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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.


Create an Observation List


Planning your observation session will help you to select easy targets and reduce frustration. With time you can increase the complexity and try to find more challenging objects. I strongly suggest using the following two resources:

  • Telescopius (formerly DSO Browser) was created by Sebastian Garcia in 2010 as a personal project to find astrophotography targets easily, as no other software allowed that. Since then, it has become an extremely valuable asset for thousands of active amateur astronomers all over the world. You can create an account for free, which allows you to define your equipment and select the suitable for your telescope and location. You can take advantage of the telescope simulator to visualise how the object will look in your telescope with a particular eyepiece and Barlow lens.

  • SkySafari Pro is a smartphone application that includes over 100 million stars, 3 million galaxies down to 18th magnitude, and 750,000 solar system objects; including every comet and asteroid ever discovered. Plus, state of the art mobile telescope control. This app provides augmented reality and helps to identify constellations and locate deep sky objects (DSO). Along with several features, it allows you to create an observation list of objects or suggest the highlighted objects for a particular night. SkySafari (not the Pro version) it's free of charge but it has limited features and catalogues. The Pro version cost ~$50 and sometimes they reduce the cost to half as a promotion.


Observing the Moon


Most probably, the first object that you will look at through your telescope is the Moon. The amount of light gathered by a small Dobsonian can hurt your eyes, especially when it's Full Moon. Quite often, the telescopes come with a not very useful Moon filter. I strongly recommend getting a neutral density filter (0.8, 0.9). That filter reduces the bright glare of the Moon and reveals highly lunar detail and surface features. In order to use it, you will need to screw the filter in the bottom of the eyepiece.


When it's Full Moon, the light from the Sun is quite perpendicular to the craters and the shadows projected are very short. That creates a very low contrast image. For that reason, you will observe our satellite better and with more detail when it's Half Moon.


Observing the Planets


If you observe a planet with your telescope when it is low on the horizon, the light coming from the planet will go through a longer path in the atmosphere. When they are higher up in the sky, the light coming from them will have a smaller journey through the atmosphere. This difference can affect the image quality. Try to plan your observing session at a time where the planet that you want to see is higher on the horizon.


The state of the atmosphere can influence quite a lot the quality of the image. If you see a planet appears very blurry through your telescope despite setting everything up perfectly, it’s more likely because of bad "seeing conditions" due to turbulence in the atmosphere.


The other important factor that will affect the image is the temperature. The telescope needs time to cool down. The different parts, as well as the mirrors, may slightly expand or retract depending if it’s hot or cold. This can be fixed by just leaving your telescope outside for at least 20mins to adapt to the outdoor temperature.


Color filters are useful to increase the contrast to reveal some details of the planet's surface. These filters are not very expensive, unfortunately, you will find very low-quality plastic-based filters that can impact your visual experience. Consider using good quality filters and select those that you are interested in rather than getting a full set of them. For entry-level filters, GSO brand can be a good option. If you want even better quality, consider getting reputed brands like Baader, ZWO, Celestron, or Orion.


The filters that I use the most for planets are #80A Medium Blue, #82A Light Blue, #25 Red, #15 Deep Yellow, and #21 Orange.


Mercury and Venus can be seen after sunset and before sunrise. Since they are orbiting closer to the Sun, these planets have phases just like our Moon. Looking at those two planets with a telescope will reveal the phase but not additional detail.


Mercury is among the most difficult planet to observe as it does not deviate much from the Sun and is often low on the horizon. Its greater elongation is only 28°. Using a #25 Red filter will enhance the planet’s disk, and #21 Orange enables you to view Mercury at twilight.


Venus is very bright because of its albedo, to the point that sometimes it's hard to see its phase. You can consider using a #47 violet filter to reduce the brightness and glare. Consider using #21 Orange to see Venus at twilight.


Mars can reveal a lot of detail without any filters. Having said that, the observation of Mars changes dramatically depending on the proximity to Earth. Choosing the best time to observe Mars will help enjoy the best viewing conditions. Its distance to us varies very strongly; from 56 to 400 million km! The most favourable moment is when the planet is opposite the Sun in relation to the Earth, which happens every two years. With 8in Dobsonian, you can see dark areas on the surface and the ice on the poles. If you want to increase the contrast and reduce the glare, consider using #21 Orange or #15 Deep Yellow and #80A Medium Blue to bring out the polar caps. The filter #82A Light Blue can help to reveal low-contrast features.


