Exposure Time by Focal Length

The longer the focal length, the less time a single untracked exposure has before stars smear into short trails. A 14mm lens on a tripod can hold a star as a clean point for close to 15 seconds. A telescope at 1,200mm has well under a second before the same thing happens. This chart runs both the old 500 rule and the modern NPF rule across the full range a beginner is likely to own, from a wide astro lens to an 8 inch Schmidt-Cassegrain, and shows exactly where a star tracker or equatorial mount stops being optional and starts being the only way forward.

What is the NPF rule, and how is it different from the 500 rule?

The 500 rule is the older shortcut: divide 500 by your lens focal length in millimeters to get a safe exposure time in seconds. It was built for a film and early-digital era of large pixels and modest resolution, and it ignores two things that matter a great deal on a modern sensor: how wide the aperture is open, and how small the individual pixels are. NPF is (35 times the f-number, plus 30 times the pixel pitch in microns), divided by the focal length in millimeters, divided again by the cosine of the declination you are shooting. The table in this article is computed at declination 0, the celestial equator, which is the worst case: exposures at any other declination can run equal or longer.

Pixel pitch is the physical width of one pixel on the sensor, in microns. Smaller pixels resolve finer detail, but they also reveal star trailing sooner, because a trail becomes visible once it crosses roughly one pixel's width. A 45 megapixel camera with tiny pixels will show trailing at an exposure time that looked perfectly clean on a 12 megapixel camera with large pixels, at the identical focal length and f-number. This is the exact gap the 500 rule cannot account for, and it is why NPF results below run consistently shorter, often by 30 to 50 percent, than the 500 rule at the same focal length.

How long can I expose without a tracker, by focal length?

The table below runs from a 14mm ultra-wide astro lens through a 2032mm Schmidt-Cassegrain, using a representative f-number for wide lenses and the real, computed focal ratio for the telescopes with a matching product on this site. Three pixel pitches are shown side by side: 2.4 microns, typical of a high-resolution small-sensor camera; 3.8 microns, typical of a mainstream DSLR or mirrorless body; and 5.9 microns, typical of a larger-pixel, lower-noise astronomy camera. All three shrink fast as focal length grows, which is the whole story of this page.

Computed at declination 0 (the worst case) with the formula (35 × f-number + 30 × pixel pitch in microns) / focal length. A target nearer the celestial pole tolerates a longer exposure than the table shows; a target on the celestial equator is exactly what the table shows.

Why does untracked deep sky imaging stop working around 100mm?

Look at the 85mm row against the 135mm row in the table above. At 85mm and f/2.8, even the most forgiving column, the 5.9 micron pixel pitch, allows only about 3.24 seconds. By 135mm that has already fallen to about 2.04 seconds. A sub-2-second untracked exposure gathers so little photon signal from a faint nebula or galaxy that no realistic amount of stacking rescues it: stacking averages down noise, but it cannot manufacture signal that was never collected in the first place. This is the practical wall, not an arbitrary number: past roughly 85 to 100mm, an untracked exposure stops being useful for anything except the Moon, a full solar disc behind a proper filter, or bright star trail and constellation photography where a soft point is not the goal.

Every telescope focal length in the lower half of the table, 250mm and up, sits at well under 1.5 seconds untracked. A SVBONY SV503 80ED at 560mm allows roughly half a second before trailing. A Celestron NexStar 8SE at 2032mm allows a small fraction of a second. None of these numbers are usable for deep sky work without a mount that tracks the sky's motion, which is exactly why every telescope-based deep sky imaging setup on this site assumes tracking from the start rather than treating it as an accessory.

What changes once you add a star tracker or equatorial mount?

A tracker rotates the camera or telescope at the sidereal rate, roughly matching the sky's own apparent motion, so a star that would trail across the frame in a fraction of a second instead stays fixed relative to the sensor for minutes at a time. A basic tracker like the iOptron SkyGuider Pro , used unguided, typically holds round stars for about 2 to 4 minutes at focal lengths up to roughly 200 to 300mm, limited mainly by how precisely it was polar aligned and by small, repeating errors in its own drive mechanism, called periodic error.

Add autoguiding, a small guide scope and camera that watches a star and feeds real-time correction back to the mount, and that ceiling extends to roughly 5 to 10 minutes per sub even at longer focal lengths, which is where a deep sky target actually starts building usable signal-to-noise ratio. A Sky-Watcher Star Adventurer GTi or a full equatorial like the Celestron Advanced VX both support this workflow, the GTi as a travel-weight first imaging mount and the Advanced VX as a heavier payload option for an 8 inch SCT or a 6 inch imaging Newtonian.

Why does declination change the exposure time?

The NPF formula divides by the cosine of declination because stars do not all move across the frame at the same apparent speed. A star sitting on the celestial equator, declination 0, traces the largest possible circle around the sky's pole in 24 hours, so it moves fastest across a fixed frame. A star near declination 60 degrees traces a much smaller circle in the same 24 hours, so it moves proportionally slower, and the cosine of 60 degrees is 0.5, meaning that star tolerates roughly twice the untracked exposure time of an equatorial target at the same focal length and pixel pitch. This is a real, physical effect, not a rounding fudge, and it is why two targets shot on the same night with the same gear can have noticeably different amounts of trailing at the identical shutter speed.

What telescope and camera pairing should a beginner actually start imaging with?

For a first deep sky imaging setup, the practical order of operations is: buy the tracking mount before the telescope, since the mount is what turns any focal length into usable exposure time, and start at a short focal length rather than a long one. A wide lens or a compact refractor like the William Optics RedCat 51 at 250mm is dramatically more forgiving of polar alignment error and mount flex than a telescope at 1,200mm or more, because the same arcsecond of mount error covers a much smaller fraction of the frame at short focal length. A dedicated astronomy camera such as the SVBONY SV305C for lunar and planetary video, or a cooled camera like the ZWO ASI183MC Pro for long-exposure deep sky work, both pair naturally with the focal lengths in the lower half of the table above. Whatever mount ends up carrying that setup, check its rated payload against the telescope and camera together at mount payload by telescope weight, since an overloaded mount tracks worse than a properly loaded one regardless of what the NPF numbers above suggest is possible. Our NPF rule exposure calculator runs this math for your exact focal length, f-number and camera, and the best star trackers compares the mounts that make exposures past 100mm possible in the first place. For the full beginner path into this hobby, see how to start astrophotography.