| review: Telephoto Lens Albinar 135 mm with M42 Thread (appr. € 50,- second hand) |
It is possible to use old and inexpensive telephoto lenses for ccd imaging.
Because of the small pixel size and the huge sensitivity of the average CCD camera - compared to film - we can achieve better pictures as might be expected by the small focal length.
My CCD camera Starlight Express MX 516 can be connected with a standard Pentax thread (M42x1).
But to get focus with the Albinar I had to use an adapter with appr. 30 mm more distance.
If you are not able to get focus beyond the infinity symbol of the telephoto lens try to unscrew the little screw blocking focusing further then infinity.
For the CCD camera I then was able to use the eye-piece nose to mount it and therefore I could screw the infrared blocking filter.
This is important because telephoto lenses are refractors and they are usually only corrected for the visual spectrum.
Without IRX filter the stars would be blurred due to the out of focus infra red light.
A focal length of 135 mm with the MX 516 gives a wide angle field of view.
At least for CCD imaging ;)
Look at the next picture of the globular cluster M10 (the little thing close to the center):
The full moon would fill a third of the picture. To enlarge the moon to fit the size of the frame I used a 3x barlow lens behind the telephoto lens.
The effective focal length can be calculated to appr. 675 mm.
Even with cheap second hand telephoto lenses you can achieve very pleasing astro photos.
My telephoto was borrowed by a friend and the objective lens was showing a lot of smaller and bigger scratches.
However the overall image quality is OK.
An advantage of using smaller focal lengths is that imperfections of the drive or the guiding cannot be seen in the photo so easy.
As another plus it is more easy to use the slew and sum function to add several ccd frames to give a deeper and less noisy view.
In other words: this is much less frustrating for the beginner than to use the prime focus of the main telescope.
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