Pentax XF 6.5 - 19mm Zoom vs Orthoscopics

[johninderby]

I tested the zoom on the moon last night at several settings, with and without the Baader Moon & Skyglow filter, and again colour wasn't any problem. There was some colour right at the very outer edge which was like the Nagler ring of fire effect, but this was only when my eye was in certain positions. This with my 6" f/8 newt with the OO 1/12PV optics.

I'll have to try the zoom with some other scopes to see if it makes any difference.

I didn't use the zoom for very long though as I switched to the Ethos 3.7 SX. Now that is the ultimate lunar eyepiece.

John

[A McEwan]

Slightly off-topic John, but did you try the 3.5mm Nagler T6?

[johninderby]

Slightly off-topic John, but did you try the 3.5mm Nagler T6?

Unfortunately I don't have the 3.5 Nagler any more (helped pay for the SX).

I remember that with the 3.5mm Nagler there was always that ring of fire if you looked at the edge of he FOV when observing the moon. I think the zoom is slightly worse, but not by much.

I did some direct head to head lunar observing comparisons between a barlowed 6mm Ethos and the 3.5mm Nagler and while the Nagler was very good the Ethos was the clear winner and just showed more detail.

John

[Ggeo]

Hi,

Has anyone used the nikon mc2 zoom for solar viewing? Performed great in my c6 on lunar and 3d on jupiter, pin sharp and no colour issues. Thinking of getting a binoviewer, not sure it's worth it or should I just get more eyepieces.

Thanks in advance,

George

[Merlin]

Maybe I'm old-fashioned, Folks, but to me it would seem to go against the laws of physics for any multi-lensed zoom eyepiece to equal the clarity of a quality ortho or Plossl.

[michael.h.f.wilkinson]

Maybe I'm old-fashioned, Folks, but to me it would seem to go against the laws of physics for any multi-lensed zoom eyepiece to equal the clarity of a quality ortho or Plossl.

I agree every surface can increase scatter, but monochromatic performance of a zoom could be good indeed. I also do not know whether we are comparing older coatings on the orthos with new ones on the Pentax. That makes a huge difference.

[neilmack]

An interesting and thought-provoking review. There can be no doubt that zoom designs have come on greatly, and offer terrific convenience. But the thing that bothers me about zooms - and complex single focus eyepieces - is the issue of light transmission. Never an issue with solar work, light transmission becomes a major bugbear for planetary work, for resolving Globulars, and for the detection of faint objects of any sort. It's an especial problem where, as Merlin rightly suggests, light is being bounced off the additional glass/air surfaces in complex designs. I recall a fascinating image of a range of well esteemed eyepieces siting in a row, all uncapped in a bright light. The best of them appeared to have no optics at all - just like looking into a black well - the rest of them had glassware that shone in various tones of mauve and purple. Almost inevitably there was a straight correlation between design complexity and reflections.

The ideal optical train is always going to be composed of fewer and better elements : obstructions, diagonal mirrors, and extra lenses always involve sacrifices of quality, and we shouldn't kid ourselves that convenience doesn't come at a cost.

[michael.h.f.wilkinson]

An interesting and thought-provoking review. There can be no doubt that zoom designs have come on greatly, and offer terrific convenience. But the thing that bothers me about zooms - and complex single focus eyepieces - is the issue of light transmission. Never an issue with solar work, light transmission becomes a major bugbear for planetary work, for resolving Globulars, and for the detection of faint objects of any sort. It's an especial problem where, as Merlin rightly suggests, light is being bounced off the additional glass/air surfaces in complex designs. I recall a fascinating image of a range of well esteemed eyepieces siting in a row, all uncapped in a bright light. The best of them appeared to have no optics at all - just like looking into a black well - the rest of them had glassware that shone in various tones of mauve and purple. Almost inevitably there was a straight correlation between design complexity and reflections.

The ideal optical train is always going to be composed of fewer and better elements : obstructions, diagonal mirrors, and extra lenses always involve sacrifices of quality, and we shouldn't kid ourselves that convenience doesn't come at a cost.

At Carl Zeiss they state quite bluntly that only the invention of anti-reflection coatings (by Zeiss ) allowed the invention of complex zooms with many lenses in many groups. At every uncoated surface, 5% of light is lost, so an 8-group design, involving 16 glass-air interfaces would have a 0.95^16 = 0.44 or 44% transmission. Cut the reflection to 1% (achievable with single coating) and we obtain 0.99^16 = 0.85 or 85% transmission. Using multi-coating (T* from Zeiss and SMC from Pentax claim 99.8% transmission) the same design reaches 97% transmission, or an almost negligible loss in transmission. Very few EPs have an 8 group design. The Nagler is a 5 group = 10 surfaces design, which gives 98% transmission with SMC-style coatings.

The thickness of the glass is probably more of a worry: 10mm of glass loses 5% of light (roughly).

