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Davesellars

Eyepiece dilemma

22 posts in this topic

I'm looking at replacing my 24mm ES68 for use in my 12" f/4.92 dob primarily without a coma corrector.

The 24mm serves as my 1st eyepiece when finding an object after getting it in the rough area with my RACI finderscope - As the object (particularly faint galaxies etc) are not in view with the RACI this eyepiece for me is absolutely critical to the success of finding the object or even again finding the exact field where it should be and having to step down to an eyepiece with around 2mm exit pupil which would be the 10mm Delos.

Because of stepping down to the 10mm Delos directly it would be beneficial to be par-focal so was directly thinking of replacing the ES with the TV Panoptic 24mm...

However!  This got me thinking that I was using with my previous setup with regards to exit pupil - Which was the 24mm with a C8 @ f/6.3 - This resulted in an exit pupil of 3.8(ish) and 1.3 degrees field of view.  90% of the time time I would see directly the DSO rather than having to step down to a more optimal exit pupil.  I have a slight worry that that if I stick with 24mm this yields an exit pupil of 4.8 and may be a bit too bright but I guess the increase in aperture will make up for it.

I have a 19mm Panoptic but the field is getting really a bit small as a finder.   Another option is a 2" eyepiece but have to be careful as a lot start to get very weighty otherwise perhaps the ES82 24mm would be a better choice than the TV Panoptic?   I don't know if the weight specified on the spec sheet is accurate but it gives 870g which is twice as heavy as the Delos.  I don't think it's sensible to go much over 500g

Sorry for rambling... :)  I'm just trying to get in my head the best choice or if there's something I've overlooked or not considered.

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Dave, in my opinion the 24mm Pan would be the best choice, it combines of a fairly lightweight ep with a great FOV.  I find the ES82 24mm sluggish and unweildly and tend to grab the Pan as it is much easier to use, also good at 1.25" barrel.

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Many thanks Robin.  Yep, I think I'd get annoyed with having to switch or faff about with any 2" to 1.25" adapters.  Being able to quickly slot in the 10mm Delos and be on focus without losing the position is the main point.  I'm having to save weight as it is by changing over my Telrad for Rigel Quickfinder so I'm not having to deal with much in the way of counterweights.

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May I ask what do you see the main benefit upgrade the 24mm ES to 24 Pan?

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Posted (edited)

24mm ES68 starts to look a bit shabby for the last 30 degrees - I think it suffers from other aberrations apart from coma at this focal ratio.  It works excellently with my f/7.5 refractors and pin-pint to the edge.  But certainly it starts to break up at f/5 (with my 120ST as well).   My 19mm Pan is sharp to the edge in comparison with the 24mm ES68.

Edited by Davesellars

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Well, that might be some simple explanations:smiley:

120ST with its 600mm focal length is known to have quite noticeable field curvature(FC), and 19mm Pan is often mentioned with some FC too, the curvatures in 120ST and 19mm might happen to be the opposive sign so that it canceals each other, therefore sharp to the edge with 19mm Pan, while through 24mm ES you get to see the FC in the scope. Have you tried to focus on the edge stars? can it be brought to sharp focus?

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Yep, I've tried re-focusing and stars can be brought to sharper focus by re-focusing - certainly in the 120ST I've seen this effect.  Can't remember doing it with the 12" dob though....

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27 minutes ago, Davesellars said:

24mm ES68 starts to look a bit shabby for the last 30 degrees - I think it suffers from other aberrations apart from coma at this focal ratio.  It works excellently with my f/7.5 refractors and pin-pint to the edge.  But certainly it starts to break up at f/5 (with my 120ST as well).   My 19mm Pan is sharp to the edge in comparison with the 24mm ES68.

But shouldn't the EP with a bigger field stop by default show more field curvature? Out of two 68 degree eyepieces the 24mm one 'slices' a wider chunk of the curve...

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Posted (edited)

12 minutes ago, BGazing said:

But shouldn't the EP with a bigger field stop by default show more field curvature? Out of two 68 degree eyepieces the 24mm one 'slices' a wider chunk of the curve...

I'd guess so, but to my eyes the 19mm has no field curvature showing as the focus is equal across the FOV.  The 24mm ES68 shows quite a lot.  If the Panoptic 24 construction is the same as the 19mm then I should see considerably less than I am at the moment I believe if any at all?

Ideally I need someone to comment that has the SW 12" flextube dob AND the 24 Panoptic to comment if this combination is good ;)

Edited by Davesellars

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well, i always see more fc in my refractor when i step up ep focal length. never compared pans and es eps in similar focal lenghts. 

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2 minutes ago, Davesellars said:

I'd guess so, but to my eyes the 19mm has no field curvature showing as the focus is equal across the FOV.  The 24mm ES68 shows quite a lot.  

Is it the same in the 12" dob as in 120ST?

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Yes, the 19mm is pin-point across the FOV in both scopes.  The only thing I don't remember doing with the 12" was re-focusing to ensure it was the same thing with the 24mm EP.  If I remember rightly it is more pronounced in the 120ST but is still there with the 12" dob in reasonable quantity.  Coma I did not notice - I know what that looks like because my little 100p f/4 shows ooodles of it! :)

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OK, good to know, good luck with your upgrade.

