Svbony 8-3 zoom

[Mr Spock]

Hmm, my 30mm UFF is flat in both the refractor and reflector. The Svbony is flat (almost) in the refractor but curved in the reflector. Same goes for my LVWs, Nirvanas (except 16mm) and TOEs - they are flat in both.

Perhaps a topic for another thread as this one is just about the Svbony.

[Ratlet]

Out of curiosity, is this field curvature?

This was taken through the svbony 3-8mm whilst testing my moveshootmove phone holder.

Never noticed it at the eyepiece with my 130pds, so I assumed it was because of the phone lens.

Still a great eyepiece.  Think this will be the default 'beginner planetary eyepiece' If you don't wear glasses.

[Mr Spock]

1 hour ago, Ratlet said:

Out of curiosity, is this field curvature?

Yes, that's the effect. Try focusing the edge and see what happens.

[Louis D]

If it sharpens up refocused, it's entirely field curvature.  If it is still mushy, just less so, then there's most likely astigmatism involved as well.  It's best to use a star to examine these effects.  Astigmatism turns stars in lines.  They'll be tangential on one side of focus and radial on the other side.  You'll also be able to see chromatic aberrations which turn stars in to radial rainbows.  Coma is rare in well corrected eyepieces in my experience.  As a rule of thumb, if an eyepiece exhibits chromatic issues, it will generally also exhibit coma.  This manifests itself as a rainbow that fans out center to edge.  It gets more diffuse the closer the star is moved to the edge.  A coma-free eyepiece shows a nice linear rainbow pointing to the center.

[Don Pensack]

From Ernest Maratovich's test of this zoom:

SVBony Zoom 3-8	8(8.1)	8.1	58(57,3)	56.9	2	6	15	diffr.	5	13	Ast. +6%
zoom..	7	7	57.3	56.5	3.3	5	20				FC,Ast.
zoom..	6	6	57.3	58.4	4	6	15	diffr.	5	11	FC,Ast. +14%
zoom..	5(5.2)	5.2	59.6(57.3)	57.3	4.5	7	20				FC,Ast. +14%
zoom..	4(4.4)	4.6	65.9(59.9)	59.6	6	7	25	diffr.	6	14	Ast. +15%
zoom..	3(3.5)	3.6	68.8(58.9)	59.8	8	8	45	diffr.	8	18	Ast. +15

 

The first column shows the focal lengths at the click stops (actual measured focal lengths in parentheses).

The next column is the field stop at each click stop.  No mystery here, because focal length = field stop at 57.3° apparent field, and this is very close.

The next column is calculated field stops based on stated (and actual) focal lengths with no distortion.  You can use these figures in Astronomy tools to get an accurate answer, or in TF = AF/M.

The next column is the measured apparent field (what we actually see). Roughly 57-60°.

Then 3 columns of spot size at f/4 (center/mid/edge) and 3 columns of spot size at f/10 (center/mid/edge). 5 is considered by most books to be perfect.  10 is fine as long as the spot stays round.  15 is OK and like many eyepieces at the edge. >20 is so-so.

The last column is the nature of the aberrations at that focal length, listed in importance from most to least, followed by a distortion %, if measured.

 

Conclusions: FC dominates from 5-7mm settings. Astigmatism is present from one end to the other.  Distortion is fairly high for the narrow fields.

f/10 performance is better than f/4, but not remarkably so except from the 3mm to 4mm settings.

It has much better performance than most inexpensive eyepieces except at the 3mm setting.

 

 

Edited April 21 by Don Pensack

[Mr Spock]

33 minutes ago, Don Pensack said:

It has much better performance than most inexpensive eyepieces except at the 3mm setting.

That isn't the finding of people who are using it. I find it just a little behind the 3.3mm TOE at the 3mm (3.5mm) setting.

[Louis D]

4 hours ago, Mr Spock said:

That isn't the finding of people who are using it. I find it just a little behind the 3.3mm TOE at the 3mm (3.5mm) setting.

On axis, off axis, edge, all?  At 3mm, it is very good across 70% of its field.  It's only in the outer portion it starts to fall apart in faster scopes.  I have no idea how well corrected the 3.3mm TOE is edge to edge.  Most folks only discuss on, or nearly on, axis performance of it and the Vixen HRs since few use these eyepieces for anything but planetary viewing or double star splitting.

[Mr Spock]

1 minute ago, Louis D said:

I have no idea how well corrected the 3.3mm TOE is edge to edge.

Sharp edge to edge.

[Louis D]

Has anyone discussed theories on the optical and mechanical organization of the Televue Nagler Zooms and the Svbony 3-8mm zoom?  My theory, they have a 4 element image forming positive group up top and a 2 element negative, Smyth, group down in the insertion tube.  To increase magnification, they simply move the two groups apart like a varifocal eyepiece such as my Speers-Waler 5-8mm "zoom".  This also maintains the size of the AFOV as in the S-W.  The mechanical trick to maintain near parfocality is to move the negative lens group downward during zooming in (higher power).  I have to do this with the focuser knob to maintain focus while zooming in with the S-W.

