Field Curvature APM 18mm UFF

[Sunshine]

Hello! It looks like I will be selling this eyepiece due to the amount of field curvature which it displays with my TSA-102,  you read correct, my "ultra flat field" APM is too field curvy, understand the effect is more evident with longer FL eyepieces. Can anyone recommend an eyepiece in the 18-25mm range which is better suited to for my scope based on experiences with similar scopes? or can field curvature be remedied by way of an additional element somewhere in the optical train? maybe a corrector directly on the diagonal or such? the 102 is an 816mm f8.

Thank you!

Edited March 17, 2021 by Sunshine

[Don Pensack]

The radius of curvature in your refractor is about 270mm.  That means a flat field eyepiece will show you field curvature.

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It's only if the curves match that you get a flat field:

( + ( = |

So, just about any flat field eyepiece will show you curvature.

If the scope had a longer radius of curvature, this wouldn't be an issue.

Or, if your scope had a flat field.

Telescope Service sells a field flattener that would work in your scope.  This might allow you to use most eyepieces successfully in that scope.

https://www.teleskop-express.de/shop/product_info.php/language/en/info/p10307_TS-Optics-REFRAKTOR-1-0x-Flattener-Bildfeldkorrektor---2--Anschluss.html

Otherwise, you'll have to confine yourself to eyepieces that have field curvature that matches (an expensive, trial and error proposition), or very narrow fields of view (narrow field stops).

Th ability to handle field curvature with accommodation in the eye diminishes as you age, too, so a correction to the scope seems the best option.

Flatten the scope's focal plane and just about every eyepiece will improve.

 

Edited March 17, 2021 by Don Pensack

[Louis D]

As Don suggests above, I use a TSFLAT2 attached in front of my GSO dielectric diagonal in place of the normal insertion tube.  For my 400m to 600mm focal length fracs, I find about 15mm of extension works well.  For your scope at about 800mm FL, you may want to dispense with any extension.  I use either the Blue Fireball M48 (2" Filter) Male Thread to SCT Male & M48 (2" Filter) Female Thread Adapter # C-05 with a 15mm extension or the Blue Fireball SCT Male Thread to 2" Nosepiece Adapter - Short # C-02 without any additional extension.  Either combination works really well to flatten the fracs' fields for my presbyopic eyes.

[Deadlake]

On 17/03/2021 at 17:03, Don Pensack said:

The radius of curvature in your refractor is about 270mm.  That means a flat field eyepiece will show you field curvature.

| + ( = (

It's only if the curves match that you get a flat field:

( + ( = |

So, just about any flat field eyepiece will show you curvature.

How do you determine the curvature of a scope?

Edit:

I’ll get number crunching

https://www.telescope-optics.net/curvature.htm#For_a_doublet

Telescope in question would be a 130mm/F6.

The field flattener reduces the focus length, presume you need a flatterner that can screw directly to a prism with a short light path?

 

Edited March 18, 2021 by Deadlake
Update web reference

[Louis D]

6 hours ago, Deadlake said:

How do you determine the curvature of a scope?

Most amateur visual scopes have planes curving away from the eyepiece at the edges.

[Don Pensack]

9 hours ago, Deadlake said:

How do you determine the curvature of a scope?

Edit:

I’ll get number crunching

https://www.telescope-optics.net/curvature.htm#For_a_doublet

Telescope in question would be a 130mm/F6.

The field flattener reduces the focus length, presume you need a flattener that can screw directly to a prism with a short light path?

 

Geometry.

The curvature of the field is indirectly related to the focal length of the scope, and can be calculated at different distance from the center axis.

The difficulty with refractors is the Radius Of Curvature, ROC, is about 1/3 of the focal length, whereas with a newtonian, the ROC is equal to the focal length.

That means field curvature issues are more present in refractors, and definitely explains the use of field flatteners for imaging, and even for visual use if the refractor is short.

That applies to doublet lenses.  Quite often, in some triplet designs, some field flattening is part of the formula, so you cannot necessarily assume a triplet will have the same FC as a doublet of the same focal length.

Some 4-lens refractors start with a long focal length objective, then add a focal reducer/field flattener later in the optical path to produce a flat field short focal length refractor.  That's a superb idea.

But, if the refractor is not inherently flat, adding a field flattener is a good thing to do for imaging.  Surprisingly few visual observers use field flatteners, but they work well, and allow the use

 of a lot more different eyepieces without seeing a lot of field curvature.

Field flatteners have a "working distance", so it's important to try to get the distance from the FF to the eyepiece focal plane at least approximately right.  To that end, it is useful to know the light path length through the diagonal

and where the focal plane is in the eyepiece.

[Merlin66]

The field curvature is opposite in reflectors.

It sometimes amazes me that a eyepiece can perform well in both types of scopes.

