Field Flattener whats its purpose?

[lifeonmars]

Hello could I ask as a beginner the purpose of a field flattener.Would i be right in saying that it improves sharpness at the edge of the FOV?

Would you use this on something with a fast focal ratio for instance an Equinox 120 which I have.or is this something you would use in the Pursuit of better Astrophotography results.

Would be really interested in members inputs on this Thank you for your help in anticipation

[TheNorthernMonkey]

The focal plane of a doublet is curved like the inside of a sphere of a radius the same as your scope (900mm in your case)

You eye is quite small, so Out of focus parts are undetectable in the few mm of your eye pupil diameter

Your camera sensor is bigger so at the edge, it's more out of focus, so a field flattened does what it says on the tin, it makes the focal plane flat, instead of curved, so reducing the out of focus parts on the corners of your frame.

I can't help you working out if you need one.

[Capricorn]

The image plane produced by a scope is not flat, it is a bit curved.

Simply way to envisage it is consider the focal plane from say 800mm from the lens/mirror.

Using the centre of the lens/mirror draw a surface that is 800mm away and you have a circle.

Well that is basically the image plane.

The longer the focal length then it is closer to being flat, but on a fast scope it is simply curved.

A field flattener simply compensates for this.

The same is true for eyepieces, their object plane is also curved.

The 2 curves most likely being opposed to each other and the edges then are the worst case, so what you see at the edge is, or can be, really bad.

For astrophotography the eyepiece is not present but the ojective is, and on the edges of the sensor the ccd and the image are not at the same place so the stars are subject to a few problems, coma/spherical aberration and just plain out of focus. So people use a flattener for this.

At the visual side a good eyepiece will be made better and would allow for this at least a little. You can get Flat Field Eyepieces, Sky's the Limit and TS do them. They are produced for f/5 and f/6 scopes to compensate for this curvature.

With the rush to wide field eyepieces then it will be more present, I tend to stick at the 60-70 degree ones. I would assume that a wide angle eyepiece is however designed to reduce this otherwise it would be close to useless.

[lifeonmars]

Thank you for your replies now i understand, comparing the focal length to a sphere and the degree of curvature will be dependent on the diameter of the sphere thats something you can visualize.

one other question when you buy a field flattener are they matched for the particular telescope or is it a bit of a gamble as to what results you will obtain from it.

i think its really great how you guys on this forum are so prepared to help us newcomers to learn more about the science thank you!!