Premium Eyepieces - good read and some personal observations

Discuss telescope eyepieces.
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OzEclipse
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Premium Eyepieces - good read and some personal observations

#1

Post by OzEclipse » Sat Dec 14, 2019 9:26 pm

This is a link to a really well-informed discussion on Ice in Space, an Australian Astro Forum, discussing correction, properties and matching of premium eyepieces to different telescope types and how eyes ability to adjust to field curvature degrades with age.

http://www.iceinspace.com.au/forum/show ... p?t=178008

I found myself nodding my head at some points where it agreed with my personal experience with some of these eyepiece-telescope combinations, and learning from other points that were made of which I was completely unaware.

In particular, Don Pensack's post #13 includes a web archive link to Pentax eyepiece aberration data no longer published by Pentax
https://web.archive.org/web/20150522023 ... xw/64.html
Pentax-eyepece-astigmatism.jpg
Don wrote:-
The 10mm, 7mm, 5mm, and 3.5mm all have negative field curvature.
The 14mm, 20mm, 30mm, and 40mm all have positive field curvature.
It depends whether that matches your scope as to whether you see a flat field or not.
Here are the astigmatism/FC curve graphs:
https://web.archive.org/web/20150522.../xo-xw/64.html
If the two curves (sagittal and meridional) deviate, that indicates astigmatism. The center of the field is at the bottom and the edge is at the top. The scale is in diopters (the eye can usually accommodate 1, but 2 would be seen as curved).
Ideally, the two curves would not deviate and they would be vertical.

The 30mm and 40mm have been out of production a long time, but are coming back soon--perhaps by the end of the year.
Examining the diagram, you can see that the sagittal and meridional curves in the Pentax 5mm are perfectly matched as in the 14mm which is why these two eyepieces are so highly regarded.

My observing partner and I compared views in my 6"f7 newtonian and a TOA150 premium refractor during a side by side comparison of the two instruments that I wrote about in AF early last year.
http://www.astronomyforum.net/astronomy ... ector.html

In that comparison, we made a statement that-
Providing the exact same eyepieces were used in both instruments, we found the views were very similar in both instruments after allowing for the slight (50mm) difference in focal length. The TOA could out resolve the reflector on all subjects but the difference in resolution was very marginal. We had to look long and hard at the finest of details to see a difference. Using eyepieces of same focal length but different brand/types/quality could elicit much greater apparent differences in views, clarity, crispness between the instruments, than the marginal difference that existed between the two instruments when identical eyepieces were used. Both instruments were resolving both the sunlit side and the shadow of the straight ridge in Dopplmayer marked in the attached photos. Each feature, the sunlit side and the shadow, measured off LROC images, was approximately 1km wide. On Saturday night, the Moon was 361000km away from Earth's geocentre, perhaps 352000 km from our topocentric location to the lunar surface suggesting that both instruments were resolving close to 0.7 to 0.6 arc sec, the TOA a little better than the Newtonian.

To elaborate on that statement highlighted in green, the differences between eyepieces in fine lunar details that I referred to through were between mid-range Baader Hyperion 5mm and Pentax XW 5mm eyepieces. The difference in resolving power and clarity of the finest details was very difficult to pick between the two instruments when using the same eyepiece however the difference between the two eyepieces was very obvious with the Pentax giving a much sharper image than the Baader. To be fair, the Pentax costs 2-3 times more than the Baader depending upon which country you are conducting your shopping.

On another occasion, Phil and I had the opportunity to compare a 14mm Pentax with a 14mm Denkmeier in Phils TOA150. We found the views between these two premium eyepieces much closer but the Denkmeier gave a slightly sharper image.

Joe
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Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops
Bortle 1-2 skies, 149 E, 35 S
web site : http://joe-cali.com/
SCOPES - ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80
MOUNTS- EM-200, iEQ45, Push dobsonian with Nexus DSC
CAMERAS : Pentax K1, K5, K01 / VIDEO CAMS : TacosBD, Lihmsec
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#2

Post by notFritzArgelander » Sat Dec 14, 2019 9:44 pm

This is, of course, very interesting. Also one could optimize eyepiece choice by matching field curvature properties.
Scopes: Refractors: Orion ST80, SV ED80 A f7; Newtonians: Z12 f5; Catadioptrics: VMC110L, Intes MK66. EPs: KK Fujiyama Orthoscopics, 2x Vixen NPLs (40-6mm) and BCOs, Baader Mark IV zooms, TV Panoptics, Delos, Plossl 32-8mm. Mixed brand Masuyama/Astroplans Binoculars: Nikon Aculon 10x50, Celestron 15x70, Baader Maxbright. Mounts: Star Seeker III, Vixen Porta II, Celestron CG5, Orion Sirius EQG
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#3

Post by j.gardavsky » Sat Dec 14, 2019 9:59 pm

Thank you Joe for this very important discussion.

