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A 6mm Delos Compared to A 5mm Pentax XO

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#1 GeneT

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Posted 18 January 2013 - 10:55 PM

For more than a year I have wanted a 5mm Pentax XO eyepiece. This was because several people whose opinion I trust, and respect, said it well may be one of the top one or two finest eyepieces ever made. I made myself a nuisance to a few of you in private e-mails asking for recommendations and opinions. I appreciate your advice. It kept coming back the same--the 5mm Pentax XO is among the best of the best. In fact, it might be the best eyepiece in this range ever made.

My telescope is a 12.5 inch, F5 Portaball, with excellent optics. I am aware of eyepiece performance differences between slower and faster telescopes.

I wanted this eyepiece for a very narrow use. I wanted to squeeze a little more detail on the planets. During this time I had been acquiring the excellent Delos line of eyepieces. But, I also wanted the best of the best. I wanted a 5XO.

A vendor told me that one would be arriving in a week or so. Two months later, no eyepiece. Pentax had quit making them. We don't know if the reason was the Japanese tsunami, or some other reason. However, now I would have to try and find a used one.

A Cloudy Nights member from Canada heard that I wanted a Pentax 5XO. He offered to sell me his. I received it a week ago, but only tonight got to try it out. I did not have time to drive to my 'sorta' dark site about 22 miles from where I live. But, since I wanted this eyepiece for planetary use, I decided my back yard would work fine on Jupiter. I got the telescope set up, collimated, and had the mirror out in the yard for two hours.

I decided to compare the 5mm Pentax XO with my Delos 6mm. They don't make a Delos 5mm.

The Pentax 5mm gives me 305X, 24X per inch, with an exit pupil of 1.
The Delos 6mm gives me 254X, 20X per inch, with an exit pupil of 1.3.

I made several back and forth comparisons. The views were excellent in both eyepieces. I kept Jupiter centered for most of the comparisons. However, I also let the planet drift to the edges. I did not use my 2X Power Mate. I viewed without glasses because the eye relief of the 5XO is very small. It wouldn't make sense to view with glasses with the Delos, and naked eye with the 5XO.

Being candid, the on axis views were almost equal in terms of how much detail I could see on Jupiter's surface. Seeing was pretty good. As the views would sharpen to the max, it appeared that the 5XO was ever so slightly sharper. I believe I have a pretty good eye for such things, but it was a photo finish, and almost too close to differentiate.

Sirus was up, so I put both eyepieces on this gem. Again, both eyepieces performed superbly. The 5XO is going to make a nice double star eyepiece.

Some closing thoughts.

--My quest for the perfect or best eyepiece is finally over. :grin: I own a 5XO.

--The Delos is probably a better planetary eyepiece than is generally realized; the Delos series is just getting out and about in the astronomy community. I am sure that as more and more people use this fine eyepiece that it will gain a reputation as a 'must have' eyepiece. The Delos can hang in there with the best of the best. Maybe, a Delos is the best of the best. In a comparison of this kind, we need to remember that the Delos has 20mm of eye relief and 72 degrees of AFOV. This is one impressive eyepiece.

--Would I recommend spending a fair amount of money for a used 5XO if you had an excellent, almost equivalent eyepiece such as a Delos? Answering for myself, yes. Sometimes in life you just have to try things, and spend some money. My kids are raised, educated, and out of the house.

Now its my turn. I feel no guilt. :lol:

#2 Jeff Morgan

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Posted 19 January 2013 - 12:35 AM

Nice report Gene. So many eyepieces, so little time.

A couple of years ago I had a chance to look through the 5XO on Ron Bauer's 5" APO, a relatively fast scope (make escapes me at the moment). I looked at Alberio and Alpha Herculis. While the eye relief was short, it was a stunner on double stars.

I find the Delos to be excellent too, almost certainly the best thing Tele Vue is currently offering. And a reasonable price too. Not as good as "specialist" eyepieces like Brandons or the 5XO, but a solid 90%.

Nice to have both types in the case.

