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A Beginners Guide to Telescopes

Author: Simon Dawes (Chairman & Trustee of the Society)

A 5 minute primer that will give you all you need to know about Telescopes

Jargon – all jargon is also available in our Dictionary of Astronomy
Aperture
The diameter of the primary optical element in a telescope
Catadioptric
A telescope that uses mirrors and lenses to focus the light
Collimation
The lining up of the optical elements in a telescope along with the tube and focuser, essential to get the best out of any telescope, refractors are usually collimated in the factory and don’t need nightly adjustment
Coma
An optical aberration that makes stars have little tails directed away from the centre of the field of view
Chromatic Aberration
This is an aberration in an optical system that results in light from different wavelengths being focused at slightly different points, so it is impossible for all the light to be in focus at the same time, typically this results in a blue or purple haze around bright objects and a loss of contrast.
Diagonal
Secondary mirror used to divert the light from the primary out to the side of a Newtonian telescope
Field of View
The area of sky can see, to see the full Moon you need a Field of View of half a degree
Focal Length
The distance the focus is away from the primary objective
Focal ratio
Simply the focal length divided by the aperture of a telescope, usual expressed as FN, where N is the number of times larger the focal length is compared to the aperture. When ‘N’ is small this is termed ‘Fast’ when N is large this is termed’ Slow’
GEM
German equatorial mount, a type of telescope mount
Meridian
An imaginary line in the sky passing north south
Newtonian
A type of reflecting telescope
Objective
The main mirror or lens in an optical system
OTA
Optical Tube Assembly, a telescope is made from a mounting system and an optical tube assembly.
Primary
Generally referring to the main ‘light gathering’ optical element in a telescope
Reflector
A telescope with a mirror that focuses the light
Refractor
A telescope with a lens that focuses the light
Resolution
Measured in arc seconds, this is the finest detail you can see, or image, it is generally limited by aperture and atmospheric conditions.
Secondary
Generally referring to a mirror in a telescope used to change the direction and or point of focus in a telescope

How to choose a telescope


The general advice is don’t choose a telescope without speaking to an amateur astronomer first, however there are a number of fundamental things you should know before you choose the right telescope for you.

Focal length:

GUIDE_Telescope_FL_Refractor.gif
Refractor

A short focus telescope (say a focal length less than 500mm) will allow you low magnification, wide fields of view and are great for extended objects such as star clusters and galaxies, a short focus telescope will be more forgiving in its mounting allowing longer un-guided exposures or poor polar alignment, by using small focal length eyepieces it is also possible to view at high magnifications making short focal lengths good for ‘grab and go’ telescopes. However if you are primarily interested in the Planets and the Moon then a long focal length telescope is more suitable for these objects.
GUIDE_Telescope_FL-Reflector.gif
Reflector

Aperture:

The larger the aperture the larger the light collecting area and the dimmer the object you will be able to see, and there is another reason to choose the largest aperture you can afford, that is ‘resolution’, or the ability to see detail, the larger the aperture the more detail you will be able to see (calculations here), however as aperture increases so does cost and weight, especially so with refractors.

Refractor, Reflector or Catadioptric?


Reflectors:

These have a parabolic mirror at the bottom of the tube and a secondary diagonal to direct the light out to the side of the telescope. Reflectors, the most common type being ‘Newtonian’s’, are the cheapest option for £ per mm so the only real option for very large telescopes or modest telescopes for astronomers on a budget, but there is a price to pay, so to speak, for this economy, the first is in physical size, reflectors tend to have large tubes, consider a 200mm (aperture) telescope with a typical focal length of 1000mm the tube will be approximately a meter long and 250mm in diameter, you wont be taking this on a plane! You can reduce the size of the tube by reducing the focal length, however F4 (a focal length 4 times the aperture) is about as short as you will be able to go.
Layout of a Newtonian reflector, there are many other types of reflector but the Newtonian design is the most common by far.
GUIDE_Telescope_Reflector.gif
Other considerations:
Central Obstruction
Reflectors have a central obstruction in the form of a diagonal secondary mirror, this reduces the total effective area for light capture but more importantly has a negative affect on resolution (calculations here).

