Spot Stars Calculator

How to use the Spot Stars Calculator

Mark Gee's winning image of the Astronomy Photographer of the Year 2013: Guiding Light To The Stars.

NOTE: The calculator above fails in most cases and it has dramatically been improved in the Spot Stars Calculator included in PhotoPills. Check next section for more accurate results.

When shooting the Milky way, the longer you keep the shutter open the better, with only one limitation: star trails.

You need to collect as much light as possible to capture stars as big bright spots, but you don’t want to get motion blurring due to the Earth’s rotation. In other words, you don’t want to see the arcs traced by the stars. And thus, you need to limit the exposure time.

To calculate the maximum exposure time using the calculator above just introduce focal length and minimum declination of the stars that are captured in your frame.

The first exposure time you get is less accurate (500 rule) but useful when you don't know the minimal star declination. Basically, to determine the optimal length of exposure, you take 500 and divide it by the effective focal length of the lens (Exposure time = 500/[crop-factor × focal length]). Thus, the shorter the focal length the longer the shutter speed, and the better images you’ll get.

The second value you get is more accurate because it takes into account sensor size, focal length and minimum declination of the stars.

The declination is the vertical angular distance between the center of a celestial body (stars) and the celestial equator. You can measure it using PhotoPills Night AR or PhotoPills Planner Night AR. A declination of +20° means that the celestial body is located 20° north of the celestial equator. The south polar cap is at a declination of –90°, the equator is at declination 0°, and the north polar cap is at a declination of +90°. Declination is to a celestial globe as latitude is to a terrestrial globe, a vertical positioning of an object.

Advanced PhotoPills Spot Stars calculator: NPF rule

As a matter of fact, the calculator above fails. It provides too large exposure times in most of the cases, getting the stars as small trails in our Milky Way shots, not dots!

For a more accurate exposure time, it’s necessary to take into account not only the focal length and the declination of the stars, but also camera sensor, camera megapixels and aperture.

Luckily, Frédéric Michaud and La Société Astronòmique du Havre have come up with a solution: the NPF rule.

And the good news! The NPF rule has been implemented into PhotoPills Spot Stars Calculator. Now you can even use PhotoPills Augmented Reality view to calculate the exposure time. Just point your phone in the sky and get the results. Check the screenshots below!

New PhotoPills' Spot Stars calculator.

Spot Stars Calculator's Augmented Reality mode.

Finally, If you're interested in learning how to imagine, plan and shoot the sun, moon and Milky Way, take a look at the following How-to articles:

How to embed the Spot Stars Calculator on your website

Take the power of PhotoPills’ Spot Stars calculator calculator with you. Just copy the following lines and paste them within the code of your website, right in the place where you want to embed it: 

<div id="ppspotstars"></div>
<script src="//" async></script>

The code will run asynchronously, without penalizing the loading time of your website.