High power (around 4mm) eyepiece tests and conclusions. **Round two!

[Mr Spock]

Tonight I had the opportunity to try out some of them on a razor sharp Saturn. For years it's been down in the mush and my previous best view was when it was overhead. Back then It was with an OMC-140.

Anyhow, not much to report. I used the LVW 3.5mm, Toe 3.3mm and Svbony at 3mm (3.5). They were all very good and I suspect you would be pleased with any of them.

On the moon it was the same as before - the LVW in front with the TOE a tad behind and the Svbony just behind that. Say 100%, 98% and 95% in terms of image quality. I didn't use the 4mm eyepieces tonight.

[josefk]

It's very interesting to see the characteristics of these EP exposed in the daylight as it were. That was a really helpful and even handed presentation of your analysis @Mr Spock - thanks 👍

[Mr Spock]

New phone (iPhone 16), new pics. 

Nirvana 4mm (not all the fov would fit in)

TOE 4mm

Svbony 3-8 @ 4mm 

LVW 3.5

And finally, a direct comparison of the centre. When doing planetary, this is where you want your image quality.

As always, eye of the beholder and all that 

 

 

[Ratlet]

Cheers for the update.  Just want to make sure I understand the terminology and what I'm seeing, but the field stop on the lvw looks razor blade sharp compared to the others.

[Mr Spock]

17 minutes ago, Ratlet said:

but the field stop on the lvw looks razor blade sharp compared to the others.

The Nirvana field stop is outside the image (82°!), there's only the middle here. The TOE and LVW are as is, the Svbony is a bit more difficult to focus than the others. They are all sharp visually, differences here are just down to getting the focus right and getting everything in the camera's depth of field.

[Louis D]

Nice results.  As I reported in my review, the Svbony 3-8mm is just okay at the edge at 4mm, and worse yet at 3mm (really 3.5mm).  The LVW reminds me of my 3.5mm XW.  It is sharp edge to edge with high contrast and a razor sharp field stop.

Try repeating your testing with the ultrawide angle camera on your phone.  It will capture the entirety of the UWAN AFOV and decrease image magnification, making eyepiece sharpness differences more pronounced.  Also, use some sort of lower contrast target in the center such as those stipple patterns of dots in the upper right and left quadrants.  Some sort of pale wood grain pattern slightly side-lit is also an excellent low contrast target.

Ideally, we'd both be using some sort of targets as used in camera lens testing:

The problem comes in with trying to maintain constant target image size on the sensor for different magnifications.  That, and these are all high contrast targets.  The real test comes with low contrast targets.

[Mr Spock]

6 hours ago, Louis D said:

As I reported in my review, the Svbony 3-8mm is just okay at the edge at 4mm, and worse yet at 3mm (really 3.5mm).

This is between the 3mm and 4mm settings, so the real 4mm... The click stops are 3.5mm and 4.4mm.

I can zoom out on the camera to fit the Nirvana in. These are at 0.8.

I should point out I've not changed the colour balance in these images and they all have the same processing. The paper is bright white and the ruler is steel. Lighting is from an LED house light.

[Mr Spock]

So, in the real world:

Nirvana 4mm: Sharp to the edge, but not 'biting' like the TOE and LVW. Very sharp with high contrast targets like the moon. Great for studying lunar detail over a wide area. Eye position is critical. I like the Nirvanas for double stars.

TOE: As good as it gets. Sharp detail on low contrast targets such as Jupiter. Also sharp edge to edge. Negative: very expensive.

Svbony: Very sharp in the centre - edges are poor. Ideal for planetary - a bit frustrating on the moon. Poor eye relief - I have to turn the rubber eyecup down. Best value for money eyepiece there is.

LVW: As good as it gets, but with 65° and 20mm eye relief. My favourite lunar eyepiece. The TOEs (4mm and 3.3mm) are just as sharp, but the LVW has an indescribable 'sense of ease' to the view. Very enjoyable. Negative: no longer available and a long wait until one comes up used.

[John]

Do these tests get at the inherant characteristics of eyepieces rather than how they behave combined with certain scope types / specifications ?

Interesting stuff  🙂

[Mr Spock]

45 minutes ago, John said:

Do these tests get at the inherant characteristics of eyepieces rather than how they behave combined with certain scope types / specifications ?

These are with the FS-60CB; they look similar with the FC-100. I've not noticed any practical observing differences with the 12" f5 Newt. That would be hard to use for tests like these due to its size.

The only real difference I noted was the field curvature of the Svbony. It seems better suited to the Newt.

[Mr Spock]

There's a possibility I could get the 6" CC inside for tests. At f12 though I wouldn't expect it to provide a challenge to any of these eyepieces.

