Hey, Ausastronomer, AKA John, I have the following question regarding your great write up. Is the Televue Paracorr needed with the 5mm Pentax XW, 7mm Pentax XW, and or 10mm Pentax XW? Maybe needed is not the right wording to use. Therefore, put another way does the Paracorr improve the views in the above listed eyepieces?
Hi,
Let me answer by saying that my 2 larger newtonains (14" and 18") are both F4.5 focal ratio telescopes and I do not use the paracorr when I use the 5mm, 7mm and 10mm Pentax XW's in those scopes. Any eyepiece be it a Nagler, ETHOS, DELOS or Pentax XW, in my opinion benefits when used with a paracorr in telescopes faster than F4.5
The 14mm and 20mm Pentax XW's benefit from a paracorr when used in any newtonian telescope.
Let me explain a little bit about the optical theory on why this is. The two predominant aberrations from using a fast newtonian are field curvature and coma. This comes from the telescope itself. An eyepiece can also introduce additional field curvature and off axis astigmatism. The astigmatism gets worse as you go further off axis. Top quality widefield eyepieces like Pentax XW's and Naglers generally have minimal astigmatism. A newtonian telescope has inherent positive field curvature. The amount is solely dependent on the focal length of the primamry mirror. The shorter the focal length the more field curvature. It has NOTHING to do with the F-ratio of the telescope which is solely a function of the focal length and the aperture. I have attached a graph showing the field curvature of all the Pentax XW eyepieces. As you can see the 3.5mm, 5mm, 7mm and 10mm Pentax XW's have inherent negative field curvature. When used in a newtonian the +ve field curvature of the telescope cancels out with the -ve field curvature of the eyepiece and you get a flat field view. The 14mm to 40mm Pentax XW's have inherent +ve field curvature and when used in a newtonian both field curvatures compound and the field curvature is noticeable to many people. Some peoples eyes have a better capacity to accomodate and adjust for field curvature so it will appear more noticeable to some people than others. Enter the paracorr. The paracorr does two things. It's main goal is to correct for coma. However, in order to achieve this, it necessarily has to flatten the field of a newtonian telescope. It does this by having inherent -ve field curvature built in, which cancels out the +ve field curvature of the telescope. Consequently when an eyepiece is used in combination with a newtonian telescope and a paracorr you are only left with the inherent field curvature of the eyepiece itself, which is usually very small and hardly noticeable. The compounding effect of two field curvatures in the same direction is eliminated.
I hope this explains the benefits of the paracorr and the Pentax XW's a little.
Might I suggest that you bundle your posts into an article. I think Mike (and the rest of us) would appreciate it should we require it for reference (instead of searching the forum archives).
Thank you John,
I have been considering another EP to add to my humble set but wanted a quality piece of glass this time and was wondering how this info relates to Refractors and SCT's as these are the types of scope I have.
A Meade 8"SCT and SW ED80.
One that would cross between the two around the 10-20mm, higher power on one while lower on the other, a dual purpose EP for want of a better description.
And the other question is "Where do ya get em" Pentax that is as I'm a bit of a Pentax fan.
Thanks for any info.
Ken I have the XW 10mm which is the best eyepiece I have ever looked through by a long shot. In my 100ED f9 frac I also like the ES 82 degree series, specially the 6.7. I got the Pentax from OPT in the US. Astronomics is a bit cheaper too, give them a try.
Matt
Hi,
As you can see the 3.5mm, 5mm, 7mm and 10mm Pentax XW's have inherent negative field curvature. When used in a newtonian the +ve field curvature of the telescope cancels out with the -ve field curvature of the eyepiece and you get a flat field view.
A lot of wishful thinking in that statement.
The cancellation would only occur at one single telescope's focal length (about 500mm judging by those graphs); and only for ONE Petzval curve (saggital and meridional curves are quite a bit apart, indicating eyepiece's residual astigmatism).
In my experience, no Pentax eyepiece gives sharper images at any given distance off axis than a corresponding Nagler. Naglers are significantly wider, sharper off axis, smaller (type 5 and 6), work better at fast f-ratios and can completely take care of residual astigmatism of the observer's eye via Dioptrix, if needed (eliminating the need to wear glasses while observing). I can see people liking Pentax eyepieces, but for me they hold no interest whatsoever. They may be a smidge more neutral in tone, but that is about it.
