HOME > New Product Information POLARIE Star Tracker
What digital cameras are suitable for taking wide field celestial photography with the Polarie?
It is essential that a digital camera for astrophotography has the ability of opening a shutter for a long period of time and setting a focus at infinity.
Use of a DSLR camera (digital camera with interchangeable photographic lens), which has functions to control the focus, shutter exposure times and lens apertures manually, is recommended.
What equipment is required besides the Polarie?
A stable camera tripod and a ball head adapter, on which to attach the camera to the Polarie, are required. In addition, a shutter release (remote) cable is needed to avoid vibration.
What photographic lenses are suitable for the Polarie?
Generally it is advisable to use a wide-angle photographic lens. The wide-angle photographic lens offers a wide frame and it is suitable for taking photos of whole constellations and the Milky Way. With the longer shutter exposure, star trails are less conspicuous in the wide frame of the wide-angle photographic lens even if the polar setting of the Polarie is not precise. The longer the focal length of a photographic lens, the shorter the exposure time that allows pinpoints star images. As a result, more accurate polar setting is required when using a telephoto lens.
What should I consider to determine the shutter exposure times?
Generally you need to take into account the brightness of the sky' background, ISO speed setting and value of the aperture stop of your DSLR camera to determine the proper duration of exposure.
When using the Polarie in urban areas, acceptable exposure times will be shorter. When using the Polarie at a remote rural site, longer exposure times will be possible. How long you can go depends on how dark your sky is. The faster the ISO speed you set, the higher the light sensitivity your camera gains with a given time of exposure. This allows you take photos of many faint stars you cannot see with the naked eye but the trade-off is an increase in electronic noise on captured images that comes from the faster ISO speed. In addition, wider lens aperture (smaller F-number) allows for shorter exposure time, but it can result in distorted stars in the corners of the frame.
The table below shows recommended maximum exposure times to deliver pinpoint star images with use of 3 photographic lenses of different focal lengths from wide-angle to standard. In the table the Polarie is aligned at a tolerance of 2 degrees from the celestial poles. It is almost the same accuracy for polar alignment when you align Polaris into the polar sight hole of the Polarie.
(Regardless of the CCD imaging size.)
|Stars on the Celestial Equator||Stars on Declination at ±60°|
|15mm||6 minutes 52 seconds||13 minutes 45 seconds|
|24mm||4 minutes 17 seconds||8 minutes 35 seconds|
|35mm||2 minutes 56 seconds||5 minutes 53 seconds|
Note: The above exposure times are calculated on the assumption that the pixel size of an imaging device is 0.001mm.
Because the apparent motion of stars differs at various areas of the sky, you view the stars as they turn around the celestial pole. This decreases as your camera is turned toward the celestial pole and the exposure times increase. Note, the shorter the focal length, the longer the exposure time.
What is the 1/2 sidereal tracking mode on the Polarie?
It is used to take "Star-Scape" photos in which you add a foreground image such as a nightscape or silhouette of landscape to your frame of the starry sky.
In the celestial tacking mode, the camera follows the motion of stars and the stars are recoded as tiny dots on the CCD imagers, but landscape and silhouette are trailed. Contrary, photography with a fixed camera-on-tripod will generate streaks of stars for short exposures. The half speed of the celestial tracking mode allows you to extend the shutter exposure times so that you can keep the stars pinpoint while minimizing the motion of the nightscape or silhouette of landscape in the frame. The faster ISO setting can also help avoiding star trails. This mode is suitable for "Star-Scape" photography with the use of wide-angle lenses.