How we compare telescopes
Every score on this site comes from the same deterministic function applied to the same stored facts. This page documents it completely, including the parts that are editorial judgement rather than arithmetic.
The short version
- Each telescope's specifications are stored once, in one file, and reused everywhere.
- Fifteen scoring factors are computed. Eleven are derived from those specifications plus structural properties of the optical design. Four cannot be derived from a specification sheet at all and are declared explicitly as editorial ratings.
- Each of the four purposes weights those factors differently. The weighted average, on a 0–10 scale, is the published score.
- Where a written verdict disagrees with the arithmetic, the verdict wins and says why.
Why there are four purposes and not one
A telescope that is outstanding for photographing large nebulae can be close to useless for photographing small galaxies, and physically incapable of visual observing. Averaging those into a single "score out of 10" would hide the only information that matters.
So every telescope is scored four times, against visual observing, nebula imaging, galaxy imaging and planetary imaging, and the comparison pages let you switch between them.
Derived factors
These are pure functions of stored facts. The same record always produces the same number, and you can check the arithmetic against the specifications on any product page.
| Factor | What it measures |
|---|---|
| Aperture | Light grasp and theoretical resolution, scaled logarithmically between a 50 mm starter refractor and a 400 mm large Dobsonian. |
| Portability | Derived from total system weight and transport length. A telescope that stays in a cupboard scores nothing in the real sky. |
| Mount usability | Whether a mount is included, whether it tracks, whether it has GoTo, and how much payload margin it has. |
| Tracking | Tracking quality available out of the box — decisive at planetary magnifications where an untracked target crosses the field in seconds. |
| Mount demand | Inverted: how forgiving the telescope is of an average mount. Long focal length and high weight lower this score. |
| Focal ratio | Speed of the system for extended nebulosity. Faster is better here, with diminishing returns below about f/3. |
| Corrected field | Star shapes across the frame in the configuration you are likely to use, including whether a corrector is built in or bolted on. |
| Image circle | Illuminated, corrected circle measured against APS-C and full-frame sensor diagonals. |
| Focal length fit | How well native focal length matches the target class: roughly 200-600 mm for large nebulae, 900-2000 mm for galaxies, and the longest practical focal length for planets. |
| Resolving power | Dawes limit from aperture, combined with image scale, capped by realistic atmospheric seeing. |
| Collimation & thermals | Derived from the optical design: how often alignment must be checked and how long the tube takes to reach thermal equilibrium. |
Editorial factors
These four cannot be computed from a specification sheet. They are declared as explicit 0–10 ratings in each telescope's record, and every score breakdown on the site labels themeditorial so you can tell them from the derived numbers.
| Factor | What it represents |
|---|---|
| Optical quality | Editorial rating of colour correction, contrast and sample consistency for the design and price band. Not a measured Strehl ratio. |
| Ergonomics | How comfortable the instrument is to actually use at the eyepiece: eyepiece position, setup steps, and whether it is usable alone. |
| Imaging workflow | Editorial rating of how much friction stands between setup and a finished sub-exposure: backfocus tolerance, adapters, tilt sensitivity, dew and collimation routines. |
| Value | Editorial rating of capability delivered per unit of cost within its price band. We do not publish prices, so this is a band-relative judgement. |
The weights
Each purpose weights the factors differently. These are the actual values used by the code that generated every score on this site.
| Factor | Visual | Nebulae | Galaxies | Planetary |
|---|---|---|---|---|
| Aperture | 30% | — | — | 30% |
| Optical quality | 20% | — | 15% | 20% |
| Ergonomics | 15% | — | — | — |
| Portability | 15% | — | — | — |
| Mount usability | 10% | — | — | — |
| Tracking | — | — | — | 10% |
| Mount demand | — | 10% | 10% | — |
| Focal ratio | — | 25% | — | — |
| Corrected field | — | 20% | 15% | — |
| Image circle | — | 15% | — | — |
| Focal length fit | — | 15% | 25% | 25% |
| Resolving power | — | — | 20% | — |
| Imaging workflow | — | 10% | 10% | — |
| Collimation & thermals | — | — | — | 10% |
| Value | 10% | 5% | 5% | 5% |
Editorial overrides
A telescope record may override its derived score for a purpose. When it does, a written rationale is mandatory, the number is labelled editorially adjusted, and the derived value is still shown in the breakdown so you can see the size of the adjustment.
Eligibility, which is not the same as a low score
Some telescopes cannot serve a purpose at all. A RASA has no eyepiece position, so it is not a poor visual telescope — it is not a visual telescope. Those cases are markedNot applicable with an explanation rather than being given a score near zero, because a zero implies a comparison that does not exist.
Winners and ties
On a comparison, the derived winner is whichever telescope scores higher, unless the difference is under 0.3 points, in which case it is reported as a tie. The scale does not support finer distinctions than that and pretending otherwise would be false precision.
Winners are never signalled by colour alone: every marker carries a symbol and a text label.
What the numbers cannot tell you
- Sample variation. Two units of the same model can differ. We score models, not the specific telescope you will receive.
- Your sky. Light pollution changes which telescope is right more than most specifications do, and we cannot know yours.
- Whether you will use it. The most consistent predictor of satisfaction is how often a telescope gets carried outside, and no datasheet records that.
- Seeing. Atmospheric turbulence caps resolution regardless of aperture, and it varies by site and by night.
Scores are relative to the telescopes on this site, not absolute quality marks. A 6.5 for galaxy imaging and a 6.5 for visual observing do not mean the same thing.
What we do not do
- We do not claim to have tested these telescopes. Nothing on this site says "we tested" or "in our lab", because it would not be true.
- We do not publish prices. A price recorded last month is worse than no price at all, so we publish coarse bands and link to retailers for the current figure.
- We do not publish ratings, review counts or stock levels we cannot verify.You will not find star ratings or "1,204 reviews" anywhere here, and our structured data does not claim them either.
- We do not generate every possible pair as a page. Currently14 telescopes would produce 91possible pairings; we publish 5 because those are the ones we have something to say about. Everything else is available as a clearly-labelled specification comparison that is not indexed.
Data provenance
Specifications are compiled from public manufacturer documentation. Records that have not been independently re-verified are labelled on the page and carry source links. Calculated values — Dawes limit, limiting magnitude, image scale — are labelled as calculated rather than presented as manufacturer figures.
If you find an error, tell us. We correct it, and we note that the page changed.