50 replies to this topic

[CrazyPanda]

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.

I can't imagine it would have to be that complicated. Surely if you designed the objective, prism, and eyepiece together as a complete system and were maybe willing to use aspheric surfaces, you could produce something that performs better as a finder scope than a generic refractor with a generic eyepiece. Doesn't have to be full apo quality, just enough to offer a wider, better corrected field with more magnification than standard finder offerings.

Edited by CrazyPanda, 06 August 2024 - 07:34 AM.

[Jon Isaacs]

A few years ago I had the idea of making a 50 mm finder from an old 200 mm F/4 Minolta camera lens. Camera lenses should have flat fields.. 

 

To make an RACI finder disassembly was required so at this point, it's a box of parts, an idea that never happened.

 

Jon

[thecelloronin]

A few years ago I had the idea of making a 50 mm finder from an old 200 mm F/4 Minolta camera lens. Camera lenses should have flat fields.. 

 

To make an RACI finder disassembly was required so at this point, it's a box of parts, an idea that never happened.

 

Jon

What would you need to go forward with this project? Are you saying disassembly was the barrier, or reassembly in the new housing?

[Tropobob]

I have a William Optics finder (50mm, F4), which can accept regular 1.25" EPs.

 

Because of this thread, I tried some experimenting to see how well or poorly other EPs perform.  (Note: Tests were done during the day, as I could not be bothered with this at night.)

 

At low magnifications (10x or below), a plossl works fine, but then becomes awful with more X.

 

I compared a 16mm Nagler, a 15mm DeLite and a 12.5mm Morpheus and found that for me, the 12.5 Morpheus was the clearly the winner.   That said, it did not seem like a great setup to use for stargazing. 

(Too low a magnification combined with too much distortion away from the centre.) 

 

 

[iKMN]

 

 

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.

I’ve never looked through one but I’ve read the Takahashi 30mm and 50mm straight through finder scopes are amazing.  I saw a post here or on SGL that TS optics sells a right angle adapter for the Tak finder scopes I could have sworn it took 1.25 eyepieces.  I use a laser pointer finder with all my scopes so I didn’t pay much attention to the Tak finder scope post.  High quality finder but probably has a bit of field curvature

 

Also if I recall correctly a few of the WO redcat scopes can be modified for visual but I am not sure which models.  I’m pretty sure they are flat field quads.  That might work as a finder scope.  Sorry if I’m giving out bad info but might be worth researching.

 

k

[Sarkikos]

Field curvature / astigmatism schema for the XW's:

 

Mike

Attached Thumbnails

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Edited by Sarkikos, 08 August 2024 - 03:07 PM.

[Starman1]

On the Pentax charts.

Lines lean right--positive field curvature

Lines lean left--negative field curvature

Lines deviate--indicates astigmatism

Figures at the bottom ate diopters.

Straight line axis in the middle = perfectly flat field.

DS = Sagittal field curvature

DM = Meridional field curvature

 

Examples:

5mm--least astigmatism of all the focal lengths

30mm--flattest field of all of the focal lengths

[Sarkikos]

It looks like the 40 XW is nearly as flat as the 30 XW.  

 

In the field, I've seen the most field curvature from the 20 and 14 XW's.  I didn't notice FC from the 10 to 3.5 XW's.  This was in a 10" f/5 Dob with a Paracorr.  

 

Mike

Edited by Sarkikos, 08 August 2024 - 04:44 PM.

[j.gardavsky]

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.

I have the Carl Zeiss E-Pl 10x/20 (f=25mm) , with the cross graticule in my finder scope, and it works fine.

 

You can adjust the field curvature in the eyepieces with the intrafocal focusing, even if the intrafocal focusing has been developed by Leica and Carl Zeiss to primarirly focus the vision on the measurement graticule.

Such eyepieces in my arsenal are:

Carl Zeiss E-Pl 10x/25 #44 42 34

Carl Zeiss S-Pl 10x/20 # 44 40 39

Leica HC Plan S 10x/22 #507807

Leica HC Plan 10x/25 Brille M #507800

Leica HC 12,5x/16 Plan M #506515

Carl Zeiss W-Pl 16x/16 #45 50 48

 

The drawback is,

when you have adjusted the intrafocal focusing, sharp on the firld stop or sharp on the graticule, then you need to get the eyepieces as a whole into the focus with the telescope. But this is done on the telescope focuser, as usual.

