Advantage of 2" eyepiece

[fox2319]

I've just started with my Skywatcher 200p Newt on EQ5 mount and was wondering about the advantages of buying a 2" eyepiece for it.

Is it worth it and, if so, are there any must have eye pieces to consider?

Sent from my iPhone using Tapatalk

[John]

Hi,

The only real advantage of 2" eyepieces is that they can provide a larger field of view than the 1.25" size. So you usually find them in focal lengths longer than 18mm - 20mm and they are used for lower power, wide views of the larger deep sky objects and star fields.

Many folks end up with a 2" eyepiece for that purpose but use the 1.25" size for medium to higher power viewing. For those who prefer to stick to the 1.25" size, the 32mm focal length can still show a reasonably large amount of sky. A 2" in that focal length can show around 50% wider field.

[ronin]

Cannot say they are a must have, I have one a 20mm 80 degree item. Never used it.

More then happy with the TV plossls and the BST's.

Not often said but they are big, 1.25 to 2.0 sounds not a lot but in terms of eyepieces it is a lot more then you expect.

Easiest approximation is think of a coke can, that is about the right size. They are not the thing you pop a couple in a pocket for a short hours observation.

The other aspect is that if you have one then you will be changing both eyepieces and adaptors.

So the change over is not as quick, may or may not bother you.

I just prefer to take out one eyepiece pop it in a pocket and drop in the next.

Likely nice to take just the 2" eyepiece outside with the scope and simply look around, that way less swapping over.

[Moonshane]

Do be aware though that some 2" eyepieces have a 50 degree apparent field and can be exceeded in overall true field by some 1.25" eyepieces. E.g. I have a 2" 26mm 50 degree apparent field eyepiece which provides 2.7 degrees true field compared with my 1.25" 24mm 68 degree apparent field eyepiece which gives 3.4 degree true field with the same scope

[RichM63]

I don't know about 'advantage' but I love the wide views given by my 32mm 2" 70° on the 200P.

Compared with the standard 10mm and 25mm the views are chalk and cheese for sheer forget where you are immersion into space.

[John]

I love wide views too which is why I have a couple of 2" eyepieces

Some of the more complex 1.25" eyepieces are now supplied with a 2" barrel, either as a hybrid design or as a screw on option, because their bulk and weight is more securely held in a 2" drawtube or diagonal. They are still optically 1.25" eyepieces though.

[Alan64]

This is my lone 2" ocular, a 70° 32mm, and compared to a 1.25" 50° 25mm...

It came with an extendable/retractable eye-cup, and to adjust any user's eye to the 17mm eye-relief.  It sold for US$79, or £54, when I purchased it back in the early 2000s.  With it, I was able to show my father the galaxy in Andromeda, and before he passed.

A 32mm(31x), with the 200mm f/5, would make for an excellent "finder" eyepiece, then to pop in one of a shorter focal-length for a closer inspection.  It's also ideal for observing said galaxy and the Pleiades in winter, and for scanning the Milky Way in the summer.  It's definitely an "eye" for the sky, and for one of the widest fields-of-view possible with most any telescope with a 2" focusser.

I'm seeing the exact same 32mm here, surprisingly... http://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-panaview-2-eyepieces.html

They also offer a 38mm, and for an even lower magnification of only 26x, but with a 7.6mm exit-pupil; still a consideration.

Personally, I've never felt the need for 2" oculars shorter than that, although I do know of amateurs whose entire eyepiece collections consist of nothing but.

[bingevader]

I have a mix.

As John has said, 2" for deep sky, 1.25" for planets and moon.

The 2" eyepieces are much bigger, but I don't find swapping between them difficult or a chore at all.

I don't tend to randomly swap between observing DSOs to planets, so once the 2" adapter is in I can swap the 2" eyepieces easily and if I need a little more magnification I have a 2" to 1.25" adapter that I can pop the shorter focal length eyepieces into.

For me, it's about framing the object as best as possible.

Great as my 25mm TV plossl may be, there are some objects for which the 24mm 82° eyepiece just gives a better viewing experience.

I'm also a fan of the 32mm Panaview.

