One person, a calculator, and a rule
Aperture is made by one person, not an editorial team: a software engineer who observes from a suburban balcony under a Bortle 6 sky, with a small refractor and a pair of binoculars, and who has watched too many people buy a telescope in November and sell it in February.
The site exists for one reason. The gap between the picture on the box and what the eyepiece actually shows is the single biggest cause of abandoned telescopes, and nobody selling telescopes has an incentive to close it.
The rule
Never show a magnification without the aperture and the exit pupil it implies. Every number on this site goes through that rule, and every picture is drawn from it.
How the advice is computed
The advisor first computes a spending ceiling from your answers alone — your sky, your home, how you will carry the instrument, who will use it — before it looks at any product. If the ceiling lands below your budget, it says so; that is a successful outcome, not a fallback.
It then ranks instrument classes — '150 mm Dobsonian', '80 mm ED refractor on an alt-azimuth mount' — against your goals, and only then names real products inside the winning classes, with prices dated to the day they were observed.
Every recommendation carries a list of what not to buy yet. Binoculars, and 'nothing yet, learn the sky first', are first-class answers.
What is physics and what is experience
Magnification, exit pupil, true field of view, resolution (Dawes' limit), surface brightness against the exit pupil squared, limiting magnitude: arithmetic, exact, unit-tested with worked examples.
'You need about 250 mm to see M51's spiral arms from a rural sky', and how much a brighter sky costs each target: observing heuristics — somebody else's eyes, somebody else's night. They are labelled as such wherever they appear, they live in one table so they are cheap to revise, and revising one means citing a source.
The sky class the Tonight page suggests for your town is a third kind of number: an estimate. Satellite images (the VIIRS Day/Night Band, in composites by the Earth Observation Group at the Colorado School of Mines, distributed by the World Bank under CC BY 4.0) measure the light going up from the ground; a published distance law (Cinzano and Falchi's generalized Walker law) turns it into the glow overhead; only a scale factor and the natural sky are fitted, on sky-meter readings published for 39 French sites; and a conventional table turns the result into a Bortle class. For a place outside the calibration it is typically half a magnitude out, about one class, and it reads the sky too bright around Paris. It says it is an estimate wherever it appears, and the slider beside it is there to overrule it.
How the eyepiece view is made
The view is a real photograph used as raw material, never shown raw. It is scaled to the true field of your eyepiece, darkened by the exit pupil, blurred by the worse of your optics and the air, thresholded against the sky glow of your Bortle class, and drained of colour wherever it is too dim for the eye's cones. The processing only takes away: nothing rendered is brighter, sharper or more colourful than the arithmetic allows.
The photographs are public-domain or Creative Commons frames from NASA, ESA/Hubble and Wikimedia Commons contributors, credited under the simulator.
Where the money comes from
Some retailer links pay a commission. The ranking is computed before the catalogue is opened, and an automated test — firewall.test.ts, in the public source — fails the build if the advice engine so much as mentions a retailer, an offer or a commission. Offers are sorted by price, never by what pays. The full arrangement is on the funding page.
How to check
Every computed number is shown beside the picture, not only inside it. Every price carries its date. Every heuristic says it is one. If you find a number that does not add up, or a claim that flatters, write: it will be fixed, and the fix will be visible.
Contact
contact@apertureastro.com — questions, corrections, and telescopes you regret buying are all welcome.