Pentax 20mm XW bad points.

[markus.a.bergh]

There's a Pentax 20mm XW eyepiece for sale on the forum. I know its a nice eyepice but what are the * bad * points of this eyepiece, especially when using it in a fast F5 ( 12" dob, FL 1500mm giving 75x magnification) ?

I have been trying to find reviews but have not had much luck finding anything that reviews this eyepiece in an F5 telescope and I dont think I'll get to "try" one any time soon.

[Starkler]

It does exhibit field curvature in fast newtonians. ie outer field focuses at a slightly different point giving bloated stars toward EOF. If you use a paracorr it flattens the field nicely. Otherwise a top eyepiece.

[markus.a.bergh]

Thank you. I appreciate the help.

[ausastronomer]

Hi,

I own this eyepiece which I use in my 18"/F4.5 and 10"/F5 dobs. I also own a 17mm Nagler T4.

Geoff is spot on in his assesment of the 20mm Pentax XW. It will show some field curvature in your scope. This is its only weakness. The field curvature will be eliminated if you use a paracorr. The 20mm Pentax XW has many strong points, including its excellent sharpness, contrast and light throughput. It gives a very "clean" view because of its cool colour reproduction. It has very good eye relief (20mm) and is very comfortable to use. If you own a paracorr you can't buy a better eyepiece around this focal length. If you don't own a paracorr and don't plan to buy one, the 22mm Vixen LVW, the 21mm Denkmeier and the 22mm and 24mm Televue Panoptics are likely better options for you in 1.25" barrel format.

Cheers,
John B

[Starkler]

Thanks John for expanding on what I said. Im not sure I agree that the panoptics are a better choice sans paracorr and they definitely do benefit from their use, but in that case the 21 xw is still better ;)

[KaStern]

Hello at all,

I have some questions upon what you wrote.







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

[Starkler]

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

[KaStern]

Hello Starkler,

thanks for your answer.

Hello marcus.a.bergh,

here is what I think about using the 20mm XP in your 12"f/5 Dob.

Off-axis your dob suffers from the following aberrations:
1) coma
2) astigmatism
3) field curvature

Coma is a very big concern. For coma please see this site:

http://www.telescope-optics.net/coma.htm

The spots show how a star gets more and more out of shape the farer away
it gets from the optical axis.

Coma is an inherent aberration of your paraboloidal mirror. You can only cure it by
1) adding a coma corrector
2) using coma-compensating eyepieces. The pretoria eyepiece such an eyepiece.

Astigmatism

http://www.telescope-optics.net/astigmatism1.htm

too is an off-axis wavefront aberration. It is caused by the difference
between the sagittal and tangential plane. With your mirror one is flat,
the other is cuved. In the case of your f/1500mm mirror it is mildly curved.
You will not see astigmatism from your 12"f/5 mirror since coma leads to
much stronger deformed spots than astigmatism.

Field curvature

http://www.telescope-optics.net/curvature.htm

Since coma and astigmatism are present one cannot speak of field curvature.
But the "best fokus" between the sagittal and the tangetial surface is curved,
so you might take this as field curvature of your mirror. It is mild.


So how about the Pentax 20mm XW in your scope?
It will not cure for coma, nor astigmatism, nor field curvature of your mirror.
In a very fast scope it will add "eyepiece astigmatism". With my f/6 mirror
is is present, but not severe. With an f/5 mirror is is much more pronounced
and with an f/4 mirror it is very strong (to my eyes).

If it was field curvature you would be able to focus the unsharpness out.
But it is not possible to focus it away, so it is not field curvature.
I allways wonder why it is often adressed as "field curvature" in american forums.
It is coma from the mirror and astigmatism from the eyepiece. Both combine
to a large chracteristically formed unsharp blurr.

I like the Penatx XW very much.
But I think for your f/5 Newt there exist better suited eyepieces.

Regards, Karsten

[ausastronomer]

Hi Karsten,

I don't necessarily agree with those comments and assesments.



I own the 20mm Pentax XW and use it in both a 10"/F5 newtonian and an 18"/F4.5 newtonian. I don't agree with this comment at all. I see the predominant aberration as "field curvature" and 95% of it "can" be focused out and that is the reason it is addressed as such in all the US forums.

This is further confirmed by my own experiences using both my scopes. In that the observed field curvature is worse on the F5 scope, than it is in the F4.5 scope, due to the faster scope having a longer focal length. The observed field curvature is related to the focal length of the scope only, not its focal ratio. Due to a longer focal length scope having a larger "radius of curvature", hence an inherently less curved focal plane.

I said in an earlier reply that there are better choices in your 12"/F5 scope. However, I also indicated that if you "owned" a paracorr that it flattens the field beautifully and you wouldn't find a better eyepiece "if" you intended to combine it with a paracorr.

Cheers,
John B

[skies2clear]

Spot on John

CS
Nick