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The Warthog

Eyepieces - the very least you need.

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I only have the basic 10mm and 25mm eyepieces that came with my Sky-Watcher Explorer 200P, so am considering upgrading to hopefully get better views.

Would the following set be a decent improvement over what I've already got - ScopeTeknix 10 part eyepiece and accessory kit

or am I better off buying individual ones and pay a bit more money?

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First of all, thanks for a fantastic post! Lots of answers to questions I was planning to ask and many I hadn't thought of.

I'm just a little confused about the difference in suggested EPS for f/8 and f/5. Why is the lowest power EP for an f/5 FIVE times the focal ratio yet only THREE times the focal ratio for an f/8?

I'm planning on buying a 150mm f/8 Dob for my 11 yr old son who is showing a great interest in all things astronomical from the moon and planets to galaxies and nebulae. I had planned on buying some extra EPS in addition to the 10mm and 24mm that are supplied in order to cover all possible viewing magnifications. I worked out that a 6mm for high power and a 40mm for low power (with 5mm exit pupil for me) would do nicely, but it would seem I am missing something??

Can anyone explain why I can't (or maybe shouldn't?) go to 40mm with this f/8?

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Thanks. I am going to add some quality EPS to my new scope purchase and this thread has really helped.

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First of all, thanks for a fantastic post! Lots of answers to questions I was planning to ask and many I hadn't thought of.

I'm just a little confused about the difference in suggested EPS for f/8 and f/5. Why is the lowest power EP for an f/5 FIVE times the focal ratio yet only THREE times the focal ratio for an f/8?

I'm planning on buying a 150mm f/8 Dob for my 11 yr old son who is showing a great interest in all things astronomical from the moon and planets to galaxies and nebulae. I had planned on buying some extra EPS in addition to the 10mm and 24mm that are supplied in order to cover all possible viewing magnifications. I worked out that a 6mm for high power and a 40mm for low power (with 5mm exit pupil for me) would do nicely, but it would seem I am missing something??

Can anyone explain why I can't (or maybe shouldn't?) go to 40mm with this f/8?

A 40mm works fine in an F/8 scope. However, to make the most of that, a 2" barrel diameter is required. Many starter scopes only have a 1.25" focuser. This means that a 24mm wide-angle EP gives about the maximum field of view possible. If you have a 2", by all means get a 40mm wide-angle.

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Thanks Michael, Yes my planned scope does only have a 1.25" focuser, so it all makes sense now.

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I only have the basic 10mm and 25mm eyepieces that came with my Sky-Watcher Explorer 200P, so am considering upgrading to hopefully get better views.

Would the following set be a decent improvement over what I've already got - ScopeTeknix 10 part eyepiece and accessory kit

or am I better off buying individual ones and pay a bit more money?

Decided not to buy that set and went for a 10mm Baader Hyperion instead, with a T Adapter and 2 fine tuning rings. All together it'll give me 5 different focal lengths.

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Another vote of thanks for this. Can someone validate my application of all this great info. to our specifics.

We are getting a Skyliner 250 / 10" which will come with usual 'adequate' quality 10 & 25 & 2xBarlow.

It is f5 Dob so we should look to add somethng c. 7mm ish for maximum magnification?

The Exit Pupil stuff I am less sure of - we are aged 46 & 51 so our age related figure is just over 6, i presume we work on 6 as it will only deteriorate over time. Is the 25 still the one for 6, or should we go for something in the 30's?

So, if we say 7 (to buy), 10 (got), 25 (got) , 30 ish (if required).

Would people upgrade the 10 / 25 first or buy the 7 first?

There is a gap 10 - 25, is that significant, would people get something 16ish?

Looking to view planets and DSO's.

Thanks for any thoughts.

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A 5mm will give you 254x magnification, which is the theoretical optimum of the scope. Whether the skies can handle that is another matter. I use that magnification on good nights quite a bit (using an 8mm in my 8" F/10). On less good nights, I stick to 203x (10mm for me, about 6.3mm for your scope).

The exit pupil for a scope of F/5 is 5mm for a 25mm EP. A 30mm yields 6mm (exit pupil = focal length of EP / focal ratio of scope). Note that there are big inter-individual differences in pupil diameter. I tend to stick to 5mm because the background stays darker. At 6.7 mm pupil (80mm F/6 with 40mm Paragon) I feel the background sometimes looks washed out. At 5.1mm (same scope with 31mm Nagler) the view is better.

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A 5mm will give you 254x magnification, which is the theoretical optimum of the scope. Whether the skies can handle that is another matter. I use that magnification on good nights quite a bit (using an 8mm in my 8" F/10). On less good nights, I stick to 203x (10mm for me, about 6.3mm for your scope).

The exit pupil for a scope of F/5 is 5mm for a 25mm EP. A 30mm yields 6mm (exit pupil = focal length of EP / focal ratio of scope). Note that there are big inter-individual differences in pupil diameter. I tend to stick to 5mm because the background stays darker. At 6.7 mm pupil (80mm F/6 with 40mm Paragon) I feel the background sometimes looks washed out. At 5.1mm (same scope with 31mm Nagler) the view is better.

Thanks thats very useful.

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K Feeling guilty but splashed out on a set of 4 Plossl. Not quite what the sums worked out at but were on offer and are close enough for a noob to start. Thanks for all the advice and help :p

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Warthog that was a great post! I've just bought an 8" Dob which came with 2 eyepieces (10mm and 25mm). I'm very excited to start my stargazing journey. Haven't a clue what I'm doing....Wish me luck!!

