starcam
sage
Reged: 09/24/07
Posts: 318
Loc: MD
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I'm using a f/7 scope, just purchased the xw20.
When looking to the stars, let's say the outer 75% stars become unfocused. When I focus on the outer stars the center stars go out of focus. Is this normal? I thought with the price tag the stars would be pinpoint to the edge. I believe I assumed wrong. I was viewing nebula's with more contrast.
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Celestron 9.25 SCT f/10 fl/2350
Stellarvue SV102ED2 F/7 fl/710
William Optics Megrez 72mm F/6 fl/432mm
Coronado PST
CG-5
UWAN 28mm
Panoptic 22mm
Televue 7T6,13T6,15wf,11
Pentax XW 7,10,20,40 / XO 5 / Zoom 8-24
RKE 8,12,15,21,28
Hyperion 5,8,17/13
TMB 2.5,4,6II,7,9
Brandon 6,8,12,16
UO HD 7mm
Oberwerk bino 15x70
Burgess 8x42 bino
WO binoviewer
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Rob S
professor emeritus
Reged: 03/16/07
Posts: 502
Loc: NZ
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For a lot of scopes this is normal.
This particular focal length XW has a lot of field curvature.
Apparently the type of scope they expected you to use it in has the opposite type of field curvature - so it cancels out.
Rob.
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8" f5 Dob with Feathertouch
Brandon: 16
Pentax: 7, 10 & 20XW
TMB: 30 Paragon
TV: 8 & 13 Ethos;
5 & 13 NaglerT6;
24 Panoptic
Leica Trinovid 8x50 BN
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slyke
super member
Reged: 02/21/08
Posts: 116
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Quote:
For a lot of scopes this is normal.
This particular focal length XW has a lot of field curvature.
Apparently the type of scope they expected you to use it in has the opposite type of field curvature - so it cancels out.
Rob.
Here is a graph from a previous discussion:
XW field curvature
If one thinks of eyepieces as corrective lenses for your telescope then you would want them to have equal and opposite curvature.
-Stephen
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3959
Loc: "Metrowest" Boston
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Quote:
Apparently the type of scope they expected you to use it in has the opposite type of field curvature - so it cancels out.
Presumably they expected you to use it in on one of their spotting scopes, and I rather doubt that these have negative curvature. I think it's more likely that they don't think you'll be bothered by this aberration nearly as much with terrestrial viewing. They've been doing this consistently. Same situation with the 12XF the 14mm and 21mm XLs. You hear people all the time saying how the curvature doesn't bother them, and naturally this is scope-dependent, but I found there were times when I could almost convince myself that I liked the 12XF and at other times hated the curvature, so I think there's a strong component to it based on what you're doing. Which is to say, I don't think it's entirely a matter of "different eyes".
Anyway, these mid-focal-length Pentax eyepieces are not, IMO, designed to be ideal astronomical eyepieces. You may like them in spite of that because of the quality of the glass and coatings, but the curvature they left in the design leaves a lot to be desired for astronomical use. The situation is strikingly different with the eyepieces they've designed presumably mainly (or in some cases solely) for astronomical use, e.g. 2" XWs or 10.5mm and shorter XLs and XWs. Even the 8.5mm XF is much better optimized for astronomical use than its 12mm sister.
For 20mm of eye relief, I'd probably go with Pentax XWs up to (and including) 10mm, switch to Vixen LVWs 13mm, 17mm, and 22mm (eschew the 14mm and 20mm XWs), and then switch back to Pentax XWs for the longer focal lengths, 30mm and 40mm.
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt and equatorial platform
- 6 eyepieces and a Barlow
- 120mm f/8.3 home-built grab-n-go Newt
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 733
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This is more typically a complaint about the XW20 at F4 or F5 than f/7. I'd like to know what you think of the eyepiece in your 9.25.
Your scopes do have their own field curvature, this is the first I've heard of someone unhappy with the XW20 at f/7 in a refractor. In any case it is true that the XW14 and the XW20 are the two eyepieces that get the most negative reactions in the XW line, usually from owners of fast scopes.
It would be interesting to know how you like the eyepiece with a field flattener in your refractor.
The XW20 may not be for you, some eyepieces just don't work for some scopes. I like the XW20 a great deal in my Vixen ED double f/6.5.
I have read that the ability of the eye to focus out the effects of field curvature varies from individual to individual, you might be more sensitive than other people.