#3 John K

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Posted 19 January 2013 - 01:03 AM

I am happy with my 6mm Delos for my planetary work.I don't feel any short comings with it.It is better than my previous Pentax 5XW.IMHO but that may have been due to FL and seeing.:-)

#4 bdcmd

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Posted 19 January 2013 - 02:05 AM


Some closing thoughts.

--My quest for the perfect or best eyepiece is finally over. :grin: I own a 5XO.
Now its my turn. I feel no guilt. :lol:


:roflmao: :roflmao: :roflmao: Enjoyed your post, as usual, Gene. I also own the Pentax XO 5 and 2.5, but not the Delos. I have stayed with everybodies favorite eyepiece to diss: the Radian. I own the 3,4,5,6,12,18 and am looking for the 8. In comparing the XO 5 to my Radian 5 and 6 tonight on Jupiter and the trapezium, and Sigma Orionis, using my Vixen 100edsf ED doublet, I came to a similar conclusion: the XO is slightly sharper on axis and no worse off axis. The XO has miserably short eye relief, which limits how long I could use it to try to break through the seeing on the Trap; never did see E and F stars tonight. The Radians allowed a much more comfortable, long duration study, due to the better eye relief, which produced averted vision star E in the Trap, as well as the C star in Sigma Orionis with direct vision; the XO could show the C star with averted vision most of the time, but trying to hold the view long enough to catch the seeing improve enough for direct vision confirmation of the C star in the XO was fatiguing on the eye. Ultimately, the comfort of the long eye relief Radian allowed it to max out the opportunities presented in the nights seeing variability. The XO is still the absolute on axis resolution champ, but the Radian is just so much more comfortable to use. I can only try to imagine the improvement that the Delos might bring to the table. But, for now, congratulations on your new, top of the class planetary eyepiece. Thanks so much for your posts. And please, continue the A::B comparisons; they are very helpful, as you have noted. :grin: bdcmd

#5 Greg77

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Posted 19 January 2013 - 02:33 AM


Nice report. Back in 2012 I've compared Pentax XO 5.1 with Pentax XL 5.2mm in my 5" scope. The eyepiece really lived up to its reputation for optical excellence. In the end I sold both and bought brand new XW 5mm. Ultimately, the comfort of the long eye relief XW is just what I need to study the planets...

#6 andydj5xp

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Posted 19 January 2013 - 03:25 AM

--Would I recommend spending a fair amount of money for a used 5XO if you had an excellent, almost equivalent eyepiece such as a Delos? Answering for myself, yes. Sometimes in life you just have to try things, and spend some money. My kids are raised, educated, and out of the house.



You are well advised to keep the XO. Having at your disposal the very best of the best is quite a comforting situation.

I'm sure that in the long run the Delos will see much more focuser time due to its ease of use (AFOV, eye relief) without sacrificing quality. And yet: whenever you want you can insert the Pentax .... and happily switch back to the Delos for its generally outstanding performance.

I've had the same experience comparing the ZAOIIs with the Leica ASPH zoom. It's a very comfortable situation.

Andreas

#7 kkokkolis

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Posted 19 January 2013 - 09:11 AM

A friend of mine (we observed some times together under dark skies) has the 5mm XO and he is enthusiastic about it. I never asked him to show me because a find even a 6mm Ortho difficult to use, that's why I got the 6mm Delos that impressed me. I used it with my 6SE that barely stands eyepieces bellow 10-8mm (for my eyes and floaters) and I had a beautiful Jupiter last week. I might add a 5mm planetary and I'm thinking about a Radian perhaps.
Since amateurism means enjoyment one should get what he enjoys without stressful effort. There's no need for remorse since any form of joy that doesn't harm others is not considered a sin any more in our western countries (in contrast with medieval or victorian times). On the other hand one shouldn't feel uncomfortable if he can extract enjoyment from cheap pieces of equipment, resisting the consumeristic frenzy the money flow tries to enforce. Cost and value aren't the same thing.

#8 Scanning4Comets

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Posted 19 January 2013 - 12:14 PM

Good report, but in able to really tell the difference in an EP back and forth test, you need to keep both FL eyepieces the same.

254x and 305x are a lot different from each other.

