Jupiter – Image taken with eyepiece projection technique (telescope: 900/120mm, eyepiece: 20mm)
Take shots at planets when they are high in the sky rather than low at the horizon. Positions high in the sky minimize air refraction distortion. Light that travels through the atmosphere is scattered by aerosol droplets and absorbed by dust. These effects cause diffraction rings and reduce the image brightness. High in the sky, light’s atmospheric path is much shorter, reducing distortion effects significantly.
There are also disadvantages of high object positions. Particularly when shooting with a large refractor, the camera position is very low. Also, a large refractor with extension tubes and camera mounted may hit the tripod legs in this position. Make sure enough space is left when moving the telescope to the desired object.
Remote controlling the camera with a computer is strongly suggested, particularly with a large refractor. Looking in upright position at the computer screen is simply much (!) more convenient than crawling on the ground trying to peek in the – very low hanging – camera screen or finder.
The image on a much larger computer screen allows more precise focusing.
Take your time when focusing. High magnifications combined with moving air layers can make this quite a challenge.
Re-check with some test shots that the focus is still optimal.
Check the histogram and ensure that neither end (black or white) is clipped. If data is lost (clipped) it is lost for good, and can no longer be used to build the image. Even the best post processing effort can not bring lost information back.
Shoot several movie clips. My recommendation is 10 by 10. Ten clips each ten seconds long. Depending on the fps rate this will provide you 1000 to 3000 single frames, a good base to work with.
Some photographers prefer much longer clips to increase the probability of catching better results. With very long clips it is more likely that shake, vibrations and drift errors are introduced as well. CCD chips get hotter and start to introduce additional noise and hot pixels. Besides, long movie clips result in very large files, making processing somewhat cumbersome.
Powerful software like RegiStax (freeware) converts the movie clip (avi) into single images. Furthermore, it aligns the images, selects the best ones and stacks them for best detail. It allows improving the resulting image even more with a great set of post processing features.
Question for Power
It is possible to calculate how much more magnification we get with eyepiece projection over a simple prime focus setup. To determine this, we need to know some dimensions: focal length of telescope and eyepiece, and the telescope aperture. Furthermore we have to measure the distance from the eyepiece lens to the camera’s CCD chip.
The dimensions used in the following example are from an actual eyepiece projection setup that was used when I shot the Jupiter image: Orion EON 120ED refractor with 20mm Eyepiece, 2 extension tubes each 2 inch ( about 50mm) and a Canon EOS T1i DSLR camera.
Focal length of telescope (FLtele): 900mm
Focal length of eyepiece (FLep): 20mm
Distance eyepiece to CCD (Depccd): 100mm
Telescope aperture (TA): 120mm
Magnification over prime focus set up (Mopf)
Mopf= (100mm-20mm)/20mm = 4
The image is 4 times larger than that of a prime focus setup.
Focal Length overall EP setup (FLoEPs)
FLoEPs = Mopf * FLtele
FLoEPs = 4 x 900mm = 3600mm
This setup has a focal length of whopping 3.6 meters (141 inches)! The number shows that eyepiece projection focusing can really be a challenge and has to be done carefully in minute steps.
Focal ratio overall EP setup (1/f oEPs)
1/f oEPs = FLoEPs / TA = 3600 / 120 = 30
The original telescope focal ratio of 7.5 has now become 30. The image will be much darker than that of a prime focus setup. Higher ISO speeds particularly for planetary images may be necessary.
Is it worth the challenge?
Most definitely: YES. Eyepiece projection astrophotography is for more advanced star shooters. It is easily among the most challenging processes in amateur astrophotography, not because of the setup but because of the effects that have to be considered and factored in. But with the right equipment and some practice it can be mastered – and the results speak for themselves: clearly visible features of the moon landscape, surface coloration and visible ice caps of Mars or detailed cloud bands of Jupiter make eyepiece projection imaging indeed quite rewarding.