FC in long Pentax XW.

[Anderscn]

Hi,

 

In the 14mm and 20mm FC is a much debated subject. However, would it be less pronounced in a long Refractor (f12) than in a faster Dob (f5)?

Best Regards

Anders

[alan potts]

Do you mean Field Curvature or Edge Distortion, the first is where things will become sharper but there is a need for a slight re-focus, the latter is where the edge can and in my Nagler eyepieces is in focus but is distorted.

E/D I would have thought not as it's a charateristic of the eyepiece. I can't say I notice any difference in eyepieces with this , and I have a few, in my F4.3 Dob and my F15 Mak, to me they look the same. So I would say the answer is basically that though I stand to be corrected.

I can only recall the 14mm Pentax XW having slight F/C and I don't remember if this was affected as it is a while back. There are some other members I know have these eyepieces but I don't know if they have a spread of Focal Lenghts scopes wise.

[RikM]

I don't know how true this is, but I read that the XW field curvature is intentional to correct for the opposite curvature in the Pentax spotting scopes. So depending of the scope, f/c may be more or less pronounced. I do know that with the 14 and 20, there is slight field curvature (differential focus between centre of field and the edge), in my 250mm f/4.7 Newtonian, which I don't notice in my 70mm f/6.4 refractor. The Newtonian view is of course made more obvious by coma. I can't comment on longer focal ratio scopes I am afraid. 

[Stu]

8 minutes ago, RikM said:

I don't know how true this is, but I read that the XW field curvature is intentional to correct for the opposite curvature in the Pentax spotting scopes. So depending of the scope, f/c may be more or less pronounced. I do know that with the 14 and 20, there is slight field curvature (differential focus between centre of field and the edge), in my 250mm f/4.7 Newtonian, which I don't notice in my 70mm f/6.4 refractor. The Newtonian view is of course made more obvious by coma. I can't comment on longer focal ratio scopes I am afraid. 

The fact that you don't see it in the refractor implies you are correct Rik, and there is some inbuilt correction for FC in the eyepieces. Shorter focal length refractors tend to show this effect, it gets worse with shortening focal length.

Following this logic, I would expect a long focal length refractor to also show some FC with the Pentax as their field tends to be flatter due to the longer focal length so the Pentax will effectively be over correcting for FC.

Does that make sense?

[Anderscn]

Thank you very much for the answers. Stu, your explanation makes good sense. So I Will have to look for other EP's in this range. Delite or Morpheus that both should have less field curvature.

BR

Anders

[RikM]

1 hour ago, Anderscn said:

Thank you very much for the answers. Stu, your explanation makes good sense. So I Will have to look for other EP's in this range. Delite or Morpheus that both should have less field curvature.

BR

Anders

It is always a compromise. The Delite, as many other Televue EP's may well show less field curvature, but will likely show more distortion. It comes down to what you prefer.

[YKSE]

On 2016-02-13 at 10:30, RikM said:

I don't know how true this is, but I read that the XW field curvature is intentional to correct for the opposite curvature in the Pentax spotting scopes. So depending of the scope, f/c may be more or less pronounced. I do know that with the 14 and 20, there is slight field curvature (differential focus between centre of field and the edge), in my 250mm f/4.7 Newtonian, which I don't notice in my 70mm f/6.4 refractor. The Newtonian view is of course made more obvious by coma. I can't comment on longer focal ratio scopes I am afraid. 

This is one of few clearly written experience about XW's FC (eyepiece+scopes).

Just a little more clarification, were the 14 and 20 used in 80mm f6.4 refractor both in night and day time?

[RikM]

51 minutes ago, YKSE said:

This is one of few clearly written experience about XW's FC (eyepiece+scopes).

Just a little more clarification, were the 14 and 20 used in 80mm f6.4 refractor both in night and day time?

I normally only use the refractor (TV Pronto) at night or with a Baader film filter for white-light solar. I just made a daytime test focussing on my garden fence and there may be a very small difference between absolute sharpest focus centre to edge with 20mm but when focussing normally, everything looks essentially perfect edge to edge.

I will look more closely next clear night and make some notes. 

[John]

FWIW I recently acquired a Pentax XF 12mm for outreach sessions. It's a very easy eyepiece to look though but I'd read that it had some FC so I was expecting some. Interestingly while I could see clear FC when I used the eyepiece in my ED120 refractor, it was much less noticable when I used the eyepiece in my 12" dob, which is my principle outreach scope.

Overall it's a pretty nice eyepiece I reckon, desipite the FC in the frac

 

[YKSE]

Thanks Rik and John for sharing your experiences.

John, your comments about XF12 in 120ED and 12" are kind of a surprise to me, have you checked how XF12 behaves in your Vixen?

[RikM]

Okay, night time test accomplished.

I must retact my earlier statement. FC is still present in 14 & 20mm XW with the Pronto. Very slight but still there. The seeing is unusually good for me tonight so I could get very small stars and look for differences. 

[YKSE]

Thanks for the updated info.

The difference in daytime and night can partly be explained by exit pupil size, in daytime, our eyes pupil open at around 2mm (source Nikon), and aberrations(FC is an aberration) is not perceiveable as in larger exit pupil in night. Stu's explanation sound reasonable too. I'm kind of puzzled by 12XF's strong FC in 120ED but not 12" Dob though...

[John]

4 minutes ago, YKSE said:

 ....I'm kind of puzzled by 12XF's strong FC in 120ED but not 12" Dob though...

There is some FC with the 12XF in the dob but it seemed more apparent in the ED120. I've not tried the 12XF in the 102 F/6.5 ED refractor yet.

With the 12XF all I really was looking for was something that was easy to look through and showed decent quality views for folks who have not used a scope before. I doubt they will be searching hard for aberrations !

 

[michael.h.f.wilkinson]

Note that with respect to the EP, field curvature in a frac is the reverse of that in a Newtonian. An EP corrected for the one is not corrected for the other. Curvature is also more severe in shorter focal lengths, so if the XW14 and XW20 were optimized for a spotting scope (like thier 65mm F=380mm), the field curvature they were optimised for is higher than that in the Pronto, and certainly than that of the 120ED. A Newtonian at F=1500 would show less FC still.