Monday, July 28, 2014

webspotting 35 - calculating the speed of the sky

As published in the August/September 2014 issue of SCOPE, the newsletter of the RASC Toronto Centre. Republished here with permission.


One of the easiest forms of astrophotography to partake in is tripod-mounted wide-field. Image the Moon, conjunctions, alignments, and appulses of planets and stars, comets, aurora, rain or moon bows, or noctilucent clouds. Don't forget sunsets and sunrises--technically, photos with a star! But can you image stars and the Milky Way? Yes!

Minimal equipment is required. Just about any camera can be used. A digital single-lens reflex camera is certainly capable even with a stock kit lens. But a point-and-shoot will work fine too, or modern smart phones, particularly if you can control the exposure time. I use "fireworks" mode on my fujifilm J20. What if you don't have a tripod? Use the nearby picnic table, ladder, or big rock.

One quickly discovers that long exposures can produce funny stars. Zoomed-in stars are not round. They are elongated or stretched. A long exposure means that stars move or drift in the camera's field. Or, more correctly, the earth's rotation causes the camera to move under the stars. The stars are blurred or trailed.

This is why powerful telescopes have motorised mounts. It explains why the iOptron Skytracker, Vixen Polarie, and new Skywatcher Star Adventurer are all the rage. Tracking mounts come with some expense and require decent polar alignment.

What you need to consider, with your basic camera (and lens) is where you're aiming and your focal length.

The location of your target is quite important for the apparent speed of the stars varies. Stars in the south are "fast;" whereas, stars in the north are slow. Think of the spokes on a spinning bicycle wheel: the angular speed of the spoke near the hub is slow but out at the edge, near the tire, is fast. Polaris is near the hub of the wheel of the sky.

When things are moving fast and you're trying to freeze the action in a photo, or reduce blur, you need to use a shorter or faster exposure. And you need lots of light. If your subject is stationary or moving very slowly then you can expose for a long time. You don't need as much light. The challenge with night time shots is that the light level is always low. This in turns puts an upper limit on exposure time. Happily, we can bump the sensitivity or ISO.

At a recent meeting, our own Dietmar Kupke shared some images, tips and tricks, and recommendations in his time-lapse presentation. He referred to the "500-rule." This suggested rule uses the following formula:

t = 500 ÷ f

where f = focal length of the camera lens,
and t = time in seconds.

The rule says with a 50 mm lens, you can shoot for 10 seconds before the stars will start to trail. Use a wide angle lens, say an 18 mm, and you can expose for half a minute.

The 500-rule is easy to remember when you're in the field and you're quickly trying to calculate value in your head. But, as previously noted, it depends where you point. If you aim to the north, say to Cepheus or Camelopardalis, you can shoot between 15 and 40 seconds. Longer, if closer to the north celestial pole.

The 500-rule really applies to objects near the celestial equator.

If you want to calculate with greater precision your maximum exposure time, check out the Astrophotography Exposure Calculator.

It allows manual operation or automatic. It has a number of camera and lens profiles on board already. It uses the declination of your target to gauge the apparent speed of the area. It also takes into account your frame orientation, landscape or portrait.

Be sure to browser the web site for other handy photographer tools.

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