1.25" or 2" EP?

[Swoop1]

So, with the visit of Rudolf and co. looming, I suspect I may be able to fund the start of my EP upgrade programme soon (or should that read lifelong EP buying frenzy?).

My question is, aprt from cost, what are the relative advantages/ disadvantages of the two formats?

The EP will be for use on my Skywatcher Explorer 150P on the EQ2/3 mount.

Thanks.

[Piero]

2" eyepieces will allow you to have a larger field of view compared to 1.25". The field stop limit for a 1.25" eyepiece is 27mm. This is the major reason why you generally find 2" eyepieces at longer focal lengths and 1.25" eyepieces at shorter ones. For longer and heavier eyepieces, a 2" barrel offers more stability. Optically, there is no distinction between the two. 

Personally, I like 2" eyepieces and I actually have 2" and 1.25" sets. The issue can be swapping eyepieces from these two sets as this can require some telescope rebalancing. Not a big deal, but something to be aware of. 

Also, if you decide to get 2" eyepieces, make sure your focuser is good enough. This is more the case for eyepieces weighing around 1kg than medium weight eyepieces of 0.5kg. The weight can cause a minimal misalignment in a focuser that is not robust enough, and this can affect the views. 

[rwilkey]

Viewing a celestial object through a 1¼" or 2" eyepiece of similar quality will look exactly the same if used in the same scope where the focal length of the scope remains constant.

However, 2" eyepieces have the following advantages and disadvantages:

- the 2" eyepiece will probably have better eye relief.
- it will feel more comfortable and give you a slightly better field of view
- at low power 2" eyepieces (25mm to 38mm focal lengths) are good for deep sky observing and not so good for planetary and lunar observing.
- 2" eyepieces may not perform so well with SCT's or Maks because of the telescopes' long focal length, making the wider field of view that the eyepiece offers irrelevent.
- one of the biggest disadvanges of the 2" eyepiece is their cost, with decent eyepieces in this diameter regularly starting at around £80, more than twice the cost of a decent 1¼" eyepiece! The other thing is that filters for 2" eyepieces will also cost twice as much and so on.
- 2" eyepieces tend to be heavy and may easily tip a lightweight telescope, making rebalancing a possibilty.

[domstar]

If I want to up the magnification, it's more of a pain to swap eyepieces from 2 inch to 1.25- especially if the temperature is low. 

[25585]

9 hours ago, rwilkey said:

Viewing a celestial object through a 1¼" or 2" eyepiece of similar quality will look exactly the same if used in the same scope where the focal length of the scope remains constant.

However, 2" eyepieces have the following advantages and disadvantages:

- the 2" eyepiece will probably have better eye relief.
- it will feel more comfortable and give you a slightly better field of view
- at low power 2" eyepieces (25mm to 38mm focal lengths) are good for deep sky observing and not so good for planetary and lunar observing.
- 2" eyepieces may not perform so well with SCT's or Maks because of the telescopes' long focal length, making the wider field of view that the eyepiece offers irrelevent.
- one of the biggest disadvanges of the 2" eyepiece is their cost, with decent eyepieces in this diameter regularly starting at around £80, more than twice the cost of a decent 1¼" eyepiece! The other thing is that filters for 2" eyepieces will also cost twice as much and so on.
- 2" eyepieces tend to be heavy and may easily tip a lightweight telescope, making rebalancing a possibilty.

2 inch barlows are more expensive.

Buying a converter to 2 inches is inexpensive, so often used 1.25 eps could have one each, making them quicker to swap with any 2 inch ep you buy.

[Paul73]

I’ve now gone completely 1 1/4". My collection is now parfocal from 6 to 24mm. Much easier for the lazy observer!

The 2” filters are staying though. Just in case I go 2” again (they cost a fortune compared to their smaller versions).

The wide view of the 2” 80°+ eyepieces can be beguiling. I am constantly tempted by the ES 24mm 82° (which would be great in your scope. Being able to view a target in its entirety at a reasonable magnification is great. However, controlling the whole field of view can be a bit of an issue and can feel a bit of a strain (unless you spend spend spend).

I would give it a try and decide for yourself.  The experience is well worth while. Some people love the experience; others prefer not to trade a kidney or sacrifice quality and stick to 1 1/4”.

