View Full Version here: : Eyepieces - what's the real truth?

10-03-2007, 04:13 PM
Okay, so I just got a very nice telescope, 2 wet and rainy days ago and eventually I know I will look through it and I can also imagine it's going to be great. I have one eyepiece on it only, a Plossil 40mm eyepiece. I have been reading every thread of this forum and there is a vast amount of discussion about eyepieces and associated filters.

Here's my question, what is the real story about eyepieces, not from a marketing point of view, brands and cost are not what I need to know. I need to know what it is intrinsically from eyepiece to eyepiece that makes you want to have a range of them.

Is it that different power or strength of eyepiece, gives various resolutions, do you see better, clearer, bigger? If you look at an object, with excellent seeing, do you go through a process of selecting the lowest size eyepiece up to your largest, if so, why not just start with your biggest?

Is it far more subtle than that, do you have different eyepieces for different types of objects, a galaxy as opposed to a single star for example? Generally speaking, without technical explanations, do you need different size eyepieces to see better quality, larger (or is that more dependant on the telescope), what does having 6 eyepieces give you that you can't get by having 3 or just 1?

This is a serious question, I expect that it is obvious that more expensive or better made eyepieces give better results, this is the same in anything that is mass produced, but why is there a need for so many eyepieces? Does it come back to changing conditions, different objects require different eyepieces, or is it just a culture where having more, bigger, more expensive is a status symbol, what is the real truth and why?

10-03-2007, 09:20 PM
good luck :)
Bit of handy stuff here under usefull infomation.


10-03-2007, 09:36 PM
Yeah, good luck...


10-03-2007, 09:55 PM
Eyepieces are to a telescope, what lenses are to an SLR camera. A telephoto lens isn't always the best choice, and likewise neither is a high powered eyepiece always the right choice.

A 40mm plossl eyepiece like the one you have will I expect give you good low magnification views with a wide field of view. (Not knowing what focal length scope you have I've just made a big assumption there!:P). For some relatively large objects like the eta Carina nebula, chances are that even with your lowest power eyepiece, you'll struggle to get the whole thing in the FOV.

The same eyepiece will give good wide field views say of the moon, but if you want to see more detail either on the moon or planets, you'll probably need a smaller EP to give you greater magnification.

But changing EP's not only changes scale and FOV, it also changes the apparent brightness of an image. Often a higher power EP provides a dimmer image of a planet or nebula than a lower power EP does, because it spreads the available light out more in your eye. So sometimes even though you'd like more magnification to see more detail, when you do go to the smaller EP you lose brightness - and that in itself might mean losing some detail. So there is a trade off.

The other trade off that often happens with EPs relates to seeing. Low power EP's usually work reasonably in a wide range of seeing. High power EPs (short FL EPs) are more affected by seeing, so if the seeing is bad, sometimes it is not worth going to higher magnification because the seeing just blurs the view.

So often when viewing, it is worth observing through a range or at least a couple of different eyepieces. Low power eyepieces give a wider, brighter view. High power EPs give a larger scale view, but at the expense of brightness, and often only at the mercy of the atmosphere (seeing).

Of course, different types and brands of EP's have different characteristics as well. Often the cheaper ones don't offer as wide a FOV as some of the more expensive EPs like Naglers, etc. The cheaper ones are also often only optically at their best near the centre of the field of view, while more expensive types and brands can give good sharp images right to the edge of the FOV.

I hope that's the sort of info you were after, Ron. I hope it doesn't seem like sucking eggs...


11-03-2007, 12:20 AM
The Televue article is very good, see also here A Pupil primer

The Basic Function of an Eyepiece. http://www.users.bigpond.com/PJIFL/page8.html

11-03-2007, 01:00 AM
Col, that 'skytonight link' was a great read. Thanks for the link. I have discovered that my pupil can enlarge beyond 8mm! Cooool!

11-03-2007, 02:39 AM
Ta (Blush)

11-03-2007, 03:28 AM
Thanks for the links, I will read everything, I promise, you have not wasted your advice on me, I will follow up every lead - Honest! Just like to thank Graham, Miaplacidus, Al, Colhut and Ken for your efforts, armed with your collective knowledge, I am certain to make improvements to my astronomical pursuits.

