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The ultimate planetary eyepiece design?

eyepieces optics
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#1 BKSo

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Posted 10 January 2020 - 10:52 PM

This excise is inspired by this comment from @Starman1

TeleVue Orthos? Well, TeleVue can make a narrow field line of eyepieces with long eye relief to satisfy the hundreds who constantly harp about that, and make them as sharp or sharper than Delites.
But you can't do it with 4 elements. It can't happen unless you only want the center 10° sharp, and you use a tracking scope, and you only look at planets, never the Moon.

I've used lots of Abbe orthoscopic eyepieces since the '70s. They're great if your scope is f/15 and tracks. Otherwise, the outer field is just not that good.
At f/5, there is just too much astigmatism.

Good planetary performance doesn't require only 3 or 4 elements, it requires excellent coatings and lens polish.
So would you buy a line of such eyepieces if they were $200 apiece? I doubt it. But that's what they would have to cost.

So the objective is to design a minimal glass eyepiece in planetary focal lengths, with long eye relief, and good field correction. I reckon for long eye relief it must be a negative-positive design. The simplest design is 1-2 or 2-1, followed by 2-2. That is what I come up with

EPPlan.png

Designed focal length is 2.5mm, designed focal ratio F/5. Eye relief is about 13mm (longer eye relief is possible by allowing even longer distance between the two groups). The design uses 4 glasses (the Brandon also use 4 glasses).

Spot.png

Spot diagram, with the white circle representing the Airy disc. The geometric FOV is 40 degrees, AFOV is 48 degrees after geometric distortion. The design seems to be well corrected, with some lateral color as the main aberration.

Not sure how good or feasible is this really?

Edited by BKSo, 10 January 2020 - 11:30 PM.

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#2 TOMDEY

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Posted 10 January 2020 - 11:53 PM

Me too. Provided the feeding scope is very slow and the needed AFOV is very small, and the tracking is accurate and smooth... then the simpler the better and would still provide great performance. Back around 1960, I built/stumbled upon my first astonishingly gratifying planetary scope... A 6-inch F/12.5 Newt of my own hands and an Edmund's (circle T)  12.5 Ortho... and sharp, 13-year-old eyes. Although I've used fancier, bigger scopes, higher mag, fancier eyeps since --- I really can't say that I've seen the planets better than way back then. A lot to be said for good slow scopes, good simple eyeps, and good young eyes... sigh...

 

PS: A 12.5mm volcano top ortho sans glasses... eye relief is actually adequate.     Tom

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#3 Starman1

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Posted 11 January 2020 - 12:11 AM

This excise is inspired by this comment from @Starman1
So the objective is to design a minimal glass eyepiece in planetary focal lengths, with long eye relief, and good field correction. I reckon for long eye relief it must be a negative-positive design. The simplest design is 1-2 or 2-1, followed by 2-2. That is what I come up with

EPPlan.png

Designed focal length is 2.5mm, designed focal ratio F/5. Eye relief is about 13mm (longer eye relief is possible by allowing even longer distance between the two groups). The design uses 4 glasses (the Brandon also use 4 glasses).

Spot.png

Spot diagram, with the white circle representing the Airy disc. The geometric FOV is 40 degrees, AFOV is 48 degrees after geometric distortion. The design seems to be well corrected, with some lateral color as the main aberration.

Not sure how good or feasible is this really?

How long is this eyepiece?

#4 BKSo

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Posted 11 January 2020 - 01:05 AM

About 4 inches.

#5 Astrojensen

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Posted 11 January 2020 - 03:54 AM

This excise is inspired by this comment from @Starman1
So the objective is to design a minimal glass eyepiece in planetary focal lengths, with long eye relief, and good field correction. I reckon for long eye relief it must be a negative-positive design. The simplest design is 1-2 or 2-1, followed by 2-2. That is what I come up with

attachicon.gifEPPlan.png

Designed focal length is 2.5mm, designed focal ratio F/5. Eye relief is about 13mm (longer eye relief is possible by allowing even longer distance between the two groups). The design uses 4 glasses (the Brandon also use 4 glasses).

attachicon.gifSpot.png

Spot diagram, with the white circle representing the Airy disc. The geometric FOV is 40 degrees, AFOV is 48 degrees after geometric distortion. The design seems to be well corrected, with some lateral color as the main aberration.

