These 2 pages give additional technical detail of the kind that I wish I had had when I first started in what I think is a sensible order!


'Ordinary' glass will reflect about 10% of the light it receives, hence the seemingly daft idea of adding coatings to it to improve light transmission.  These anti-reflective coatings gives the green appearance of eyepieces, lens, etc., with some coatings claiming to enable up to 99% light transmission.


Ideally all air to glass surfaces should be coated and the better eyepieces, usually containing 3 or more lens will be described as fully multi-coated.  Below that are those which are multi-coated, then fully coated and finally just coated where just one side of the lens, the one you can see, has been coated with a single layer.


If you wear glasses and are NOT astigmatic, then it is best not to wear them when looking through and eyepiece as you are presenting yet another lens in the way of the very faint light from distant stars and galaxies.  And how many of us have well-coated, scratch-free lenses?  It can be a bit of a bind having to take them on and off but worth doing.  If you must wear glasses then


Eye Relief becomes very important.  This is the distance that you need to place your eye in front the eyepiece in order to get the best view. 15 -20mm is about the minimum if you wear glasses.  Without glasses an eye relief of 5mm or less becomes uncomfortable and eyelashes start to get in the way. Eye Relief is a function of the make and type of eyepiece, as is the True Field of View (TFoV) - that is how much you can actually see. 


All eyepieces are rated as to their Apparent Field of View.  Plossl eyepieces, the most common and the type usually supplied with your scope, have an AFoV of around 50 degrees.  


The True Field of View depends on the magnification you use - a higher magnification means a smaller True field of view.  Thus a typical 25mm eyepiece in a 80mm Refractor telescope of focal length 400mm (focal ratio 5) gives a magnification of 16.  This gives you  a TFoV of just over 3 deg (50/16) , which is what you see through the eyepiece.  A 10mm eyepiece gives a magnification of 40 and a TFoV of 1 deg.  


In contrast, the Catadioptric Schmidt-Cassegrain and Maksutov types of telescopes will usually have a focal ratio of at least 10 - an effective 800mm focal length with the same size of telescope (80mm). Thus you would have a magnification of 32 giving you half the TFoV of the same size (aperture) of Refractor - just over 1.5 deg with the 25mm eyepiece and just over 1/2 deg (37.5 min) with the 10mm eyepiece.


The Moon presents a disc of around 1/2 degree so will be fully visible in all these cases.  But when you move up to larger scopes you get higher minimum magnifications so the TFoV becomes much more important.  Hence the popularity of wide angle eyepieces.  These typically start at around 66 - 70 deg APoV.  Then you might have the Nagler type with 82 deg, whilst others like the Ethos and Delos will boast 100 or even 110 deg.  And the wider the AFoV the higher the price and weight. The latter can seriously unbalance the scope, particularly if you happened to use  a really heavy one with the smaller type of scopes we have been considering.


Another important feature eyepieces is the Exit Pupil.  This is size of the focused beam of light that enters your eye.  It is defined in mm as the eyepiece focal length divided by the focal ratio of the scope.  Thus with a 25mm eyepiece and a 80mm Refractor with a focal ratio of 5 the Exit Pupil is 5mm (25/5) and 2.5mm (25/10) with a f/10 Catadioptric scope.  A 10mm eyepiece would give 2mm (10/5) and 1mm (10/10) respectively.  


The human eye will typically have a pupil size of 7mm when we are young, but as we age this will gradually diminish down to around 5mm or less.  So a longer focal length eyepiece with the 80mm scope could result in a Exit Pupil larger than the pupil of the viewer - light would be lost.  However, a more likely problem is having too small an Exit Pupil.  Anything less than 1mm becomes uncomfortable and is likely to result in diminution of light.  And the maximum magnification you can use will depend on the viewing conditions.  So be very careful not to get carried away and get an eyepiece that is too powerful for your scope both because of the Exit Pupil and also because of the Maximum sensible magnification that can be used.


A very useful piece of software which will enable you to calculate

all the combinations of eyepieces and telescopes can be found here.


Barlow lenses effectively stretch the focal length of any scope and thus increases the magnification.   You may well get a 2x Barlow with your scope which would effectively result in a 12.5mm eyepiece when used with the 25mm eyepiece and a 5mm eyepiece with the 10mm eyepiece.  However, the field of view is also halved and the quality of some of these is not that good so you may be better off either buying a good quality Barlow, or investing in good quality higher power eyepieces.  You can  get higher magnification Barlows up to 5x, but anything other than a 2x is only really suitable for AstroPhotography.  Even then they are only really suitable for planetary and lunar work and a 3x Barlow is a more sensible limit for most viewing conditions, depending on the focal ratio of the Telescope.


A term which you will come across when looking at eyepieces is Parfocal. This simply means that eyepieces within a certain range will all have the same Eye Relief, so when changing from one to another in the same range you won't need to refocus, though you may have to recentre if the object isn't exactly in the centre of view.  Thus all the quality Televue Radian eyepieces, 18mm, 14mm, 6mm, 5mm, 4mm, 3mm, offer a 20mm Eye Relief - they are also quite heavy, the 18 and 14mm being about 1/2lb and the others 3/4lb - and pricey at around £180 - £190.


Good quality eyepieces really do make a difference, though you don't necessarily have to pay a lot unless you really want wide views.  But do remember that the higher the magnification, the less often you are likely to use that eyepiece simply because viewing conditions will seldom be good enough.   Each evening it is always best to start off with a low magnification eyepiece.  In that respect a good quality 25mm or 32mm is a really good investment.


Using Manual Scopes



Without being too technical