<p>It really doesn't make much sense to talk about magnification unless you are actually looking into an eyepiece.</p>

<p>When you have a "backyard" telescope, for direct observation (not photographic) the magnification is a sensible thing. The light recording surface is fixed (your eye). The primary focus of the telescope is a small image of the sky, so an eyepiece is used to magnify the primary focus. The overall magnification of the sky depends focal length of the main mirror, and the focal length of the eyepiece. $M=F_O/F_e$</p>

<p>For a small telescope, the maximum useful magnification is limited by the size of the main mirror. A larger mirror would allow for greater resolution, and so a more powerful eyepiece can be used. </p>

<p>But the Keck is a different beast. The limit on resolution is determined not by the size of the mirror, but by the blurring due to the atmosphere. It doesn't matter how big the mirror is, there is no more information that can be gained by greater magnification.</p>

<p>Nobody is sticking an eyepiece at the primary focus and looking down it. Instead the light at the primary focus is passes into various scientific instruments. Most of the are spectrometers of one sort of another, and so they don't produce magnified images.</p>

<p>If you have an image, the concept of magnification is unclear. If I have a photo of Jupiter, and I move my head closer to the image, it gets bigger. So what is the magnification? It depends on the distance of the observer to the image. It really doesn't make much sense to talk about magnification unless you are actually looking into an eyepiece.</p>

<p>There are some cameras that are used, such as the <a href="https://www2.keck.hawaii.edu/inst/nirc2/genspecs.html" rel="nofollow noreferrer">NIRC2</a>. It has a narrow view mode with 0.009942 arcsec/pixel. On a typical 100 pixels per inch display, Jupiter (at 50 arcseconds) would be 50 inches across.</p>

CC BY-SA 3.0
2017-06-02T18:53:38.847Z
Thanks for the informative answer. When the NIRC2 camera is displaying Jupiter 50 inches across, can you say roughly what magnification that appears to be?
larry909
CC BY-SA 3.0
2017-06-02T22:55:07.653Z
No, because that would depend on how far your head is from the screen. It really doesn't make much sense to talk about magnification unless you are actually looking into an eyepiece.
James K
CC BY-SA 3.0
2017-06-02T16:01:27.160Z
2017-06-02T16:01:27.160Z
James K
<p>What is the powers the telescope is used at? </p>

CC BY-SA 3.0
2017-05-29T11:51:40.707Z
What do *you* understand magnification to mean? The properties of the primary mirror alone cannot give you the answer.
ProfRob
CC BY-SA 3.0
2017-05-29T16:09:51.180Z
@RobJeffries edited question
larry909
CC BY-SA 3.0
2017-05-30T13:21:45.073Z
aperture has nothing to do with magnification.  It does affect resolving power.  I suspect you're not really clear about what you are trying to understand.
Carl Witthoft
CC BY-SA 3.0
2017-05-30T16:13:54.960Z
@CarlWitthoft isn't it true that a bigger lens has more magnification power than a smaller lens? Ok so maybe they weren't aiming for more magnification, only for light gathering power. But I think it's a reasonable question: What magnification power is the telescope used at?
larry909
CC BY-SA 3.0
2017-05-30T18:59:48.517Z
@larry909 Absolutely not.  Please take the time to look up things like "focal length" and "magnification" in wikipedia or any introduction to optics.
Carl Witthoft
CC BY-SA 3.0
2017-06-02T04:45:19.783Z
@CarlWitthoft edited the question. even if I don't understand how optics work it's still a reasonable question to want to know at which magnification the telescope is used at.
larry909
<telescope>
What is the magnification of the Keck telescopes in Hawaii?
larry909
# What is the magnification of the Keck telescopes in Hawaii?

(Asked by: username_0 on 2017-05-29)

What is the powers the telescope is used at?

**Tags:** telescope
> **Comments:**
> **username_1**: What do *you* understand magnification to mean? The properties of the primary mirror alone cannot give you the answer.  
> **username_0**: @RobJeffries edited question  
> **username_2**: aperture has nothing to do with magnification.  It does affect resolving power.  I suspect you're not really clear about what you are trying to understand.  
> **username_0**: @CarlWitthoft isn't it true that a bigger lens has more magnification power than a smaller lens? Ok so maybe they weren't aiming for more magnification, only for light gathering power. But I think it's a reasonable question: What magnification power is the telescope used at?  
> **username_2**: @larry909 Absolutely not.  Please take the time to look up things like "focal length" and "magnification" in wikipedia or any introduction to optics.  
> **username_0**: @CarlWitthoft edited the question. even if I don't understand how optics work it's still a reasonable question to want to know at which magnification the telescope is used at.  

---

## 1 answers

### Answer by username_3 on 2017-06-02. Score: 1 | Accepted answer

It really doesn't make much sense to talk about magnification unless you are actually looking into an eyepiece.

When you have a "backyard" telescope, for direct observation (not photographic) the magnification is a sensible thing. The light recording surface is fixed (your eye). The primary focus of the telescope is a small image of the sky, so an eyepiece is used to magnify the primary focus. The overall magnification of the sky depends focal length of the main mirror, and the focal length of the eyepiece. $M=F\_O/F\_e$

For a small telescope, the maximum useful magnification is limited by the size of the main mirror. A larger mirror would allow for greater resolution, and so a more powerful eyepiece can be used.

But the Keck is a different beast. The limit on resolution is determined not by the size of the mirror, but by the blurring due to the atmosphere. It doesn't matter how big the mirror is, there is no more information that can be gained by greater magnification.

Nobody is sticking an eyepiece at the primary focus and looking down it. Instead the light at the primary focus is passes into various scientific instruments. Most of the are spectrometers of one sort of another, and so they don't produce magnified images.

If you have an image, the concept of magnification is unclear. If I have a photo of Jupiter, and I move my head closer to the image, it gets bigger. So what is the magnification? It depends on the distance of the observer to the image. It really doesn't make much sense to talk about magnification unless you are actually looking into an eyepiece.

There are some cameras that are used, such as the [NIRC2](https://www2.keck.hawaii.edu/inst/nirc2/genspecs.html). It has a narrow view mode with 0.009942 arcsec/pixel. On a typical 100 pixels per inch display, Jupiter (at 50 arcseconds) would be 50 inches across.

> **Comments:**
> **username_0**: Thanks for the informative answer. When the NIRC2 camera is displaying Jupiter 50 inches across, can you say roughly what magnification that appears to be?  
> **username_3**: No, because that would depend on how far your head is from the screen. It really doesn't make much sense to talk about magnification unless you are actually looking into an eyepiece.  

---

