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Recommend an eyepiece for a very fast telescope for viewing planets

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#26 213Cobra

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Posted 15 September 2019 - 02:11 AM

Ok, I know you’re not supposed to use a very fast scope (like an f4 or less) to look at planets.  But suppose you’re about to pack up for the evening and you see Saturn clearing the trees.  Assuming that you’re absolutely certain that the amateur astronomy community will never find out, what eyepiece  do you use to view it?  Do you combine it wIth a specific Barlow?  With some other supplementary tool?  Money is no object answers and thrifty answers are encouraged. 

Jeeze Louise, where does this stuff start?

 

I use f/3.3, f/5, f/5.3, f/6 all the time for planets with great results. And yes, I have f/10 and f/15 instruments too.

 

Takahashi TOE 2.5, 3.3 4.0mm; Takahashi UW 3.3mm; Tele-Vue Nagler 2.5mm, 3.5mm are all outstanding with fast scopes on planets. If you need a barlow boost, a 2.5X Powermate works well, or if you have a Takahashi scope, their dedicated Extenders are exceedingly effective.

 

Phil



#27 sixela

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Posted 16 September 2019 - 04:01 PM

 so since even simple eyepieces tend to perform fairly well on-axis, 

Simple eyepieces do not perform fairly well on-axis at f/4 (even at f/4.5 spherical aberration is already trivially detected in a star test). The only eyepiece that I would call "simple" (2 groups) that still works fairly well there is the Zeiss Abbe ortho, but if you have a fast scope you'll barlow it for very high power since they don't come in very short focal lengths.

The Pentax XP and XO also work fairly well but they aren't really "simple" anymore -- they have three groups. Ditto for the excellent Vixen HR.

 

The TMB supermono is even simpler and tack sharp in the middle, but the sweet spot in the field is quite small to my eyes -- I can no longer accommodate well for different focus -- because of really brutal field curvature (which bizarrely seems to be less noticeable to me when using a barlow).

The advice to combine simple eyepieces with a barlow, however, is spot on, unless you want to go the DeLite et al route.

 

On an f/3.7 a Paracorr and below can be quite handy, since the spherical aberration added can usually compensate that of the eyepiece train, especially since it depends on the position of the Paracorr with respect to the focal plane (allowing you to "detune" the Paracorr to tweak things, in some cases).

It's really hard to generalise -- every eyepiece combo will have slightly different spherical aberration, which will reveal itself in a star test. As long as it's not much more than 1/6th wave PTV on the wavefront there's no need to have angst about it.


Edited by sixela, 16 September 2019 - 04:03 PM.


#28 MartinPond

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Posted 16 September 2019 - 07:02 PM

The field curvature in an F4.5/F5 scope might seem trivial

  by itself, but the curvature results in a deviation after a while

  (ie, at the edge of the field stop)..

 

 

  Therein lies the rub.   A tiny deviation of a big objective

    in a short-fl eyepiece is in fact a major problem.

  The two focal planes do not match and the short one has big problems.

 

Direct compensation only corrects at one F/value.

The full job could be described as "transforming" the curved

   focal plain to flat,  whether this is at F4 of F15...

 

A trivial eyepiece that does this transformation right to the field edge

   is a 2,2,2 of weak doublets, flipped the right way,

     but the field is limited to 50 degrees.

 

You are transforming the curvature of the short barrel 

   with a Smyth or Barlow, by carving an area of less

   curvature from the middle....this works for low-fl eyepiece

    with high eye relief.

 There are other ways, though..


Edited by MartinPond, 16 September 2019 - 09:00 PM.


#29 Jon Isaacs

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Posted 16 September 2019 - 07:37 PM

The field curvature in an F4.5/F5 scope might seem trivial

  by itself,  but once it is played against an eyepiece for, say, 100x.,

the impact of the curvature is 100 times greater!

 

 

The field curvature of a Newtonian is equal to it's focal length.

