<p>How do I find the apparent field of view (aFOV) of a eyepiece using simple math (if possible)? I need the aFOV to give as a value to Stellarium’s custom eyepiece</p>
<p>I found a Q&amp;A about this subject but, I don’t fully understand the math:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/questions/40492/how-to-estimate-my-eyepiece-fov">How to estimate my eyepiece FOV?</a></li>
</ul>
<p>On my eyepiece it says “<strong>Gskyer® 5mm</strong>” Also see photo below.</p>
<hr />
<p><a href="https://i.sstatic.net/4SBQa.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/4SBQa.jpg" alt="Photo of my 5 mm eyepiece" /></a></p>

CC BY-SA 4.0
2022-01-19T22:27:47.293Z
Note that the aFOV depends on the detailed design of the eyepiece; it can't be deduced from magnification or focal length.  That's why the linked question advises you to measure the field stop or time how long it takes a star to cross the field; some measurement will be required to get the aFOV.  Inexpensive eyepieces will have aFOV of about 40 degrees or less; specialized wide-field eyepieces, usually larger and heavier with larger glass elements, can achieve 80 degrees or more.
antlersoft
CC BY-SA 4.0
2022-01-20T00:19:14.003Z
@GrapefruitIsAwesome please read the question first before voting to block everyone else from posting an answer; the OP has already linked to that question and asks for further clarification. Closing as duplicate in this case (without offering help) is counterproductive.
uhoh
CC BY-SA 4.0
2022-01-20T00:21:23.143Z
Hi @Donnie *Welcome to Stack Exchange!* What information do you have now? Do you have the eyepiece itself? Does it have writing on it anywhere? Any words or numbers? Is it possible to attach a photo of it? You can do that using the image import tool after you click "edit"  https://i.sstatic.net/btuiD.png
uhoh
CC BY-SA 4.0
2022-01-20T01:06:44.677Z
@uhoh it has the name of the company on it and “5mm” on it. I think “5mm” is the focal length.
Donnie
CC BY-SA 4.0
2022-01-20T01:08:08.860Z
@uhoh thank you for welcoming me to Stack Exchange
Donnie
CC BY-SA 4.0
2022-01-20T01:09:19.513Z
@GrapefruitIsAwsome no that link does not answer my question
Donnie
CC BY-SA 4.0
2022-01-20T01:12:37.707Z
@antlersoft ok thank you for the note
Donnie
CC BY-SA 4.0
2022-01-20T01:17:13.890Z
@Donnie understood, it was not clear to me how it did not address your question.
GrapefruitIsAwesome
CC BY-SA 4.0
2022-01-20T01:30:45.943Z
@GrapefruitIsAwesome that's precisely what comments are for, to clarify issues. You can just comment "Can you elaborate in more detail what it is in answers to your linked question that you need answered differently here?"
uhoh
CC BY-SA 4.0
2022-01-20T01:32:06.223Z
@uhoh, I've retracted my close vote
GrapefruitIsAwesome
CC BY-SA 4.0
2022-01-20T01:34:21.050Z
@GrapefruitIsAwesome *Thanks!*
uhoh
CC BY-SA 4.0
2022-01-20T01:45:46.203Z
@uhoh I updated my post to include what is printed on the eyepiece
Donnie
CC BY-SA 4.0
2022-01-20T02:46:53.490Z
@Donnie looks great, *thanks!*
uhoh
CC BY-SA 4.0
2022-01-20T15:38:55.507Z
Does this answer your question? [How to estimate my eyepiece FOV?](https://astronomy.stackexchange.com/questions/40492/how-to-estimate-my-eyepiece-fov)
WarpPrime
CC BY-SA 4.0
2022-01-20T17:36:18.007Z
That eyepiece is going to be a simple design with an AFoV of around 45° (something between 42° and 52°, but probably towards the lower end). The simplest way to estimate it is to look at the Moon with it, remember what you see, and compare it with a [FoV calculator](https://astronomy.tools/calculators/field_of_view/) (fill in all the numbers you know: telescope aperture and focal length, and eyepiece focal length, then play with the FoV until what the calculator shows you matches what you saw) To *calculate* it then: https://www.cloudynights.com/topic/544944-how-do-you-measure-afov/?p=7348189
Aaron F
CC BY-SA 4.0
2022-01-21T23:05:47.410Z
@fasterthanlight did you not notice that the OP has already linked to that question? Did you not see [this comment](https://astronomy.stackexchange.com/questions/48239/how-to-find-the-afov-of-a-eyepiece#comment105284_48239)?
uhoh
<observational-astronomy><telescope><stellarium>
How to find the aFOV of a eyepiece?
Donnie
# How to find the aFOV of a eyepiece?

