This section contains lots of information contained in the other guides,

but in an easy to find page with fewer details.  

 

If you are buying for someone else who will use the instrument, you don't need to learn all this stuff!

This page will tell you what you should be looking for.

 

As well as all the information about telescope perameters and capabilities this page also includes a useful websites section at the bottom.

 

Hopefully, it will save you time scouring through the other guides for the information you need, but be aware the information contained on this page is available in more detail for you in the site.

 

The recommendations here relate to small to medium sized equipment.  The maths is rigid for all equipment!

 

Each heading below is a link to the relevant detailed page in this website.

If you have further questions, beyond this page's detail, the relevant guide will answer them.

If you know what subjects the telescope is to be mainly used for, just look up those subjects below and the telescope you'll need will become aparrent:

 

If you want to know what size and type of telescope you'll need for your requirements, read on.

 

Telescope Type and MINIMUM Size Recommendations. (Why would you settle for the minimum? NO smaller!)

 

This is a good "Children and adult's" sharer size:

Short focus (Focal ratio of f5 to f10 : For focal ratio divide the focal length by the diameter) AZ Mounted

Minimum size: 60mm on AZ mounting - Ideally 70mm - 80mm

Best for: General Observation / Planets / Double Stars / Deep Sky / Moon.

Recommended Example: Bresser SkyLux AZEQ 70mm f10

 

Refractor for Children's Shared Telescope Observation (See specific 'Supporting Parents' section): Click this link. 

 

 

"Teen / Adult" Telescope Minimum Sizes: Telescopes get heavy, teens and adults can manage these sizes:

 

Refractor:  Short focus (Focal ratio of f5 to f10 : For focal ratio divide the focal length by the diameter) EQ Mounted

Minimum size: 70mm - Ideally 90mm - 120mm

Best for: General observation / Deep Sky / Moon.

Recommended Example:

SkyWatcher StarTravel 102 on EQ2 or EQ3

 

Refractor:  Long focus (Focal ratio of f10 to f16 : For focal ratio divide the focal length by the diameter) EQ Mounted

Minimum size: 70mm - Ideally 90mm - 102mm

Best for: Planetary observation / double stars / Moon.

NOT GOOD FOR: Deep Sky / Comets

Recommended Example:

SkyWatcher EvoStar 90 EQ2 or EQ3

 

Newtonian (Focal ratio of f5 to f8 : For focal ratio divide the focal length by the diameter) EQ Mounted

Minimum size: 130mm Parabolic - Ideally 150mm parabolic.

Best for: General Observation / Deep Sky / Moon

NOT GOOD FOR: Solar

Recommended Example:

EQ = SkyWatcher Explorer 130P on EQ3

AZ = SkyWatcher Heritage Flextube 130P Dobson Mount

 

'Dobson Mounted' Newtonian (Focal ratio of f6 to f8 : For focal ratio divide focal length by the diameter) AZ Mounted

Minimum size: 150mm Parabolic - Ideally 200mm parabolic.

Best for: General Observation / Deep Sky / Moon

NOT GOOD FOR: Solar / Photography

Recommended Example:

SkyWatcher Classic 150P Dobsonian

 

 

Maksutov-Cassegrain (Focal ratio of f12 to f16 : For focal ratio divide the focal length by the diameter) EQ Mounted

Minimum size: 90mm - Ideally 127mm - 180mm

Best for: Planetary observation / double stars / Moon.

NOT GOOD FOR: Deep Sky

Recommended Example:

SkyWatcher SkyMax127 on EQ2 or EQ3

 

Schmidt-Cassegrain (Focal ratio of f11 to f14 : For focal ratio divide the focal length by the diameter) EQ Mounted

Minimum size: 127mm - Ideally 127mm - 252mm

Best for: Planetary observation / Moon.

NOT GOOD FOR: Deep Sky

Example:

Celestron C5

 

Binoculars (Hand held)

Minimum 8x40 or 7x50  - Ideally 10x50 with porro prisms for your first pair. (NOT ROOF PRISM)

There are many larger sizes available, and roof prism types, but they would not be a good idea for your first pair!