I'm sorry to read of your disappointment. I am a big booster of these eyepieces. But I wouldn't recommend the XW14 or XW20 as a FIRST XW eyepiece due to the mixed reactions. I would have recommended the XW10.
regards
greg n
Greg N
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"Aperture will get you through times of no money better than money will get you through times of no aperture."
featuring selected astrojunk:
bunch o' widefield eyepieces
bunch o' narrowfield eyepieces
couple o' Barlows
couple o' scopes
couple o' mounts
couple o' tripods
and a pier 'n' stuff
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Deep13
Carpal Tunnel
Reged: 01/25/05
Posts: 1843
Loc: NE Ohio
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That seems surprising in an f/7 refractor. I have the 14 which has field curvature in my 12.5" f/5, but I don't notice it in the 8" f/6 or the TV101.
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That which can be asserted without evidence can be dismissed without evidence.
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RAKing
Carpal Tunnel
Reged: 12/28/07
Posts: 2092
Loc: West of the D.C. Nebula
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This is always quite interesting. I also had problems with the 20XW, but the 14XW is superb in my C925.
I suspect our eyes are also contributing to the differences, as mentioned in the above posts.
Ron
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Time spent looking at the stars is added to your life.
Tak FS-128, C925-CF, C6SE, other stray cats and refractors.
A-P Mach1 GTO
Zeiss orthos to Ethos - and some stuff in between.
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BillP
Post Laureate
Reged: 11/26/06
Posts: 3955
Loc: Vienna, VA
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Quote:
I'm sorry to read of your disappointment. I am a big booster of these eyepieces. But I wouldn't recommend the XW14 or XW20 as a FIRST XW eyepiece due to the mixed reactions. I would have recommended the XW10.
FWIW I got a peek thru the XW40 this week thru the 12" Clark Refractor at the US Naval Observatory. Having previously owned things like the 40 Paragon, 35 Pan, 40 Titan-II ED I have to say I was taken at just how more compact the 40XW was...not to mention its superb performance and really easy eye placement. It's on my list now for a future purchase  A really nice 40!
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Bill Paolini
XT10i Dob---TSA-102 S-APO---APM80/480 S-APO--- P.S.T.
TMB Supermonos---Meade UWAs---TV Panoptic---AT Titan II ED
To your own eyes be true...
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3959
Loc: "Metrowest" Boston
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Quote:
Having previously owned things like the 40 Paragon, 35 Pan, 40 Titan-II ED I have to say I was taken at just how more compact the 40XW was
Bill, I love my 40XW, but could this perception about it being much more compact have anything to do with it being in a 12" Clark refractor at the time? 
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt and equatorial platform
- 6 eyepieces and a Barlow
- 120mm f/8.3 home-built grab-n-go Newt
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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BillP
Post Laureate
Reged: 11/26/06
Posts: 3955
Loc: Vienna, VA
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--------------------
Bill Paolini
XT10i Dob---TSA-102 S-APO---APM80/480 S-APO--- P.S.T.
TMB Supermonos---Meade UWAs---TV Panoptic---AT Titan II ED
To your own eyes be true...
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starcam
sage
Reged: 09/24/07
Posts: 318
Loc: MD
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A few things spoiled me the naglers and being on here learning to be more critical of the views. I luv the xw10,7 and xo5 and the zoom is awesome for the sun and night sky observing,when I'm lazy. I picked the xw20 because it supposely had less curvature than the 14 and I already have the 13t6. The xw40 works well the sv102ed2 f/7, but in the c9.25 f/10 there was a lot of curvature. I will try the xw20 in the c9.25 maybe this weekend. All is not bad as for nebula viewing, the XW20 smoked the 22pan, in light through-put. The 22 pan had less curvature to these aging eyes. I know I don't have top of the line scopes, but I saw more rave reviews then negative one's about the xw20. Wasn't sure if Hoya changed the eyepiece.
--------------------
Celestron 9.25 SCT f/10 fl/2350
Stellarvue SV102ED2 F/7 fl/710
William Optics Megrez 72mm F/6 fl/432mm
Coronado PST
CG-5
UWAN 28mm
Panoptic 22mm
Televue 7T6,13T6,15wf,11
Pentax XW 7,10,20,40 / XO 5 / Zoom 8-24
RKE 8,12,15,21,28
Hyperion 5,8,17/13
TMB 2.5,4,6II,7,9
Brandon 6,8,12,16
UO HD 7mm
Oberwerk bino 15x70
Burgess 8x42 bino
WO binoviewer
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gaz-in
sage
Reged: 12/17/07
Posts: 443
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Should using the XW 14 and/or XW 20 with a Paracorr allow one to correct the field curvature issues?
