Focal reducer field flattener

[anthony.tony]

I've just purchased a 0.6 focal reducer for $95.00.What is the difference between it and a focal reducer field flattener. Tony

[rogerg (Roger)]

What brand is it?

I have meade ones (F/6.3 and F/3.3), I think they are advertised as "focal reducer/field flatenner". I don't understand the field flatenner bit, as by reducing the focal length they bring the image in smaller hence increasing vinyetting and edge distortion, from what I have experienced. Set up in the right position of the image train it's not a problem (no distortion of vinyetting visible).

Roger.

[Harb]

Thats an interesting point..............where do you all put your focal reducer in the optic chain.......

cheers
John

[rogerg (Roger)]

For me:

Simplistically:

Rear Cell of LX200 -> NGF-C focuser -> Focal Reducer -> Spacer -> Camera


However.... I actually have the focal reducer optics now inside a 2" draw tube adaptor so the focal reducer slides inside the focuser, hence shortening the distance between rear cell and reducer, which is important to minimise.

Roger.

[[1ponders] (Paul)]

The "flattener" part of it apparently counteracts for curvature of the mirror reducing the "comet" shape of stars towards the edge of the field.

From memory the meade/celestron (same animal) are optimised so that the last glass element is 105mm from the imaging plane (ie surface of the ccd chip/film)

[jase (Jason)]

Focal reducer, field flattener and telecompressor terms are all used rather loosely. Some telescopes have integrated field flatteners as part of the optical design i.e TV-NP101 has a two element objective at the rear.
There is some great information from Optec with raytrace and spot diagrams of their telecompressors. http://www.optecinc.com/optec_026.htm

Fundamentally, they all operate the same, but you do need to pay attention to the focal plane distance to obtain optimal performance. In some cases, spacers maybe required. In addition to this, some products introduce vignetting as the focal plane is shifted.
Russ Croman has provided some excellent focal reducer positioning formulas that can be adapted to most telescopes - http://www.rc-astro.com/resources/reducer.html

I guess the important point to understand is that using a focal reducer doesn't always mean your getting the advertised reduction. Rarely does an SCT roll off the production line at F/10 (as advertised). The old LX200's for example were actually F/10.7. Therefore this and other accessories can increase/decrease the effectiveness of the focal reducer.