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General Astronomy >> General Observing and Astronomy

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mrramsey
super member


Reged: 11/19/11

Loc: Hudson, Oh
2" occular question new
      #5533497 - 11/22/12 08:01 AM

I currently use 1.25" plossls in my 8" f/10 LX90. I do have a 2" visual back. Question is, what can I expect if I were to get a 2" - 70 or 80 degree 38mm eyepiece? Would my views be much wider or better than say a 38mm 1.25 @ 52 FOV?

I am just trying to get a bit wider field of view. I could get a 6.3 focal reducer I suppose but really don't want to add extra glass. I was considering these from university optics.


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FirstSight
Duke of Deneb
*****

Reged: 12/26/05

Loc: Raleigh, NC
Re: 2" occular question new [Re: mrramsey]
      #5533543 - 11/22/12 08:47 AM

In general, the potential gain from going to 2" eyepieces from 1.25" eyepieces is proportional to the extra range of field stop widths the 2" format allows, e.g. the largest diameter field stop possible in a 1.25" format is around 26-27mm, whereas the largest diameter field stop possible in a 2" format is around 46mm. Of course, there are other considerations for any given focal-length and optical design (including the designed AFOV) that constrain the useful width of the field-stop, but the maximum constraints imposed by the barrel size and therefore max field stop diameter is a "hard" physical limit, rather than one dictated by other design considerations.

You can get an approximate idea of the practical differences by going to the Televue Eyepiece Calculator and plugging in any two out of three fundamental parameters for your scope (focal length, aperture, or focal ratio), and be sure to check the box indicating that your scope is capable of accepting 2" eyepieces. NOTE: EVEN IF you plan to go with a EP brands other than Televue, the results generated will nevertheless be very usefully illustrative for answering your question. You'll at least get an accurate ball park idea of what practical benefits you may get going with 2" instead of just 1.25" EPs with your scope. You can un-check Ethos or Nagler if you want to limit results to 70 deg AFOV EPs such as Panoptics, Delos, and Plossls (again, looking at the values generated for these EPs will give you a useful rough idea of the potential results from some other brands as well).

Edited by FirstSight (11/22/12 09:12 AM)


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killdabuddha
Pooh-Bah
*****

Reged: 08/26/11

Re: 2" occular question new [Re: FirstSight]
      #5533628 - 11/22/12 09:53 AM

We'd originally built with these that you linked in mind. Thing is, nobody talks about them anymore. I think that they were better maybe 10 years ago? I was hard-pressed to find anything recent on them and got pulled into the Explore Scientific line for "bang for buck." But at f/10, and for the price, neither have I read anything bad about them. If we'd maintained our "discipline," no doubt we'd have them.

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Tony Flanders
Postmaster
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Reged: 05/18/06

Loc: Cambridge, MA, USA
Re: 2" occular question new [Re: mrramsey]
      #5533633 - 11/22/12 09:56 AM

Quote:

I currently use 1.25" plossls in my 8" f/10 LX90. I do have a 2" visual back. Question is, what can I expect if I were to get a 2" - 70 or 80 degree 38mm eyepiece? Would my views be much wider or better than say a 38mm 1.25 @ 52 FOV?




First of all, it's impossible to make a 38-mm eyepiece with a 52-degree field of view in a 1.25-inch barrel. About the longest eyepiece that can deliver a full 50-degree field of view in that format is 32 mm.

Second of all, for f/10 SCTs in particular, because of their long focal ratio, 2-inch eyepieces are especially beneficial. For instance, a relatively inexpensive 40-mm eyepiece with a 70-degree apparent field of view would give you a field of view 75% bigger in diameter and three times bigger in area than anything you can get in the 1.25-inch format.

Buying a focal reducer is another means to the same end.


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mrramsey
super member


Reged: 11/19/11

Loc: Hudson, Oh
Re: 2" occular question [Re: Tony Flanders]
      #5533694 - 11/22/12 10:31 AM

Quote:

First of all, it's impossible to make a 38-mm eyepiece with a 52-degree field of view in a 1.25-inch barrel. About the longest eyepiece that can deliver a full 50-degree field of view in that format is 32 mm.

Second of all, for f/10 SCTs in particular, because of their long focal ratio, 2-inch eyepieces are especially beneficial. For instance, a relatively inexpensive 40-mm eyepiece with a 70-degree apparent field of view would give you a field of view 75% bigger in diameter and three times bigger in area than anything you can get in the 1.25-inch format.

Buying a focal reducer is another means to the same end.




Thanks Tony, I may have been wrong about my current EP, I was going from memory. The second part of your response was just what I was after. The search is on for a 2" EP or two -


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