Author Topic: What is the difference between 1.25" and 2" eye pieces  (Read 121 times)

ricoperte

  • Active Astronomer
  • ***
  • Posts: 120
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
What is the difference between 1.25" and 2" eye pieces
« on: December 29, 2017, 01:17:25 AM »
Hey folks, would choosing a 2" eye piece over a 1.25" be any advantage? I know the 2" is more expensive and not as standard as the 1.25. Thanks!



naiciareamu

  • Active Astronomer
  • ***
  • Posts: 115
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #1 on: December 31, 2017, 06:29:44 PM »
Good question. For low power views the simple answer is yes. A 2" eyepiece will have a larger field stop than a 1.25" giving you a wider field of view. Typically this would be for eyepieces 30 - 40mm focal length. Usually you only need one eyepiece in this range for your lowest power views. Any powers higher than this are easily accommodated by 1.25" eyepieces which have many more sizes and are less expensive.

therpomercu

  • Active Astronomer
  • ***
  • Posts: 113
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #2 on: January 04, 2018, 12:35:17 PM »
depends on design of Ep and focal length. All things(design) being equal the 2" is better at lower powers

tecretalhei

  • Active Astronomer
  • ***
  • Posts: 123
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #3 on: January 04, 2018, 06:51:52 PM »
I converted to all 2" to eliminate swapping 1.25"-2" adapters out. This can get expensive but I only buy used EPs. 2" EPs are considerably heavier than 1.25". My scope is balanced for their bulk.

Ivan Kim

  • Active Astronomer
  • ***
  • Posts: 116
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #4 on: January 09, 2018, 05:21:04 AM »
Let's start by considering a single eyepiece design like a basic plossl...

A standard plossl has an apparent field of view of 52 degrees. For moderate and high power eyepieces, the design easily fits in a 1.25" barrel. However, once you get beyond 32mm or so, the barrel of the eyepiece (rather than the eyepiece design) starts to become the limiting factor on the apparent angular view provided by the eyepiece. You can buy longer (lower power) plossls in a 1.25" format, but you won't get a 52-degree apparent view (a 40mm plossl will in 1.25" format will probably be 42-44 degrees). The 2" eyepiece can accommodate the full plossl design in much longer focal lengths (e.g., the Meade 56mm 2" plossl hasa 52-degree apparent field). So there would be no advantage to a standard plossl in a 2" design at 20mm, but at 40mm, the 2" plossl should provide a wider field of view.

As you move into wider designs, the need totransition to 2" barrels occurs earlier. Wide-angle designs may need to move to 2" barrels in the mid-20mm focal lengths, and super/ultra-wideswill switch in the teens.

Rick Reiter

  • Active Astronomer
  • ***
  • Posts: 129
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #5 on: January 10, 2018, 08:17:35 AM »
You are correct about the two inch format being more expensive than the 1.25" format, but remember that in the 2" format you are generally getting a higher quality product than so many of the 1.25" format eyepieces. In addition the 2"ers require more of that expensive glass to make the larger eyepiece. That is not to say that all 2" eyepieces are great quality, it's just that more 2" eyepieces are made to higher standards than the 1.25" eyepieces. In the 1.25" format there are some that will blow most of the 2" eyepieces out of the water, but then again some 1.25" eyepieces suffer from field stops caused by the internal parts and the barrels. Not so much in the 2" format. For my scopes I have 10, 18 26 and 38 mm focal lengths plus 2X and 1.5X Barlow 2" eyepieces for my 5" refractor and 10, 15, 25, 40 and an 8 to 24 mm variable and a 2X Barlow in 1.25 mm format for my 5" Maksutov. That gives me a good evenly spaced range of 31X to 236X for the big refractor. All of my 2" eyepieces are at least 65 degrees AFOV. The 1.25" pieces are a mixed bag of tricks as far as AFOV goes but they give me 30X to 236X magnification. It works out just fine for me in that I only do visual observing.

The bottom line to your question is the 2" EP will generally give you wider field of view and they provide a wider path for the light column so the views will be somewhat brighter.

enmumenge

  • Active Astronomer
  • ***
  • Posts: 113
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #6 on: January 10, 2018, 06:50:13 PM »
Quote
You are correct about the two inch format being more expensive than the 1.25" format, but remember that in the 2" format you are generally getting a higher quality product than so many of the 1.25" format eyepieces.

This is the wrong way to look at it!

It is absolutely untrue that 2-inch eyepieces are inherently better than 1.25-inch eyepieces. Many of the world's most highly respected eyepieces are in 1.25-inch barrels. Conversely, in recent years in particular, there have been many mediocre 2-inch eyepieces.

In general, manufacturers use the smallest barrels that will accomplish their purpose. If all the necessary lenses fit into a 1.25-inch barrel, that's great! All other things being equal, small and light is obviously better than big and heavy. Some eyepieces that are pushing the 1.25-inch format are put into hybrid barrels that fit both into 1.25-inch focusers and 2-inch focusers without adapters.

Which barrel size is needed depends almost entirely on the eyepiece's focal length and its apparent field of view. For any given design, the longer the focal length, the bigger the eyepiece. A 32-mm Plossl is simply a 16-mm Plossl where all the lenses have been scaled up by a factor of two in every dimension. Therefore, the 2-inch format is rarely used for medium-focal-length eyepieces such as a 16-mm, and hardly ever used for short-focal-length eyepieces such as an 8-mm. It's not needed, and as I said before, if you can get away with using the 1.25-inch format, it's obviously better.

