Magnification in the Astro-Telescope: What you need to know!

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Magnification is not the be-all-and-end-all of telescopes!
While I'm looking around the internet I get lots of adverts directed at me for tiny little telescopes with magnification claims of 525x and the such-like. 
I thought I'd better write a guide for beginners who are captivated by this thought and who might sensibly look up magnification before buying one of these disappointing instruments.
You can see a surprising amount with relatively modest magnifications - If you were to pin me down and say I could only have ONE eyepiece (therefore one magnification) I would choose one which gave about 150x magnification.
Not 250x or 300x or even the fantastic 525x magnification - Just 150x.
The larger the telescope, the higher the maximum magnification, but this is always dependent on the 'seeing' or atmospheric turbulence.  Useful magnifications vary with the size of the telescope and the seeing conditons.
For general viewing around the heavens, whatever size your telescope, you will require several magnifications:
A fairly low power for wide field views (30x – 50x)
A medium power around (80x - 100x)
And a higher power for normal viewing nights (150x - 175x).
Above this the image deteriorates due to the atmosphere on all but the calmest nights where you might be able to employ 200x - 300x magnification.

Generally you will find that you will use magnifications in the region of 50x - 175x most of the time regardless of the size of your telescope!

'Big Telescope' Examples:
I have used the 9" (225mm) Refractor at the Godlee Observatory, Manchester University. We had excellent views of Jupiter at 175x magnification.

I used the 30" (750mm) Newtonian at the Amateur Astronomy Centre Nr. Bacup. And, once again, we had some lovely views of Jupiter, at 200x magnification.  (See Pic. at top)

The best view I ever had of the globular cluster M13 was in a 22" (550mm) amateur owned Newtonian, at just 200x magnification.

So, to make my point:
Even 'huge' telescopes utilise 'reasonable and usable' magnifications.  If the telescope at the top of this page only needs to use 200x, why would a tiny 60mm refractor need 525x?  The answer is that they're appealing to the 9 year old inside you that thinks that they'll get a great view at huge magnifications!   This is not the case.

Why Do We Need Different Magnifications?
The objects we look at are of different sizes and sometimes we want to see the whole, and sometimes the detail.
This means that we need to be able to 'close in' on something. For instance, if we want to look at the whole of the star cluster "The Pleiades" (Seven Sisters) a 6"/150mm f6 reflector would need a magnification of only 35x to fit them all in to the view - To view Alcyone, the bright multiple star of the cluster, you would require a magnification of around 100x - 120x to show the stars to their best advantage.

Likewise, if you're interested in planetary detail, you'll want magnifications from about 80x to the maximum your telescope and the atmosphere will allow.  If you want to observe the moons of Jupiter and Saturn, you will only need to use 60x - 100x to fit them in the field of view.

The main consideration is the atmospheric turbulence.  If the atmosphere is unsettled, then you will need a lower magnification to see detail.  The higher magnifications can only be used effectively when the atmosphere is calm and 'quiet'. 

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How do we work out magnification?

Each eyepiece (The small lens you look through) has it's own focal length (Usually printed on the side) If you know the focal length of the telescope object glass or mirror, then all you need to do to find the magnification is divide the focal length of the telescope by the focal length of the eyepiece.
Eg1. 900mm / eyepiece 15mm = 60x magnification.
Eg2. Focal length 1250mm / Eyepiece 8mm = 156x magnification.
It can be seen then that the same eyepiece in different telescopes will give different magnifications!
Eg: a Refractor, focal length 1200mm with eyepiece 10mm = 120x magnification. The same eyepiece in a 150mm Newtonian with a focal length of 750mm = 75x magnification.
Magnification is a big selling point for telescope sellers so you have to accept that they will throw in an eyepiece that is virtually useless just to give that 525x magnification claim. As long as they give other eyepieces that cover the 50 - 100 range and the 100 - 150 range you'll be OK.

But do you know what? I have noticed that once you get away from the ridiculously small telescopes manufacturers tend to give more reasonable magnifications! Quite often a 76mm Newtonian (too small for astronomy) will come with 525x (useless), but a 150mm Newtonian (Entry level size) will more likely have 300x as it's 'best' magnification (One that could actually be used on the best viewing nights).

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Working it Out!

If you don't know your primary focal length you can measure the outside of your telescope to approximate (to about 1cm) the focal length.
Remove the eyepiece, wind the focuser right in, measure exactly as shown in the diagram.

Once you know this length in millimetres, you can find magnifications easily.
Don't forget you can always buy more eyepieces to get the usable magnifications you require.
So... Following my advice further it stands to reason that to utilise the higher magnifications to the full larger telescopes are essential! A five inch (130mm) Newtonian can manage 250x on an excellent night, but a six inch 150mm Newtonian will stand 300x magnification on the same night.
Logically, then: If you buy smaller than a 130mm Newtonian Reflector, or an 80mm Refractor you won't see the detail that's available. You can really only expect to use magnifications up to 50x per inch of aperture for Newtonians, and 100X for refractors. (To be 'scientific' let's use millimetres!)

THE MAXIMUM USEABLE MAGNIFICATION on a very good seeing night is:

About 2x per millimetre diameter for Newtonians (Half that on a normal night!)

About 3.75x per millimetre diameter for Refractors. (Half that on a normal night!)

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The atmosphere is a major player in the 'What's the highest magnification I can use?' debate.  The rule is to use the highest magnification that still shows a steady clear image.
Sometimes the atmosphere is really steady and you can use the highest magnification your telescope will allow.  But, more often, there will be warm and cold currents churning up the view.  In that case you have to moderate your magnification so you can see the best view you can get.  This usually means that you have to use a maximum magnification of about 150x on a 150mm Newtonian telescope, one that boasts a maximum of 300x.
When I went to live in Lanzarote, I was looking forward to 300+ clear nights a year, but what I didn't realise was that the atmosphere would boil from sunset until four a.m.  The views before midnight were nearly always rubbish!  The best viewing was before dawn once the ground had cooled down.
In temperate latitudes we have it a bit better.  The atmosphere in the UK is usually good after the Sun's been down an hour, getting very good after midnight any time of year.  Once the atmosphere is calm you can regularly use half your telescope's maximum magnification.  On really clear and still nights you can observe at the max.
A slight mist can sometimes be a good thing - It can act like a filter when looking at the bright planets or the Moon and actually enhance the image! Clouds can be observed round by looking through the gaps.  But rain is a definite no-no.
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Further Reading:

My other telescope guides may interest you:

I have written these guides to help amateur astronomers choose the right equipment for themselves.

Choosing Your First Telescope: Complete Essentials.

Setting Up an Equatorial Mounting (Simple!)

Best Value Planetary Telescope  (90mm Refractor)

Best Value Beginner's All-Rounder Telescope  (130mm Newtonian)

What Size Telescope Will Show Jupiter's Moons?

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I will answer as soon as I can and it's no bother – Honestly!

More information? Google "supercooper telescope help".

Clear skies and good seeing... I hope this guide has been useful. :o)
All text and images © Barry Cooper 2008-16 unless otherwise credited.
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