Eyepiece question

John · Guest

I received an ETX-80AT-TC for Christmas. My wife ordered it through Opticsplanet. I'm looking at the following additional eyepieces:

1. Meade 07650 (5.5 mm, 4 mm eye relief)
2. Cel 93387 (5mm, 20 mm eye relief)
3. Cel 93317 (6 mm, 2 mm eye relief)

I'm leaning towards #2, as it seems to be ideal for short refractors. However, with a 2x Barlow would that be pushing it? (400/5=80 x 2=160x magnification)

Just looking for opinions.

Thanks,
John

opticsplanet.com · Posted: Fri Jan 20, 2006 12:08 pm Post subject:

Hi John

Thanks for posting with us.

Yes, 160x is pushing it with the ETX-80, though it may give you some useful images of something very bright like the moon. You'll probably be much happer with 100-120x as a top end on this scope. This will still be enough to provide nice images of the planets.

As you can see, eye relief becomes a problem with short focal length (high magnification) eyepieces. 2 or 3mm of eye releif makes it impossible to use eyeglasses, of course, but it is also a pain in the neck even without eyeglasses, since it requires you to get your eye snug down on the eyepiece.

One option you listed is an example of how some eyepiece designs address this problem. The Celestron X-Cel has 20mm of eye relief for all focal lengths. You can also see this in other eyepiece designs such as the Vixen LV and the Televue Radian. This is acomplished by incorporating a another lens in the barrel at various distances - essentially a barlow lens - to decrease focal length while maintaining eye relief. It means adding more lenses with a subsequent loss of image brightness, but in a quality eyepiece, it's a more than acceptable trade-off, especially if you wear galsses.
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John · Guest

Thanks for the reply. I wear glasses but I do not have astigmatism.

Based on what you said, ideally I would like a Celestron X-Cel in the 6mm to 6.5mm range. However, it does not seem that make that size. Looks like they go from 5mm to 8mm.

The Vixen LV and Televue Radians are a little pricey right now.

Thanks again,

John

opticsplanet.com · Posted: Fri Jan 20, 2006 5:08 pm Post subject:

Hi John

I understand about the Televue. In the X-Cel, you could always go with the 12.5mm and add a 2x barlow. This won't be quite as optically good as a 6mm by itself, but it will get you there.

Since you don't have astigmatism, there is no penalty or loss of resolution when you take your glasses off and at this magnification, the astigmatism is probaby not a factor, anyway. A standard 6mm plossl is probably your best bet.
_________________
----------------------
Your personal optics expert
Joanie (Jne) K
http://www.OpticsPlanet.com
Phone: (888) 263-0356
Fax: (847) 574-6820

John · Guest

My wife has astigmatism, and I'm trying to get her hooked as well Smile

Will the Vixen LV 6mm Eyepiece work/fit with the Meade #126 Barlow Lens and the ETX-80? I am "assuming" that eyepieces are standardized and that one company's 1.25 eyepiece will fit other scopes.

Also, any recommendations for a solar filter for the ETX-80?

Thanks for your help and responses.

John