2inch v 1.25 inch eyepieces

[CSM]

Sometime in the future I will probably upgrade my scope and in looking around at future possibilities I began musing on the matter of eyepieces. As someone commented, 40 years ago eyepieces were .925 inches, then they standardised on 1.25. But that is a long time ago and a helluva lot has happened since then. So maybe 2 inch eyepieces should now be standard.

But what exactly are the advantages and disadvantages of 2 as opposed to 1.25 - visually, photographically, economically?

[Mr Spock]

2" eyepieces are used for wide fields, the reason being the field lens can be much larger than 1.25". There's no good reason for 'normal' focal length eyepieces to come in 2" sizes.

[haitch]

There is a limit as to how much of the sky you can fit up a 1.25" tube. As a rule of thumb its about 52^o afov for a 32mm f/l eyepiece and something like 20mm I think at 82o an inverse proportion in between. So anything above those figures will be vignetted by the focuser tube and you'll need to go to 2". Explore Scientific are, I think, bringing out a 30mm 100^o eyepiece in a 3" fitting.

[ronin]

HAve you seen the actual size of a 2" eyepiece ?

They sound a reasonable step up in size, they are actually monsters. :eek:

You do not carry a set of 2" eyepieces round, you load them in a trolly bag so that it is easy to move them. :rolleyes:

The main "problem" will be that they need bigger glass and that costs money, as does the extra grinding and figuring and all that costs money.

I would say that the cost and the physical size would prevent them becoming the dominent eyepiece size, and there is not a 1.5" eyepiece.

[Stargazer_00]

Until the "normal" AFOV for eyepieces is 100 degrees 1.25" will remain the standard.

All my eyepieces are 82 degree AFOV and most of them are 1.25" still. It's a great size to keep the weight down. 2" eyepieces are nice, for sure but only because they serve to give even wider views. you want, where possible, to keep the weight down as much as possible in my opinion.

[moogoomonkey]

I like my 2" EP. I have to say I just it 80% of the time. I sometimes switch out when I've found an interesting object just to see what it looks like in my other EPs. The only thing I use my 1.25s for exclusively is planetary but for everything else I use my trusty 32mm (Which i picked up for £30 )

[houston]

Its a pain changing from one size to the other and having barlows with adapters for both sizes. Filters dont fit both either. Saying that, i use 26mm 2inch most.

[Stargazer_00]

Its a pain changing from one size to the other and having barlows with adapters for both sizes. Filters dont fit both either. Saying that, i use 26mm 2inch most.

the easy way around this is (assuming you use a newt) is to replace the extension inthe focuser to a 2" one. get a 2" to 1.25" coverter.

then get all yours filters at 2" and screw them into the focus extension rather than the eyepieces. This way you only screw the filter on once and get to use it on all your eyepieces regardless of barel size.

Threaded 2" extensions are usually about £17-£20 in 35mm, 50mm or even more if you need it. GSO clones

2" to 1.25" converters are again about £20 for a decent twist lock one. Again GSO clones.

Obviously 2" filters are a bit more but this method gives you alot more use out of the filters in my experience.

If you aren't using a newt then just screw the filters onto the part of your 2" diagonal that goes into the scope (SCT/Frac)

[E621Keith]

As other said, there are good reasons why some eyepieces are 2", while others are not 1.25".

Reason 1: Optical restrictions. 2" eyepiece is needed when the field stop becomes larger than a 1.25" barrel. This happens at 32mm for 55deg, 24mm for 70deg, 16mm for 82deg and 13mm for 100deg. 2" barrel has the same restrictions at longer focal lengths (40mm for 70deg, 31mm for 82deg, 25mm for 100deg), which is why companies like Explore Scientific is now experimenting on giant 3" eyepieces.

Reason 2: Mechanical strength. Some modern wide field eyepieces has became so heavy that a 1.25" push fit connection simply cannot hold it securely and a 2" connection is needed. e.g. Explore Scientific 100deg. The 9mm is optically 1.25", but mechanically 2".

Generally, 2" eyepiece are big and heavy. It will put your scope completely off balance. 1.25" is still the standard size, while 2" is used only when it is necessary.