Nothing spotted yet....

:p:clouds1:

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!

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Hi all, good read but being a newbe i would like your opinion on a zoom EP as an alternative to several different EPs. i have an astro master 130 witch is an F/5 i think ?

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Hi all, good read but being a newbe i would like your opinion on a zoom EP as an alternative to several different EPs. i have an astro master 130 witch is an F/5 i think ?

Some people like zoom EPs, others do not. A zoom is in principle a compromise, and might struggle to get the same quality view (or size of the field of view (FOV)) as a similarly priced fixed focal length. Having said that, there are some very good zoom EPs out there. A lot of people like the Hyperion 8-24mm zoom (68 to 50 deg FOV), and the more expensive Nagler zooms are also very good (these have a 50 deg FOV rather than an 82 deg FOV, proving the point about difficulties in correcting aberrations in zooms). Good zoom EPs do not come cheap, but they can replace a range of EPs, which can be handy when travelling.

A special mention must be made of the Antares Speers-Waler zoom. This is a 5-8mm zoom, with 65 deg FOV and is supposed to be very good. The only downside is that it is MASSIVE!!:)

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Thanks Warthog. This is a really useful post. I have a new C9.25 which is f10. My existing eyepieces were 4mm, 9mm and 32mm. The new scope came with a 25mm. I have really really struggled with the 4mm and now I know why! The 9mm is just about ok but still quite a strain. My favourite is the 32mm but it looks like I should be aiming for a 15mm

regards

Gullsrock

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Great post! I've a c8 f10 with a set of omni plossl eyepieces. Get great views more off the lower power eyepieces like the 32mm and the 40mm than the 6mm which is the highest power one I have. I have a 13mm eyepiece I use quite often. Anyone got any advice which one to use to get the best views of venus and mars with? Been struggling to get sharp views of them. Venus seems to have a colour spectrum around its edge dunno is that a problem with my equipment or what and I can never get decent views of mars just a red spot no matter how much I focus. Frustrating!

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........

A special mention must be made of the Antares Speers-Waler zoom. This is a 5-8mm zoom, with 65 deg FOV and is supposed to be very good. The only downside is that it is MASSIVE!!:D

:headbang: Not that massive. You do get used to it.

Point of note, the Antares Speers zooms(not a true zoom, really a variable focal length eyepiece) are around 82 degrees afov. As well as the 5-8(which I have and can confirm its awesomeness), there is also a 8.5-12 version.

An adapter/spacer is also available that, when screwed in, increases the magnification ranges by 60%.

In my opinion if this eyepiece range had green lettering;), they'd be more internet chatter regarding them. A hidden gem, in my honest opinion.

Andy.

Edited by AndyH

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:D Not that massive. You do get used to it.

Point of note, the Antares Speers zooms(not a true zoom, really a variable focal length eyepiece) are around 82 degrees afov. As well as the 5-8(which I have and can confirm its awesomeness), there is also a 8.5-12 version.

An adapter/spacer is also available that, when screwed in, increases the magnification ranges by 60%.

In my opinion if this eyepiece range had green lettering;), they'd be more internet chatter regarding them. A hidden gem, in my honest opinion.

Andy.

You are right, I see I made a typo, I meant to say 85 deg FOV (doh).

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Hi, so I was thinking about eye pieces and barlows and was wondering about the benefits around quality over price. I have seen that you can spend serious money on eye pieces and barlows. From my end of things I wondered what the difference in image quality would be between the Barlow that was supplied with my Skywatcher 200 and The Astro Engineering AC 710 or 730 that sit in the 50 to 60 pound bracket or if it is better to buy reasonable quality eye pieces as and when I can afford them.

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There is a law of diminishing returns when investing in optics. I started out with pretty decent Polarex Symmetrics and the odd Circle-T ortho thrown in. These were a clear step up from the garbage Huygens and Huygens-Mittenzwei (plastic) crud EPs often supplied with scopes in those days. I had loads of fun with those (0.965") EPs. I then got my Celestron C8 and used Vixen (36mm) and Celestron (26 and 10mm) Plossls. Again, very good value for money, except that the Plossls have short eye relief at short focal lengths. Nonetheless, I used these for years, until I got fed up with the 10mm and got a Vixen LV 9mm and later 7mm. These were again a clear step up both in quality and comfort. This set up was again used for years, until I got a 2" visual back for my flip mirror (for planetary photography). I then invested in a 40mm TNB Paragon, which completely blew the 36mm Plossl out of the water: a bit sharper, clearly less internal reflection, WAY bigger field of view, and much more comfortable to use. Once you have such a premium EP, you want to replace the rest as well, because you notice the shortcomings of your current kit. This is generally my philosophy: replace things only when you are bothered by the limitations they have.

That I think is the best guideline I could give: buy things at a price you can afford, and give you an image quality that impresses you. If you like it, and like the price, it is a good buy. Try before you buy is important (go out to star parties and look through different EPs (preferably in your own scope) to see what you like best).

Remember that each small improvement is more generally disproportionately more expensive (exception: the Pentax XF 8.5 mm which is better than a TV Radian 8mm at a considerably lower price). If you cannot see the difference in your scope, it is a waste of money to buy the expensive kit.

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