50 replies to this topic

[CrazyPanda]

My understanding about FC is that if the eyepiece FC matches the scope FC, it cancels out - that is ) ) or ( ( results in the minimization of visible FC.

 

I have a couple of finder scopes with short focal lengths (225mm and 160mm) that accept 1.25" eyepieces. The eyepiece I'm using in my 225mm focal length finder is good but definitely shows the FC inherent to the finder. This means faint stars near the edges can be mistaken for galaxies or nebulae and you need to center them to see if what you're looking at is actually the target or not.

 

I was wondering if anyone knows of eyepieces that either deliberately or serendipitously are good for use in finder scopes or otherwise short focal length refractors.

[Cotts]

I remember trying to start a thread on this years ago and, for the most part, got ignored.

 

I still think that the field curvature of eyepieces (in terms of the radius of the curvature) should be published by the manufacturers as much as apparent field and focal length.   It is an important statistic because, as you say, the radius of curvature of these shorter Newtonians of f/4 and f/3 is absolutely NOT a trivial phenomenon and could, perhaps, be matched by choosing the right eyepiece.... (This idea won't beat coma, though....

 

If you want to start another thread to be ignored try requesting the accuracy of the figure of the glass in eyepieces - the "strehl" of the eyepiece as a metaphor - its wavefront error.  I did and lots of smart people told me it wasn't important.

 

Then there is the 'colour' imparted by eyepieces.  This one is 'warm', while this other one is 'cool'.  Surely a band-pass report from 400 to 700 nm could easily be done by the manufacturer and included.  

 

We pay lots of money for high-end eyepieces and can only receive anecdotal reports about them - we rely on the reputation of the manufacturer.

 

Dave

[SeattleScott]

It is easiest to find informative reviews on high end gear (like Delos versus Morpheus, Ethos versus HW) because the people who are really hardcore tend to have nice gear. One can find an extensive amount of details on 7XW versus 7SW versus 7 Delite versus 6.7 Morpheus. But one will struggle to find the same level of detail for 17 Hyperion versus 18UFF versus 16 UWA. The people who really know what they are doing generally aren’t using low to midtier eyepieces that have field curvature that might match a short refractor/finder. The people who have low to midtier eyepieces with field curvature typically don’t have three other eyepieces of the same focal length in order to compare and see which shows the least FC. If they could afford a few of the same focal length, they would just buy one premium eyepiece instead. So there very well could be an eyepiece that does a reasonable job for what you seek, but it probably isn’t common knowledge, because people buying this tier of eyepieces are rarely buying multiples and comparing them. And even if they do, users at this level might struggle to distinguish FC from astigmatism, coma, etc.

[Lazaroff]

See what you can find out about Erfle eyepieces. I believe at least some have the direction of field curvature you're looking for. 

[Astrojensen]

the radius of curvature of these shorter Newtonians of f/4 and f/3 is absolutely NOT a trivial phenomenon 

Compared to refractors and SCTs, the field curvature of Newtonians is indeed trivial. The radius of curvature for a Newtonian is 95% of its focal length. It's 35% (on average, non-Petzval designs) for a refractor and 25% for an SCT (the classic, non-Edge SCT). 

 

This means that a 150mm f/4 Newtonian has the same field curvature as a 1600mm focal length refractor or a 2280mm focal length SCT... 

 

 

Clear skies!

Thomas, Denmark

[russell23]

The way I look at this is to evaluate which eyepieces seem to perform the best across the field with my refractors.  The performance is a combination of focal ratio and focal length of the scope.

 

For example, eyepieces that struggle some at the edges are always better near the edges in my AT115 f/7 Triplet (805mm FL) than my 94mm f/6.8 (640mm FL) Vernonscope Brandon Triplet.  But I don't have any eyepieces that struggle near the edges of my 102mm f/11 ED refractor. 

 

The last time I had the 94mm Brandon out the only eyepiece in my signature that was still sharp near the field stop was the 14mm Delos.  Most of the others varied from quite a bit of edge falloff to really bad.  Strangely enough the 20mm Brandon does pretty well in all my scopes.  Including the AT 72 EDII. 

