Intermittent CA in Svbony 406P (2 Viewers)

[looksharp65]

I got the 406P in the autumn of 2022, and wrote a quite favourable report in the thread. This scope is used in the evenings after work so I can go directly to the reserve without fetching the usual stuff at home, which would delay me too much because I work until 6 pm and always ride the bike.

The supplied zoom is not to my liking because after work I use spectacles, and the zoom's eye relief is a royal PIA , in particular since my MHG 8x42 is so perfect.
So I took the plunge and ordered a Baader Hyperion 13 mm, which seemed fine when I tested it directly upon arrival.
The same evening, I took the scope to a place with the setting sun in my back. I thought the scope was lacking in sharpness, contrast and in particular it showed quite a bit of longitudinal CA. I had seen it before with the zoom, too.

So I started to investigate the used market and offerings from different sourcesto find something better. Used scopes are ridiculously expensive in Sweden and I guess people sell the lemons, too. There were a few Fieldscope ED II and another ED82A available from Japan (my main scope is a super sharp ED82A) but I really felt hesitant. Then I could have bought a Celestron Regal 65 M2 with the zoom which was down 50%...but evenings and large distance viewing favour fullsized scopes.

Yesterday I took the scope home for some thorough testing. These two days were overcast with some haze.
The scope delivered flawlessly. I'd say the Nikon with 30x DS is sharper, but the 10% more magnification in the 406P with the 13 mm Hyperion (33x) rectifies the difference. The Svbony may appear a wee bit brighter despite the 25% loss in geometrical brightness, but it is also less contrasty, almost a bit washed out.
The edges are as sharp as the center but require some refocusing. I think the Hyperion 13 does justice to this very fast scope. In comparison the William Optics 15 mm 72° SWAN showed excessive field curvature.

And the longitudinal CA? Virtually non-existent! There was no way I could provoke it by looking through the scope, only a digiscoped photo suggested some minor traces of CA.
How can this happen? One reason I took the scope home was to experiment with tripod height. I had used my ordinary setting but the scope and eyepiece reach higher than the Nikon. But using that setting at home was still OK.

While the CA was very apparent that evening, it didn't behave a bit like the non-ED Fieldscope where it's present all the time as fringes outside otherwise sharp images.
Is there a thing about Schmidt-Pechan prisms with dielectric coatings where the Nikon is a porro? Svbony's homepage says BAK-4 and 'FMC' prism coatings, so not much help there. After all I'm relieved to see that the scope hasn't developed a fault, I must just force myself to neglect the CA when it shows.
But I'm puzzled with the Jekyll and Hyde behaviour of this scope.

//L

 

[William Lewis]

I think it's down to the positioning of the field lens along with how well the lenses are figured to deal with a steep light cone. The zoom will also more than likely have more of those lenses- all for less cost.

It's more than likely the latter in the case of the standard zoom as the field lens should be in the optimum position, given the zoom is designed for the scope!

Prisms do make a big difference but I wouldn't think they would manifest a difference between the two eyepieces the way you describe.

As a note by your calculation of 33x magnification for the 13mm eyepiece suggests a figure of 430mm for the focal length for the Sv bony, by comparison your ed82 has a focal length around 500. The difference is f5.3 (Sv bony) to f6.09 (Nikon) in terms of focal ratio.

I only mention this as for comparison purposes I would think the Sv bony may have a sightly longer focal length. In this case if your running the zoom at 33x to test against the 13mm the 13mm would have a higher magnification and therefore generally show more ca anyway if the focal ratio of the Sv bony is more than 430mm - more good points to the Hyperion!

 

[looksharp65]

Thanks William!

I believe my post was slightly unclear.
The intermittent CA seemingly occurs in sunlight. Nothing related to the eyepieces, which also is confirmed by the location in the middle of the view, not towards the edges.
I'd say the CA is equal with either of the eyepieces.

It has been confirmed that SV406P 80's focal length is as short as 430 or 432 mm which is remarkably short when it has such good performance. The Nikon ED82A is 525 mm.

I compared the zoom, set at about a third of the way between 30x and 40x, and it seems ro match the Hyperion 13's magnification, and also slightly more than the Nikon at 30x.

Usually I wouldn't think I'm overly bothered with traces of CA so that I look for it to have something to complain about. I mentioned the non-ED Fieldscopes and their easily detected colour fringing. The Nikon Monarch X 10,5x45 had intolerable CA and the old Nikon HG 10x32 was less than ideal. Other than that everything's been fine until now.
I guess the Svbony's objective cell isn't the most advanced ever, and the short focal length complicates things further. Bearing the price in mind, this is still a real good scope.

//L

 

[William Lewis]

Oh I see. Blame late night reading! So the c.a is not generally noticeable on overcast days but becomes more prominent when it's sunny?

We get quite a few little egrets round here and they can be frustrating to observe at a distance in less well corrected optics. Did you try the usual targets, TV aerials, the tops of trees etc against the sky?

I would also maybe revisit when you first noticed it but try having something blocking the sun from behind you just in case some was getting through to the eye lens.

Could be down to poor seeing conditions on the sunny day? I was caught out by atrocious seeing a couple of months back, worse than I'd ever had before, it made me think what turned out to be a very good scope was total garbage.

 

[Jinac]

I had a similar experience with the UFF18 svbony, it has more CA of the original zoom and the first day I tested it, the CA was really important but since then, there is more CA than in zoom or Hyperion 13mm but at level very acceptable.
I'm wondering if some days, the day light is very purveyor of aberrations. I don't know anything about optic and so I ignore if the atmosphere could filter the light in such a way that it is more CA...

 

[looksharp65]

Oh I see. Blame late night reading! So the c.a is not generally noticeable on overcast days but becomes more prominent when it's sunny?

We get quite a few little egrets round here and they can be frustrating to observe at a distance in less well corrected optics. Did you try the usual targets, TV aerials, the tops of trees etc against the sky?

I would also maybe revisit when you first noticed it but try having something blocking the sun from behind you just in case some was getting through to the eye lens.

Could be down to poor seeing conditions on the sunny day? I was caught out by atrocious seeing a couple of months back, worse than I'd ever had before, it made me think what turned out to be a very good scope was total garbage.

Right now the scope is on my roofed glass balcony and weather is overcast, so conditions are perfect. Looking at dark objects with the white sky for background, I can't see CA regardless how I push myself. But when I watch a white object against a darker background, there's a little CA, characteristically magenta fringing on one side and dark yellow on the other. Nothing too obtrusive. But when it's brighter and sunny, and the object (read: bird) is clean white, then it kicks in and gets annoying. This is with the 33x. With Nikon at 50x, there's no CA whatsoever.
Guess that's not a really unusual behaviour with CA, but it highlights (pun intended) how good the CA correction is in the ED Fieldscopes and that all ED glass isn't created equal.

//L

 

[looksharp65]

I'm wondering if some days, the day light is very purveyor of aberrations. I don't know anything about optic and so I ignore if the atmosphere could filter the light in such a way that it is more CA...

Similar to my line of thinking, and I considered an idea that UV radiation might be altered in dielectric coatings and reflected as visible light.
This, however, is not a realistic explanation, since the percentage of UV vs. visible light on a sunny day is less than on an overcast day.
So I guess I'm dealing with the usual run-of-the-mill CA 😂

//L

 

[hopster]

This is interesting because the only time I ever notice CA in the Noctivid is on bright and sunny days, especially when there is a lot of blue light so at altitude or by the sea.