Example:

Swift Ranger II, 10x50s

 

Binoculars (AZ Mounted)

Mounted bins are essential over 20x70  - Ideally 20x80 with porro prisms for your first pair. (NOT ROOF PRISM)

There are many larger sizes available, and roof prism types, but they would not be a good idea for your first pair!

Example:

Celestron SkyMaster 20x80s

 

 

Travel Scope AZ mounted.

The Celestron Travel 70 is ideal in many ways, it's the obvious choice.  However, it is not recommended that you try using a travel scope as your main telescope. They don't have the image scale to give enough detail, but are good for wide field views.

 

 

This is a list of 'trusted' makers who usually delight.  As with any telescope, have a look at the manufacturer's specifications.

Make sure your Newtonian is a parabolic one! Refractors must be over 400mm focal length.

 

I would recommend any of the following makes as having a very good chance of getting good optics, good views and excellent build quality:

 

SkyWatcher   Their EvosStar and StarTravel refractors are very good. You want a "P" in their Newtonian models (eg: 130P / 150PL / BKP1145)

 

Celestron        Their AstroMaster refractors are nice. LT model refractors are great for wider angle views. Powerseeker refractors are good.

                          Watch the Powerseeker Newtonian telescopes, they are spherical or Bird-Jones rubbish! AstroMaster Newtonians are                                              all spherical tat!  To get a parabolic Celestron you need to look at C6-N at least! Celestron are not nice to their customers!

 

Orion               Nice instruments. Check for parabolic Newts (They do mostly parabolic scopes, but make sure!) Nice build quality.

 

Bresser           Refractors are all excellent.  Newtonians: Watch for Bird-Jones and non-parabolic versions.

 

Meade             Usually good, but their small refractors (60mm to 70mm) can have poor images.  Always check Newtonians for parabolics.

 

Konus             Good refractors of 60mm - 90mm.  Nicely made. Don't let the tube colour put you off, these are good scopes!

 

Obviously there are lots of other manufacturers, but I would suggest you have a safe bet of getting a good telescope with these.

Do I have a list of telescopes I would definately NOT recommend?   I certainly do, my friend:

In all honesty, I have never come across a telescop made by any of these manufacturers that I would be happy to ask someone to use! 

As a rule of thumb, PLEASE AVOID THESE:

 

They tend to have poor quality optics, poor design, poor manufacture, poor materials, a combination of, or all of these failings!

 

Astral

Bushnell

Edu Science

Jessops

National Geographic (Own Make Bargain Basement models)

Opticraft

Optus

Science Tec (Avoid any instrument that comes in a see through front carry box!)

Seben

Simmons

Tasco

Vision King

Vivitar

Zennox

 

 

Below is a list of telescope types and their abilities:

 

"Types" of telescope to AVOID:

ANY "Bird-Jones" / "Catadioptric Newtonian" design with no full diameter corrector plate.

They all give awful image quality and include: Celestron PowerSeeker127 & PS1000, SkyWatcher 1141.

 

ANY Spherically figured Newtonian:

These include popular scopes - Celestron AstroMaster 114 and 130, SkyWatcher 1309 and BK1145, (BKP1145 is OK= Parabolic)

 

ANY 76mm Newtonian:

They are just too small  (SkyWatcher do make a 76P, but it's just too small to be useful in astronomy!)

 

ANY refractor telescope under 60mm diameter:

They are just too small to be useful in astronomy.

Long focus (f13 - f16) Japanese 60mm refractors represent the best of the 60mm brigade but, are the absolute minimum size.

 

 

Reference section

Regarding telescopic capabilities and variables:

Magnification: 

 

To find the magnification:

Divide the telescope primary focal length by the eyepiece focal length.                               Eg: 1,000mm FL / 8mm eyepiece = 125x

 

Remember, if you don't know your telescope's focal length, you can only measure the tube of a refractor or a Newtonian to calculate the focal length. Catadiapotric telescopes have incalculable (by tube measurement) focal lengths.  Measure the exit pupil for catadioptric scopes. (See magnification section)

 

Maximum Recommended Magnification:

Don't exceed 2.5x per millimetre of aperture in any telescope type (50x per inch)              Eg: 80mm refractor 80 x 2.5 = 200x

 

Barlow Lens:

My recommendation is to use a 2x Barlow if you need one to reach magnifications without resorting to eyepieces under 6mm.  Make sure it is a good quality Achromatic Barlow.

