Advantages of 2 inch?

[domstar]

Hi everyone. I wanted to ask what the advantages of 2 inch eyepieces are over 1.25. I assume the field of view would be bigger notwithstanding that 1.25 eyepieces can have large fields of view, but I would really like to know if they let in more light than 1.25 eyepieces. Do faint objects appear brighter? Probably not.

Yours hoping

Dominic

[michael.h.f.wilkinson]

It is just a matter of field of view: a larger section of the image plane of the telescope objective can be seen

[Louis D]

2" eyepieces allow for a either a larger field stop (determines true field of view) or a more secure fitting for heavy eyepieces like some of the ES-92, ES-100, and ES-120 eyepieces that could be mounted in a 1.25" fitting.

The exit pupil determines the "brightness" of an object, and that is strictly dependent on the focal length of the eyepiece divided by the focal ratio of the telescope.  It has nothing to do with either the apparent or true fields of view of an eyepiece.

[xtreemchaos]

and theres the safety bit, a 2inch eyepiece hurts when you drop one on your foot . I was going to say something the same as Michael. charl.

[MarsG76]

Once I heard that the difference 2" makes is the difference between oooh and AHHH!!!

But seriously, I think that the available max AFOV would be the biggest difference mm for mm... If you want 100 or 110 degrees than 2" are the only way to go, but TV does go upto 82 deg 1.25" and are great wide view EPs.

[MarsG76]

7 minutes ago, xtreemchaos said:

and theres the safety bit, a 2inch eyepiece hurts when you drop one on your foot . I was going to say something the same as Michael. charl.

Would hurt the bank account even more... The purchase and than the dropping of the expensive glass.

[Ruud]

The  2" advantage is not in the eyepiece, but in the focuser. In a 2" focuser you can use both 2" and 1.25" eyepieces. In a 1.25" focuser you can't use 2" eyepieces.

A larger true field of view requires a larger image circle at the focal plane of the telescope. The focuser and eyepiece must be able to accommodate for this larger circle. For instance: a 68° 24mm eyepiece shows an image circle that will fit a 1.25" barrel, whereas an 82° 24mm eyepiece shows an image circle that is too big for 1.25" barrel. So an 82° 24mm needs a wider barrel, and the next bigger barrel size is 2".

Manufactures mention it when an eyepiece is 2" to inform buyers that the eyepiece will not fit a 1.25" focuser.

It has nothing to do with quality. Just like size 11 shoes are not of a better quality that size 9 shoes. Here's a diagram of which afov/fl combinations will and won't fit in a 1.25" format:

[John]

1 hour ago, MarsG76 said:

Once I heard that the difference 2" makes is the difference between oooh and AHHH!!!

But seriously, I think that the available max AFOV would be the biggest difference mm for mm... If you want 100 or 110 degrees than 2" are the only way to go, but TV does go upto 82 deg 1.25" and are great wide view EPs.

Some of the 100 and 110 eyepieces are 1.25" eyepieces - the Ethos 13mm and shorter for example. They are large eyepieces though so most folks use them in 2" eyepiece holders. Now always though

 

[Stu]

38 minutes ago, Ruud said:

The  2" advantage is not in the eyepiece, but in the focuser. In a 2" focuser you can use both 2" and 1.25" eyepieces. In a 1.25" focuser you can't use 2" eyepieces.

A larger true field of view requires a larger image circle at the focal plane of the telescope. The focuser and eyepiece must be able to accommodate for this larger circle. For instance: a 68° 24mm eyepiece shows an image circle that will fit a 1.25" barrel, whereas an 82° 24mm eyepiece shows an image circle that is too big for 1.25" barrel. So an 82° 24mm needs a wider barrel, and the next bigger barrel size is 2".

Manufactures mention it when an eyepiece is 2" to inform buyers that the eyepiece will not fit a 1.25" focuser.

It has nothing to do with quality. Just like size 11 shoes are not of a better quality that size 9 shoes. Here's a diagram of which afov/fl combinations will and won't fit in a 1.25" format:

That's a very useful graph Ruud  

[MarsG76]

50 minutes ago, John said:

Some of the 100 and 110 eyepieces are 1.25" eyepieces - the Ethos 13mm and shorter for example. They are large eyepieces though so most folks use them in 2" eyepiece holders. Now always though

 

I stand corrected..

[MarsG76]

33 minutes ago, Stu said:

That's a very useful graph Ruud  

Thanks for sharing the graph... It actually answers some questions/informs facts on the reasons for FOVs in some of my EP collection.. Such as why my 40mm LV is only 42deg FOV... It's a 1.25" EP and that's the possibility limit of the barrel size vs FL.

