What you’ll actually see
Everything below is computed. The circle is the true field of view your instrument and eyepiece produce; the object is drawn at its real angular size inside it; how bright it appears comes from your aperture and your sky. Nothing here has been made prettier than it is.
Grey because it is grey. The eye has almost no colour response at these light levels — the photographs you have seen are long exposures.
The numbers
120× on 150 mm at f/8.0 — 1.3 mm exit pupil
True field 0.42° = 25′ · focal length 1200 mm
Brightness 39% of a 2 mm exit pupil · detail limit 2.50″
A 1.3 mm exit pupil — comfortable. All 150 mm of aperture is reaching your eye.
Start from a real instrument
How much light and how much detail. The only number that sets a ceiling on what is possible.
Focal length 1200 mm. Changes the magnification an eyepiece gives, not how much light arrives.
120× — 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
Recognisable — it looks like what it is
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.
It does not fit
At 85′ across, this target is wider than the 25′ your eyepiece shows. A longer eyepiece — or binoculars — will frame it better than more magnification ever could.
The sky is the limit here, not the telescope
Under a Bortle 4 sky this same 150 mm would show detailed — structure is visible. To match that from where you are, you would need about 200 mm of aperture. A drive is cheaper.
What this setup reaches — 14 of 14
150 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.
Why it is grey
Colour vision comes from cone cells, which need far more light than any nebula delivers. Below that threshold you are seeing with rods, which are more sensitive and completely colourblind. The Orion Nebula is the usual exception — bright enough that some people catch a green tint — and Mars, Jupiter and Saturn are bright enough for colour throughout.
This is not a limitation to be disappointed by. Those photons left the Orion Nebula around the time the Norman Conquest was being planned, and they ended their journey on your retina rather than a sensor. The photographs are beautiful. This is different.