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HfxObserver
scholastic sledgehammer
Reged: 11/12/04
Posts: 794
Loc: Regina, SK, Canada
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So you are saying that because the Radius is large in the longer Focal Length scopes that the light beam is flatter as the larger radius appears shallow, compared to a smaller aperture same Ratio scope, when it intersects the eyepieces field element making it less curved.
I'm imagining two circles, one the size of an Orange, we can see this has a steep curve as we hold it in our hand....the other...lets say the size of the Earth...this appears flat yet they both have the same degree or ratio of angle.
If this line of reasoning was true then f/1 scopes would be all the rage....but it's the opposite since steeper cones are more easily gobbled up by eyepieces.
My understanding of optics is that we are taking the light into a cone, not a circle. So that in essence, the focal ratio does play a pivotal role. The steep cone of the slow scope is easier on eyepieces then the shallow cone of the flat Earth...er, I mean faster scope.
Feels good to put my philosophy degree to good use 
-Chris
P.S I have tested my 20XW in several 8" f6, 10" f4.7 and 12" f5 scopes the results were best performance on the higher f the scope, not on the longest focal length.
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Chris
7X50 Vixen,22X100 Antares
80mm William Optics Megrez II ED
Santel MK6
Borg 125SD f6 (Pentax/Oasis version)
Tak-Lapides
Pentax XW's 40,20,14,10,5mm XO, 3.5, 3.8XP, Speers 5-8, 30mm Widescan III
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Tiny
super member
Reged: 05/02/08
Posts: 197
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not to ask a stupid question here but why would a coma corrector/paracorr correct curvature in an eyepiece when the light is passed through the eyepiece last?
wouldnt the image simply be distorted by said eyepiece?
It also goes against the suggestion by televue to use eyepieces that themselves have good edge correction. If the above was true then you'd be able to use any piece of junk with amazing results, which... isnt true in my experience.
By the way OP, if that was a deal breaker for you, you'd probably enjoy a Meade UWA. Very flat field with pinpoints to the edges. You lose the great Eye Relief though.
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ausastronomer
scholastic sledgehammer
Reged: 06/30/03
Posts: 937
Loc: NSW Central Coast (Australia)
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Quote:
So you are saying that because the Radius is large in the longer Focal Length scopes that the light beam is flatter as the larger radius appears shallow, compared to a smaller aperture same Ratio scope, when it intersects the eyepieces field element making it less curved.
I'm imagining two circles, one the size of an Orange, we can see this has a steep curve as we hold it in our hand....the other...lets say the size of the Earth...this appears flat yet they both have the same degree or ratio of angle.
Your analogy is close to how it works but not quite there. You are correctly understanding that the optical surface of a longer focal length optic has less curvature than the shorter focal length optic and your example is a good one. However the intercept of the light cone with the field lens of the eyepiece has nothing to do with the field curvature of the telescope. The field curvature of the telescope primary optic (not the eyepiece) is dependent on the Radius of Curvature (ROC) of the primary optic and its inherent design. The observed field curavture at the eyepiece is then dependent on matching and cancelling the field curvature of both the telescope and the eyepiece.
Quote:
If this line of reasoning was true then f/1 scopes would be all the rage....but it's the opposite since steeper cones are more easily gobbled up by eyepieces.
No, this isn't so. As the light cone gets steeper aberrations other than field curvature, like coma and off axis astigmatism get infinitely worse.
Quote:
My understanding of optics is that we are taking the light into a cone, not a circle. So that in essence, the focal ratio does play a pivotal role. The steep cone of the slow scope is easier on eyepieces then the shallow cone of the flat Earth...er, I mean faster scope.
Absolutely true, but not in respect of field curvature. The shallower light cone will greatly improve eyepiece performance in terms of visible coma, off axis astigmatism and lateral colour. This is the reason you can use cheaper eyepieces in slower F-ratio scopes and they can perform reasonably well, yet perform poorly in fast scopes.
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P.S I have tested my 20XW in several 8" f6, 10" f4.7 and 12" f5 scopes the results were best performance on the higher f the scope, not on the longest focal length.
That's because all of those scopes are still fairly close in focal length and other aberrations could also be coming into play. In addition the slight increase in focal length may not be sufficient to compensate for the shallower depth of focus. Compare the 10"/F4.7 to an 18"/F4.5 and you will see what I am talking about. The eyepieces will work much better in the 18" scope.
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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:
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.
I don't think anyone presumes that f-ratio is the only factor. Other factors may indeed override the effect of f-ratio because they have a bigger effect, but this is a far cry from being able to say that f-ratio doesn't determine anything.
If you run the numbers on the curvature of a 10" f/5 versus an 18" f/4.5 using a 20mm Pentax XW, the curvature of the primary optic doesn't look very promising as an explanation of anything. We're talking about 0.06mm of focuser travel at the edge of the field in the 10", 0.04mm in the 18". That's a difference of 0.02mm. A typical eyepiece might need 1mm of focuser travel, often more. Maybe it's the difference in astigmatism rather than curvature in the longer instrument.