In addition, the bigger an eyepiece's apparent field of view, the bigger it needs to be for any given focal length. A 25-mm eyepiece with a 50-degree apparent field of view, such as a Plossl, is nice and compact. A 25-mm eyepiece with a 68-degree field of view is typically a bit longer than a 25-mm Plossl and quite a bit fatter; it's the widest field of view that fits into a 1.25-inch barrel in that focal length. A 25-mm eyepiece with an apparent field of view in the 80-degree range is huge. It requires a 2-inch barrel.

However, a 13-mm eyepiece with an apparent field of view in the 80-degree range, while not exactly svelte, fits nicely into the 1.25-inch barrel. It is, essentially, that same 25-mm eyepiece with all the lenses scaled down to 50% of their original size.

The bottom line is that you should first figure out what eyepieces you want and then discover what barrel size they come in. It's crazy to pick an eyepiece on the basis of its barrel size -- unless your scope simply doesn't accept 2-inch eyepieces, in which case you have no choice.

abtempoecar

  • Active Astronomer
  • ***
  • Posts: 114
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #7 on: January 11, 2018, 05:59:23 AM »
Actually I was going to start a similar thread but, as usual, Tony nailed all my questions.

From what I have read, no real experience, It looks like 1.25" up to about 66 degrees are available and at not to steep costs, say less than $100 per eyepiece.

And I have seen some series 70 degrees and overwhere the longer FL, 25 mm and longer,are 2" and the shorter are 1.25" so I was thinking that2" eyepieces might be preferable for wide field useful to about 25 mm but after that there are 1.25 mm eyepieces that offer pretty wideAFOV seem to be workable.

That seems to be in line with what Tony posted. Howeverit looks like once you get over 80 degrees you almost have to go to 2" Would that be right?

buddderpdrivla

  • Active Astronomer
  • ***
  • Posts: 130
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #8 on: January 11, 2018, 01:17:57 PM »
Ed,

That is not it. A Nagler 17mm T4 is 82* and 1.25 inch. An Ethos 13mm is 100* and works very well as a 1.25 inch eyepiece.

You can very well build a great eyepiece collection for your Dob8, all in 1.25. But to go beyond the maximum field of you reachable with a 1.25 inch eyepiece, 1.3 degree, you will need to get a 2 inch eyepiece.

I also own a Dob8. Look at my sig where you will see my eyepieces. I just ordered an ES 82-24 that will give me around 1.7*. That will be my sole 2 inches eyepiece. No obvious need for more.

Ben

seucamthepo

  • Active Astronomer
  • ***
  • Posts: 119
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #9 on: January 12, 2018, 03:39:18 AM »
The 17mm Nagler T4 has a 2" barrel. The 16mm Nagler T5 has a 1.25" barrel. All of the Nagler T6s have 1.25" barrels.

The Tele Vue Ethos line (100 to 110 degrees) from 13mm to 3.7mm can be used with 1.25" focusers, as can the 12mm Nagler T4.

http://www.televue.c...14#.VoCe7vkrIdU

The 5.5, 8.8, and 14 Meade Ultra Wide Angle 82 degree eyepieces have 1.25" barrels, as do the 4.7, 6.7, 8.8, 11, and 14mm Explore Scientific 82 degree eyepieces.

The Speers-WALER 82 degree eyepieces have 1.25" barrels.

The new 3.5, 5, 7, 10, and 14mmVixen SSW Ultra Wide 83 degree eyepieces also have 1.25" barrels.

The 12.5mm Docter Ultra Wide Angle UWA 84 degree eyepiece has a dual 1.25"/2" barrel, as do some of the Naglers.

The 3.3, 5.7, 7, and 10mmTakahashi Ultra Wide/Flat Field 90 degree eyepieces have 1.25" barrels.

Dave Mitsky

Johnny Jeep

  • Active Astronomer
  • ***
  • Posts: 131
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #10 on: January 12, 2018, 09:12:27 AM »
At the same exit pupil, all eyepieces, assuming similar light throughputs, will produce the same image brightness. The field of view will vary with the design.

Dave Mitsky

Zack Tucker

  • Active Astronomer
  • ***
  • Posts: 148
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #11 on: January 12, 2018, 12:45:04 PM »
In addition to the sound advice already shared, I'll add that lots of folks have only one 2"ep.
For that widest field low power option.
Because of present-day wide field ep options, they do the rest of their magnifications in 1.25", for the previously stated reasons.

nasapehe

  • Active Astronomer
  • ***
  • Posts: 129
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #12 on: January 13, 2018, 12:28:29 AM »
Having a 2" eyepiece to maximize a telescope's true field of view is certainly a good idea, in my opinion.

Dave Mitsky

ryarejalo

  • Active Astronomer
  • ***
  • Posts: 132
  • Activity:
    0%
  • Reputation: +0/-0
    • View Profile
Re: What is the difference between 1.25" and 2" eye pieces
« Reply #13 on: January 13, 2018, 02:16:19 AM »
Quote
Having a 2" eyepiece to maximize a telescope's true field of view is certainly a good idea, in my opinion.Dave Mitsky
Some objects deserve the lowest powers you can manage. The Veil Nebula is stunning at various powers but I most enjoy it at my lowest with an O-III filter.