I have considered the question of what a person needs in his eyepiece kit, as a bare minimum, for quite a while. Personally, I don't have a lot of disposable income, and I recognize that a lot of amateur astronomers are getting along on a shoestring budget. So, if you can afford to go out and buy a full set of Naglers, or even Radians, go ahead, this article isn't for you. It is for those of us who have to choose between a new eyepiece and a new spring jacket, and are already garnering disapproving looks from our partners for buying that natty little refractor at a higher price than they really, truly expected.

I will talk first about scopes on equatorial or tracking mounts, and later about Dobsonians.

I am assuming that, as we don't have a lot of money, we are not buying large catadioptics or refractors, and cannot afford a Newtonian of larger than 8". These general principles apply to most scopes, however.


SCOPES ON EQUATORIAL, GOTO, OR TRACKING MOUNTS

I am going to talk about Plossls, mostly, as they are the best value for money. If you get a branded Plossl, you will seldom get a piece of junk. You can expect reasonable sharpness across most of the field in all but the fastest scopes. Plossls also have a field of view of 50 - 52º, which is quite reasonable. I am also going to suggest a set of three or four eyepieces, and no Barlow,except in the case of a fast scope.

You should have a high power, a medium-high and/or medium-low power eyepiece, and a low power eyepiece. The eyepieces that came with your scope probably fill the medium-high and low power slot. If they are satisfactory, keep them for now. If they are marked 'H' or 'SR' don't even think about keeping them! If they are marked with a 'K', they are Kellners, which are generally acceptable eyepieces, but a little limited on field of view, being about 45º, usually.

Find out the focal ratio of your scope. It should be printed on a plate on the scope, usually near the focuser, and be represented by a number like f/5 or f/8. F/6 or lower is a fast scope, and f/7 or higher is an intermediate to slow scope. Scopes with focal ratios of f/8 or higher are generally more forgiving of lower-quality eyepieces, while fast scopes tend to reward lower-quality eyepieces with fuzzy stars anywhere from 1/3 to 1/2 way from the edge to the centre.

If you can't find the focal ratio, but you know the aperture and focal length, the focal ratio is (focal length/aperture).

Take your focal ratio, and multiply it by 3/4. So, if you have an f/8 scope, the result is 6. If you have an f/10 scope, the result is 7.5. This result is the length in millimetres of your high power eyepiece. It will give about 2/3 of the theoretical maximum power of your scope. This is the actual maximum if you do not always enjoy perfect seeing and transparency. If you have a 100mm scope, this eyepiece will give 133x.

IF YOU HAVE A FAST SCOPE, say, f/5, this formula will suggest a 3.75mm or 4mm eyepiece. Looking through a Plossl at this length is a miserable experience. If this is the case, I would suggest you buy an eyepiece with a length equal to 1½ times your focal ratio, and buy a 2x Barlow lens in the same price range as your eps. These purchases give you your high power and medium-high power magnifications, so skip the next paragraph.

Now multiply your focal ratio by 1¼. For our f/8 scope, the result is 10, and for an f/10 scope, the result is 12.5. This is the length of your medium-high power eyepiece. For our 100mm scope, it gives a magnification of 80. Eyepieces in these lengths are not hard to find, and you can go up or down a millimetre if your dealer doesn't stock them.

Multiply your focal ratio by 2, now. By now, you can do the math yourself! In our 100mm scope, this gives a magnification of 50. This is your medium-low power eyepiece, and your low power eyepiece is given by multiplying your focal ratio by 3, and you get a magnification of 33 in your 100mm scope. IF YOU HAVE A FAST SCOPE, you want an eyepiece of 3 to 4 times your focal ratio, or 15 to 20 mm for an f/5 scope as your medium-low power eyepiece, and about 5 times your focal ratio for your low power eyepiece.

An eyepiece of 5 times your focal ratio also gives you an 'exit pupil' of 5mm. This is the longest eyepiece you want to use if you are older, as this exit pupil is approximately equal to an older (45+) person's maximum pupillary dilation. You can't use more light than that. If you are younger, you could go up to 7 times your focal ratio, or an exit pupil of 7mm.

To summarize, for an f/8 scope, we suggest a kit consisting of 6, 10, 16 and 24mm. For an f/10 scope, 7.5, 12.5, 20 and 30mm. For an f/5 scope, 2x Barlow, 8, 18, and 25mm.

If your budget allows for only three eyepieces, drop one of the medium power eyepieces. If you are a lunar/planetary observer, then we would suggest dropping the medium-low eyepiece, and if you are a DSO observer, the medium-high eyepiece. In the latter case, we could suggest dropping the high power, but let's face it, there will always be times you want to get a good look at Saturn, or a good planetary nebula, so keep the high power.


DOBSONIANS

Dobsonians tend to be large, fast scopes. If your Dob is 6" or less, you can safely follow the guidelines for the scopes listed above, as the highest magnification this will give you is 200.

At about 200x, it gets hard to follow things with a Dob. Some people can do it, and your ability to follow objects will improve with time, but 200x is a good start. You will want to have an eyepiece kit between 200x, and a 5mm (or 7mm if you are a youngster) exit pupil. Suppose you have a 10", f/5 Dob. You will have a focal length of 1250mm, and will get 200x with a 6.25mm eyepiece. In practical terms, a 6.5 to 7.5mm eyepiece will be what you will find available. To get a 5mm exit pupil out of a 250mm mirror, you will need an eyepiece that gives you 50x. This means a 25mm eyepiece. To get a 7mm exit pupil out of the same mirror means a magnification of 36, and a 35mm eyepiece.

