For those interested about eyepiece or optics.. or for those Pentax owners... or for those just interested. Hey.. I don't own a Pentax but maybe next time I will get one.. hmm anybody wants to sponsor me one?
Ok, I got these graphs for Pentax japan website. I did some analysis of the performance of these eyepieces based on the graphs they provided. I do not guarantee that what I anaylsed to be 100 percent correct, but it should have some degree of accuracy. Reference is Telescope Optics book.
1) Pentax 5XO, 3.5XO and O-5
http://img401.imageshack.us/img401/8960/p1cl4.jpg
Comparison between Pentax XO5, XO2.5 and O-5 eyepieces.
This graph is the designed specs of the eyepieces. I would just give my analysis of them.
All eyepieces suffer from astigmatism, field curvature and distortion. On-axis (centre) of the
Eyepiece is designed to be the best. That means at the centre it suffers no astigmatism,
field curvature and distortion but has perhaps some spherical aberration.
Astigmatism= occurs when rays in one plane (Tangential/ Meridonial) [DM] and another plane Sagittal [DS],
these two planes being at 90 degree with respect to each other, when focused, focuses at different
points along the optical axis. Therefore, at best focus (that means trying to get the best point) between
them results in an elliptical shape instead of a circle shape. Causes the image to be less sharp.
Field curvature = is a phenomenon that the centre if focused while the edges of the field is not in focused.
This results because the eyepiece and telescopes do not focus as a vertical plane but the focus plane is
curved. The more the field curvature, the less sharp at that point of focus.
Distortion = change of shape of an object. Distorted object.
From these graph, the field curvature, astigmatism and distortion information can be extracted.
The yellow dotted line indicates the field curve of the Sagittal [DS] plane while the bold yellow line indicates
the field curvature of the Meridonial [DM] plane. The y-axis is the AFOV with centre at the origin, and the edge of
AFOV being the highest point on the vertical axis. The x-axis is the distance along the optical axis. As you see
The yellow lines are curved because the eyepiece focus plane is curved. The graph does not show the true field curvature.
The true field curvature would be at best focus, meaning in between the DM and DS curve. I have drawn a bold RED line
to indicate the estimated true field curvature. The Nearer the RED line is to the vertical axis, the better as it means less
field curvature.
The sharpness off-axis (away from the centre) can be analysed by the DS and DM curves. The bigger the separation of
these 2 curves, the greater the astigmatism. Remember that the y-axis is the off-axis performance. An ideal (impossible)
case is that the DM and DS are vertical lines along the y-axis. This means there is zero astigmatism and zero field curvature.
Design dilemma:
As you can see, some eyepieces are very sharp but suffer from more field curvature. Some are less sharp (more astigmatism) but
suffer less field curvature. DESIGN IS A BALANCE OF ALL THESE ABERRATIONS! There is no eyepiece that corrects both
Astigmatism, distortion and field curvature at the same time. There must be some balance.
Analysis:
Just basing on the graphs supplied by Pentax. It can be measured (roughly though) that the XO5 has the least astigmatism
but highest field curvature. If the eye can accommodate its field curvature, it would give sharpest images from centre to edge than
the X02.5 and O-5. The Pentax XO2.5 has about the same field curvature as the Pentax O-5; they have flatter fields than XO5.
The XO2.5 is better corrected in astigmatism than the O-5.The O-5 has the largest astigmatism meaning off-axis is the worst
performance. Hence, I would say the XO2.5 has the most balanced correction among the 3. The distortion is 5 percent max
for all of them. For all of them the ‘sweet spot’ in the centre is about 20 percent of the AFOV or about 9 degree AFOV.
2) Pentax XW field curvature
http://img410.imageshack.us/img410/6154/p2nc2.jpg
Field curvature note
Note that the field curvature has different signs (concave or convex). In the case of XW14 to XW20, their signs are opposite
to that of telescopes.
For XW3.5 to XW10 have similar direction FC to telescopes. Telescopes are usually –ve FC. If the FC of the eyepiece
matches that of the telescope, there will not be FC. It is difficult for exact match of FC or for other aberrations and hence
the designer will try to reduce the FC of the eyepiece and other aberrations. In general, the lower the astigmatism, FC, and
distortion the better. Note that when the telescope and eyepiece mix, there will be different results. A eyepiece with FC can
be flat if the FC matches the telescope. The image can be sharper if the eyepiece aberration is opposite that of the telescope
and hence matches.
Analysis
The XW20 and XW14 suffers the worst field curvature because not only it has large FC, its sign adds
to the telescope’s FC. That is the reason why people complain that the FC is bad in these 2 eyepieces.
