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Hi Marc,
I have been having issues with my f/10 LX200 and the Meade f/6.3 focal reducer using my Canon 350D, given yours would be a different set up slightly re: equipment, I too was told 115mm for my set up, yes I could focus in the center of the field but around the outside I had "Pac-Man type stars, I later email Bintel with one of my 2 min exposures showing the errors and Don emailed me with this reply:
"Is the scope a SCT or ACF.
If the later the f/6.3 will not work properly and spacing will be of the utmost critic.
As the ACF scopes are already produced a flat field image and the problem is that a reducer / corrector can further flatten the field so that curvature may be reversed.
If it is an SCT see the below details from S&T Review about correct spacings.
Popular f/6.3 reducers sold by Celestron and Meade for their f/10 Schmidt-Cassegrains are designed to be used with a 105-mm separation between the back surface of the reducer and the detector (be it film, CCD, or whatever) altering this spacing changes the compression factor — great for fine-tuning a CCD system. Increasing the separation increases the amount of compression and thus reduces the effective focal length. Ideally we could increase the separation enough to accommodate small pixels. In practice, however, either image quality or, more likely, mechanical restrictions imposed by the telescope’s focusing system will limit the amount of compression that can be obtained.
If you are using a microfocuser it should be placed after the microfocuser and before T- Adapter and T-Ring.
Hope this is helpful"
As you can see I need the 105mm separation to which I've tried and did another 2 min exposure on the same object.
The results were clearly better, the stars around the outside had lost this "Pac-Man type feature" but was faced with another problem, zooming in on these stars around the outside showed rotational/un-polar aligned characteristics... in the centre of the image the stars are elongated NE to SW, at the bottom right corner the stars are elongated NW to SE and then halfway in between they're bl#$%y round!
I have later found the O-Ring holding the lens's in place on the focal reducer was somewhat loose, only slightly, so after a tightening up I am now on the testing once again to try this out, as my problem may be the focal reducer it's self, one of the dud ones that was marketed.
A mate also has the same focal reducer to which we will try if mine fails then we will know for sure.
Sorry if I went on a bit but the spacing for my set up is definitely 105mm.
Paul
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