Not sure how good or feasible is this really?

It's basically just a Dollond eyepiece with a smyth lens. It is a REALLY interesting design, if you have somehow been able to extend the AFOV from the Dollond's measly 15° to a very usable 40°. 

 

I know from experience with my own homemade Dollonds, that they are VERY comfortable and easy to look through (apart from the narrow AFOV) and are exceptionally free from ghosting, glare and reflections, with stunning clarity and contrast. Having that at 40° AFOV would be nothing short of astounding. 

 

A downside of the Dollond is its relatively pronounced field curvature, especially at shorter focal lengths, but a barlow goes a long way in correcting that. I am unsure whether the barlow has field curvature with the opposite curvature of the Dollond? If so, they could possibly be matched to eliminate it. I think. 

 

 

Clear skies!
Thomas, Denmark


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#6 Mark Harry

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Posted 11 January 2020 - 08:15 AM

Tom said it in spades. I have a set of circle "T"s, and with a driven mount, can't be beat. I usually get a lot of "poo-poo's" when I relate I saw 8 stars in the Trap on one spectacular night 10 years ago with an 8" F/5.9 that I reworked the optics on. The best combination with regard to mag vs brightness was a 9mm Ortho @ 133x.
A 7mm would lose the two faintest ones (G&I)---just a bit too dark.
Minimal amount of glass, central area of image tack-sharp, and the mount allowed me to pay better attention to just observe.
I haven't seen a night of transparency and steadiness that good since. Everything fell into place for about 3/4 of an hour. Guess I was fortunate!
Tom Trusock recommended me to get the Ortho's for planetary, and they work just as well there, too. At the time I was somewhat of the opinion-
"How could a $50 eyepiece beat a Nagler?"  Suffice to say, I found out.


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#7 j.gardavsky

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Posted 11 January 2020 - 09:33 AM

This excise is inspired by this comment from @Starman1
So the objective is to design a minimal glass eyepiece in planetary focal lengths, with long eye relief, and good field correction. I reckon for long eye relief it must be a negative-positive design. The simplest design is 1-2 or 2-1, followed by 2-2. That is what I come up with

attachicon.gifEPPlan.png

Designed focal length is 2.5mm, designed focal ratio F/5. Eye relief is about 13mm (longer eye relief is possible by allowing even longer distance between the two groups). The design uses 4 glasses (the Brandon also use 4 glasses).

attachicon.gifSpot.png

Spot diagram, with the white circle representing the Airy disc. The geometric FOV is 40 degrees, AFOV is 48 degrees after geometric distortion. The design seems to be well corrected, with some lateral color as the main aberration.

Not sure how good or feasible is this really?

For anyone with the technical capability for DIY, making such EP should be doable.

The lenses in high quality are available, like from Zeiss HQO/AR surplus here: https://www.ebay.de/...FcAAOSwWxxd3k8Z

 

The only problem emerging: the manufactured batch would be limited by the size of the surplus from Zeiss.

 

At f=3.5mm to complement the Pentax SMC XO 2.5, I would be a potential buyer,

JG


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#8 BKSo

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Posted 11 January 2020 - 09:36 AM

It's basically just a Dollond eyepiece with a smyth lens. It is a REALLY interesting design, if you have somehow been able to extend the AFOV from the Dollond's measly 15° to a very usable 40°.
[snip]
A downside of the Dollond is its relatively pronounced field curvature, especially at shorter focal lengths, but a barlow goes a long way in correcting that. I am unsure whether the barlow has field curvature with the opposite curvature of the Dollond? If so, they could possibly be matched to eliminate it. I think.


