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Pentax XF 6.5 - 19mm Zoom vs Orthoscopics


A McEwan
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Hi all.

I bought a Pentax XF zoom for use as my solar observing eyepiece in hydrogen alpha with my Lunt 60 PT B1200 and white light through my Baader wedge, a while back, after asking for opinions here on SGL.

I was initially very pleased with the performance of the eyepiece and found it very handy to be able to vary the magnification without having to swap over eyepieces. This was the first zoom eyepiece I had ever used, and I didn't want to "rise through the quality ranks" so went for the best option first - saves money in the long run.

Now, having used it for a while, I have some horrible stuff to write about it. The thing is, you see, I feel guilty about using a zoom eyepiece. The perceived wisdom is that zooms lose something to the optical quality of individual eyepieces, and so I have always used individual ones of fairly high quality in my observing. I suppose you could consider this the "old skool" argument employed by "purists".

<thing is, I'm ashamed to admit that I actually like to think of myself as a bit of a traditionalist - and I bet some of you out there do too!>

So... the other day, as it was sunny and I was going to use the Lunt for a while anyway, I decided to test the zoom directly against my selection of orthoscopics. These comprise University Optics 18mm and 12.5mm, Baader Genuine Ortho 9mm, Antares (Baader-clone) 7mm and Circle T 6mm. I know from experience that Orthoscopics give the best views in the Lunt. As well as the sharpness and contrast, their narrower field of view makes any hint of "sweet spot" less apparent - something that wider field eyepieces are noticeably worse for.

So, I selected the active regions 1163 and 1164 and started off with the 18mm Ortho, then switched to the 19mm setting on the zoom. Then the 12.5, then the appropriate setting on the zoom, and so on through the range of eyepieces.

Now the really dreadful appalling thing that I have to report is this. The Pentax XF zoom eyepiece shows more detail in the Lunt than any of my Orthoscopics, regardless of focal length or manufacturer. Focal length for focal length, there was always a slight edge to the view through the zoom. I tested this thoroughly by examining a particular part of one of the active regions which had a dark swirl with a slightly fainter colouration in its interior. The difference was more noticeable in the zoom than in the Orthoscopic of comparative focal length.

I hope you understand how this upset me? Zoom eyepiece that's better than Orthoscopics? But... Orthoscopics are supposed to be the sharpest eyepices for solar system objects out there, with minimal reduction in throughput due to their small number of air-to-glass surfaces. And a zoom eyepiece beat them? Pah.

So, the Pentax Zoom is either an incredible eyepiece or all my Orthoscopics are rubbish! Only I don't think they are, as I've used many of them extensively on planetary observations through my refractors! The only time I used the Pentax zoom on the night time sky I was less impressed with it - it didn't appear as sharp as the Ortho that I had used before it (can't remember which one it was) and had a small amount of CA.

So, for me at least, the Pentax has to rate as pretty incredible for solar viewing, as I can't really write off the Ortho's, based on other observations made with them.

You learn something every day...

Ant

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Hi Ant,

what a fantastic report! I feel very sorry for you, and simultaneously happy for you, that your traditionalist ideals are being challenged!

I have used four zooms in the past and never been disappointed, and at present I almost exclusively use my Baader 8-24mm zooms for solar viewing, although I admit that sometimes I switch to one of my Zeiss Abbe-II orthoscopics if I want to concentrate on a particular object.

The Pentax zoom sounds like it's optically in a different league from the Hyperion, but it's also in a different price range... One day I may have a pair for binoviewing.....

Thanks for sharing your experiences

Andrew

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I remember when I first got my Pentax XF 6.5 - 19mm Zoom for use with my Lunt 60. I did the BO ortho comparison with the Pentax zoom and found that the zoom was just as good as if not better than the orthos. The sharpness and contrast are the best of any solar eyepiece I've ever used. :p

I have tried the Baader zoom with the Lunt, and while it was quite good, the Pentax is simply in another league altogether for solar viewing. Also it's a fairly small and lightweight eyepiece as well.

John

PS

Should we start a Pentax Zoom Appreciation Society ? :)

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The Pentax zoom sounds like it's optically in a different league from the Hyperion, but it's also in a different price range...

Actually, at just over £200 they're in serious competition with Baaders, even considering price! Wow - the temptation is going to be very hard to resistl!

Andrew

Edited by Andrew*
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Thats a very interesting and frank report Ant :)

I wonder if there is something about solar viewing that makes zoom lenses excel at it ?. When I had a Baader MkII 8-24 I was really impressed with it's performance in a PST but it was not quite as good as the fixed focal length Hyperions on dark sky objects.

Will you run a similar comparison under dark skies ? - it would be very interesting read about that :p

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Now, having used it for a while, I have some horrible stuff to write about it. The thing is, you see, I feel guilty about using a zoom eyepiece. The perceived wisdom is that zooms lose something to the optical quality of individual eyepieces, and so I have always used individual ones of fairly high quality in my observing. I suppose you could consider this the "old skool" argument employed by "purists".