 

So I kind of adapt which eyepieces I emphasize using on a given night to the refractor I take out.  Last night I had my 102mm f/11 out.  I ended up using the 2" 32mm University Konig (with and without a 2x barlow) almost exclusively for the last 2 hours.   It actually presented the best view in the lower transparency skies I had last night.   The 32mm Konig is a mess in the 94mm Brandon.

[Jon Isaacs]

 

I was wondering if anyone knows of eyepieces that either deliberately or serendipitously are good for use in finder scopes or otherwise short focal length refractors.

 

Dave:

 

Finders have short focal lengths and are very fast, typically F/4 or somewhat faster. I have several RACI finders that take 1.25 inch eyepieces. They're a bit unusual in that the Astro-Tech 50 mm is about F/5, the 70 mm is based on a Carton 70 mm F/4.5 air spaced objective, the only normal finder is a StellarVue 50 mm that F/4.

 

At F/4, you need eyepieces that are well corrected for astigmatism if you want, clean sharp views. That's where it all starts. Correcting for field curvature is a second order priority.

 

The catch 22 is that eyepieces that are sharp across the field at F/4, they also have flat fields. 

 

I have heard that Vic Maris recommends the 16 mm Type 5 Nagler as the sharpest finder eyepiece. I don't always agree with Vic but in this case, I do agree. At F/4, the 16 mm Type is so sharp across the field that the field curvature seems nonexistent. 

 

My friend Bruce likes the 13 mm Type 6, I imagine the 24  Panoptic is also quite sharp, the 22 mm Panoptic seems very sharp.

 

I know you're hoping to correct for field curvature but my experience is if you using high quality eyepieces, like the TeleVues, the field curvature takes care of itself.

 

Jon

[Starman1]

The information the OP requests doesn't exist in public information.
The only exception I know about is Pentax XW 70° eyepieces, where the FC and astigmatism information was published many years ago and can be found in the web archive.
In general, IF the information is known at all, it is proprietary and the manufacturers don't want it known.

[CrazyPanda]

Dave:

 

Finders have short focal lengths and are very fast, typically F/4 or somewhat faster. I have several RACI finders that take 1.25 inch eyepieces. They're a bit unusual in that the Astro-Tech 50 mm is about F/5, the 70 mm is based on a Carton 70 mm F/4.5 air spaced objective, the only normal finder is a StellarVue 50 mm that F/4.

 

At F/4, you need eyepieces that are well corrected for astigmatism if you want, clean sharp views. That's where it all starts. Correcting for field curvature is a second order priority.

 

The catch 22 is that eyepieces that are sharp across the field at F/4, they also have flat fields. 

 

I have heard that Vic Maris recommends the 16 mm Type 5 Nagler as the sharpest finder eyepiece. I don't always agree with Vic but in this case, I do agree. At F/4, the 16 mm Type is so sharp across the field that the field curvature seems nonexistent. 

 

My friend Bruce likes the 13 mm Type 6, I imagine the 24  Panoptic is also quite sharp, the 22 mm Panoptic seems very sharp.

 

I know you're hoping to correct for field curvature but my experience is if you using high quality eyepieces, like the TeleVues, the field curvature takes care of itself.

 

Jon

 

I'm not sure astigmatism is as bad as FC in some cases.

I've been testing various eyepieces to upgrade my current SV 60mm finder because I've been wanting more magnification.

 

On the short list were the 15mm TV Delite, and the 14mm Oberwerk: https://oberwerk.com...1-25-eyepieces/

 

The finder is F/3.75.

Both eyepieces have just about the same effective field stop size (I measured the Oberwerk with a star drift) so both will experience the same degree of FC at the focal plane.

 

The 15mm DeLite certainly puts up sharper stars across the field with very little refocusing needed. The 14mm Oberwerk requires quite a bit more refocusing. When I do refocus to get the stars sharper at the edges, they are considerably tighter and better than if I don't refocus. There is more astigmatism present than in the TV DeLite, but star size shrinks to about 1/5th the size vs when I don't re-focus. Stars at the edge of the Oberwerk are indistinguishable from galaxies or small nebulae.

