1.25 and 2" EP Question

[SpaceFish]

Hi All,

 

Sorry if this has been asked before but I can't seem to find an answer, hoping this is the right section if not sorry.

 

So, I know EPs come in two sizes 1.25 and 2" and I have been looking at scopes that come with diagonals and adaptors that take both. Just wondering what the difference is between the two and do you get any benefits or losses using say a 1.25" EP with the adaptor as apposed to using the 2 inch directly?

 

Thanks.

 

SF.

[rockystar]

2" allows for wider views at lower magnifications, you hit an upper limit with 1.25".

some EPs are 2" because they are heavier and a 2" tube will support them better.

[ronin]

2" eyepieces tend to be wider, just not sure how as the numbers are much the same.

They are usually at longer focal lengths as a wide diameter lens of short focal length is heavily prone to assorted aberrations.

They cost more, well when compared to the standard run of the mill EP's.

They are BIG, 1.25 to 2 sounds a small step but not when it is an eyepiece think of BST Starguider to Coke can, weight is about a full coke can as well. I have a 20mm 2" eyepiece in it's box, the 2" EP box will hold 4 TV Plossl boxes.

[SpaceFish]

Thanks for the quick response. Very informative.

 

I was looking at the Skywatcher 80ED that came with one two incher and wondering if I would have to save up more to only go the 2" route. It means if I do go for that SW I don't need to worry about saving up for the coke cans , but as always, Expect more questions closer to the time on what I should be going for.

 

Cheers.

SF.

[westmarch]

Hi there,

I found Robin's (rwilkey) explanation both concise and helpful:

http://www.swindonstargazers.com/beginners/eyepieces.htm  Part 2.

John

 

[Alan64]

In my experience, I have only one 2" ocular, a 32mm, and shown here compared to a 1.25" 30mm...

14 hours ago, SpaceFish said:

Hi All,

Sorry if this has been asked before but I can't seem to find an answer, hoping this is the right section if not sorry.

So, I know EPs come in two sizes 1.25 and 2" and I have been looking at scopes that come with diagonals and adaptors that take both. Just wondering what the difference is between the two and do you get any benefits or losses using say a 1.25" EP with the adaptor as apposed to using the 2 inch directly?

Thanks.

SF.

Hello,

In my experience, I ended up with just one 2" ocular, a 32mm, and shown here compared to a 1.25" 30mm...

Do you have a telescope in mind?  What type of objects would you like to see?

Eyepieces have come a long way since the 1950s and '60s, and before.  Back then, the .965" format was the standard...

Some still like to use them, and for their compact size.  Such were born of eyepieces used for microscopes.  For the most part, however, the views of the sky are quite narrow.

Then, eyepieces in the 1.25" format became available, and with wider fields-of-view; a 1.25" 20mm compared to a .965" 20mm...

I've found that I've not needed anything much above a 1.25" 20mm; for, to me, a telescope is for observing faraway objects up close.  I have binoculars for low power and wide fields when desired, although I do have that 1.25" 30mm, too, and I enjoy the low power and wide view it affords as well...

Then there are others who like to view wide expanses of the sky on a regular basis with a telescope, and the larger of objects such as the galaxy in Andromeda and the Pleiades star-cluster, in addition to the congested star-fields of the Milky Way.  A telescope is then like one half of a giant pair of binoculars, a monocular, but with the ability to change the magnifications.  It is then that 2" eyepieces and a 2" diagonal are considered, among other instances.

[Stu]

20 hours ago, SpaceFish said:

Thanks for the quick response. Very informative.

 

I was looking at the Skywatcher 80ED that came with one two incher and wondering if I would have to save up more to only go the 2" route. It means if I do go for that SW I don't need to worry about saving up for the coke cans , but as always, Expect more questions closer to the time on what I should be going for.

 

Cheers.

SF.

You only need 2" eyepieces if you want lower powers with a wide apparent field of view.

The 80ED you are looking at is actually a wonderful scope for low power views of the Milky Way and other large objects such as the Pleiades and North America Nebula.