Clear skies!
Thomas, Denmark

The Dollond with a Smyth section was the starting idea. Then the glass and curvature were fine tuned so that abbreviation from the two groups cancel each other — that’s what the likes of OSLO and Zemax do. Note that the eye lens is opposite of the Dollond. From what I can tell the eyepiece is quite flat field, like no more than 2 diopter.

Edited by BKSo, 11 January 2020 - 10:05 AM.

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#9 BKSo

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Posted 11 January 2020 - 09:58 AM

For anyone with the technical capability for DIY, making such EP should be doable.
The lenses in high quality are available, like from Zeiss HQO/AR surplus here: https://www.ebay.de/...FcAAOSwWxxd3k8Z
JG


Err... not so easy. The doublets are not just any achromatic but have under/over correction built in. That is how you get good 40 degrees afov instead of just 15 as pointed out by Thomas.
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#10 dan_h

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Posted 11 January 2020 - 10:35 AM

So the objective is to design a minimal glass eyepiece in planetary focal lengths, .....

 

 

 

Planetary focal lengths?  I guess this assumes that the scopes in use are comparatively short focal length.

 

For my use in long refractors, a planetary eyepiece would be 10-15mm.    In an 8" SCT, 2.5mm would provide over 800X. 

 

The 8mm RKE from Edmunds has always had a following as a rather sharp planetary eyepiece.  Close to minimal glass. 

 

The easiest way to clean up edge problems in an eyepiece is to use tracking. 

 

dan



#11 Starman1

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Posted 11 January 2020 - 01:55 PM

You guys might laugh, but I've seen Jupiter many times with high-end AP scopes using minimalist glass eyepieces, and seen some wonderful images.

But my lifetime-best view of Jupiter was with a 2X 2" PowerMate + Paracorr II + 8mm Ethos eyepiece at 456X in literally perfect seeing.

That's 18 lenses in about 11 groups.  The image was so good, it made most of Christopher Go's images of Jupiter look blurry.

I think the number of lenses has very little to do with the details you see.


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#12 AndresEsteban

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Posted 11 January 2020 - 07:57 PM

You guys might laugh, but I've seen Jupiter many times with high-end AP scopes using minimalist glass eyepieces, and seen some wonderful images.

But my lifetime-best view of Jupiter was with a 2X 2" PowerMate + Paracorr II + 8mm Ethos eyepiece at 456X in literally perfect seeing.

That's 18 lenses in about 11 groups.  The image was so good, it made most of Christopher Go's images of Jupiter look blurry.

I think the number of lenses has very little to do with the details you see.

"I think the number of lenses has very little to do with the details you see.", yes, if you have top optics elements like "powermates", "paracorrs", "TV Ethos", ok you may say that. But if not working with top qualty hardware on the eyepiece side, then minimalist aproach is the best, I mean, abbe orthos can't be beated, either old Circle T - Volcano top orthos or the Baader BCO versatile orthos with slight wider AFoV. That's my 2 cents...

Clear skies for us all!
Andy

 


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#13 Mark Harry

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Posted 11 January 2020 - 08:29 PM

Well, my experience I guess will be different than these 2 guys above.
To see the difference, I say the conditions have to be perfect to see the difference.

There is one other rule of thumb I use. I take the inch of aperture, and multiply it by about 15-16x/inch. This assures a really good brightness to resolution level. This, an object will reveal anything possible. Lower powers, the scale is too tiny. Higher, the dimmer, and less contrast available. At least, it works well with my eyes. Really bright high contrast, (major planets and moon) of course you could double-triple the power to make it easier for older eyes.
And do I have to mention collimation??? That 8" F/5.9 I had, was easy to tell if it was even a miniscule twitch off regardless what was being observed.



#14 don clement

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Posted 11 January 2020 - 08:30 PM

IMO Clavé  Plössl eyepieces are the best planetary eyepiece from experience.

 

Don


Edited by don clement, 11 January 2020 - 10:48 PM.