 

This makes FC the greater of the two evils. It's likely that the Oberwerk's FC direction is opposite that of the finder.

That's what prompted this thread.
 

Edited by CrazyPanda, 05 August 2024 - 02:20 PM.

[Olimad]

In my F4 newton (400 mm FL), I have found that the 6mm redline stacked with a 0.5x FR reduce drastically the astigmatism. I think its like a 10-12mm fl. 

 

The view is brighter, more than the 9mm alone. I find It a bit disturbing, but at dark site, views of stars fields are really greats. Better than my others eyepieces, because It offers wide field ( approx 2° -in comparison with other EPs with known characteristics- at pprox 36x), and almost no astigmatism and focused on almost all the field of view. The other night, with great seeing and Oiii, M8 (for exemple) was stunning, and really defined, offering more context than my KUO 7mm alone (which was great too).

 

For me It works well. May be worth a try, i don't know if It works with other scopes or eyes, but It is really cheap.

Edited by Olimad, 05 August 2024 - 04:23 PM.

[Jon Isaacs]

The 15mm DeLite certainly puts up sharper stars across the field with very little refocusing needed. The 14mm Oberwerk requires quite a bit more refocusing. When I do refocus to get the stars sharper at the edges, they are considerably tighter and better than if I don't refocus. There is more astigmatism present than in the TV DeLite, but star size shrinks to about 1/5th the size vs when I don't re-focus. Stars at the edge of the Oberwerk are indistinguishable from galaxies or small nebulae.

 

This makes FC the greater of the two evils. It's likely that the Oberwerk's FC direction is opposite that of the finder.

 

 

Which eyepiece provides sharper stars without refocusing?  It seems to be Delite by a wide margin.

 

When the Oberwerk is refocused, is it sharper at the edge than the Delete when it's not refocused?

 

Do you consider it realistic to refocus a finder to get sharper stars at the edge and then refocus to get sharp stars in the center? I certainly don't. I focus the finder and just leave it. It's easier to move the star field by moving the scope. 

 

I'm not worried about seeing galaxies in the finder scope, the galaxies I am generally looking for are hard enough to see in the main scope. 

 

You said the Delite is sharper than the Oberwerk and requires very little refocusing. That is telling me that it's the freedom off-axis astigmatism that's most important.. 

 

That said, the 16 mm Type 5 provides a very sharp field in a 50 mm F/4 finder.

 

Jon.

Edited by Jon Isaacs, 05 August 2024 - 02:53 PM.

[CrazyPanda]

When the Oberwerk is refocused, is it sharper at the edge than the Delete when it's not refocused?

 

Yes, marginally so.

 

 

 

Do you consider it realistic to refocus a finder to get sharper stars at the edge and then refocus to get sharp stars in the center?

 

No, hence the purpose of this post. I'm wondering if anyone has found a solid "finder scope eyepiece" that seemingly, for whatever reason, negates the FC of the finder itself so that refocusing is not needed in order for stars at the edge to be small enough that they aren't mistaken for other DSO targets.

 

 

 

You said the Delite is sharper than the Oberwerk and requires very little refocusing. That is telling me that it's the freedom off-axis astigmatism that's most important..

 

Well no...

 

I explained why.

 

If I'm using the Oberwerk, FC is significantly worse than the DeLite. This makes stars at the edge significantly more bloated than the DeLite. I have to refocus the DeLite a tiny bit. I have to refocus the Oberwerk more. When both are refocused appropriately, stars are acceptable. Very good in the DeLite, acceptable in the Oberwerk. Therefore FC is the dominant aberration.

 

If I were to rank edge quality from best to worst, it would go like this:

1. TV DeLite - focused for the edge
2. Oberwerk - focused for the edge
3. TV DeLite - focused for the center
4. Oberwerk - focused for the center

Put another way - refocusing the DeLite = marginal improvement. Refocusing the Oberwerk = big improvement.

Edited by CrazyPanda, 05 August 2024 - 03:10 PM.

[Starman1]

What happens when you focus 1/2 way from center to edge?  Does your eye have enough accommodation for that to work?

Otherwise, I think that finding the eyepiece that matches perfectly will be hit or miss.  You might have to try a lot of eyepieces.

