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Telescope Test
In the following article, we will compare the GPU Optical 127/890AS oil spaced triplet apochromatic telescope to the RR 154/1200 air spaced achromat. The main purpose of this comparison is to demonstrate photographic evidence of the differences of the compared telescope types.
GPU 127/890AS
Some time ago Pal Gyulai asked me to make a few photographs using the new product of his company, a 127mm aperture F/7 focal ratio oil spaced apochromat OTA. This lens is a new design, it uses an aspheric surface to further decrease optical aberrations. I was interested in the task, so I happily took it.
The APO lens is build into a standard GPU OTA, with a 3.6" Crayford style focuser and a retractable dew shield. The length of the scope is about 715mm when collapsed and the mass is 6.9kg (without accessories). The scope was used on a Gemini G10 mount, which handled the OTA easily.
The eyepieces used were a 32mm TeleVue Plössl, a 10mm focal length TeleVue Radian.
The reference telescope is an RR Achromat from Holland, with interferometric test data showing 96.4% Strehl ratio in green (546nm wavelength) color.This lens is really excellent, but it is an achromat, with some natural false color. It has about 1" more aperture than the APO, and one of the questions to answer in this test was whether or not the extra aperture could balance for the contrast loss due to false color.
In the first series of images, the TeleVue Radian eyepiece was used as a projection eyepiece. In the past, I have made photos of the same objecs using other telescopes, I included these images for further reference. These old images were made using a 127/1140 Yulin achromat (from China), and another lens from Holland, an RR 104/1200. At the first look, it is visible that the currently compared scopes are in another league, they easily outperform the olde/smaller lenses.
In the following images, it is visible that the RR 154/1200 achromat is really excellent, in resolution it challenges the several times more expensice APO. But the contrast of the APO is several times "harder", it can not be compared to the achromat.
After making the first series of images, I detected some slight false color at the edges where black meets white, and I was interested whether this color is raised by the GPU lens or by the Radian eyepiece. To decide in this question, I removed the eyepiece and shot a few images using a Philips ToUCam webcamera, in primary focus. This removed even the last bit of false color from the images, so it is proven that the Radian eyepiece was responsible for the slight false color of the first series of images (using a digital camera).
The result of the test is the proof of the fact, that an APO can outperform an achromat, even if the achromat has significantly more aperture. The color free image means more advantage to the APO than the 1" aperture advantage of the achromat.
It is also important to note, that the Radian eyepiece looked about "perfect" with the achromat, but in the APO now I can easily see the false color of the eyepiece during visual observations. No eyepiece is perfect, but it seems today high-end APO lenses are better corrected for false color than even the best eyepieces.
Istvan Zentai
*Original version of this article in hungarian language can be found here: http://zentai.csillagaszat.hu/test127vs154.htm
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1. object: electric connection in about 80m distance
Eyepiece projection: TeleVue Radian 10mm, Camera: Nikon Coolpix 990
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104/1200 RR Achromat
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127/1125 YULIN Achromat
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RR 154/1200
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GPU 127/890AS
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RR 154/1200
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GPU 127/890AS
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2. Object: sat receiver head from 150m dist.
Eyepiece: TeleVue Radian 10mm, Camera: Nikon Coolpix 990 |
RR 154/1200
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GPU 127/890AS
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RR 154/1200
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GPU 127/890AS
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| 3. GPU 127/890 AS Webcam images in prime focus |
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Please note, that common wisdom says webcams are not usable to make deep sky images. We believe it is only partially true. Even if the above deep sky shots are not comparable to high-end CCD images, they prove that webcams can be successfully used for basic deep sky photography.
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