Just a quuote of optical measurement information the ES 24mm vs 24 Pan in a f4 scope:wink: (Those angular miniutes etc size means how big a star's spot size)

----------------------------------------

ES 24mm 68°
 - in the center less than 5 angular.  Minutes.
 - on the zone (70% from the center to the edge) 11 angular.  Minutes due to curvature and astigmatism.
 - on the edge of the field of view - 20 ang.  Minutes.
The main aberration is curvature and astigmatism.  There is a noticeable (5-6%) positive distortion ("pincushion").  The image of the diaphragm edge is sharp and unpainted.  There are also no traces of increase chromaticity.

Pan 24mm
With a 1: 4 lens.
Excellent image of the luminous point on the axis of less than 2-3 angular.  Minutes. 
On the zone (70% of the center to the edge) the star image increases to 6-7 angles.  Mostly due to astigmatism. 
At the edge of the field of view (one and a half degrees from the edge of the diaphragm), the aberration spot grows to 10 angles in size.  Minutes, the main aberration is astigmatism and coma.  The chromaticity of the increase reaches 20 ang.  Minutes (i.e., of the order of 1%).  Distortion is positive (pincushion) not less than + 3-4%.

----------------------------------

 

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6 minutes ago, YKSE said:

 

----------------------------------------

ES 24mm 68°
 - in the center less than 5 angular.  Minutes.
 - on the zone (70% from the center to the edge) 11 angular.  Minutes due to curvature and astigmatism.
 - on the edge of the field of view - 20 ang.  Minutes.
The main aberration is curvature and astigmatism.  There is a noticeable (5-6%) positive distortion ("pincushion").  The image of the diaphragm edge is sharp and unpainted.  There are also no traces of increase chromaticity.

Pan 24mm
With a 1: 4 lens.
Excellent image of the luminous point on the axis of less than 2-3 angular.  Minutes. 
On the zone (70% of the center to the edge) the star image increases to 6-7 angles.  Mostly due to astigmatism. 
At the edge of the field of view (one and a half degrees from the edge of the diaphragm), the aberration spot grows to 10 angles in size.  Minutes, the main aberration is astigmatism and coma.  The chromaticity of the increase reaches 20 ang.  Minutes (i.e., of the order of 1%).  Distortion is positive (pincushion) not less than + 3-4%.

----------------------------------

 

Great many thanks for the info.

What I read from that is the Pan is almost twice as sharp in all aspects?   Also from this - "Distortion is positive (pincushion) not less than + 3-4%."  - would this mean that the Panoptic is pretty flat then?

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Yes, you can read as twice as sharp by some numbers, but under 5-6 argmin is not very easily seen except on bright stars.

Distortion has nothing to do with FC, it's the geometrical distortion, easily seen in daytime targets with straight lines.

You might seen more lateral colours with 24pan than 24 ES. That's what that chromaticity  means, I guess.

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i believe the data is from ernest's measurements at astro-talks.ru

basically, between es 24 and pan 24 it is pick your poison. pan has tighter stars, and that might be notieceable not necessarily in the center of f/5, but more in the zone. at the edges the total aberration of the pan (including chromatic smudge) is comparable to the aberration of the es24.

so overall pan is a bit tighter and has different aberrations at the edge, which do not include noticeable curvature.

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Ok.  I think it'll be worth it.  At the very very least I'll have par-focal eyepieces which is something which annoys me intensely using the dob particularly.  Plus a weight saving which is useful for my alt/az setup as well as the dob.

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I had a chance last night towards the end to do a bit of a test and compare view of the Panoptic 19mm properly with the 24mm ES.  I pointed it a decent star field in the middle of Cygnus where there wa a good mix of bright stars and faint stars.  Interestingly the Pan was sharper across the entire view, I'd say better colour definition with the Pan as well.  The surprised me because the Pan with higher power should not show tighter stars than the ES? Still, the ES was very good and perhaps it's drop off in quality of stars was not quite as pronounced as I'd gathered before and was rectifiable with a touch on the focuser for the stars from 70%.  The Pan was in a different league though and showed only a drop off in quality in the last 5% otherwise was pretty flat.

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21 hours ago, Davesellars said:

The surprised me because the Pan with higher power should not show tighter stars than the ES?

Tightness of stars has more to do with the level of optical correction and quality of polish than magnification level since stars remain point sources at any magnification level in amateur telescopes.  Large globular clusters resolve as grains of back-lit salt in my 5.2mm Pentax XL even though I'm running at over 200x as an example.

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OK, after a lot of thinking and particularly after the weekend's session I've taken the plunge and purchased the 24mm Panoptic.

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6 hours ago, Davesellars said:

OK, after a lot of thinking and particularly after the weekend's session I've taken the plunge and purchased the 24mm Panoptic.

As long as you don't need to wear eyeglasses at the eyepiece, you won't regret it.  It is probably the best corrected of all the Panoptics, being the newest.

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Nope, no eyeglasses. :). I find the 19mm really comfortable so will be fine with the 24mm.  All I need now is a clear night tomorrow to give it a good go!

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