 

A 300 mm F/5  has a focal length of 1500 mm. 200x with an 82 degree eyepiece, (10.6 mm field stop), I calculate the curvature of the field as 9 microns.

 

At F/5, the depth of focus is 55 microns, at F/4 Its 35 microns. This means the center and the Edge can be in focus simultaneously. 

 

Field curvature is a function of the square of the linear distance off axis so it quickly diminishes as magnification is increased and the field narrows.

 

Jon



#30 MartinPond

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Posted 16 September 2019 - 08:30 PM

"---

 The field curvature of a Newtonian is equal to it's focal length.

 ---"

 

----"

Field curvature is a function of the square of the linear distance off axis so it quickly diminishes as magnification is increased and the field narrows.

---"

 

It is linear,,, and then in a subsequent statement you say it falls off

    as the square??

The second statement seems to be talking about the actual deviation,

    not the 2nd derivative, curvature..

You are probably talking about two things and calling them by the same name.

 

 

#2 seems closer to what is really going on with this trouble, though...

 

 

The eyepiece wants to see a flat flocal plane.

Unfortunately, it sees a focal  sphere section instead,

and if you focus on the center, the outer parts are not at the correct distance.

...and start to blur.  

The effect is observable and undeniable.

 

 

Somehow, we both must be talking about the "Petzval Surface":

 

https://en.wikipedia...field_curvature

 

There is a nifty example of how the Kepler space telescope has

   a grid of sensors laid out on a curved surface.

Each sensor will experience only the curvature-caused deviation of focal points

      of a flat facet that is 1/5th of the cell mount width,

    so, roughly....1/25th of the deviation...

 

That z-positions look like your second definition of "curvature", the deviation from a plane.

The radius of the curved mount seems to  fit your first definition of  "curvature".

 

When the smoke clears, though, the layout of the sensors for the Kepler

  on a curved surface demonstrates exactly what an eyepiece is up against..

The focal points at the edge are not where the eyepiece wants them to be.

And a little "curvature"(math and physics definition) adds up to an obvious deviation.

 

 

 

I was wrong about higher power making the edges more out of focus, though.

I have edited that out.

The higher power means an eyepiece with a smaller field stop...

   so it all cancels out:  hi and low power see the see the same field trouble.

 

I have edited that error out..


Edited by MartinPond, 16 September 2019 - 09:03 PM.


#31 Jon Isaacs

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Posted 16 September 2019 - 09:59 PM

It is linear,,, and then in a subsequent statement you say it falls off

    as the square??

 

 

Martin:

 

There are two factors:

 

In a Newtonian, the radius of curvature is equal to the scope's focal length.

 

Think of a beach ball. The diameter of the beach ball is equal to twice the focal length.

 

The actual difference in focus is dependent on how far from the center of the field, how far off-axis the star is. 

 

This is dependent on the second power of the distance from the center.

 

Think of the beach ball, one with a 3 meter diameter.  A small section of the ball, 10 mm in diameter is very flat. It's like earth, over small distances, it is essentially flat. But as you look at larger and larger distances, the curvature becomes less and less flat.

 

What have done is calculate the change in flatness over a 10 mm patch of a sphere with a radius of 1500 mm, it's a few microns.  This is significantly smaller than the depth of focus at F/5, at F/4.

 

Jon



#32 RichA

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Posted 17 September 2019 - 12:05 AM

Ok, I know you’re not supposed to use a very fast scope (like an f4 or less) to look at planets.  But suppose you’re about to pack up for the evening and you see Saturn clearing the trees.  Assuming that you’re absolutely certain that the amateur astronomy community will never find out, what eyepiece  do you use to view it?  Do you combine it wIth a specific Barlow?  With some other supplementary tool?  Money is no object answers and thrifty answers are encouraged. 

2.4mm TMB.



