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Eyepiece Limitations

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#1 Theman785

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Posted 19 November 2012 - 11:50 PM

Hi, this may have been addressed previously but I'm having quite a hell of a time finding information related to what the operating limitations are on eyepieces.
I suppose a more specific approach to what I'm trying to find out is; how and what factors would help me in determining what size eyepiece to purchase?
As it stands now i have a very respectable 10"schmidt-newtonian, but I only have a Meade Series 4000 25mm Plossl and a 2x Apochromatic Barlow Lens. I can't figure what the next best step in EP should be, I live about 15miles S of Denver, so moderate light pollution, but I don't know if that would be a factor in deciding between say a 6mm or 12.5mm EP.
I am also wondering how to determine how exactly the whole magnification factor affects my viewing ability/limits? Does that make sense? Is there some magic formula providing a link between magnification and, say, magnitude?
To simplify my rambling nonsense, how to you figure what size EP you need to see "X" object, and what needs to be factored in?
Thanks for clearing this up guys!

#2 GOLGO13

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Posted 20 November 2012 - 12:12 AM

Well...couple quick things.

Aperture of the telescope plays a role. But with a 10 inch scope you should have no problem there. In general scopes can handle (theoretically) 50-60x per inch of aperture. But that's not normally what the atmosphere will allow.

Believe your scope has an F4 focal ratio with a focal length of 1016. So a 10mm eyepiece gives you about 100x (Telescope focal length 1016mm divided by eyepiece focal length 10mm). I'd say you'd want to stick with your 25mm for low power...28mm gives you a exit pupil of 7...which is in general the limit of exit pupil a human eye can handle. As you get older that gets worse and you may only handle 6 or so.

Now, in your situation, a nice 10mm eyepiece and a good barlow would be pretty darn good for you. That gives you 40x (with your 25mm), 100x, and 200x (assuming 2x barlow).

Since your scope is F4 (though i don't know a lot about Schmidt Newts), I'd suggest not going too cheap on eyepieces. I mean, you could get a $40 plossl and a $40 barlow if money is tight. That wouldn't be too bad. If money isn't tight I would suggest a Pentax XW10mm and a nice barlow. But, the Pentax is $280 on sale right now (pretty pricey).

You can go above 200x if atmosphere permits. I'd say 300x is about the max most nights. That being said, I've had my 10 inch newt at 480x and the image was still very sharp (was a good night).

#3 GOLGO13

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Posted 20 November 2012 - 12:40 AM

200x is pretty good for smaller DSOs. Supposedly an exit pupil of 2 is best for DSOs. So an 8mm eyepiece would be exactly at 2 for exit pupil. Couple ways to calculate exit pupil. One is eyepiece focal length divided by Focal ratio.

What magnification really depends on the object. 40x on M13 (a globular cluster) provides mostly a fuzzy ball. Crank that up to 200x and it's a ton of stars. Some smaller nebulas need some higher power to get the best view. I always star out lowest power then go up from there.

Planets and the moon are nice at higher powers. I usually go from 150x to 250x for planets.

A good book for explanations of scopes and eyepieces as well as just about everything else is Star Ware by Phillip Harrington. It's a little older now (2007), so some of the products being described may or may not still be for sale. but the content in the book is quite good for learning about scopes and accessories.

#4 TexasRed

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Posted 20 November 2012 - 05:59 AM

We really need to know the focal lenth or f/ratio of your telescope to give you specific advice. How much you want to spend would help, too.

Without knowing that, I'd say you need to do some math and determine what focal length of eyepiece will give you an exit pupil of about 0.5 mm. Double that, and look for something with a focal length very close to that.

Avoid a 12.5mm eyepiece, since you can already get that focal length with your 25mm and your Barlow.

An eyepiece providing about a 2mm exit pupil will also be very useful.

#5 REC

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Posted 20 November 2012 - 10:54 AM

Just some simple advise to add to the above. An exit pupil of two is the sweet spot for a lot of objects. I do a lot of observing of DSO's larger planets, Moon and a filter Sun. Next most used EP would be one with an exit pupil of one for high power for planets, moon globular clusters. After that you can fill in as you go. There are some nights the the seeing just won't support high power, so I have to back off those EP and wind up with moderate power.

Anyway....a lot of this depends on what kind of scope your using.

Welcome to Cloudy Nights!

Bob

#6 csrlice12

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Posted 20 November 2012 - 11:23 AM

Here is an eyepiece chart. Fill in the particulars about your scope and the eyepiece you are considering. It will give you the magnification, the exit pupil, The AFOV, etc...

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