SVBony SV238 Off-Axis Guider — image 1
SVBony

SVBony SV238 Off-Axis Guider

Off-axis guider · 8 x 14 mm prism · M54 and M48 · W9196A

Guides through your main telescope's own optics instead of a second tube bolted on top. The answer for long Schmidt-Cassegrains, where a separate guide scope cannot see the errors that matter.

82/ 100
Polaris Score
The right way to guide a long focal length telescope
From$115.99
Where to buy
Affiliate link — we earn a commission, you pay the same. We rank on fit, not payout.
What problem does this solve?

A guide scope cannot see your main mirror move.

Autoguiding assumes that where the guide camera is pointed is where the main camera is pointed. On a small refractor with a guide scope clamped to the top, that assumption holds well enough. On a long Schmidt-Cassegrain it quietly fails in two ways. The tube flexes a fraction of a millimetre as it swings, so the guider and the main scope slowly disagree. Worse, focusing an SCT moves the primary mirror, and that mirror keeps shifting under its own weight through the night. A separate guide scope is bolted to the outside and sees neither.

An off-axis guider removes the assumption entirely. A small prism sits in the light cone ahead of your camera and picks off a sliver of the field from the edge of the image circle. The guide camera therefore looks through the same primary mirror, the same corrector and the same focuser as the photograph. Anything that moves the image moves the guide star by exactly the same amount, and the software corrects it. There is nothing left to flex between them because there is no between.

You need one if
You need one if you image at 1500 mm or beyond, especially on a Schmidt-Cassegrain, or if your guiding graph looks fine while your stars still come out slightly elongated.
You can skip it if
Skip it if you image with a short refractor under about 1000 mm, where a guide scope is simpler, cheaper and entirely sufficient, and where the flexure this fixes is too small to see.
How it actually works

What the setup looks like.

An off-axis guider is a spacer with a prism in it. It goes into the imaging train, which is why setting one up takes more thought than clamping a guide scope on top:

An off-axis guider is a spacer with a prism in it. It goes into the imaging train, which is why setting one up takes more thought than clamping a guide scope on top:

1
Work out your back focus first
The body adds 12.5 mm to the optical train, plus 4 mm for the M48 adapter or 5 mm for the M54. Your camera and any flattener have a fixed spacing requirement, and this has to fit inside it. Do this arithmetic before you order anything else.
2
Fit it between the telescope and the main camera
The M54 or M48 thread goes toward the telescope, the main camera bolts to the back. The prism now sits in the light path just outside the area your sensor records.
3
Set the prism depth
The prism height adjusts. Push it too far in and it casts a shadow on the corner of your photograph. Leave it too far out and it collects no light. Adjust it while looking at a real frame, not by eye.
4
Focus the guide camera separately
The guide camera slides in the 1.25 inch port and must reach focus at the same point as the main camera. This is the step people find fiddly. Once set for a given camera pairing, it stays set.
5
Hand it to PHD2
From here it behaves like any guider. The software picks a star and corrects the mount, except now it is watching through your main optics.

The prism looks at the very edge of the corrected field, where star shapes are at their worst. A slightly bloated or comatic guide star is normal here and does not stop the software finding its centre.

01Will it fit my setup

Compatibility.

What this accessory works with — check the fit before you check the price.

What this accessory works with — check the fit before you check the price.

Schmidt-Cassegrains, Maksutovs and long refractors from roughly 1200 mm upMain camera sensors up to APS-CM54 and M48 telescope-side threads, with M42 adapters includedGuide cameras with a 1/3 to 1/1.8 inch sensor on a 1.25 inch barrelPHD2, N.I.N.A., ASIAIR and Ekos
Note✕Needs a guide camera, sold separately✕Consumes 12.5 mm of back focus before adapters, which not every imaging train can spare✕Guide stars are scarcer than with a guide scope, because the prism sees a very small patch of sky✕Setting the guide camera to the same focus as the main camera takes patience the first time✕Sensors larger than APS-C will start to vignette against the prism
02The numbers