[Louis D]

Most Newtonians have such a large radius of curvature (equal to their focal length) that the field is very nearly flat for most eyepieces.  Refractors, especially short focal length ones, present a very curved field having a radius of curvature of about 1/3 the focal length.  Thus, the latter are far more demanding on eyepieces and the observer's accommodation.

Edited March 19, 2021 by Louis D

[Sunshine]

The effect in my Pentax XW's and, my Baader 9mm was such that it never crossed my mind, if there is it is not enough to catch my attention. My 18mm APM on the other hand

was like putting my eye to a fish bowl, enough so that when I would scan around it made me slightly dizzy, who knows to what extent my own eye was to blame for it 

Edited March 19, 2021 by Sunshine

[Merlin66]

I refer to:

https://www.telescope-optics.net/curvature.htm

[Deadlake]

The other option is a Q extender,

https://www.cloudynights.com/topic/751644-apm-xwa-100°-eyepieces/?p=10889469

this gets rid of abberations around the edge by increasing the field curvature (focal length increased), but I suspect the flatterer works with more EPs.

Edited March 19, 2021 by Deadlake

[John]

The Pentax XW's have a range of field curvatures from positive, through neutral to negative. I guess some focal lengths will suit some scope types better than others ?:

Pity that similar data is not available for other eyepieces so that we can make our selections taking that into account. We seem to have to learn the characteristics of most eyepieces by trying them out and sharing the information on forums like this.

 

[Don Pensack]

One other thing that occurs to me is that I have run into a couple KUO eyepieces that were misassembled.

Once assembled correctly, they worked fine.

I just last night tried the 18mm APM UFF in my 102mm f/7 triplet refractor (no field flattener) and noticed no abnormal field curvature

and the moon images were good across the field.

That scope is short enough that I think I would have seen any extensive field curvature in the eyepiece if it was there.

 

One 13mm XWA had incredible distortion at the edge of the field and the field looked like a deep bowl.

I was going to return the eyepiece, but took it apart and discovered one element had been installed upside down.

I corrected it, and the eyepiece worked great, with just the typical edge distortion common to 100° eyepieces.

The same thing happened on a 10mm UFF several months later.

 

So I wonder if the eyepiece is misassembled.  The 18mm I checked out was very nice and had a quite flat field presentation.

You can find the assembly orientations for each lens right here:

https://www.landseaskyco.com/altair-ultraflat-24mm-65-eyepiece-stainless-steel.html

Edited March 19, 2021 by Don Pensack

[Louis D]

14 minutes ago, Don Pensack said:

The same thing happened on a 10mm UFF several months later.

Which lens in the diagram was flipped?  It looks like only the negative lens in the lower barrel could be flipped and still get the whole thing to screw together.

[Don Pensack]

17 hours ago, Louis D said:

Which lens in the diagram was flipped?  It looks like only the negative lens in the lower barrel could be flipped and still get the whole thing to screw together.

In the 13mm, they had flipped the mushroom-shaped lens right where the lower barrel joins the upper barrel.

In the 10mm, they had flipped the lowest main lens so the flat surface was up instead of down.

Both were returns from customers complaining of distortion and aberrations.

[Deadlake]

On 18/03/2021 at 16:54, Don Pensack said:

Geometry.

The curvature of the field is indirectly related to the focal length of the scope, and can be calculated at different distance from the center axis.

The difficulty with refractors is the Radius Of Curvature, ROC, is about 1/3 of the focal length, whereas with a newtonian, the ROC is equal to the focal length.

That means field curvature issues are more present in refractors, and definitely explains the use of field flatteners for imaging, and even for visual use if the refractor is short.

That applies to doublet lenses.  Quite often, in some triplet designs, some field flattening is part of the formula, so you cannot necessarily assume a triplet will have the same FC as a doublet of the same focal length.

Some 4-lens refractors start with a long focal length objective, then add a focal reducer/field flattener later in the optical path to produce a flat field short focal length refractor.  That's a superb idea.

But, if the refractor is not inherently flat, adding a field flattener is a good thing to do for imaging.  Surprisingly few visual observers use field flatteners, but they work well, and allow the use

 of a lot more different eyepieces without seeing a lot of field curvature.

Field flatteners have a "working distance", so it's important to try to get the distance from the FF to the eyepiece focal plane at least approximately right.  To that end, it is useful to know the light path length through the diagonal

and where the focal plane is in the eyepiece.

If you are using the flattener on a fast scope F6 will there still be an advantage of using a mirror diagonal over a prism at all? I ask this as BBHS mirror diagonals are hard to come by at present. I want the stars at the edge to be sharp.

[Don Pensack]

On 20/03/2021 at 10:52, Deadlake said:

If you are using the flattener on a fast scope F6 will there still be an advantage of using a mirror diagonal over a prism at all? I ask this as BBHS mirror diagonals are hard to come by at present. I want the stars at the edge to be sharp.

I don't think the type of diagonal relates to whether or not field curvature is present or visible.