A lot of inexpensive 50mm finders have a lot of astigmatism at the edge of the field no matter what eyepiece is used.

I sold a Stellarvue F50 that did not have a lot of astigmatism, and I could kick myself for doing so.

It was excellent, by the way, paired with a 24mm Panoptic.

[SeattleScott]

In my F4 newton (400 mm FL), I have found that the 6mm redline stacked with a 0.5x FR reduce drastically the astigmatism. I think its like a 10-12mm fl.

The view is brighter, more than the 9mm alone. I find It a bit disturbing, but at dark site, views of stars fields are really greats. Better than my others eyepieces, because It offers wide field ( approx 2° -in comparison with other EPs with known characteristics- at pprox 36x), and almost no astigmatism and focused on almost all the field of view. The other night, with great seeing and Oiii, M8 (for exemple) was stunning, and really defined, offering more context than my KUO 7mm (which was great too).

For me It works well. May be worth a try, i don't know if It works with other scopes or eyes, but It is really cheap.

Panda is using achro finder scopes not reflectors so field curvature will present differently than with a reflector.

Honestly I’m quite surprised that a .5x reducer reportedly works well on a F4 newt. These reducers have a reputation for poor edge performance outside of the central area. I suppose with a 6mm eyepiece, you are mostly just using the central area of the reducer, so the performance might be respectable. Normally what you hear is people wanting to use these reducers with a max FOV 1.25” eyepiece rather than just filling a gap in their eyepiece lineup. Now I am a little curious about trying my .5x reducer visually at F4 to see at what point the edge correction suffers.

[CrazyPanda]

What happens when you focus 1/2 way from center to edge?  Does your eye have enough accommodation for that to work?

Otherwise, I think that finding the eyepiece that matches perfectly will be hit or miss.  You might have to try a lot of eyepieces.

A lot of inexpensive 50mm finders have a lot of astigmatism at the edge of the field no matter what eyepiece is used.

I sold a Stellarvue F50 that did not have a lot of astigmatism, and I could kick myself for doing so.

It was excellent, by the way, paired with a 24mm Panoptic.

I have *juust* enough accommodation but it takes some effort. I can't just glance through the finder and see what's going on with both my central and peripheral vision.

 

So let's say I'm aiming for some NGC galaxy or even a planetary nebula and I think I'm close. When I focus half way, basically nothing is in focus at a glance. To see if the desired target is in the field of view, I have to look specifically all around the field to let my eyes focus the stars and see if the fuzzy patch is just fuzzy stars or the target.

 

Just takes more work.

Edited by CrazyPanda, 05 August 2024 - 04:11 PM.

[Olimad]

Panda is using achro finder scopes not reflectors so field curvature will present differently than with a reflector.

Honestly I’m quite surprised that a .5x reducer reportedly works well on a F4 newt. These reducers have a reputation for poor edge performance outside of the central area. I suppose with a 6mm eyepiece, you are mostly just using the central area of the reducer, so the performance might be respectable. Normally what you hear is people wanting to use these reducers with a max FOV 1.25” eyepiece rather than just filling a gap in their eyepiece lineup. Now I am a little curious about trying my .5x reducer visually at F4 to see at what point the edge correction suffers.

Sorry Panda, havn't thought about refractor.

 

Only a small add, from all my eyepieces It only works with my redline 6mm. I am not ising parracor.

[luxo II]

I was wondering if anyone knows of eyepieces that either deliberately or serendipitously are good for use in finder scopes or otherwise short focal length refractors.

Yes - Erfle, and derivatives.

Edited by luxo II, 05 August 2024 - 08:14 PM.

[Jon Isaacs]

If I'm using the Oberwerk, FC is significantly worse than the DeLite. This makes stars at the edge significantly more bloated than the DeLite. I have to refocus the DeLite a tiny bit. I have to refocus the Oberwerk more. When both are refocused appropriately, stars are acceptable. Very good in the DeLite, acceptable in the Oberwerk. Therefore FC is the dominant aberration.

 

 

See if you can find a 16mm Type 5 Nagler to try.  It sure looks good to my eyes.. 

