Need to know what certain parts are on a telescope are

Bought a little starter telescope recently. It's a Newtonian
reflector-style scope. Anyways, it's got two eyepieces included. One is
called a "1.5x erecting eyepiece", while the other one is called a "3x
Barlow lens". What is each used for, and when?

Second, the kit seems to come with a little green filter that screws
into the various eyepieces. What type of filter is this?

Yousuf Khan

On Aug 23, 5:15*pm, Yousuf Khan wrote:
Bought a little starter telescope recently. It's a Newtonian
reflector-style scope. Anyways, it's got two eyepieces included. One is
called a "1.5x erecting eyepiece", while the other one is called a "3x
Barlow lens". What is each used for, and when?

Second, the kit seems to come with a little green filter that screws
into the various eyepieces. What type of filter is this?

* * * * Yousuf Khan

1) An erecting eyepiece is designed for terrestrial viewing.

(Most astronomical telescopes turn things upside down)

2) A Barlow lens is an image magnifier.

3x larger may be too much for most telescopes.

Since it magnifies errors of alignment, mounting vibration,
atmospheric problems, optical inaccuracy, reduces field of view,
increases difficulty of pointing and greatly reduces the brightness of
the final image. The "fuzzy blob syndrome" is typical of inexpensive
telescopes and over-ambitious beginners.

3) The green filter may be useful on the moon and planets to bring out
detail by increasing contrast.

The filter should NEVER be used to view the Sun as this is VERY
dangerous. Due to the concentrated heat in the light beam coming out
of the eyepiece! The eye has no pain sensors for stopping the
instantly blinding light from entering the eye.

On Aug 23, 9:15*am, Yousuf Khan wrote:
Bought a little starter telescope recently. It's a Newtonian
reflector-style scope. Anyways, it's got two eyepieces included. One is
called a "1.5x erecting eyepiece", while the other one is called a "3x
Barlow lens". What is each used for, and when?

The Barlow lens, if it is a Barlow lens, isn't an eyepiece.

So what you have is one eyepiece, the "1.5x erecting eyepiece", which
you will have to use for all your viewing, astronomical as well as
terrestrial, even though in general erecting eyepieces are not used
for astronomical use - because the extra lens degrades performance
slightly, and is not really needed for astronomical viewing, in which
it doesn't matter if things are upside-down.

The 3x Barlow lens turns the 1.5x eyepiece into a 4.5x eyepiece by
being put between it and the telescope. It's basically equivalent to
the telenegative lenses that are sometimes put between a lens and an
SLR body to cheaply give one an additional set of telephoto lenses.

While high magnifications and cheap telescopes don't mix, it's not
clear that a 4.5x combination will provide your telescope with an
excessively high magnification.

John Savard

"Yousuf Khan" wrote in message
...
Bought a little starter telescope recently. It's a Newtonian
reflector-style scope. Anyways, it's got two eyepieces included. One is
called a "1.5x erecting eyepiece", while the other one is called a "3x
Barlow lens". What is each used for, and when?

Second, the kit seems to come with a little green filter that screws into
the various eyepieces. What type of filter is this?

Yousuf Khan

Does you "1.5X erecting eyepiece" look like this:
http://www.skywatcher.com/swtinc/pro...1=5&class2=502
If so, it is not actually an eyepiece. It is an erecting prism that rotates
the upside down image, and it also increases the magnification by a factor
of 1.5. You still need an actual eyepiece. You will likely only need to
use the erecting prism if you want to look at birds, boats and other
terrestrial objects.

Likewise, the Barlow is not an eyepiece. It is used together with an
eyepiece to increase magnification. You probably will never want to use it.
The magnification is determined by the telescope focal length divided by the
eyepiece focal length. The quality of the eyepiece is perhaps even more
important than the quality of the telescope. The are many optical designs
for eyepieces, Kellner, orthoscopic, Plossl, Nagler etc. You can read about
the pros and cons of each. The important features of an eyepiece are low
aberration on and off-axis, large apparent field of view, and long eye
relief.

I would start with an eyepiece of around 25 mm f.l., eventually you might
want others in the range of 36 to 9 mm. You need to determine what size
eyepiece you need (the barrel diameter). Most small astronmical telescopes
use 1.25", but some use 0.965". A good place for inexpensive eyepieces is
he
http://www.surplusshed.com/pages/cat...cessets_1.html

On 23/08/2012 7:23 PM, anorton wrote:

"Yousuf Khan" wrote in message
...
Bought a little starter telescope recently. It's a Newtonian
reflector-style scope. Anyways, it's got two eyepieces included. One
is called a "1.5x erecting eyepiece", while the other one is called a
"3x Barlow lens". What is each used for, and when?

Second, the kit seems to come with a little green filter that screws
into the various eyepieces. What type of filter is this?

