> I am looking for a feild flattener only. I have an LX200
> 10" F6.3 scope and the edges are out of focus when the
> center is in focus. There are lots of focal reducer/feild
> flatteners, but I don't need further focal reduction, and I
> don't suppose these combo items do any more flattening beyond
> that created by their focal reduction anyway (?). Can anyone
> point me to a source of just field flatters?
Focal reduction, and field flattening, are seperate functions in a sense
(though there is a slight tendency to reduce/increase from a normal
flattener - doing a 'unity gain' device would require yet another lens
to compensate for the gain/reduction introduced by the flattener). Field
flattening, is achieved by having a meniscus lens, that is
a bit like the corrector on a Mak (concave on one side, and convex on the
other), but with a slight +ve or -ve magnification as well. Focal reduction,
just requires a normal +ve lens (this will also increase the effective field
curvature). The combi units do contain seperate lenses to perform both
functions, but the flattener lens, also provides part of the colour
correction. Borg do a flattener, that has a very small +ve effect (nominally
1.04*). It is designed for the field curvature from a refractor, rather than
an SCT. However it is possible to use this, by spacing it to the right
distance from the target. A similar mildly -ve flattener, is available from
Takahashi. With both these, it'd take some calculation/experiment, to get
the right spacing for the best effect on an SCT. To calculate this, the
'easiest' way (assuming you have Excel), is to download the telescope design
spreadsheet from Oldham optical (http://www.oldham-optical.co.uk/), and put
together an imaginary SCT, based on the dimensions of your scope (remember
the primary is about f/2). This package outputs the radius of the field
produced by the scope. The Borg unit, has the seperation needed for
different refractors specified (and the radius these generate can be
predicted the same way), allowing a very close figure to be calculated. For
your scope, the spacing should be close to that needed for about an 1m
uncorrected refractor.

Best Wishes

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