ED refractorOTA only
Sky-Watcher Evostar 72ED (OTA)
- Aperture
- 72 mm (2.8")
- Focal length
- 420 mm (16.5")
- Focal ratio
- f/5.8
The most sensible entry into wide-field deep-sky imaging, provided you budget for the flattener it does not include.
Optical design
ED refractors
Doublets using extra-low-dispersion glass — most of the correction of an apochromat for a fraction of the money, and the reason small ED refractors have become the standard first astrograph.
Best suited to:Nebula imagingVisual observing
How the design works
An ED refractor is a two-element design in which one element is made from extra-low-dispersion glass such as FPL-51 or FPL-53. Low-dispersion glass spreads wavelengths less as it bends them, so the pair can bring red, green and blue much closer to a common focus than a conventional achromat can. The result is a telescope with only a faint violet residual on the very brightest objects, at a price much closer to an achromat than to a triplet. The field remains curved, so imaging still requires a flattener.
Strengths
- Colour correction close to apochromatic in ordinary use, at roughly a third of the cost of a triplet.
- Light and short, so a small tracking mount can carry one along with a guide scope and a camera.
- Sealed and collimation-free, which removes an entire category of maintenance and troubleshooting.
Limitations
- No field correction of its own — a matched flattener or reducer/flattener is required for imaging and is an additional purchase.
- A faint violet halo remains on very bright stars, the Moon's limb and Venus, visible if you look for it.
- Small apertures. These are supplementary visual telescopes, not primary deep-sky instruments.
Why ED doublets took over entry-level astrophotography
Three things happened at once. Extra-low-dispersion glass became affordable, small tracking mounts became affordable, and cameras got good enough that a 70 to 80 mm aperture could produce images worth keeping.
The result is the current default first astrograph: a short ED doublet between about 60 and 90 mm, a matched reducer/flattener, and a mount with real payload margin because the tube weighs under two kilograms.
What separates the glass types
FPL-51 and FPL-53 are the two extra-low-dispersion glasses you will see quoted, with FPL-53 having lower dispersion and therefore better correction. In a small, slow doublet the difference is subtle; in a faster or larger one it is more visible.
It is worth much less than the marketing implies. Mount quality, spacing accuracy and total integration time all affect your final image more than the glass designation does.
Using one visually
Better than people expect. A 72 mm ED doublet with a decent diagonal and eyepieces is a genuinely enjoyable grab-and-go: sharp, colour-clean, ready instantly, and wide enough to frame large clusters properly.
What it will not do is compete with aperture. On faint galaxies and globular clusters, a 200 mm reflector costing similar money is in a different class. Think of an ED refractor as a second telescope for wide views and travel, not as the one that shows you the faintest things.
Maintenance and setup
Essentially none. Sealed optics, no collimation, quick cooldown and no cleaning schedule beyond removing dust occasionally. The recurring practical issues are dew on the objective, solved with a dew shield or heater, and — for imaging — the one-off task of getting the flattener spacing correct, which is measured once and then left alone.
What you will need alongside it
- A matched field flattener or reducer/flattener for imaging, with the correct spacer rings
- A tracking mount for imaging, or any stable mount for visual use
- A star diagonal and eyepieces if you intend to observe visually