Having decided on your low and high power, it is fairly easy to pick two more eyepiece focal lengths that will fill in the gap. If your spread is 6mm to 25mm, try 10mm and 16mm as your intermediate lengths. If the spread is 6mm to 35mm, then use 12mm and 20mm as your intermediate eyepieces.

So, for an 8" f/5 Dob, you would be getting something like a 5mm, 10, 16 and 25mm.

These guidelines will give you a useful set of eyepieces without breaking the bank. You can buy one eyepiece a month until you have your set, and use the eyeieces you have until your set is complete.If you can afford slightly better eyepieces, then buy those, with the length guidelines still in mind. If you have a fast scope, ask specifically if the eyepiece you are considering is appropriate for a fast scope. Some less expensive wide-angle eyepieces perform well only in a f/8 or slower scope, and you don't want to buy a set of these with a fast scope.

Best wishes, and enjoy your new hobby!

Great Post, should be a Sticky

A very imformative post WH must have took some time to type out.

I agree with everything you say and eventually my plan is also to slim my collection down to 4 good eyepieces.

I do not like barlows so my ideal collection to accompany my dob will be.

26T5
17E
13E
8E

Better start playing the lottery

Hi Warthog,

Thanks very much, that's a really helpful post. There's so much to learn in this hobby!

I'm really looking forward to when my barlow arrives but the lovely weather and clear skies will probably have finished by then!

Liz

good post! I completely agree that 200 is about the limit for a dob.

Under our skies yes but I've read so many times on Cloudy nights that they in the USA can get x600 + from their dobs. Their seeing is far superior to ours but you still need to track, unless they have tracking mounts as well.

I am still at the 'about to buy a scope' stage so this was a fantastic read. Many thanks Warthog.

+1 for a sticky

Yeah, my dream kit would look like that, too, but I find my Antares Speers-WALERs good value. This took me about two weeks to write and edit, and there are still a couple of minor things I could change.

Thanks!

for me Doc, it's not the seeing that's the issue but the tracking. At 150x an planet moves across the fov in about 2 minutes (I guess ?) and the image isn't that good at the edges anyway so at 600x you'd be moving the scope about every 20 seconds . Unless their dobs are so smoooooooooooooooth that that can keep the object in the middle of the fov all the time without any vibration (I know I can't) I don't get it

ps I think there's quite a lot of bull**it on CN as in my scopes bigger than yours, my scopes good up to 1,000 times etc etc

Warthog,

Thanks for the excellent post. I like the fact that it's a relatively simple method to follow, so people with little experience can work out an EP selection without getting too bogged down by jargon.

Dave.

Great post, should be a sticky methinks!
Amanda

I agree here, most of what I read I do not really believe and take it with a grain of salt, but there's alot of people who do use very high mags with dobs.

It is possible just needs mega practice.

great post, Warthog and a nice way of simplifying it all. Easy to understand and will help people get the most out of their money but more importantly most of what they can get out of their scope.

Andrew

Fantastic advice, WH!!
Wish i'd have known all that about 10 years ago.

Certainly, if one can PLAN these things better in advance!
For "accessorising" proved much my financial downfall...

A great insight into eyepiece selection WH.
Your post is a boon for those wishing to get a selection of occulars to suit their telescope.
Very informative.
Ron.

This was so helpful for me. Reading through my user manual was pretty basic but this thread really cleared some things up.

My Scope is a F/8.6 and came with a 9mm, 25mm and 2x Barlow lens. So from what was posted above it appears that I have the standard high power and low power eye pieces.

Would you all recommened any other sizes?

You might want to post this question in the beginners section as well to get some other views but I'd suggest a 32mm Plossl which will give you nice low power views. With the barlow lens you would have quite a comprehensive range of powers then.

John

Great post and a big help. Thank you very much.

Hi

Been reading up a lot on EP. Seems like I am going to get an 8 inch f/5 reflector. It will come with a 20mm plossl so thats 50x.

I've read that fast telescopes need better EP and more expensive barlow lenses, I was planning on getting the cheapest ones which were the Skywatcher plossls/barlows, do you think they will work well, and if so, what size, if I only buy one I would buy the 7.5mm one and maybe a barlow lens with it

+2

That would be a good choice with that scope, but if you are going to buy only one ep, spend a little more money and get a good one. Apparently the Meade 4000 Superwides are a good choice. You can use the 20mm as a low power ep and add eps as you save for them. It doens't take long to accumulate a good set of eps.

Brilliant post, thanks!

I have recently purchased SW explorer 150PL so using your guide I should go for 6, 10, 16 and 24mm EP's. The EP's supplied with this scope are 10mm, 25mm and 2x barlow, giving effective range of 5, 10, 12.5 and 25mm which seems like a reasonable match.

I'd be interested in upgrading the supplied EP's, and/or adding to the existing EP's but I'm not sure which way to go. Should I add to existing ones, or replace the ones I have (or both!)

Secondly, it seems that you can spend £20 on an eyepiece, or upwards of £400!! What sort of EP would be considered "good quality" for most people? Is £40 enough, or should I be looking at spending more. Any recommendations on makes/models of reasonably priced quality EPs would be appreciated.

Thanks

You get what you pay for with eyepieces but, for your scope, the TAL, Skywatcher, Meade or Celestron plossls will work pretty well.

First Light Optics have a good deal on Skywatcher Plossl's at the moment:

Skywatcher - Skywatcher SP Plossl eyepieces

John