"HAT TRICK" METHOD FOR ASTROPHOTOGRAPHY
Last updated: 19 January 2010
In many articles on my ETX Site and my Cassiopeia Observatory blog I have mentioned using the "hat trick" method for my astrophotography. I decided to document my "Hat Trick" method of making short and long exposures with my telescopes and camera.
Using the "hat trick" method is required with many cameras to avoid image vibration at the start and end of the exposure due to camera-induced movement. This vibration can result from the camera's shutter and/or mirror movement, or even from just pressing the camera's shutter release button with your finder. In the early days of photography before cameras had mechanical shutters (or electronic shutters), photographers would use their hat to cover the camera lens before pulling a slide out of the film holder to expose the film. They would then flip the hat away from the lens exposing the film to make the exposure. When the exposure was completed, they would cover the lens again with the hat and move the slide back over the film. Today, we do the same thing for some types of astrophotography.
You can use a hat, cap, or almost any object that blocks light from reaching the camera as a "shutter". I made my "shutter" back in the early 1960s for use with my Edmund 3" reflector using a piece of cardboard. It is white on one side and black on the other side. It is about 8" in diameter and works fine for my ETX telescopes, my 8" LX200-ACF telescope, and my Nikon D70 DSLR camera lenses.
To make the exposure, you first cover the telescope's aperture or camera lens with your "hat". Here you can see my "hat" covering the aperture of my Meade ETX-90RA:
You then open the camera's shutter in its "Bulb" setting. If the camera does not have a Bulb setting to leave its shutter open, then use the slowest shutter speed on the camera and make your hand exposure shorter than that. I use a Nikon wireless remote shutter release with my Nikon D70 DSLR to avoid hand-induced vibrations. However, you can also use the camera's self-timer (if it has one), or just press the camera's shutter release to open its shutter and wait a few seconds for any vibrations to dampen out.
Now that the camera's shutter is open, you move your "hat" away from the aperture or camera lens to let light reach the camera and start the exposure:
Flip the cover to the side in a rapid motion and make the motion perpendicular to the optical axis. Do not flip the cover forward as seen here:
This can cause air currents that can affect the image quality.
Leave the aperture (or lens) uncovered for the duration of your exposure. When you are ready to stop the exposure, reverse the process; slide the cover back over the aperture (or lens) and then release the camera's shutter to close it. Simple.
It is easy to make long exposures. And with practice, you can make exposures less than 1 second, even down to about 1/2 and 1/4 of a second.
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