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New Meade 26mm MWA

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18 replies to this topic

#1 Ronofthedead07

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Posted 20 August 2019 - 02:46 PM

So Meade appears to be offering a new 100-degree MWA eyepiece in a 26mm focal length (#607019). Meade does not appear to have provided a whole lot of information but are claiming a 7-element design with 25mm of eye relief. Price is $289.99.

 

At this focal length the only real comparable eyepiece is the 8-element Explore Scientific 25mm 100* which retails for $749.99 (currently on sale for $599.99). I haven't used the ES but it seems to receive somewhat mixed reviews, so I wonder how this new Meade may compare.



#2 dan_h

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Posted 20 August 2019 - 04:29 PM

A 100 degree field for a 26mm focal length would require a field stop very nearly 62mm diameter. 

How does one fit this in a 2" barrel?

I would expect edge problems of all sorts.

 

dan  



#3 25585

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Posted 20 August 2019 - 05:04 PM

Same optics as the ES?



#4 SeattleScott

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Posted 20 August 2019 - 05:54 PM

At half the price? Not likely. Pretty sure the ES has more than 7 elements.

Scott

#5 DRodrigues

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Posted 20 August 2019 - 06:52 PM

These are similar to the Omegon Panorama II  https://www.cloudyni...n-panorama-ii/ so have <90º AFOV.

I'm curious about the ER mentioned and would like to see if it allows viewing all AFOV with glasses on - the 20mm ER mentioned for the 15mm version didn't...


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#6 Riccardo_italy

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Posted 21 August 2019 - 09:10 AM

A 100 degree field for a 26mm focal length would require a field stop very nearly 62mm diameter. 

How does one fit this in a 2" barrel?

I would expect edge problems of all sorts.

 

dan  

(26*100)/57.3=45.4mm

 

That's in theory!


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#7 dan_h

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Posted 21 August 2019 - 01:23 PM

(26*100)/57.3=45.4mm

 

That's in theory!

My bad. Thank you for the correction.

 

dan



#8 Vla

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Posted 21 August 2019 - 04:57 PM

My bad. Thank you for the correction.

It's not. The 57.3 route is accurate only for small angles, much smaller than 100 (which btw has negative tangens). Exact field stop diameter is given by 2f*tan(GFOV/2), where "f" is the focal length and GFOV the geometric field of view,  i.e. field with zero distortion. Since we only have AFOV, which is inflated by distortion, we could approximate the field stop by assuming 20% distortion, which would make the GFOV about 83 degrees. The corresponding field stop would be 2f*tan41.5=46mm.


Edited by Vla, 21 August 2019 - 06:01 PM.


#9 Vla

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Posted 21 August 2019 - 06:00 PM

So Meade appears to be offering a new 100-degree MWA eyepiece in a 26mm focal length (#607019). Meade does not appear to have provided a whole lot of information but are claiming a 7-element design with 25mm of eye relief. Price is $289.99.

 

At this focal length the only real comparable eyepiece is the 8-element Explore Scientific 25mm 100* which retails for $749.99 (currently on sale for $599.99). I haven't used the ES but it seems to receive somewhat mixed reviews, so I wonder how this new Meade may compare.

The 7-element Nagler-like eyepieces can have excellent correction. An example by Smith/Ceragioli/Berry in their book has flat field and 0.77 Strehl in the green 40 degrees off. With little modification, perhaps some mild field curvature, it can go close to 100. Its disadvantage is the size.



#10 dan_h

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Posted 21 August 2019 - 07:54 PM

It's not. The 57.3 route is accurate only for small angles, much smaller than 100 (which btw has negative tangens). Exact field stop diameter is given by 2f*tan(GFOV/2), where "f" is the focal length and GFOV the geometric field of view,  i.e. field with zero distortion. Since we only have AFOV, which is inflated by distortion, we could approximate the field stop by assuming 20% distortion, which would make the GFOV about 83 degrees. The corresponding field stop would be 2f*tan41.5=46mm.

Thank you. I had used the 2f*tan(GFOV/2) and for 100 degrees it yields a field stop of 61.96mm.  I hadn't realized how significant the distortion is.  

 

dan



#11 Vla

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Posted 22 August 2019 - 07:01 AM

Distortion in a 100-deg unit is probably more than 20%. The "Modern Nagler" mentioned above has 24% 40 degrees off axis, and it's (very) unlikely that a similar design will have less at 50 degrees. Which implies  that the field stop is smaller than 46mm.



