While many astrophotographers follow the “rule of 500” (or 300), some experimentation can help find the right exposure time for your setup.
When a child is taught to ride a bike, we often put training wheels on the back; they straddle the rear wheel to ensure the bike cannot tip over while the child learns to keep their balance. Training wheels are great for building confidence and familiarity with the operation of a bicycle.
What does this have to do with astrophotography? Well, there is a rule of thumb called “The Rule of 500,” that acts a lot like training wheels. And at some point, it's time to put it away.
Rule of 500 (or 300)
If you've never heard of the Rule of 500, it's quite simple. When taking an untracked photo of the night sky using a camera on a tripod, this rule tells you how long you can expose before the stars begin to trail. You take the number 500 and divide by the focal length of your lens. For example, if you have a 20-mm wide angle lens, then 500 / 20 = 25. You can shoot for 25 seconds on a tripod before the stars start to streak.
Rules of thumb have their place. When I helped a friend teach a Milky Way photography class at the Grand Canyon Star Party a few years ago, some needed the help of the lab assistant (i.e., me) in the dark. “How do I set my exposure length again?” There’s no shame in this. They needed the training wheels until they gained a better familiarity with their camera.
When DSLRs with crop sensors (smaller than 24×36 mm) came on the market, the “Rule of 500” evolved into the “Rule of 300.” Stars trailed more quickly with these cameras, so they required shorter exposures.
Some reasoned that the shorter exposure times came from a longer focal length, but that still makes me squirm. If you replace the camera on your telescope with one that has a smaller sensor, does it change the focal length of your telescope? Most certainly not.
The real reason those crop-sensor cameras had to expose for less time is because they had smaller pixels than full-frame cameras at the time, not because there was any change in focal length. In fact, today’s full-frame cameras have smaller pixels, so any new DSLR or mirrorless camera will be more likely to obey the “Rule of 300.”
But enough about rules: If you're at that point and are ready to put away the training wheels, read on.
How Long Can You Expose?
Calculating the true exposure time rather than using a rule involves pixel scale, which measures how much sky one pixel covers. In a previous blog, I explained how to calculate the pixel scale for your telescope, and the same formula works for camera lenses as well. Given Earth's rotation and a fixed camera, you can calculate how long will it take for a star to drift from one pixel onto another. But if you do the math (and I have), you end up with a number that’s too large. That’s because most stars will start out overlapping more than one pixel to begin with.
So let’s take off the engineer’s hat and put on the chef’s hat. No good cook follows a recipe without tasting the soup to make sure they’ve added enough salt. Start with the recipe: Use the rule of 300 as a starting point and take a photo of a starfield. Now, zoom in on your resulting photo.
If your camera’s zoom won’t let you see close enough, take a range of exposures and download the images to your computer to look at them carefully. (When picking a camera for nightscape photography, try to choose one that lets you zoom in on completed exposures.)
Note that you don’t need a pristine dark sky site to do this. Turn down the camera’s ISO, close the aperture a bit — do whatever you need to do to take a long exposure from your light-polluted yard and still show some stars.
Now, play with the rule. You can characterize the camera-lens system for yourself and see how far you can push it before the stars start to noticeably trail. Note that you’ll want to focus on examining stars in the middle of the frame, as even the best camera lenses will add some distortion to stars near the edge of the field, which can easily be mistaken for trailing stars.
For the honors students I’ll add this: Stars near the celestial equator are going to drift across your pixels much more quickly than the stars closer to the celestial poles. If you have a wide-enough field of view, you may get differential drift; stars in one part of your frame may trail while others stay nice and crisp. The key remains trial and error. Evaluate your exposures on the spot and determine just how long you can really shoot before the stars transform from points to little inchworms of light.