Flat field eyepieces in a newtonian

[runway77]

Hello, I am having a problem with a relatively expensive flat field eyepiece in a combination with a newtonian reflector. I am unable to achieve proper focus with that eyepiece because when I focus it on axis, the edges are blurry. When I focus it at the edge, the center field is out of focus. I don't want to mention the concrete type of this eyepiece because I received it with a spot on the bottom barrel, therefore I cannot be sure whether it has been dropped. This eyepiece has very good reviews on this forum. Optically it looks good and there is no rattle. My question is whether these flat field eyepieces are usable with newtonians as these telescopes doesn't generally suffer from the field curvature. Many thanks for help.

[Carbon Brush]

Can you identify the model of scope and eyepiece please?

[runway77]

skywatcher 150 PDS and the eyepiece is APM 24mm ultra flat field. But as I say, it might be that this eyepiece is faulty. Hence my question to fellow observers whether these flat field eyepieces work for you with the newtonian reflectors or do you have similar experience? Thank you.

Edited October 10, 2020 by runway77

[Carbon Brush]

An F5 newtonian is going to be 'picky' on eyepieces.
Sorry, I have no experiece of the APM eyepiece, though it does appear to be intended for a fast scope.
Hopefully another member can contribute.

Do you have the opportunity to take your eyepiece t try in another (similar focal ratio) scope?

You might be interested in this thread.

Good luck,

David.

[runway77]

I have tried it already in two f/5 newtonians with the same outcome. I will try it in my 8 inch f/6 dob as well, but it's cloudy. The best test would be with a refractor but unfortunately I don't have any ;)

[Ricochet]

Newtonians have much flatter fields than refractors. I don't recall anyone complaining of field curvature in this "flat field" eyepiece. If you are seeing field curvature (and not coma) then I would suggest that you return the eyepiece.

[runway77]

2 hours ago, Ricochet said:

Newtonians have much flatter fields than refractors. I don't recall anyone complaining of field curvature in this "flat field" eyepiece. If you are seeing field curvature (and not coma) then I would suggest that you return the eyepiece.

this doesn't look like coma and I don't have this problem with any other eyepieces. I have not noticed anyone as well to have similar problems with flats field eyepieces in newtonian telescopes. I will probably return this eyepiece and ask for a replacement. Thanks for your response. 

[andrew s]

Stop the Newtionian down with a  mask to say f10. If it's  still an issue I would suspect the eye piece.

Regards Andrew 

[Louis D]

I've used this eyepiece in multiple telescopes that are flat field or close to it and have never noticed any appreciable field curvature in it.  The only thing I've noticed is that the last 5% to 10% starts to fuzz out which no amount of refocusing will correct.  You can see this loss of edge sharpness in my eyepiece AFOV comparison image below:

These were taken through an AT72ED refractor with a TSFLAT2 field flattener ahead of the diagonal at the proper spacing.