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#15 Jeff Morgan

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Posted 11 January 2020 - 11:39 PM

The easiest way to clean up edge problems in an eyepiece is to use tracking. 

 

 

For the planetary enthusiast striving for that extra nth percent of eyepiece performance ... a tracking mount is almost certainly a box that was checked off long ago.


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#16 ed_turco

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Posted 12 January 2020 - 02:01 PM

The comments by the OP and others give me some hope that I haven't lost my sanity -- as of yet.

 

I still have my set of Ramsden eyepieces I made in 1974 that I used for many years. It is a truism that most eyepieces can beat mine but I have one certain advantage --  I can drop these with impunity and not worry about the consequences.  :)

 

Try that with a Nagler!


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#17 Mark Harry

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Posted 12 January 2020 - 04:36 PM

Atta-boy, Ed!!!
********
 In my experience, there's nothing that can beat a simple Ramsden or Ortho. Everything else takes a back seat.

And while the subject is up, to use a long focal length telescope of sane aperture, helps.


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#18 j.gardavsky

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Posted 12 January 2020 - 05:21 PM

Atta-boy, Ed!!!
********
 In my experience, there's nothing that can beat a simple Ramsden or Ortho. Everything else takes a back seat.

And while the subject is up, to use a long focal length telescope of sane aperture, helps.

in my case it's Kellner (1 - 2), ortho (3 - 1), König (2 - 1)

 

JG



#19 MartinPond

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Posted 13 January 2020 - 02:50 AM

This excise is inspired by this comment from @Starman1
So the objective is to design a minimal glass eyepiece in planetary focal lengths, with long eye relief, and good field correction. I reckon for long eye relief it must be a negative-positive design. The simplest design is 1-2 or 2-1, followed by 2-2. That is what I come up with

attachicon.gifEPPlan.png

Designed focal length is 2.5mm, designed focal ratio F/5. Eye relief is about 13mm (longer eye relief is possible by allowing even longer distance between the two groups). The design uses 4 glasses (the Brandon also use 4 glasses).

attachicon.gifSpot.png

Spot diagram, with the white circle representing the Airy disc. The geometric FOV is 40 degrees, AFOV is 48 degrees after geometric distortion. The design seems to be well corrected, with some lateral color as the main aberration.

Not sure how good or feasible is this really?

 

 

I think you are describing something almost exactly like a modified Dollond with a

   super-long very-high-gain Barlow.before it.   As such, the tracing would not

   be much affected by a change in the strength or position of the negative

   achro.....as long as extra length made up for a weaker negative doublet.

   Looks a lot to me like just a modified Dollond with a Barlow that makes it see a super-long barrel.

 

    In a nutshell, I think is is tremendously feasible to at least simulate this.

    If the long distance is lined with flat-black threading or the rolled-sanding-screen I use sometimes,

   you have a tremendous potential for contrast improvement (ie, killing stray light)...

 

    Looks quite worth a try, though my theory behind how glass surfaces matter tells me

    that putting a 30mm Plossl after 5-6X of BArlow gain would give the same dazzling results

    with 50 degreees afov.

 

     But....I believe a strong Barlow element and/or long extension would be very feasible to test your idea.


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#20 MartinPond

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Posted 13 January 2020 - 03:05 AM

)

Finished a round of testing, with my 

   "vertical test bench" (ie, a thin glass slide and ring to hold the group)

 

-----with an Orion Tri-Mag and 3" extension, there is a very flat field..

     ...I have to keep swapping up in power (down in FL) to open out the field,

       but it's about  35--38 degrees afov, flat, and 95% sharp.

 

------contrast wasn't super at this low angle.  I looked down the barrel,

     and the Tr-Mag started picking up a little sheen.

    I mean....it wasn't made for this, exactly.

 

----If  I resume testing with a 5" 1.25"-OD hdpe "lavatory" pipe section

     lined with the sanding screen, the contrast jumps up.

 

You can get similar results if you use a 3X shorty, which has a smaller

   window (oddly enough, like your drawing), plus an extension, 3-5 inches.

