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40mm - 50mm Eyepiece Question


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I am looking to add a lower magnification  eyepiece to my collection and have been looking at the 40/50mm range. Is it not as important to have a wider fov on this size of eyepiece as it is on the higher magnification ones as most seem to be in the 50/60 fov range ?

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There are wider ones. The TMB Paragon 40mm had 68 deg FOV (clones are the TS Paragon and the Skywatcher Aero). There is also the Meade S5000 40mm SWA (or its MaxVision/ES clones) at 68 deg FOV. I have a Vixen LVW 42 at 65 deg FOV, and the Panoptic 41 mm is also 68 deg. These offer the widest true FOV in a 2" barrel EP. this is why 50 mm EPs generally have a smaller apparent FOV. If you can get your hands on a TMB Paragon 40mm I would pounce on it, as it is very good, comparatively light, and extremely comfortable.

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You don't state the f-ratio of your scope.  Multiply 7 by your f-ratio to determine the typically longest focal length eyepiece you would want to use with your scope.  This is because you don't want to go much above a 7mm exit pupil because that about the biggest your eye's iris will dilate during dark adaptation.  That, and the skies get really washed out making it difficult to pick out DSOs.

Let's say you have an f/6 scope, 7*6=42mm would be the longest you'd probably want to go.  For an f/10 SCT, theoretically, you could go to 7*10=70mm, but the view would be like looking through a straw due to the limits of a 2" visual back.  For an f/4 Dob, it would only be 7*4=28mm, thus the popularity of the 21mm Ethos or 25mm ES-100.

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3 hours ago, Louis D said:

 you don't want to go much above a 7mm exit pupil because that about the biggest your eye's iris will dilate during dark adaptation.

It varies by age too...
image.png.9d10d9070d0c2171c30d6d4d989a03f7.png

... but naturally YMMV.  I'm 52 and my optician measured mine at 4 light and 6 dark... so I'm telling everyone I'm 40 from now on....

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My 31mm Nagler gets used a lot less than my 21mm Ethos and the 40mm Aero ED much less than either.

If I observed regularly under very dark skies perhaps the result would be different perhaps ? :dontknow:

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11 minutes ago, John said:

My 31mm Nagler gets used a lot less than my 21mm Ethos and the 40mm Aero ED much less than either.

If I observed regularly under very dark skies perhaps the result would be different perhaps ? :dontknow:

The skies are Bortle 5 where I live but my daughter lives in Ireland and the skies are amazing as there are no street lights. We visit several times a year normally 🙄 so I am planning for the times we are able to visit again.

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44 minutes ago, John said:

My 31mm Nagler gets used a lot less than my 21mm Ethos and the 40mm Aero ED much less than either.

If I observed regularly under very dark skies perhaps the result would be different perhaps ? :dontknow:

It could be you are expressing a preference for a darker background in the eyepiece, but it is equally possible you are expressing a desire for a higher magnification.

I had the 31 Nagler and 21 Ethos for a few years, but found I only reached for the 31 a few times in that period, so I sold it.  After moving away from the 100° fields of the Ethos eyepieces at longer focal lengths

due to astigmatism and needing glasses at lower powers,  I found that narrowing the fields brought back a need for the longer focal length again.  If limited to 76-82°, I find I can use a smaller gap between eyepieces.

It's not obvious that having 100° eyepieces makes possible a wider spread between magnifications, but it seems to be the case.

As Al Nagler explains the "majesty factor" (unfortunate name), if the higher power eyepiece has the same field size, most people will prefer it because the background will be darker and the apparent field less bounded.

Those 3 eyepieces have true fields of a 10 8.3 : 6.2 ratio, but a magnification ratio of 25 : 32 : 48, so the field area decreases by 38% while the magnification increases by 92%.  That makes the higher power feel as if it has not narrowed in keeping with the magnification increase, giving more of a feeling of a huge expanse of sky.  The 21mm Ethos is narrower in field stop than the 31mm Nagler, but the impression of the field size, contrast, and sharpness makes the 21 more impressive to look through.  And that would be true in dark skies as well as brighter ones.

Edited by Don Pensack
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The diameter of the eyepiece barrel limits the diameter of the field stop which is what defines the edge of the eyepiece apparent field of view. Once the eyepiece focal length gets over a certain length the AFoV just gets smaller and smaller so no more sky is shown, ie: the true field of view remains the same.

If you step up to a larger barrel size, you can get the wider fields of view with these longer focal length eyepieces. The is a matter of size and weight to consider though. These are 3 inch accessories:

https://www.cloudynights.com/uploads/monthly_01_2015/post-99-0-64539800-1420184496.jpg

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2 hours ago, banjaxed said:

My eyepiece collection is 4,8,12.16,20,27 and 32mm

The mm size of an EP is only one of it's key stats. AFOV is just as important, especially when your aim is to see more of the sky. Without knowing the AFOV of your EPs it's hard to help you find one that gives a noticeable difference.

I have a Celestron E-Lux 32mm which has an AFOV of 56.  In my scope this gives a TFOV of 0.84.
While my F/10.5 scope is never going to show a huge chunk of sky I figured I could do a little better and with the Pentax XW 40mm, with it's AFOV of 70, I get 1.31 degrees of sky. I'm very pleased with the results.

If my 32mm had had an AFOV of 82 then it would have given 1.23 degrees of sky and there would have been little point in the XW40 (from a TFOV point of view).

Edited by globular
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8 hours ago, banjaxed said:

As it has to be transportable I usually take my ST 120 or the 127mak.

Is the ST120 F/5 and the 127mak F/11.8?

If so and you are after lower mag / wider views then I assume you'd use the ST120. 
And your existing 32mm in that scope is giving you 18.8x mag and 2.67 degrees TFOV at exit pupil of 6.4mm.

I'm not sure I'd go any larger than 32mm in that scope because of the exit pupil and already very low magnification.
If you're keen on seeing more sky then you could get something like a 31mm Nagler giving 4.24 degrees TFOV and 19.4x mag with 6.2mm exit pupil.  Not cheap though.
Or there is the 30mm ES 82 giving 4.10 degrees TFOV at 20x mag and 6mm exit pupil.  This is cheaper (still not cheap though) but I'm not sure it performs as well in faster scopes like this.

If you have young eyes and think you could take an 8mm exit pupil then something like my Pentax XW40 would give you 15x mag and 4.67 degrees TFOV.  I wouldn't though.

You might even consider something like the APM HDC XWA 20mm.  With its 100 degree AFOV it will give you more sky than your 32mm plossl at 3.33 degrees, at higher magnification of 30x and with a still very nice exit pupil of 4mm.  The price is very good too for this hyper wide design.  But eye relief is only around 15mm so it might not be suitable if you wear glasses.

There are so many options... 🤔

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A 2 inch wide field / low power eyepiece that will exploit the ST 120mm F/5 refractor capabilities will simply not be useful with the 127mm Mak-Cassegrain as far as I can see:icon_scratch:

 

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Compatible in both scopes: 1.25"

Widest possible field of view in the scope (all these have the same TRUE field of view):

40mm 40° Plössl

32mm 50° Plössl

24mm 68° widefield

Formula to compute true field:

True Field in degrees= (field stop of eyepiece ÷ focal length of telescope) x 57.296

 

The eyepieces above all have a 27mm field stop, the approximate size of the inside of the barrel.

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If the OP already has a 2" diagonal, all he needs to add is a Mak to SCT thread adapter and a 2" visual back to use it with his 127 Mak, and then he can use 2" eyepieces with it.

I've found the vignetting to be minimal to the eye as I discussed in the following thread:

 

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