Is a coma corrector also a flattener?

[Cody]

I'm a little confused with the terminology. For example a Quattro reflector doesn't seem to have any flatteners for AP but people do mention to Baader coma corrector which in turn talks about reducing trails on stars at the edge of field.

[RickS]

Coma and field curvature are different aberrations. Coma distorts objects towards the edge of the field of view making stars look like comets with tails pointing away from the centre of the FOV. Field curvature results in a difference in focus between the centre and edges of the FOV. You can have things in focus at the centre or at the edges but not both at the same time.

Coma is the main aberration affecting Newtonian reflectors, so you'll generally need a coma corrector for AP using one unless you have a small sensor or a slow scope (f/ratio like f/8 or greater.)

Field curvature is an issue with other designs such as refractors. That's why field flatteners are available for these scopes. Some refractors designed specifically for AP, the Takahashi FSQ scopes for example, have extra lens elements to produce a flat field without further correction.

Cheers,
Rick.

[Cody]

Thanks so does this mean Newts already have flat fields after coma correction?

[RickS]

Yep, if you get rid of the coma a well-figured Newtonian is well corrected for AP.

Cheers,
Rick.

[Sundog]

So coma corrector and field flattener are only needed for AP, not for visual viewing?

It seems their effect would also be desirable for visual.

[RickS]

A coma corrector is probably worthwhile for visual with a fast scope and eyepieces with a wide FOV.

Mild field curvature is not such a big deal because the eye can accommodate for small focus changes as you look around the FOV.

I'm mostly an imager so it would be advisable to get confirmation from a visual expert before spending money ;)

Cheers,
Rick.

[Wavytone]

Many modern low power eyepieces will benefit, but some won't - it depends on the eyepiece. For example edmunds RKEs were designed to match the f/4 Astroscan and won't benefit from a coma corrector.

To some extent its a case of try it and see if its better or worse.

With high power eyepieces on the planets or bright stars you'd be better off without it, as it may be a source of ghosts or internal reflections.

If it corrects for field curvature as well that may be plus or a minus, depending on which eyepieces you use, for example plossls used with a fast newtonian may be better off without the corrector because plossls have a curved field that is a fair match for the average f/4 Newtonian.