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Astro Vs Spotting Scope - Difference in glass (1 Viewer)


Well-known member
United Kingdom
Hello All

I understand some of the benefits that an astro scope has over a terrestrial scope i.e comfortable eyepiece viewing angle, larger objective (reflectors), motorized mounts, longer focal length

But what is it about the actual glass of a high end 80mm astro scope that is superior for viewing the heavens to a high end spotting scope with similar objective size?


Well-known member
But what is it about the actual glass of a high end 80mm astro scope that is superior for viewing the heavens to a high end spotting scope with similar objective size?
1. Quality Control

Astronomical scopes are often used at powers of 100x or higher. Spotting scopes usually max out at 60x. So scope manufacturers may accept comparatively poorer quality, because the difference is less obvious at low powers and uncritical users may never notice. Discussion threads about the Kowa scopes suggest a meaningful percentage are lemons.

2. Erecting Prisms

The prisms used to produce "correct" images can result in spikes on bright stars. Astronomical scopes can use mirror diagonals to avoid the spikes, at the cost of flipped left-right images.

3. Focal Ratio

For compactness most spotting scopes are f6 or faster, this can be more demanding of manufacturing tolerance. CA can also be an issue unless (and even when) ED glass is used. Astronomical scopes can be much slower, which means easier manufacturing and less CA.

4. Sealing window

Waterproof scopes have a sealing window to make the scope airtight. This often acts as an additonal field stop and reduces the maximum TFOV, "max TFOV" eyepieces like 24mm 68° designs may have a portion of the FOV cut off. Astronomical scopes lack this sealing window.


Well-known member
The above post has it pretty good, but I'll add: even on spotting scope designs that use a prism type that doesn't split the light beam, it's still a much larger prism with many more light bounces than a standard telescope diagonal (simple prism or mirror).

Spotting scopes also use a lens to focus internally vs. a telescope which moves the eyepiece.

The objective element on a spotting scope has to be designed to be much more rugged. Telescope lens cells often have room for the lens to expand and contract with temperature.

All these extra things in a spotting scope can only degrade the image. Even with the same objective lens.


Well-known member
I have noticed that a correct image prism worsens the sharpness. Maybe not so much at low magnification but definitely at high magnification, for example used at planet observations. I doubt that any optics with correct image prisms works good at 100x. While a mirror diagonal still can provide tack sharp image at 200x providing the optics are very high class as well.
My Pentax 65mm ED spottingscopes(I have both the straight and angled version) starts to noticeably degrade after above ~35-40x.


Well-known member
I’ve taken an 80mm Astro refractor above 100x with an erecting prism, there was a diffracted spike off the prism, switching to a mirror diagonal improved the view.



Well-known member

it is possible to get an astro correct image prism which performs well even at higher magnifications in the form of the Baader 2" or T2 Amici prisms... but they don't come cheap...

And the spike is still there with brighter objects... but many astronomical observers are used to that from the spider on newtonian telescopes...


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