<p>It seems that almost every article online regarding planetary viewing recommends a telescope with a high focal ratio (f/8 or more), for revealing the finer details of the planets. But I find this a bit confusing because the resolving power of a telescope is solely a function of the aperture, or at least that is my understanding.</p>
<p>Given the above, shouldn't it be a matter of a large enough aperture and then the eyepiece to use to zoom on the detail of choice in formed image?</p>

CC BY-SA 4.0
2020-06-29T15:00:27.520Z
The f/number controls the total light collection **per solid angle**  - if you don't get enough photons, you won't be able to see details due to insufficient contrast.
Carl Witthoft
CC BY-SA 4.0
2020-06-29T18:00:40.807Z
But isn't aperture what decides how many photons you "collect"?
cad86
CC BY-SA 4.0
2020-06-29T18:51:07.013Z
but also how many photons get to your eyeball depends on f/# because of the effective magnification implicit in the value
Carl Witthoft
CC BY-SA 4.0
2020-06-29T21:00:55.057Z
Here is where it gets confusing for me. Isn't higher focal ratios associated with "slower" image taking? Why is that the case if higher focal ratios equate with more photons?
cad86
<jupiter><saturn>
Focal ratio importance for planetary viewing
cad86
# Focal ratio importance for planetary viewing

(Asked by: username_0 on 2020-06-29)

It seems that almost every article online regarding planetary viewing recommends a telescope with a high focal ratio (f/8 or more), for revealing the finer details of the planets. But I find this a bit confusing because the resolving power of a telescope is solely a function of the aperture, or at least that is my understanding.

Given the above, shouldn't it be a matter of a large enough aperture and then the eyepiece to use to zoom on the detail of choice in formed image?

**Tags:** jupiter, saturn

> **Comments:**
> **username_1**: The f/number controls the total light collection **per solid angle**  - if you don't get enough photons, you won't be able to see details due to insufficient contrast.  
> **username_0**: But isn't aperture what decides how many photons you "collect"?  
> **username_1**: but also how many photons get to your eyeball depends on f/# because of the effective magnification implicit in the value  
> **username_0**: Here is where it gets confusing for me. Isn't higher focal ratios associated with "slower" image taking? Why is that the case if higher focal ratios equate with more photons?  

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## 0 answers