#33 213Cobra

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Posted 17 September 2019 - 01:11 AM

>>Ok, I know you’re not supposed to use a very fast scope (like an f4 or less) to look at planets.  But suppose you’re about to pack up for the evening and you see Saturn clearing the trees.  Assuming that you’re absolutely certain that the amateur astronomy community will never find out, what eyepiece  do you use to view it?  Do you combine it wIth a specific Barlow?  With some other supplementary tool?  Money is no object answers and thrifty answers are encouraged.<<

 

Tele-Vue 2.5mm Nagler; 3.5mm Nagler; 3mm DeLite; Takahashi TOE 2.5mm, 3.3mm, 4.0mm. All will work fine at f/4. I use them with my f/3.3 reflector when that's what I have out, and routinely with f/5, 5.3 and 6 refractors. You can use longer FL with Powermate or Barlow, but with the eyepieces I list, I don't bother.

 

Phil



#34 Sarkikos

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Posted 17 September 2019 - 05:46 AM

What is the f number and aperture of your telescope?

 

Mike

Steve Reecy, where are you? 4.gif

 

Mike



#35 stevereecy

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Posted 17 September 2019 - 07:56 AM

Agreed, we have been making suggestions for days now without actually knowing what the scope is. Or receiving any more guidance from OP. Based on signature I am assuming it is a Comet Catcher which would eliminate several of the suggested eyepieces simply because the FL do not go low enough for high power planetary viewing with a Comet Catcher (at least without using a barlow). And yes he is asking for everything from cheap to money is no object, but is one really going to spend $330 or whatever on a Tak TOE to view planets with what, a 50 year old budget scope with something like 70% CO? Ok I exaggerate but putting a TOE in a Comet Catcher feels like putting a 500 hp engine in a Honda Fit. The Comet Catcher is going to be the limiting factor. So a bit of guidance from the OP to specifically identify the scope, and a realistic budget for what he might spend on a planetary eyepiece for one of the worst planetary scopes ever made, would be quite helpful.

 

Op here:   Yes my interest is the comet catcher.  F3.6???  But I figured the answers would be helpful to others so I kept my question generic. The irony is that the comet catcher has a built in coma corrector, so my plan may have backfired. 


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#36 stevereecy

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Posted 17 September 2019 - 07:59 AM

To all: Thank you so much for the answers. You guys are the best!  I was digging through a box of stuff and found a 9mm Nagler that I thought I sold.  I’m going to try barlowing it using 2x and 3x to see what I get. 


Edited by stevereecy, 17 September 2019 - 08:47 AM.

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#37 Jon Isaacs

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Posted 17 September 2019 - 11:23 AM

To all: Thank you so much for the answers. You guys are the best!  I was digging through a box of stuff and found a 9mm Nagler that I thought I sold.  I’m going to try barlowing it using 2x and 3x to see what I get. 

 

An excellent plan.. and wise and fortunate decision in the past not to sell the 9 mm. :)

 

Jon



#38 George9

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Posted 17 September 2019 - 01:29 PM

Is that a type 1 (original) 9mm Nagler? Good you kept it, and don't sell it in the future. It has almost a full mm of eye relief more than the type 6, even though both are listed as 12mm due to rounding. I think it was the best of the original Nagler series.

 

George



#39 25585

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Posted 18 September 2019 - 07:45 AM

Is that a type 1 (original) 9mm Nagler? Good you kept it, and don't sell it in the future. It has almost a full mm of eye relief more than the type 6, even though both are listed as 12mm due to rounding. I think it was the best of the original Nagler series.

 

George

1mm is such a tiny measurement! 



#40 George9

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Posted 18 September 2019 - 10:27 AM

1mm is such a tiny measurement! 

True but it matters when you are right on the border. E.g., between eyelashes just touching or not, or seeing the full AFOV or not. And 12mm is short enough that some people will be on the border.

 

But is it really worth older coatings? I guess I just found it more comfortable than my 7mm Nagler6. (But less portable.)

 

George




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