# Thread: 25mm eye piece vs 10mm eyepiece??

1. ## 25mm eye piece vs 10mm eyepiece??

Could someone explain something to me, I'm pretty clueless when it
comes to telescopes and eyepieces? I have a Celestron 114mm telescope
with two eyepieces, a 25mm SMA 1 1/4" and a 10mm SMA 1 1/4". They are
what came with it. Why is it, things are larger with the 10mm than
with the 25mm?

Also, I tried to look at Mars the other night and all I saw was an
orange dot (Iowa, USA 11:30pm 8/23/03). What can I expect to be able
to see with a 114mm telescope. Can I buy an eye piece that will allow
me to see more detail, or is my telescope just too small?

Thanks.

p8oust7eh+

2. ## 25mm eye piece vs 10mm eyepiece??

Thank you very much for the info.

D.

3. ## 25mm eye piece vs 10mm eyepiece??

triumpht5@yahoo.com (D. Buck) wrote in message news:<f433d5cf.0308241902.1723ea84@posting.google. com>...

The little orange dot is Mars. It looks bigger in the 10mm eyepiece
than in the 25mm eyepiece because the 10mm eyepiece gives your more
magnification. Shorter eyepiece = greater magnification.

Do a bit of math.

Your 114mm scope has an aperture of 114mm, or, approx 4.2 inches.
Rule of thumb is that the maximum magnification you can expect from
magnification will be on the order of 4.2 x 50 = 210X to 4.2 x 60 =
252X.

Consult your owner's manual and find the "focal length" of you scope.
I found the Celestron 114 on a web site and the focal length is listed
as 900mm.

Magnification equals scope focal length divided by eyepiece focal
length.

Thus, if your scope has a focal length of 900mm, and you use a 25mm
eyepiece, your magnification is 900/25 = 36X. With the 10mm eyepiece,
your magnification is 900/10 = 90X. Thus, Mars will be 2.5 times as

Assuming, then, that your maximum magnification is around 225X, then a
4mm eyepiece will get you to your maximum magnification.

However, it may not be the smartest thing to do to purchase a 4mm
eyepiece because all it will do is give you your maximum magnification
-- and when you encounter poor atmospheric conditions, you may not
want max mag. Instead, get an 8mm eyepiece and a 2X Barlow. The
Barlow multiplies the power of each eyepiece by a factor of 2. Thus,
a 25mm eyepiece with 36X, when coupled to a Barlow acts like a 13mm
eyepiece at 72X. An 8mm eyepiece will give you 900/8 = 112.5X; with a
2X Barlow, the same eyepiece will yield 225X -- about the most you

The BArlow effectively doubles the number of eyepieces you have. Your
eyepieces with a 2X Barlow will give:

25mm = 36X; with Barlow = 72X
10mm = 90X; with Barlow = 180X
And if you add an 8mm, you will get:
8mm = 112.5X; with Barlow = 225X

So -- with 25, 10, and 8 eyepieces and a 2X Barlow you will have
magnifications of 36X, 72X, 90X, 112.5X, 180X, and 225X.

I use an 8-inch Dob with 1200mm focal length. My 7mm eyepiece yields
171X, with a Barlow I get 340X from the same eyepiece. Under normal
atmspheric conditions, 340X is pushing it for Mars where I live, which
is a high-humidity area and if I try to view Mars at much above 250X,
it starts to wash out in the atmospheric haze. Folks with bigger
scopes and better atmosphere are seeing Mars at 500X and up.

If you purchase an 8mm eyepiece and a 2X Barlow, or if you buy a 4mm
eyepiece, add a set of planetary filters to your order. I have found
Mars is improved by using an orange filter (#21), light red (#23), red
(#25), and a Sirius Planetary Enhancement Filter (try HandsOn Optics
-- I ordered one from them and received in in 4 days). Mars is very
bright; the filter will knock back some of the brightness so the
brightness does not wash out the details, and, the colors in the
filter enhance or reduce certain colors so you can see surface
details.

Your 10mm eyepiece yields 90X, which is just not enough to see details
on Mars -- you need to get up around 150X.

Also, remember -- no matter how clearly your scope shows Mars, no
matter which filters you use, you WILL NOT SEE MARS THE WAY THE PHOTOS
SHOW IT. Those photos were made either from satellites that flew past
Mars or from huge, earth-based telescopes shooting long (several
hours) exposures, or from the Hubble.

You will see a round disk with the polar cap and some dark, shadowy
areas - and what you see depends on how high Mars is in the sky, the
amount of crap in the atmosphere, and the amount of light pollution at

By the way, you are welcome to post an article on this newsgroup

You may want to consider a University Optics 7mm orthoscopic eyepiece
(they don't make an 8mm). 7mm will give you 128X, with a Barlow =
256X -- which is pushing your scope but should work. The orthoscopic
eyepieces require you to get your eye right up against the eyepiece
but I have UO 9mm, 5mm, and 4mm and have had excellent results with
them on Mars. The price is reasonable.

http://www.universityoptics.com/uoep.htm#Orthoscopics

SPQR
<<<<

4. ## 25mm eye piece vs 10mm eyepiece??

> Also, remember -- no matter how clearly your scope shows Mars, no

Several hours exposure of Mars... That's a bit much, don't you think?

Jarle