<p>Yes, potentially.  There are a few limitations:</p>
<ol>
<li><p>It will be easier to use in alt azimuth mode, which in practice means setting the polar axis to 90 (ie straight up).  Not all equatorial mounts can do this.</p>
</li>
<li><p>With the standard astronomical diagonal, images will be inverted left to right.  You can get diagonals for terrestrial viewing that will fix this.  If you don't use a diagonal, the image will be upside down.</p>
</li>
<li><p>The field of view will be quite restricted if you use a typical eyepiece.  For example a 20mm plossl would give a magnification of 45 and a field of view of about 1 degree.  To get more useful field of view, you would need longer focal length eyepieces, with with a wide angle design.  These are more expensive than ordinary eyepieces.</p>
</li>
<li><p>Unless your scope is an apo, you may see quite noticeable chromatic aberration, particularly at the edges of objects with a bright background (eg birds).</p>
</li>
</ol>

CC BY-SA 4.0
2021-01-13T20:20:40.207Z
Thank you for your very useful warnings and recommendations.
JACQUES MERRAN
CC BY-SA 4.0
2021-01-11T10:08:51.620Z
2021-01-15T10:14:50.507Z
2021-01-15T10:14:50.507Z
Dr Chuck
<p>Supplemental to @DrChuck's <a href="https://astronomy.stackexchange.com/a/40814/7982">thorough answer</a> and in reference to advice there about the use of a standard astronomical diagonal leaving images inverted left to right, I'd like to point out that there are image erectors that can solve this problem.</p>
<p>Binoculars are two telescopes where each has either <em>a pair</em> of prisms or newer fancy prism to flip the image up-down and left-right so that it is the same orientation as what we see without them.</p>
<p>These have been available with refractors for a long time so that we don't have to turn star charts upside down so that they look the same as they do through the telescope. My old Unitron 2.4 inch (50 mm) (like the one below but smaller) came with a big circular object containing a pair of prisms just like the old binocular configuration, but fancier, smaller prism image erectors now exist.</p>
<p>Out with the old...</p>
<p><a href="https://i.sstatic.net/zFbsm.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/zFbsm.png" alt="enter image description here" /></a></p>
<p>(cropped &amp; annotated from <a href="https://i.sstatic.net/isu79.jpg" rel="nofollow noreferrer">this larger image</a> from <a href="https://astromart.com/classifieds/astromart-classifieds/telescope-refractor/show/unitron-model-114-60mm-f-900mm-pending-to-jerry" rel="nofollow noreferrer">here</a>)</p>
<p>and in with the new.</p>
<p><a href="https://i.sstatic.net/n4JCZ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/n4JCZm.png" alt="newer style telescope image erector" /></a> (from <a href="https://www.harrisontelescopes.co.uk/acatalog/skywatcher-45-degree-erecting-prism-1.25.html" rel="nofollow noreferrer">here</a>; 45° Amici roof prism erector)</p>

CC BY-SA 4.0
2021-01-15T02:53:33.530Z
2021-07-10T23:54:31.173Z
2021-07-10T23:54:31.173Z
uhoh
<p>Can I use a refractor telescope 5&quot; aperture, 900mm focal length on an equatorial mount for terrestrial viewing.</p>

CC BY-SA 4.0
2021-01-15T02:22:27.193Z
This question is quite clear and *has been* answered as-written; no need to close as "unclear". **keep open!**
uhoh
<telescope>
Using a refractor telescope 5" aperture, 900mm focal length for terrestrial viewing
JACQUES MERRAN
# Using a refractor telescope 5" aperture, 900mm focal length for terrestrial viewing

(Asked by: username_0 on 2021-01-11)

Can I use a refractor telescope 5" aperture, 900mm focal length on an equatorial mount for terrestrial viewing.

**Tags:** telescope

> **Comments:**
> **username_1**: This question is quite clear and *has been* answered as-written; no need to close as "unclear". **keep open!**  

---

## 2 answers

### Answer by username_2 on 2021-01-11. Score: 3

Yes, potentially. There are a few limitations:

1. It will be easier to use in alt azimuth mode, which in practice means setting the polar axis to 90 (ie straight up). Not all equatorial mounts can do this.
2. With the standard astronomical diagonal, images will be inverted left to right. You can get diagonals for terrestrial viewing that will fix this. If you don't use a diagonal, the image will be upside down.
3. The field of view will be quite restricted if you use a typical eyepiece. For example a 20mm plossl would give a magnification of 45 and a field of view of about 1 degree. To get more useful field of view, you would need longer focal length eyepieces, with with a wide angle design. These are more expensive than ordinary eyepieces.
4. Unless your scope is an apo, you may see quite noticeable chromatic aberration, particularly at the edges of objects with a bright background (eg birds).

> **Comments:**
> **username_0**: Thank you for your very useful warnings and recommendations.  

---

### Answer by username_1 on 2021-01-15. Score: 0

Supplemental to @username_2's [thorough answer](https://astronomy.stackexchange.com/a/40814/7982) and in reference to advice there about the use of a standard astronomical diagonal leaving images inverted left to right, I'd like to point out that there are image erectors that can solve this problem.

Binoculars are two telescopes where each has either *a pair* of prisms or newer fancy prism to flip the image up-down and left-right so that it is the same orientation as what we see without them.

These have been available with refractors for a long time so that we don't have to turn star charts upside down so that they look the same as they do through the telescope. My old Unitron 2.4 inch (50 mm) (like the one below but smaller) came with a big circular object containing a pair of prisms just like the old binocular configuration, but fancier, smaller prism image erectors now exist.

Out with the old...

[![enter image description here](https://i.sstatic.net/zFbsm.png)](https://i.sstatic.net/zFbsm.png)

(cropped & annotated from [this larger image](https://i.sstatic.net/isu79.jpg) from [here](https://astromart.com/classifieds/astromart-classifieds/telescope-refractor/show/unitron-model-114-60mm-f-900mm-pending-to-jerry))

and in with the new.

[![newer style telescope image erector](https://i.sstatic.net/n4JCZm.png)](https://i.sstatic.net/n4JCZ.png) (from [here](https://www.harrisontelescopes.co.uk/acatalog/skywatcher-45-degree-erecting-prism-1.25.html); 45° Amici roof prism erector)


---

