Visual Astronomy Tools
Real-World Double-Star Resolving & Eyepiece Calculator
Most split calculators stop at Dawes’ limit and leave you disappointed at the eyepiece. This one adds the things that actually decide a split — seeing, the brightness gap between the two stars, central obstruction, colour, altitude and your own experience — then shows you the magnification and eyepiece to use and simulates the view.
Your telescope
Sets aperture, focal length & typical obstruction — all editable.
0 for a refractor; ~0.2 Newtonian; ~0.32–0.37 SCT/Mak.
The double star
You & your sky
Optional — enables tonight’s best time & altitude.
01The numbers
02Best time to look
03Magnification & eyepiece
04Simulated view
This is a physical simulation of the pair in the eyepiece — the two stars are blurred by the seeing you chose, dimmed by their brightness difference, and the bright primary throws the glow that can bury a faint companion. It is the double-star equivalent of “what will I actually see,” and it is the honest counterpart to the theoretical limits above. For framing whole star fields and deep-sky objects, use the Eyepiece & field of view tab, which overlays your true field on a real sky image.
05How double-star splitting really works
Why two telescopes of the same aperture can disagree on the same pair — and why the textbook limit is only the start of the story.
The theoretical limits — and why they mislead
Every telescope has a diffraction limit set purely by aperture. Two well-known rules describe it, both for a pair of equally bright stars in perfect air:
Dawes limit (″) = 116 / D Rayleigh limit (″) = 138 / D (D = aperture in mm)
Dawes is empirical — the closest a keen eye can detect a notch between two touching 6th-magnitude stars. Rayleigh is stricter, the point where one star’s diffraction disk sits on the first dark ring of the other. For a 100 mm scope that’s about 1.16″ and 1.38″ respectively. The trouble is the fine print: equal brightness and perfect air. Real pairs and real skies rarely oblige, which is why a calculator that reports only Dawes is almost always too optimistic.
The four things that actually decide a split
This tool layers four real-world effects on top of the diffraction limit:
- Seeing. The atmosphere smears every star into a blob typically 1–4″ across. If that blob is wider than the pair’s separation, no aperture on Earth splits it that instant. Seeing is usually the real ceiling on tight doubles.
- Magnitude difference (Δm). A faint companion next to a brilliant primary hides inside the primary’s glare, not because the optics can’t resolve the geometry but because contrast fails. This is the single biggest thing ordinary calculators ignore.
- Central obstruction. A reflector or SCT’s secondary mirror pushes light out of the central Airy disk and into the surrounding rings, brightening the halo that swamps a faint companion.
- Colour & experience. A colour contrast (gold and blue) makes a pair easier to separate; a practised observer catches splits in fleeting moments of steady air that a beginner never notices.
The Sirius B problem
Sirius’ companion, the Pup, sits about 11″ away — roughly twenty times the Dawes limit of a modest 100 mm scope. By the textbook it should be trivial. In practice it is one of the most notorious targets in the sky, because the companion is nearly 10 magnitudes (about 8,000×) fainter and drowns in the primary’s glare. Aperture is not the limiting factor here — contrast is. That is exactly the kind of case this calculator is built to flag: it will tell you the geometry is easy while the brightness gap is brutal.
Altitude and air
A star low in the sky is seen through far more turbulent, absorbing atmosphere. At 20° altitude you look through nearly three times the air of the zenith, and seeing degrades accordingly. From Canadian latitudes Sirius never climbs much above 30°, which compounds its difficulty. Enter your location and the tool computes when the pair transits (its highest point) and how high it gets — the single best thing you can do for a hard split is to catch it near the meridian.
Magnification, exit pupil and the eyepiece
Resolving the pair optically is only half the job; you then need enough magnification to enlarge the gap so your eye can see it. As a rule of thumb a tight double wants an apparent separation of a few arcminutes, which for a 1″ pair means 200× or more. Magnification comes from the eyepiece:
Magnification = telescope focal length ÷ eyepiece focal length Exit pupil (mm) = aperture ÷ magnification
Push too far and the exit pupil shrinks below about 0.5 mm (roughly 2× the aperture in mm as a magnification), where the image dims and floaters intrude. The Eyepiece & field of view tab lays out every eyepiece we stock on your scope — magnification, true field and exit pupil — and the split calculator names the one closest to the magnification your target needs.
A quick note on the numbers
The theoretical limits (Dawes, Rayleigh) are exact. The real-world difficulty model — how much the magnitude gap, obstruction, seeing and experience move your practical limit — is a transparent empirical estimate calibrated against well-known targets, not a guarantee. Doubles also drift: the separations built in are recent-epoch values for fast binaries. Treat the verdict as a well-reasoned expectation, and let the eyepiece be the final judge.
Recommended eyepieces
06Frequently asked questions
Why does the tool say a pair is “within my aperture” but still hard?
Because resolution and visibility are two different things. Your aperture may resolve the geometry easily — the stars are far enough apart — while the brightness difference means the faint one is lost in the bright one’s glare. Sirius B is the classic example: wide open geometrically, nearly impossible in contrast. The tool separates these two questions and tells you which one is the wall.
What seeing value should I choose?
Seeing is the size of the blurred star disk in arcseconds. On an average night it’s 2–3″; a steady night is 1–1.5″; a turbulent one is 4″ or worse. If you don’t know, leave it on Average. You can also judge it live: if a bright star boils and shimmers, seeing is poor; if it sits as a tight steady point, it’s good. Tight doubles are a seeing game as much as an aperture game.
How accurate is the difficulty verdict?
The theoretical limits are exact physics. The clean/touching/elongated/single verdict comes from an empirical model that blends the diffraction limit with seeing, the magnitude gap, obstruction, colour and experience. It’s calibrated against well-known targets and is a genuinely useful expectation, but it isn’t a promise — the atmosphere has the final say on any given night. Use it to plan, and to understand why a pair is hard.
Does central obstruction really matter?
For a near-equal pair, only slightly. For an unequal pair it matters more: the secondary mirror shifts light from the central disk into the diffraction rings, brightening the halo around the primary and making a faint companion harder to lift out. That’s why an unobstructed refractor often out-splits an obstructed scope of the same aperture on tough contrast pairs, even though the reflector technically has the same diffraction limit.
How do I pick the right eyepiece?
Open the Eyepiece & field of view tab. It shows the magnification, true field and exit pupil of every eyepiece we carry on your telescope, and flags magnifications past the useful ceiling. For a double, the split calculator also tells you the target magnification and names the closest match in stock. As a guide: high power (small exit pupil) for tight doubles and planets; low power (large exit pupil) for framing clusters and nebulae.
Where do the sky images come from?
The real-sky preview uses Aladin Lite from the Centre de Données astronomiques de Strasbourg (CDS), showing the Digitized Sky Survey (DSS). It renders the actual patch of sky around your target with your eyepiece’s true field drawn on top, so you can see how a cluster or nebula will frame. Note that survey photographs won’t show a tight double resolved — for that, the split calculator’s simulated view is the right tool.
Is any of my data sent anywhere?
All the calculations run entirely in your browser; nothing you type is uploaded. The only external request is to the CDS sky-survey service to fetch the background image for the field-of-view preview, and only when you open that tab.