This post is to document and hopefully gather some input on a 10" F/7.7 Harmer Wynne Cassegrain design that I plan to build. For the past few years, Rajesh (RajG) and I have been discussing this idea privately, and I’m now considering moving forward with it. The project involves creating a Harmer Wynne design using off-the-shelf components, specifically a GSO 10" F/4 parabolic mirror and commercially available lenses for the corrector.
Past Experience and Motivation
Why build a Harmer Wynne when I already own a 12.5" F/8 Starizona Hyperion (which is also a Harmer Wynne design)? The answer is simple: for the spirit of ATMing and to pursue a design that has always intrigued me. While my 12.5" F/8 is an excellent optical system, building a smaller, custom version with off-the-shelf components is a challenge that excites me.
Six years ago, I started a similar project to build a 14.5" Harmer Wynne, but I eventually abandoned it due to the challenges of making the corrector lenses. While I got far with the secondary mirror, mirror cell design, and baffle assembly, I ended up converting the project to a Newtonian setup with a 3" Paracorr. I always wished I had completed the Harmer Wynne version, and I’d like to take a moment to personally thank Mike Jones for his help with the optical design and optimization. I feel a bit ashamed for abandoning the project.
With Rajesh’s help and my renewed enthusiasm, I’m revisiting the Harmer Wynne design, this time with the 10" F/4 parabolic mirror I already have on hand and using OTS lens for the corrector elements.
The Harmer Wynne Design: Overview
The Harmer Wynne design is a unique and specialized Cassegrain configuration that features:
- Parabolic primary mirror
- Spherical secondary mirror
- Two corrector lenses in the baffle assembly
This design is especially suited for astrophotography, offering a wide corrected field of view (FOV). The spherical secondary mirror simplifies collimation significantly compared to traditional RC or Classical Cassegrains. While there will be some challenges in collimating and centering the lenses within the baffle, I think these issues are manageable and will address them when the time comes.
Optical Design and Corrector Lenses
Here is the optical design made by Rajesh:
The heart of this project revolves around the corrector lens assembly. I reached out to Ross Optical for a quote on the lenses I’ll need for the corrector, specifically:
- L-PCC118 (Plano-Concave)
- L-PCX358 (Plano-Convex)
They offered both lenses with broadband anti-reflective coatings (400-700nm) for a total of approximately $650. This price is reasonable when compared to the cost of a high-quality coma corrector, and I’m happy with the balance of cost and expected performance. Of course, I’ll also need to fabricate the lens cell and baffle assembly, but I’m committed to this project in the spirit of ATMing and building an advanced optical system.
Measuring the Parabolic Mirror Radius of Curvature
Before proceeding with the lens order, I need to obtain precise measurements of the radius of curvature for my 10" F/4 parabolic mirror. I have two methods in mind to achieve this:
- Radius bar with knife-edge (KE) reading: A straightforward method to get accurate curvature measurements.
- Astrometry and plate solving: Using plate solving, without any correctors in the light path, to determine the curvature.
I’ll choose the method that provides the most reliable results for this mirror.
Making the Secondary Mirror
The secondary mirror will be a 4.5" diameter F/7 convex mirror. The curve is fairly shallow, so it should be relatively easy to fabricate. This time (compared to my previous 14.5" Harmer Wynne project), instead of creating a matching concave test plate and using fringes to measure the figure, I plan to polish the secondary and test it directly in the telescope as an assembly. Any necessary touch-ups will be made to the secondary mirror as a complete optical system.
Challenges and Thoughts
One challenge with this design is the short backfocus distance, which requires a very low-profile focuser. I’ve considered designing and using a secondary mirror focusing system, which could eliminate the need for a traditional focuser. However, I’m uncertain about how changing the primary/corrector lens-to-secondary spacing will impact performance.
Rajesh has suggested using a diagonal mirror behind the corrector lens light path and converting the system to a Nasmyth-style configuration to reach focus. This is an option I’ll explore further as the project develops.
Note on optimization: based on the experience I gained from my 14.5" Harmer Wynne build, I know that the ideal process involves measuring each component after it’s made and then re-optimizing the optical design using the measured surface data. However, since the corrector elements for this project will be purchased, I don’t plan to directly measure these surfaces. Instead, I’ll rely on the manufacturing tolerances provided in the engineering drawings (hopefully this is not a mistake).
For the rest of the system, I’ll do my best to measure the secondary mirror radius, primary mirror radius, mirror distances, and corrector lens placement. I hope to fine-tune everything to create a fully functioning optical system.
I’m excited to revisit this design and build a unique, custom Harmer Wynne Cassegrain. I’d appreciate any input or feedback on the optical design, challenges, or solutions others may have encountered when building a similar system. Looking forward to sharing more updates as I progress!
Edited by Etendue645, 22 June 2025 - 03:33 AM.