[John]

21 minutes ago, Louis D said:

Has anyone discussed theories on the optical and mechanical organization of the Televue Nagler Zooms and the Svbony 3-8mm zoom?  My theory, they have a 4 element image forming positive group up top and a 2 element negative, Smyth, group down in the insertion tube.  To increase magnification, they simply move the two groups apart like a varifocal eyepiece such as my Speers-Waler 5-8mm "zoom".  This also maintains the size of the AFOV as in the S-W.  The mechanical trick to maintain near parfocality is to move the negative lens group downward during zooming in (higher power).  I have to do this with the focuser knob to maintain focus while zooming in with the S-W.

The Nagler zooms are a 5 element design. 

They do seem to work in a similar way though. I have been using the Svbony 3-8 and Nagler 2-4 this evening. Both very good performers although one costs nearly 4x as much as the other 🙂

I wonder how the APM Super zoom works ?

 

 

 

Edited April 21 by John

[Don Pensack]

1 hour ago, Louis D said:

Has anyone discussed theories on the optical and mechanical organization of the Televue Nagler Zooms and the Svbony 3-8mm zoom?  My theory, they have a 4 element image forming positive group up top and a 2 element negative, Smyth, group down in the insertion tube.  To increase magnification, they simply move the two groups apart like a varifocal eyepiece such as my Speers-Waler 5-8mm "zoom".  This also maintains the size of the AFOV as in the S-W.  The mechanical trick to maintain near parfocality is to move the negative lens group downward during zooming in (higher power).  I have to do this with the focuser knob to maintain focus while zooming in with the S-W.

In order to maintain parfocality, the upper and lower sections must move apart but one moves up while the other moves down.

That's why they look like mushrooms at the shortest focal lengths.

The Nagler Zoom has 5 elements in a 3 segment upper and 2 segment lower.

Al Nagler told me once that he could make a 15-30 zoom with an 82° field in the same manner, but it would be too expensive to sell, too heavy to use, and very fragile.

Edited April 22 by Don Pensack

[Don Pensack]

5 hours ago, Mr Spock said:

That isn't the finding of people who are using it. I find it just a little behind the 3.3mm TOE at the 3mm (3.5mm) setting.

The center and 50% points are very good.  It's only at the edge the eyepiece image quality suffers.

As exemplified by the Moon photos and Ernest's measurements.

However, the figures at f/10 are very reminiscent of many inexpensive eyepieces.

It's only by f/4 (and likely f/5) that the eyepiece really falls down in edge quality, and then, likely outside the center 50% of the field.

[Louis D]

35 minutes ago, Don Pensack said:

The Nagler Zoom has 5 elements in a 3 segment upper and 2 segment lower.

I kind of wondered if/suspected based on the 50 degree AFOV.

[Nakedgun]

~

14 pages on this one eyepiece? 

Brother!

 

 

 

.

[Louis D]

14 hours ago, Nakedgun said:

14 pages on this one eyepiece? 

Brother!

Well, it's like having a minimum of 6 eyepieces in one, perhaps more if you dabble in half-mm focal lengths.  So, that makes just over 2 pages per covered focal length!  That doesn't seem so excessive then.😁

[Highburymark]

20 hours ago, Nakedgun said:

~

14 pages on this one eyepiece? 

Brother!

 

 

 

.

Probably a symptom of almost no new eyepieces for the past 4/5 years, combined with the unique popularity of the 3-8. 
What other launches have we seen? 
Tak TPL. Two new Pentax XWs. APM zoom. Maybe a couple of others I can’t think of.
Think this is the new normal. It’s a completely saturated market. SVB did well to exploit a rare remaining niche, where the only alternative is 4x the price. APM to a lesser extent with its zoom. 
 

 

 

 

[Stu1smartcookie]

13 hours ago, Highburymark said:

Probably a symptom of almost no new eyepieces for the past 4/5 years, combined with the unique popularity of the 3-8. 
What other launches have we seen? 
Tak TPL. Two new Pentax XWs. APM zoom. Maybe a couple of others I can’t think of.
Think this is the new normal. It’s a completely saturated market. SVB did well to exploit a rare remaining niche, where the only alternative is 4x the price. APM to a lesser extent with its zoom. 
 

 

 

 

Also a sympton of "bad weather Syndrome" as talking about equipment fills the void of actually being able to use it . IMO its a good EP for the money ... just about sums it up

 

[Mr Spock]

This eyepiece is popular. It's the best thing since sliced bread 

But let's be honest, you can't see much with sliced bread...

[Stu1smartcookie]

1 hour ago, Mr Spock said:

This eyepiece is popular. It's the best thing since sliced bread 

But let's be honest, you can't see much with sliced bread...

... true but you cant eat the EP 🤣

[Ratlet]

1 hour ago, Stu1smartcookie said:

... true but you cant eat the EP 🤣

I mean, you probably could.  Once 🤣

[Stu1smartcookie]

1 hour ago, Ratlet said:

I mean, you probably could.  Once 🤣

pass the salt please