UV Barlow Lens
UV Barlow lens
The well known planetary astrophotographer Arnaud van Kranenburg works with the famous Celestron C9,25. Like many others, he uses a Barlow lens to obtain the proper focal ratio, according to the Nyquist criterion.
For Venus, more demands are required. Details in the clouds are visible only beyond the visual light, in ultraviolet light (340nm). The biggest challenge for a Barlow lens is that most materials are opaque and the system has not been corrected for this wavelength. Also the coating has not been optimized for this wavelength.
What needs to be done?
The famous optical designer Harrie Rutten has invented a solution: Why don't you use a dedicated UV Barlow that is transparent for UV and corrects the aberrations in the system? However, to correct for the aberrations, Harrie Rutten needed the optical specifications of the telescope. These specifications were very difficult to obtain from Celestron, even with a nondisclosure agreement.
At first glance it seemed not possible at all to obtain this information. As a last resort we measured the telescope ourselves. Besides the thicknesses and radii, Harrie Rutten also needed the shape, curvature and position of the neutral zone of the Schmidt corrector.
On my own milling machine I measured the position of the neutral zone and this was later confirmed with an autocollimation test setup.
After the corrector was removed we immediately measured the radius of curvature of the secondary mirror.
Meanwhile Celestron had given the optical properties to Harrie Rutten after a nondisclosure agreement had been reached. However, the optical data showed some inner discrepancies. Due to the calculation work and our measurements the correct optical data could be retrieved.
The only remaining issue was the type of glass of the corrector. It was odd that even Celestron was not able to give a unambigious answer.
From the calculations of Harrie Rutten found out that a C9.25 is a good all purpose telescope but not suitable for ultraviolet imaging. However, this was not a surprise. Nearly all telescopes are not corrected, or are opaque for UV.
After all the optical properties were known, Harrie was able to design the Barlow lens. An extra challenge was the desire to correct the remaining aberrations of the Schmidt-Cassegrain.
The final design was made from 4 COTS lenses of a wellknown optical supplier.
The total assembly is mounted in a 2" as well as a 1.25" housing.
Seeing is believing! Watch the beautiful results of Arnaud one can conclude that the UV Barlow fulfills all demands.