But the prismatic smear that occurs with prisms gradually gets worse below f/8.  I wouldn't use a prism diagonal at f/6.

Baader BBHS mirror diagonals are not the only good star diagonals out there, by the way.

[Deadlake]

14 hours ago, Don Pensack said:

I don't think the type of diagonal relates to whether or not field curvature is present or visible.

But the prismatic smear that occurs with prisms gradually gets worse below f/8.  I wouldn't use a prism diagonal at f/6.

Baader BBHS mirror diagonals are not the only good star diagonals out there, by the way.

Unfortunately the 2" dielectric coated diagonals absorb the infra-red spectrum and so are not suitable for NV usage. I cannot think of another 2" non-dielectric mirrored diagonal that does not absorb in the infra-red spectrum apart from the BBHS. Thanks for your help Don. BBHS back in stock mid too late April apparently.

Edited March 22, 2021 by Deadlake

[JTEC]

I believe that the ‘prismatic smear’ that Don refers to might be somewhat greater with the 2” prism over the 1.25” because more glass is being traversed? I have both the Baader-Zeiss prisms and like them a lot; much prefer them to the AstroPhysics dielectric I used to own.  
However, I recently got hold of the Baader 1.25” BBHS and the difference in colour rendition and range is obvious.  This is now my preferred diagonal in the 140 f7 apo.
I am so taken with these qualities that I began to wonder about replacing the alu coatings on my 12” Dob with modern, suitably protected silver.  That’s a different thread I guess.  I expect it would be pricey to do but that’s academic at the moment because I can’t find anyone offering to do it 🙂. Any suggestions welcome!

Edited March 22, 2021 by JTEC

[Don Pensack]

One thing to note about silver--it's response rolls off below 500nm, and transmission is poor at 400nm.

Aluminum doesn't roll off significantly in the 400-500nm range.

The way to bring silver up to the aluminum level at the 400-500nm range is with special UV-enhancing coatings added.

It think it possible those are applied in some cases, but you would want to find out if they are.

Dielectric coatings can be optimized for any wavelength bandwidth and reflective angle desired, but might require custom coatings if going deep into the infrared is desired.

https://www.photonics.com/Articles/Mirrors_Coating_Choice_Makes_a_Difference/a25501

For pure infrared use, gold is great--high reflectivity and it doesn't corrode at all.  Not good for all visual, though perhaps OK for planets?

https://www.cloudynights.com/topic/346824-gold-diagonal-better-contrast-on-planets/

Someone out there offers a gold-coated star diagonal--Vernonscope?

Gold is soft, though, and should be overcoated for protection from scratches, just like silver and aluminum.

 

[Deadlake]

6 minutes ago, Don Pensack said:

One thing to note about silver--it's response rolls off below 500nm, and transmission is poor at 400nm.

Aluminum doesn't roll off significantly in the 400-500nm range.

The way to bring silver up to the aluminum level at the 400-500nm range is with special UV-enhancing coatings added.

It think it possible those are applied in some cases, but you would want to find out if they are.

Dielectric coatings can be optimized for any wavelength bandwidth and reflective angle desired, but might require custom coatings if going deep into the infrared is desired.

https://www.photonics.com/Articles/Mirrors_Coating_Choice_Makes_a_Difference/a25501

For pure infrared use, gold is great--high reflectivity and it doesn't corrode at all.  Not good for all visual, though perhaps OK for planets?

https://www.cloudynights.com/topic/346824-gold-diagonal-better-contrast-on-planets/

Someone out there offers a gold-coated star diagonal--Vernonscope?

Gold is soft, though, and should be overcoated for protection from scratches, just like silver and aluminum.

 

This is the CN thread: https://www.cloudynights.com/topic/593410-nv-tips-for-viewing-globular-clusters/?hl=+dielectric++infra#entry8373739 on the article.

[JTEC]

Thank you, Don.  That’s tremendously interesting and informative.  This feels like (yet) another instance of horses for courses. I wonder if, with the increasing use of blue-rich LED lighting and ubiquitous scattering, that drop of in reflectivity/transmission in the blue might be advantageous in some circumstances and with some targets.
 

[JTEC]

1 hour ago, Deadlake said:

This is the CN thread: https://www.cloudynights.com/topic/593410-nv-tips-for-viewing-globular-clusters/?hl=+dielectric++infra#entry8373739 on the article.

Thanks, Deadlake - interesting link!

[Deadlake]

2 minutes ago, JTEC said:

Thanks, Deadlake - interesting link!

I'm beginning to think BBHS mirror is the way forward, apart from any scope > F7. I'm going to wait and use a known performer.
It is hard to sort one dielectric coating from another as the silver Baader BBHS also have a dielectric coating applied as well.  

[JTEC]

To my eye, the differences are slight but real.  If the budget allows, there may be a case for having one of each! Not an absurd extravagance when you think what we spend on eyepieces. 🙂