Re: Eyepiece Question
- From: "Kickin' Ass and Takin' Names" <PopUlist349@xxxxxxxxxxx>
- Date: 31 Dec 2006 15:27:00 -0800
vinylanach@xxxxxxx wrote:
I bought a Meade ETX telescope last year for my kids and myself. A few
weeks ago, I purchased a 9.7mm Super Plossl eyepiece on e-bay to
replace the 26mm one supplied with the ETX. Basically, I just wanted a
better view of the planets.
I received it a few days ago, and while certain things are certainly
larger, such as the moon, I find that some objects, such as Saturn and
Mercury, are distinctly fuzzy and grainy. So I'm trying to figure out
why I can't get the same clarity as I get with the 26mm eyepiece. Is
the 9.7mm eyepiece too much for the ETX? Are atmospheric conditions
the problem? Or is there a chance that the eyepiece is defective?
Thanks in advance.
There are several reasons why these objects are fuzzy and grainy.
First, let's review some math.
Maximum magnification
The maximum magnification you can expect from a telescope equals
(aperture in inches) X 60. Some folks use (aperture in inches) X 50.
Whatever.
You don't tell us which ETX you have so I'm just guessing. The ETX-80
has an aperture of 3.15 inches; ETX-90 is 3.5 inches and ETX-125 is 5
inches. Applying the factor of 60 would yield the following maximum
magnification figures:
ETX-80: 3.15 x 60 = 180X
ETX-90 = 210X
ETX-125 = 300X
Eyepiece magnification
The magnification provided by an eyepiece equals (telescope focal
length) divided by (eyepiece focal length). The focal lengths for the
ETX scopes are: ETX-80, 400MM; ETX-90, 1250MM. With a 9.7 mm
eyepiece, your magnifications would be:
ETX-80: 400mm divided by 9.7mm = 41X
ETX-90: 1250mm divided by 9.7mm = 129X
ETX-125: 1500mm divided by 9.7mm = 196X
Now, what does this tell us? If the eyepiece is yielding a
magnification that is close to or more than your scope's maximum
magnification, that could cause fuzzy seeing because you are pushing
the scope to its optical limits. HOWEVER -- we see that this is not
the situation here -- for example, the ETX-90 has a maximum mag of 210X
while the 9.7mm eyepiece gives a 129X magnification, well within the
scope's capabilities.
So, your problem is not caused by too much magnification. Here are
some possiblities.
1. Not focused correctly. I suspect when you put the eyepiece into
the scope you insert the eyepiece barrel all the way into the eyepiece
holder. It could be that the eyepiece needs to be pulled slightly out
of the holder because the travel of the focuser is not quite enough to
focus with that eyepiece. So -- insert the eyepiece into the scope so
that about 1/3 of the silver barrel is sticking out of the eyepiece
holder and try to focus the scope.
In this same vein, are you certain you are focussing the scope? The
ETX scopes use that tiny little silver knob that's impossible to grip
and you may not be focussing the scope -- can you focus other objects
such as stars, or, can you focus on terristreal objects -- that is,
point the scope at a hilltop several miles away and make certain you
can focus.
As a former ETX-90 owner, I recommend you replace the stock focussing
knob with a Flexi-Focus -- several dealers sell them -- do a google
search for "ETX flexi-focus" or "ETX accessories." Here is an example:
http://www.shutan.com/Merchant2/merchant.mv?Screen=PROD&Store_Code=1&Product_Code=595-8699&Category_Code=a
2. Seeing conditions. Atmospheric conditions control what you see.
The sky may appear to the naked eye to be clear when, in fact, it's
anything but. It doesn't take much in the way of haze, light
pollution, or humidity to cause objects to look fuzzy.
If you are focussing correctly, try viewing over a period of several
nights. Wait for a cold front to move through and clear out the air.
3. You may have a bad eyepiece. Can you focus on other objects, both
celestial and terrestrial?
.
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