zurück zur Sektion 30
Im folgenden ein Test- und Erfahrungsbericht von Herrn Tom Licha zu unserem Multi Purpose Coma Korrektor. Text und Abbildungen (mit Ausnahme des MPCC) © 2003 by Tom Licha
Review: Baader / Celestron MPCC Multi Purpose Coma Corrector (appr. Euro 120,-) - written on April 10, 2003 - ranked 9.0 out of 10 (1 Votes)


The MPCC coma corrector		 The MPCC is a coma corrector designed to eliminate the most severe off-axis aberration in Newtonian telescopes: the coma. This is achieved without changing the focal length of the telescope. The basic features are as follows:
  • Suitable for primary mirrors from f/4 to f/6
  • 2 inch barrel at the focuser's side
  • T-thread (M42x0.75) at the camera's side
  • 12 micron spot size (0.012mm) across the field of 35mm film
  • Fully multi-coated
  • Best results at 55 mm distance from T-thread to film or detector
  • Focus is shifted to appr. 10 mm to out, so no back focus is needed
The MPCC coma corrector Telescope and Camera used for test images
The telescope used for the test images was a 12 inch f/4 Newtonian with a Strehl ratio of 0.96. The system was first collimated with a laser to maximum precision. This laser was connected with a T-thread-fast-connector to be at 100% the same position and angle as the camera later. The CCD camera was the HX916 with it's small pixels hence demanding high optical quality and small spot size. Each pixel covered an area of 1.21 square seconds of arc in the sky or 1.1 seconds of arc in width. According to the Nyquist theorem the maximum spot size of the optics should be 2 times the diagonal of a pixel. With 6.7 micron pixel width this translates to 19 microns of maximum spot size. The MPCC should hence be well in this specification or in other words the HX916 should slightly oversample the image. Of course this is only within reach when good seeing is present. At the time of the test the seeing was a bit worse than that but not too bad as we will see soon.
Baader is recommending to have the film or detector exactly at 55 mm in distance to the T-thread. My filter wheel and CCD camera combination is giving 53 mm of distance so the test was performed being 2 mm out of spec.

Test object 1:
An open cluster Two images of 30 seconds exposure were taken. Both images' histograms were scaled in exactly the same way. The equipment was well prepared for a fast change and thanks to the quick connector there was only one or two minutes in between these images. So the seeing conditions as well as the angle above horizon are fairly the same. The first two images are the full field of view of the HX916 (11.2 mm diagonal, 32 minutes of arc) but at 2x software binning in PhotoPaint.
 
Full field without MPCC Full field with MPCC
The center of the CCD camera is slightly to the upper right. Hence the coma is most severe at the lower left corner. The MPCC shows a perfect coma correction, slight loss of light in the background sky but a deeper field with dimmer stars at the same time. The next two images are showing the lower left corner at the original resolution.
 
Lower left corner without MPCC Lower left corner with MPCC
If you have a close look you will see more stars in the image with the MPCC. Despite the loss of light by the air-glass transitions the gain of magnitude by the less spread star light through the absence of coma is obvious.
 
2x enlarged lower left corner without MPCC 2x enlarged lower left corner with MPCC
Here the gain in magnitude is easy to be seen as well as a more contrasty image with the MPCC. The smallest stars are slightly blurred to a bit more than the 19 microns. This is most probably due to the just not good enough seeing.

Test object 2:
Moon in prime focus. This is a single frame with 0.01 seconds exposure time. An H alpha passband filter was used not to overexpose the image. The original frame of 1300x1030 showed very high sharpness all over the image.


Moon with MPCC, image scaled down
 


Moon with MPCC, upper right corner at original
resolution of 1.1 sec. of arc per pixel
 
Conclusion
The images are proving that the MPCC is doing a very good job. Especially there is no increased and fuzzy star size at the optical center. For the next time I will take all of my images with the 12 inch through the MPCC. Sooner or later (most probably later!) the seeing will be good enough for the final test. Then the MPCC can show whether it is just very good or excellent
 
Tom Licha, April 10, 2003   http://www.licha.de/AstroWeb

zurück zur Sektion 30