However, note that these 2 eyepieces has almost no astigmatism and if used with a long focal length Reflector
or flat field refractor (like TV’s or Pentaxes ones), it will give very sharp to the edge (if the eye can still accommodate).
XW5 has also almost no astigmatism and would give less FC problem because of the its FC’s sign. It has the potential
to be the sharpest XW among all. If people find the XW3.5, XW7 and XW10 field flat, the XW5 would be just as flat but sharper
than the them across the field.
The least corrected for astigmatism are XW10, XW7 and XW3.5. The XW30 and XW40 is the most well balanced giving
the least FC and very good correction for astigmatism. Note that these correction is already many times better than Erfle eyepieces.
Hence for best correction:
For XW3.5 to XW10 => XW5 is the best
For XW14 to XW40=> either XW30 and XW40 as they are close
For best centre correction (largest sweet spot) = XW10
Pentax eyepiece analysis!
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Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
Pentax eyepiece analysis!
AstroDuck
Hi Canopus (and it is up tonight),
The analysis is quite interesting. I would like to comment on the XW line in particular, having owned a couple of these now and before.
Just to verify: For FC in the XW line, shouldn't it be positive from 14 to 40, no?
The conclusion for the longer XW FLs is quite consistent. It seems that the 30 is a favourite compared to the 31Nag, if AFOV is not an important factor for the observer, especially when contrast comes in. XW30 excels in this respect. Comparing to the Panoptic 40, the XW40 is a thumbs-up performer. These longer XW FLs have less pincushion distortion and is sharp (consistent with your findings), and with excellent SMC coatings. If your own astigmatism is affecting you, the XW30 will be a better bet than the XW40.
Likewise, the XW20 has the FC problem which could be an issue for some, and the XW14 shows less of this effect. Some have compared the XW20 with the Pan22, and preferred the former for reasons of greater depth, and higher contrast. But nevertheless, with an EFL in the range of 20 to 22, it is better to have an eyepiece that has a flat field. So most would have opted for XW30 if $$$ not an issue. If you are using the XW20 in a fast dob, it could be a problem when it comes to FC.
Note that Pentax had originally designed the XW to work well with refractors and most have claimed that the XWs work well the F6 (or longer) environment particularly.
However, even with considerable FC in XW14 (though lesser than XW20), one may be less inclined to look at the edges of the field of an XW14 at the same time as the center. Putting this aside, sharpness and contrast is excellent in the XW14 but for those who have compared with the Nag13T6, the latter seems to yield a sharper on axis performance. However, the XW14 has less lateral colour. Not forgetting the better flat field performance (which is expected) as well. When it comes to contrast however, the XW14 is a better candidate and with less light scatter.
One point to note though, that the XW14 does not exhibit any trait of the "kidney bean" effect that is prevalent in some Nags like the Nag12T4. So even with the wider AFOV, moving one's eye around to look at the edges of the field, you will see this black out. It does not appear to be so for the XW14. This is further made more pronounced if the eye relief is short which is so for the Nags in comparison with the XW line.
Next, many may cringe when they notice the great amount of elements (ranging from 6 - 8) used in XWs, but Pentax have claimed 98% light transmission for these eyepieces. Having seen faint galaxies with them before, I would be convinced. Not forgetting the renowned contrast performance for this line in general.
And for those who are bothered or particular about eye relief, then one may go for the XW rather than competing models like the Nags.
Lastly, I would mirror my thoughts with Canopus on the shorter XW FLs (3.5 - 10) but in practice I would think XW10 and XW5 is close.
Still, in all, all eyepieces mentioned above are already very good, and it is like comparing priced high end cars. The descerning observer will have to find his/her "nooch" or comfort attributes for the respective optical system.
Meanwhile, I can't wait to test the XW30 on the TOA......soon! ;P
My thoughts!
The analysis is quite interesting. I would like to comment on the XW line in particular, having owned a couple of these now and before.
Just to verify: For FC in the XW line, shouldn't it be positive from 14 to 40, no?
The conclusion for the longer XW FLs is quite consistent. It seems that the 30 is a favourite compared to the 31Nag, if AFOV is not an important factor for the observer, especially when contrast comes in. XW30 excels in this respect. Comparing to the Panoptic 40, the XW40 is a thumbs-up performer. These longer XW FLs have less pincushion distortion and is sharp (consistent with your findings), and with excellent SMC coatings. If your own astigmatism is affecting you, the XW30 will be a better bet than the XW40.
Likewise, the XW20 has the FC problem which could be an issue for some, and the XW14 shows less of this effect. Some have compared the XW20 with the Pan22, and preferred the former for reasons of greater depth, and higher contrast. But nevertheless, with an EFL in the range of 20 to 22, it is better to have an eyepiece that has a flat field. So most would have opted for XW30 if $$$ not an issue. If you are using the XW20 in a fast dob, it could be a problem when it comes to FC.