BTW, Newtonian's field curvature is rather unimportant (unless you use 8" f/4 or shorter as your main scope). At 14" (or 18") at f/4.5 telescope's curvature is negligible (10mm off axis, the curvature would amount to about 30 microns, worst case. At that amount, defocus blur is barely larger than Airy disc, and is completely swamped by coma).
I think a lot of people prefer large eyelens because they use eyepiece as a screen (to look AT, as opposed to THROUGH). I actually find huge eyelenses annoying and rather distracting.
Now, let’s see where all the above gets us. My current telescopes are a 10”/F5 newtonian, a 14”/F4.5 newtonian and an 18”/F4.5 newtonian. The eyepieces which I currently have to use in these scopes are:-
At the risk of wandering too far off the original topic, I notice you have a pretty complete set of UO HD orthoscopics. Any comments on them??
Hi David,
I have the full set of UO HD orthos (5mm,6mm,7mm,9mm,12mm and 18mm). The Baader Genuine Orthos (BGO) are the same eyepiece with possibly slightly different coating specifications. For all practical purposes the two perform the same. The original UO volcano top orthos came in extra focal lengths (4mm, 5mm, 6mm, 7mm, 9mm, 12.5mm, 18mm and 25mm). Because of the top design the old volcano tops were slightly easier to use, but slightly inferior optically. The HD's (and BGO's) go a bit deeper and have better scatter and internal reflection control.
Like all orthoscopics, the UO HD's have a narrow FOV and short eye relief, which gets shorter as the focal length gets shorter. I have no problems using any of them with my glasses off. Optically they are very good. They offer 90% to 95% of the performance of the premium orthos like the Zeiss Abbe II's and the Pentax XO's, at a greatly reduced cost. They are a clear notch ahead of the cheaper Chinese/Taiwanese orthos and plossls. I would rate them as about the equal of Televue Plossls and maybe a touch ahead of the Takahashi LE's. They are much cooler in their colour tone than Televue Plossls. They "were" made in Japan and build quality on them was typical of all Japanese manufactured optics, very good. I believe they are no longer made following the Tsunami in Japan and any new items you might find, will be shelf stock. Over time you should be able to put together a set 2nd hand. They are certainly worth getting if you can find them and can't afford the very top end orthos, for lunar/planetary viewing etc, or can't afford premium widefields.
The cancellation would only occur at one single telescope's focal length (about 500mm judging by those graphs); and only for ONE Petzval curve (saggital and meridional curves are quite a bit apart, indicating eyepiece's residual astigmatism).
In my experience, no Pentax eyepiece gives sharper images at any given distance off axis than a corresponding Nagler. Naglers are significantly wider, sharper off axis, smaller (type 5 and 6), work better at fast f-ratios and can completely take care of residual astigmatism of the observer's eye via Dioptrix, if needed (eliminating the need to wear glasses while observing). I can see people liking Pentax eyepieces, but for me they hold no interest whatsoever. They may be a smidge more neutral in tone, but that is about it.
BTW, Newtonian's field curvature is rather unimportant (unless you use 8" f/4 or shorter as your main scope). At 14" (or 18") at f/4.5 telescope's curvature is negligible (10mm off axis, the curvature would amount to about 30 microns, worst case. At that amount, defocus blur is barely larger than Airy disc, and is completely swamped by coma).
I think a lot of people prefer large eyelens because they use eyepiece as a screen (to look AT, as opposed to THROUGH). I actually find huge eyelenses annoying and rather distracting.
Bratislav,
You can sit here and type whatever math you like, but the eyes do not lie. I have used Naglers and Pentax eyepieces for years. The off axis distortion in the Naglers I tried in my 10" f/4.7 was all too evident as compared to a 10mm, 7mm, 5mm and 3.5mm XW. The same goes for the Pentax XL eyepieces in those focal lengths, I am not talking coma here, I am talking AMD and RD. Off axis, these Pentax eyepieces show a lot less distortion than the Naglers I have tried.....which were 9mm and 7mm T1 and T6. I see ZERO distortion in any of these I just mentioned. The Naglers have a lot of rectilinear distortion and AMD off axis. The Pentax eyepieces I mentioned showed ZERO RD or AMD.
