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Help understanding the correlation between eyepiece AFOV and Focal Length

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#1 BMANN2

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Posted 28 May 2022 - 08:36 PM

I have been reading a few posts that have been asking about eyepiece recommendations and I see a couple different ones brought up often. So I decided to look at them in the Field of View Calculator.

 

This is when I saw something I didn't entirely understand. I used EP's with different FL and different AFOV for each EP, but each gives 'basically' the same view. If you take a look at the picture below. I used a Celestron C8 as the telescope, pointed at M31. Given the somewhat noticeable price differences in the eye pieces used, but getting the same view, what is the difference?

 

What else is there to look for in an EP? Is there a difference from a narrower low power (Tele Vue 32mm) and a wider higher power (ES 24mm) if they give 'the same view'? Or possibly the even cheaper X-Cel LX with similar power to the ES EP but just a tad smaller FOV?

 

KXYh3da.png



#2 james7ca

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Posted 28 May 2022 - 09:31 PM

Although the true field of views look similar the eyepieces are producing different magnifications and different exit pupil sizes. Both of the latter are important to determine the apparent size of the object (the higher the magnification the larger the object will appear) and the surface brightness of any extended object (related to magnification but also to the size of the exit pupil).

 

Determining which eyepiece is "best" might come down to personal preference. That said, there are other factors to consider such as eye relief, edge of field performance, image distortion, and color correction (to name four out of several other factors).


Edited by james7ca, 28 May 2022 - 09:32 PM.

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#3 N3p

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Posted 28 May 2022 - 09:47 PM

You pick what you like really.

For instance on my equatorial mount with tracking I like to use my 6mm orthoscopic on the moon with 42d AFOV, but I never use it with the manual Dobson because it's too difficult to keep the object inside of the FOV with something as narrow as 42d. On the  EQ mount, these eyepieces are fantastic. The type of mount can dictate some limits.

Then I discovered at some point that I am comfortable with a AFOV of 68~70d for general observation, I don't need to move my eye too much around to see the edge, it's all there easily. The eyepiece can be a bit lighter also compared to 82, 90, 100d, again, in my case because of the EQ mount "limitations".

 

Some eyepieces are easier to use then others with the eye placement, the eye relief. It's really a life time of experimentation.

 

I think with standard C8 you have some limitation with the maximum AFOV you can use without getting some vignetting, with longer FL eps (?), it needs part modification in the back.  I have picture of these cool but expensive modifications somewhere.



#4 BMANN2

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Posted 28 May 2022 - 10:17 PM

Although the true field of views look similar the eyepieces are producing different magnifications and different exit pupil sizes. Both of the latter are important to determine the apparent size of the object (the higher the magnification the larger the object will appear) and the surface brightness of any extended object (related to magnification but also to the size of the exit pupil).

 

Determining which eyepiece is "best" might come down to personal preference. That said, there are other factors to consider such as eye relief, edge of field performance, image distortion, and color correction (to name four out of several other factors).

So I am understanding the exit pupil. I forgot about that (diameter of aperture / magnification). But I am having troubles wrapping my head around how these will change the size of the object. If you have a low power narrow FOV, versus a high power wide FOV. Like demonstrated looking at the ES EP and the Tele Vue EP in my original picture. Wouldn't the size of what ever you are looking at be the same? Because the object itself it taking up the same area I guess you could say of your field of view? 

 

Even though one is more magnified, you don't see a lot around it so it takes up your whole FOV. And if one is less magnified, but narrow. Then it still takes up your whole FOV so wouldn't both seem the same size? I feel like I am thinking about this completely wrong or something sorry about that. 



#5 Jon Isaacs

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Posted 28 May 2022 - 10:22 PM

What else is there to look for in an EP? Is there a difference from a narrower low power (Tele Vue 32mm) and a wider higher power (ES 24mm) if they give 'the same view'? Or possibly the even cheaper X-Cel LX with similar power to the ES EP but just a tad smaller FOV?

 

 

Your questions are good ones.  There are three things to consider:

 

-The TFoV which is what you thinking about now.

 

- The Magnification. The 32 mm provides 64x in you C-8, the 24 mm provides 85x, 33% greater. Your posted images are deceiving because the show Andromeda as being the same size in the eyepiece. You see the same part of Andromeda but in the 24 mm, it's 33% larger so you see more detail. I have scaled your images to show what they would look like at the different magnifications relative to each other. The top is the 32 mm eyepiece, the bottom, the 24 mm.

