F/3.3 Focal reducers compared
By Chris Vedeler
A f/10 Schmidt-Cassegrain telescope can be reduced from f/10 to f/6.3 and even f/3.3 by using a focal reducer. A focal reducer works by concentrating the light cone produced by the scope into a smaller, yet brighter circle. Because the light cone at focus is actually smaller and the detector stays the same size, the field of view also increases. As one shrinks the light cone it inevitably reaches a point where the edges of the light cone are no longer hitting the detector, which causes vignetting. This is why f/3.3 focal reducers do not work with 35mm film. The light cone is shrunk so small that it would produce only a small illuminated circle in the center of the film, but would not illuminate the edges of the film at all. Most CCD cameras have much smaller detectors than 35mm film, and so one is able to use a very powerful focal reducer. Vignetting still occurs, but due to the small size of the CCD chip the images are still useable.
Along with producing a brighter image, a focal reducer also makes the apparent star sizes on the detector smaller. This makes for a more pleasing and sharper looking image. With most Schmidt-Cassegrain telescopes, using a focal reducer is a must for most objects while using a CCD camera due to the small detector size of the CCD and the long focal lengths of the Schmidt-Cassegrain telescopes. It is important to match the pixel size of the CCD detector to the image produced by the telescope. It doesn't make much sense to have a pixel size equal to 1/2 an arc second if your seeing is 2 arc seconds. It is wasted resolution.
There are two manufactures of f/3.3 focal reducers for Schmidt-Cassegrain telescopes. The Optec Maxfield and the Meade f/3.3 reducer/corrector. I own the Meade unit, and my friend Chuck Greenberg loaned me his Optec for these tests. The Optec unit has been out for a number of years, and was once the only f/3.3 reducer available. The Optec unit sells for around $345 plus another $50 for the adapter mount. The Meade unit sells for $149 and comes with everything necessary to connect most CCD cameras to most Schmidt-Cassegrain telescopes and was just released the early part of 1999.
The question I hope to answer in this article and my tests is if the Optec reducer is better than the Meade reducer, and if so, is it enough better to justify the cost difference.
All of these tests where performed with the same Meade 10" f/10 LX200, on the same night, with the same ST7E CCD camera using the same camera settings.
A- Dome Flats: The first test of the focal reducers was to do a dome flat using each one. You can see that the Meade focal reducer showed significantly more vignetting than the Optec. Both reducers show a fairly significant cutoff a the top of the image. This is caused by the pick off prism for the guide chip of the ST7. At f/3.3 the light cone comes in at such a wide angle that this small prism actually casts a shadow on the chip. In practice this isn't a huge deal as it is only on the upper 5% of the image, and can generally be flat fielded out without a problem.
From this test it is clear that the Optec produces a much less vignetted field than the Meade reducer. The focal reducing lenses of the Optec mounts much closer (about 10mm) to the CCD chip than the Meade (about 50mm). The Optec uses much stronger and shorter focus magnification than the Meade, which permits a less vignetted field of view.
|Meade f/3.3 focal reducer dome flat.||Optec Maxfield f/3.3 dome flat.|
B- Imaging tests: The second test of the focal reducers was to take real
world images. In the small images below both focal reducers
appear to perform about as well. The Meade actually gives a
little bit wider field of view than the Optec, indicating that it
is a little stronger focal reducer (f/3.2 maybe?). The Meade
image appears to be a little brighter in the center than the
Optec, which is probably due to the vignetting of the image which
would be removed with flat field. In terms of image
sharpness, they both performed about the same. The center of the Optec
image appears slightly sharper and has more pleasing contrast than the center of
the Meade image. The Meade image however has much sharper edge of field
stars than the Optec. Although I was very careful to adjust the focus
of each image, these differences may be due to slight differences in the
|M16 using the Meade f/3.3 focal reducer.||M16 using the Optec Maxfield f/3.3.|
C- Field curvature and optical quality: Neither reducer produced great edge of field star images, but surprisingly the Meade did better than the Optec in terms of roundness of stars near the edge of the field. I suspect this is due to the distance that the Meade reducer is from the CCD chip, and not needing to bend the light as much. The Optec produced obvious coma near the edge of the field, while the Meade produced acceptably round stars. I've been told that this is due to chromatic aberration and a fringing of blue light by the Optec reducer. This effect can be significantly reduced by using a yellow filter but I see this as a major liability as it basically nullifies the increased sensitivity to the blue of the new "E" chip.
|Blown up cropping of the lower left corner of the M16 image using the Meade f/3.3 focal reducer.||Blown up cropping of the lower left corner of the M16 image using the Optec Maxfield f/3.3.|
The Optec Maxfield focal reducer beats the Meade in terms of quality of workmanship and evenly illuminated field, while the Meade wins in terms of edge of field optical quality and price. Both reducers work about as well in real world testing as the vignetting of the Meade reducer can be easily flat fielded out. Given that the Meade reducer is less than half the price of the Optic and the fact that both reducers perform about as well in terms of optical performance and image quality, I must conclude that the Meade reducer is a much better value while not sacrificing anything that effects the final image.
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