 

Best,

JG

 

 

 

 

[CrazyPanda]

On the Pentax charts.

Lines lean right--positive field curvature

Lines lean left--negative field curvature

Lines deviate--indicates astigmatism

Figures at the bottom ate diopters.

Straight line axis in the middle = perfectly flat field.

DS = Sagittal field curvature

DM = Meridional field curvature

 

Examples:

5mm--least astigmatism of all the focal lengths

30mm--flattest field of all of the focal lengths

What direction is the FC of a typical refractor?
 

[Starman1]

What direction is the FC of a typical refractor?
 

Negative.

If field curvatures match, you see a flat field.  If field curvatures oppose, you see egregious FC at the edges.

[CrazyPanda]

Negative.
If field curvatures match, you see a flat field. If field curvatures oppose, you see egregious FC at the edges.

Darn, so that rules out the 14xw

[quilty]

a typical doublet refractor has negative FC? Is the off axis fl then longer or shorter than on axis?

A typical Erfle ep has positive FC?

[Starman1]

Darn, so that rules out the 14xw

Other 14s with long eye relief to consider:

Tele Vue Delos 72° 1.25"

Baader Morpheus 78.3° 1.25" or 2" **

Nikon NAV-SW 72° 1.25"

Long Perng/Orion/Stellalyra/Founder Optics 80° 2"

(**used as 2", you need 21mm + the height of the 1.25" adapter as additional outfocus)

 

I can vouch the 14mm Morpheus has only a trace of FC in my 102mm f/7 apo.

Ditto on the Long Perng 80°.

On the Long Perng, to make them have enough effective eye relief for glasses, you need to replace the eyeguard.

[Sarkikos]

Negative.

If field curvatures match, you see a flat field.  If field curvatures oppose, you see egregious FC at the edges.

Refractors have negative FC?  Newtonians have positive FC?  

 

Years ago, I had all the XW's except the 40 and 30.  In my 10" f/5 Dob with Paracorr, I saw FC in the 20 and 14, but not in the 10 through 3.5.   And yet the 20 and 14 have positive FC and the 10 through 3.5 have negative FC.   The visibility of FC should have been the opposite of what I saw.  

 

Mike

[Starman1]

https://www.telescop...t/curvature.htm

[Sarkikos]

https://www.telescop...t/curvature.htm

Can you translate this for those who don't speak math?    In fig. 60, which curvature is considered positive, which is negative?  I wish Vlad would give a simple verbal summation of the main points at the end of each page of his website for those who are not math mavens.  I read through his articles looking for summations, but he doesn't always provide them. 

 

What I know is what I've experienced in the field:  Field curvature in the 20 and 14 XWs, no field curvature in the 10 through 3.5 XWs, in a 10" f/5 Newt with Paracorr.  I know I've read reports from others with fast Newts who have seen FC in the 20 and 14 XWs.  This is pretty much common knowledge.  

 

If the 20 and 14 XWs have positive FC according to the schema in post #31, Newtonians have positive FC, and matching FC cancel out, why would I see FC through those two eyepieces in a Newtonian?  And why would I not see FC through the 10-3.5 XWs, although they have negative FC, the opposite of the positive FC of Newtonians?

 

Mike

Edited by Sarkikos, 08 August 2024 - 08:55 PM.

[Starman1]

Bruce MacEvoy explains it a little better:

https://www.handprin....html#curvature

Positive field curvature is when the focal plane is convex toward the observer (i.e. the edges are farther away from the eye),

and negative field curvature is when the focal plane is concave toward the observer (i.e. the edges are closer to the eye).

The most salient point:

"However, in eyepieces, where the focus is at the image plane, positive curvature means the bowl is facing toward the observer. Thus, if the objective has a positive curvature (as most telescope objectives do), and the eyepiece has a negative curvature, then the two focal surfaces will curve in the same direction..."

 

Vlad Sacek discusses it further:

https://www.telescop...Petzval surface.

Go way down the page to Eyepiece Field Curvature

Edited by Starman1, 08 August 2024 - 10:29 PM.

