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CCD Imaging for the Budget Minded (Part III)

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CCD Astro Imaging for the Budget Minded Novice - Part Three
CCD Imaging for the Budget Minded Novice - Part Three
By Jim Chung , February 2006.

Apo refractor versus inexpensive Maksutov Cassegrain

In the original installment (December 2005) we performed a cost analysis for all our imaging equipment except the telescope itself! I'm very content with my 20 year old 76 mm Tele Vue aporefractor but like all astronomers yearn for extra aperture while keeping portability and GOTO functionality foremost.

The Konus 130mm MotoMax is a 5.2" Maksutov with a 2000 mm focal length and a 39 mm secondary meniscus yielding an impressively low 9% central obstruction. It is also found as the Celestron C130 spotting scope. Made in China by Guangzhou Bosma and introduced about three years ago, the Internet consensus has been mixed about its performance and quality control. As a result new old stock is being sold for under $300 USD and much less in the used market. I mounted the TV refractor on top of the Konus, sold its original 8x50 finder and had a net outlay of $140!

The Konus also comes with an internal flip mirror system allowing one to view through an eyepiece on the dorsal rear end of the OTA and to mount a CCD on the conventional visual back location. A small glass/dust barrier was removed and a visit to Home Depot for more plumbing supplies resulted in a functional visual back.

Taking into account the central obstruction, this scope represents 2.7x the optical area or 166% more light gathering ability than the TV refractor and is functionally equivalent to a 5" aperture. Casual star testing confirmed decent collimation, there was very minimal mirror image shifting during focusing and the oversized focus knob allowed fine control. I was very optimistic!

Imaging Saturn (Mag 0) and Jupiter (Mag -2) at prime focus on the Konus was very impressive as compared to the small images resultant from the Tele Vue. This was expected when comparing a focal length of 2000 mm to 560 mm. The internal flip mirror worked well and was accurately aligned allowing easy placement of the image onto the CCD chip. Further savings result because a fine focuser is not needed so the net cost falls to zero as compared to the Tele Vue setup. Better light gathering ability also allowed me to image at 30 fps with 1/6 s shutter speed, an essential factor in webcam planetary imaging success. To get comparable image sizes would require a Barlow on the Tele Vue and further reducing the intensity of light transmission.

Using Tele Vue's 5x Powermate (which is a Barlow with an additional pair of lenses to reduce vignetting and flatten the view) on the refractor yields images some 7-8x larger.
It reduces light transmission so much that I have to decrease shutter speed to a full second in order to image. This reduces my sampling rate to less than 1 frame per second and it takes me more than 20 minutes just to shoot 1000 frames. At 30 fps this would have taken less than two minutes. The reason why this is problematic is three fold. Good viewing conditions in Toronto in the winter invariably mean subfreezing temperatures so my exposed hands are slowly going necrotic. I'm also imaging in AltAz orientation so the less than perfect tracking of the scope means constant manual correction during this extended imaging time further exacerbated by the increased image size. Lastly the longer time means greater chance of viewing to degrade and decreased ability to take advantage of moments of enhanced viewing.

Surprisingly, the results were far beyond my expectation.

Clearly, contrast, resolution and color were superior in the Tele Vue. The GRS is clearly visible on Jupiter as is one of its moons (Io?) despite pushing the refractor to well beyond the theoretical 40x magnification per inch limit rule. To try and make the comparison more level I imagined Jupiter with a Meade 2x Barlow but was unable to place the Barlow at the visual back due to clearance issues with the fork mounting system. I had to image off the less than ideal flip mirror rather than a clean unobstructed direct path of light. Taking this into account the image of Jupiter at 2x on the Konus still has significantly less detail or contrast and despite a larger aperture still required a shutter speed in excess of 1 second!

Does this confirm the superiority of refractor to folded optic design? I think not. People with Questars will undoubtedly get fine performance and aperture does ultimately rule. What this exercise does confirm is that optics is the one place where one should not compromise too heavily and that even a small 3 inch scope can deliver surprising results if well engineered.

Next ? Final Part 4: DSO imaging on a budget. I recoup the Konus by using it to autoguide and image with a Canon 300D DSLR and the Tele Vue.


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