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The Warthog

Eyepieces - the very least you need.

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Great overview!  As i am a newbie and just pulled the trigger on order my XT10i, this helped me to pick out a couple of eyepieces that I wanted to start with and try.  +1

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Ok with the eyepieces that came with my new scope my eps are : two 20mm , a 10mm and a 6mm along with a 3x barlow lens which I am a bit confused about using because each time I try I get no views at all(I'm new to barlow lens) is there any other ep I could benifet

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Well that has cleared a few thing up and I as I don't know any better I will follow it. 

Thanks 

Brian.

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I

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.

I have an f/13.8 scope and after the doing the math I get 10.35 mm for my high power eyepiece. My question is with such an odd number should I go with an eyepiece that is 15 millimeters or eyepiece that is 9mm?

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Or maybe the closest to it? That'd be a 10mm or an 11mm EP.

However, what's the focal length of your 'scope?

Mine's 1200mm and my 'scope has a focal ratio of f5.9.

So this calculation gives me a maximum EP size of 4.4mm with a magnification of 273x in this 'scope.

That's rather a lot for almost everything but the moon.

But, my advice would be to follow the advice Warthog gives with regards to short focal length plossl EPs and get a longer focal length that you're likely to use more often and a 2x barlow lens.

I have a lovely 8mm plossl and a 2x barlow. The 8mm (@ 150x) is very good generally in my 'scope and especially when conditions are poorer.

On the moon you might even be able to push a little further.

Pop in the barlow and the 8mm EP and I get some cracking views of the moon (300x).

On better nights, I've enjoyed a 7mm and the barlow (343x) on the moon.

I'm lucky enough to regularly use a 6mm EP (200x) from my back garden with this 'scope.

I'd just like you to consider the focal length and magnification of your 'scope also and the general conditions of the site from which you regularly observer, before trying to get the maximum out of your 'scope.

Cheers

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The focal length of my scope is 1250. I am looking at getting 15mm eyepiece. I am going to have to more research on barlow lenses. I don't know much about them.

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I have one of those too!  :grin:

Don't they still come with a 9.7mm? I enjoyed using mine with only the 2 supplied EPs and a barlow on the 26mm for over a decade!

A 15mm would be a good choice in the middle though.

Cheers

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The focal length of my scope is 1250. I am looking at getting 15mm eyepiece. I am going to have to more research on barlow lenses. I don't know much about them.

I am not brilliant on barlows either but pound for pound a celestron xcel x2 barlow is pretty hard to beat for quality. There are loads out there though and if cash is an issue a standard "deluxe" skywatcher 2x is still a reasonable piece of kit to be getting on with at under £30

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I have one of those too! :grin:

Don't they still come with a 9.7mm? I enjoyed using mine with only the 2 supplied EPs and a barlow on the 26mm for over a decade!

A 15mm would be a good choice in the middle though.

Cheers

When I bought my ETX-90, I just got the 26mm eyepiece.

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Ah, then your options are many and varied.

You might want to take a look at the 68° Maxvisions which are Meade 5000 SWAs that never made it to market, there's a 16mm and a 20mm. Either (or both) would be rather splendid and they're at a very decent price.

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How do you like you ETX-90? This is the first time I've pulled mine out in ten years. I am worried about if I am going to be limited in what I can see and do with it . I want to get into doing astro photography. I think ETX-90 won't be able to do very well when it comes to astrophotography but if it's a decent scope to do some observing with I can put off buying another scope for a couple of years.

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It was my only 'scope for over a decade and I still think it's great.

It comes on holiday with us every year which the 8" can't do!

We still often get the two 'scopes out side by side and the etx seems to do better on the poorer evenings, of which there have been many this past few years.

It's definitely a decent 'scope to do plenty of observing and as I said, last me over 10 years before I decided I'd like something bigger.

It suited me down to the ground to begin with, because I didn't know my way around and the Autostar made that a doddle.

But for the last few years before I got the dob, I was finding I wasn't using the computer at all and all the setup and alignment seemed a bit of a faff, hence the dob!

I can't help you on the astrophotography front, but I would say that the etx is a great deal more than adequate 'scope for getting you back into astronomy.

Cheers

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I just tried out a new 9mm eyepiece last night. The eyepiece has a 50 degree apparent field of view. I think it might be too much magnification for my scope. I think it will work for lunar observing, but I am not sure. Any thoughts?

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It should be great on the moon.

Why do you think it will be too much magnification?

I bought an 8mm TV plossl some months back, primarily for the 8" dob, but have been using it successfully in the etx90.

In fact, it gave the best images of Mars for me at the time.

Cheers

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I seemed to have a hard time getting saturn in view and keeping it in view with the 9 mm. But that could have been due to my alignment being off.

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I do not understand any of this. Too much maths - I was never any good.

But I need eye pieces, as I have lost my set (in a nice metal flight case) after a break.

Well, I say a break - I have yet to use my scope properly - have had it 4 years now (disgusting, I know)

I stored the eyepieces so safe, I have lost them.

I need eyepieces as I am attending Herstmonceux next weekend - no good turning up without lenses.

I am going to buy a very basic 4 pack of eyepieces - 4mm, 6mm, 12mm, 20mm.

And then I might buy a 10mm and a 25mm individually.

I have a skywatcher explorer 200P on an eq5 mount.

Sarah

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I do not understand any of this. Too much maths - I was never any good.

But I need eye pieces, as I have lost my set (in a nice metal flight case) after a break.

Well, I say a break - I have yet to use my scope properly - have had it 4 years now (disgusting, I know)

I stored the eyepieces so safe, I have lost them.

I need eyepieces as I am attending Herstmonceux next weekend - no good turning up without lenses.

I am going to buy a very basic 4 pack of eyepieces - 4mm, 6mm, 12mm, 20mm.

And then I might buy a 10mm and a 25mm individually.

I have a skywatcher explorer 200P on an eq5 mount.

Sarah

Have you considered buying a zoom eyepiece rather than a basic set, it would probably serve you better

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Good post WH. I melted the inside of a cheap eyepiece while doing a solar projection. I know what to replace it with now-NOT for solar projection.

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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!

A brilliant post that has helped me no end in deciding what eyepieces I need for a reasonable collection! Many thanks Warthog.

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