Coma
Newtonians, especially fast Newtonians suffer from coma, this can be reduced using a coma corrector at the eyepiece but expect to pay around £100 for a reasonable one, an alternative is the corrected Newtonian, most common being the Maksutov-Newtonian which has a corrector plate at the front of the tube to correct for the coma.

Collimation
Newtonian’s are very easy to collimate, however choose a ‘scope with a strong tube so that the ‘scope stays in collimation and needs less tweaking, some commercial 'scopes have thin rolled steel tubes that flex easily so check this before buying.

Colour free
Reflectors do not suffer from chromatic aberration and on this front they cannot be bettered.

Refractors:

These have a lens, usually a doublet or triplet at the top of the tube, apertures greater than 127mm (5 inch) are very rare and very expensive. However a quick look in an astronomy magazine will show you that the price of refractors vary wildly, and the reason for this is to correct their primary fault, that of chromatic aberration, this presents itself in two ways, first as a halo of purple around bright objects and second in a loss of resolution, as you focus on the dominant colour all other colours will be out of focus to a greater or lesser extent resulting in blurring of the image. Properly corrected refractors are called apochromatic, then come ‘ED’ telescopes sometimes referred to as ‘semi-apochromatic’, then achromatics and finally uncorrected telescopes.
The layout of the refractor, the main difference in refractors tends to be their ability to focus light of different colours at the same point.
GUIDE_Telescope_Refractor.gif
Other Considerations:
Dew
Refractors suffer from dewing up, this can be alleviated with a ‘dew cap’ which is simply a tube that extends over the primary, many refractors have these built in and getting a ‘scope with a dew shield that slides back for storage will help keep the whole package a reasonable size. In addition to a dew shield many astronomers invest in dew heaters or 12v hair dryers!

Focal length
As a rule of thumb, the higher the F ratio the less chromatic aberration that will be suffered and this should be considered when choosing a refractor.

Flat Field:
Refractors can also suffer from not having a flat field, this manifests itself as stars appearing slightly oval towards the edge of the field, 'field flatteners' are available but you need to research which is the best one for your specific scope as some don't work as well as claimed.

Catadioptric:

There are two very common type of Catadioptric telescope, the Schmidt-Cassegrain and the Maksutov-Cassegrain, both are enhancements to the ‘classical Cassegrain’ design and have easy (read cheap) to produce spherical primary mirrors, and correcting optics in the form of a corrector plate at the front of the telescope. They are compact designs but have a high F ratio with Schmidt’s typically being F10 and Mak’s around F14. Mak’s have a smaller secondary obstruction so tend to have a higher resolution.
Schmidt-Cassegrain
Optical set-up
GUIDE_Telescope_Schmidt-Cas.gif
Maksutov-Cassegrain
Optical set-up
GUIDE_Telescope_Schmidt-Mak.gif
Other Considerations:
Size:
These are very compact telescopes, but don’t expect a 300mm version to be easy transport unless you are a weight lifter! The small 80 – 120mm versions are very compact and reasonably priced.

Dew
The same considerations regarding dew on refractors are equally relevant here.

Focal length:
Even a small 90mm version will have a huge focal length, great for planets and the moon but if you plan deep sky imaging you will need a very good mount for even short exposures, usually focal reducers are available to reduce the effective focal length.

Focuser:
These ‘scopes focus by moving the primary mirror in or out, this can cause the field of view to move as you focus, which can be very annoying, consider adding a Crayford focuser to your purchase.

Flat Field:
Catadioptric telescopes can also suffer from not having a flat field, this manifests itself as stars appearing slightly oval towards the edge of the field, 'field flatteners' are available but you need to research which is the best one for your specific scope as some don't work as well as claimed.

The Mount:

The mount can be easily forgotten when choosing a telescope, but in all honesty it is the most important part of your telescope and you should expect to spend a significant proportion of your budget on the mount. There are 3 main types in use, the German Equatorial Mount (GEM), the Fork Mount and the Dobsonian.

German Equatorial Mount

These range in price from £50 to £30,000 and generally you get what you pay for. When considering a GEM you need to know if you will be carting it in and out of the house every time you observe, the larger more expensive GEM’s are very heavy, also remember that you will need a counter weight roughly the same weight as your OTA, these usually come with the mount but add to it's weight when carting around.
GUIDE_Telescope_GEM.gif
Other considerations:
Capacity:
All good manufacturers will publish the weight limit the mount is designed to handle, general advice is try to stick to half the mounts capacity.

