How to use the astrophotography calculator
This is an all-in-one calculator for telescope and astrophotography math. Pick your telescope, camera and eyepiece from the built-in databases — or switch on Custom specs and type your own numbers — and every result on the page recalculates instantly from the same inputs. A focal reducer or Barlow is applied across all of the figures that depend on focal length, and the framing preview draws your sensor rectangle and eyepiece circle to scale against the target you choose.
The page is organised into three result groups that answer three different questions:
- Imaging — what part of the sky lands on your sensor, how finely it is sampled, and whether that matches your seeing.
- Visual — how much an eyepiece magnifies, the true field it shows, and the exit pupil reaching your eye.
- Optics — the resolving power and light-gathering ability of the aperture itself.
Everything you set is saved in your browser, and the Copy share link button puts your whole configuration into a URL so you can bookmark a setup or send it to an imaging buddy.
Field of view and image scale
A telescope on its own has no single "field of view." The field is set by the focal length working together with the size of the sensor (for imaging) or the apparent field of an eyepiece (for visual). For a camera, the field of view is found separately for the sensor's width and height:
With a 23.5 mm-wide sensor on a 550 mm refractor, the horizontal field works out to about 2.45° — roughly five Moon-widths across. Add a 0.8× reducer and the effective focal length drops to 440 mm, widening that field to about 3.06°; add a 2× Barlow and it narrows to about 1.23°. The framing preview above shows exactly how the resulting rectangle sits over real deep-sky targets, which is the quickest way to tell whether the Andromeda Galaxy will fit in one frame or needs a mosaic.
The companion number is image scale — how much sky each pixel covers, in arcseconds per pixel:
A 3.76 µm pixel at 550 mm gives 1.41 ″/px. Image scale is the bridge between your gear and the sky's detail, and it is what the sampling check below is built on.
Sampling: matching pixels to the sky
Resolution in a deep-sky image is almost never limited by the telescope — it is limited by seeing, the blurring caused by turbulence in the atmosphere, measured as the full-width-half-maximum (FWHM) of a star in arcseconds. Good sampling means your pixels are fine enough to record that blur without being so fine that you are just spreading the same blur over more pixels (and collecting less signal per pixel, for no extra detail).
A widely used target is to place roughly three pixels across the seeing disk:
| Pixels per FWHM | Verdict | What it means |
|---|---|---|
| < 2 | Undersampled | Stars land on too few pixels; fine detail and round stars are lost. Common with short lenses and large pixels. |
| 2 – 3.5 | Well sampled | The sweet spot — detail preserved without wasting signal. |
| 3.5 – 5 | Slightly oversampled | Usually fine; you trade a little signal per pixel for headroom to crop or deconvolve. |
| > 5 | Oversampled | Pixels far finer than the seeing allows; longer exposures for the same depth, with no real gain in detail. Consider binning. |
Because the verdict depends on your sky, set the Seeing dropdown to match your site. A 1.4 ″/px scale that is perfect under 3–4″ suburban seeing can look oversampled at a 1″ world-class site and undersampled in poor conditions. The gauge above moves as you change gear or conditions so you can find a combination that lands in the green.
Focal ratio, with reducers and Barlows
The focal ratio — the famous "f-number" — is simply focal length divided by aperture:
It controls how quickly your camera accumulates signal from extended objects like nebulae: a faster (smaller) f-ratio means shorter exposures for the same depth. A focal reducer multiplies the focal length by its factor (e.g. 0.8×), which lowers the f-ratio and widens the field; a Barlow or extender does the opposite. Switch reducers and Barlows in the input panel and watch the f-ratio, field of view, image scale and sampling all shift together — they are not independent settings, and seeing how they trade off against each other is most of the battle in choosing a configuration.