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ausastronomer
scholastic sledgehammer
Reged: 06/30/03
Posts: 937
Loc: NSW Central Coast (Australia)
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Quote:
Should using the XW 14 and/or XW 20 with a Paracorr allow one to correct the field curvature issues?
Hi,
Yes it does and extremely well. I use the XW's from 5mm through to 20mm in my 14" and 18" F4.5 scopes without issue.
Something to note, which 95% of people get wrong and several in this thread have already got wrong.
The field curvature perceived has nothing to do with the F-RATIO of the telescope.
It is dependant on many things, including
1)the FOCAL LENGTH OF THE TELESCOPE and the RADIUS of CURVATURE OF THE PRIMARY OPTIC . Of course for a given aperture a long focal length scope will have a slower F-ratio than a short focal length scope and less field curvature. This is where the incorrect assumption comes from that field curvature is related to F-ratio. My 18"/F4.5 Obsession has clearly a lot less field curvature with the 14mm and 20mm XW's than does my 10"/F5 newtonian. This is beautifully corrected in both scopes with a paracorr.
2)The design of the telescope is critical as to whether it has +ve or -ve field curvature and whether that matches and compensates the field curvature inherent in the eye piece design.
3) The ability of the observers own eyes to "auto adjust" or compensate for slight out of focus. This ability can vary enormously from one observer to the next and is generally the reason why field curvature bothers some people yet doesn't bother others.
In regard to the performance of the 14mm and 20mm Pentax XW's in SCT's I offer the following. Notwithstanding the scopes are slow at F10 (see my comments above re F-ratio) the C9.25 has an F2.5 primary mirror with a very short focal length of about 23" and a very tight radius of curvature. Because of this there is a large amount of field curvature inherent in the SCT design and they will not work well with these eyepieces.
Cheers,
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John Bambury
AS of NSW
AS of Hunter Valley
18"/F4.5 Obsession fully loaded with OMI optics
14"/F4.5 SDM fully loaded with Zambuto Optics
10"/F5 Newtonian with Argo Navis
Lots of Pentax XW's. 12mm & 17mm Nagler T4's, 31mm Nagler T5 & 13mm ETHOS. TV Paracorr and 1.8X TV Barlow and 2.5X TV Powermate
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Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3959
Loc: "Metrowest" Boston
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Quote:
The field curvature perceived has nothing to do with the F-RATIO of the telescope.
Sorry, but you're totally wrong about that.The f-ratio determines the exit pupil diameter, which in turn affects the depth of focus of the eyeball. It's a simple optical fact, quite incontrovertible.
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt and equatorial platform
- 6 eyepieces and a Barlow
- 120mm f/8.3 home-built grab-n-go Newt
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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russell23
sage
Reged: 05/31/09
Posts: 427
Loc: Upstate NY
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My 20mm XW is great in my 120mm f/8.3 refractor - very sharp almost to the edge. However, I tried it in a 150mm f/5 refractor and found the same problem in the outer 25% of the field that you describe.
Dave
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ausastronomer
scholastic sledgehammer
Reged: 06/30/03
Posts: 937
Loc: NSW Central Coast (Australia)
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Quote:
Quote:
The field curvature perceived has nothing to do with the F-RATIO of the telescope.
Sorry, but you're totally wrong about that.The f-ratio determines the exit pupil diameter, which in turn affects the depth of focus of the eyeball. It's a simple optical fact, quite incontrovertible.
I don't agree. The F-Ratio in fact affects nothing, including determining the exit pupil diameter. The exit pupil is affected by 3 things:-
1) The focal length of the telescope
2) The aperture of the telescope
3) The focal length of the eyepiece
It is convenient mathematics that the exit pupil can be calculated by dividing the eyepiece focal length by the f-ratio of the telescope. That happens because the F-Ratio is a function of the first two things. Change either of the first two things and you also change the exit pupil and because it is a function of them, you also change the F-ratio.