Yousuf Khan

Does you "1.5X erecting eyepiece" look like this:
http://www.skywatcher.com/swtinc/pro...1=5&class2=502
If so, it is not actually an eyepiece. It is an erecting prism that
rotates the upside down image, and it also increases the magnification
by a factor of 1.5. You still need an actual eyepiece. You will likely
only need to use the erecting prism if you want to look at birds, boats
and other terrestrial objects.

Yes it does look like that, though not quite as nice, more plasticky.
And yes, there are actual eyepieces included with it, separately, but I
knew what those are for, I wasn't aware of what this thing was for.

Yousuf Khan

On 23/08/2012 1:29 PM, Quadibloc wrote:
On Aug 23, 9:15 am, Yousuf wrote:
Bought a little starter telescope recently. It's a Newtonian
reflector-style scope. Anyways, it's got two eyepieces included. One is
called a "1.5x erecting eyepiece", while the other one is called a "3x
Barlow lens". What is each used for, and when?

The Barlow lens, if it is a Barlow lens, isn't an eyepiece.

So what you have is one eyepiece, the "1.5x erecting eyepiece", which
you will have to use for all your viewing, astronomical as well as
terrestrial, even though in general erecting eyepieces are not used
for astronomical use - because the extra lens degrades performance
slightly, and is not really needed for astronomical viewing, in which
it doesn't matter if things are upside-down.

The 3x Barlow lens turns the 1.5x eyepiece into a 4.5x eyepiece by
being put between it and the telescope. It's basically equivalent to
the telenegative lenses that are sometimes put between a lens and an
SLR body to cheaply give one an additional set of telephoto lenses.

Does the Barlow lens go first on the telescope body, or the erecting
eyepiece first? I assume that they get stacked in series with each other.

While high magnifications and cheap telescopes don't mix, it's not
clear that a 4.5x combination will provide your telescope with an
excessively high magnification.

Why would it be excessive magnification?

Yousuf Khan

On 23/08/2012 12:38 PM, Chris.B wrote:
1) An erecting eyepiece is designed for terrestrial viewing.

(Most astronomical telescopes turn things upside down)

Thanks.

2) A Barlow lens is an image magnifier.

3x larger may be too much for most telescopes.

Since it magnifies errors of alignment, mounting vibration,
atmospheric problems, optical inaccuracy, reduces field of view,
increases difficulty of pointing and greatly reduces the brightness of
the final image. The "fuzzy blob syndrome" is typical of inexpensive
telescopes and over-ambitious beginners.

So the Barlow lens would be overkill?


3) The green filter may be useful on the moon and planets to bring out
detail by increasing contrast.

Okay thanks. So it's not something special like an h-alpha filter right?

Yousuf Khan

On Aug 23, 5:51*pm, Yousuf Khan wrote:

Yes it does look like that, though not quite as nice, more plasticky.
And yes, there are actual eyepieces included with it, separately, but I
knew what those are for, I wasn't aware of what this thing was for.

Oh! So that thing isn't an eyepiece either, it's an erecting lens. So,
like the Barlow lens, you put it between the lens and your eyepiece to
gain additional magnification (thus you have basically a larger
selection of eyepieces this way).

If you have also more than one real eyepiece, of course you will
notice that the ones with shorter focal lengths also have a smaller
area for your eye to look into. So using an eyepiece with a longer
focal length with a Barlow lens provides greater eye relief - greater
comfort in looking through a bigger eyepiece - for the same
magnification.

John Savard

On 2012-08-23, Yousuf Khan wrote:

3) The green filter may be useful on the moon and planets to bring out
detail by increasing contrast.

Okay thanks. So it's not something special like an h-alpha filter right?

No. The hydrogen alpha wavelength is red. H-alpha filters cost hundreds,
if not thousands, of dollars.

Bud

On 24/08/2012 03:09, William Hamblen wrote:
On 2012-08-23, Yousuf Khan wrote:

3) The green filter may be useful on the moon and planets to bring out
detail by increasing contrast.

Okay thanks. So it's not something special like an h-alpha filter right?

No it is just a piece of boring green coloured glass. Removes colour
fringes from your image which might help on high contrast moon views.

No. The hydrogen alpha wavelength is red. H-alpha filters cost hundreds,
if not thousands, of dollars.

A deep sky H-alpha filter (not solar) can be had for about £100. eg

http://www.telescopehouse.com/acatal...er_1_25__.html

Notch filter technology pricing has improved enormously with modern
vacuum coating deposition techniques. Not much good visually but helpful
for CCD imaging since it cuts down skyglow a lot.

A solar H-alpha is a much more expensive and exotic affair but its
passband of 0.7A or so is 10x smaller than the deepsky. They also have
to be used with a red glass energy filter on the input to the scope.

Regards,
Martin Brown