Even in small telescopes, Jupiter will look majestic and you should be able to see the four largest moons (Io, Europa, Ganymede, Callisto), also known as the Galileans moons. With a telescope of 8inch or more, you should comfortably see the Great Red Spot and enjoy the different colours made out by the belts and zones that constitute the planet’s surface. The filter #21 Orange sharpens the contrast in the belts and helps to bring out the Great Red Spot. The filter #82A light blue is superb for Jupiter in the twilight, it definitely improves belt contrast and reveals low-contrast features.


Saturn, through an 8inch Dobsonian telescope, provides amazing views with excellent details. Saturn has four main moons: Titan, Rhea, Tethys and Dione. These celestial bodies can be observed in scopes of medium aperture, assuming that viewing conditions are good and the Moon are not positioned too far from the planet. Mimas is another moon that can sometimes be seen but can be difficult due to its closeness to the rings. Titan is the easiest to catch and can be seen in almost any scope. The Cassini Division is a region 4,800 km in width between Saturn's A ring and B Ring. It was discovered in 1675 by Giovanni Cassini at the Paris Observatory using a refracting telescope that had a 2.5inch objective lens with a 20-foot-long focal length and a 90x magnification. Your Dobsonian telescope has a bigger diameter and better quality optics compared to the one used by Cassini. You should be able to see the Cassini Division without any problem with a moderated magnification.

The filters #80A Medium Blue and #82A Light Blue can help to get more details from Saturn's rings and surface features. The filter #11 Yellow Green introduces additional contrast in the Cassini Division.


The other planets, Uranus and Neptune will represent a challenge for new amateur astronomers with a Dobsonian. Even with a very large Dobsonian, you will be unable to see too much detail from those planets' surfaces.


Observing Deep Sky Objects


Planets and the Moon are very cool, but soon you and your kids will get bored of observing just a few objects. The next natural step will be to explore a list of Deep Sky Objects (DSO), which is almost endless. Dobsonians are really good telescopes for visual observation of DSOs. Obviously, the bigger is the mirror, the fainter objects you will be able to observe and the more details you will be able to see.

Some objects will be easy to find just by using the finder scope. With a 50mm finder scope you can already see some bright DSOs like Orions nebula, Eta Carinae, Omega Centauri, etc. Some other objects will require more effort. As mentioned in the previous sections, I strongly suggest getting a PUSH TO finder adapter. It will make your journey to DSO observation easier.

Before you start looking for DSOs, I strongly suggest you visit Telescopius website and generate a list of objects that you can see with your telescope, and start with the brightest ones. Click on targets located on the top menu and follow these instructions:

  1. Fill up in search parameters the day and time that you will be observing. Set the inclination over the horizon to avoid any potential visual obstacle (trees, buildings, etc).

  2. Select the apparent magnitude. The drop menu provides information about the magnitude limit attending to the telescope aperture. For example, a 200mm telescope is 14.2 mag.

  3. Sort by magnitude (brightness).

For the night of 11 December 2021, and considering an 8inch aperture telescope, I have up to 4,048 objects to target. With my 16inch Dobsonian, I can choose among 5,243 targets on the same night.


Observing from light polluted areas


Light pollution filters try to reduce as much as possible the artificial light coming from the city. While those filters are transparent for a wide range of the spectrum, it blocks certain particular regions of well-known artificial light bulbs. Knowing the dominant light pollution in your city will help you choose the right filter to save time and money.


I highly recommend the Ultra-High Contrast filter (UHC), which allows the transmission of nearly 100% of the radiation from both O-III and the H beta lines. All annoying, scattered light from other wavelength sources, including local artificial light pollution, is filtered out. With this strong blocking of the sky, a great detail becomes visible for gas nebulae and planetary nebulae.


UHC-E variant increases the contrast of emission nebulae and also comets. It’s recommended for observation of emission nebulae and comets with small telescopes under light-polluted skies.



Do you still need help?

I hope this information has been useful and it helps you on the amazing journey ahead. 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.


Credits: The main image of the article is from Roel Weijenberg, who developed his Dobsonian inspired by the popular ultralight Strock 250 model. http://www.roelblog.nl/2021/09/filmpje-het-opbouwen-van-de-400mm-propdob/

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