[johninderby]

My choice in zooms would be the Leica ASPH zoom. Very expensive but as good as any fixed fl eyepiece. It's worth reading the comparison of the Leica ASPH Zoom and Zeiss Abbe Ortho IIs on Cloudynights.

http://www.cloudynights.com/item.php?item_id=2390

Their conclusion was.

"All tests performed so far have not shown differences in favour of the ZAOIIs. As mentioned before this isn't sound proof for differences not to exist. But if there are any they will be that subtle that only very experienced observers under the most favorable seeing conditions could see may be a hint of differences."

John

[michael.h.f.wilkinson]

My choice in zooms would be the Leica ASPH zoom. Very expensive but as good as any fixed fl eyepiece. It's worth reading the comparison of the Leica ASPH Zoom and Zeiss Abbe Ortho IIs on Cloudynights.

http://www.cloudynights.com/item.php?item_id=2390

Their conclusion was.

"All tests performed so far have not shown differences in favour of the ZAOIIs. As mentioned before this isn't sound proof for differences not to exist. But if there are any they will be that subtle that only very experienced observers under the most favorable seeing conditions could see may be a hint of differences."

John

It costs about as much as three or four Naglers in its focal range . I have become used to 82 deg AFOV, so the FOV of the zoom looks cramped. Compared to a collection of orthos it would be great of course. Like the eye relief too

[johninderby]

Yes I'd love to own the Leica zoom but I'd have to sell a couple of Ethos to pay for it

I think I'll just have to "make do" with the Ethos eyepieces for now.

John

[michael.h.f.wilkinson]

Yes I'd love to own the Leica zoom but I'd have to sell a couple of Ethos to pay for it

I think I'll just have to "make do" with the Ethos eyepieces for now.

John

We all feel your pain

[E621Keith]

My choice in zooms would be the Leica ASPH zoom. Very expensive but as good as any fixed fl eyepiece. It's worth reading the comparison of the Leica ASPH Zoom and Zeiss Abbe Ortho IIs on Cloudynights.

http://www.cloudynights.com/item.php?item_id=2390

Their conclusion was.

"All tests performed so far have not shown differences in favour of the ZAOIIs. As mentioned before this isn't sound proof for differences not to exist. But if there are any they will be that subtle that only very experienced observers under the most favorable seeing conditions could see may be a hint of differences."

John

It's not fair to compare the £250 XF zoom with the £600 Leica. A more appropriate comparison will be the £400 Pentax SMC 8-24 zoom, but even that is significantly cheaper.

[johninderby]

It's not fair to compare the £250 XF zoom with the £600 Leica. A more appropriate comparison will be the £400 Pentax SMC 8-24 zoom, but even that is significantly cheaper.

I wasn't comparing them. I was simply stating that it was possible to get a zoom that was equal to the best fixed focal length eyepieces if cost was no object.

John

[E621Keith]

I wasn't comparing them. I was simply stating that it was possible to get a zoom that was equal to the best fixed focal length eyepieces if cost was no object.

John

Sorry I misunderstood your point.

I agree some zoom can be on par with or better than fixed focal length eyepieces. Given the same budget, fixed eyepiece will lead in quality, but then there will be a point where the zoom will be good enough.

[Ggeo]

I think Keith has hit the nail on the head. A modern zoom like the pentax or nikon will give great performance and convenience in a single eyepiece. The trade off is in fov at the wider end of the scale and in the case of the nikon getting a suitable adapter. I've had to get a batch made as the adapters are unavailable, but it still makes the nikon zoom cheaper than the pentax and allows me to use my nikon eyepieces in both astro and spotting scopes.

[Neil English]

Medium powers

Same 90mm ED Triplet.

Same target: Luna.

Moon very high and good seeing so I brough out my 9mm BGO and 7mm Antares Orthos for this round. I also brough out a single polarising filter to use with the zoom to see what effect it had on the spurious colour.

At the medium magnifications offered by the Ortho's (9mm = 66x, 7mm = 85x) the views of Mare Imbrium, Mare Frigoris, Plato and Montes Appeninus were stunning. Ultra sharp with huge variety of tone across the lunar landscapes. Focus was exactly the same across the entire field of view, and once again there was no false colour to be found - although once the limb hit the field stop of the 9mm there was a very brief instant of slight CA but it only occured on contact with the Moon's limb abd couln't be replicated anywhere else in the field of view.

I inserted the Pentax XF zoom and dialled in the approximate mag to match the 9mm and then examined the view. Ok, how can I put this tactfully... "Gobs of colour" is about as close as I can get. The central 60-70% of the field was sharp, contrasty and extremely detailed and the outer 30-40% was soft and tinted various hues from blue to pinky-orange. Eye placement varied the colour! The difference in focus across the field was quite noticeable but when I adjusted the focus so that the outer portion of the view was focused, that did not help with the false colour issues.

The central portion of the view ws as good as the Orthoscopics had shown. I can't say it was noticeably any better though. That's not to put the view down - if I ignored the outer 30-40% of the field the view was lovely, but as a whole it left a lot to be desired.