A lot depends on coatings (especially when looking at the Sun, when low angle scatter is important), and Pentax's coatings are as good as they get.

But you shouldn't make any conclusions about the ortho design. Even a Baader (not "clone" as in "shares the same housing") Genuine Ortho or a University Optics HD ortho are going to be better than the ones you tried, and old Pentax SMC orthos and Zeiss Abbe Ortho I and IIs are also going to be better than even the zoom.

I like the Pentax XF zoom a lot, but it does have a lot of field curvature, so the edges of the field can be a bit blurry unless your eyes accommodate for focus easily. It's more easily noticeable if you have star fields at the edge.

Edited by sixela
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The answer is simple. If you look in H-alpha (or any other line emission) you are testing the monochrome performance of the lens, not the chromatic correction. When using a single spectral line, even a single lens can be very decent, so long as other aberrations are kept under control. This explains why the zoom is so much better in H-alpha than on the night sky, where CA occurs. It is quite possible that at that given wavelength, the zoom just happens to outperform the orthoscopics, but say at O-III the story is different (you could test this with very narrow band filters). In white light you apparently do see CA defects in the zoom.

I hope this puts the universe right :(

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I have been testing the Pentax zoom for regular night time observing and it is proving to be very good indeed. No traces of CA at all and excellent sharpness and contrast.

I'd rate it as similar to a WO SPL eyepiece in performance, and given that it's a zoom that's rather impressive.

John

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.............I'd rate it as similar to a WO SPL eyepiece in performance, and given that it's a zoom that's rather impressive.

John

Certainly is, John !

The only zooms that I've loved/love, are the Televue Nagler and Speers Waler units.

Others I've tried have been a bit average, optically.

Andy.

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Got a very brief nighttime update to this. Fri 11th I spent a half hour or so with my 18mm and 12.5mm Ortho's and the Pentax Zoom. I used the 90mm ED Triplet f6.67 and studied the Moon. Seeing was good though there were some intermittent drifts of high thin cloud which did not detract from the view too much at all - a natural Moon filter, perhaps.

I started with the 18mm Ortho. Focus was flat across he entire field of view and there was no fals colour to be observed anywhere. Features were very sharp and focus snap was very easy to find. Beautiful view.

I inserted the zoom and selected the magnification that appeared to make features the same size as when I had used the 18mm Ortho. The zoom is not clickstop so there is no "pre-set" for the different focal lengths. Once found, I compared the view. Wider field but when the Moon drifted to the edge of the field focus needed to be adjusted slightly. False colour also became apparent once the Moon was off-axis. I found that shifting my eye position did help resolve the false colour issue but not entirely.

Features on the disc were also extremely sharp and contrasty and again it was a beautiful view.

I switched back and forth a few times to make sure of the differences. Then I put the 12.5mm Ortho in.

More features became easier to see, obviously and ones already seen became larger. The view was extremely sharp and again, no false colour and a perfectly uniform field of focus. Tone vatiations in the maria were very apparent and some isolated peaks beyond the terminator became observable.

Finding the same focal length in the zoom, I compared the view again. Same false colour and focus isses as at the longer focal length. Again, eye positioning helped some. Interestingly though, the isolated peaks I'd seen through the Ortho were "slightly" more obvious in the Pentax. Other features were very similar in clarity and sharpness.

So.... there's obviously more to follow on this but I'm finding it a very interesting comparison. Thus far I have found a reason to keep the Ortho's. The ease of maintenance when viewing. Off-axis false colour is non-existant and flatness of field is perfect (in this 'scope anyway). The Pentax threw up just as good an overall image, and even outperformed on the isolated sunlit peaks beyond the terminator, but the off-axis colour and need to refocus off axis were definately there.

On a driven telescope and with the subject kept on axis these would not be much of an issue, but with large clusters? That remains to be seen. Hopefully tests will continue once the weather improves...

Ant

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Interesting update. I think this does suggest the earlier observations were due to working in monochrome light. If you insert say an OIII filter on the moon (weird I know) the observed difference would be smaller. Regarding large clusters, I prefer the Paragon and Nagler over an orthoscopic EP any time.

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Thanks. This is quite incomplete yet as I was only using two relatively low magnification settings on the zoom (~33x and ~48x) so I still need to do further testing at more zoomed-in powers. Yes, will have to try it in the Dob too at some point...

Ant

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Thanks. This is quite incomplete yet as I was only using two relatively low magnification settings on the zoom (~33x and ~48x) so I still need to do further testing at more zoomed-in powers. Yes, will have to try it in the Dob too at some point...

Ant

yeah! get on with it! we're waiting.....:o

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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

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