[umadog]

I think the simplest way to understand 1.25" vs 2" is to know the formula for calculating the true field of view:

True field of view = eyepiece field stop diameter ÷ telescope focal length x 57.3

The field stop is the solid metal ring in the eyepiece barrel that lets the light through. You see the field stop when observing: it's the edge of the field of view. To increase the area of sky that's visible you have to increase the field stop diameter. So to get wide fields with really long focal length telescopes you need a very wide eyepiece. This company makes such eyepieces in diameters in excess of 4":

http://www.siebertoptics.com/SiebertOptics-eyepieces-observatory.html#4%20%20Eyepieces

[Galilean]

As someone commented, 40 years ago eyepieces were .925 inches, then they standardised on 1.25.

Apologies for being pedantic, but wasn´t the ´old´ standard size 0.965" ?

[CSM]

Galilean - yes, apologies. Slip of the pen. And thanks to all you other guys. Confirms what I have been thinking. Unless someone invents a lighter glass, or some equivalent material which is cheap enough to use, or we develop fantastic computer algorithms which somehow balance the scope, the 1.25 eyepiece is going to remain the 'standard'. And presumably until mega-chips are developed the 2 inch has nothing to offer Astrophotography.

[E621Keith]

I wonder what caused the shift? The optical giants like Zeiss, Pentax and Nikon were all making eyepieces in 0.965 in the 90's.

Who had the market leverage to shifted the standard to 1.25"? Was it Meade and Celestron, or was it TV, Vixen or somebody else.

[John]

I thought the 1.25" standard was from the US with the UK being on RAS thread and the Japanese and Germans on .965" back in the 1950's / 1960's ?.

I'm not 100% sure though.

[E621Keith]

I thought the 1.25" standard was from the US with the UK being on RAS thread and the Japanese and Germans on .965" back in the 1950's / 1960's ?.

I'm not 100% sure though.

That made sense, 0.965" is a strange number in Imperial unit it's hard to imaging why would anyone came up with this standard. However, it's 24.5mm once you convert it to metric, which made more sense. Still, I wonder why they didn't just opt for 25mm.

[CSM]

As I understand it the reason for the shift from .965 to 1.25 was better field of view - as you would get going up to 2 inch.

[NotSoSuperNova]

Not contributing to the 'vs' argument at all and I only refer to myself but it's my opinion that when I'm of the calibre of amateur that needs to move up to the 2" EP's i'll know the reason that I'm doing it. Not that I'm against the OP asking the question, I'm a research junkie myself. I just think lots of things will become evident as you progress naturally.

Ryan

[John]

Theres no "competition" involved here. The two sizes enable different specifications to be achieved. 3" eyepieces for the amateur are in the pipeline which will offer more possibilities for those that want them.

The primary thing you can do with a wider barrel format is to deliver a wider apparent field of view than the same focal length can deliver in the smaller size, so when folks want that, they consider the larger fitting eyepiece, provided that their scopes can accommodate them.

Looking at the long running thread on eyepiece cases in the eyepieces section, many folks end up with one or two 2" eyepieces for low power / wide angle views and use the 1.25" for the medium to high power views.

Many folks get on just fine with a 1.25" set though - there are wonderful quality eyepeices in both sizes if you have the desire and budget

[E621Keith]

If you just want big eyepiece, you can even get 4" eyepiece from Siebert Optics in the US.

http://www.siebertoptics.com/SiebertOptics-eyepieces-observatory.html

[Moonshane]

Basically I don't see a difference between good quality 2" and 1.25" eyepieces. They both show good levels of contrast, detail and sharpness in a field of view dependent upon magnification and apparent field of view. The wider field you want the larger the eyepiece needs to be for a given focal length to accommodate a wider field stop.

The wider the AFOV, the more difficult/costly it gets to create good quality (visual) images across the whole field and hence the reason 2" eyepieces are often exponentially expensive.

I agree with one wide field 2" eyepiece in every set; I have eleven eyepieces and only one 2". I think that's more than enough. However, don't underestimate how excellent a good quality 25mm or 32mm plossl is for the given field. each different type of eyepiece gives a slightly different experience when in use and that's a part of the fun of choosing your own set and one of the reasons it's often so difficult to choose.