SVBony

SVBony SV165 40mm Guide Scope

40 mm f/4 achromatic mini guide scope · 160 mm focal length · W9125B

The 30 mm guide scope with a third more focal length and a bracket that does not wobble. The one to buy if your imaging scope is around a metre of focal length and the smallest guider is running out of resolution.

84/ 100

Polaris Score

The middle rung, and the one most short refractors actually want

From$81.99

Where to buy

SVBONYPartnerBest price$81.99→

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What problem does this solve?

The 30 mm works until your main scope gets longer.

A guide scope measures how far a star has drifted, and how finely it can measure that is set almost entirely by its focal length. At 120 mm, the smallest SV165 spreads a star across roughly five to six arcseconds of sky per camera pixel. Software can find the centre of that star to about a tenth of a pixel, so the measurement is good to well under an arcsecond. That is plenty for a 500 mm refractor and thin for anything approaching 1200 mm.

The 40 mm version stretches the same idea to 160 mm of focal length, which brings the sampling down to roughly four arcseconds per pixel on a typical small guide camera. It also swaps the tiny finder foot for a machined bracket that clamps the tube along its length. In practice the bracket is the bigger upgrade: flexure between guider and main scope produces drift the software corrects in exactly the wrong direction, and a stiffer mount removes a failure mode that no amount of focal length can fix.

You need one if

You need this one if your imaging scope is between roughly 700 and 1300 mm, or if you have been guiding with a 30 mm on a finder shoe and seeing corrections that never quite settle.

You can skip it if

Skip it if your main scope is 600 mm or shorter, where the 30 mm is genuinely enough, or if you are past 1500 mm, where you want the 50 mm or an off-axis guider instead.

How it actually works

What the setup looks like.

There is nothing electronic in this tube. It is a lens, a barrel and a bracket, and it becomes a guiding system once a camera and a laptop join in:

Bolt the bracket to the main scope

The integrated bracket takes a standard finder shoe or bolts to a spare dovetail. Clamp it along the tube rather than at one end, which is what makes this version stiffer than the 30 mm.

Thread a guide camera into the back

The rear accepts a 1.25 inch nosepiece or a direct T2 thread. The T2 route is the better one: fewer joints means less to shift on a cold night.

Focus by rotating the front section

The front part of the tube turns through 15 mm of travel. Find tight stars once, lock the thumbscrew, and it holds through transport.

Hand the drift to PHD2

The software picks a star, measures its wander several times a minute and pushes correction pulses to the mount through its guide port or an ASIAIR.

Check your camera's back focus before you buy. This tube reaches focus at 12.5 mm behind the T2 thread, which is exactly the sensor depth of the common mini guide cameras. Add a filter or an extra adapter and you can run out of travel.

01Will it fit my setup

Compatibility.

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

Any main telescope with a finder shoe or a spare dovetailZWO, QHY, Player One and SVBony mini guide cameras1.25 inch nosepiece or direct T2 threadPHD2, N.I.N.A., ASIAIR and EkosMain scopes from roughly 700 to 1300 mm focal length

Note✕Needs a guide camera, sold separately✕Only 12.5 mm of back focus, so a filter or extra spacer may not reach focus✕Achromatic doublet, so it guides well and photographs badly✕Will not reach focus with a star diagonal fitted✕Still short for a 2000 mm SCT, where the 60 mm or an off-axis guider belongs

02The numbers

What These Numbers Mean

5 numbers in plain English · full specifications

Focal lengthMedium

160 mm

This is the number that decides whether the tube suits your rig. With the 2.9 to 3.75 micron pixels of a typical mini guide camera, 160 mm samples the sky at roughly 3.7 to 4.8 arcseconds per pixel. Guiding software locates a star to about a tenth of a pixel, so the resulting measurement is good to a few tenths of an arcsecond. That comfortably serves an imaging scope out to about 1300 mm.

ApertureAdequate

40 mm

Forty millimetres gathers about eighty percent more light than the 30 mm version, which is the difference between hunting for a guide star in a sparse field and simply having one. It still will not match a 50 mm guider near the galactic poles, where usable stars genuinely run thin.

Focal ratioFast

f/4

Fast optics let the guide camera expose for one or two seconds instead of four. Short guide exposures matter because they let the software catch a drift while it is still small, rather than reporting it after the error has already been written into your photograph.

Back focusTight

12.5 mm

This is the specification most likely to cause you a bad night. Twelve and a half millimetres is precisely the sensor depth of the standard mini guide cameras, so a bare camera threads on and reaches focus. Insert a filter, a longer nosepiece or an extra spacer and the focus point moves behind the sensor, where no amount of turning the barrel will retrieve it.

Optical designGuiding only

Achromatic doublet

An achromat brings red and blue light to slightly different focus, which wraps bright stars in a faint violet halo. Guiding software measures a star's centre of brightness and is entirely untroubled by this. Photography is not, which is why this tube guides well and should never be pointed at a camera you intend to keep the picture from.

Optical

Aperture40 mm

Focal length160 mm

Focal ratiof/4

LensTwo element, fully multi-coated

Physical

FocuserFront section, 15 mm of travel

Back focus12.5 mm

Camera connection1.25 inch and T2

Body179 x 114 x 59 mm

Weight450 g

MaterialSandblasted anodised aluminium

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 bracket is the real upgrade.

Compared with the 30 mm on a plastic finder shoe, the machined bracket that clamps this tube along its length removes a genuine source of drift. Flexure between the guider and the main scope makes the software correct in the wrong direction, and no amount of focal length compensates for it.

Measure your camera before you order.

The 12.5 mm of back focus is generous for a bare mini guide camera and unforgiving of anything else. If you were planning to put a filter in front of the sensor, plan on the 50 mm instead.

It will not take pictures.

Two elements of ordinary glass at f/4 is a fine way to find a star's centroid and a poor way to make a photograph. If you want a guide scope that doubles as a small imaging telescope, look at a 50 mm ED guider.

Focus it once and stop touching it.

The front section rotates through 15 mm and locks with a thumbscrew. Set tight stars on the first clear night and it will hold through car journeys and cold snaps for a season.

Editor’s Take

The interesting thing about this one is that the aperture is the least important number on the page. Forty millimetres over thirty buys you a slightly easier time finding a guide star. What actually improves your images is the extra 40 mm of focal length and, more than that, a bracket that holds the tube along its length instead of pinching one end of it in a finder shoe. Guiding failures are far more often mechanical than optical, and this is the cheapest rung on the ladder where the mechanics stop being the weak point.

— The Skynocular verdict

84/ 100

SVBony SV165 40mm Guide Scope

40 mm f/4 achromatic mini guide scope · 160 mm focal length · W9125B · $81.99

SVBONYPartnerBest price$81.99→

SVBony SV165 40mm Guide Scope

The 30 mm works until your main scope gets longer.

What the setup looks like.

Compatibility.

What These Numbers Mean

Good to know.

Related accessories.