 

IMG_28052022_201827_(360_x_360_pixel).png

 

IMG_28052022_201711_(480_x_480_pixel).png

 

- The exit pupil. The exit pupil is the beam of light you actually look it. The greater the magnification, the smaller it is. The brightness of Andromeda is proportional to the Area of the Exit Pupil. Your scope is F/10, the exit pupil is the Aperture / Magnification or the Focal Length of the Eyepiece / Focal Ratio 

 

For the 32 mm = 32/10 = 3.2 mm, for the 24mm it's 24mm / 10 = 2.4 mm Since it's the Area, the brightness ratio is (3.2/2.4)2 = 1.78x, the 32mm is 78% brighter.

 

Jon


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#6 GlennLeDrew

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Posted 28 May 2022 - 11:12 PM

The magnification determines image scale. 50X makes the object appear to be 50X nearer to you and hence 50X larger than as seen by the unaided eye. This is independent of the field of view. Whether the eyepiece is a narrow or a wide field design, the apparent size of the object, or some part of it, on your retina will be the same.

 

The eyepiece design determines whether it delivers a narrow, moderate or wide field. When you peer into an eyepiece, the image is framed by a (nominally) sharp-edged circle of blackness. The apparent angular diameter of this circle on your retina sets the apparent field of view (AFoV). As the AFoV gets wider, the view looks like you are getting closer to a porthole on a spaceship. For instance, an ultra-wide AFoV of 100° appears 2X wider than the standard 50° AFoV, and because area is a squared function, covers 4X more area. It's like the difference between a basic cinema screen and IMAX.

 

The true field of view (TFoV) depends on both the magnification and the AFoV. For instance, if a 50X eyepiece has a 50° AFoV, it's TFoV is 50° / 50X = 1°. By comparison, a different eyepiece delivering the same 50X eyepiece but having a 100° AFoV (twice as wide an image circle) will provide a TFoV of 100° / 50X = 2°.

 

Note that in the foregoing both eyepieces deliver the same 50X, meaning the image scale will be the same, the amount of detail will be the same, and the image brightness will be the same. The principal difference will be the factor of 2 difference in both the AFoV and the TFoV, and the 4X difference in sky area and retina covered with imagery. The wider AFoV at given magnification covers more sky, thus permitting to accommodate larger objects, and it makes panning about or star hopping more efficient. And it better exploits the more sensitive outer retina. And it makes for a more immersive viewing experience.

 

Consider an eyepiece of 25mm focal length and AFoV of 50°. Now consider an eyepiece of 12.5mm focal length and AFoV of 100°. The second eyepiece, having 1/2 the focal length of the first, will deliver 2X the magnification. But it has 2X the AFoV, compensating for the doubled power, hence resulting in it still providing the same TFoV as the lower powered eyepiece.


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#7 RichA

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Posted 28 May 2022 - 11:24 PM

I have been reading a few posts that have been asking about eyepiece recommendations and I see a couple different ones brought up often. So I decided to look at them in the Field of View Calculator.

 

This is when I saw something I didn't entirely understand. I used EP's with different FL and different AFOV for each EP, but each gives 'basically' the same view. If you take a look at the picture below. I used a Celestron C8 as the telescope, pointed at M31. Given the somewhat noticeable price differences in the eye pieces used, but getting the same view, what is the difference?

 

What else is there to look for in an EP? Is there a difference from a narrower low power (Tele Vue 32mm) and a wider higher power (ES 24mm) if they give 'the same view'? Or possibly the even cheaper X-Cel LX with similar power to the ES EP but just a tad smaller FOV?

 

KXYh3da.png

You can double field diameters going from Plossls and the like to 100 degree eyepieces.  But there is magnification as a benefit to consider in deepsky viewing. Using higher power suppresses background sky brightness leading to seeing more detail within the same area of coverage.  For example, in a light polluted area, a 12mm eyepiece with an 84 degree design will provide (likely) more object detail (stars, nebular) than a 20mm eyepiece with a 50 degree APV.  Going from say 50x to 100x in a larger scope will often net you a near magnitude extra gain which is a benefit depending on the object being viewed.  Even lower magnification increases will produce noticeable gains.



#8 Michael Covington

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Posted 28 May 2022 - 11:54 PM

In your example, you're bumping into a very simple thing -- in the focal length range 25 mm and up, the diameter of the field of view is limited by the eyepiece tube.  So you can have either a shorter focal length with a wider AFOV, or a longer focal length with a narrower AFOV, and either way, the true FOV is limited by the tube.  

Also, 25mm to 32mm is just not a big difference.



#9 Dave Mitsky

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Posted 29 May 2022 - 12:02 AM

The 24mm 68-degree AFOV will provide a more "immersive" view at a higher magnification, which darkens the background sky somewhat and makes for a more pleasing view in that regard.  The 32mm 50-degree eyepiece may work a bit better with nebula filters, which tend to perform better at larger exit pupils.

 


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#10 BMANN2

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Posted 29 May 2022 - 10:16 AM

Thank you all for the super detailed and well explained responses. I am understanding the topic much better now! biggrin.png


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