STEP BY STEP

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

Sky at Night Magazine - - IMAGING FOR SCIENCE -

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).

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