Cheers,
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John Bambury
AS of NSW
AS of Hunter Valley
18"/F4.5 Obsession fully loaded with OMI optics
14"/F4.5 SDM fully loaded with Zambuto Optics
10"/F5 Newtonian with Argo Navis
Lots of Pentax XW's. 12mm & 17mm Nagler T4's, 31mm Nagler T5 & 13mm ETHOS. TV Paracorr and 1.8X TV Barlow and 2.5X TV Powermate
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gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 733
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Quote:
A few things spoiled me the naglers and being on here learning to be more critical of the views. I luv the xw10,7 and xo5 and the zoom is awesome for the sun and night sky observing,when I'm lazy. I picked the xw20 because it supposely had less curvature than the 14 and I already have the 13t6. The xw40 works well the sv102ed2 f/7, but in the c9.25 f/10 there was a lot of curvature. I will try the xw20 in the c9.25 maybe this weekend. All is not bad as for nebula viewing, the XW20 smoked the 22pan, in light through-put. The 22 pan had less curvature to these aging eyes. I know I don't have top of the line scopes, but I saw more rave reviews then negative one's about the xw20. Wasn't sure if Hoya changed the eyepiece.
Actually the XW20 probably didn't smoke the 22 Pan in light throughput; possible, Televue tends to run a few % behind Pentax (so far as results are posted anywhere). But more likley is that the manipulation of the field in the Pan Optic smears out stars a bit with the resulting tendency to make the fainter ones disappear. Which is throughput of a sort.
The only defect I've noticed in my C14 and f/6.5 refractor with the XW40 is my own eyes: when I got the dioptrx running, my XW40 mysteriously became crystal sharp edge to edge. If you haven't been evaluated for astigmatism lately, you might consider that. When we evaluate eyepieces we are pushing everything to the max, including our eyeballs.
Greg N
--------------------
"Aperture will get you through times of no money better than money will get you through times of no aperture."
featuring selected astrojunk:
bunch o' widefield eyepieces
bunch o' narrowfield eyepieces
couple o' Barlows
couple o' scopes
couple o' mounts
couple o' tripods
and a pier 'n' stuff
|
gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 733
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Quote:
Should using the XW 14 and/or XW 20 with a Paracorr allow one to correct the field curvature issues?
I got a number of reports after I posted my astromart article on Pentax that in fact the XWs become wonderful with the paracorr. But I'm not sure I'd want to put a paracorr on original post's f/7 Stellarvue refractor.
Greg N
--------------------
"Aperture will get you through times of no money better than money will get you through times of no aperture."
featuring selected astrojunk:
bunch o' widefield eyepieces
bunch o' narrowfield eyepieces
couple o' Barlows
couple o' scopes
couple o' mounts
couple o' tripods
and a pier 'n' stuff
|
Mike Hosea
Post Laureate
Reged: 09/24/03
Posts: 3959
Loc: "Metrowest" Boston
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Quote:
It is convenient mathematics that the exit pupil can be calculated by dividing the eyepiece focal length by the f-ratio of the telescope.
I think I see where you're coming from. For some reason you don't want to regard f-ratio as a first-class variable. Suppose I have a function f(x,y) = c*(x/y). All we're doing here by using f-ratio is studying instead the relationship g(z) = c*z, where z = x/y. Obviously, g(z) = c*z = c*(x/y) = f(x,y). People are pointing out (correctly) that their perception of curvature depends on z, whereas you are trying to convince us that it only makes sense to think of it as depending on both x and y. You're not wrong to say that it depends on both x and y, but you are wrong to assert that it doesn't depend on x/y, i.e. on z.
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Mike
Stuff that I use:
- 7" f/6.7 home-built Newt and equatorial platform
- 6 eyepieces and a Barlow
- 120mm f/8.3 home-built grab-n-go Newt
- Canon 15x50 IS and Eagle Optics 12x50 Ranger binoculars
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ausastronomer
scholastic sledgehammer
Reged: 06/30/03
Posts: 937
Loc: NSW Central Coast (Australia)
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Mike,
I think you are clearly missing the point I was originally trying to make. Possibly I also didn't choose my words carefully enough.
People are "incorrectly" assuming that because they have a fast F-ratio telescope they will see field curvature. I am trying to say that because of the F-ratio alone this isn't necessarily so.
For instance a 30"/F4.5 newtonian will show less field curvature with these eyepieces than say an 8"/F6 newtonian, by virtue of its much longer focal length and radius of curvature of the primary optic, notwithstanding that it has a faster F-ratio and a shorter depth of focus. My 18"/F4.5 to 10"/F5 comparison also demonstrates this clearly.
Cheers,
--------------------
John Bambury
AS of NSW
AS of Hunter Valley
18"/F4.5 Obsession fully loaded with OMI optics
14"/F4.5 SDM fully loaded with Zambuto Optics
10"/F5 Newtonian with Argo Navis
Lots of Pentax XW's. 12mm & 17mm Nagler T4's, 31mm Nagler T5 & 13mm ETHOS. TV Paracorr and 1.8X TV Barlow and 2.5X TV Powermate
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