Quote:
Originally Posted by KaStern
Hello at all,
1) What is field curvature?
2) does the field curvature of the XW 20 only appear in fast scopes?
3) does the paracorr eliminate field curvature? Is it a field flattener?
4) why does TeleVue tell that the paracorr works as a coma corrector,
not as a field flattener?
http://www.televue.com/engine/page.asp?ID=230
Regards, Karsten |
1) Field curvature means that the focal plane is not 100% flat. It means that the focal point at the centre of field is different to that at the edge. ie optimum focus giving sharp stars at the centre will give slightly bloated stars at the edge. Alternately focusing sharp stars at the edge gives slightly out of focus stars at the centre.
2) the type of scope used does have an effect as some scopes present a more curved focal plane than others. Hopefully someone else can elaborate.
3) It does appear to have that effect.
4) Coma correction is its main aim, field flattening is a fortunate by product of its design
P.S I've seen lots of talk of field curvature attributed to certain pentax xw eyepieces, but they're by no means the only high end ep's to suffer from this. I've seen it in nagler type 4's also. Meade s5000 60 degree plossls are shockers