<p>It depends on what you plan on using it for. For dimmer, deep sky objects you should be concerned with the balance of aperture and focal length (with a focus on aperture for light gathering power). For brighter objects, like the Moon or the solar system planets you probably don't need much light gathering power (aperture) so a long focal length for imaging finer details should be ok. Just a reminder though: never look at the Sun without proper filters, you'll go blind instantly, and looking at the Moon without filters will wreck your night vision and may hurt at first.</p>

CC BY-SA 3.0
2013-10-10T07:00:24.700Z
+1 on the Moon tip. I once accidentally pointed my scope at part of the moon and had a shadowy streak on my vision for some days. Good neutrally grey polarization filters that can be tuned from 50%-95% absorption can be had for around 50 EUR (for 1.25" eyepieces).
Arne
CC BY-SA 3.0
2013-10-09T03:49:38.017Z
2013-10-09T03:49:38.017Z
Sarah Szabo
<p>Right or wrong, myself as a beginner I started my decision based on aperture to get the appropriate light gathering ability for my area.  At that point focal length became pretty much a non-issue.  My choices were down to 2, and 1 of them would have been long enough that getting it outside easily would have been difficult.</p>

CC BY-SA 3.0
2013-10-09T12:31:39.447Z
Point being that focal length probably is not going to be an issue at all.  Once you satisfy your other more important parameters, you probably won't have a choice.
Brian Knoblauch
CC BY-SA 3.0
2013-10-16T14:40:47.557Z
-1 No justification given, very little explanation.
called2voyage
CC BY-SA 3.0
2013-10-16T19:37:53.267Z
Yeah, I'm sorry, I really have no idea what you guys are looking for.  A question was asked on what one should do, I answered it, I thought that's what this site was for?
Brian Knoblauch
CC BY-SA 3.0
2013-10-08T22:38:07.933Z
2013-10-08T22:38:07.933Z
Brian Knoblauch
<p>There is one rule that is generally true for all deep sky objects (nebulae, stars, galaxies,...): Aperture matters!</p>

<p>For solar system objects, aperture is not that important. </p>

<p>The second most important thing is: What size are the objects you want to look at: Small objects need long focal lengths and high magnifications, large objects need short aperture for low magnifications.</p>

<p>With 400mm you could watch objects like:</p>

<ul>
<li>Andromeda galaxy core</li>
<li>Orion nebula, other large emission or reflective nebulae (e.g. Pleiades)</li>
<li>large star clusters</li>
<li>low magnification lunar observations</li>
</ul>

<p>With 900mm you could watch objects like</p>

<ul>
<li>Planets (Saturn, Jupiter, Mars, ...)</li>
<li>high magnification lunar observations</li>
<li>planetary nebulae (e.g. ring nebula)</li>
</ul>

<p>Note that 60 and 70mm aperture are still very small for telescopes! The aperture influences two things:</p>

<ul>
<li>Light sensitvity: The larger the aperture, the more light you can collect. Very important if you live in a city!</li>
<li>Maximum resolution: Rule of thumb is that you can do aperture in mm times two as maximum magnification. I.e. for 60mm a 120x magnification is the absolute maximum which still is feasible.</li>
</ul>

<p>The magnification is created by the eyepiece. E.g. when you have a 400mm focal length telescope and use a 10mm eyepiece, you get 400mm/10mm = 40x magnification.</p>

<p>Note: the shorter the eyepiece focal length, the more difficult it is to build. Good 5mm eyepieces can cost 100 USD and up. I personally started with a 750mm Newtonian with 150mm aperture and 25mm and 10mm eyepieces. That's a good allrounder, even though planets will appear rather small with the 10mm eyepiece. But you can later invest more money in good eyepieces, which you can re-use on better telescopes which you may buy later on.</p>

<p>Edit: One more thing -- the telescope mount is equally important as are the eyepieces and the telescope itself. A mount that fits the telescope easily is as expensive as the optical tube assembly itself. Hence many beginners start out with a Dobson telescope, which uses a very, very simple yet sturdy mount.</p>

CC BY-SA 3.0
2013-10-09T09:21:23.340Z
2013-10-09T09:21:23.340Z
Arne
<p>Let's assume there are three types of telescopes with the following specifications:</p>

<pre><code>    Telescope                  Aperture              Focal Length
    --------------------------------------------------------------
    Example 1                     70 mm                    400 mm
    Example 2                     60 mm                    700 mm
    Example 3                     60 mm                    900 mm
</code></pre>

<p>As a beginner, should I look for a higher value in the aperture or the focal length?</p>

<telescope>
Should I focus more on Aperture or Focal Length for a telescope?
www
# Should I focus more on Aperture or Focal Length for a telescope?