Note that Pentax had originally designed the XW to work well with refractors and most have claimed that the XWs work well the F6 (or longer) environment particularly.
However, even with considerable FC in XW14 (though lesser than XW20), one may be less inclined to look at the edges of the field of an XW14 at the same time as the center. Putting this aside, sharpness and contrast is excellent in the XW14 but for those who have compared with the Nag13T6, the latter seems to yield a sharper on axis performance. However, the XW14 has less lateral colour. Not forgetting the better flat field performance (which is expected) as well. When it comes to contrast however, the XW14 is a better candidate and with less light scatter.
One point to note though, that the XW14 does not exhibit any trait of the "kidney bean" effect that is prevalent in some Nags like the Nag12T4. So even with the wider AFOV, moving one's eye around to look at the edges of the field, you will see this black out. It does not appear to be so for the XW14. This is further made more pronounced if the eye relief is short which is so for the Nags in comparison with the XW line.
Next, many may cringe when they notice the great amount of elements (ranging from 6 - 8) used in XWs, but Pentax have claimed 98% light transmission for these eyepieces. Having seen faint galaxies with them before, I would be convinced. Not forgetting the renowned contrast performance for this line in general.
And for those who are bothered or particular about eye relief, then one may go for the XW rather than competing models like the Nags.
Lastly, I would mirror my thoughts with Canopus on the shorter XW FLs (3.5 - 10) but in practice I would think XW10 and XW5 is close.
Still, in all, all eyepieces mentioned above are already very good, and it is like comparing priced high end cars. The descerning observer will have to find his/her "nooch" or comfort attributes for the respective optical system.
Meanwhile, I can't wait to test the XW30 on the TOA......soon! ;P
My thoughts!
You can guess how popular it is -
Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
Remus,
Yes, that the FC direction is the intringuing part. For the XW20 to XW40, it is +ve while that of the XW3.5 to XW10 is -ve. Most telescopes are -ve and requires a +ve field curvature of an eyepiece to match. Actually, the words are all in Jap so I do not know if they reverse the graph. As usually optical design is from left to right, that means for eyepieces it is designed for minimum spot size or highest MTF. Hence the parallel rays are simulating that of the exit pupil rays and done from exit pupil to the field stop of the eyepiece.
If that is the case, it does not tally with that of the graph shown by pentax. I am not too sure if Pentax reverse the direction as it can be reversed if you are referring from the field stop of the eyepiece to the exit pupil. Anyway, the graphs will show the field curvature. Direction wise, I am not too sure but from reports of XW20 and XW40 having field curvature and the XW5 to XW10 not having, although they have about the same field curvature diregarding sign. It could mean that the sign of the XW20 and XW40 is the opposite of that of normal telescopes and hence some mismatch and becomes more evident if the telescope's field curvature is more. For the XW30 and XW40, the field curvature is slight so it will not be evident to the eye. Its field curvature is comparable to orthos. Flat. That is my guess on the sign. I may be wrong as I cannot read Japanese.
Yes, that the FC direction is the intringuing part. For the XW20 to XW40, it is +ve while that of the XW3.5 to XW10 is -ve. Most telescopes are -ve and requires a +ve field curvature of an eyepiece to match. Actually, the words are all in Jap so I do not know if they reverse the graph. As usually optical design is from left to right, that means for eyepieces it is designed for minimum spot size or highest MTF. Hence the parallel rays are simulating that of the exit pupil rays and done from exit pupil to the field stop of the eyepiece.
If that is the case, it does not tally with that of the graph shown by pentax. I am not too sure if Pentax reverse the direction as it can be reversed if you are referring from the field stop of the eyepiece to the exit pupil. Anyway, the graphs will show the field curvature. Direction wise, I am not too sure but from reports of XW20 and XW40 having field curvature and the XW5 to XW10 not having, although they have about the same field curvature diregarding sign. It could mean that the sign of the XW20 and XW40 is the opposite of that of normal telescopes and hence some mismatch and becomes more evident if the telescope's field curvature is more. For the XW30 and XW40, the field curvature is slight so it will not be evident to the eye. Its field curvature is comparable to orthos. Flat. That is my guess on the sign. I may be wrong as I cannot read Japanese.
AstroDuck
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Canopus Lim
- Posts: 1144
- Joined: Fri Jun 03, 2005 12:46 pm
- Location: Macpherson
Just in case people get the wrong impression of this analysis. Firstly, the Pentax eyepieces are all very good. I am not comparing against other eyepieces because other companies do not post their designs unless it is Patented like the Nagler eyepiece. What was written was just based on the data Pentax show to everyone. I am just teaching on how to read these graphs.
