Monday, November 21, 2011

Shoot the Geminids

Photographing meteors such as the Geminids is possible using a Single Lens Reflex (SLR) camera. The camera must have a "T" (time) or "B" (bulb) setting for taking time exposures. You will also need a cable release, a tripod or a very stable surface to place the camera on, for best results. The camera will need a lens that is between "fisheye" and 50mm. Lenses larger than 50mm may capture too small a field of view. This article is written for the film camera but can be used as it is for digital SLRs.

Here is what you will need to do.

Set up your photography equipment in an area that is shaded from any stray lights that may interfere. This is absolutely necessary!

Set the camera on a tripod or some other surface that is very stable. Make sure the focal ratio is set to the lowest possible setting. This means that the aperture of the camera is "wide open". Make sure the camera is set to "B" or "T" for time exposure. Set the focus to infinity.

Aim the camera at the area of the sky that you intend to photograph. Once positioned properly, make sure the tripod is locked down to prevent its "head" from moving under the weight of the camera. If you do not have a tripod, use things to prop the camera up in a way so that it is stable and so that you can still look thru the view finder.

Depress the cable release button and lock it in place. Allow the camera to take a picture for anywhere from 15 to 30 seconds, maybe more. Once the desired time has elapsed, release the cable release lock which will end the exposure. While taking the picture, do not move the camera at all.

The length of time that you should expose the film can be a tricky thing. Light pollution will shorten the amount of time that an exposure can be made before the file reaches it's "Sky Fog Limit" or is effectively overexposed. Experimenting with the length of time an exposure is well worth the effort!

The speed and grain size of the film is an important consideration. Generally speaking, ASA 400 film is "fast" enough for these purposes. Fine grained film such as ASA 100 will give you sharper images than ASA 400 but the pictures would be much darker. With low light levels, "fast" film is highly desired. ASA 800 will expose quicker than ASA 400 but will be grainier. Kodak Gold and Fuji Film's of ASA 400 or ASA 800 should suffice.

The "F" stop or "focal ratio" setting is very important. As mentioned above, you want the camera to be "wide open" or set to lowest focal ratio setting. The reason is that the film will be able to gather more light if the aperture is wider. Your pictures will capture more fainter meteors at a lower focal ratio than at a higher focal ratio. If your camera produces fuzzy results and it is indeed correctly focused, close the aperture down one stop.

Go out on a clear night and test your setup! On a clear night prior to the Geminid meteor shower, set your camera up and take a few pictures. This will help you determine what works best for your camera and how the film reacts to the night sky in your area. Try a few exposures of 15 seconds, 30 seconds, 1 minute, 2 minutes, 3 minutes and 4 minutes using the lowest focal ratio, and record the frame number and exposure time on a scratch pad. Repeat the process with the focal ratio backed off one stop. When you get the film developed, you'll be able to compare the results with your notes and determine what works best for your camera and sky conditions. What you will see are called "star trails". Every star in the photo will appear to be "trailed" for all photos over 20-30 seconds in duration. This is OK though as many meteor photographers do use this method. You would need an "equatorial" mount with a tracking motor to eliminate this effect. Knowing how your camera records light before the main event is essential!

Warn the film developer that your pictures may be very dim! When getting your photographs developed, it is a good idea to make sure they know your photographs are dim and to the untrained eye, may appear to be of nothing at all. If the developer uses normal processing, you should at least get some kind of results. The important thing is that they print them! It is a better idea to have them developed locally, where you can discuss what's on the film prior to processing it.

Some Geminids are exceedingly bright and may possibly overexpose or ruin a time exposure. If a very bright fireball crosses the camera's field of view, end the exposure shortly thereafter. Know where your camera is pointing! Sometimes, a bright meteor will leave a "train" or trail. These make very interesting photographs as the trails become twisted and contorted by winds high in the upper atmosphere.

Framing your picture with natural landscapes will help to make it more interesting and will provide the viewer with a better perspective of the event. Trees, cactus, rock formations and distant mountains are all good objects to try this on. A wide field photograph of meteors while looking across a very still lake or pond may make for astounding poster quality shots, especially if the water surface is very still and reflects the meteor well. If you have a lake or pond nearby, give this a try. It may actually be possible if the meteor counts are high and if they are bright. Although the moon will significantly interfere with meteor observing and photography this year, it may provide a bit of foreground illumination to the setting.

Enjoy the show and Good Luck!

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