Eyepieces - what's the real truth?

[DobDobDob (Ron)]

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?

[GrahamL]

good luck
Bit of handy stuff here under usefull infomation.

http://www.actonastro.com/index.htm

[Miaplacidus (Brian)]

Yeah, good luck...

http://www.televue.com/engine/page.asp?ID=140

[sheeny (Al)]

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!). 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...

Al.

[ColHut (Colin)]

The Televue article is very good, see also here A Pupil primer
http://skytonight.com/howto/basics/3304201.html

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

[ballaratdragons (Ken)]

Col, that 'skytonight link' was a great read. Thanks for the link. I have discovered that my pupil can enlarge beyond 8mm! Cooool!

[ColHut (Colin)]

Originally Posted by ballaratdragons

Col, that 'skytonight link' was a great read. Thanks for the link. I have discovered that my pupil can enlarge beyond 8mm! Cooool!

Ta (Blush)

[DobDobDob (Ron)]

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.

[DobDobDob (Ron)]

Originally Posted by sheeny

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).

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?

[Adrian-H]

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

[sheeny (Al)]

What Adrian said!

Al.

[DobDobDob (Ron)]

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

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?

[sheeny (Al)]

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...

Al.

[DobDobDob (Ron)]

Originally Posted by sheeny

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...

Al.

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

[bird (Anthony Wesley)]

Originally Posted by tailwag

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?

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

[DobDobDob (Ron)]

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

[Starkler (Geoff)]

Originally Posted by tailwag

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

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

[DobDobDob (Ron)]

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) ?

[Miaplacidus (Brian)]

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).

Cheers,

Brian.

[DobDobDob (Ron)]

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