Cheers,

#9 russell23

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Posted 19 January 2013 - 02:35 PM

Good report, but in able to really tell the difference in an EP back and forth test, you need to keep both FL eyepieces the same.

254x and 305x are a lot different from each other.

Cheers,


Oh I don't know Mark. I think this comparing eyepieces at exactly the same FL criteria is a bit overblown. We compare eyepieces that have a wide range of variation in optical characteristics. Just look at BillP's 24-26mm comparison which had the following % differences between the largest and smallest value in the review:

Magnification: 8%
AFOV: 66%
Usable eye relief: 285%
Eye lense diameter: 81%

The eye relief, eye lense diameter, and AFOV may all play a role in the usability of the eyepiece to an observer which could ultimately impact the ability of the observer to relax and observe and therefore the ability to see details with the eyepiece.

An eyepiece is a package of characteristics and focal length is just one of those characteristics. I don't think a 20% magnification difference is all that significant. We don't see a large demand on the market for 1.20x barlows. :)

Dave

#10 Scanning4Comets

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Posted 19 January 2013 - 03:37 PM

Overblown? Hardly.

@ 254x, Jupiter may show sharp detail, and @ 305x you may NOT see the same sharp detail, or you may. It all depends on the sky and conditions. There is quite a difference at these two magnifications on any given night. Now, put those two same mags on Saturn, and both will work a lot better as Saturn can take more mag than Jupiter on most nights.

I used to think the same thing, but a REAL COMPARO can only be done on two diff eyepieces at the same magnification, or VERY CLOSE to it.

Cheers,

#11 BillP

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Posted 19 January 2013 - 04:26 PM

An eyepiece is a package of characteristics and focal length is just one of those characteristics. I don't think a 20% magnification difference is all that significant. We don't see a large demand on the market for 1.20x barlows. :)

Dave


Dave, very true that folks don't generally demand 1.2x Barlows! IMO, the magnification difference can be important depending on the circumstances.

In the case of the OPs scope, a 24mm vs a 26mm eyepiece will generate little difference in magnification (61x vs 66x). The brightness though will be 17% brighter in the 26mm due to the larger exit pupil. 17%, while sounding like a lot, visually is noticeable but not significant..so not a WoW 17% brighter. However, it will well explain why fainter or adverted vision stars might seem more easily seen. Switch to an 6mm vs 5mm in same scope and now the brightness difference is to have a 44% brighter image in the 6mm and 50x less magnification (significant when potentially working at near the top magnification an atmosphere may be allowing). So yes, while the magnification difference may be just 20%, the brightness difference is a whopping 44%. And with that extra brightness the image will appear more contrasty visually as well in the 6mm. At higher magnifications, these differences can be more significant because of dwindling brightness and contrast, whereas at lower magnifications a similar difference might not show quite so dramatically.

However, like you, I do not think it is a problem when evaluating eyepieces of different focal lengths...even 20% different because the report will still tell a story. Plus, any evaluation using a scope under a particular location's skys is really a system-test and not just an eyepiece test. So one needs to work in the aspects of the system to come away with how significant any noted differences might be.

Working at the high magnifications of the OP where exit pupils are approaching 1mm, to take away a result that the eyepiece pushing 50x more magnification and showing an image that is 44% less bright, to visually appear slightly sharper is actually saying quite a bit about that 5mm eyepiece! Phenomenal to pull off that kind of subjective result when the math is telling us what is happening between the two different exit pupils being generated. Just another report IMO that characterizes how that little 5XO was quite ahead of its time in 2003 and how it is still ahead of the current time. A fantastic eyepiece. It takes nothing away from the Delos which is phenomenal in other ways. But it does highlight just how much a very finely executed minimum glass eyepiece can be. Rare to have things like this produced, and lucky when one can get hold of one. Not an every day eyepiece IMO...but certainly the best of the best when that level of performance can be capitalized upon in a telescope.

I agree that visually, looking at a target overall, a 20% jump in magnification is well, not worth it because it is just not that impressive. But when a scope is operating near a limit for the atmosphere on a target, then the 20% jump can become something significant. But to have it still perform on-par or slightly sharper while operating at this end of the spectrum is pointing out just how special that 5mm XO is!