 

Jon

[CrazyPanda]

See if you can find a 16mm Type 5 Nagler to try. It sure looks good to my eyes..

Jon

I have no doubt. I had a 12 T2 for a while and used it in my SV 60 finder and it was perfect to the edge. The only issue is I prefer to use glasses for looking through the finder since I wear them to look at star charts, and the 16 T5 is short on ER. I suppose if necessary I can always adapt.

Edited by CrazyPanda, 05 August 2024 - 08:40 PM.

[Jon Isaacs]

I have no doubt. I had a 12 T2 for a while and used it in my SV 60 finder and it was perfect to the edge. The only issue is I prefer to use glasses for looking through the finder since I wear them to look at star charts, and the 16 T5 is short on ER. I suppose if necessary I can always adapt.

 

The 16mm T5 is very sharp but has limited eye relief.  It seems hopeless.. 

 

I wonder how one of the Morpheus's would work?  I have the set but it will be sometime before I get out to skies dark enough to really see how well corrected the edge is.. 

 

Jon 

[CrazyPanda]

 

The 16mm T5 is very sharp but has limited eye relief.  It seems hopeless.. 

 

I wonder how one of the Morpheus's would work?  I have the set but it will be sometime before I get out to skies dark enough to really see how well corrected the edge is.. 

 

Jon 

Good question. I will try my 9.

 

I did try one of my Zeiss E-PL microscope eyepieces with adjustable focus mechanism. The focus mechanism seems to change the field curvature characteristics and I'm able to sharpen the view right to the edge with the eyepiece. The downside is it's 25mm, which is not the focal length I'm looking for and it doesn't have any threads to use with a thread-in barlow cell. It also seems like a sacrilege to relegate a high-end Zeiss eyepiece to finder scope duty
 

It's too bad there isn't a premium finder scope on the market that is fully end-to-end optimized to be both extremely light weight, but have solid edge correction, long eye relief, and adjustable magnification (either through interchangeable optimized eyepieces or integrated zoom mechanism). The StellarVue finders come close. They are light for their size, but the stock eyepiece is hot garbage - not enough magnification, too narrow a field of view, mediocre edge performance (mix of astig and FC plagues the view). If you want to bring your own eyepiece to the table... well here we are in this thread.

[CrazyPanda]

Yes - Erfle, and derivatives.

Is there a particular one you might recommend?

[luxo II]

Well... the SWA eyepieces are Erfle derivatives and some (the 20mm SWA) have quite severe field curvature that would suit really short refractors. Conversely pretty bad in a real flat field refractor (I have one), maks (I have a few) and horrid in Newtonians.

[Starman1]

Good question. I will try my 9.

 

I did try one of my Zeiss E-PL microscope eyepieces with adjustable focus mechanism. The focus mechanism seems to change the field curvature characteristics and I'm able to sharpen the view right to the edge with the eyepiece. The downside is it's 25mm, which is not the focal length I'm looking for and it doesn't have any threads to use with a thread-in barlow cell. It also seems like a sacrilege to relegate a high-end Zeiss eyepiece to finder scope duty
 

It's too bad there isn't a premium finder scope on the market that is fully end-to-end optimized to be both extremely light weight, but have solid edge correction, long eye relief, and adjustable magnification (either through interchangeable optimized eyepieces or integrated zoom mechanism). The StellarVue finders come close. They are light for their size, but the stock eyepiece is hot garbage - not enough magnification, too narrow a field of view, mediocre edge performance (mix of astig and FC plagues the view). If you want to bring your own eyepiece to the table... well here we are in this thread.

It would have to have a triplet objective and have a built in field flattener, or be like the Tele Vue refractors and have a long focal length objective that is reduced and flattened by an internal lens.

It would be expensive and probably not sell as a finder, like:

Sharpstar 50EDPH APO Refractor--reducible to f/4.6 (231mm focal length)--2.2 lbs.

[quilty]

See what you can find out about Erfle eyepieces. I believe at least some have the direction of field curvature you're looking for.

yeah, I had the same idea a year before. For instance the ES 26 mm 62° seems to be an Erfle type ad seems to partially mend the field curvature of the Bresser 102/460 mm frac.
Like that combination