It is worth getting familiar with apparent field of view and how it relates to actual field of view.

In 1.25" format, the widest fields you can get are with a 40mm Plossl which has around a 43 degree apparent field of view (afov), a 32mm Plossl which has a 50 degree afov, or a 24mm 68 degree afov eyepiece. All give roughly the same amount of sky in the view, but the 24mm shows this at higher power which gives a darker sky background. The 40mm can feel a little like looking down straw and can appear washed out with any sort of light pollution.

In 2", you can get much wider fields such as a 20mm 100 degree afov, 30mm 82 degree afov, 40mm 68 degree or 55mm 50 degree.

What does this all mean? Well to show some examples in the 80ED.

The first image is a 20mm 68 degree vs a 20mm 100 degree.

The second image is a 32mm Plossl vs a 30mm 82 degree.

The last image is a 40mm 43 degree vs a 40mm 68 degree.

So you see you get a much bigger field going to 2". But, below around 16 or 17mm eyepiece there is no benefit using 2" format, the quality is just the same and you can get fields of view just as large in 1.25".

Some people prefer using a 2" diagonal anyway as they feel more solid than 1.25" and you have the option of using both formats without changing out the diagonal.

Even with a 40mm eyepiece you get x15 in the 80ED, and with 20mm you get x30 which is way more than any standard binoculars, and of course you have the stability of having it mounted on a tripod. The 80ED is a lovely scope to get out under a dark sky and will show you widefield views as well as higher power up to say around x160.

[SpaceFish]

Thanks again all,

 

As always, really great responses and lots to think about. Part of me is desperate to get the scope (well all of me really) But another part (em Percentages not my forte) is glad I am spending the time getting my head around the basic  concepts too. Between you guys and my local club its truly an eye opener how much there is to learn if willing. And I am very willing.

 

SF.

[Louis D]

On 3/19/2017 at 05:20, Stu said:

You only need 2" eyepieces if you want lower powers with a wide apparent field of view.

Or high powers with a wide apparent field of view as with the 9mm ES-120 or 14mm ES-100 or my favorite, the 17mm ES-92.

[Stu]

45 minutes ago, Louis D said:

Or high powers with a wide apparent field of view as with the 9mm ES-120 or 14mm ES-100 or my favorite, the 17mm ES-92.

Of these, the 17mm 92 degree looks like it is the only one which actually needs a 2" barrel. The 14mm has a field stop of 24.4mm and the 9mm is 15.7mm, both less than the 27mm of the 1.25" 24mm Panoptic or 32mm Plossl. The 17mm is just over this at 27.46mm. I guess both marketing and other engineering reasons (such as secureness of connection to the diagonal) come into play in the decision to go 2".

[Louis D]

1 hour ago, Stu said:

Of these, the 17mm 92 degree looks like it is the only one which actually needs a 2" barrel. The 14mm has a field stop of 24.4mm and the 9mm is 15.7mm, both less than the 27mm of the 1.25" 24mm Panoptic or 32mm Plossl. The 17mm is just over this at 27.46mm. I guess both marketing and other engineering reasons (such as secureness of connection to the diagonal) come into play in the decision to go 2".

You also have to consider the size of the field lens.  Even though the 17mm Nagler T4 has a field stop of 24.3mm which should fit in a 1.25 barrel, it has a field lens which is larger than 27mm in diameter (negative-positive design).  I don't know the field lens diameters for the 9mm and 14mm ES eyepieces, but they're probably pretty close to 27mm if not beyond.  The original 10 pieces of the 9mm ES-120 didn't come with a field stop and had an approximately 140 degree AFOV, about the same TFOV as the 13mm Ethos according to one person who compared them side by side.  Given that the latter does come in a 1.25" barrel, it is likely ES could have crammed the 9mm into a 1.25 as well if it has a similar field lens to the Ethos which does just about fill a 1.25" barrel.  It's probably the enormous weight of the 9mm ES-120 that drove them to choose a 2" barrel.