11-03-2007, 03:44 AM
Just quickly Al, what is considered high and low power, or more specific, what would a normal range of eyepiece sizes be? Is my 40mm near the top, middle or bottom?

11-03-2007, 04:04 AM
40mm is low powered, 9mm is high powered, it dose depend on the scope you are using too, but that is just in general.

9mm will give you a very narrow feild of view but higher magnification

while 40mm will give you a huge feild of view fitting lots of things in

objects in the sky vary in size, so the larger objects like some nebulae need lower powered eyepeices for a larger feild to fit them into

while smaller objects like galaxies are small and need higher powered eyepeices to get a decent look at them,

while you will probly still need the lower powered eyepeice to find the galaxy at first.

hope that helps

11-03-2007, 09:57 AM
What Adrian said!


11-03-2007, 04:51 PM
Thanks Adrian & Al, that is EXACTLY in plain English what I needed to know. As always, it was the complete opposite of what I was thinking, it seems that everything to do with astronomy is reversed and backwards.

Last night I used my first scope for the first time, and was in a lather of sweat despite it being a cold night/morning. When I wanted to go left, I lost the object, it worked out that to go left in my brain I needed to go right in reality, the same with up and down....you guys know this but think back to your first time ever, boy was it frustrating :doh:

I'll do another thread about what I eventually observed, but to support what you have told me, I can tell you that when I got Saturn in my sights, despite it being crystal clear, it was so tiny and only covered the tiniest portion of my FOV. I was thinking to myself, why isn't this larger, the other night through my giant binoculars it was massive.

With your explanation and my own observations, I am now coming to terms with the need for various eyepieces. It is more or less as I suspected, I just wanted to be sure before I lash out and start buying a whole heap of eyepieces.

This brings me to Barlow's, these from what I have read are meant to sit between the lens and the existing eyepiece and increase the magnification, because there is so much written about them, I will assume they are a must in every eyepiece collection. From your collective experience is there a nasty eyepiece manufacturer to stay clear of or should I just go to the closest shop and buy a 9mm of what ever brand name they are flogging?

In other words, is there a difference between a 9mm by company A as opposed to company B, other than a pretty box and great marketing hype, is 9mm the same regardless, or does the quality of the leading manufacturer deserve their premium because they are genuinely better.

If the latter, which are the better brands over time price not withstanding?

11-03-2007, 06:20 PM
The short answer there Ron is you get what you pay for in EPs generally.

You can expect the expensive ones to be good - really good. The cheaper ones can be more variable. The best thing to do is to get along to some observing nights and have a look through some other peoples EPs before you lash out too much.

There are some excellent EP's if you are not worried about cost, such as Naglers, etc. Remember a good EP is essentially an investment for life...


11-03-2007, 06:36 PM
Thanks Al, I figure I don't drink, I don't smoke and I'm almost too old for the other stuff, so spending some money on a hobby that you can sustain till your last night on Earth isn't such a bad thing ;)

11-03-2007, 07:42 PM
If you peruse the catalogues you'll see that the longest fl eyepieces tend to be around 40mm, purely from a physical ease-of-production standpoint, and also that they tend to work well in most commercial scopes as a "low power" eyepiece.

Remember that the "power" of an eyepiece will depend on the focal length of the scope you are using it in, so a 40mm ep will be much lower power in a short-tube refractor (750mm fl) than a commercial SCT (2500mm fl approx).

cheers, Bird

11-03-2007, 08:00 PM
Thanks Bird, I recall there is a formula around somewhere to work out your power, it's something like the focal length divided by the mm size, or something similar, not sure where I read it, obviously in a thread on this forum (there isn't anywhere else to go for reliable info)...

