I Was Wrong About f/2. Big Time. Hyperstar Fail ( C8 Edge vs Z61 II ) | F/2 vs F/5.9
The “f/2 should crush it”… test (and why my first result didn’t)
If you’ve been around astrophotography for more than five minutes, you’ve heard it:
“Faster focal ratio = faster imaging.”
So in this Versus episode, I decided to put that idea on trial with a super simple setup:
Celestron C8 + HyperStar (around f/2)
William Optics Zenithstar 61 II (a small refractor around f/5.9) Agena Astro Products+1
Same camera, same filter, same night
Same target: The Veil Nebula YouTube
On paper, this should’ve been a blowout. f/2 vs f/5.9 isn’t close.
But in practice… my first comparison didn’t look like the f/2 system “crushed” anything. YouTube
So I stopped, assumed I was the variable (as usual), and dug into what can make an f/2 rig not look like the obvious winner.
What we were really testing
This wasn’t a “which scope is better” video.
It was a test of a very specific (and very common) assumption:
Can you compare imaging “speed” by just looking at two stacks and declaring a winner?
Because when the focal lengths (and framing) are different, the answer gets messy fast.
Why my first f/2 result looked “wrong”
Here are the biggest gotchas that can make an f/2 setup look underwhelming if you don’t normalize the comparison.
1) Image scale can trick your eyes
If one setup is framed wider/tighter than the other, you’re not comparing the same sampling of the nebula and background.
A stack can look “cleaner” just because:
the stars are smaller at that scale,
the noise is being averaged differently,
or you’re judging detail at different magnifications.
Beginner rule: if you want a fair “speed” comparison, you need to compare at the same framing / same resolution, or at least resample one image to match the other before judging noise and detail.
2) Narrowband/duo-band filters can behave differently at fast focal ratios
Most duo-band filters target the big emission lines: H-alpha (656.3nm) and OIII (500.7nm). ZWO Astrophotography+1
But interference filters can shift their passband to shorter wavelengths when light hits the filter at an angle (angle-of-incidence effects). Fast systems like f/2 send more steeply angled light through the filter, which can change transmission where you care about it most
That’s a big reason “high-speed” filters exist (built specifically to perform better in very fast systems like HyperStar/RASA). firstlightoptics.com
3) f/2 is unforgiving (focus, collimation, tilt, and calibration)
At f/2:
tiny focus errors look huge,
collimation sensitivity is brutal,
tilt shows up immediately,
vignetting can be dramatic (so flats matter a lot).
So even if the system is “faster,” it’s also easier to accidentally throw away that advantage with one small setup mistake.
Practical takeaways (aka: what I’d tell Past Francisco)
If you want maximum speed per clear hour
HyperStar-style f/2 systems can be insane—especially when you’re time-limited or the weather is unreliable. YouTube+1
But… they demand more attention to setup and they can be pickier about filters. 宏惠光電股份有限公司+1
If you want “easy mode” widefield imaging
Small refractors like the Zenithstar 61 II (360mm, f/5.9) are:
easy to focus,
easier to keep stars happy,
easier to travel with,
and generally less dramatic. Agena Astro Products+1
They might not be “fast” on paper, but they can feel fast in real life because you waste less time fighting the system.
How to run your own fair “speed test” (quick checklist)
If you want to replicate this idea with your own gear:
Pick a bright emission target (Veil, North America, Rosette, etc.)
Use the same camera + same filter if possible
Keep sub length and total integration identical
Calibrate both stacks the same way (bias/darks/flats)
Match framing or resample to match resolution
Compare:
detail in faint nebulosity
background noise at matched scale
star size / star control
total processing effort to get a pleasing result
Gear mentioned in this Versus episode
Celestron C8 + HyperStar (f/2-class system) Starizona
William Optics Zenithstar 61 II (61mm, 360mm, f/5.9) Agena Astro Products+1
Duo-band filter (passes Hα and OIII lines) ZWO Astrophotography+1
Want to support Deep SkyLab?
If you like this “beginner lab notebook” style of astrophotography testing, you can support the channel here:
https://buymeacoffee.com/deepskylab
And if you want to partner, sponsor, or send something for a future Versus test:
francisco@deepskylab.org