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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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Quote:
not to ask a stupid question here but why would a coma corrector/paracorr correct curvature in an eyepiece when the light is passed through the eyepiece last?
wouldnt the image simply be distorted by said eyepiece?
I don't know why it works but you can take it to the bank that it works and works very well (with a fast newtonian). The paracorr also does a good job of cleaning up some aberrations other than coma inherrent in some other eyepieces. For example, the 17mm Nagler T4 isn't a great eyepiece in a fast newtonian without the paracorr. It is an exceptionally good eyepiece with a paracorr and one of my favourites.
Quote:
It also goes against the suggestion by televue to use eyepieces that themselves have good edge correction. If the above was true then you'd be able to use any piece of junk with amazing results, which... isnt true in my experience.
There is a limit to what the paracorr can fix and be rest assured the 14mm and 20mm XW's and the 17mm Nagler are far from "any piece of junk". They are outstanding eyepieces which display slight single aberrations. With cheap eyepieces they have a dozen issues all going on at once. Also keep in mind that the totally aberration free perfect eyepiece, has yet to be designed or created.
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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ausastronomer
scholastic sledgehammer
Reged: 06/30/03
Posts: 937
Loc: NSW Central Coast (Australia)
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Quote:
If you run the numbers on the curvature of a 10" f/5 versus an 18" f/4.5 using a 20mm Pentax XW, the curvature of the primary optic doesn't look very promising as an explanation of anything. We're talking about 0.06mm of focuser travel at the edge of the field in the 10", 0.04mm in the 18". That's a difference of 0.02mm. A typical eyepiece might need 1mm of focuser travel, often more. Maybe it's the difference in astigmatism rather than curvature in the longer instrument.
Mike,
There is minimal astigmatism in both eyepieces. The EOF aberration is field curvature. I do know the difference between the two.
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:
There is minimal astigmatism in both eyepieces. The EOF aberration is field curvature. I do know the difference between the two.
My comment was referent to the telescopes, not the eyepiece. Astigmatism is lower in the longer focal length optic, i.e. the tangential and sagittal surfaces are closer to one another, minimizing the possibility of any unusual reinforcement or cancellation of that in the eyepiece (the 20mm XW). I don't know if this matters. I'm having a hard time figuring a theoretical explanation for your observations that would have the right magnitude of an effect. My own experience masking down an aperture only demonstrates the importance of f-ratio to perceived curvature. I have no experience trying to compare field curvature of eyepieces in both small and large Newts of the same f-ratios.
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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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Quote:
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.
I'm guessing you mean the outer 25% of the stars were unfocused? In any event (PMed you also), I gave the 20XW vs the 19 Pan a whirl in my f/8 TSA-102 tonight. Of course field curvature is evident using either eyepiece. Choosing the Double Cluster as the target I centered it in the FOV and chose an out of focus star near the field stop using the 19 Pan. It took approx a 160 deg turn of my fine focus knob to bring that star into focus again. Switching to the XW and redoing the experiment, the same star needed approx a 180 deg turn of the fine focus knob to refocus. So although the XW needed a little more, field curvature was shown by both EPs (obviusly a combo of that coming from my objective and some contribution from the EP).
However, even with the little extra out of focus way off axis, overall I felt the rendering of the view in the XW was quite a bit better than what I was seeing in the Pan. Dimmest stars were obviously more pronounced in the XW and much easier to see with direct vision. Overall the impression thru the XW was of a "clearer" view and the 19 Pan in comparison seemed slightly veiled in appearance. Whether it was better transmission or contrast or some combo of attirbutes is anyone's guess since hard to really pin down what is the driver. Regardless, better looking star field thru the XW and was very easy to notice the cleaner looking view. Off-axis I didn't feel the stars looked any more non-pinpoint in either EP. Extra eye relief of the XW was much appreciated also...could have used that last evening as it was cold and damp and my Pans were fogging after each view thru them :-/
As a side note, was interesting that the XW needed a whole bunch more infocus compared to the Pan (or put the other way, the Pan needed a lot more out focus than the XW). I always find it interesting when same FL EPs need such vastly different focus travel.
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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...
Edited by BillP (11/03/09 11:02 PM)
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starcam
sage
Reged: 09/24/07
Posts: 318
Loc: MD
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Yes, i said it wrong, the outer 25%.
That's why I don't do reviews, what I mean and what I type, make people confused.I'd be interested in your viewing of nebula's comparing the 2 eyepieces. I really like the 24 pan my 22pan is almost in the for sale bin.
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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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BillP
Post Laureate
Reged: 11/26/06
Posts: 3955
Loc: Vienna, VA
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In the XW, M42 showed itself a little brighter overall, with the dark lanes/regions being a little more pronounced, and the extent of the "wings" appearing just a little greater. I would not put the difference as being a big one, but noticable with careful viewing (except for the overall brightness which was a easy first impression when switching from the 19 Pan to the 20 XW).
Both great EPs, but the XW definitely edges the 19 Pan out IMO in many areas. Nice EP. Now if Pentax would only come out with a 24 XW 
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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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