#12 GeneT

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Posted 19 January 2013 - 05:03 PM

Good report, but in able to really tell the difference in an EP back and forth test, you need to keep both FL eyepieces the same. 254x and 305x are a lot different from each other. Cheers,


They are not the same, but neither are eyepieces of the same focal length that differ significantly in AFOV and eye relief. Comparing a 5 to a 5 would have been better. However, I am comfortable with what I saw and what I reported.

#13 Scanning4Comets

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Posted 19 January 2013 - 06:48 PM

Guess I will just throw my arms up in the air. One minute people on here say that comparing eyepieces of the same FL is a must, and the next minute they say it doesn't matter.

Oh well....I give up.



#14 Rick Woods

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Posted 19 January 2013 - 07:05 PM

Good report, but in able to really tell the difference in an EP back and forth test, you need to keep both FL eyepieces the same. 254x and 305x are a lot different from each other. Cheers,


They are not the same, but neither are eyepieces of the same focal length that differ significantly in AFOV and eye relief. Comparing a 5 to a 5 would have been better. However, I am comfortable with what I saw and what I reported.


Hey - you compare what you have!
Congrats on a nice acquisition.

#15 herrointment

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Posted 19 January 2013 - 07:15 PM

I'll take the Ely Kid at his word.

#16 BillP

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Posted 19 January 2013 - 07:20 PM

Guess I will just throw my arms up in the air. One minute people on here say that comparing eyepieces of the same FL is a must, and the next minute they say it doesn't matter.

Oh well....I give up.


:funny:

"Important" if the goal is to try to just ferret out eyepiece-specific behaviors (long with using a flat-field scope with known aberrations). Not all that critical if you are either assessing "system" behavior or are making adjustments based on the know "system" biases. Why I always say ALL reviews and reports are very valuable.

#17 GeneT

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Posted 19 January 2013 - 10:39 PM

Good report, but in able to really tell the difference in an EP back and forth test, you need to keep both FL eyepieces the same.


I agree with the above. However, let me come at this again. What if someone wants an eyepiece in the range of 5 to 6mm focal length?
--The 100 degree AFOV Ethos comes in 4.7, 3.7, and 6;
--The 100 degree AFOV Explore Scientific comes in 5.5;
--The 82 degree AFOV Naglers come in 5mm;
--The 82 degree AFOV Explore Scientific comes in 4.7 and 6.7;
--The 72 degree AFOV Delos comes in 3.5, 4.5 and 6;
--The 72 degree AFOV Pentax comes in 3.5 and 5.
When you line all these up, you will have a difficult time lining up all these eyepiece lines in 5's and 6's. It would be nice if they did line up in nice exact columns for each; but, they don't. In fact, the variances in focal lengths are all over the place.

Now, what if you want to compare the above product lines with two of the finest eyepieces ever made--the 5mm Pentax XO and the 6mm ZAO? Again, the columns don't line up nicely into 5's and 6's. What if you are luck and find a good used 5mm Pentax XO and a good used 6mm ZAO? What if either would fit in an acceptable range for your telescope in a high power eyepiece? Now, you test both. You look for subtle differences in planetary detail and stellar points. Remember, one is a 5 and one is a 6mm focal length eyepiece. You can't test a 6 XO against the 6 ZAO and you can't test a 5 ZAO against a 5 XO. All you can do is test a 5XO against a 6 ZAO.

Please note that you now are back exactly in the same place where I originally posted. I compared a 5XO against a 6 Delos.

Of course your points about differences in magnification as it relates to contrast and so on are correct. However, the fact is we consumers often have to make these inexact comparisons between eye piece types. I am sure you have a trained eye. I believe I do also. Over the years I have learned to trust what I see. I agree it would have been nice to have lined up two 5's and two 6's and made a direct comparision. However, it can't be done. I believe what my eye showed me and I am comfortable in drawing the conclusions I did in my original post.

