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When is it worth buying 2" eyepieces


jnb

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Having just acquired a new scope with a 2" focusser I'm wondering when it would be worth getting 2" eyepieces. It seems that a lot of 2" eyepieces are are just 1.25" eyepieces in a 2" barrel presumably because there's no technical advantage due to stops etc. So apart from a few wide angle low magnification eyepieces is there any reason to use 2" eyepieces?

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The 2" format can and usually does deliver a wider field of view than a 1.25" eyepiece of the same focal length. For example, a 32mm eyepiece in the 1.25" format has a maximum of a 52 degree apparent field whereas a 32mm in the 2" format can have 80+ degrees.

For the above reason, many folks have 1.25" eyepieces in the short to medium focal lengths and then a 2" eyepiece or two at longer focal lengths where wide fields of view are desired.

In terms of optical quality there is no advantage in the 2" format itself.

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I have had a 2" eyepiece for some years now and never used it.

Equally I am not looking for the greatest magnification or the widest emersive views.

I really would not like to say how much use or advantage one is these days since you can get 82 and 100 degree eyepieces at 1.25" fitting.

Biggest problem I would say would be swapping between 2" and 1.25" while viewing, not only will it be anything but parfocal there is an adaptor to take out/put in at the time. I get the idea that anything will not just have moved out of view it will likely have set below the horizon. :eek: :eek: :eek:

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I only have 1x 42mm with a 2-inch fiiting.

They will be useful to avoid the need for fiddling about with 1.25 adaptors.

And for larger eyepices around 500g to 1kg, the 2-inch fitting to be more secure in the focusser tube for less flex = more accurate viewing.

The dual fitting eyepieces which come to mind are the Teleview Ethos, and the Skywatcher SWA 70-degree.

You don't want either one of them dropping out onto your foot.

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The right time is when you find what you want at the right price, the difference is not worth worrying about on the whole unless you are a very dedicated widefield viewer.

Dont forget though if you use visual filters and want to use 2" and 1.25" eyepieces you will need to get 2" filters  and a 2 to 1.25 eyepiece adapter that has enough depth and a filter thread. (to stop the 1.25" eyepiece bottoming out onto your fllter)

2" filters are a fair bit more expensive.

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as others have said. they are no better or worse, just (potentially) offer a wider field of view compared with 1.25" eyepieces of the same focal length. in fact the difficulty of making larger eyepieces with a wider field sometimes means they are worse than their smaller cousins.

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The true field of view on the sky that any eyepiece can see is determined by its field stop.  That's the diameter of the focal plane in the eyepiece.

As focal lengths get longer, the field stop increases until it bumps into the barrel.

In 1.25" eyepieces, that'll be about a 27mm field stop.

But 2" eyepieces can continue all the way up to about 46mm, or 70.4% wider.

So, for example, that would yield a true field of 0.97 degrees in a 12.5" f/5 dob for the 1.25" eyepiece, or 1.66 degrees in the 2".

When you look at it that way, it's obvious what 2" eyepieces are for: to expand the true fields at low powers.

When the field stop gets small enough to fit in a 1.25" eyepiece, there's no reason for the eyepiece to be larger.

That happens at about 13-18mm of focal length depending on the amount of apparent field required (larger apparent fields require larger field stops).

So if you see eyepieces shorter than that with 2" barrels (and there are some), it is primarily for convenience.

The eyepiece could be so big and heavy that the manufacturer felt a 2" size was safer, or just because they felt most

users would prefer not to have to use adapters in 2" focusers.

But a 13 Ethos (100deg.) is 1.25", as is a 16mm Nagler (82deg), as is a 24mm Panoptic (68deg.) and a 32mm Plossl (50deg)

The really short eyepieces could be even smaller, but aren't because the 1.25" size is more universal now.

So, just think of 2" eyepieces as big, low-power, wide field of view, eyepieces and you'll have the answer to when it is worth

buying 2" eyepieces.

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Don's answer is on the money. There is no point in having a short focal length eyepiece in 2 inch format other than for convenience. But once you want the widest possible fields (which some observers do and some don't) you have to go to 2 inch. Ronin says you can get 1.25 EPs with ultra wide fields and you can, yes, but only in shorter focal lengths. These will take you to the 1.25 field stop at higher power but they won't do anything else.  Some 2 inch ultra-wide positives;

- If you have a long FL scope only a 2 inch ultra wide EP can give you anything like a widefield view.

- If you have a short FL scope a 2 inch ultra wide can give you binocular-like views with the possibility of easy use of filters. (l like the Rosette or the entire Veil, for instance, in a 26mm Nagler/TeleVue Pronto 70mm scope with O111 filter.)

- In a Dob an ultra-wide EP gives you a long time between nudges, which can be nice.

If you do go for ultra wide it is worth paying for quality or the edge of field (the bit you are paying so much for!) will fall off in quality.

Olly

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