DIY 70mm f/4 Telescope (guidescope?)

I finally decided to start pursuing some budget webcam autoguiding to enhance my astrophotography (I will post a write-up about that project when it is complete). One of the most important components of a budget autoguider is a guidescope.

One of the (many) problems with webcam-based autoguiders is having a bright enough star in the field of view to pick as a guide star. Regardless of that issue, guidescopes tend to be pretty expensive. Spending $80.00 on a little 50mm guidescope did not seem very budget-friendly. One of the best ways to avoid this expense is to purchase a very cheap telescope and use it as a guidescope. There are many cheap 70mm f/5 or f/6 telescopes available. I decided to have just a little more fun.

Back to the “finding a bright enough star issue”. I figured that a 70mm aperture would reveal more stars than a 50mm aperture, so I pursued a 70mm setup (I image with a 80mm f/5, so it seemed silly to go over 70mm for the guidescope). In addition, I wanted very fast optics (lower f/#) to make it even easier to find a suitable guide star. So I set out to collect some very cheap materials and build my own fast 70mm guidescope. What did I find?

  1. 70mm telescope. I bought one at a flea market for $15.00. It was in horrible condition! The telescope was a Celestron 70az f/13 astromaster with a horrible objective lens. But that was okay. We only need the tube, the dovetail, and the focuser.
  2. 70mm f/4 objective lens. I found a used 70mm f/4 lens out of an old pair of Meade binoculars for $25.00 on ebay. We only need the objective lens and objective lens cell. The objective lens I got was a cemented achromat, so it was easier to use.

That’s it. Total cost: $40.00. Of course, you can buy a 70mm f/5 (or even f/4) on ebay for slightly less than that but using quality Meade and Celestron parts will make the final scope much better. Okay, enough! Let’s make the telescope.

Step 1. Dissemble the Celestron 70mm telescope.

20180627_004933

We can see the focuser (LHS), dovetail attached to the tube, and the lens assembly (RHS). Just a few screws hold all of these parts together. The disassembly is easy.

Step 2. Cut the tube. This was the funny part. I guess I didn’t realize how tiny a 70mm f/4 would be. It is important to define your goal before you do this. Do you want to use this telescope for visual use or for imaging use?

If you plan to use this little guy visually (which would probably be pretty cool) all you have to do is:

  1. Cut the tube. Guess. Use the focal length of the new objective lens to approximate the length.
  2. Roughly assemble the scope (with the new objective lens and focuser).
  3. Place a diagonal and an eyepiece in the focuser. Go outside and see if you can focus on a star. If you can, you are done.
  4. If you cannot focus, repeat steps 1-3 until you are done.

If you plan to use this little guy for imaging (like a guidescope) all you have to do is:

  1. Cut the tube. Guess. Use the focal length of the new objective lens to approximate the length.
  2. Roughly assemble the scope (with the new objective lens and focuser).
  3. Place a DSLR or webcam in the focuser. Go outside and see if you can focus on a star. If you can, you are done.
  4. If you cannot focus, repeat steps 1-3 until you are done.

Once you obtain your final cut, the rest is pretty easy. You can cut your tube using many different tools, just be careful. I used a hacksaw and it certainly was not the easiest way to get the job done. My final cut is shown below.

20180627_005044

 

I was surprised how small it was! After roughly attaching the new objective lens and focuser with a DSLR, I was able to achieve focus on Jupiter. Please note that you need to check the ability to focus on a very, very far away object to ensure the cut is good. If you are doing this during the day or on a cloudy night, at least check the focus on an object very far away.

This is when I ran into some problems. The tube length was much shorter than I expected. The cut tube was shorter than the dovetail. After attempting to cut the dovetail and failing, I decided to use the original length dovetail and mount it in a very unorthodox manner. This is when you are going to have to get creative! The following image shows my solution.

70mmf4

After some struggling, the final product was assembled. A little 70mm f/4! I call it, “miniscope”!

20180627_005129

Of course, it is not perfect. (1) The used optics were not in the best condition. (2) The focuser is not really suitable for a f/4 telescope (it was designed for a f/13!). To overcome this issue, I cut the focuser tube on the end facing the objective cell. You want the focuser tube to be as far away from the objective cell as possible, but you still need it to be stable. Overall, I am sure the focuser is still hurting the performance. (3) As a guidescope, this 70mm f/4 only has a 280mm focal length. For auto-guiding and shorter imaging focal lengths, this is probably fine.

Maybe I will test this little guy as an imaging telescope and post the results…

Thanks for reading! Follow me @budgetastronomy on instagram!

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