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How to Use Dobsonian Reflector Telescope?

How to Use Dobsonian Reflector Telescope?
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Last Updated: 08 August 2019

Astronomers, thank you for reading my short guide. In this article, I’m going to clarify the basics of using a Dobsonian telescope. First of all, the instructions in this article are aimed at new astronomers, looking for some help just to get started. It is also aimed at returning astronomers, who may have forgotten how to use Dobsonian telescopes. And if you’re of the latter, I think this is a good refresher article.

The Origins of the Dobsonian Reflector Telescope

Secondly, the instructions in this piece pertain to telescopes without go-to mounts, which means that no computer will automatically slew the telescope to objects in the sky. This is only for basic mounts that are not motorized. A Dobsonian telescope is a Newtonian or a reflector telescope, using an altazimuth mount called a Dobsonian. This was named after John Dobson, who is credited for popularizing the use of this style of mount, allowing for larger, compact and more affordable telescopes.

This style of mount uses the same 2-axis mount, having the horizontal or azimuth axis, and vertical or altitude axis. Many consumer-grade mounts use this same principle of a mount at the opposite end of the spectrum, the largest optical-infrared observatory, the VLT. That one also uses telescopes mounted on altazimuth mounts. So I’d say you’re in pretty good company.

Without further adieu, here are the steps for using a Dobsonian telescope.

How to Use a Dobsonian Telescope: Step by Step

Step 1

Let’s start with step one, which is to inspect and assemble the telescope. It all begins with a tube called the optical tube assembly, which is made up of several components. Check your OTA for any missing or broken parts including:

  • The primary mirror, which is located at the bottom of the tube
  • The secondary mirror: which is the smaller mirror mounted near the top of the OTA
  • The finderscope
  • The focuser tube
  • The base: make sure that it can swivel
  • The control knobs: make sure that they can turn and lock the vertical axis
  • Your eyepieces

Step 2

Collimate. The telescope collimation is the process of adjusting the primary and the secondary mirror, using the knobs. However, some telescopes have fixed primary and secondary mirrors. So you can throw them in the back of your car for transport, and the mirrors will stay in place.

However, if your telescope is like this one, and it’s sensitive to bumps, you may have to collimate the telescope frequently or before each use. I’m not going to cover the process of collimation in this article.

Step 3

Align your finderscope. Your finderscope gives you a low magnification view of the sky so that you can slew your telescope in the general direction of the object that you would like to observe.

If it is properly aligned, you should be able to see that same object in your eyepiece. However, because the finderscope is independently mounted, it could get knocked out of alignment during transport. The basics of aligning your finderscope are to first locate a distant object like a light post, and center that image in your lowest powered eyepiece’s field of view. Then lock down your telescope and look through your finderscope, center that same object using the adjustment knobs on the finderscope’s mount.

Step 4

Observe through the finder scope, but first, make sure that the base is as level as possible. This is important because the horizontal axis will usually not have a locking mechanism. So if, for example, you were to situate it in a slight tilt, like in a slope, the heaviest part of the telescope is going to tilt towards the bottom of that slope. Next, don’t forget to remove the dust caps from the OTA and the finderscope. I’m sorry to state the obvious, but some of you might skip this step because it’s so obvious. If you’re at a dark sky site, these dust caps are dark, and they’re kind of difficult to see. Or, if you’re like somebody I know, who owns a collapsible Dobsonian for the first time observing, probably didn’t realize that the bottom one also has a dust cap.

Next, aim the telescope in the general direction of the object that you want to observe. Rotate the telescope about its base horizontally, then loosen the adjustment knobs. Next, tilt the telescope up or down about its altitude axis. Look through your finderscope to locate the object, and then lock it down. Remember that your finderscope is going to have slightly higher magnification, so the distances between stars will be a little bit greater. Also, if you have a right-angle finder scope, the images might be upside-down. This particular one has a prism that automatically corrects them so that they appear right-side-up as if you are looking through a straight-through finder scope.

Step 5

Observe using your lowest-powered eyepiece, and by power, I mean magnification. Take a look at your eyepieces. If there is an X beside them, then you will need to choose the one with the lowest number. If not, they are probably in millimeters like the ones I have on my telescope. In that case, choose the one with the highest number. Here, it’s going to be 24. Take that eyepiece, and insert it into the focuser tube. And lock it down. Then turn the focuser knob to sharpen the image. If you wear glasses like me, then you might find that you can’t reach focus because your eye is too far away from the surface.

If your eyepiece has a rubber shield around it, you can fold it away to get closer to the eyepiece. And if your finderscope is aligned, you’re going to see the same object in the eyepiece. Enjoy! If you’re starting, I recommend that you first practice observing bigger and brighter objects like the moon. And also, keep in mind that the Earth rotates, so this causes objects in the field of view to drift away, and to use a high magnification eyepiece will make objects drift away quickly.

Conclusion

So, I also recommend using first low magnification eyepieces. That way, you can observe longer by making fewer slew adjustments. Well, that about does it for now. If you have any questions or concerns, please let me know in the comments section. And also I’d like to point out that the methods and procedures that I gave you in this article are just the way I like to do things.

If you have your own ways, and if you have tips and tricks, then I would love to know them. I hope that this article was helpful to you! Clear skies, and thanks for reading.

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