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Old 02-11-2007, 05:20 PM
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Can You really get F2 out of an LX90?

Subsequent to my thread on " how do you estimate focal reduction" i am still unsure whether its possible to get F2 out of an LX90 without the image deteriorating very much.
regards philip
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Old 02-11-2007, 05:30 PM
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[1ponders] (Paul)
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Don't know about an LX90 (though I believe the mirror is f/2 or there abouts) but Celestron have an 8" F2 SCT called the Farstar system.

http://home.freeuk.com/m.gavin/fastar.htm

It is only for imaging though and the camera is used inplace of a secondary mirror.
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Old 02-11-2007, 05:50 PM
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There exists also a 'Hyperstar' system for some Meade OTAs. I think it is only available for 10" & 12" and requires a secondaru holder having 6 screws instead of three, which I believe the later models have. Google 'hyperstar'

cheers,
Doug
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Old 02-11-2007, 05:50 PM
Shawn
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Yeah you can get those for Meade too, but really only practical for large aperture otherwise the camera gets in the way....
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Old 02-11-2007, 06:09 PM
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Phillip I assume you are using a GstarEX.
For any focal reducer there is an optimum distance from the sensor to get the flattest field. For full frame or even c size sensors you are basically limited to a reduction of 0.85 to at best 0.70. The field of any simple optic is curved concave towards the optic and if you go for too much reduction it then curves the other way ie away from the optic.

As the GstarEX is at most 8mm (diagonal) so even a highly curved field for a large sensor will be approximately flat for the centre part of the image plane.

So a reduction of 0.2 is quite feasible as long as the focal length of the FR is many times the size of the imaging sensor diagonal.

Bert
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Old 02-11-2007, 07:31 PM
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Quote:
Originally Posted by avandonk View Post
Phillip I assume you are using a GstarEX.
For any focal reducer there is an optimum distance from the sensor to get the flattest field. For full frame or even c size sensors you are basically limited to a reduction of 0.85 to at best 0.70. The field of any simple optic is curved concave towards the optic and if you go for too much reduction it then curves the other way ie away from the optic.

As the GstarEX is at most 8mm (diagonal) so even a highly curved field for a large sensor will be approximately flat for the centre part of the image plane.

So a reduction of 0.2 is quite feasible as long as the focal length of the FR is many times the size of the imaging sensor diagonal.

Bert
Thanks for the info bert i was a bit perplexed at F2 but the images i posted seemed to indicate that. regards philip
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