[madbadgalaxyman (Robert)]

Hello, Vi,

I honestly don't know anyone, apart from a few very odd people, who would recommend a 3 inch refractor as a primary instrument for an adult. These telescopes are either recommended as a children's telescope, or as a second telescope for ease of use and a "fun and quick look-see". I used a three inch refractor when I was a teenager, and I persisted with it, though a lot of adults would have probably stashed it in the attic very quickly.

The idea that a three inch telescope can show rich planetary detail and show deep sky objects well.....is totally incorrect and bizarre.

I do like a refractor as much as the next man, but the smaller ones, particularly 70mm to 100 mm aperture, have very limited Light Gathering power for nebulae, galaxies, globulars etc.........
A 3 inch (75mm or 80mm) refractor cannot really resolve globular clusters, for instance. Furthermore, even in a dark sky, a four inch refractor shows virtually all galaxies as indistinct blobs without any individuality.
A four inch can (but only in excellent conditions) be OK for viewing nebulae, both bright and dark, but (apart from the Eta Carinae & Orion & Tarantula & Lagoon nebulae), a small aperture like this requires a very dark and very transparent sky to see nebulae well in a four inch.

The slightest light pollution when viewing with a four inch, and you find yourself limited to viewing about two dozen deep sky objects!

Also, even the highest quality 3inch / 80mm refractor can show only very modest detail on the planets, as the best angular resolution that can be achieved is around 1.5 arcseconds. This is quite easy to see, numerically, when we consider that the angular diameter of Jupiter is only 50 arcseconds at best, and the disk of Jupiter is usually larger than that of the other planets, when viewed in the telescope. Thus, the number of resolution elements stretching across a planetary disk is too small for seeing substantial detail when using a three inch (75/80 mm) refractor.

You can't even split a lot of the famous double stars with a three inch! (a three inch refractor is a crazy recommendation as a primary instrument for an adult)

A four inch is substantially better for planetary viewing than a three inch, as it has a nominal Best Angular Resolution of 1 arcsecond. It is probably necessary to put about 60 resolution elements across the disk of Jupiter in order to see a lot of planetary detail, so you need 1 arcsecond resolution, or preferably somewhat better to see substantial planetary detail. A three inch doesn't get anywhere near the angular resolution necessary! Therefore, somewhere within the aperture range 4-6 inches, telescopes begin to show rich and intricate planetary detail. A 5 inch (or preferably a 6 inch) can get below 1 arcsecond resolution, and this is what is necessary to glimpse that fine planetary detail when the seeing is good.

(( In defense of refractors, I would day that Short-tube apochromatic refractors of 4 or 5 inches in aperture are a convenient instrument to use, and can give extremely high contrast images, without the necessity of collimating a reflector; the main reason that some Newtonians have a bad reputation is usually the fact that the owner does not know how to collimate the telescope properly.))

cheers, madbadgalaxyman