[John]

Duplicate post. Please ignore.

 

Edited 21 hours ago by John

[John]

23 minutes ago, Mr Spock said:

There's a possibility I could get the 6" CC inside for tests. At f12 though I wouldn't expect it to provide a challenge to any of these eyepieces.

I thought that classical cassegrains exhibit field curvature themselves (as do some other optical designs) so how would you separate that out from optical characteristics inherant to the eyepiece ?

 

[Mr Spock]

5 minutes ago, John said:

I thought that classical cassegrains exhibit field curvature themselves (as do some other optical designs) so how would you separate that out from optical characteristics inherant to the eyepiece ?

It's not something I've seen visually.

[Mr Spock]

All scopes, unless they are designed as flat field, will have field curvature. I'm not sure how much eyepiece designers have this in mind when designing an eyepiece.
I believe the curvature is called the Petzval surface.

[Zermelo]

2 hours ago, John said:

Do these tests get at the inherant characteristics of eyepieces rather than how they behave combined with certain scope types / specifications ?

Interesting stuff  🙂

I've wondered that too. In particular, I believe field curvature can sometimes be accommodated by the observer's eye, when moving from centre of field to edge, depending on the amount of FC and the capabilities of the particular eyeball. An image obviously can't replicate that, and will just show fuzziness at the edge (assuming that the central portion has been focused).
Is it possible to examine the out-of-focus area in the test image and determine if it is (potentially not an issue) FC rather than some other aberration?

[vlaiv]

Just now, Zermelo said:

Is it possible to examine the out-of-focus area in the test image and determine if it is (potentially not an issue) FC rather than some other aberration?

Very difficult if not impossible.

Best thing to do would be to have manual focus lens, use single star / artificial star and measure focus difference between - the best focus in center of EP and the best focus at the edge.

Then repeat above image two times - one that will focus at center of the image and other that will focus camera lens at the edge of the image (by using measured focus offset).

I think that from focus offset and parameters of the lens one could calculate dioptre difference which would translate into ability of one's eye to adjust the focus and also it would isolate field curvature from other aberrations in the second image.

[josefk]

16 hours ago, Ratlet said:

... the field stop on the lvw looks razor blade sharp compared to the others.

"wanted - 3.5mm LVW" LOL.

I use the field stop as an asset all the time on Saturn trying to pop out the dimmer moons. I think i might actually observe Saturn longer with it just outside the FOV than i do with it inside the FOV 🙂 🤷‍♂️

[josefk]

59 minutes ago, Mr Spock said:

All scopes, unless they are designed as flat field, will have field curvature. I'm not sure how much eyepiece designers have this in mind when designing an eyepiece.
I believe the curvature is called the Petzval surface.

it's why Tele Vue use a Petzval for EP development (and i think testing). i also seem to think, but may be dreaming it, that this was even the primary initial motivator for the NP petzval series before offering it for sale to the public...

[Mr Spock]

3 minutes ago, josefk said:

"wanted - 3.5mm LVW" LOL.

It took years for me to find mine 

[Mr Spock]

According to what I've read, a Petzval lens corrects for coma and SA but makes field curvature worse.

[josefk]

Just now, Mr Spock said:

According to what I've read, a Petzval lens corrects for coma and SA but makes field curvature worse.

no i don't think so - I may need to do some more reading myself - but i was fully under the impression the designs "primary" corrective asset was for field curvature. That's why they are favoured for AP because they're flat for the sensor...

On the other hand other lens aberrations such as SA still need attention to the glass and the figure etc. and are not inherently or automatically corrected by the petzval lens arrangement per se.

[vlaiv]

I think that you both correct - but you are probably talking about different things.

One is Petzval lens design and other is Petzval-Nagler or rather Nagler-Petzval telescope design.

First has a lot of field curvature and later is flat on 50mm diameter.

[josefk]

3 minutes ago, vlaiv said:

I think that you both correct - but you are probably talking about different things.

One is Petzval lens design and other is Petzval-Nagler or rather Nagler-Petzval telescope design.

First has a lot of field curvature and later is flat on 50mm diameter.

i've just come to the same realisation with a bit of googling triggered by this thread - i didn't realise in the photography world (and the original design invention) it was so different to its application in astro. Possibly/probably why Tele Vue and Tak sometime append the description with "modified" - "modified Petzval"

[Louis D]

24 minutes ago, josefk said:

"wanted - 3.5mm LVW" LOL.

Try the 3.5mm Pentax XW until then.  I have yet to find fault in mine.  They're $269+tax here in the US.