The bottom line here is that both the 5mm Pentax XO and the 6mm Delos are superb eyepieces. The CN'rs who turned me on to the 5XO did me a great favor. I now own one of the best one or two eyepieces ever made in this focal length range.

On a related note, I was seriously thinking of purchasing a 13mm Ethos. I have a 12mm Nagler. I don't know if TeleVue used to make a 13mm Nagler, but if they did, they no longer do. Therefore, I had to compare my 12mm Nagler against a 13mm Ethos that a friend owned. Again, the focal lengths did not line up. I had to draw conclusions with these eyepieces. I decided that the main issue for me was the 17 degrees of eye relief of the Nagler vs. the 15 of the Ethos. I could get all of the field of view in the Nagler while wearing glasses where I could not with the Ethos. I liked the 'whiter' tones of the Ethos better than the warmer tones of the Nagler. However, I also did not feel that the 100 AFOV suited my viewing needs compared to the 82 of the Nagler. Therefore, I stuck with the Nagler and did not upgrade to the Ethos.

I would be interested in someone comparing a 6 ZAO with a 6 Delos. Has anyone compared a 6 ZAO with a 5 XO? (With all the caveats that we have to remember that they are of differing focal lengths.)

#18 russell23

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Posted 19 January 2013 - 11:17 PM

Overblown? Hardly.

@ 254x, Jupiter may show sharp detail, and @ 305x you may NOT see the same sharp detail, or you may. It all depends on the sky and conditions. There is quite a difference at these two magnifications on any given night.


But sky conditions are a different issue. You don't draw conclusions about any eyepiece that seeing conditions won't allow you to test. You wait until you have seeing conditions that allow both eyepieces being compared to be fairly tested. The day my 2x Explore scientific barlow arrived the seeing conditions were so horrid I couldn't even get a sharp focus at 53x ... and I'm still waiting to test that barlow because I haven't had clear skies since.

You can absolutely compare eyepieces of different focal length. The limitations should be pointed out where the focal length difference might be relevant. For example, the 5mm XO would be expected to give a blacker sky background than the 6mm Delos - just based upon exit pupil. If you can see more finely resolved details in the 5mm XO as compared to the 6mm Delos you can note that the increased detectability of details can possibly be attributed to the higher magnification provided by the 5mm eyepiece. But that doesn't change the fact that you saw more details in the 5mm eyepiece than the 6mm eyepiece.

I used to think the same thing, but a REAL COMPARO can only be done on two diff eyepieces at the same magnification, or VERY CLOSE to it.

Cheers,


I'm not sure why identical focal lengths are necessary for a report to be a "real comparo"? Why not identical AFOV too then? Why not identical eye relief? Why not identical coatings?

"or Very close to it"? What is very close? I think 5mm vs. 6mm is very close. If you put an eyepiece into a telescope and told an observer that it is either a 5mm giving 305x or a 6mm giving 254x, do you really think that people would correctly identify the magnification more than ~50-60% of the time? I've had times in which I've been switching back and forth with different eyepieces w/barlows and all of the sudden realized that I'm looking at something at 160x instead of the 120x I thought I had in (~30% magnification difference).

Will you be able to detect differences resulting directly from using a 5mm vs. a 6mm eyepiece? Absolutely. But that doesn't mean those differences are so extreme that comparing a 5mm to a 6mm does not constitute a "real comparo".

Dave

#19 russell23

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Posted 19 January 2013 - 11:29 PM

An eyepiece is a package of characteristics and focal length is just one of those characteristics. I don't think a 20% magnification difference is all that significant. We don't see a large demand on the market for 1.20x barlows. :)

Dave


Dave, very true that folks don't generally demand 1.2x Barlows! IMO, the magnification difference can be important depending on the circumstances.

In the case of the OPs scope, a 24mm vs a 26mm eyepiece will generate little difference in magnification (61x vs 66x). The brightness though will be 17% brighter in the 26mm due to the larger exit pupil. 17%, while sounding like a lot, visually is noticeable but not significant..so not a WoW 17% brighter. However, it will well explain why fainter or adverted vision stars might seem more easily seen. Switch to an 6mm vs 5mm in same scope and now the brightness difference is to have a 44% brighter image in the 6mm and 50x less magnification (significant when potentially working at near the top magnification an atmosphere may be allowing). So yes, while the magnification difference may be just 20%, the brightness difference is a whopping 44%. And with that extra brightness the image will appear more contrasty visually as well in the 6mm. At higher magnifications, these differences can be more significant because of dwindling brightness and contrast, whereas at lower magnifications a similar difference might not show quite so dramatically.

