What telescope to buy? Part I – visual observations

Geralt

End of the year is coming, nights getting longer at the northern hemisphere, many people start to think about gifts and presents. Winter night sky can impress many of us, and we may start to seek for some telescope that will enable us to see more in the sky. There is whole lot of telescopes to choose among in almost any price range. So what to look for and what telescope to buy?

Conditions and observing location

The first thing to look for are the conditions of the location we would like to observe night sky. It may be a balcony in the city, backyard in the suburbs or a parking lot in the remote location. All these places provide different conditions and different observing impressions. There is a useful light pollution scale called Bortle scale. If the sky we have access to is Bortle 1-3 than our observations will not be limited by light pollution. Bortle 4-5 is still good, but faint objects with low surface brightness may be out of reach. Level 6-7 Bortle will limit available objects to brighter targets (star clusters, brightest galaxies, nebulae). Bortle 8-9 narrows available targets further down to star clusters, double stars, planets and Moon. The darker sky is priceless. It is also valid for astroimaging, but for deep sky visual observations it is crucial.

Small refractor put on handy AZ4 mount
Small refractor put on handy AZ4 mount

Once we know where (mostly) we observe it is a good moment to think about logistics. Telescopes are made in a wide range of sizes. The most important telescope attribute is aperture (diameter). Not length, not magnification, but aperture. And it comes for a price – not only money, but also portability and ease of use. If we are among a few percents of the most happy astronomy amateurs in the world, then we just live in the area, where light pollution is not a concern. But usually we need to choose and find a compromise between night sky quality and our willingness and ability to relocate to dark place for observations.

The rule of the thumb is that the best telescope is the one we use the most. It makes little of sense to purchase big 16 inches diameter telescope that we will use once per year, because we need to pack it up and travel with cart to remote, dark location. It also not the best option to have long refractor or newtonian on equatorial mount at modest size balcony – it will be pretty difficult to operate and observe. But actually before the purchase I really recommend to do something else:

Go to starparty

XV starparty in Zatom, Poland
XV starparty in Zatom, Poland

Or get into some local astronomy amateurs association. Nowadays in the internet era it is quite easy to find people with similar interests and who live nearby. If we want to spend some money (sometimes significant amount) we may also spend some time on getting into the subject. Starparty is a great opportunity to get familiar with different telescopes and also to ask questions.

Starparty participants are open people with knowledge and experience. And usually they are more than happy to share the eyepiece views with others. These discussions and observations can be real time and money saver. After few observing sessions with experienced amateurs we may already know what we should aim for. Is it worth to have small telescope at balcony to use every clear night, or maybe it makes more sense to have large telescope that we need to pack to car and drive one hour to dark location. Or maybe to have both? If we are not able to participate in starparty (due to travel problems or inconvenient time) we can still seek for astronomy amateurs in our local neighbourhood.

Brand new or second hand telescope?

There are good reasons for buying both new and second hand telescope. If you buy new, you get the warranty and the purchase risk is very low, because even if instrument does not work it can be returned. But if you buy telescope for the first time, there is 99% percent probability it will not be your target telescope and you will change it in the future (or sell if you give up on astronomy). If you purchase and sell second hand instrument, you will just lose less.

There are dedicated forums for astronomy amateurs basically in each country, but also several ones with international scope (Stargazers Lounge, Cloudy nights). It is usually quite safe to purchase second hand gears at such places, but we always should be vigilant. If we are not sure about the equipment or equipment owner, then it is always a good idea to ask more experienced colleagues.

There is also one more important aspect of purchasing the first telescope. We probably do not want to purchase some exotic or unpopular item, especially not the new one. Assuming there is 99% chance we will want to exchange it or sell in the future, such exotic instrument may be quite hard to sell. On the other hand less popular equipment can be sometimes less expensive than popular ones – this is important if you are a buyer.

What sky – what objects?

Depending on the sky quality and local light pollution some night sky targets can be out of our reach. There are two aspects of the night sky quality: light pollution and seeing. Light pollution can be estimated well with Bortle scale that already has been introduced in this blog post. Astronomical seeing is the attribute that describes amount of apparent blurring of the image due to turbulent moves of air layers. The more the atmosphere is perturbed, the more image is degraded.