[Stu]

9 minutes ago, MarsG76 said:

Thanks for sharing the graph... It actually answers some questions/informs facts on the reasons for FOVs in some of my EP collection.. Such as why my 40mm LV is only 42deg FOV... It's a 1.25" EP and that's the possibility limit of the barrel size vs FL.

Yep, that's right. 24mm 68 degree, 32mm 50 degree and 40mm 42 degree all show around the maximum fov in a 1.25" format.

A 40mm can still be useful under certain circumstances such as providing a bigger exit pupil on a Mak.

[MarsG76]

5 minutes ago, Stu said:

A 40mm can still be useful under certain circumstances such as providing a bigger exit pupil on a Mak.

Or a C8... The nebulae are definitely brighter through it than comparing to say a 25mm EP...exit pupil is definitely the reason.

[Stu]

7 minutes ago, MarsG76 said:

Or a C8... The nebulae are definitely brighter through it than comparing to say a 25mm EP...exit pupil is definitely the reason.

Yep, same principle. Anything with a relatively longer focal ratio, including say an f12 or f15 refractor.

[John]

With these longer focal length scopes, I guess that there a trade off between getting a more effective exit pupil and the gradual brightening of the background sky that seems to happen at lower magnifications ?

Maybe this is something that you need to find out for yourself ?

[domstar]

So if I've got a 2 inch focuser and a 2 inch diagonal, is there something I'm missing if I use a 1.25 eyepiece? What happens to all that area outside the 1.25 diameter. Is that extra light, extra field of view or neither? Apologies if I'm being thick- it won't be the last time.

[N3ptune]

I have an alternate question, what exactly cause the limitation of the eyepiece with a 1.25" barrel to prevent it to access maximum wide fields of view? A some point an eyepiece with a larger apparent field of view will show more true field of view at the condition it has a 2" barrel. So, there must be something lost at some point with the 1.25 barrel..?

This is an example with the ES eyepieces with my 1000mm FL telescope.

EP 24mm 68d
TFOV = 68/ (telescope focal length 1000 / EP focal length 24 = 41.66) = 1.63 degree TFOV

EP 24mm 82d
TFOV = 68/ (telescope focal length 1000 / EP focal length 24 = 41.66) = 1.96 degree TFOV

I don't know maybe it's a very bad question.. for a non mathematician. If it's not possible to explain this to a 10 years old, or with maybe with a drawing, I'll to let go of this...

[Louis D]

11 minutes ago, N3ptune said:

I have an alternate question, what exactly cause the limitation of the eyepiece with a 1.25" barrel to prevent it to access maximum wide fields of view? A some point an eyepiece with a larger apparent field of view will show more true field of view at the condition it has a 2" barrel. So, there must be something lost at some point with the 1.25 barrel..?

This is an example with the ES eyepieces with my 1000mm FL telescope.

EP 24mm 68d
TFOV = 68/ (telescope focal length 1000 / EP focal length 24 = 41.66) = 1.63 degree TFOV

EP 24mm 82d
TFOV = 68/ (telescope focal length 1000 / EP focal length 24 = 41.66) = 1.96 degree TFOV

I don't know maybe it's a very bad question.. for a non mathematician. If it's not possible to explain this to a 10 years old, or with maybe with a drawing, I'll to let go of this...

As I said in my initial response, the barrel determines the maximum size of the field stop.  The field stop determines the maximum true field of view.  The field stop is the physical boundary outside of which there is only blackness in the eyepiece apparent field of view.  Flip a classic positive only eyepiece upsidedown and look for a ring up inside the barrel with a tapered edge.  That ring is the field stop and it is located at the focal plane of the eyepiece.

To better understand the maximum true field of view visible with a telescope and a given focuser, hold a piece of tissue or tracing paper across the opening of the focuser after you've gotten the moon in the field of view and removed the eyepiece.  Refocus the moon on the paper.  The size of the moon on the paper is the native magnification of the telescope.  All your eyepiece does is magnify that image and convert it into a form your eye can focus on.  If you move the telescope a bit, you'll see the moon move about the field and then disappear off the edge.  Clearly, if you had a larger focuser opening, you'd be able to move the moon further before it moved out of the focuser field of view.  A 3" or 4" focuser takes this even further and can be useful for large format imaging.

[N3ptune]

@Louis D

Ahh oki it's because of the field stop, I am on the verge of understanding a part of the concept now with your explanation.

