70/700 refractor, alt-az: what you actually see
The 100–150 € first telescope. Fine on the Moon and the planets; the wobbly tripod, not the glass, is what people give up on.
With the eyepieces in the box
A 70 mm aperture through its 25 mm · 10 mm eyepieces, under a suburban sky. Every circle is the true field; nothing is enlarged.
Try it yourself
Grey because it is grey. The eye has almost no colour response at these light levels — the photographs you have seen are long exposures.
Built from a photograph — NASA, ESA, M. Robberto (heic0601a), CC BY 4.0 — used only as input: scaled to the true field, then darkened, blurred and desaturated by the arithmetic. Nothing here is brighter, sharper or more colourful than the exit pupil allows.
The numbers
28× on 70 mm at f/10.0 — 2.5 mm exit pupil
True field 1.79° = 107′ · focal length 700 mm
Brightness 1.6× of a 2 mm exit pupil · detail limit 2.50″
A 2.5 mm exit pupil — comfortable. All 70 mm of aperture is reaching your eye.
The ones people buy first
The 100–150 € first telescope. Fine on the Moon and the planets; the wobbly tripod, not the glass, is what people give up on.
Or start from a catalogue instrument
How much light and how much detail. The only number that sets a ceiling on what is possible.
Focal length 700 mm. Changes the magnification an eyepiece gives, not how much light arrives.
28× — a shorter eyepiece magnifies more, and dims everything as it does. The slider runs the way the boxes are numbered: left is short and powerful.
Plössl — the standard bundled eyepiece
6 — bright suburban, Milky Way only overhead if at all
How steady the air is, in arcseconds. Tonight the atmosphere (2.5″) is the detail limit, not the optics (1.66″) — a bigger telescope would not show more.
Detectable — you can tell something is there
A grey-green fan of light wrapped around four close stars. It is the one deep-sky object that survives a city sky, and the only nebula most people ever see colour in.
The four stars at the centre are the Trapezium; a 100 mm splits all four. Larger apertures add wings and texture rather than colour.
Winter evenings in the northern hemisphere; summer in the southern.
The sky is the limit here, not the telescope
Under a Bortle 4 sky this same 70 mm would show recognisable — it looks like what it is. To match that from where you are, you would need about 100 mm of aperture. A drive is cheaper.
What this setup reaches — 9 of 14
70 mm · Bortle 6Struck through means not visible at all from your sky with this aperture. Drag the aperture slider and watch which ones come back — then drag the sky slider and watch how much more it buys you than the aperture did.
Which targets are reachable is an observing heuristic — somebody else’s eyes, somebody else’s night — and so is the 2.5″ of atmospheric seeing assumed above. The magnification, exit pupil, field of view and surface brightness are arithmetic and are exact.