[CrazyPanda]

Can you translate this for those who don't speak math?    In fig. 60, which curvature is considered positive, which is negative?  I wish Vlad would give a simple verbal summation of the main points at the end of each page of his website for those who are not math mavens.  I read through his articles looking for summations, but he doesn't always provide them. 

 

What I know is what I've experienced in the field:  Field curvature in the 20 and 14 XWs, no field curvature in the 10 through 3.5 XWs, in a 10" f/5 Newt with Paracorr.  I know I've read reports from others with fast Newts who have seen FC in the 20 and 14 XWs.  This is pretty much common knowledge.  

 

If the 20 and 14 XWs have positive FC according to the schema in post #31, Newtonians have positive FC, and matching FC cancel out, why would I see FC through those two eyepieces in a Newtonian?  And why would I not see FC through the 10-3.5 XWs, although they have negative FC, the opposite of the positive FC of Newtonians?

 

Mike

Newtonians have very minor FC compared to the equivalent focal length refractor. Astrojensen above said this:

 

The radius of curvature for a Newtonian is 95% of its focal length. It's 35% (on average, non-Petzval designs) for a refractor

 

So in your 1250mm focal length Newt, radius of curvature is roughly 1190mm. Meanwhile in the same FL refractor, the radius of curvature would be just 440mm. So chances are the FC of the Pentaxes is quite a bit steeper than the Newt. FC is partially cancelled, but not fully. Meanwhile it would seem the 14 and 20 would be downright awful in a refractor - shorter ROC & opposite sign.
 

[Sarkikos]

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.

If it's a small aperture finder, I wouldn't bother trying to actually see most galaxies or planetary nebulae in the finder.   Instead, look at the location of the object on a printed atlas or better yet, on SkySafari on a smartphone or tablet.  Compare that location with what you see in the finder.  Position the location of the object behind the crosshairs of the finder.  It does not matter one bit if you cannot see the object itself in the finder.  Now look in the eyepiece of the telescope.  If the finder was closely aligned with the telescope, the object should be in the field of the eyepiece.

 

That being said, it would be nice if the finder had a flat field to the edge and no astigmatism.  Good luck with that. 

 

Mike

Edited by Sarkikos, 08 August 2024 - 11:42 PM.

[RichA]

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.

The concept goes all the way to Sidgwick's book, "Amateur Astronomer's Handbook."  In addition, he suggested that overcorrecting (or under) the eyepiece could fix the problem with a scope that was the opposite.   This all comes back to the idea of eyepieces matched to scopes.

Edited by RichA, 09 August 2024 - 12:03 AM.

[CrazyPanda]

If it's a small aperture finder, I wouldn't bother trying to actually see most galaxies or planetary nebulae in the finder.   Instead, look at the location of the object on a printed atlas or better yet, on SkySafari on a smartphone or tablet.  Compare that location with what you see in the finder.

 

That's how I had been doing it in my 60mm finder at 10x. At 10x it's easy to see targets like M27 and M51 etc. But the Ring Nebula was just too small to identify and always tripped me up as to which point was the target. So I stuck a 14mm eyepiece in the finder for 16x and the Ring Nebula now had dimension in the finder and totally eliminated the ambiguity.

 

I'm also able to see / identify many other targets as long as they are in the central 75% of the field of view. It just improves my overall observing efficiency. I realized I didn't need a very wide field of view because I always got within a couple of degrees with my Rigel. What was more important was increasing the chances of a direct identification of the target in the finder so I could snap to it.

Edited by CrazyPanda, 09 August 2024 - 12:02 AM.

[Sarkikos]

Bruce MacEvoy explains it a little better:

https://www.handprin....html#curvature

Positive field curvature is when the focal plane is convex toward the observer (i.e. the edges are farther away from the eye),

and negative field curvature is when the focal plane is concave toward the observer (i.e. the edges are closer to the eye).

The most salient point:

"However, in eyepieces, where the focus is at the image plane, positive curvature means the bowl is facing toward the observer. Thus, if the objective has a positive curvature (as most telescope objectives do), and the eyepiece has a negative curvature, then the two focal surfaces will curve in the same direction..."