The meridian
GEM’s need to ‘flip’ to pass the meridian this can be annoying but is essential to stop the counterweight hitting the pillar or tripod legs.

Versatility
A GEM can take any type of OTA, or even many OTA’s making it very versatile mount.

Fork Mount:

Generally a fork mount comes with an OTA, the two are inseparable, the fork has an advantage over the GEM in that it usually holds the OTA in two places rather than one, so if choosing this sort of telescope you need to consider both the OTA and mount performance as one.
GUIDE_Telescope_fork.gif
Other Considerations:
The Zenith:
Usually the zenith is out of reach and if you have a camera attached to the OTA don’t expect to be able to image directly above as the camera is likely to snag the bottom of the mount.

Field Rotation:
Generally fork mounts come set-up for Alt/Az use, so they have to drive in altitude and azimuth to track the sky, this results in field rotation making long exposures difficult, the problem is easily solved by buying a wedge to convert the scope to Equatorial mode, but be warned a good wedge will cost quite a bit.

Go-to:
A fork mount comes with it’s drive, so don’t expect to be able to upgrade from a mount that is driven to full go-to you usually have to buy a whole new telescope.

Dobsonian:

Like fork mounts these are built for the telescope, they are inherently simple (read cheap) and can be very compact, but without some very special equipment don’t expect these mounts to be driven or have go-to, it is possible (see here for one way to do it) but more advanced than I want to go into here.
GUIDE_Telescope_Dob.jpg

Included equipment

The last consideration in your telescope choice is in the equipment included.

Focuser:
If it doesn’t have a Crayford focuser, then make sure it has a high quality rack and pinion focuser, check the focuser is smooth and not easy to push in with your hand. If you are buying a Catadioptric or reflector the focuser will be relatively straight forward to upgrade to a Crayford but they are quite expensive as after market items, refractors are much harder to upgrade if it doesn’t come with a Crayford to start with.
Finder:
Many smaller scopes (especially Catadioptric ones for some unknown reason) have very small finders, you should aim for a 50mm finder, preferably one that can be focused and one that can accept different eyepieces (this last bit is not essential) a red dot finder is also a nice addition but not essential, details to make a 50mm Finder can be found here.

Finally:

The best scope you can buy is the one you are most likely to use, in my experience these tend to be small, short focus, portable and quick to set-up.

So what do I recommend?
I can’t really recommend a telescope for you but I can tell you how I would spend a fixed budget, there isn’t really a simple correct answer so don’t go and buy a scope based on what I would do and these are my preferences and thus include my biases, make sure you understand the basics and make an informed decision yourself!
Under £200
For this budget, I would buy a second hand 200mm Dobsonian, with a view to later mounting the OTA on a GEM when the budget becomes available. The Dob is easy to transport and doesn’t require any set-up so I won’t need an observatory either.

Under £1000
I would buy a short focus 80mm ‘ED’ semi-apo refractor and spend the majority of the money on a decent GEM mount,this will be a high quality portable 'scope.

Telescopes I have owned

• 300mm F4 Newtonian on an EQ6pro pillar mounted in a dome
• 127mm Achromatic Refractor on a Vixen GPDX GEM mount and SkyWatcher (EQ6) tripod (despite it’s limitations this is the ‘scope I use the most)
• 90mm Mak on a cheap but sturdy GEM mount, the main reason for this is I need it to be light weight and not take up too much room as it is my holiday ‘scope, it lives in an ABS tool box with everything that I need except the tripod which is packed separately.
• 150mm home built Dobsonian – currently being converted to a split ring Newtonian.
• 150mm classical Cassegrain (shares the vixen GPDX mount)
• 3" F15 Steinheil (Victorian) doublet Refractor - lovely scope (shares the vixen GPDX mount)
• ED80 Refractor (my grab and go 'scope) (shares the vixen GPDX mount but also have a cheap, small GEM for holidays)
• Equinox 100mm F10 modified PST (shares the vixen GPDX mount)



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