The Sky Rover 90 GPS is a 90mm f/6 Super ED triplet — 540mm of focal length built for wide-field deep-sky imaging. Like every refractor, it forms its sharpest focus on a slightly curved surface, while your camera sensor is flat. The SR90GPSFF 1× field flattener resolves that mismatch. It's the matched corrector for the 90 GPS, and for an imager it's the piece that lets the scope deliver round, sharp stars all the way into the corners of the frame.
It does one job and doesn't touch anything else. The 90 GPS stays 540mm at f/6 — same image scale, same field of view, 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 come to focus at the same point as the center. Instead of stars that stretch and soften toward the edges, you get clean, well-corrected stars across a full-frame chip.
Flattener, Not Reducer
This is a 1× flattener. It preserves the native focal length and focal ratio of the 90 GPS — the full 540mm f/6, unchanged. Your framing, image scale, and exposure times stay exactly what they were. The only thing you gain is a flat, corrected field: sharp stars from the center of the frame to the corners. If you like the 540mm focal length and simply want the whole sensor usable, this is the corrector for the job. (If you'd rather go faster and wider instead, that's the separate 0.8× reducer's job — not this one.)
Threads Directly to the Camera Angle Adjuster
The 90 GPS has an integrated Camera Angle Adjuster (CAA) — the rotating interface at the rear of the focuser that lets you square up a composition and return to the same framing later. The SR90GPSFF threads straight onto it. There are no intermediate adapters to add flex or introduce a spacing error — just a solid, repeatable mechanical connection built to keep the optical train aligned. Rotate the CAA to frame your target, and the flattener rotates with it.
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 sensor has to land at the correct back-focus distance for the corners to come out fully corrected — confirm the required spacing for the SR90GPSFF and add or remove spacers so your camera lands on the number.
What's Included
- Sky Rover SR90GPSFF 1× field flattener for the 90 GPS
Features
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1× field correction — no focal reduction. Keeps the 90 GPS at its native 540mm f/6. Same scale, same speed, same working distance — just a flat field.
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Matched to the 90 GPS triplet. Designed for the specific field curvature of the 90mm f/6 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; APS-C and smaller chips simply use the center of the corrected field.
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Threads directly to the CAA. Mounts to the 90 GPS's integrated Camera Angle Adjuster with no intermediate adapters, for a rigid, repeatable connection.
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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 90 GPS?
No. It's a 1× flattener, not a reducer. The scope stays at its native 540mm focal length and f/6 focal ratio.
Is this the correct flattener for my 90 GPS?
Yes. The SR90GPSFF is matched to the 90 GPS's optical prescription and threads onto its Camera Angle Adjuster. Sky Rover builds a dedicated corrector for each model — the GPA scopes, for example, use a different flattener with different threading — so this one is specific to the 90 GPS.
Will it cover a full-frame camera?
Yes. It's designed to deliver a flat, corrected field across a full-frame sensor, and it 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?
It is 55mm the industry standard for back focus.
Do I need it for visual observing?
This is an astrophotography accessory and is not designed to work visually.
Final Thoughts
If you image with the Sky Rover 90 GPS, the SR90GPSFF is the natural finishing piece. It keeps everything you bought the scope for — the Super ED triplet's correction, the fast 540mm f/6 field, the rotating CAA — and adds the one thing a flat sensor needs: a flat focal plane to match it. Thread it onto the CAA, set your spacing once, and the whole frame goes to work instead of just the middle.