 

Vlad Sacek discusses it further:

https://www.telescop...Petzval surface.

Go way down the page to Eyepiece Field Curvature

Well, maybe a little better.

 

Once, I asked MacEvoy to explain some optical concept to me in simple verbal terms without recourse to mathematics.  He seemed offended and told me to look it up on the internet. 

 

The diagrams at Vlad Sacek's link are helpful.

 

So, positive FC is convex toward the observer.  This means that the refractor in fig. 60 mentioned above has positive FC.  Negative FC is concave toward the observer.  The Newtonian in fig. 60 has negative FC.  

 

Positive FC + Negative FC = Field Curvature.   Negative FC + Negative FC = Flat Field. 

 

This is why the 20 and 14 XW's (positive FC) showed FC in my f/5 Newtonian (negative FC), while the 10 through 3.5 XW's (negative FC) did not show FC in the f/5 Newt (negative FC).  

 

Now if I can just remember this for when the subject comes up next time.  I'll have to invent some kind of mnemonic to remember it, otherwise I probably won't. 

 

Mike

Edited by Sarkikos, 09 August 2024 - 12:11 AM.

[quilty]

Which is correct now?

 

a typical doublet refractor has negative FC? Is the off axis fl then longer or shorter than on axis?

A typical Erfle ep has positive FC?

This I gathered from the posts above

or this one?
 

Bruce MacEvoy explains it a little better:
https://www.handprin....html#curvature
Positive field curvature is when the focal plane is convex toward the observer (i.e. the edges are farther away from the eye),
and negative field curvature is when the focal plane is concave toward the observer (i.e. the edges are closer to the eye).
The most salient point:
"However, in eyepieces, where the focus is at the image plane, positive curvature means the bowl is facing toward the observer. Thus, if the objective has a positive curvature (as most telescope objectives do), and the eyepiece has a negative curvature, then the two focal surfaces will curve in the same direction..."
 
Vlad Sacek discusses it further:
https://www.telescop...Petzval surface.
Go way down the page to Eyepiece Field Curvature

which says that normal doublet fracs have positive fc and need to match with a positive curved ep in order to zero the overall curvature
Are Erfle types positive or negative?

I answered to myself: Doublet lenses are positve, Erfle eyepieces, too. In combination both curvatures cancel each other when they have the same extent, right?

Edited by quilty, 09 August 2024 - 02:38 AM.

[Sarkikos]

Newtonians have very minor FC compared to the equivalent focal length refractor. Astrojensen above said this:

 

 

 

 

So in your 1250mm focal length Newt, radius of curvature is roughly 1190mm. Meanwhile in the same FL refractor, the radius of curvature would be just 440mm. So chances are the FC of the Pentaxes is quite a bit steeper than the Newt. FC is partially cancelled, but not fully. Meanwhile it would seem the 14 and 20 would be downright awful in a refractor - shorter ROC & opposite sign.
 

Yes, I knew that Newtonians have less FC than refractors given the same f number.  And after parsing through the links about FC above and looking at the diagrams in them, and considering my own field experience, I've come to the conclusion that Newtonians have negative FC while refractors have positive FC. 

 

As I said in post #48 above, referring to the diagrams at Vlad Sacek's link:

 

So, positive FC is convex toward the observer.  This means that the refractor in fig. 60 mentioned above has positive FC.  Negative FC is concave toward the observer.  The Newtonian in fig. 60 has negative FC.

If anyone thinks this is wrong, let them explain to me why I am wrong, and not just throw a formula at me, shoot a link to me or advise me to look it up on the internet.

 

In my experience, the residual negative FC of f/5 Newtonians must be enough to counteract the negative FC from at least some eyepieces.  In my f/5 Dob with a Paracorr I saw FC in the 20 and 14 XW (positive FC) but no FC through the 10 - 3.5 XWs (negative FC).  The FC from the 20 and 14 XWs did appear worse without the Paracorr. 

 

Take into account that my eyes no longer accommodate for focus.  If there is FC in an optical system, I will see it.

 

The 14 and 20 XWs have positive FC.  Refractors have positive FC.  These eyepieces should show mimimal or no FC in refractors.  

 

Mike

Edited by Sarkikos, 09 August 2024 - 08:08 PM.