[fox2319]

This is my lone 2" ocular, a 70° 32mm, and compared to a 1.25" 50° 25mm...

comparison7.jpg

It came with an extendable/retractable eye-cup, and to adjust any user's eye to the 17mm eye-relief.  It sold for US$79, or £54, when I purchased it back in the early 2000s.  With it, I was able to show my father the galaxy in Andromeda, and before he passed.

A 32mm(31x), with the 200mm f/5, would make for an excellent "finder" eyepiece, then to pop in one of a shorter focal-length for a closer inspection.  It's also ideal for observing said galaxy and the Pleiades in winter, and for scanning the Milky Way in the summer.  It's definitely an "eye" for the sky, and for one of the widest fields-of-view possible with most any telescope with a 2" focusser.

I'm seeing the exact same 32mm here, surprisingly... http://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-panaview-2-eyepieces.html

They also offer a 38mm, and for an even lower magnification of only 26x, but with a 7.6mm exit-pupil; still a consideration.

Personally, I've never felt the need for 2" oculars shorter than that, although I do know of amateurs whose entire eyepiece collections consist of nothing but.

Thanks for the pointers. Looking at the 32 or 38mm and trying to decide. 

[John]

Thanks for the pointers. Looking at the 32 or 38mm and trying to decide. 

I think the 32mm would prove more effective with your F/5 scope.

[bingevader]

Me too.

[bingevader]

You can compare the aFOV quite easily in Stellarium, here's my 24mm MV and 25mm plossl:​

[John]

....As I understand it, the limiter on the 1.25" eyepieces is the barrel diameter.  I don't know off hand what the absolute limit might be but it is probably around the 76 degree mark based on what I see for sale.  After that you have to go to 2"....

The size of the apparent field of an eyepiece is defined by the field stop which is an aperture ring inside the eyepiece. With a 1.25" eyepiece barrel the field stop cannot physically exceed around 28mm and with a 2" barrel that limit is around 46mm.

As the focal length of the eyepiece gets longer the diameter of the field stop required to deliver a certain apparent field increases so the limit of the field of view that a 1.25" or 2" eyepiece can deliver depends on it's focal length.

In the 1.25" fitting I have a 13mm eyepiece that delivers a 100 degree apparent field of view but it's field stop requires nearly the whole internal diameter of the 1.25" barrel to achieve that. At the 18mm focal length around 82 degrees is the limit, at 24mm it's 68 degrees, at 32mm 52 degrees etc. etc. This is why a 40mm eyepiece in the 1.25" fitting can only have a maximum apparent field of 43 degrees.

Currently eyepiece apparent fields go from a tunnel-like 32 degrees (eg: a 1.25" Supermonocentric) to the massively expansive 120 degrees (a 2" eyepiece with a 9mm focal length). There is one 3" format eyepiece readily available and a few 4" ones which are generally intended for observatory class instruments.

The range of eyepieces available these days is really rather bewildering as are some of their prices !

[Ruud]

An eyepiece is like a loupe. You use it to study the image that the objective forms at its focal plane. An eyepieces has a field stop to prevent you from seeing those parts of the focal plane that it can't show sharply. The larger the field stop, the more sky you see.

The field stop cannot be bigger than the barrel of the eyepiece. It has to fit. The size of the field stop is proportional to the apparent field of view of the eyepiece and to the focal length of the eyepiece

Expressed in a formula:  Field Stop (mm)  =  0.0175 * apparent field of view of eyepiece (degrees) * focal length of eyepiece (mm)  When a field stop is bigger than about 29 mm it won't fit is a 1.25" barrel, but it may fit in a 2" barrel.

Eyepieces with a 29mm field stop are on the blue line in this graph

Eyepieces below the blue line fit in a 1.25" barrel, those above don't. We often recommend a 32mm 52° Plössl or a 24mm 68° Wide Field to those that want to maximize the amount of sky in their 1.25" focuser.

All eyepieces on the blue line show the same amount of sky, but they do so at different magnifications.

Additional remarks

Some (bad) eyepieces fit in a 1.25" focuser, yet they are above the blue line. Those eyepieces are bad in the sense that that have excessive pincushion distortion, which inflates their  apparent field of view.

Some people think 2" eyepieces are the better choice. That is not true. 2" eyepieces aren't necessarily better than 1.25" ones, just as size 11 shoes aren't necessarily better than size 9 shoes. But, if you want to fit more sky in your view, or a bigger foot in a shoe, you may run into the need for a larger size.

