EdgeHD 8 vs C8: Corner Stars, Backfocus, and Who Should Buy Which
EdgeHD 8 vs C8 (Beginner-Friendly, Real-World Test)
If you’ve ever looked at SCT images and thought: “Why are the stars in the corners kind of… weird?” — you’re not alone.
I’m still very much learning, so this post is the written version of my EdgeHD 8 vs standard C8 comparison, with the goal of answering one beginner question:
Does EdgeHD actually matter for imaging… or is a regular C8 basically the same?
TL;DR (the honest version)
In the center of the frame, both scopes can look great.
EdgeHD is mainly about the edges/corners (flatter field, better star shapes across a larger sensor).
If you’re using a small sensor, a standard C8 can be totally fine — the “bad corners” might be outside your camera anyway.
If you’re using APS-C or larger, EdgeHD’s advantage becomes way harder to ignore.
(And yes — I also overlayed rectangles showing what smaller sensors would see so you can sanity-check what you’d actually experience.)
Watch the full video
What I tested
This wasn’t a lab test — it was a practical “how does this look in real life” comparison.
I focused on:
Star shapes in the center vs corners
How sensitive each setup is to backfocus spacing
What the image looks like when you crop down to:
a 533-sized sensor
a 585-sized sensor
(and whatever camera I used in the actual test)
The big concept: why EdgeHD exists
A regular SCT like a C8 can produce a sharp image in the middle, but the optical design tends to create a curved focal plane — so the “best focus” isn’t a perfectly flat sheet across the whole camera sensor.
That usually shows up as:
corner stars looking stretched or “seagull-ish”
stars getting worse the farther you go from the center
frustration that feels like bad collimation or tilt (even when it’s not)
EdgeHD scopes add corrective optics so the field is flatter and stars behave better across more of the frame.
Results: what changed (and what didn’t)
1) Center sharpness
In my test, center sharpness wasn’t the deciding factor. Both scopes can deliver good center stars when focus and collimation are in the ballpark.
2) Corner stars (this is where EdgeHD earns its name)
This is the part most people actually care about.
On the regular C8, the corners degraded more noticeably (especially if you’re viewing the full frame from a larger sensor).
On the EdgeHD, the corners held up better — not magically perfect, but much more consistently sharp across the frame.
3) The “small sensor reality check”
This was the most useful part of the whole comparison for me:
When I overlayed what smaller sensors would capture…
A 533 crop often avoids the worst corner behavior entirely.
A 585 crop can still be pretty forgiving.
APS-C and larger sensors expose way more of the field — and that’s where EdgeHD starts to feel “worth it.”
Backfocus: the part that can make you think you’re doing something wrong
If you’re new, backfocus can feel like black magic.
Here’s the simple version:
Both setups can be sensitive to spacing
But EdgeHD systems are often treated like “precision spacing matters” rigs, because the corrected field is designed around specific geometry
My takeaway: if your corners look bad, don’t immediately assume the scope is bad — spacing can be the difference between “ugh” and “ohhh!”
Who should buy which?
Buy / keep the regular C8 if:
You’re imaging with a small sensor (533-class especially)
You plan to crop anyway
You want maximum value per dollar
You’re okay with corners not being perfect on larger sensors
Choose EdgeHD 8 if:
You’re using APS-C or larger
You care about stars across the full frame
You want fewer “is it tilt or is it optics?” nights
You want an SCT that scales with you as you upgrade cameras
The beginner verdict
If you only look at the center, you can convince yourself these scopes are basically the same.
But if you care about the whole frame — or you’re shooting with a larger sensor — EdgeHD isn’t just marketing. It’s solving a real problem.
And the best part: if you’re on a small sensor right now, you don’t have to rush into the expensive choice. You can grow into it.
Support Deep SkyLab
If these beginner-first experiments help you, you can support the channel here:
https://buymeacoffee.com/deepskylab
Business contact: francisco@deepskylab.org
FAQ (easy win: pull from your comments)
Q: Can I just fix corner stars in processing?
Sometimes you can hide it, but it’s way easier when the raw data is better.
Q: Is this tilt or field curvature?
It can be both — but field curvature has a very “symmetric corner worsening” vibe. Tilt usually looks like one side is worse than the other.
Q: What if I’m only shooting galaxies in the center?
Then a regular C8 can be a killer value, especially on small sensors.