Roll-off shed observatory during session
Roll-off shed observatory during session

For the bigger instruments seeing is the limiting factor of the achieved resolution. Seeing is very dynamic quality that is affected by many factors, both local but also larger scale. General weather conditions, wind speeds at different heights, temperature drop over altitude – it all affect the seeing.

But seeing can be also increased by local environment. If our observing location is surrounded with big heat accumulators (house roofs, concrete plazas, pavements, big buildings) then air will be even more disturbed, when these objects will radiate out heat during night. If we are concerned about good resolution (planetary/Moon observations, double stars, small planetary nebulae), then we should avoid such places and do observations at more remote locations. Big meadows area usually improves seeing, also lakes and large water reservoirs provide better seeing conditions. But seeing also changes from night to night, is different at summer and winter. There are also premium seeing locations around the world, and dedicated planetary observers knows them well.

And as for the light pollution and Bortle scale – low light pollution provides of course better observing conditions. But also atmosphere transparency is important. Even if you are far away from city lights, but smog or haze is present in the air, observations will not be a pleasure. In such case it is always better to be higher. So if there are any hills or mountains in the area, go higher.

Light pollution, transparency and seeing – these three factors we should consider when selecting observing location.

What telescope – what objects?

Actually this is kind of myth, that some telescope types are more suitable for specific targets. It is vice versa – some telescope types are less suitable for some targets, due to their flaws. If we would have perfect telescope it would be perfectly suitable for all targets regardless of its construction type. So what we should be aware of?

First of all – we do no talk about toys here. No plastic telescopes, no red or orange coating on the lenses. Quite often as first telescope we are presented with refractor – classic construction with glass objective at the front and eyepiece at the back. Entry level refractors have usually achromatic objective lens. Short focal ratio achromatic refractors (like around f/5) usually have significant chromatic aberrations, and are less suitable for planetary and Moon observations. Longer refractors (like around f/10) will do these tasks better. But fast focal ratio refractors are nice instruments for observing deep sky objects at dark locations. Chromatic aberrations are not so painful then, and good contrast and large field of view provide remarkable views. More expensive refractors are more complicated constructions with exotic glass used (ED glass, triplet objective, apochromatic objective lens). Prices rise quickly in this segment, but image quality and contrast are getting close to perfect in these constructions.
Popular refractors among amateurs are: 70 or 90mm aperture f/10 as entry level achromatic refractor, short f/5 achromatic refractors in the aperture range 80 to 150mm for deep sky observations. Small 60-70 mm ED refractor as travel scopes. Bigger 80-120mm ED refractor for general observations. Triplet apochromatic refractors are usually used for astroimaging.

Another instrument type often suggested as first scope is reflector, so the telescope that uses mirror as primary objective. Most common and also most all-purpose reflector is Newtonian telescope equipped with parabolic mirror. Good quality Newtonian telescope can provide very good quality images – assuming they are colimated well. This construction is open tube instrument, so over time mirrors may get dirty and require cleaning. In Newtonian telescope the eyepiece is attached to the side of the tube, so the orientation during observations is not obvious.
Popular Newtonian telescopes among amateurs are: fast f/5 small all-purpose instruments with aperture 135 and 150mm that can be put on alt-az mount (like AZ4). Larger Newtonians are usually constructed on Dobsonian mount. Dobsonian telescopes exists in the range from 150mm aperture till 500mm and more. Such large telescope transported under dark sky provides extraordinary experience.

200mm SCT telescope and 130mm refractor on GoTo mounts
200mm SCT telescope and 130mm refractor on GoTo mounts

Third main type of telescopes are catadioptric instruments. This construction is a mix of reflective optics (primary and secondary mirrors) and refractive optics (usually corrector plate). Schmidt-Cassegrain, Maksutov-Cassegrain and Maksutov-Newtonian are the most popular catadioptric constructions among amateurs. Catadioptric instruments provide large aperture and large focal length (usually focal ratio is f/10 for SCTs and f/12 or more for Maksutov-Newtonians) at short tube. These telescopes are real general purpose instruments with one only flaw – they cannot provide very wide field of view, like fast Newtonian or refractor. Other than that catadioptrics are usually good quality performers, with large aperture and convenient focus point placement (at the tube back), so observations are quite comfortable.
Popular catadioptrics among amateurs are: small 125, 150 or 200mm SCT as portable telescopes for visual (on alt-az mount or also on GoTo mount), Maksutov-Cassegrains at 102 or 127mm as small Moon/planetary and terrestial scopes.