I popped on the polarising filter and refocused. The false colour was reduced though not completely removed. The extent of "good" field was extended. The difference in focus between the centre and the edge was unaffected. The improvment in colour came at he cost of reduced brightness.

I made the same comparisons with the zoom set to 6.5mm (its highest power) and the 7mm, swapping back and forth, with the same results.

There simply is no comparison. On lunar viewing the Ortho's lead by a long, long way. They seem in fact to be "perfect" in every observable way. Flat field, zero false colour, sharp. Yes limited eye relief but I'm used to it and find them quite comfortable to use.

The Pentax is more comfortable from an eye relief point of view, but eye placement is more critical I found and only the central portion of the view was anything like what the Ortho's produced.

I will not use the Pentax XF zoom for lunar viewing again, but I am still completely wowed by its H-alpha solar performance and I'll continue to use it solely for this function. I've yet to do a comparison between the zoom and Ortho's in whilte light solar viewing. I suspect it would be similar to the lunar experience but we'll see hopefully sometime soon.

Just as a thought, do you think I could have a sub-par model? Do these experiences tie in with any other people's that you know of?

Ant

Hi Antony,

That's a very interesting report! I had a very similar experience with this zoom. I bought the unit as I was very impressed by the fixed focal length Pentax XF eyepieces. To my eye, they were about as sharp as the more expensive XWs. So how could I go wrong with a pentax zoom?

Well, I tested the unit out on my TV 76 SD doublet about 18 months back. It was quite sharp on axis, but I was shocked to discover the same 'gobs of colour' as you reported off axis. It was particularly pronounced at the longer (lower power) settings. I thought I was going mad (LOL). The amount of lateral colour was unacceptable in my books. I tried it out on my Tal 100R (F/10) and although it was a lot better, it was still present. I could even detect a little in my 4" F/15.

In fact, I think I commented on it when I met fellow SGLer Stuart Ross, when I was checking out his WO 132. We both looked at the full Moon and we compared it to a basic 20mm Plossl. The plossl had nada lateral colour in the F/6.3 TV, but the Moon had a bright coloured ring round it with the Pentax XF zoom.

Shockeroonie!

Regards,

Neil.

[Rossco72]

Neil,

Thankfully the Pentax 8-24mm SMC Zoom is much better. The moon looks just as good as with fixed length EP's although a little narrow in the FOV not unacceptably so.

Cheers

[johninderby]

Strange as this isn't an issue with my XF zoom with any of my scopes. No more "coloured ring" around the moon than with a Nagler. Perhaps a different batch of XF zooms.

John

[luisevan]

I allready see throught s Leica zoom, and is a fantastic eyepiece, its indeed so good as a Televue Nagler.

Actually there is one zoom eyepiece that I think would would be better than the Pentax, but given the price I don't think I'll be buying one any time soon.

Leica Vario zoom eyepiece ASPH.

17.8 - 8.9mm

http://www.apm-telescopes.de/leica-zoomokular-vario-okular-17.8-8.9mm-asph._proinfo_65509.html

Only £687.00

John

[Rossco72]

Strange as this isn't an issue with my XF zoom with any of my scopes. No more "coloured ring" around the moon than with a Nagler. Perhaps a different batch of XF zooms.

John

I remember Neil's disappointment well and in my 132FLT the XF Zoom did have bands of colour on the crater rims of the moon and at all the borders of light and dark.

My own WO eyepieces ate the time, SPL's and SWANs didn't show any false colour.

Maybe Neil was just unlucky and got a dud'un but then again that would be unfortunate given that quality control is what we pay the premium for.

[johninderby]

I remember Neil's disappointment well and in my 132FLT the XF Zoom did have bands of colour on the crater rims of the moon and at all the borders of light and dark.

My own WO eyepieces ate the time, SPL's and SWANs didn't show any false colour.

Maybe Neil was just unlucky and got a dud'un but then again that would be unfortunate given that quality control is what we pay the premium for.

Damaged during shipping maybe?

John

[Neil English]

Hi Guys,

I don't know what the issue was with it TBH. It wasn't like the 'ring of fire' you get in a large Nagler. It was present too near the centre of the fov for that. I can't remember off hand the size of the apparent field but the full Moon only took up about one third of the field and had a ring round it. I showed it to Stuart. Bright stars also showed it up all too clearly. I was getting rainbows off axis.

The eyepiece didn't look damaged but it could have been assembled wrongly. I eventually sent it back for a full refund but didn't take a chance on another unit.

Pentax produce great stuff, so odds are I got a lemon.

Regards,

Neil.

[Rossco72]

And to be clear Neil's fixed XW (20mm if i recall correctly) was very good and I now own an SMC 8-24mm and I am VERY happy with it...

[Neil English]

Hi Stuart,

The XWS are great eyepieces without a doubt. The XW zoom looks fab, but I guess it ain't cheap. Still 8-24mm is a very useful range of focal lengths. Might give that one a closer look.

Cheers,

Neil.