Imaging
Putting a camera on a telescope is a mechanical problem before it is an optical one. Everything turns on two thread designations and one distance — and the distance is the figure most listings leave out.
Telescope, then any reducer or corrector, then spacers, then the T-ring, then the camera. Each piece adds length, and the total has to land within the telescope’s focus travel.
T2 is M42 × 0.75. M48 is the wider standard used where a larger sensor would otherwise be vignetted. The 1.25 in and 2 in figures describe barrels, not threads, and the two systems are joined by adapters rather than converted.
At prime focus the telescope is the lens and there is no eyepiece in the path. In eyepiece projection an eyepiece enlarges the image before the sensor, which raises magnification and lowers brightness sharply.
Required spacing. Where a reducer publishes it, this site prints it; where it does not — and one on this site does not — the page says so, because a reducer with an unknown spacing requirement is a trial-and-error purchase.
1.25 in · holds an eyepiece to 35 mm · 140 g · T2 ring required
A projection adapter that takes a 1.25 in eyepiece up to 35 mm in diameter and couples it to a camera through a T2 ring bought separately.
Best for: Lunar and planetary photography where extra magnification is wanted
≈ £39 / €39
1.25 in · 68 mm extended barrel · M42 × 0.75 inside and out
A 1.25 in projection adapter with an extended 68 mm barrel carrying M42 × 0.75 threads on both the inside and the outside, fully threaded internally against reflections.
Best for: Eyepiece projection on a telescope with generous focuser travel
≈ £17 / €20
50 mm optical path · 2 in · spring-ring clamp · filter thread
A 2 in extension sleeve of 50 mm optical path with a spring-ring clamp, a filter thread at the lower end and internal ribbing against reflections.
Best for: A telescope whose focal plane sits too far out for its eyepieces to reach
≈ £36 / €39
f/10 to f/6.3 · four elements · SCT rear thread
A four-element fully multi-coated reducer-corrector that threads onto the rear cell of a Schmidt-Cassegrain and converts f/10 to f/6.3.
Best for: A Schmidt-Cassegrain used for imaging, where f/10 makes exposures impractically long
≈ $95 / £93 / €109
Canon EOS T-ring · 1.25 in to M42 coupling · rotating inner ring
A set containing a Canon EOS T-ring and a 1.25 in to M42 coupling, with an inner ring that rotates to orient the camera.
Best for: A Canon EOS owner who wants both halves of the coupling in one purchase
≈ £15 / €16
The piece that converts the T2 thread — a 42 mm thread with a 0.75 mm pitch — to a specific camera bayonet. It is camera-specific, and it is the only part of the train that is.
The diameter is, the pitch usually is not. T2 is M42 × 0.75; the old M42 screw mount is M42 × 1. They start to thread together and then bind, which is how the mistake is discovered.
It is the distance from the last optical surface to the sensor. A telescope has a limited range of it, and a reducer or corrector usually requires an exact figure — miss it and the corners never come to focus.
Any figure written into the body of this page was true on the day it was written. The amount shown next to each instrument is requested from the retailer as the page loads, and it is that amount, not the text, that stands.
Telescopes and mounts are discounted over and over through the year — hardest of all in the weeks before Christmas, which is exactly when most of them are bought — and a struck-through figure on its own says nothing whatever about what the same model was charged at four weeks ago. UniverTrack keeps a week-by-week record for each retailer, so a discount is measured against a figure somebody genuinely paid instead of one invented for the sale — and it says when the moment to buy has genuinely arrived.