Accessories

Once we decide what telescope we will look for and got some knowledge about observing, we wonder what else do we need, what eyepieces, what filters… Let’s not jump into it too quickly. One thing we need for sure is mount for the telescope. It can be intrinsic part of the telescope (like in dobsonian telescope) or separate device.

For visual observations usually sturdy alt-az mount is a good choice: AZ4 or similar construction for small and medium size instruments. Larger AZ5 for larger scopes. For SCT and refractor telescopes where eyepiece is at the instrument back also equatorial mount can be a solution, also with tracking or GoTo system. Equatorial mounts are larger, heavier and it takes more time to set it all up than alt-az mount with similar capacity. Smallest telescopes can be used also with large photo tripod. Another option is ALT-AZ fork mount with GoTo capabilities, that is often available for small SCT telescopes.

Another thing in the to do list will be preparation to session. Once we know sky very well we know by heart position of many targets and we may jump from one to another with ease. But since you are reading this, it is probably not the case. Even if we have GoTo system available, we should prepare to observations. Use planetarium software to locate objects, prepare star hopping, print charts or use printed sky atlas (Sky&Telescope Pocket Sky Atlas is perfect at the beginning, Interstellarum Sky Atlas would be experienced amateurs choice). Without a plan we can just hop between brightest stars (some of them may be planets) and we will not observe anything meaningful.

So what about actual accessories? Most probably one or two eyepieces are already included. If we purchased second hand telescope, then there may be more accessories. Let’s use the ones we already have and check what we miss. Consult with other observers what they use. Go to starparty and check other gears. Take your telescope and borrow eyepiece or filter from others to check if you like it. If we get into astronomy, then many eyepieces and filters will pass through our hands. It is normal. When we change telescope it may turn out that our favourite eyepieces do not fit well. Let’s not get attached to them, these are only instruments, not an end in itself.

Telescope recommendations

Refractor 102mm ED on AZ4 mount
Refractor 102mm ED on AZ4 mount

Well, it was quite a long entry, I hope you would find it useful. Below my recommendations of several instruments that are pretty popular among people who look for first telescope. Smallest telescopes below can be put on small ALT-AZ mount (AZ3, AZ4), or even photo tripod. Larger will work better on bigger AZ5 class mounts. Also single arm GoTo mounts will do the trick, and equatorial EQ3-2/CG4 class mounts (with or without GoTo/tracking)

150, 200 or 250mm Newtonian telescope on dobson mount. Popular telescope for visual observations, best bang for the buck ratio. Not quite handy, pretty heavy, but large aperture provide excellent observations under dark sky.

Long achromatic refractor 70 or 90mm aperture, f/10 focal ratio. Portable (though not very short) instrument, easy to use. Usable power up to 100-120x. Can reveal some planet and Moon details, also several deep sky objects under dark sky.

Short achromatic refractor 100 to 150mm aperture, f/5 focal ratio. Mostly for deep sky objects with long focal length eyepiece, low power, wide field under dark sky.

60-80mm aperture ED doublet refractor with focal ratio around f/7. Good glass as travel scope, for wide fields, terrestrial observations, Moon views.

100-120mm aperture ED doublet refractor with focal ratio around f/7. These telescopes are good performers for visual observations – provide excellent views of both wide fields and also can work with powers up to 200x at planets, double stars, Moon.

102 or 127 mm Maksutov-Cassegrain (MAK) telescope. Very compact telescope with long focal length. Comfortably observations at balcony. Portable. Good optics, max power 200-250x. Not for wide fields.

125, 150 or 200mm Schmidt-Cassegrain (SCT) telescope. Very compact general purpose telescope, portable. Very good images (when properly collimated). Not for wide fields, though better than MAK.

Small 135 or 150 mm aperture Newtonian at f/5 focal ratio. Popular general purpose telescope, suitable for both wide fields observations and also planetary/Moon work. Provide good quality images (when properly collimated).

Clear skies!

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