most compelling image below showing it is an optical design issue is the in-focus reflection showing the CCD pixel matrix. CCD surfaces are substantially reflective compared to the AR coated cover slips on some CCD arrays.
20 Second exposure Keller Test Image - Regulus Position 01
20 Second exposure Keller Test Image - Regulus Position 02
20 Second exposure Keller Test Image - Regulus Position 03
20 Second exposure Keller Test Image - Regulus Position 04
20 Second exposure Keller Test Image - Regulus out of focus, ghost reflection in focus showing reflection originating at CCD surface ( pixel matrix reflection)
30 Second 1x1 bin full field Keller test image..( scope collimation slightly off) taken during first quarter moon
90 Second 1x1 bin full field Keller test image M3 region showing ghost reflection from 6.2 mag, K3III spectral class star
Televue new STL Paracorr test image - I only had a one hour window to take this image before clouds rolled in but the correction looks very good ( the scope was slightly miscollimated but you can see based on the spot sizes across the field with spot on collimation the stars in full field would be tight corner to corner. More test images to follow when clear weather returns......
30 second exposure Televue test image of Regulus star field in 1x1 bin (scope collimation slightly off) taken during first quarter moon
Summaries and Notes: (See June note above.) The Keller and Televue both exhibit excellent field correction over the field of the Kodak 6303E CCD area. The Keller corrector has received much interest because it was the first large aperture field corrector for fast Newts. Many advanced imagers are excited about the ability of fast large aperture instruments to go very deep in a much shorter time than Cass designs.. The Newt corrector is an enabling optical component for imaging with a fast Newtonian but it appears that with the high reflectivity of front illuminated CCD's substantial entrance pupil ghost images are formed from brighter field stars in this corrector. In equivalent tests ( very limited at this point) the Televue STL Paracorr optical design better suppressed internal or ghost reflections. There is more vignetting with the Televue than the Keller design due the the smaller clear aperture but some of that vignetting ( which can be seem on both products test images ) is due to the high focuser of my scope design which causes the 5" secondary mirror to clip some of the light path. A lower design (ie: FLI PDF) would be ideal for these correctors.
Update: October 2007
Astro Systems Austria (ASA) provided a newly designed corrector for me to test on my 12" f/4 Astrograph. This new corrector was superb in all aspects including the elimination of reflections across the visible and NIR spectrum. The physical size of the corrector was identical to the older design however the new design has a slightly different power ( approx. 10% more than the old design).
I was very appreciative of their support in solving this problem and can say the new corrector is really an enabler for deep sky wide field pinpoint imaging. It has been very rewarding to see the re-emergence of the fast Newtonian as a potent CCD imaging platform. They are not trivial though to design for pinpoint imaging in all orientations!