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These Celestron HD scopes sound very good and a big step forward for Celestron.
As far as flat field goes there is usually some curvature present even with a flattener.
For example my TEC180 with flattener is quite flat and so is an FSQ but if you use CCD inspector it will report some curvature. Take the flattener off and redo the test and that curvature will be far greater.
Same with CDK. It is quite flat. The term flat is relative and some are more "flat" than others. Nuch like the term "APO" for refractors. Some are clearly not APO but for marketing reasons they are termed APO as that is a desirable quality helping sales.
But having said that you want to see an image from a large chip camera that shows round stars to the corners.
How is the Celestron HD for mirror flop and dewing? I believe it has a mirror lock but how well does it work?
These seem to be the 2 problems associated with the SCT design - a dewing up of the corrector, mirror flop and tube currents/thermal imbalance between mirror and ambient. To a lesser degree may be the chromatic aberration a corrector plate adds. That does not seem to be too bad from images I have seen.
CDK17 for example you cannot get a precise focus if the mirror temp is more than about .8C different to ambient. This highlighted for me how crucial that is and how many scopes out there are not performing to what they could be if the thermal issues were properly sorted out.
The main adavantage as I see it of this corrector is typical SCT images show bloated stars away from the centre of the image. If it handles that then they have just given SCTs a new lease of life as they are very good in many other ways.
Their mirrors don't get dirty, they are compact and relatively light for their size. If you put a reducer and get below F7 then their imaging speed would be good as well. F10 is a pain. They also have long focal length for their size. So a HD could well be an awesome scope especially if you modified it to blow cool air through it to cool the mirror 3 degrees below ambient and have a flow to get rid of tube currents.
Greg.
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