A couple of questions about eyepieces...

[kered]

Hello everyone! Its my first post in this interesting forum.
I am very new to observational astronomy and have few newbie questions about eyepieces. Here it goes..

What's the difference between a 1.25 and 2 inch eyepiece?

How do you calculate the true and apparent field of view?

If you're looking through an eyepiece with a short eye relief, don't your eyelashes get in the way? How do you deal with that?

Why are some eyepieces so much more expensive than others (eg. TeleVue)?

Are there any other characteristics of eyepieces that I should know about?

Thanks in advance!
I'm currently studying abroad. Hope to see you guys at as many ob sessions as possible when I come back to Singapore in May!

[Canopus Lim]

Hi Kered, welcome to Singastro.


There are a lot of things to know about telescopes, eyepieces, optics, astronomy etc.. so it is not easy to tell everything and to understand everything. It would be best if you can join those group sessions here in Singapore as you would learn more about astronomy and equipment.


What's the difference between a 1.25 and 2 inch eyepiece?
The 1.25inch and 2inch eyepieces denote the barrel of the eyepiece. The telescope has af focuser which accepts 2inch or 1.25inch eyepieces; depends also as some focusers or diagonals are just 1.25inch capable. These are standard USA dimension for eyepieces. Anyway, because of the barrel size, it restricts the amount of True Field (meaning the actual field or area of the sky seen). 1.25inch barrel is smaller than 2inch so 2inch eyepieces have the potential of having a larger field seen.

True field of view (TFOV) = Apparent Field of view (AFOV)/ magnification (M)

The TFOV is the actual area of sky that is seen through the eyepiece.

Magnification (M) = Focal length of telescope/ Focal length of eyepiece

Magnification is the number of times the image is enlarge. 7x means 7 times larger as compared to what is seen with our eyes only.

Apparent field of view (AFOV) is the "circle" you see when you look through the eyepiece. The larger the "circle", the larger the apparent field of view and hence more sky can be seen. For example a Plossl eyepiece has about 50 degree AFOV while a Televue Nagler Eyepiece has 82 degree field of view. The area of the circle in the Nagler eyepiece is 82^2/50^2=2.7x larger than the area of the circle in the Plossl. This means, with the Nagler eyepiece one can see 2.7x more field relatively to that of a same focal length Plossl eyepiece. The larger the apparent field of view, the more immessive it is because the circle is larger and you will feel less 'constrained' as looking through a small circle (like a pinhole).

If you're looking through an eyepiece with a short eye relief, don't your eyelashes get in the way? How do you deal with that?
For these eyepieces, the only way is to clean them or buy other eyepieces that has longer eye relief. Usually eyepieces of short focal length (and hence higher magnification) has shorter eye relief. There are some that has longer eye relief at short focal length but they tend to cost more.

Why are some eyepieces so much more expensive than others (eg. TeleVue)?
Astronomy equipment like in many other equipment has a price. There are reasons why things are more expensive. For example, a ferrari cannot be priced as a Toyota and the price difference comes with different performances or features. Televue eyepieces have many different models. The widest selection of eyepieces compared to many other brands. They are expensive because of build quality and ability to have good images at fast scope like f/4. Slow scopes like those of f/10 can do without such eyepieces but of course these eyepieces will do even better in a f/10 scope. There are other brands like Pentax that do well at fast scopes.

Are there any other characteristics of eyepieces that I should know about?
There are a few things you have to know before purchasing:
1. Get more information about the eyepiece like reviews etc...
2. Know the AFOV and TFOV
3. Know what telescope you have
4. Know your eyes whether you have astigmatism etc.
5. Know the eye relief and whether you are comfortable with the short eye relief of some eyepieces

The best way is to join observing sessions so that you know exactly what kind of astro equipment is out there.

[kered]

Hi Canopus. Thanks for the quick reply and excellent explanation.

I have joined the local astronomy club and have looked through about 10 different scopes so far. The reason I'm asking these questions is because I'm choosing eyepieces for a friend who has an 8 inch SCT but knows even less about these stuff than I do.

Regarding the apparent field of view, I know most eyepiece manufacturers state the AFOV in their specs. But some don't. So, when you said Plossl eyepieces have 50 deg AFOV, how did you calculate that? Or is it something you just know from memory?

I actually have 2 more questions, if you don't mind..

If an object is focused when I look through an eyepiece, will it definitely be focused when someone else looks through it? I showed Saturn to a 7 year old kid, but she didn't seem to be impressed, which is weird.

Also, will all eyepieces work with all telescopes as long as the barrel sizes match?

