Wallace: A Journey with a 6-inch f/2.8 Telescope

By Lucas Sifoni

🌲 Walking with Wallace

I wanted to experiment with a different style of storytelling to introduce Wallace, a 153mm $f/2.8$ ultra-wide-field telescope I constructed last year. Beyond the hardware, I wanted to capture the "outdoor" essence of astronomy.

Lately, my life has been centered around hiking and running. This led me to a recurring question: How can I share the visceral experience of the outdoors and the views of the cosmos?

"We can never truly share a visual or sensorial experience—much like the feeling of a trail run—but we can attempt to convey the impression of it."

I find inspiration in people like Serge Vieillard, who uses astronomical drawing to bridge this gap, and the YouTube creator Kelp and Fern. The latter is a master of outdoor content because he blends a raw, authentic feel with a polished final product, often removing himself from the center of the frame to let the environment speak.

The Expedition

My goal was to create a "photo-video dump" documenting a trek to a cliffside: the sounds of the forest, the setup of Wallace from my pack, the act of observing, and the return journey.

It was a grueling but rewarding evening. Despite the sweltering $\approx 36^\circ\text{C}$ heat, I carried a significant amount of gear to document the story:

Surprisingly, the act of documenting didn't detract from the experience. Instead, it acted as a mindfulness exercise. Because I was searching for things to capture, I became more attuned to the environment:

The whistling of the wind.
The rhythmic chirping of insects.
The crunch of limestone beneath my boots.

While I am no professional videographer, the process made me deeply appreciate the craft of those who do it for a living.

🛠️ Technical Specifications

Wallace is my most successful instrument to date because it has a singular, well-defined purpose: ultra-wide deep-sky observation. There is something truly magical about fitting the entirety of NGC 7000 (the North America Nebula) into a wide-field eyepiece.

The Optical Path

The primary mirror is $153\text{mm}$ with a focal ratio of $f/2.84$ . Interestingly, this ratio was somewhat accidental. I originally aimed for $f/3$ , but a miscalculation regarding the shim thickness on the sine table during the curve-grinding process resulted in nearly $f/2.7$ . I decided to embrace the challenge and finished it, eventually raising the focal ratio slightly during the final fine-grinding stages.

Mirror Fabrication History

Attempt	Outcome	Note
First	~~Failure~~	A deep scratch occurred while attempting to smooth a central bump.
Second	Success	A central hole remained, but it is completely masked by the $59\text{mm}$ secondary.

The final in-instrument figure is $0.92$ . While not perfect, figuring such a fast asphere was a significant challenge, and I chose to stop there rather than risk introducing new scratches.

The Corrector

I utilized a 4-element GSO corrector (designed by Roger Ceragioli), sourced from leboncoin for $65\text{€}$ .

Many astronomers dismiss this corrector as low-quality due to its price and incorrect factory spacing. ~~The stock backfocus is $\sim 56\text{mm}$~~ , but the optical design actually requires $> 70\text{mm}$ . For Wallace, I found the optimal distance to be $\approx 79\text{mm}$ . When spaced correctly, it outperforms my MPCC + hyperbolic setup in my K2 telescope and exhibits significantly less chromatism.

Mechanical Build

The construction is a tribute to the Coulter CT-100, featuring a two-module design joined by a dovetail.

Material: 3D printed in deep-blue ASA.
Support: Six $6\text{mm}$ solid carbon fiber rods.
Field of View: With a $100^\circ$ eyepiece, the system provides a true field of $> 4^\circ$ . Correction is excellent across $66\%$ of the field, with slight degradation at the extreme edges (which mostly serves as "contextual" space).

Community Discussions:

🚀 The Next Iteration

The second version of Wallace will be significantly more ambitious. We have already diamond-generated a new blank featuring a meniscus, which will drastically reduce the weight.

Key improvements for V2:

Thermal Management: The current mirror takes $\sim 30$ minutes to reach ambient temperature. The new thin blank will cool almost instantly, making it a true "grab-and-go" instrument.
Optical Precision: I am seeking "revenge" on the central figure errors of the first mirror; I will not settle for anything less than a $0.95$ figure.