However, like you, I do not think it is a problem when evaluating eyepieces of different focal lengths...even 20% different because the report will still tell a story. Plus, any evaluation using a scope under a particular location's skys is really a system-test and not just an eyepiece test. So one needs to work in the aspects of the system to come away with how significant any noted differences might be.

Working at the high magnifications of the OP where exit pupils are approaching 1mm, to take away a result that the eyepiece pushing 50x more magnification and showing an image that is 44% less bright, to visually appear slightly sharper is actually saying quite a bit about that 5mm eyepiece! Phenomenal to pull off that kind of subjective result when the math is telling us what is happening between the two different exit pupils being generated. Just another report IMO that characterizes how that little 5XO was quite ahead of its time in 2003 and how it is still ahead of the current time. A fantastic eyepiece. It takes nothing away from the Delos which is phenomenal in other ways. But it does highlight just how much a very finely executed minimum glass eyepiece can be. Rare to have things like this produced, and lucky when one can get hold of one. Not an every day eyepiece IMO...but certainly the best of the best when that level of performance can be capitalized upon in a telescope.

I agree that visually, looking at a target overall, a 20% jump in magnification is well, not worth it because it is just not that impressive. But when a scope is operating near a limit for the atmosphere on a target, then the 20% jump can become something significant. But to have it still perform on-par or slightly sharper while operating at this end of the spectrum is pointing out just how special that 5mm XO is!


I agree with everything you said Bill. I've always felt that smaller magnification jumps make more sense at higher magnifications than at lower magnifications. Like I said to Mark - the seeing conditions have to be removed as a factor before you should even be making a comparison between the 5mm and 6mm eyepiece. I think GeneT did that.

It is a good point you make that the brightness will be a larger factor than the magnification. So the focal ratio of the scope will be a factor in play as well.

I just hate to see us become so restrictive in what constitutes a valid comparison that people become afraid to report on comparisons they've made that a lot of people might find useful - even given limitations potentially introduced by slight differences in eyepiece focal length. For the example in this thread it is quite clear that for a person that likes planetary observations, but also would prefer more viewing comfort - the 6mm Delos seems to do quite well up against the 5mm XO and for some people that might sway them to go with the 6mm Delos. Others might be impressed by the fact that despite the disadvantages the XO faces, it still seemed to slightly outperform the 6mm Delos - and so those people might opt for the 5mm XO. If we're discouraged from making such comparisons then we don't get the benefit of such information to guide our decisions.

Dave

#20 russell23

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Posted 19 January 2013 - 11:38 PM

Guess I will just throw my arms up in the air. One minute people on here say that comparing eyepieces of the same FL is a must, and the next minute they say it doesn't matter.

Oh well....I give up.


I'm sure it won't take too long for someone else to come along and support you that the "FL must be the same" approach is the most valid approach. I disagree for reasons I've given, but that doesn't mean that you are wrong to prefer that requirement. It is just another example of the YMMV that underlies much of what gets discussed here on CN.

For my part, if I see "conventional wisdom" that I think has gotten to the point that nobody even thinks to question it anymore, then I question it if I think the conventional wisdom has become detrimentally restrictive. I happen to think many of these "requirements" that discourage people from wanting to share their reviews and observations are detrimental.

:)

Dave

#21 Mike B

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Posted 20 January 2013 - 12:03 AM

I happen to think many of these "requirements" that discourage people from wanting to share their reviews and observations are detrimental.