Note that this is just only the eyepiece. When combined with the telescope there will be different results. It is just a rough guide on how the eyepiece will perform. When designing optics, the software will generate such curves so that the designer can choose if that should be the design or not. So please take this analysis with a pinch off salt. It tells roughly how it will perform like in the case of the ortho. It shows clearly that off-axis is not that good and it is true in real life that orthos do not perform that well off-axis. It also shows that the ortho has a flat field and that is true in real life. Some may be true, some may not as it depends on the telescope used as the telescope also has its own DM and DS curves and these curves will interact with the eyepieces' DM and DS curves. Also, it depends on the user's eyes. Lastly, I just want to emphasize that from these curves one can see that designing optics is not so simple. There are balances to be made.
For camera lenses..they post their performance graphs like MTF, distortions etc. Therefore sometimes it is good to know how to read these graphs and you will know roughly how it will perform. The best test though, is to try it!
Note that this is just only the eyepiece. When combined with the telescope there will be different results. It is just a rough guide on how the eyepiece will perform. When designing optics, the software will generate such curves so that the designer can choose if that should be the design or not. So please take this analysis with a pinch off salt. It tells roughly how it will perform like in the case of the ortho. It shows clearly that off-axis is not that good and it is true in real life that orthos do not perform that well off-axis. It also shows that the ortho has a flat field and that is true in real life. Some may be true, some may not as it depends on the telescope used as the telescope also has its own DM and DS curves and these curves will interact with the eyepieces' DM and DS curves. Also, it depends on the user's eyes. Lastly, I just want to emphasize that from these curves one can see that designing optics is not so simple. There are balances to be made.
For camera lenses..they post their performance graphs like MTF, distortions etc. Therefore sometimes it is good to know how to read these graphs and you will know roughly how it will perform. The best test though, is to try it!
AstroDuck
Yang Beng, thanks for the informative guide and interpretation of the graphs.
As we have discussed this afternoon, I brought up a few points:
1. It would be more interesting and of better value to compare X brand 10mm vs Y brand 10mm. Unfortunately other eyepiece manufacturers do not release sufficient information or graphs so that we can compare them 'on paper'.
2. I would be careful to use terms like 'worst' and 'bad' as these are not to be taken at face value but have to be measured relative to other similar eyepieces of the same focal length. For example, I would say that FC is slightly more in these two eyepieces compared with similar focal lengths of other brands. Likewise, FC in these two eyepieces are not as flat as other focal lengths in this XW series.
3. These graphs also do not tell us the 'final' performance of the eyepieces in terms of field curvature, astigmatism etc since the actual image quality is the resultant of the interaction between the eyepiece aberrations with the aberrations of the scope's main objective.
4. There are some other important factors to consider. For wide-field eyepieces intended for observing extended deep sky objects, these factors such as field curvature, astigmatism and distortion are relatively important. However for planetary eyepieces, these factors are relatively not so important. The critical factors for planetary observation are mainly contrast, sharpness and light throughput.
5. For me, an actual side-by-side comparison between the astronomical images seen in different eyepieces of the same focal lengths as observed through the same scope under good seeing/transparency sky conditions would give a more realistic and qualitative assessment of each eyepiece relative to another eyepiece.
As we have discussed this afternoon, I brought up a few points:
1. It would be more interesting and of better value to compare X brand 10mm vs Y brand 10mm. Unfortunately other eyepiece manufacturers do not release sufficient information or graphs so that we can compare them 'on paper'.
2. I would be careful to use terms like 'worst' and 'bad' as these are not to be taken at face value but have to be measured relative to other similar eyepieces of the same focal length. For example, I would say that FC is slightly more in these two eyepieces compared with similar focal lengths of other brands. Likewise, FC in these two eyepieces are not as flat as other focal lengths in this XW series.
3. These graphs also do not tell us the 'final' performance of the eyepieces in terms of field curvature, astigmatism etc since the actual image quality is the resultant of the interaction between the eyepiece aberrations with the aberrations of the scope's main objective.
4. There are some other important factors to consider. For wide-field eyepieces intended for observing extended deep sky objects, these factors such as field curvature, astigmatism and distortion are relatively important. However for planetary eyepieces, these factors are relatively not so important. The critical factors for planetary observation are mainly contrast, sharpness and light throughput.
5. For me, an actual side-by-side comparison between the astronomical images seen in different eyepieces of the same focal lengths as observed through the same scope under good seeing/transparency sky conditions would give a more realistic and qualitative assessment of each eyepiece relative to another eyepiece.
rlow