(Asked by: username_0 on 2022-01-19)

How do I find the apparent field of view (aFOV) of a eyepiece using simple math (if possible)? I need the aFOV to give as a value to Stellarium’s custom eyepiece

I found a Q&A about this subject but, I don’t fully understand the math:

* [How to estimate my eyepiece FOV?](https://astronomy.stackexchange.com/questions/40492/how-to-estimate-my-eyepiece-fov)

On my eyepiece it says “**Gskyer® 5mm**” Also see photo below.

---

[![Photo of my 5 mm eyepiece](https://i.sstatic.net/4SBQa.jpg)](https://i.sstatic.net/4SBQa.jpg)

**Tags:** observational-astronomy, telescope, stellarium

> **Comments:**
> **username_1**: Note that the aFOV depends on the detailed design of the eyepiece; it can't be deduced from magnification or focal length.  That's why the linked question advises you to measure the field stop or time how long it takes a star to cross the field; some measurement will be required to get the aFOV.  Inexpensive eyepieces will have aFOV of about 40 degrees or less; specialized wide-field eyepieces, usually larger and heavier with larger glass elements, can achieve 80 degrees or more.  
> **username_2**: @username_3 please read the question first before voting to block everyone else from posting an answer; the OP has already linked to that question and asks for further clarification. Closing as duplicate in this case (without offering help) is counterproductive.  
> **username_2**: Hi @username_0 *Welcome to Stack Exchange!* What information do you have now? Do you have the eyepiece itself? Does it have writing on it anywhere? Any words or numbers? Is it possible to attach a photo of it? You can do that using the image import tool after you click "edit"  https://i.sstatic.net/btuiD.png  
> **username_0**: @username_2 it has the name of the company on it and “5mm” on it. I think “5mm” is the focal length.  
> **username_0**: @username_2 thank you for welcoming me to Stack Exchange  
> **username_0**: @GrapefruitIsAwsome no that link does not answer my question  
> **username_0**: @username_1 ok thank you for the note  
> **username_3**: @username_0 understood, it was not clear to me how it did not address your question.  
> **username_2**: @username_3 that's precisely what comments are for, to clarify issues. You can just comment "Can you elaborate in more detail what it is in answers to your linked question that you need answered differently here?"  
> **username_3**: @username_2, I've retracted my close vote  
> **username_2**: @username_3 *Thanks!*  
> **username_0**: @username_2 I updated my post to include what is printed on the eyepiece  
> **username_2**: @username_0 looks great, *thanks!*  
> **username_4**: Does this answer your question? [How to estimate my eyepiece FOV?](https://astronomy.stackexchange.com/questions/40492/how-to-estimate-my-eyepiece-fov)  
> **username_5**: That eyepiece is going to be a simple design with an AFoV of around 45° (something between 42° and 52°, but probably towards the lower end). The simplest way to estimate it is to look at the Moon with it, remember what you see, and compare it with a [FoV calculator](https://astronomy.tools/calculators/field_of_view/) (fill in all the numbers you know: telescope aperture and focal length, and eyepiece focal length, then play with the FoV until what the calculator shows you matches what you saw) To *calculate* it then: https://www.cloudynights.com/topic/544944-how-do-you-measure-afov/?p=7348189  
> **username_2**: @fasterthanlight did you not notice that the OP has already linked to that question? Did you not see [this comment](https://astronomy.stackexchange.com/questions/48239/how-to-find-the-afov-of-a-eyepiece#comment105284_48239)?  

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## 0 answers

