What’s the world’s smallest useful astronomical telescope?

I’m not looking for a toy here.

https://en.wikipedia.org/wiki/Small_telescope

Isn’t that like asking what is the World’s smallest mountain?

https://en.wikipedia.org/wiki/Small_telescope

A small telescope is generally considered by professional astronomers to be any reflector-type telescope with a primary mirror of less than 2 metres (80 in) diameter. By amateur standards a small telescope can have a primary mirror/aperture less than 6–10 inches (150–250 mm) in diameter. Little if any professional-level research is performed with the refracting type of telescope in the modern era of astronomy.

Small telescopes dominate astronomical research in the fields of asteroid and comet discovery and observation, variable star photometry, and supernova and nova discovery, and colorimetry and polarimetry of the solar system’s planets.

Because of their limited light-gathering capability, small telescopes are usually not well-suited to spectroscopy, although some useful spectroscopic work can be performed with reflecting-type telescopes with a primary mirror as small as 14 inches (35 cm) when equipped with the increasingly sophisticated modern ccd imaging and spectroscopic instrumentation recently becoming available to amateur astronomers.

Most telescopes within the field of amateur astronomy are considered to be small, ranging in general from 2-inch (50 mm) achromatic refracting types, to reflecting type telescopes featuring primary mirrors up to (and sometime exceeding) 36 inches (90 cm) in diameter. Most small telescopes are dedicated to visual observing, although many are applied to such uses as gathering scientific data, or astrophotography.

The range of amateur astronomer’s telescopes is wide, with numerous types and designs, such as achromatic and apochromatic refractors, Newtonian reflectors, Schmitt-Cassegrain, Maksutov–Cassegrain and Maksutov–Newtonian compound reflectors. However in more recent years manufacturers of telescopes for the amateur market have begun offering more sophisticated designs, such as the Ritchey–Chrétien and (corrected) Dall-Kirkham, which have traditionally been the preserve of large professional-grade instruments.

What exactly constitutes a “useful” telescope? I’ve got a pocket telescope (about 1”, refracting) that I find quite useful, and have even used it for astronomy, though I haven’t discovered anything significant.

mollwollfumble said:

What’s the world’s smallest useful astronomical telescope?

I’m not looking for a toy here.

Did Galileo do useful work with his telescope? It had an aperture of 26 mm

Yes it is like “smallest mountain”.

Wocky said:

What exactly constitutes a “useful” telescope? I’ve got a pocket telescope (about 1”, refracting) that I find quite useful, and have even used it for astronomy, though I haven’t discovered anything significant.

That looks useful to me. How long is it and how much does it weigh?

I’ve seen attachable magnifying lenses for mobile phones. Not sure of the power they have though.

Here ya go.
Lenses

http://www.aliexpress.com/item/Special-Design-8X-Zoom-Phone-Telephoto-Camera-Lens-with-Clip-Universal-for-iPhone-Samsung-HTC-Smart/32331947744.html?spm=2114.01010108.3.152.eVdRwW&ws_ab_test=searchweb201556_7_79_78_77_80,searchweb201644_5,searchweb201560_9

?

Wouldn’t lens size determine usefulness, below a certain size not enough light entering to have a image

Spiny Norman said:

http://www.aliexpress.com/item/Special-Design-8X-Zoom-Phone-Telephoto-Camera-Lens-with-Clip-Universal-for-iPhone-Samsung-HTC-Smart/32331947744.html?spm=2114.01010108.3.152.eVdRwW&ws_ab_test=searchweb201556_7_79_78_77_80,searchweb201644_5,searchweb201560_9

?

The attachment looks amateurish, everything else looks spiffingly modern and then you attach it with a clothes peg.

Will follow up on your replies soon.

But have follow-on question:

What is the most powerful telescope (including digital sensor) that will fit in a box 5*10*10 cm?

i’d say the usefulness of a telescope is determined by what you are looking at and why you are looking at it

if you are thinking space telescope to fit in a small box you’ll most likely have to determine what you want to look at and why

I’m delighted to see that you think an off-the-shelf telescope attachment for mobile phone could have real astronomy value. I think so too, and I want to do better if possible.

wookiemeister said:

i’d say the usefulness of a telescope is determined by what you are looking at and why you are looking at it. if you are thinking space telescope to fit in a small box you’ll most likely have to determine what you want to look at and why

Am thinking space telescope to fit in a small box.

I proposed a space telescope that would fit in a box 40*50*220 cm, thinking that was very small. Got back the reply that they are interested, but only if I can fit the space telescope in a box 5*10*10 cm. That severely limits what it can do. There is a space telescope almost that small already in orbit. http://www.brite-constellation.at/.

What to look at: supernovae, novae, comets, asteroids, irregular variable stars, eclipsing binaries (not necessarily galaxies). In general, looking at everything that varies in brightness over the course of a few days or months. To operate both in discovery mode (eg. looking for new comets) and alert mode (looking in detail at something discovered earlier).

Why? Prototype for a larger digital telescope, testing of hands-off sub-pixel stacking algorithm with real data. Pushing the limits to find the boundary between the possible and the impossible.

mollwollfumble said:

Yes it is like “smallest mountain”.

Wocky said:

What exactly constitutes a “useful” telescope? I’ve got a pocket telescope (about 1”, refracting) that I find quite useful, and have even used it for astronomy, though I haven’t discovered anything significant.

That looks useful to me. How long is it and how much does it weigh?

Collapsed: 132mm; fully extended: 343mm. Mass: 225g.

I got it from York Optical in Flinders St. in Melbourne, but they’ve gone out of business.

mollwollfumble said:

I’m delighted to see that you think an off-the-shelf telescope attachment for mobile phone could have real astronomy value. I think so too, and I want to do better if possible.

wookiemeister said:

i’d say the usefulness of a telescope is determined by what you are looking at and why you are looking at it. if you are thinking space telescope to fit in a small box you’ll most likely have to determine what you want to look at and why

Am thinking space telescope to fit in a small box.

I proposed a space telescope that would fit in a box 40*50*220 cm, thinking that was very small. Got back the reply that they are interested, but only if I can fit the space telescope in a box 5*10*10 cm. That severely limits what it can do. …

Almost by coincidence, attended a talk today about giving that space telescope a diameter of 8 cm and a focal length of about 4 cm. The proposed telescope was a Schmidt-Cassegrain with a resolution possibly as fine as 1.5 arcseconds. So, quite different to the small refractor telescopes discussed above.

Still designing the thing. A space telescope 8 cm diameter and less than 10 cm long.

Using the freeware version of Atmos software. Looks like I’ll have to shell out $400 US for the full version.

Even the freeware version is giving me some real insights, such as folding the light path of a refractor telescope back using two flat (rather than curved) mirrors to triple the focal length. Also introducing me to Maksutov-Cassegrain and Houghton-Cassegrain as alternatives to Schmidt-Cassegrain.

Maksukov-Cassegrain

Houghton-Cassegrain