<p>I'm trying out a DSLR on my <a href="https://www.celestron.com/products/powerseeker-70eq-telescope-with-motor-drive-and-phone-adapter" rel="nofollow noreferrer">Celestron 70EQ</a> refractor telescope with a DSLR T-ring.</p>
<p>The first try didn't magnify very much, and I learned that I need to extend the focus length to magnify the image; I've found both <a href="https://rads.stackoverflow.com/amzn/click/com/B0BDV5652K" rel="nofollow noreferrer" rel="nofollow noreferrer">fixed-length</a> and <a href="https://rads.stackoverflow.com/amzn/click/com/B07QTQ5BBB" rel="nofollow noreferrer" rel="nofollow noreferrer">variable-length</a> tube extenders.</p>
<p>Question:</p>
<p>How does the &quot;tube extension length&quot; in millimeters compare to an &quot;eyepiece focal length&quot;?  For example, if I have a 10mm eyepiece, how long does the tube extension need to be approximately equivalent when it hits the DSLR CCD?</p>

CC BY-SA 4.0
2023-10-20T21:57:34.073Z
You can't use extension tubes on the fixed optics of a telescope like you can with a camera lens.  What you need is a "Barlow lens" which makes the telescope appear longer.
Greg Miller
CC BY-SA 4.0
2023-10-21T07:39:13.827Z
The tube extension is completely unrelated to the eyepiece focal length. You would expect them to be similar in principle, but eyepieces don’t have the same position of the focal plane as compared to their drum.
Pierre Paquette
<observational-astronomy><telescope><photography><eyepiece>
How do eyepiece mm values equate to telescope tube extension mm values?
KJ7LNW
# How do eyepiece mm values equate to telescope tube extension mm values?

(Asked by: username_0 on 2023-10-20)

I'm trying out a DSLR on my [Celestron 70EQ](https://www.celestron.com/products/powerseeker-70eq-telescope-with-motor-drive-and-phone-adapter) refractor telescope with a DSLR T-ring.

The first try didn't magnify very much, and I learned that I need to extend the focus length to magnify the image; I've found both [fixed-length](https://rads.stackoverflow.com/amzn/click/com/B0BDV5652K) and [variable-length](https://rads.stackoverflow.com/amzn/click/com/B07QTQ5BBB) tube extenders.

Question:

How does the "tube extension length" in millimeters compare to an "eyepiece focal length"? For example, if I have a 10mm eyepiece, how long does the tube extension need to be approximately equivalent when it hits the DSLR CCD?

**Tags:** observational-astronomy, telescope, photography, eyepiece

> **Comments:**
> **username_1**: You can't use extension tubes on the fixed optics of a telescope like you can with a camera lens.  What you need is a "Barlow lens" which makes the telescope appear longer.  
> **username_2**: The tube extension is completely unrelated to the eyepiece focal length. You would expect them to be similar in principle, but eyepieces don’t have the same position of the focal plane as compared to their drum.  

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## 0 answers