The Barlow used with an eyepiece doubles the magnification by doubling the focal length of the primary, but also the focal ratio. It preserves the eye relief (The distance your eye is from the lens when observing).  For this reason it is better to use an 8mm eyepiece with a 2xBarlow than a 4mm eyepiece on the planets, but the Barlow has to be very good quality!

 

My MAX magnification recommendation for Planetary views: is around 150x to 175x in general atmospheric viewing.

Very calm nights upto 250x may be used in larger sized equipment (90mm refractor or 150mm Newtonian)

 

My MAX magnification recommendation for Deep Sky views: is up to 100x in general atmospheric viewing.

Very calm nights upto 150x may be used in larger sized equipment (90mm refractor or 150mm Newtonian)

 

Focal Ratio (f Number)

Divide the Focal Length of the primary by the Diameter of the primary.                           Eg: 750mm FL / 150mm D = f5

Exit Pupil Size

Focal length of the eyepiece divided by the telescope focal ratio.                                    Eg: 30mm / f5 = 6mm exit pupil

As long as the exit pupil is 7mm or under, you will be ok.

Longest focal length eyepiece usable in a scope (ie: lowest sensible magnification for astronomy)

Focal ratio multiplied by 7.                                                                                                           Eg: f6 telescope would be 6 x 7 = 42mm

Apparent Field of View:

This is the width of the view when we look into the eyepiece.                                             Eg: AFOV is stated on new eyepiece blurb

 

Actual Field of View:

This can be worked out by dividing the Apparent field of view of the eyepiece by the magnification of the optical system.

                                                                                                                                                      Eg: AFOV is 35 degrees and scope 120x   

                                                                                                                                                      So:  35/120 = 0.29 degrees of sky observed

 

Resolution and Magnitude Limit :

 

Magnitude works on a logarithmic scale:

 

A 2nd mag star is 2.512 times fainter than a 1st Mag star, and on it goes.

To see one magnitude fainter, your larger telescope has to present 2.5+ times the light to the eye than the original scope.  Remember the secondary obscures some light if you have one, and reflection light reducytions need to be taken into account if you are comparing refractors and Newtonians.  Generally, if you double the diameter comparing refractors then you have four times the light collection. If you're comparing refractor with Newtonian then double the diameter is only about three times the light gathering!

 

1st Mag = Star Magnitude

2nd Mag = 2.512 x dimmer

3rd Mag = 6.310 x

4th Mag = 15.85 x

5th Mag = 39.82 x

6th Mag = 100.0 x

 

Note: You can shift these values up or down to find relative values:

A 2nd Mag star is 100 times brighter than a 7th mag star or a 3rd Magnitude star is 15 times dimmer than a 0 mag star.

 

Magnitude scale is bipolar!  There are positive (dimmer than 0 magnitude) and negative (brighter than 0 magnitude) objects + & - values.

 

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Pedantic: Purists might argue that a long focal ratio Newtonian and a short one will have different light gathering areas due to the different size of the secondary, but visually the faster ratio of the shart focal ratio scope will generally outweigh the small magnitude gain given by the smaller secondary shadow.

 

Eg: The difference between a 150mm / f4 and a 150mm / f8 is about 9% more light in the f8, but that equates to only a difference between a magnitude limit of 13.55 and 13.45 for stars! Meanwhile, DSOs will 'look brighter' in the f4. Not worth calculating. Magnitude limit calculations really only relate to stars. Atmospherics and eyepiece choice will negate the difference between focal ratios in same diameter telescopes regarding stars!

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Resolution, The Dawes Limit:

 

This is the formula for finding the resolution limit of your telescope (If it has diffraction limited optics).

 

R = 4.56/D       (D in inches, R in arc-seconds)

R = 11.6/D       (D in centimeters, R in arc-seconds)

where :
D = Diameter of the main lens (aperture)
R = Resolution: The resolving power of the instrument