How to Use a Dobsonian Telescope
Dobsonian telescopes are ideal for visual observers and kids. With a small budget, you can get a big aperture and observe very faint and distant objects without dealing with the complexity of an equatorial mount. Several websites say that Dobsonian telescopes are very easy to use, but taking full advantage of what that telescope can offer, may represent a challenge for people getting started in astronomy.
In this article, I'm going to provide important information to use your new Dobsonian telescope appropriately and avoid common frustrations when getting started.
The first thing you need to know is that the standard kit that comes with a Dobsonian telescope is usually not enough to start using it. So, be prepared to spend a small amount to get important tools to help you set up the telescope, find objects, and observe them.
The best advice I can give you is to join the local astronomical society. You will find people with the same interest and they can teach you how to use it. If you are based in Auckland, NZ, consider booking one of the Telescope Labs. In these sessions, we will teach you how to assemble and balance your telescope properly, align the finderscope, collimate the mirrors, choose the appropriate eyepieces and find objects in the sky with your own telescope. During these sessions, you can also try different eyepieces, filters, tools and gadgets provided by Skylabs NZ. That can help you to decide what you need to buy next.
Observation Area
Consider setting up your telescope on a tidy and flat area. Ideally, avoid any object or building blocking a large region of the sky. If you can also level the base of your Dobsonian, it will make the PUSHTO and/or manual alignment easier and even more accurate (I will discuss this in detail in the following sections).
Avoid using a deck or other surfaces that can inject vibrations when walking around. At home, my big Dobsonian telescope sits on top of a 600x600x37mm stone paver carefully levelled and surrounded by bark. During the Stargazing Labs (outreach sessions), the Dobsonian telescopes are placed on the grass.
Collimation
The mirrors of the telescope need to be well aligned to deliver a high-quality image. If you end up with a Dobsonian telescope, you should consider buying a Cheshire eyepiece. Single beam laser collimators are relatively affordable and easy to use. Unfortunately, they can not help correct the off-axis position of the secondary mirror or ensure the collimation of both mirrors’ surfaces. While the laser collimator helps speed up the process, to ensure the perfect collimation between the two mirrors, a Cheshire is always required. If your main mirror has no central spot, you should also consider getting a mirror spotting template and center spots stickers.
Please, follow the steps described in the following article to achieve the best visual experience with your telescope. https://www.skylabs.co.nz/post/how-to-collimate-a-newtonian-telescope
Finding Objects
If you are not very familiar with the night sky, you can lose many hours trying to find a particular object. Nothing is more frustrating than a star cluster, a nebula, or a galaxy that you want to look at but you simply cannot find. The red dot, laser pointer, or projector finders can help to reduce this frustration. These types of finders in combination with a regular finderscope will help you to get success. If you don't want to drill new holes in your telescope to incorporate a new finderscope, consider getting a dual finder bracket. I strongly suggest using the Skylabs dual finderscope bracket with laser pointer mount which allows using a regular finder scope in combination with an inexpensive laser pointer and a PUSH TO finder adapter.
PUSH TO Finder adapter
Computerized mounts can help a lot to find objects, especially in light-polluted areas. These types of mounts are quite expensive but there is a very cheap option to turn your Dobsonian into a computerized telescope, just using a smartphone. SkEye is a very popular Android OS application that enables PUSHTO guide capability to manual telescopes. It is ideal for Dobsonian and other visual telescopes to find deep-sky objects. Unfortunately, this application is only available for Android. I'm an iPhone user but I purchased a second-hand Samsung Galaxy only for this purpose. If you are interested in adopting this option, consider getting a Skylabs NZ Smartphone Finder Mount, which has been optimized for a smartphone with SkEye. As mentioned before, if the mount of the telescope is levelled, it will help the PUSHTO alignment and you will achieve even more accuracy. Follow the SkEye instructions to enable PUSHTO capability in your telescope.
Magnification and Eyepieces Selection
The theoretical maximum magnification that you should consider is two times the diameter of the telescope aperture in mm. For example, a telescope with 200mm of aperture can deliver a magnification up to x400. This assumes that your telescope is perfectly collimated, the sky and atmospheric conditions are excellent and the quality of the mirrors and eyepieces are exceptional.
Usually, you will try high magnification with the Moon, planets and/or planetary nebulas in order to see more detail.
How to calculate the magnification
The magnification is calculated by dividing the focal length of your telescope by the focal length of the eyepiece. The focal ratio is the 'speed' of a telescope's optics. It can be calculated by dividing the focal length by the aperture. The smaller the f/number, the lower the magnification, the wider the field, and the brighter the image with any given eyepiece.
Magnification = telescope focal length / eyepiece focal length
For example, if you have a Dobsonian 200mm f/6, that means that the focal length is 1200mm. If you are using a 32mm eyepiece you will get a magnification of x37.5. If you are using a 6mm eyepiece you will get x200 magnification.
I strongly recommend using a Barlow lens. These types of lenses multiply (by a particular factor) the effective focal length of the telescope, multiplying the magnification and also the "number of eyepieces" that you have. For example, using a Barlow lens x2 in combination with the 32mm eyepiece, you will get x75 magnification, and x400 using the 6mm eyepiece. Somehow, with just one Barlow lens and two eyepieces, you have the equivalent to four eyepieces, 32mm, 16mm, 6mm and 3mm.
How much magnification?
Unless you have a computerised and motorised Dobsonian, you will need to follow the object manually. That will introduce a lot of vibrations and it will become not a pleasant observation when the magnification is quite high.
If you want to observe objects under high magnification, consider getting an equatorial platform. If you observe from home, it can be a good asset. Otherwise, be prepared to carry another bulky and heavy component. While most of the online references point that equatorial mounts are not accurate to follow objects, there are some options available in the market that not only allow you to follow objects with high magnification but also make a Dobsonian suitable for astrophotography. From my point of view, equatorial platforms are not recommended for a beginner, and they are out of the scope of this article.
Planets and the Moon are very cool, but soon you will get bored of observing just a few objects. The next natural step will be to explore an almost endless list of Deep Sky Objects (DSO). For that reason, I recommend investing in medium to long focal length and wide field eyepieces to target DSOs. For eyepieces with long focal lengths (+32mm) consider using 2" rather than the 1.25", otherwise the field of the image will be cropped by the diameter of the eyepiece.