Astro Exposure Calculator

by Michael Oates

This exposure calculator is designed for both astronomical and terrestrial use.


  1. First download each of the 3 discs and print out onto thick paper or even card if your printer will allow this.
  2. Paste the prints onto card.
  3. Cut out the 3 disks, then carefully with a sharp knife cut out the 2 shaded portions.
  4. Next pierce the centres of each card with a sharp point.
  5. Pass a paper fastener through all 3 disks, the smallest on top and the largest on the bottom.
  6. Spread out the fastener on the back an secure with tape.
  7. The calculator is now ready for use.

How to use the Calculator

I will explain with an example...

Suppose you wish to photograph the moon at quarter phase and the film you are using has a speed of 100 ISO.

  1. First use the pointer marked LUNAR and move the outer disk until it is aligned with QUARTER.
  2. Then making sure the two outer disks don't move rotate the inner disk until '100' is seen in the small window.
  3. You can now read off the exposure required from the 2 scales 'f' and 'sec', such as 1/60th sec at f8, or 1 sec at f64.
For planetary photographs use the PLANET pointer.

You can even work out the correct exposure for ordinary daytime photography by using the DAYLIGHT pointer.

Exposures should be bracketed either side of the indicated exposure for a number of reasons.


The figures used in the exposure calculator were based on tables published in Michael Covington's book "Astrophotography for the Amateur".

I've taken many photographs with this calculator and I find the exposures always spot on. (but then I may be slightly biased :-))

You are free to use the calculator and pass it on to friends. But I do ask, please do NOT re-distribute, without permission, or sell/pass this on to other parties for profit.

Copyright 1995 Michael Oates

The Disks

Small Disk

Medium Disk

Large disk

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