    An irising effect is important.   

 

So....yeah, it looks like your layout can be emulated, if you can tinker.

The power of the eye doublet will open the field until it degenerates at the edges..

So...that needs critical fiddling.  I get nice results already,

but mainly I have realized the baffling is even more critical in this application.

 

And I do like it, mysefl, with a nice 30mm Plossl.

 

For your app and mine, irising and/or stray light suppression will be very critical

  for planetary....maybe beyond the normal.  I'm getting a hankering for a

   3X shorty w/ doublet now.  .No wonder the reviews are all weirdly high.


Edited by MartinPond, 13 January 2020 - 03:07 AM.

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#21 luxo II

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Posted 13 January 2020 - 07:34 PM

Martin this is pretty much what occurred to me 40 years ago when pursuing high magnification for the planets. Despite having two old-school long Newtonians with very good mirrors the available eyepieces under 9mm and Barlows were all pretty dreadful things in that era.

So the aim was to achieve very nice high magnification with quite modest eyepieces (such as 25mm Kellner or Plossl).

but... instead of using a negative lens (with all that follows) associated with the eyepiece, the alternative view is to amplify the primary focal length of the scope:

a) use a single reflecting surface to maintain perfect achromatism;
b) a magnification factor of 3-5 is entirely feasible, and
c) ideally, reduce or cancel the 3rd order aberrations of a fast primary mirror.

Bearing in mind that for high magnification to be useful you also must have some decent aperture or it will be crippled by diffraction. So small refractors are out and the likely target is a sizable reflector of some sort.

There were two solutions evident:

- Classical cassegrain (and dual Newtonian) or
- folded Newtonian, with a parabolic convex secondary mirror sending the light back to the primary for a second bounce then back to the Newtonian focus through a hole in the secondary.

Both can give very nice high magnification with a modest eyepiece and I would suggest much better than any Barlow.

After building one at f/18 I concluded scopes with focal ratios around f12-f18 were quite superior to f/10 or less. They’re just not very popular though with the imaging types.

Edited by luxo II, 13 January 2020 - 07:47 PM.

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#22 MartinPond

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Posted 13 January 2020 - 08:43 PM

That fits with the idea that the long 2,2 is really the Strong Smyth with mod. Dolland..
Yes....a very long fl reflector scope would make the mod. Dollond quite happy.

Very long scopes are better in many ways, but they can get awkward and the true field gets a bit low..

 

I was surprised to see how important and tricky the lining/baffling becomes at such low angles,

be it long Barlow or long barrel...it's not easy.   In the main barrel, it's a lot easier to trap the

  stray light in a refractor, unless the reflector has a well-oversized tube. 

I dropped a basic 4" F/10 Newtonian a long time ago because it was so noisy...

...maybe a better lining would have helped.

 

 

When all is said and done, I would prefer a long, strong 5x Barlow+extension 

  with a 1,2 (the 1 being next to the eye) in this notation Konig-1  or a Plossl to the restrictive

   doublet's  afov.     So...if I ran the 80mm/F9-----3xshorty-----extension for 5x----

    hmmmm, it would take a 25mm fl to get 5mm effective. fingers crossed on vignetting.

     It's worth a try to just use my ES 3X telecentric with anything 15mm----30mm..

     It really kills of the stray light.  But I haven't looked at a 3x Shorty before...

   maybe the small window will cast a great  shadow and save the day.  


Edited by MartinPond, 13 January 2020 - 08:55 PM.

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#23 BKSo

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Posted 14 January 2020 - 06:45 AM

Martin,
You could well stop down the Trimag. I do that with my barlow. The required aperture is
A = l/(m F) + s /m
where l is the length of the barlow (from the nose to the eyepiece focal plane), m is the barlow magnification, F is the telescope focal ratio and s is the diameter of the eyepiece field stop (assuming simple eyepiece).

And please continue doing research on this. It is useful beyond this particular eyepiece.