In the case of the previous ES eyepieces, the 24mm 68d 1.25" has 27.2mm diameter field stop and the 24mm 82d 2" has 33.5mm diameter field stop.  So with 33.5 mm diameter you get more clearance over the central object so the field of view is wider because one field stop diameter is 6mm wider then the other.

But what about the central object in the FOV, the moon for instance, is it going to be the same size in both eyepieces ?  or the ratios of the 82d will make it appear a bit bigger?

 

[25585]

On 30/08/2017 at 16:08, John said:

Some of the 100 and 110 eyepieces are 1.25" eyepieces - the Ethos 13mm and shorter for example. They are large eyepieces though so most folks use them in 2" eyepiece holders. Now always though

 

Some eyepieces that come with both 2" (50mm) & 31.7mm should only be used in the former. And for the monster eps, that is only just sufficient.

[John]

44 minutes ago, 25585 said:

Some eyepieces that come with both 2" (50mm) & 31.7mm should only be used in the former. And for the monster eps, that is only just sufficient.

Agreed - I don't make a habit of doing that !

 

 

[John]

1 hour ago, N3ptune said:

@Louis D

Ahh oki it's because of the field stop, I am on the verge of understanding a part of the concept now with your explanation.

In the case of the previous ES eyepieces, the 24mm 68d 1.25" has 27.2mm diameter field stop and the 24mm 82d 2" has 33.5mm diameter field stop.  So with 33.5 mm diameter you get more clearance over the central object so the field of view is wider because one field stop diameter is 6mm wider then the other.

But what about the central object in the FOV, the moon for instance, is it going to be the same size in both eyepieces ?  or the ratios of the 82d will make it appear a bit bigger?

 

The apparent  angular size of the object depends on the magnification. With a wider field stop you see more of a large target. With a narrower field stop you see less of it but the angular size remains the same as long as the focal length of the eyepiece is the same.

With my 100 degree apparent field of view 8mm Ethos eyepiece in my 12 inch F/5.3 dobsonian I can see a true field of view of half a degree which is the same diameter as the full moon so I can just about fit the whole of the moons disk in the field of view. The magnification is 200x. With a 50 degree 8mm plossl the moons disk looks the same size in the eyepiece, the magnification is the same but I can only fit half the moons disk into the field of view. From this you can deduce that diameter of the field stop of the 8mm Ethos eyepiece is around twice that of the 8mm plossl and when you look at the specs of the eyepieces that is in fact the case.

 

 

 

 

[N3ptune]

5 hours ago, John said:

The apparent  angular size of the object depends on the magnification. With a wider field stop you see more of a large target. With a narrower field stop you see less of it but the angular size remains the same as long as the focal length of the eyepiece is the same.

With my 100 degree apparent field of view 8mm Ethos eyepiece in my 12 inch F/5.3 dobsonian I can see a true field of view of half a degree which is the same diameter as the full moon so I can just about fit the whole of the moons disk in the field of view. The magnification is 200x. With a 50 degree 8mm plossl the moons disk looks the same size in the eyepiece, the magnification is the same but I can only fit half the moons disk into the field of view. From this you can deduce that diameter of the field stop of the 8mm Ethos eyepiece is around twice that of the 8mm plossl and when you look at the specs of the eyepieces that is in fact the case.

 

 

 

 

Ok now I fully understand the concept thanks, I am saving this explanation in my file collection too, to be able to re read it in case I forget.  To be able to see the whole moon disk at 200x must be truly amazing! I just come back from an observation of the moon at 212x and I don't even see one whole quarter of it.

Thanks again.

@domstar Sorry for the thread hijacking..

[domstar]

3 hours ago, N3ptune said:

@domstar Sorry for the thread hijacking..

Not at all. I want to know this stuff. I think it's helped me understand my problem too. So if the moon is fully in your field of view at 200x, it must be very bright indeed. In my head there is missing light between the outside of a 1.25 eyepiece and the inside of a 2 inch but it's not light from the central object, rather light from objects that would've been in my field of view if I'd had a 2 inch instead of a 1.25 (notwithstanding field stops).

[rwilkey]

On 30/08/2017 at 18:21, domstar said:

So if I've got a 2 inch focuser and a 2 inch diagonal, is there something I'm missing if I use a 1.25 eyepiece? What happens to all that area outside the 1.25 diameter. Is that extra light, extra field of view or neither? Apologies if I'm being thick- it won't be the last time.

There is nothing lost or gained, what you see through any eyepiece is absolute to the view it offers.