How do "Ultra Wide Angle" Ep's work?

[Ccolvin968]

I'm slightly confused by this whole thing.

I'm branching off of another active topic of mine right now where I was recommended the Meade Series 5000 UWA Ep's for my Z10.

How exactly do these Ep's work? 

If the focal length of the EP is for example, 8.8mm how is the FOV still 82degrees? Does this mean that my target will appear much smaller in the EP? All of the Meade UWA EP's from what I've seen have that 82 degree angle. So as I get a shorter and shorter focal length, does the amount of space I can see change? I can't figure out a really clear way to ask this question. Pretty much, will my target look a lot smaller in an 82 degree FOV or will it be the same size as it is in other Ep's but with more space around it? I'm pretty sure I want to buy at least two of these today. Just want to be sure I'm buying the right thing. How are they for planetary observations? Or are they more suited for DSO's like nebulas, galaxies, and star clusters? Thanks for the help!

[stadaik]

Im suppose you are guessing right but lets wait someone expert (older) to analyze it.

[John]

The focal length of the eyepiece determines the magnification it will give in a given scope and the apparent size of an object being viewed. The edges of the field of view of an eyepiece are defined by an aperture called the field stop. The larger the field stop, the wider the view but the object remains the same apparent size within that wide field.

The figure in degrees that is part of an eyepieces specification is the apparent field of view (AFoV) of the eyepiece. This is not the true field of view (TFoV), ie: the amount of sky that you will actually see when you use the eyepiece in a scope. The TFoV depends on a combination of the magnification that the scope / eyepiece provides and the AFoV.

This web page is quite useful. You will notice that the objects being shown in the examples of normal and ultra wide fields stay the same size. It's the amount of sky that frames them that changes:

http://www.eaas.co.uk/cms/index.php?option=com_content&view=article&id=4:a-short-guide-to-choosing-eyepieces-by-neil-paterson&catid=5:learning-zone&Itemid=8

[ronin]

Yes the amount of space you see changes.

THere are 2 terms TFOV and AFOV, trouble is I cannot recall which is which.

Quick search:

AFOV is the field of view of the eyepiece so in this case 82 degrees.

TFOV is the True Field of View, basically what you end up with when you put that eyepiece in a scope.

They are related as:
TFOV = AFOV/Magnification

So 12mm 82 degree eyepiece delivers 1270/12 = 106x mag so you get 82/106 = 0.77 degree view. 10" f/5 = 1270mm focal length.

Any object in it is magnified 106x the rest is space around it.

If you were looking at a planet in a 12mm 82 degree EP and in say a 12mm 50 degree plossl they would be the same size planet image and you would have more space around it in the 82 degree EP then in the 50 degree EP.

Which would likely make it "appear" bigger in the plossl, a pea in the middle of a bucket looks smaller then a pea in the middle of a tea cup. Yippee I manages top spell both with an "a". :grin:

A wide field eyepiece comes into use when the target is big and is also bright enough to take some magnification, M42 being the obvious one.

To get all M42 in you need at least 1 degree field, in a plossl that is 50x in the Meade that is 82x.

Just to throw in more there is the point that if the field is really big you may have to move your eye to sort of peer round the side to see things.

[Ccolvin968]

Good to know. Thanks for the clarification!

[Ccolvin968]

Perfect! You both answered this question exactly how I had hoped it would be. I'm glad I got the Meade Series 5000 UWA 14mm and 8.8mm. 

I also have a Celestron 8-24mm laying around to help me out until I can afford a couple more of the Series 5000. 

[bingevader]

The other advantage of an EP with a wider field of view, if your 'scope isn't motorised, is that you get more drift time.

[Ju Piter]

This web page is quite useful. You will notice that the objects being shown in the examples of normal and ultra wide fields stay the same size. It's the amount of sky that frames them that changes:

http://www.eaas.co.uk/cms/index.php?option=com_content&view=article&id=4:a-short-guide-to-choosing-eyepieces-by-neil-paterson&catid=5:learning-zone&Itemid=8

thank you for the link john and what is that meade eyepiece in the picture looks MASSIVE  lol

[ollypenrice]

To give the same answer from a slightly different angle we could say that 'magnification' refers to how much the image on your retina is magnified by the scope-EP. This is governed exclusively by the scope FL divided by the EP focal length and by no other numbers. The EP FOV simply governs how much sky you can see.

Personally I love the wide FOV even when the object doesn't need it. I like the 'context' of stars and space around the target. Some prefer the opposite.

Olly

[Pig]

I simply pop them into the diagonal and look through them     they seem to work fine.

[John]

thank you for the link john and what is that meade eyepiece in the picture looks MASSIVE  lol

I think it's a Meade UWA 30mm. It's one of the heaviest and fattest you can get in the 2" format. I thought my Nagler 31 was pretty large and heavy but the Meade 30 ecplipses it. These things are really heavy too so you do need to consider scope balance when you put one in the drawtube. I think the Meade 30 weighs a little over 1.3 kg / 2.9 lbs.

[Piero]

Add 200g, less than the weight of a Nagler T6, and the Meade 30mm matches my TV60 on a balance!

Quite shocking!

Another aspect of magnification is that the more one magnifies, the dimmer the image becomes. This is the exit pupil. Certain objects, e.g. extended nebulae or galaxies, are very sensitive to this, and that's why you should opt for longer eyepiece focal length, or larger exit pupil. Other objects, e.g. planets, are not so sensitive, and therefore you can magnify more, seeing permitting.

Just a note, the telescope function is to collect light on a focal plane, not to magnify. The eyepiece function is to move more or less forward this focal plane so that you can see the object closer or more distant, hence it magnifies it.

[stevecoe]

I can provide some input about the eyepieces you are considering.  I have the series 4000 version of the 14mm and 8.8mm and I use them often on all of the objects in the sky that there are to view.  I enjoy the wide field of view, sharp star images and excellent contrast.  I have found them easy to use for decades and have never looked through an eyepiece that makes me want to buy anything new.  I also have the 6.7mm and it provides very good views of the planets and double stars at high power.

For truly wide field I love my Panoptics and I have 22mm, 27mm and 35mm.  And again, I have had them for decades and am not looking for anything else.  I have a chance at the Texas Star Party to observe with Al Nagler for several hours.  I told him he could have stopped making eyepieces with the Panoptics and I would have never noticed.  He chuckled and said thay he had too many designs in his head to do that.

Clear skies;

Steve Coe

[bingevader]

For truly wide field I love my Panoptics

And then for Super truly wide field there's the Delos,

and for Ultra truly wide field the Nagler,

and Extremely truly wide field, the Ethos! :D

[JamesF]

And then for Super truly wide field there's the Delos,

and for Ultra truly wide field the Nagler,

and Extremely truly wide field, the Ethos! :D

And for super-massively truly wide field, the 120 degree ES 9mm.  If they get much wider you'll be able to see what's behind you

James