(Asked by: username_0 on 2013-10-08)

Let's assume there are three types of telescopes with the following specifications:
    Telescope                  Aperture              Focal Length
    --------------------------------------------------------------
    Example 1                     70 mm                    400 mm
    Example 2                     60 mm                    700 mm
    Example 3                     60 mm                    900 mm

As a beginner, should I look for a higher value in the aperture or the focal length?

**Tags:** telescope

---

## 3 answers

### Answer by username_2 on 2013-10-09. Score: 13 | Accepted answer

There is one rule that is generally true for all deep sky objects (nebulae, stars, galaxies,...): Aperture matters!

For solar system objects, aperture is not that important.

The second most important thing is: What size are the objects you want to look at: Small objects need long focal lengths and high magnifications, large objects need short aperture for low magnifications.

With 400mm you could watch objects like:

* Andromeda galaxy core
* Orion nebula, other large emission or reflective nebulae (e.g. Pleiades)
* large star clusters
* low magnification lunar observations

With 900mm you could watch objects like

* Planets (Saturn, Jupiter, Mars, ...)
* high magnification lunar observations
* planetary nebulae (e.g. ring nebula)

Note that 60 and 70mm aperture are still very small for telescopes! The aperture influences two things:

* Light sensitvity: The larger the aperture, the more light you can collect. Very important if you live in a city!
* Maximum resolution: Rule of thumb is that you can do aperture in mm times two as maximum magnification. I.e. for 60mm a 120x magnification is the absolute maximum which still is feasible.

The magnification is created by the eyepiece. E.g. when you have a 400mm focal length telescope and use a 10mm eyepiece, you get 400mm/10mm = 40x magnification.

Note: the shorter the eyepiece focal length, the more difficult it is to build. Good 5mm eyepieces can cost 100 USD and up. I personally started with a 750mm Newtonian with 150mm aperture and 25mm and 10mm eyepieces. That's a good allrounder, even though planets will appear rather small with the 10mm eyepiece. But you can later invest more money in good eyepieces, which you can re-use on better telescopes which you may buy later on.

Edit: One more thing -- the telescope mount is equally important as are the eyepieces and the telescope itself. A mount that fits the telescope easily is as expensive as the optical tube assembly itself. Hence many beginners start out with a Dobson telescope, which uses a very, very simple yet sturdy mount.


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### Answer by username_1 on 2013-10-09. Score: 10

It depends on what you plan on using it for. For dimmer, deep sky objects you should be concerned with the balance of aperture and focal length (with a focus on aperture for light gathering power). For brighter objects, like the Moon or the solar system planets you probably don't need much light gathering power (aperture) so a long focal length for imaging finer details should be ok. Just a reminder though: never look at the Sun without proper filters, you'll go blind instantly, and looking at the Moon without filters will wreck your night vision and may hurt at first.

> **Comments:**
> **username_2**: +1 on the Moon tip. I once accidentally pointed my scope at part of the moon and had a shadowy streak on my vision for some days. Good neutrally grey polarization filters that can be tuned from 50%-95% absorption can be had for around 50 EUR (for 1.25" eyepieces).  

---

### Answer by username_3 on 2013-10-08. Score: 3

Right or wrong, myself as a beginner I started my decision based on aperture to get the appropriate light gathering ability for my area. At that point focal length became pretty much a non-issue. My choices were down to 2, and 1 of them would have been long enough that getting it outside easily would have been difficult.

> **Comments:**
> **username_3**: Point being that focal length probably is not going to be an issue at all.  Once you satisfy your other more important parameters, you probably won't have a choice.  
> **username_4**: -1 No justification given, very little explanation.  
> **username_3**: Yeah, I'm sorry, I really have no idea what you guys are looking for.  A question was asked on what one should do, I answered it, I thought that's what this site was for?  

---

