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Pentax XW - Field Curvature?

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23 replies to this topic

#1 Dave Bush

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Posted 12 December 2020 - 10:27 PM

In a coma corrected f/4.9 Newt (f/5.6) does the FC in the 20mm XW still exist?

 

Please only respond if you have actual experience with this.  



#2 areyoukiddingme

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Posted 13 December 2020 - 12:21 AM

Would you settle for F5 and F5.75 corrected?



#3 Dave Bush

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Posted 13 December 2020 - 12:32 AM

Would you settle for F5 and F5.75 corrected?

Huh?



#4 Ernest_SPB

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Posted 13 December 2020 - 03:44 AM

In a coma corrected f/4.9 Newt (f/5.6) does the FC in the 20mm XW still exist?

 

Please only respond if you have actual experience with this.  

In F5 scope with plane focal surface (alike Newton) 20 mm XW shows strong FC (spots 30' in diameter)



#5 adamckiewicz

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Posted 13 December 2020 - 07:27 AM

So does that mean that in an edge hd there will be more fc than in standard sct?

#6 ad701xx

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Posted 13 December 2020 - 10:47 AM

I remembered someone posted a chart showing the field curvatures and astigmatism of the XWs not long after they first came out. I found it in the archives here:

 

https://www.cloudyni...eld-curvature/ 


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#7 areyoukiddingme

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Posted 13 December 2020 - 11:36 AM

At F5 20 XW does show significant FC in a Newtonian.

 

At F5.75 (with paracorr), 20 XW shows mild FC, to my eyes.


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#8 Starman1

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Posted 13 December 2020 - 11:46 AM

Same here: coma-corrected scope at f/5.75, focal length 1826mm, only very mild field curvature.

The 20mm has almost 4 diopters of positive field curvature at the edge of the field, likely out of the range of accommodation,

which ranges from 1-3 diopters by wavelength for average vision.

Of course, the older you are, the less accommodation you have, so what might be tolerable at 20 might be intolerable at 60.


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#9 adamckiewicz

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Posted 13 December 2020 - 11:48 AM

I remembered someone posted a chart showing the field curvatures and astigmatism of the XWs not long after they first came out. I found it in the archives here:

https://www.cloudyni...eld-curvature/


Thanks a lot!
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#10 Dave Bush

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Posted 13 December 2020 - 03:28 PM

Same here: coma-corrected scope at f/5.75, focal length 1826mm, only very mild field curvature.

The 20mm has almost 4 diopters of positive field curvature at the edge of the field, likely out of the range of accommodation,

which ranges from 1-3 diopters by wavelength for average vision.

Of course, the older you are, the less accommodation you have, so what might be tolerable at 20 might be intolerable at 60.

Well, I'm 61 so this sure sounds like a no-go for me.


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#11 junomike

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Posted 13 December 2020 - 05:09 PM

Well, I'm 61 so this sure sounds like a no-go for me.

Dunno, I'm 50 and see very little FC in the XW's in any OTA.  Others, much younger complain about it so it's possible you might be ok with it.

Problem is trying before buying.



#12 jjack's

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Posted 13 December 2020 - 05:57 PM

Is the 20 XW work better with short refractors or 8"SCTs ? Is the field curvature of this eyepiece fits the field curvatures of theese scopes ?



#13 Dave Bush

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Posted 13 December 2020 - 08:24 PM

 

Problem is trying before buying.

Exactly. 



#14 daniel_h

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Posted 13 December 2020 - 08:54 PM

Is the 20 XW work better with short refractors or 8"SCTs ? Is the field curvature of this eyepiece fits the field curvatures of theese scopes ?

dsigned for refractors -the pentax spotting scopes which are short f/l, but should work fine in a f10 sct 


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#15 25585

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Posted 14 December 2020 - 03:36 AM

Look out for pre-owned, my 20mm was & I saved a lot buying it. With a 2x Barlow,  you get a good flatter field 10mm equivalent.



#16 Sarkikos

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Posted 14 December 2020 - 07:43 AM

In a coma corrected f/4.9 Newt (f/5.6) does the FC in the 20mm XW still exist?

 

Please only respond if you have actual experience with this.  

