The Sky Rover 155 GPS is the flagship of the line — a 155mm Super ED triplet at 1240mm and f/8, built for the demanding end of deep-sky imaging: planetary nebulae, tight clusters, and small, distant galaxies where aperture and focal length both matter. Like every refractor, it forms its sharpest focus on a slightly curved surface, while your camera sensor is flat. Your eye ignores that curvature at the eyepiece; a sensor doesn't. Behind a camera — a full-frame one especially — the center comes to a tight focus while the corners start to stretch. The SR155GPSFF 1× field flattener is the matched corrector that fixes it.
It corrects the field and leaves everything else alone. The 155 GPS stays 1240mm at f/8 — same image scale, same field, same exposure times. What changes is the shape of the focal plane: the flattener bends the field flat to match a flat sensor, so the corners focus at the same point as the center. Instead of elongated stars at the edges, you get round, well-corrected stars across the whole frame, right out to the corners of a full-frame chip — the full resolving power of 155mm of aperture, used across the entire sensor.
Flattener, Not Reducer
This is a 1× flattener. It preserves the native 1240mm focal length and f/8 focal ratio of the 155 GPS — your framing, image scale, and exposure times don't change. The only thing you gain is a flat, corrected field. Choose it when you want the full 1240mm focal length and the resolving power that comes with it.
If you'd rather trade some of that reach for a faster, wider system, Sky Rover makes the SR155GPSRFF 0.8× reducer for this scope, which brings it to 992mm at f/6.4. Native focal length and maximum resolution with the flattener, or a shorter, faster field with the reducer — pick the one that fits your targets and your sky.
Connecting Your Camera
The camera side uses standard 48mm (M48) T-threads. Dedicated CMOS and CCD astronomy cameras usually have an M48 back plate and thread on directly. DSLR and mirrorless bodies connect through a T-ring matched to your camera mount (sold separately). As with any flattener, the corners are only fully corrected when the sensor sits at the specified back-focus distance — confirm the required spacing and count the depth of any filter drawer, off-axis guider, or spacer so the sensor lands on the number.
What's Included
- Sky Rover SR155GPSFF 1× field flattener for the 155 GPS
Features
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1× field correction — no focal reduction. Keeps the 155 GPS at its native 1240mm f/8. Same scale, same speed, same working distance — just a flat field.
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Matched to the 155 GPS triplet. Optimized for the specific field curvature of the 155mm f/8 Super ED optical system, not a generic corrector adapted to fit.
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Full-frame coverage. Corrects the field for round, sharp stars across a full-frame sensor when properly spaced; smaller sensors use the center of the corrected field. The 155 GPS's redesigned Camera Angle Adjuster carries a 3.7-inch clear aperture, so the light path isn't pinched ahead of the flattener.
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Standard M48 camera-side threads. Dedicated astronomy cameras attach directly; DSLR and mirrorless bodies connect via a T-ring (sold separately).
Frequently Asked Questions
Does this change the focal length of the 155 GPS?
No. It's a 1× flattener, not a reducer. The scope stays at its native 1240mm focal length and f/8 focal ratio.
What's the difference between this and the SR155GPSRFF reducer?
This flattener keeps the native 1240mm f/8 and simply corrects the field — choose it for maximum focal length and resolving power. The SR155GPSRFF 0.8× reducer shortens the system to 992mm and speeds it to f/6.4 for a wider field and faster exposures, at the cost of some image scale. Pick based on your target size and how much exposure time you want to spend.
Will it fit any other Sky Rover scope?
No. The SR155GPSFF is matched to the 155 GPS's optical prescription and mechanical connection. Each Sky Rover model has its own dedicated flattener; this one is specific to the 155 GPS.
Will it cover a full-frame camera?
Yes. It's designed to deliver a flat, corrected field across a full-frame sensor, and works equally well with APS-C and smaller sensors.
Do I need a T-ring?
Dedicated CMOS and CCD cameras with an M48 back plate usually thread on directly. DSLR and mirrorless cameras need a T-ring matched to your camera mount, sold separately.
What is the correct back-focus distance?
The sensor must sit at the flattener's specified back focus for the corners to be fully corrected. Confirm the exact figure before building your imaging train, and include the depth of every adapter, filter drawer, off-axis guider, and spacer in the total.
Do I need it for visual observing?
No. Field curvature only matters to a flat camera sensor — your eye accommodates the curved focal plane on its own. This is an astrophotography accessory.
Final Thoughts
At 1240mm and f/8, the 155 GPS is built for the targets that reward both aperture and focal length — small galaxies, planetary nebulae, tight clusters. Used at native focal length behind a camera, it needs a flat focal plane to match a flat sensor, and the SR155GPSFF is the corrector made for exactly that. Thread it onto the Camera Angle Adjuster, set your spacing once, and the whole frame delivers the resolution the scope is capable of — not just the middle of it.
