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2" vs. 1.25"

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#1 CharlesW



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Posted 06 November 2012 - 10:19 AM

I bought an 8" Celestron and the 1.25" Celestron lense kit a couple of weeks ago. I'm quite thrilled with that purchase but I was back at OPT last week to get a focal reducer and the sales guy mentioned that I should consider getting into 2" eyepieces. I didn't have time at the moment to ask him, "Why?" Would someone mind pointing me to an existing thread or sharing their thoughts on this?

#2 Glen A W

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Posted 06 November 2012 - 10:32 AM

Hi Charles, and welcome!

The views through even lower-priced 2" eyepieces will absolutely blow you away. Take a 32mm Meade 2" eyepiece costing $80 - the view of open clusters and so forth is just awesome. I am no eyepiece snob, but at low powers, I have just never seen things in any 1.25" eyepiece like in the big 2" ones. I suppose some of this is simply due to the small barrel limiting options for designing for wide field, though it is also a fact that the huge eye lenses and barrels tend to help immerse you into it.

You are missing out if you don't get a 2" diagonal and an eyepiece or two, especially with the Celestron 8" which I assume is the SCT, and which has a long focal length and rather narrow field in the first place. It is not necessary to spend a lot to reap the benefits. There are many eyepieces available at reasonable prices and you may find you need just one 2" eyepiece to make you happy. That is how I am lately, but I would not part with that two-incher in a million years.


#3 csrlice12


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Posted 06 November 2012 - 10:35 AM

If you got an 8" SCT, or even a standard 8" Dob, these have a long f/l and the larger the focal legnth the larger the fstop of the eyepiece is needed for low power. About 27mm is the largest you will find in a 1.25" eyepiece as that is the maximum size of the field stop, in a 2" eyepiece, the field stop is much larger, allowing you a wider possible FOV. If that 8" is a CAT with a 1500+mm f/l, you'll need those big, 2" eyepieces to be able to use low power on your scope. Some 8" Cats are over 2,000mm fl, so as, you can see, a 50mm lens in a 2000mm scope = 40X; and you will need a 2" eyepiece at 50mm.

As you can see, any one eyepiece will not work the same in variuos scopes. That 50mm that gives you 40X in that 2000mm Cat, would only give 24X in a 1200mm Dob (and may not even be usable as it might start showing the shadow of the secondary in the view).

#4 Glen A W

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Posted 06 November 2012 - 10:36 AM

I would look into a 32-38mm for that scope. By the time you hit 50 you will likely have trouble. That's how it is on mine, anyhow.

#5 Alan French

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Posted 06 November 2012 - 10:44 AM

Your telescope has a certain image scale, based on its focal length. If you put a digital camera at the focal plane of your telescope it would record a certain portion of the sky. The shorter the focal length of your telescope, the more sky the camera would see. If you have an 8" f/10 SCT, it has a relatively long focal length, and so the camera would not record a large portion of the sky.

Inside an eyepiece is a field stop. When you focus your eyepiece, the field stop defines the outer edge of your field of view. So the size of the field stop is what limits how much of the sky you can see. The maximum field stop in a 1.25" eyepiece is about 27mm. In a 2" eyepiece, it is about 46mm, so at any given focal length, the 2" eyepiece can show more of the sky.

The illustration below shows a view of the Veil complex with a telescope of 600mm focal length. Superimposed on it is a 27mm field stop, and a 46mm field stop, so you can see than a long focal length 2" eyepiece will show sky around the Veil, while a long focal length 1.25" eyepiece just barely squeezes it into the field of view.

With the longer focal length of an 8" SCT, you can't see this much sky, but the 2" will still show a far wider area.

Clear skies, Alan

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#6 Pharquart


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Posted 06 November 2012 - 10:55 AM

In a different thread, I provided information on converting an 8" SCT to accept 2" eyepieces.


The high focal ratio of an 8" SCT and the internal baffle will limit how much wider field of view you'll get with 2" eyepieces. It'll be wider than 1.25", but not as good as you'd get on a F/4.5 Dob.


#7 Jon Isaacs

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Posted 06 November 2012 - 10:58 AM

With the longer focal length of an 8" SCT, you can't see this much sky, but the 2" will still show a far wider area.

A focal reducer allows for wider fields of view by shortening the effective focal length. It also corrects for the significant field curvature of the SCT.

In choosing between the focal reducer and the 2 inch diagonal, the maximum possible field of view is about the same, 1.25 degrees TFOV for focal reducer, 1.29 degrees TFoV for the 2 inch eyepiece.

One can try to combine them but vignetting, which is a small problem with either a focal reducer or a 2 inch eyepiece, becomes a real problem with both. A focal reducer only changes the effective focal length of the scope, the basic limitations, 2032 focal length and the 38mm rear baffle/rear port diameter are inherent in the design.

Also, there are some scopes that have issues clearance issues with 2 inch diagonals...


#8 REC


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Posted 06 November 2012 - 11:26 AM

As above comments, you can use those 2" 30-40mm WA EP's which are great for star clusters and some nebula's. Feels like you are looking out a porthole in space!

Also, if you get a 2" dialectic mirror diagonal, it will be a little brighter than the stock prism one that comes with the scope.

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#9 drbyyz


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Posted 08 February 2013 - 09:34 PM

+1 for the Agena SWAs, I own the 38mm and love it. I'm sure it wouldn't hold a candle to some of the higher end EPs, but for $85 it's a freakin steal. No complaints here.

++1 or 2 for the ES eps if they are in your budget. Great, high quality eps at a reasonable price.

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