Magnification, true field and exit pupil
For visual observing, the eyepiece sets the magnification:
The patch of real sky you see — the true field of view — is the eyepiece's apparent field divided by that magnification:
And the exit pupil is the width of the light beam leaving the eyepiece, which should not greatly exceed your eye's dark-adapted pupil (about 7 mm for young eyes, less with age):
Two practical bounds fall out of this. The lowest useful magnification is the one that produces a ~7 mm exit pupil (below it, light is wasted around your iris); the highest useful magnification is roughly twice the aperture in millimetres, beyond which you magnify the blur faster than any new detail. The Visual panel reports both so you can see where your current eyepiece sits in that range.
Resolution and light grasp
Two classic formulas describe the finest detail an aperture can separate, both in arcseconds for an aperture D in millimetres:
A 100 mm aperture resolves about 1.16″ (Dawes) — enough to split tight double stars, though atmospheric seeing usually has the final say. Light grasp compares how much more light the telescope collects than your naked eye:
That 100 mm scope gathers roughly 200× more light than a 7 mm pupil — which is why faint galaxies snap into view through even a modest telescope. These figures depend only on aperture, so they stay put as you change cameras and eyepieces; they are the fixed character of the glass.
Ideal sub-exposure: swamping read noise
For deep-sky imaging, the best length for a single frame — a "sub" — is the one where noise from the sky background grows just large enough to swamp the camera's fixed read noise. Beyond that point, longer subs add almost nothing to the final stacked image, while every minute you add is more data lost when a satellite trail, a wind gust or a clipped bright star ruins a frame. The calculator finds that crossover from your sky brightness, optics and sensor:
Here RN is read noise in electrons and p is the extra noise you are willing to accept versus an infinitely long exposure. The 5% standard gives a swamp factor of about C ≈ 9.8; the strict 2% setting needs much longer subs (C ≈ 24.7), and the relaxed 10% setting allows shorter ones (C ≈ 4.8). The sky rate is how fast light pollution fills each pixel, built from your site and gear:
This is why the recommended sub shifts so much with conditions. A darker sky (higher SQM) or a narrowband filter slashes the sky rate, so you need far longer subs to swamp read noise — narrowband at f/7 from a Bortle 2 site can want 10-minute subs, while broadband at f/4 under a bright suburban sky can be swamped in 15–30 seconds. A faster focal ratio, larger pixels or a more sensitive sensor all raise the sky rate and shorten the ideal sub. Because the page already knows your telescope and camera, all of that is filled in for you — just set your sky and filter.
The swamp curve plots the extra noise a given sub length adds compared with infinitely long exposures. It falls steeply at first and then flattens; the recommended sub sits where the curve drops into the green zone at your chosen tolerance. Past the knee, you are climbing a curve that is already nearly flat — spending exposure for almost no gain in depth. Dark current matters mainly for long narrowband subs, where it can rival or exceed the sky rate; set your camera's measured value in the advanced panel for those.
Need the full filter database and one-shot-colour analysis? The in-page calculator above is a quick single-channel estimate. For per-Bayer-channel swamping on OSC cameras, a verified filter catalog (Baader, Chroma, Astrodon, Antlia, Optolong and more), full-well saturation checks and a complete session planner, use the dedicated Advanced Sub-Exposure Calculator. And to see your frame on real sky imagery, the Telescope FOV Simulator overlays it on DSS, 2MASS and DESI Legacy survey data.
Frequently asked questions
How do I calculate a telescope's field of view?
2 × arctan(sensor size ÷ (2 × focal length)), calculated separately for the sensor's width and height. For visual use, it is the eyepiece's apparent field of view divided by the magnification. This calculator does both at once and draws the result to scale on your chosen target.What is a good image scale for astrophotography?
Does a focal reducer change my field of view and sampling?
What is exit pupil and why does it matter?
What is the highest useful magnification of my telescope?
Why is the resolution limit different from what I actually see?
How long should my sub-exposures be?
Do longer sub-exposures always give a better image?
Equipment specs are drawn from manufacturer data and community measurements and are approximate — verify against the manufacturer before any purchase decision. Built by Stellar Nomads.