Paul

[ronin]

2" eyepieces are big. Always sounds a small step from 1.25 to 2, but that is a 1.6x increment and that has to be in all 3 dimensions so the reality is close to 1.6 cubed. Which is 4x the volume.

I have one 2" eyepiece, it sits with my TV plossl's and I can fit easily 3 TV plossl boxes inside the 2" eyepiece box with space around them.

With the range of good 1.25" eyepieces I simply never use the 2". Even the BST's give good fields of view and good eye relief and 3 or 4 of those in a bag weight less then a single 2". ES82's will deliver the wider views and the higher magnification. 2" eyepieces seem good for the wide low power views, partially I suspect because there is not really a 30mm ES82 type available in 1.25".

[Stu]

4 hours ago, ronin said:

2" eyepieces seem good for the wide low power views, partially I suspect because there is not really a 30mm ES82 type available in 1.25".

That is exactly the point @ronin, it is not possible to make a 30mm 82 degree afov eyepiece in 1.25" format because the field stop of the smaller barrel limits the design to around 50 degrees.

As said, the key advantage of 2" eyepieces is the wider apparent field of view which they can offer at the same focal length. The apparent field of view of eyepieces is limited by the field stop of the eyepiece. If you look at different formats available, you will see this limit coming into play.

In 1.25" format you have:

24mm 68 degree afov

32mm 50 degree afov

40mm 43 degree afov

All three have basically the same field stop and offer the same field of view at different magnifications. The relative merits of each one are for the 24mm - smaller exit pupil giving darker sky background, for the 32mm - eye relief and in the case of the 40mm it has a benefit of giving maximum exit pupil in scopes like small maks when using narrowband filters.

Exactly the same principle applies with the 2" format:

21mm 100 degree afov

31mm 82 degree afov

41mm 68 degree afov

55mm 50 degree afov

The 55mm and 41mm have 46mm field stops and show maximum fov in the 2" format. The 31mm and 21mm have smaller field stops than the maximum so there are other design limitations leading to these afov sizes.

If you compare two eyepieces of equal focal length, you will see where the benefit of the 2" format lies, i.e. in allowing much larger afovs at the same focal length.

A 30mm 50 degree 1.25" eyepiece gives a magnification of x40 and a field of view of  1.25 degrees in a Skywatcher 200p. A 30mm 82 degree 2" eyepiece gives the same mag but a field of view of 2.05 degrees in the same scale. In the picture attached, this is the difference between the object filling the fov vs framing it nicely with black sky around to give context on an object like M45.

Looking at this in an alternative way, a 100 degree afov eyepiece can give the same actual field of view whilst giving a smaller exit pupil which helps to darken the sky background and gives a better impression of contrast.

If we take a 32mm 50 degree afov eyepiece, it gives 1.33 degree field of view in our 200p, with an exit pupil of 5.3mm. A 17mm 100 degree afov eyepiece gives a very similar 1.42 degree field of view, but with an exit pupil of 2.8mm. On target like the Double Cluster this will give a much darker sky background giving a much more pleasing image, particularly if you live under a light polluted sky.

Two example images attached. First is the 30mm 50 degree and 82 degree afov example on M45, second is the 32mm and 17mm example on the DC.

Hope this is all of some help  

[Swoop1]

Thanks for your replies everyone. Definite food for thought.

I might have to badger some society members to see if they have a 2" I could borrow for a month to experiment.

[25585]

For a dob I find wider FOV indispensible, for checking alignment with Telrad & finder, then starting to zone in. 

Parfocal is a help as well. 

One of the ironies of scope designs is that cassegrains with their long focal lengths, which could really be helped for unguided viewing by a 2 inch ep FoV, limit use effectiveness by vignetting etc. 

[Stu]

3 minutes ago, 25585 said:

For a dob I find wider FOV indispensible, for checking alignment with Telrad & finder, then starting to zone in. 

Parfocal is a help as well. 

One of the ironies of scope designs is that cassegrains with their long focal lengths, which could really be helped for unguided viewing by a 2 inch ep FoV, limit use effectiveness by vignetting etc. 

Actually, although there is vignetting on 2" eyepieces caused by the baffle tube on SCTs and Maks, I have found that 2" eyepieces are quite useable. Your eye is not anywhere near as sensitive as a camera to vignetting, so although it is there, it does not detract from the view too much. There are a few threads on the forum discussing this topic, worth searching out.