FYI, Field curvature is there in my 10" f/4.7 scope in case you were wondering, but I see NONE in the 10mm, 7mm, 5mm or 3.5mm XW's or 10.5mm, 7mm 5.2mm XL's.
Don't get me wrong, Naglers are fine eyepieces indeed in certain focal lengths, but what you said above was completely wrong.
The Pentaxes are superb, probably the benchmark and they are not overtly expensive.
My Pentex 10mm XW has pride of place in my limited ep set.
But other ep like the ES are no push over and they provide excellent value.
My favorite ES is the 8.8mm 82d and you can pick it up at VTI yourself now, if you are in Melbourne for a miserly $149!
I have the same set in Orthoscopics and they are awsome in my long focal length Zeiss refractor . very good luna , planetary and double star eyepieces ,
I have 3 Naglers ( 13mm 17mm 20mm ) and love them all .
Brian.
Quote:
Originally Posted by AstralTraveller
At the risk of wandering too far off the original topic, I notice you have a pretty complete set of UO HD orthoscopics. Any comments on them??
Would any of the 82' ES owners be able to say where these EPs are made? Akin to their Televue progenitors do they have any letters in fine print stamped on their sides such as 'Japan' or 'Taiwan ROC' by any chance?
You are right thers Profiler , my 13mm T1 and 17mm T4 are 'Taiwan ROC ' and my 20mm T2 is ' Japan ' .
I cant see any differance in the build quality and views , both are of very high quality , good QA I suppose ?.
Brian.
Quote:
Originally Posted by Profiler
Would any of the 82' ES owners be able to say where these EPs are made? Akin to their Televue progenitors do they have any letters in fine print stamped on their sides such as 'Japan' or 'Taiwan ROC' by any chance?
Would any of the 82' ES owners be able to say where these EPs are made? Akin to their Televue progenitors do they have any letters in fine print stamped on their sides such as 'Japan' or 'Taiwan ROC' by any chance?
Having used both the Pentax XWs and T5 & 6 Naglers (side by side for 10/11mm, 13/14mm and 5mm) I can only agree with John's excellent post, particularly his comments on colour fidelity and sharpness.
I would add only one other factor: the Naglers are more forgiving with eye relief IMO. While the Naglers 'allow' up to about 15-20mm, the Pentax EPs positively require it. I found 'kidney-beaning' and blackout to be major issues with the XW, unless my eye was perfectly placed. This I found pretty uncomfortable and so settled on Naglers.
I can't however fault the actual view with the Pentax XW series - beautifully crisp across the field and very neutral colour.
Since my last entry, it seems VTI has knocked the price of their 8.8m ES to $99.95!
A ES at less then a hundred dollar,if you pick up, is indeed a milestone in Australia?
Having used both the Pentax XWs and T5 & 6 Naglers (side by side for 10/11mm, 13/14mm and 5mm) I can only agree with John's excellent post, particularly his comments on colour fidelity and sharpness.
I would add only one other factor: the Naglers are more forgiving with eye relief IMO. While the Naglers 'allow' up to about 15-20mm, the Pentax EPs positively require it. I found 'kidney-beaning' and blackout to be major issues with the XW, unless my eye was perfectly placed. This I found pretty uncomfortable and so settled on Naglers.
I can't however fault the actual view with the Pentax XW series - beautifully crisp across the field and very neutral colour.
Hi Peter
From my experience I think it depends on the eyepiece.
With the Nagler type 1, I found eye placement very difficult each time I tried it out. I hear kidney beaning was particularly inherent in this eyepiece tho. Yet for the owner, it's not an issue.
The Nagler 17mm (type6 I think), I had no problem.
The Pentax 10mmXW (a workhorse) I have no problem with eye placement (If I did in the beginning, I can't remember). Yet I find the 7mm XW trickier. It doesn't get a whole lot of use so perhaps that's why and I do notice if I use it for a long session I'm fine with it.