I very much look forward to seeing you in one of those sessions when I'm back in Singapore.

[mrngbss]

If an object is focused when I look through an eyepiece, will it definitely be focused when someone else looks through it? I showed Saturn to a 7 year old kid, but she didn't seem to be impressed, which is weird.

Perhaps the little girl didn't know how big and how far Saturn is from Earth. Perhaps the little girl needs to have some relative sizes to compare with before she can feel the excitement.

Try these:
1. Speed of Cassini to reach Saturn in a few months.
2. Size of Singapore, compared to Earth, then earth compared to Saturn, etc.

[kayheem]

If an object is focused when I look through an eyepiece, will it definitely be focused when someone else looks through it? I showed Saturn to a 7 year old kid, but she didn't seem to be impressed, which is weird.

1) If there is different refractive correction between 2 observers (i.e. differing power in the spectacles), then you will need to re-focus.

2) My experience with kids is that they tend to hold the eyepiece or focuser when looking into the eyepiece. This may move the scope just enough to move the object out of the FOV.

If they are not impressed, look at the moon at low power (100X or so).

[Canopus Lim]

There are reasons:
1. She is never impressed with Saturn in the first place. Some people do expect pictures like that in magazines and expect the images to be very big and detailed.
2. She did not focus it properly and Saturn looked like a blurred blop. This happened once when a kid looked through my scope, during the Singastro Mars watch. My scope was pointing at Mars. He did not know how to focus and he focused till it turned out to be a 'donut'... so he thought Mars looked like that. This focusing thing is a big issue to many newbies as they always think there is no need to focus the scope and most people will shrug off focusing when you tell them to do so. If her eyesight and yours is different, or if you are wearing glasses and she does not, the focusing point will be different and it will require focusing.
3. The scope may not be pointing at Saturn when she was looking.

[Canopus Lim]

Regarding the apparent field of view, I know most eyepiece manufacturers state the AFOV in their specs. But some don't. So, when you said Plossl eyepieces have 50 deg AFOV, how did you calculate that? Or is it something you just know from memory?

I actually have 2 more questions, if you don't mind..


Also, will all eyepieces work with all telescopes as long as the barrel sizes match?

Plossl is a type of eyepiece design that comprises 4 elements (2 glasses) and 2 groups. It has a symmetric design of the 2 doublets but different manufacturers can vary the glasses and curves used to have better performance. Plossls are known to have about 50 degree AFOV and eye relief of about 0.6 that of the focal length of the eyepiece.

As long as the barrel of the eyepiece is the same as the telescope focuser it would work. The only issue is of the exit pupil. Exit pupil is that projected circle from the eyepiece when it is placed into the telescope focuser. That projected circle is where we would put our eyes on and that position is where we would see the image without vignetting (darkening of the edges).

Exit pupil will vary with different eyepiece and telescope combination. It is calculated as:
Exit pupil = focal length of Eyepiece / (telescope f/#).

So a 8inch SCT of f/10 when used with a 40mm eyepiece will give an exit pupil of 4mm. The larger the exit pupil, the brighter the image but less magnified. Our eyes can dilate in the dark to about 7mm (depends on people and age. 7mm is rather optimistic). Therefore, when buying eyepiece, consider the exit pupil too as if too large (exit pupil larger than the dark adapted eye), light will be wasted. Wasted meaning the telescope will not perform at full aperture (8 inch performing like 6 inch etc... example only when using that eyepiece).

[kingkong]

with kids, i find they are more easily impressed by venus than saturn...

put venus in the eyepiece, show the kid and ask: what do you see? they will look in the ep, look up at the bright star that you pointed out earlier and looked at the ep again amazed... works every time. even my 3 y.o. knew to say: "moon!"

[weixing]

Hi,

when you said Plossl eyepieces have 50 deg AFOV, how did you calculate that? Or is it something you just know from memory?

The specification is given by the manufacturer. For me, I don't remember the specification due to bad memory, but some eyepiece are so common and mention so many times by others that I was "force" to remember it... ha ha ha Usually, most eyepiece of the same type have similar AFoV:
1) Orthoscopics: around 43 degree,
2) Plossl: 50 degree,
3) Super Plossl: 52 degree,
4) Wide to Super Wide Angle eyepiece: 60 degree, 65 degree, 68 degree & 70 degree
5) Ultra Wide Angle eyepiece: above 80 degree

By the way, Plossl and Super Plossl are also consider as wide angle eyepiece in some astronomy book.

Have a nice day.

[yanyewkay]

time to add into wiki..heheh... good read for me too.