#12 Jon Isaacs

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Posted 22 August 2019 - 12:35 PM

My thinking:

 

AFoV is something that can be directly measured. Any scaler relationship between the focal length, the field stop and the AFoV will require a fudge factor which accounts for distortion.

 

Interestingly, if one uses the relationship:

 

AFoV = 57.3 deg/rad x Focal length / Field stop, eyepiece's like the 21 mm and 13 mm Ethos, the 20 mm ES 100 degree, work out to within a degree or two of 100 degrees.

 

Jon



#13 dan_h

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Posted 22 August 2019 - 04:34 PM

My thinking:

 

AFoV is something that can be directly measured. Any scaler relationship between the focal length, the field stop and the AFoV will require a fudge factor which accounts for distortion.

 

Interestingly, if one uses the relationship:

 

AFoV = 57.3 deg/rad x Focal length / Field stop, eyepiece's like the 21 mm and 13 mm Ethos, the 20 mm ES 100 degree, work out to within a degree or two of 100 degrees.

 

Jon

I did some research. As I understand it, that formula is derived from the formulas for the image size from the objective. 

 

Basic trigonometry gives, Primary  Image size = tan(FOV) X Objective FL. 

 

The Objective FL is equal to EPFL X Magnification,  and magnification is defined as AFOV/FOV

 

So Image Size or Field Stop size = tan(FOV) X EPFL X (AFOV/FOV) 

 

At small angles (<0.1 radians),  tangent very nearly equals angle so the factors Tan(FOV) and FOV cancel leaving 

 

Image Size or Field Stop = AFOV * EPFL and we need to add the conversion factor of 57.3 for degrees to radians .

 

Field stop(mm) = AFOV * EPFL(mm) /57.3  which as you stated, seems to agree with actual field stops. 

 

Working backwards as I originally did, from the AFOV using EPFL,  results in an apparent size for the field stop, not the actual size. 

 

dan


Edited by dan_h, 22 August 2019 - 04:35 PM.


#14 Starman1

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Posted 22 August 2019 - 05:31 PM

(26*100)/57.3=45.4mm

 

That's in theory!

Correction.  100° would require 61.97mm of field stop, but that is for 0% distortion, which doesn't correspond to reality.

With distortion, it could be as small as 44.6mm (45.4 calculated).  That figure is the 13mm Ethos field stop x 2 for the 2 x 13mm focal length, assuming equal distortion.

However, what are the odds it would be better than the 8 element ES 25mm?

The ES eyepiece has noticeable vignetting and chromatic aberration of the exit pupil, and astigmatism in the outer 10-15% of the field at f/5-f/6 (probably worse at faster

f/ratios, but I have no experience there with this eyepiece)

Now, if the Meade is, indeed, only around 90°, it could be quite OK.  We need some test reports.  Who's the guinea pig?


Edited by Starman1, 22 August 2019 - 05:36 PM.

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#15 Miranda2525

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Posted 22 August 2019 - 07:05 PM

So Meade appears to be offering a new 100-degree MWA eyepiece in a 26mm focal length (#607019). Meade does not appear to have provided a whole lot of information but are claiming a 7-element design with 25mm of eye relief. Price is $289.99.

 

At this focal length the only real comparable eyepiece is the 8-element Explore Scientific 25mm 100* which retails for $749.99 (currently on sale for $599.99). I haven't used the ES but it seems to receive somewhat mixed reviews, so I wonder how this new Meade may compare.

Link?



#16 nicoledoula

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Posted 22 August 2019 - 09:20 PM

https://www.meade.co...-eyepieces.html


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#17 Miranda2525

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Posted 24 August 2019 - 09:07 AM

https://www.meade.co...piece-26mm.html

 

Ahhh, ok, I see it now. 

 

That will be a hard task. I have a 23mm Axiom LX which is 84.5 degrees. IMO, the 26mm is a stretch of the imagination. lol.


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#18 25585

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Posted 24 August 2019 - 03:48 PM

Waiting for a UK dealer to stock one.....



#19 Ronofthedead07

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Posted 01 September 2019 - 08:29 PM

Thanks for the input, everyone. I am skeptical of some of the claims, so it will be interesting to see how it fares.

 

I'm sticking to 82s for the time being!




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