And here's the weirdest one of the whole lot... The Denkenmeir 14mm was really bad on my first use. Someone else got theirs the same time as me and sent me a pm of how hard the eye placement was on the first use. A message from him the next day told me it wasn't a problem anymore & had now become his favorite ep. Exactly the same thing happened to me- the second night I didn't know what all the fuss was about- eye to eyepiece and off I went straight away, just like him.
Dare I say, eye placement is just a matter of getting used to I think. .
Hi Peter
From my experience I think it depends on the eyepiece.:.
Hi Suzy,
I think some of your following comments, which I have quoted, indicate that the individual observers physiology has a lot to do with things as well as the eyepiece itself
Quote:
Originally Posted by Suzy
With the Nagler type 1, I found eye placement very difficult each time I tried it out. I hear kidney beaning was particularly inherent in this eyepiece tho. Yet for the owner, it's not an issue.
Could not agree more. I have problems with blackouts on a a lot of the type 1 Naglers and the 16mm T2 Nagler. I find the 35mm Panoptic (which others have no issues with) absolutely horrendous.
Quote:
Originally Posted by Suzy
The Nagler 17mm (type6 I think), I had no problem.
One of the easiest eyepieces to use from a comfort viewpoint that I have used, outside my Pentax XW's. My 17mm Nagler T4 sees a lot of focuser time, as it hits the sweetspot in the 18" for a lot of DSO's. It does need a paracorr to give a good view across the entire FOV in a fast newtonian but works beautifully when combined with a paracorr. As does the 22mm Nagler T4.
Quote:
Originally Posted by Suzy
The Pentax 10mmXW (a workhorse) I have no problem with eye placement (If I did in the beginning, I can't remember)..
Never had a problem with this one either.
Quote:
Originally Posted by Suzy
Yet I find the 7mm XW trickier. It doesn't get a whole lot of use so perhaps that's why and I do notice if I use it for a long session I'm fine with it..
Here's where it comes down to the individual user. This is my absolute favourite high power eyepiece for lunar/planetary in both my 18" (300X) and my 14" (230X) and I have never experienced a blackout or kidney bean with it, and I have owned it for 10 years
Quote:
Originally Posted by Suzy
And here's the weirdest one of the whole lot... The Denkenmeir 14mm was really bad on my first use. Someone else got theirs the same time as me and sent me a pm of how hard the eye placement was on the first use. A message from him the next day told me it wasn't a problem anymore & had now become his favorite ep. Exactly the same thing happened to me- the second night I didn't know what all the fuss was about- eye to eyepiece and off I went straight away, just like him.
I know you spent your very hard earned dollars on the 14mm Denkmeier based on my recommendation, a year or more ago. Me recommending the 14mm Denk was pretty funny in itself on the basis that I own a 12mm UO HD ortho, 12mm Nagler T4, a 13mm ETHOS, a 14mm Pentax XW and don't own a 14mm Denk I also know that you initially had some reservations with the 14mm Denk. Am I correct in assuming that on the basis you still own and use the 14mm Denk, you are happy with it? While I don't own the 14mm Denk I rate it as optically the best eyepiece around this focal length outside the 12.5mm Docter and the 12mm Nikon NAV HW which are a serious amount of coin, and a major waste of $$$$ on my old burned out retinas.
Quote:
Originally Posted by Suzy
Dare I say, eye placement is just a matter of getting used to I think. .
Of equal importance to eye placement is the ability to keep your head/eye still while you observe. If you move your head around like "Noddy the Clown" you will have a lot of difficulty with a lot of eyepieces. The ability to "find" the exit pupil and then hold your head still is of critical importance.
I have the same set in Orthoscopics and they are awsome in my long focal length Zeiss refractor . very good luna , planetary and double star eyepieces , 
You are right thers Profiler , my 13mm T1 and 17mm T4 are 'Taiwan ROC ' and my 20mm T2 is ' Japan ' .
.
I also know that you initially had some reservations with the 14mm Denk. Am I correct in assuming that on the basis you still own and use the 14mm Denk, you are happy with it? While I don't own the 14mm Denk I rate it as optically the best eyepiece around this focal length outside the 12.5mm Docter and the 12mm Nikon NAV HW which are a serious amount of coin, and a major waste of $$$$ on my old burned out retinas.