Edited by BKSo, 14 January 2020 - 06:50 AM.


#24 MartinPond

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Posted 15 January 2020 - 08:59 AM

I concur. 

Continue the charge! 

More empiricals.....

 

Picking  through the parts/EPs pile,

 

   I first concentrated on the  stopping down of the Tri-Mag.

----I found a vinyl eyecup from a 7x25 pair that tucked snugly onto the Tri-Mag.

----The opening was consequently reduced from 22mm to 17mm

----Looking into the Tri-Mag barrel, there is a huge jet-black shadow zone from this,

      and only the dull sheen of ~20% of the Tri-Mags's internal ribbing.

----The improvement of contrast in regular EPs, ands especially the 

     'flat-top Dollond, was striking.   I use a test target of successively fading

     printed words (the r/g/b values climb from 40 to 255 ea. in a few steps)

     The stray light is a powerful chandelier 4-6 ft from the axis at 25 out of  36 ft.

      A bit more torture than most sky targeting, though less than the Moon.

-----The 53 degree field of a 30mm Plossl was not vignetted at all. 

         Awesome.

-------------------> I think I should leave the 17mm iris in there, to make it a

                       "planetary Barlow".  I'm sure the 3X Tv would work in a similar way

 

Then, a look at the EP..

    ---I found the most "domed flat-face' achromats from homebrew Plossls

    ---I can almost reach the 40 degrees afov as one achromat, but not quite....it's 38 degrees.

            Some of that is also how the fl of the achro effects afov...it is hard to pick

          exactly the right values.  The surfaces in the achro do much of the regular

          'field opening". 

    -----But:   in pairs, as the thick-group-Plossls I made,

          I got: *16mm FL (before Barlow)

                    *65 degree afov , clean to the edge

                    *A little better contrast than the single-achro..

                          I suspect there is some stray-rejection to the basic Plossl type.

          There was some geometric bending near the edges, but...hey, it's 

                  unavoidable at 65 afov. Perhaps even better contrast if

                 field-trimmed to 55 degrees?

 

 

So...I'm sticking with my "super-contrast thick-symmetric 5mm effective planetary" scheme,

    despite 6 elements total. Irising makes the Barlow elements an asset.

 

If you want to fully explore the '2,2  with modified Dolland' idea, you need a fairly

   exacting achromat.   Ray-tracing that ideal achromat to figure the 

    back-fl and forward-fl might give you the ability to seek out surplus or

     commercial parts.   If someone manufactured the thing, they could

    get what they wanted, but it would take an order of hundreds to avoid pricing issues.

     Using an irised Tri-Mag or 5x-long would help on feasibility...I don't see 5X longs, though,

     just shorts and tele-extenders ($$$)


Edited by MartinPond, 15 January 2020 - 09:02 AM.

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#25 Chuck Hards

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Posted 15 January 2020 - 09:16 AM

I've been using my old UO flat-top "Professional Series" eyepieces preferentially the last few years, Barlowed when necessary with an Orion Tri-mag.  It's a tough call at times but I think I see a tad more detail using the flat-tops than I do with the volcano-top orthos.  Seeing is always the biggest factor so it's tough to draw conclusions based on experiences from different nights.

 

My volcano-top orthos are a motley bunch, several different brands.  They started out mostly UO and Edmund back in the 70s, but as the years went by, when I came across a FL that I already had, I kept the better one and sold the other.   So in theory my set is now composed of the best examples of each FL, rather than a homogeneous collection of all the same brand.  Yet the flat-tops usually best them by the slimmest of margins- and the flat-tops are a mix of Konigs and orthos.  I also can't use the super-short FL eyepieces anymore due to lack of eye relief.  Barlowing the eyepieces adds a tad more eye relief.

 

This is with objectives of f/7 and slower, up to f/20.   The fancy stuff and "hand grenades" are for the fast light-buckets.

 

I picked up some older Tak orthos last fall and haven't had a chance to try them yet, stay tuned.




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