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Old 05-04-2010, 09:16 PM
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DavidTrap (David)
Really just a beginner

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Join Date: Jun 2009
Location: Brisbane
Posts: 2,828
Focal Reducer Test

I bought a Televue Focal Reducer (0.8x) / Flattener earlier this year. Unfortunately, the weather in SEQ has hampered my efforts to test this until now.

I was also loaned a Baader MPCC to test as well - thanks Gerald.

The refractor is a William Optics ZenithStar 70mm Doublet. F6.2 and focal length 430mm. Without correction, its coma is pretty shocking...

I setup tonight and fired off 5x30sec exposures of the Orion Nebula. These were unguided, with the scope tracking in Alt-Az mode. Fortunately with Orion in the west and at ~45 degrees altitude, field rotation shouldn't be a huge issue with such short exposures.

I processed these with some darks in DSS and adjusted levels in PS - they're certainly not masterpieces!

The images in order are: without a flattener, MPCC and Televue.

As you can see, the coma is pretty shocking on a large chip (Nikon D200). It doesn't get much better with the MPCC (as expected for a device designed for fast newtonians). I'm pretty stoked with the results for the Televue. It is designed for 400-600mm focal length refractors.

It is expensive (over half the cost of the scope) and heavy - the focuser struggles to hold it and the camera up... However, the results are impressive.

I'd be keen to hear any learned opinions - especially regarding any visible collimation problems.

Thanks for looking,

David T
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