The IceInSpace offers quite a few good reviews, and here is another one on the Pentax XW against the Naglers,
http://www.iceinspace.com.au/forum/arch ... 85557.html

I have recently added the Pentax XW 5mm, completeing the 3.5, 7 and 10mm group.

Best,
JG
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#4

Post by OzEclipse » Sat Dec 14, 2019 11:30 pm

j.gardavsky wrote:
Sat Dec 14, 2019 9:59 pm
Thank you Joe for this very important discussion.

The IceInSpace offers quite a few good reviews, and here is another one on the Pentax XW against the Naglers,
http://www.iceinspace.com.au/forum/arch ... 85557.html

I have recently added the Pentax XW 5mm, completeing the 3.5, 7 and 10mm group.

Best,
JG
Thank you JG. This is also an excellent discussion that was published around the time I first joined that forum. John Bambury is one of this countries more experienced visual observers and I find everything he writes to be worth reading.

I notice that he has published some eyepiece collection recommendations for young and old eyes

John Bambury wrote-
I am an eyeglass wearer and my preference is to observe with my eyeglasses on using long eye relief eyepieces. If I was to start over again I would build the following eyepiece collection for use in these scopes

5mm Pentax XW
7mm Pentax XW
10mm Pentax XW,
12.5mm Nikon NAV HW (or 12.5mm Docter)
17mm Nikon NAV HW
22mm Nagler T4
31mm Nagler T5

Televue Paracorr

If I was a young bloke and didn’t need eye glasses I would build the following eyepiece collection

5mm Pentax XW
7mm Pentax XW
10mm Pentax XW,
12.5mm Nikon NAV HW (or 12.5mm Docter)
17mm Nikon NAV HW
20mm Nagler T5
26mm Nagler T5
31mm Nagler T5
As an older, astigmatism suffering, eyeglass wearer, my collection recommendations would differ.
His biggest scope is an 18" f4.5, mine an 18" f5.6.
I have 31mm and 17mm Naglers and while I considered buying the 22mmT4. I have tried the 22mmT4 and no longer feel the need to fill the gap in between.

My astigmatism is light, 0.5 diopter in my dominant eye. I don't notice the astigmatism when observing even at wide exit pupils and certainly not at magnification and small exit pupils and consequently never wear glasses when observing. Perhaps a better way to express this is that for my eyes, the overall visual experience, clarity and view is much better without glasses than with glasses.

My 18" has a 100" focal length. The 31mm gives me 80X magnification with a 1 degree field and a 5.7mm exit pupil. The 17mm gives me 150X magnification with a 0.5 degree field and a 3mm exit pupil.
I find the 31mm well suited to wide field viewing of large subjects. Orion Neb, Tarantula, M31, Magellanic Clouds, NGC 253, galaxy clusters etc etc.
I find the jump to 150X 0.5 deg 3mm exit pupil with the 17mm is ok for me. For many years when i started out, I owned 25mm and 12mm orthoscopic eyepieces with a 6"f7 which also saw me jump from 1 deg to 0.5 deg FOV's. So perhaps it's a case of old dogs and new tricks ;-)

As the eyepiece FL shortens, there is a definite advantage to having smaller steps between eyepiece focal lengths particularly with large diameter, long focal length instruments where the maximum magnification needs to be matched to the seeing conditions. I have 7mm & 12mm Naglers. I use the 12mm frequently, the 7mm only goes in the 18" on nights of very good seeing. I would consider adding the 10mm & 14mm Pentax XW's and a 5mm mainly for my 6"f7.

We don't get many 500x nights where I could put a 5mm in the 18" and even if I could, the image flies across the 70 deg field very fast in the 18". The 82 deg field of the 7mm Nagler and 350X is more than adequate for showing remarkable levels of detail on the planets. The fine detail visible in the brown clouds of Jupiter's equatorial belts is a glorious sight.

In short one high end wide field eyepiece and then put your money into a greater number of high quality short focal length eyepieces.


cheers
Joe
Amateur astronomer since 1978
Astronomical interests : astrophotography, visual observing, nightscape photography, solar eclipse chasing
asteroidal occultations, nightscape astrophotography workshops
Bortle 1-2 skies, 149 E, 35 S
web site : http://joe-cali.com/
SCOPES - ATM 18" Dob, Vixen VC200L, ATM 6"f7, ED80
MOUNTS- EM-200, iEQ45, Push dobsonian with Nexus DSC
CAMERAS : Pentax K1, K5, K01 / VIDEO CAMS : TacosBD, Lihmsec
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