Agreed. And i suspect that's the frustration Mark is alluding to... i think we've ALL seen someone's comparo get critiqued rather bluntly because of some perceived failure to match conventional wisdom, when in fact there was much good to be derived... possibly even including the questioning of said "wisdom". :lol:

On EP focal lengths & comparo's- i've got a goodie for ya:
I once stuck a 9.4mm Speers-Waler in my BVer, with a Pentax 10.5mm XL in the adjacent holder; the views actually MERGED! Quite nicely, too. Had me stumped, but good!

So, was the 10.5mm not really, exactly 10.5mm?... or was the 9.4mm not really, precisely 9.4mm? :scratchhead:

What i went on to perceive, tho, was that star groupings near the FoV centers merged well, whereas star groupings in the periphery were slightly off. My eye was accommodating & forcing the "merge" when the angular distance diff was small enough... but when the FoV stretched out far enough, the angular distance diff between, due to the slight magnification diff, exceeded my eyes' accommodation abilities.

Yet my eye's own ability to override a ~10% magnification difference remains. For reference, the mags involved that night would've been ~169x and ~189x, with a 10" SCT. So, i'm wondering what ELSE my eyes might be capable of "overriding" without my being aware?
:foreheadslap:

#22 GeneT

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Posted 20 January 2013 - 04:27 PM

I once stuck a 9.4mm Speers-Waler in my BVer, with a Pentax 10.5mm XL in the adjacent holder; the views actually MERGED! Quite nicely, too. Had me stumped, but good!


Gueeze. I would never have thought of doing that. Very interesting!

#23 GeneT

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Posted 20 January 2013 - 04:42 PM

I'm sure it won't take too long for someone else to come along and support you that the "FL must be the same" approach is the most valid approach.


There are times when the choice of eyepieces for comparision do not match up with exact focal lengths. For example, what if someone was trying to decide between some Pentax XW's or Delos eyepieces? Both make a 10mm for a direct comparison. However Delos than goes to an 8 and a 6. Pentax XW goes to a 7 from the 10. Delos makes a 4.5 and Pentax XW a 5--pretty close. However, how does one compare the Pentax 7? Do you compare it to the Delos 6, or 8? In any case, there is no way to compare these similar eyepieces directly against the criteria of matching exact focal lengths. And in this and other cases, I believe there still can be dome valid comparisons made when trying to determine which eyepiece(s) to buy even when one of the criteria does not exactly match.

#24 mountain monk

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Posted 20 January 2013 - 05:56 PM

Me too. Thanks for your review.

Dark skies.

Jack

#25 ausastronomer

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Posted 20 January 2013 - 06:06 PM

I am happy with my 6mm Delos for my planetary work.I don't feel any short comings with it.It is better than my previous Pentax 5XW.IMHO but that may have been due to FL and seeing.:-)


I have no doubt seeing and thermal equilibrium factors are what contributed to the perceived difference in performance between the 5mm Pentax XW and the 6mm Delos. If 6mm is a better more frequently useable focal length for your situation then you cannot go past the 6mm Delos, it is excellent. At the shorter focal lengths and smaller exit pupils a 1mm difference in focal length can make a huge difference. At the longer focal lengths it doesn't matter much, if at all.

I have a 5mm Pentax XW, a 6mm Delos and a 7mm Pentax XW and I am yet to see any performance difference in any criteria where I feel the 6mm Delos has an edge over either of the other two eyepieces. In fact the XW's outpoint the DELOS in some performance criteria, which are not critical to me. What happens quite frequently is that the 6mm DELOS will hold up under the conditions in my 10"/F5 scope and the 5mm Pentax XW will not. That's the conditions, not the eyepiece. When conditions are good enough for both eyepieces to deliver their best, there is nothing between them.

I have also used the 5mm Pentax XO on many occasions and confirm that it marginally outpoints the 5mm Pentax XW, under the very best observing conditions, in a premium telescope. However, I came to the conclusion that on the basis the performance gain was very marginal and the Pentax XW was infinitely more comfortable to use for long observing sessions, there was no place in my eyepiece case for a 5mm Pentax XO. In fact after a 2 hour lunar / planetary observing I have no doubt I am seeing more in the 5mm Pentax XW, or 6mm Delos, than I would be seeing in a 5mm Pentax XO due to eye fatigue and strain.

Cheers






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