My focal length is 1200mm, and I know what certain objects look like (size wise) with the 40mm EP, I can't wait to try a 9mm to see the comparative difference. It's a bummer the shops aren't open on Sunday's :whistle:

11-03-2007, 08:47 PM
Magnification = scope focal length / eyepiece focal length

True field of view = apparent fov / magnification

exit pupil = scope aperture / magnification , or eyepiece focal length/ scope focal ratio

So for a 20mm plossl we get:

Mag= 1200/20 = 60x
Tfov = 50degrees/ 60 = 0.83 degrees
exit pupil = 150mm/60 = 2.5mm

11-03-2007, 09:03 PM
Hi Geoff,

Thank you for the formulae, how do you arrive at 50 degrees apparent fov, is this an estimate only?

Given I have 1200/40 = 30 mag and your 20mm example yielded twice that of 60 mag, so it should go that the higher the mag the lower the power. Is this a proven immutable law that mag is the inverse of power (in this context) ?

11-03-2007, 09:12 PM
Plossls are generally about 50 degrees in apparent field of view. (Superwides are 70 degrees, Naglers and ultrawides are 80-82 degrees. Orthoscopics less than 40 degrees.)

The shorter the focal length of your eyepiece, the higher the magnification.

9-10 mm is a good focal length eyepiece for a 1200 mm scope. If you get a 2x barlow lens, you effectively have magnifications of 30x (40mm EP), 60x (40mm with barlow), 120x (10mm EP), and 240x (10mm EP with barlow, probably seldom used).



11-03-2007, 09:19 PM
Thanks Brian, that's nailed it for me. I now know exactly what I have to do with that silly plastic card in my wallet :whistle:

11-03-2007, 09:31 PM
Ron your scope is a 150mm f8 and it is said that the greatest visual acuity is with an ep that gives you a 2mm exit pupil. This corresponds to a 16mm eyepiece. I would try a 15mm for general medium power dso viewing at 80x, barlowed will give 160x or a little more. I doubt a 150mm achromat can be pushed much higher in power without the colour being objectionable.

If you buy a 40mm plossl, make sure its not in 1.25" barrel format or you wont get any wider field than you would with a 32mm plossl. This is why we have 2" format eyepieces to accomodate these larger fields

11-03-2007, 09:37 PM
That's magic advice, thank you so much, I'll take your tip and get a 15mm to go along with the 40mm that came with it. Apart from the obvious increase in size and cost, is a 3x Barlow different than a 2x Barlow in any other ways?

11-03-2007, 09:47 PM
Hi tailwag, one thing is re-branding of ep's, meaning you get the $29 one for $200 with a nice name on it. Wide field eyepieces are so expensive because it is really difficult to reproduce the same quality viewing as an orthoscopic at 80deg field of view. What your interested in viewing will dictate what you will need(so take your time). It truly is a mine field of information and consumer bolg, let your own eyes make the decisions, there is a lot of very sound advice and also chest beating about mine is better than yours(tread carefully). Good luck with the hair, I wish mine would leave home.

12-03-2007, 12:46 AM
hello ro,
as far as barlows go....a barlow fitted to the forward end of the diagional (rather than between the diagional and eyepiece) will yeild a 50% increase in mag. therefore a 2x barlow fitted between the diagional and scope would become a 3x.

12-03-2007, 03:54 AM
Ron there's plenty of good advice in this thread already, but remember also that these kinds of topics have come up previously. Have a read back through a lot of the older threads as well - change your settings when viewing this category to see threads posted within the last year for instance instead of the last month.

I know there was a whole heap of extremely valuable reading in this category because when I joined about a year ago I read back over this stuff for 2-4 hours a night, almost every night, for weeks and weeks and weeks. I'm no expert, no where near it, and I haven't had opportunity too try most of the ep's mentioned, but I learned a hell of a lot about good and bad eyepieces, barlows, etc.

There is great advice on budget, medium, & top priced eyepieces - yes, they do differ, but different ep's suit different people too. Try to buy as good as you can comfortably afford I guess, there's little value in buying cheap ep's if you can afford better - remember that a good eyepiece can go with you on your journey as you grow in this hobby. No matter what scope you buy next, or how many scopes you leave behind, you can always take your eyepieces along with you.

In particular if you want to cut to the real gold in this topic search for any posts by Ausastronomer - read everything he has written, listen to what he says ... because most others listen pretty carefully to his opinions re eyepieces.

