STEP BY STEP

How to record the data you need to determine a shower’s ZHR photographically

Step 1

Use a fast mid- to wide-angle lens (eg 14-40mm). A 12V hairdryer or heater band can help keep it moisture free. Set to manual focus and use lowest f-number. Set the camera to manual, and ISO to half to two-thirds of maximum.

Step 2

Tracking or fixed mounts are fine. Focus at infinity. Selecting JPG over RAW reduces frame transfer times albeit with reduced quality. Choose exposures of 30-60 seconds. Up to 30 seconds, set camera to continuous shooting and lock the button down on a remote shutter release.

Step 3

For longer exposures, use a programmable release with the camera in bulb mode. Record the limiting magnitude (dimmest star visible) and radiant altitude every 15 minutes. Estimate the radiant altitude using a planetarium program or app.

Step 4

Download images and check for trails. Add a prefix of ‘meteor_’ onto every image in which you find one. Count how many belong to the shower. For the relative ZHR calculation, you’ll also need to estimate the fraction of the sky covered by the camera.

Step 5

To calculate relative ZHR unique to your setup, chop the session into hours. Determine the shower trail count (N), fraction of sky covered (F; for example, for one-quarter sky coverage, F would equal 4), radiant height (Rh) and the average limiting magnitude (lm).

Step 6

Calculate your camera’s relative ZHR (which isn’t equivalent to a true ZHR) for the time spent observing (T obs). The population index (r) is unique for each shower and can be obtained from sources such as the International Meteor Organisation (www.imo.net).