Orion Field Flattener for Skywatcher ED80 and SW120 Refractors

[Foxinsox (Graeme Fox)]

I have been using an Orion Field Flattener for Short Refractors ( 400mm – 650mm) on my ED80 refractor to image with a DSLR. This has worked well.

I am now considering upsizing to a 120mm refractor for DSLR imaging with similar focal length of my ED80.
Will my existing Orion Field Flattener that has worked with my 80x600 refractor work with a proposed 120x600 achromatic refractors ?
What are the technical factors that I need to understand to determine the compatibility of a field flattener with different refractors ?




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[Wavytone]

For refractors of similar construction (ie doublets in this case) the radius of curvature of the focal plane scales with focal length. What this means is that for an objective with longer focal length than the ED80 had, the uncorrected focal plane will be flatter than that of the ED80, requiring less correction. You might be able to achieve that with the flattener closer to the camera body but you may also find this isn’t satisfactory and need another flattener to match the scope.

[OzEclipse (Joe Cali)]

Graeme,

As Wavy said, the field flattener corrects curvature which is a function of focal length not diameter. Both scopes you mention have 600mm focal length, the Orion flattener is designed to work on 400-600mm focal lengths so the flattener should work on both scopes.

The SW120 being an achromatic objective (two colours focus simultaneously) won't produce image quality comparable to the ED80 which is an apochromatic objective (three colours focus simultaneously). The flattener only flattens field curvature, it does not correct for the problems that will be introduced by the achromatic objective.

Joe