Planets

Eyepiece projection is an excellent way to take detailed shots of the moon and planets. The objects photographed in these images are considerably larger and show more detail than those taken with primary focus techniques. Primary focus techniques replace the camera lens with an OTA (no diagonal, no eyepiece) telescope, but eyepiece projection adds an eyepiece into the optical path, which greatly increases the focal length and magnification. The image below shows the typical eyepiece projection configuration.

The increase in magnification and focal length comes at a price, however. A higher focal length (at the same aperture) results in a higher focal ratio number (1/f). The higher the focal ratio number, the weaker the image. This requires longer exposure times or higher ISO speeds to achieve decent image brightness. In addition, the constantly moving layers of air diffract the incoming light. This means that the higher the magnification, the greater the distortion. The same goes for any wobble or vibration of the mount and telescope.

How to do it?

Mount

  • The mount must be solid and sturdy. It must support the weight of the telescope, camera and all accessories, and must remain stable, even in a light breeze.

  • Never maximize the load of a mount. The old astrophotographer's rule still applies: the actual weight of the equipment should not exceed half the specified load capacity of the mount.

  • Many astrophotographers do not extend the tripod legs for better stability and minimal vibration.

  • Align German equatorial mounts (GEM) with great care. This helps to "keep the object in the field of view" even at the highest magnification.

Telescope and accessories

  • The axis of the finder and the main telescope must be perfectly aligned. This allows to "find" the object and to frame it in the very narrow field of view (FOV).

  • Screwed connections, such as tube extensions, are preferable to sliding connections. Screwed connections offer better stability, less bending, and are less susceptible to shaking and vibration.

  • Eyepiece projection usually requires a significant amount of back-travel of the focuser. The required length may exceed the focuser's travel, which will make the projection blurry. An extension tube will provide the extra focusing distance needed.

The Camera

  • It is strongly recommended that you use a remote control for the camera. Pressing the shutter release manually causes shaking and vibration. If your camera is not equipped with a remote control, use the longest shutter release time, at least 10 seconds. Some cameras offer only 2 seconds of shutter release time. This is usually too short because many media still shake 2 seconds after you press the shutter button.

  • Most cameras allow you to shoot video clips (avi). Even if the video mode offers lower pixel resolution, shoot video clips, especially for planetary imaging. Video clips are made up of many individual frames and software such as RegiStax converts the video clip into a string of individual frames, which can be stacked. With a frame rate (fps) of 10 fps to 30 fps, a 10-second clip produces a large number of individual frames. This is important because air movement and other distortions blur many images. The probability of getting a few good images increases with the number of images available.

  • Stacking good images helps to pronounce the features and texture of objects.

  • If your camera does not have video (avi) capability, take at least 30 or even 50 (or more) images to increase the likelihood of getting very good ones with little or no air movement.

  • DSLR cameras use mirrors that turn over during exposure. If you are shooting images (not video clips), use the mirror lock if it is available. Even if the mirror is very light, the rapid movement can create enough momentum to cause shaking, which again blurs the image.

Object position

  • Photograph planets when they are high in the sky rather than low on the horizon. Positions high in the sky minimize distortion due to air refraction. Light passing through the atmosphere is scattered by aerosol droplets and absorbed by dust. These effects cause diffraction rings and reduce the brightness of the image. At height, the atmospheric light path is much shorter, which greatly reduces the effects of distortion.

Post-processing

  • Powerful software like RegiStax (freeware) converts the video clip (avi) into individual images. You can also use DeepSkyStacker to do image stacking and many other post-processing functions.