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1.25" vs 2" diagonal


mac19

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Thank you in advance for takin the time to provide me with some insight. I'm new to Astronomy/Telescopes and I'm having a hard time finding finding the disadvantages/advantages between the two. I'm purrchaseing a CGEM 1100 and the C11 comes with the 1.25" star diagonal. Is there a benefit to swapping the diagonal out to a 2"? The reason I ask is I have no eye pieces and if there is an destinct advantage to using 2" eyepieces on the C11 I'd like to start aquiring 2" eye peices rather than 1.25".  Thank you and Have a great Day!

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At longer focal lengths a 1.25 can restrict the apparent field of view,eg. a 30mm limit is around 40 degrees fov, if a two inch barrel is used an eyepiece of 30mm can be designed to have a 80 degree fov. It follows that little advantage can be had by a 2.0inch eyepiece unless the fov is larger than the barrel will allow. hence most short focal or narrow fov eyepieces are made to 1.25 format.

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Mac,

If you  buying a C11 it is probably best to buy a 2 " diagonal that comes with a 1.25 adapter (most do).

This will allow you to use both, higher powered eyepieces tend to be 1.25" and you do not want to restrict your options.

The link shows an excellent diagonal :laugh:

http://www.teleskop-express.de/shop/product_info.php/info/p89_TS-2--Star-Diagonal---99--Dielectric---Compression-Ring---2-.html

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You'll definitely want a 2" diagonal with the C11 to get the most from it. The largest true fov possible with the 1.25" diagonal and a 32mm 52deg Plossl will be 0.60deg. That is truly tiny. Where as with a 2" diagonal and a Vixen LVW 42mm that grows massively to 1.11deg.

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Hi, not sure what C 11 you have, Edge or standard xlt, but the xlt is supplied with a 1.25" visual back. So to upgrade to the 2" format you will also want a 2" visual back for the scope. There is an advantage with using the 2" format as in my experience some 1.25" diagonals can cause vignetting with eyepieces like a 32 mm plossl. :smiley:   

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Hi and welcome,

In short there is no difference in my book of the view you can get with both types of diagonal as long as you buy reasonable quality ones. You don't have to get Televue like me. However for you to get the widest FOV with your scope you will need to go down the more expensive path and larger 2 inch eyepieces and diagonal. If you are happy with higher magnifications and narrow FOV and only intend looking at Planets then the smaller diagonal and eyepieces will work fine.

If you stick to the smaller size your widest eyepiece will be something like a 32mm 50 degree Plossl.

Alan

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