Yes.  Absolutely it does have obvious FC.  I have a 20 XW and a 10" f/4.8 Dob.  But putting the 20 XW in a Paracorr helps a lot.  I was able to have a pretty flat field with a Paracorr if I focused the 20 XW about half-way between center of field and edge of field ... with a Paracorr.  

 

My eyes have virtually no accommodation for field curvature.  So if there is FC to see, I will see it.  To my eyes, the 20 XW has obvious and ugly FC in a 10" f/4.8 Dob without a Paracorr.  With a Paracorr, not so much.

 

Mike


Edited by Sarkikos, 14 December 2020 - 07:47 AM.

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#17 jjack's

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Posted 14 December 2020 - 12:14 PM

Thanks Daniel_h. Good to know. Shurely some eyepieces work better on some kind of scopes and are not so good on anothers scopes. But nobody could tell us realy.



#18 Dave Bush

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Posted 14 December 2020 - 12:44 PM

Yes.  Absolutely it does have obvious FC.  I have a 20 XW and a 10" f/4.8 Dob.  But putting the 20 XW in a Paracorr helps a lot.  I was able to have a pretty flat field with a Paracorr if I focused the 20 XW about half-way between center of field and edge of field ... with a Paracorr.  

 

My eyes have virtually no accommodation for field curvature.  So if there is FC to see, I will see it.  To my eyes, the 20 XW has obvious and ugly FC in a 10" f/4.8 Dob without a Paracorr.  With a Paracorr, not so much.

 

Mike

Thanks Mike.  That’s the kind of info I was looking for.  Bummer though.  Sure wish Tele Vue made a 20mm Delos. 


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#19 sanbai

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Posted 14 December 2020 - 04:17 PM

AFAIK, the paracorr 2 corrects residual field curvature of the primary mirror (the trade of is a 1.15x focal extension factor). Also, as discussed some months ago, the field curvature of the telescope and the eyepiece neutralize each other if both are of the same sign. If they are of opposed sign, it results in a more pronounced field curvature.

 

Thus, my guess is that the field curvature of the Pentax is of different sign of a non-coma corrected newtonian (call it a bad combination). Add the paracorr 2 and you basically see the native field curvature of the eyepiece (because the telescope gives now essentially a flat field). Dave's telescope is has a bit more focal length, so flatness is even better.

 

According to Sarkikos it seems to be usasble, but the hassle is that one have to put a (bright) star in the middle of the AFOV to find a compromise.

 

In that thread about FC of eyepiece+telescope there were some combinations that cancelled each other in "non-edgeHD" SCT scopes. Ideal is to have a flat field telescope.

 

Well, 17.3 mm of a Delos is not that difference in magnification. Or 17.5 of a Morpheous, where the larger AFOV compensates (in terms of TFOV) those 2.5 mm. The Morpheous is 19 mm eye reliefe (like the 22 t4 you let go), but you may still see the full AFOV (76 °) you are paying for (and it's cheaper than the 22T4). However, I don't know if the Baader have field curvature (my guess is that it does not have).



#20 Sarkikos

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Posted 15 December 2020 - 06:54 AM

Field curvature produces a different position of focus for center of field versus edge of field.  If you focus the telescope for a star at center of field, a star at edge of field will be out of focus.  If you focus the telescope for a star at edge of field, a star at center of field will be out of focus.  If the field curvature is not so bad, you can focus at a star between center of field and edge of field, and both center of field and edge of field will be nearly in focus.  Not perfect, but better than if you focus at either center of field or edge of field.

 

Mike



#21 adamckiewicz

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Posted 15 December 2020 - 07:21 AM

And if you look 2 small objects such as m65/66 or m81/82 you cat have them at same distance from center and both perfectly focused.

What is important if you have field curvature is to center a bright star of the field, focus on it and then put the object in the center of the field.

#22 Sarkikos

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Posted 15 December 2020 - 07:26 AM

Unless you want to see a flat field in focus across the entire view.  Ideally, that is what I always want to see.

 

Mike


Edited by Sarkikos, 15 December 2020 - 07:27 AM.

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#23 adamckiewicz

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Posted 15 December 2020 - 07:50 AM

And so do I. But cannot at low power with my sct :(

#24 Sarkikos

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Posted 15 December 2020 - 07:56 AM

You can if you use a field flattener/coma corrector in the SCT, along with flat field eyepieces.

 

Mike


Edited by Sarkikos, 15 December 2020 - 07:56 AM.



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