What These Numbers Mean

5 numbers in plain English · full specifications

5 numbers in plain English · full specifications

Body thicknessSlim
12.5 mm
This is the first number to check and the one most likely to stop the purchase. Every imaging train has a fixed distance from the telescope's rear thread to the camera sensor, usually 55 mm behind a flattener, and everything in between competes for it. At 12.5 mm plus a 4 or 5 mm adapter, this is at the slim end for an off-axis guider, but it still has to be budgeted for before you buy.
Prism sizeGenerous
8 x 14 mm
The prism is the window through which you hunt for a guide star, and its area sets how likely you are to find one. Eight by fourteen millimetres is large for this class. It does not make guide stars as easy to find as a 50 mm guide scope does, because it is still sampling a small patch at the edge of the field, but it is meaningfully better than the small square prisms that made early off-axis guiders unpopular.
Main sensor supportSufficient
Up to APS-C
The prism has to sit outside the area your camera records but inside the telescope's corrected circle. That gap is what limits sensor size. APS-C covers the great majority of cooled astronomy cameras. A full-frame sensor would push the prism further out, into a part of the field where the star shapes are poor and the illumination is falling away.
Guide sensor sizeSmall on purpose
1/3 to 1/1.8 inch
A larger guide sensor would not help. The prism only illuminates a small circle, so anything beyond about 1/1.8 inch collects dark corners rather than extra stars. This is why the cheap mini guide cameras, which are the wrong tool for almost everything else, are exactly right here.
Telescope interfaceFlexible
M54 and M48
M54 is the thread on most modern flatteners and larger reducers, M48 is the two inch standard, and M42 adapters are in the box for both. Between them these cover most imaging trains without a trip to a machinist, which is not something every off-axis guider can claim.
Optical
Prism size8 x 14 mm
Prism heightAdjustable
Main sensor supportUp to APS-C
Recommended guide sensor1/3 to 1/1.8 inch
Mechanical
Telescope interfaceM54 and M48
Guide camera interface1.25 inch
Body thickness12.5 mm
M48 adapter4 mm
M54 adapter5 mm
Adapters includedM48, M54 to M42, M48 to M42
03Before you buy

Good to know.

The honest caveats — the things that save you a return, a bad first night, or a second purchase you didn't need.

The honest caveats — the things that save you a return, a bad first night, or a second purchase you didn't need.

Do the back focus arithmetic before you order.
This is the single most common reason an off-axis guider ends up in a drawer. Add the body, the adapter, your filter drawer if you use one, and your camera's own sensor depth, then check the total against what your flattener demands. If it does not fit, no amount of adjustment will make it.
Finding a guide star is genuinely harder.
A 50 mm guide scope surveys a degree of sky. This prism sees a few arcminutes at the edge of the field. Most of the time a star is there. Sometimes you will need to rotate the whole assembly to bring one into view, which is normal practice and not a fault.
It removes a problem a guide scope cannot.
Mirror flop in a Schmidt-Cassegrain moves the image without moving the tube. A guide scope bolted to the outside is blind to it and will happily report perfect guiding while your stars trail. This is the reason off-axis guiding exists.
Expect ugly guide stars.
The prism looks at the outer edge of the corrected field, where coma and astigmatism are worst. A guide star that looks like a small comma is entirely normal. Guiding software measures a centre of brightness and is untroubled by the shape.
Editor’s Take

Off-axis guiding has a reputation for being difficult, and the reputation is half deserved. The setup genuinely is fiddlier: you are inserting an optical component into a train that has a fixed length, and then focusing two cameras to the same plane. What people underrate is that this is a one-time cost. Once the spacing is right and the guide camera is parfocal, an off-axis guider is the more reliable of the two approaches, because it has removed the failure mode rather than mitigated it. On a short refractor none of that matters and you should buy a guide scope. On a two metre Schmidt-Cassegrain, a guide scope is measuring the wrong thing carefully, and no amount of care fixes that.

— The Skynocular verdict
82/ 100
SVBony SV238 Off-Axis Guider
Off-axis guider · 8 x 14 mm prism · M54 and M48 · W9196A · $115.99

Guides through your main telescope's own optics instead of a second tube bolted on top. The answer for long Schmidt-Cassegrains, where a separate guide scope cannot see the errors that matter.