[bingevader]

Nice graph.

I do enjoy how these topics keep coming around, with each time a little more added to the conversation.

I think I made a short list of tv EPs with the same field stop but varying fl and fov to demonstrate this somewhere...

[fox2319]

Thanks for all the advice. I've ordered a 32mm 2" eyepiece from FLO and am hoping I'll get it and some clear skies for the weekend

[fox2319]

Which one?  

http://www.firstlightoptics.com/skywatcher-eyepieces/skywatcher-panaview-2-eyepieces.html

32mm at £69. I've also downloaded Stellarium so that I can plan my evenings a bit more carefully

[Alan64]

A 32mm(31x) with a 200mm f/5 affords an exit-pupil of 6.4mm, and ideal.  A 38mm(32x) with a 200mm f/6, as Ed's, results in an exit-pupil of 6.3mm, and also ideal.

Splendid!

[John]

A 32mm(31x) with a 200mm f/5 affords an exit-pupil of 6.4mm, and ideal.  A 38mm(32x) with a 200mm f/6, as Ed's, results in an exit-pupil of 6.3mm, and also ideal.

Splendid!

As Ed observes under quite light polluted skies, I wonder if an eyepiece that delivers a smaller exit pupil at a little more magnification might be more effective in terms of actually seeing fainter deep sky objects though ?

Thats how I've found things and I reckon my back yard skies are darker than Ed's.

In my F/5.3 12" dobsonaian my 21mm 2" eyepiece gets more use than my 31mm does for this reason.

[Ruud]

Hi fox, congratulations on the new eyepiece.

In Stellarium, go to Configuration (F2) > Plugin > Oculars and check "Load at startup". Then close and restart Stellarium. Again, go to Configuration > Plugin > Oculars and click on configure. You can now populate the plugin with your eyepieces (and those you're thinking about buying) and your telescope. After this, you can use the plugin to see the true fields that you get using the eyepieces/barlows/telescopes you have entered. Like in Ben's post above.

Quite useful if you want a sense of scale.

Don't forget to let us know how you fare with the Panaview!

[Alan64]

My 150mm f/5 Newtonian came equipped with a 1.25" plastic focusser; a pity, being of plastic, but usable. 

I had purchased a Tele Vue 43°40mm Plossl many years ago.  I didn't know any better at the time, and I've come to regard it as not much more than a paperweight.  To correct that mistake, I've finally settled on one of a somewhat shorter focal-length.  It came down to either the GSO or the Baader 32mm Plossl.  However, after careful research, it came down to the Baader, and this one...

I haven't tried it out yet, but I am hopeful.

[John]

I compared the Baader 32mm plossl with the Vixen 30mm NPL a couple of years back:

http://stargazerslounge.com/topic/179005-baader-classic-32mm-plossl-meets-vixen-30mm-npl-plossl/

I'll be interested to hear what you think of them

[jetstream]

John have you tried the 40mm Vixen NPL 1.25"? , I wonder if the twist up eyecup gives enough lift with both this and the 32mm NPL? I'm still keeping my eye out for Quark friendly eyepieces and I dislike floating eyerelief, but a long enough twist up would fill the bill. Optically I really like both the 32mm and 25mm TV plossl's.

[John]

John have you tried the 40mm Vixen NPL 1.25"? , I wonder if the twist up eyecup gives enough lift with both this and the 32mm NPL? I'm still keeping my eye out for Quark friendly eyepieces and I dislike floating eyerelief, but a long enough twist up would fill the bill. Optically I really like both the 32mm and 25mm TV plossl's.

I've not tried that one Gerry.

I'm not a great fan of 40mm eyepieces in the 1.25" fitting to be honest with you. I can see that the twist up eye cup might help with the eye positioning, provided that it moves far enough.

[Alan64]

I compared the Baader 32mm plossl with the Vixen 30mm NPL a couple of years back:

http://stargazerslounge.com/topic/179005-baader-classic-32mm-plossl-meets-vixen-30mm-npl-plossl/

I'll be interested to hear what you think of them

"I hope the above notes prove useful for anyone considering a ~32mm 1.25" eyepiece..."

Indeed they did.  Thank you!