Cheers Ron, and always remember to have fun with your observing!

12-03-2007, 08:00 AM
Blimy, that's something I wouldn't have thought about, great tip :thumbsup: this makes me immediately want to stick to long established and respected distributors and manufacturers. The old adage they must be doing something right to be in business so long, is one I pretty well believe in. Thanks for this useful and unexpected advice.

12-03-2007, 08:02 AM
There you go, another thing I have learned, this gives new meaning to the term, 'Position, position, position'....yes I know it's 'Location', I just changed it a little for dramatic effect ;)

12-03-2007, 08:07 AM
Thanks Steve, your three main points (snipped above) are well made, I'll do all three out of respect, but I'll especially do the third point because there is little point to it, if you're not having fun. I appreciate your input Steve :thumbsup:

UPDATE: I see what you mean Steve, I searched and found 95 threads and read just two so far and you are right, interesting to say the least - Thanks again.

12-03-2007, 04:43 PM
Hi Ron,

No one seemed to pick up on a few of the questions about number of eyepieces, and barlows.

I would suggest on a 1200mm scope you probably dont want to go below 9mm in focal length. So I would suggest 3 eyepeices, 9-10mm, 20mm and your existing 40mm. Personally I prefer 9, 15, and 25mm but since you already have the 40 you need something more inbetween...

I wouldnt suggest a barlow, for cheap ones they arent all that good, and if you are buying one eyepiece and a barlow or two eyepeices there probably isnt much $ difference anyway so just get the two eyepeices...

Oh the reason for not going below 9mm (at least to start with) is they are more difficult to focus, and have a shorter focal length which generally means you need to be right up close with your eye (essentially glued to it) if you arent used to this is can be a bit difficult at first to do comfortably... especially if all you are used to is a 40mm.

12-03-2007, 05:15 PM
Ah Shredder, you hit the jackpot for me, I thought I was going crazy, I was trying to press my eye as hard as I could against the rubber surround of the eyepiece and every other moment the whole FOV would go black. I eventually worked out that if I had my eye back (an inch I would guess, but have no real way of measuring accurately), that my view stayed constant and gave me less eye strain as well.

In the beginning, I was squeezing my left eye so hard and observing with my right eye, that I was aware that I was constantly telling myself to relax the left eye. I attribute all this to being a beginner and understand that I'll find my groove in time and all these tiny things will be distant memories.

Thanks also for your input on the 3 EP sizes, given my telescope and the fact that I already have a 40mm. I think you are right and this concurs with the overwhelming majority of advice given by the many members who have assisted me on this issue.

I can tell you that I am just waiting for the means to get to the shop to purchase the EP's and I have also made a decision to wait on the Barlow's until I can go to the Societies Dark Site and see the actually difference in a range of scopes and sizes as to what a Barlow really can and can't do.

From all I have already, given I have only had one night out with my scope so far, that just the new EP's I purchase later this week, will give me a heap of added flexibility I currently don't have and keep me swapping and learning about each EP for a good while. So a pause on the Barlow's until I can field test them and understand them more.

Thank you Shredder and also all the other members who helped out, I plan to simulate my first night's observing as close as possible on my next night, for about an hour to get back to where I left off, then I will introduce the new EP's in an effort to truly observe the differences (to me) that the EP's bring. Doing this as opposed to just jumping straight into the new EP's without a systematic approach won't teach me as much as if I take my time and use a controlled and pre-planned methodology.

I very much remember the differences (to me) of both Saturn and Jupiter and they will be the focus again of my testing. provided I write my observations down relatively quickly in my log book, I should establish, as far as I'm concerned what the real differences the EP's mean to me.

I presume this will give me a very good baseline as a beginner for the future when the telescopes I either purchase or have access to are better scopes, doesn't overshadow the need to select the correct EP for the object/conditions at hand.

What I mean by that last sentence is I don't want the equipment to dictate to me what I select for each new session, but rather use experience and knowledge to select the correct equipment for the task at hand. I hope I have made that somewhat tricky point clear.