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.

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. 

The simplest way to estimate the maximum exposure time is to use PhotoPills’ Spot Stars calculator.

Just introduce focal length and minimum declination of the stars that are captured in your frame to calculate the maximum exposure time to get stars as spots.

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.

This calculator is also available in PhotoPills app and much more.

PhotoPills' Spot Stars calculator.

Spot Stars Calculator's camera selector. 

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.