The Equipment Comparator lets you compare telescopes side by side, spec against spec, and does the same for cameras and eyepieces. Pick one kind, add up to three models from the equipment catalogue and read their specs, with the few numbers worth working out from them, in one table. It is for anyone weighing real models before choosing one.
How to use it
- Choose a kind with the buttons at the top: Telescopes, Cameras or Eyepieces. A comparison holds one kind only, so changing the kind empties the shortlist.
- Press the add button, which reads Add a telescope, Add a camera or Add an eyepiece to match the kind. A window opens over the equipment catalogue for that kind.
- Type in Search brand or model. It matches the brand, the model and the numbers, so 200 finds a 200 mm aperture.
- Select a row to add it. An item you already added stays in the list, dimmed and marked already added, so it cannot be picked twice.
- Repeat for a second and a third item. Each appears as a chip with a cross to remove it. At three, the add button is disabled and reads Three is the most you can compare. Reset empties the shortlist and keeps the kind.
- Read the table. Each spec is a row and each item a column, with the unit in brackets after the row’s name. Read across a row to compare one number. On a narrow screen every item becomes its own card with the same rows.
How it works
The rows depend on the kind. Telescopes: Aperture (mm), Focal length (mm), Focal ratio, Dawes limit (″) and Rayleigh limit (″). Cameras: Pixel size (µm), Resolution (px), Megapixels, and Sensor width, Sensor height and Sensor diagonal (mm). Eyepieces: Focal length (mm), Apparent field (°) and Eye relief (mm).
Aperture, focal length, pixel size, resolution, apparent field and eye relief come straight from the catalogue. The rest are worked out. A camera’s sensor width is its pixel size times the pixels across, divided by 1000 to give millimetres; the height works the same way, the diagonal follows, and the megapixels are the pixels across times the pixels down, divided by a million. Numbers are rounded for display. Where the catalogue holds no value, as with the eye relief of some eyepieces, the cell shows a dash and never a zero.
The Comparator looks at one item at a time, so it shows only what an item has by itself. The focal ratio is the telescope’s own, with no Barlow or reducer. An eyepiece has no magnification or true field here, and a camera no pixel scale or field of view, because each needs a telescope. For a pairing, use the Astrophotography Framing Assistant (telescope and camera) or the Visual Astronomy Assistant (telescope and eyepiece).
The two resolution limits depend on the aperture alone. They ignore central obstruction, the quality of the optics and the Seeing, so they give the smallest separation an ideal telescope could resolve, not what you will see when the air is unsteady. The Comparator does not highlight the higher number or say which item is better. It sets the numbers side by side and leaves the reading to you.
Formula
N = f ÷ D ; Dawes = 116 ÷ D ; Rayleigh = 138 ÷ D
N is the focal ratio, a number with no unit, shown as f/N. f is the focal length and D the aperture, both in millimetres (mm). Dawes and Rayleigh are the two resolution limits in arcseconds (″); 116 and 138 are constants in ″·mm.
Worked example
For D = 103 mm and f = 700 mm: N = 700 ÷ 103 = 6.8, shown as f/6.8. Dawes = 116 ÷ 103 = 1.13″ and Rayleigh = 138 ÷ 103 = 1.34″.
Common questions
How many telescopes can I compare at once, and can I mix kinds?
You can add up to three items, all of one kind: telescopes, cameras or eyepieces. A single item shows its own specs as a card, and the comparison starts at two. Changing the kind empties the shortlist, because a telescope and a camera have no spec in common.
Does the tool say which telescope is better?
No. It marks no difference, gives no score and picks no winner. It sets the numbers in one table so you can read across each row. Which ones matter depends on your use: aperture sets the light gathered and the resolution limits, while focal length and focal ratio decide how a camera frames the sky.
What do the Dawes limit and the Rayleigh limit mean?
Both are the smallest angle between two stars, in arcseconds, that an ideal telescope of that aperture can still show apart. Dawes’ limit is 116 divided by the aperture in millimetres and Rayleigh’s is 138 divided by it, so Rayleigh’s is a little larger.
Why does an eyepiece show no magnification or true field?
Both depend on the telescope the eyepiece goes into, and the Comparator holds no telescope. It shows the eyepiece’s focal length, apparent field and eye relief. To read the magnification and true field, pick both in the Visual Astronomy Assistant.
What if my model is not in the catalogue?
Then it cannot be compared here. The catalogue lists real, purchasable models and is read-only: you can search it, but you cannot type in numbers of your own.
Does the comparison tool need an account, and is my shortlist saved?
No account is needed and nothing is saved. Your shortlist lives on the page, so a reload empties it. The only things your browser keeps are your theme and language preferences.
Related tools
In the store
Equipment this tool helps you choose, at David Astro: