Re: ATM 2" vs. 1,25" barrel. + an eyepiece primer

[Lynne and Larry Manuel <lynneandlarry.manuel@sympatico.ca>]

N. Foldager wrote:
> 
> If I obtain a 27mm Panoptic (2") and a 7mm Nagler (1,25"), wouldn't I
> get the problem that the 27mm cannot be adapted to a 1,25 inch barrel?
> 

If you could make an adapter [they are not commercially available] to
attach the 27mm Panoptic to a 1.25 inch focuser, it would work, and the
view would be dim around the edges.  Such dimness around the edge of
field of view is called vignetting,  Also, you can imagine that the
whole assembly is going to have to stick out quite far, since a 2 inch
adapter cannot go inside a 1.25 inch focuser, while a 1.25 inch adapter
_CAN_ go inside a 2 inch focuser.  For an eyepiece to focus so far back,
the telescope must be built with sufficient ‘back focus’ distance. 
Enough back focus is usually available in telescopes other than
Newtonians with low profile focusers.


You may not be very familiar with eyepiece technology - and
terminology.  I suggest you read the Tele Vue web page information on
basic telescope and eyepieces information - several times over a period
of weeks.  There is a lot of good stuff there.

> Would the 27mm not have the same field of view (68 deg) if it where
> made for a 1,25 barrel?

No, it would not have the same true or apparent field of view.

I suggest you become very familiar with the following terms:

- field stop diameter

- apparent field of view

- true field of view


> If yes, what then is the benefits of 2 inch barrels?

The benefit is:  ability to have a larger field stop diameter, and
therefore a larger true field of view.

The apparent field of view varies with the eyepiece type or design. 
Example:  All Panoptics have a 68 degree apparent field of view.  This
is the angle of the circle of view you see when you look through the
eyepiece [even when it is not in a telescope].

All Nagler type eyepieces have an apparent field of view of 82 degrees. 
Tele Vue Plossls are 50 degrees.

In any given telescope, the field stop diameter determines the true
field of view.  This is how much of the sky you can see when the
eyepiece is in a telescope.  The eyepiece focal length determines how
large something will appear in that filed of view.

Recycled letter follows:
....................
Divide your telescope's focal length [in mm] into 3,437.  This will give
you the IMAGE SCALE, in arc minutes of sky you will see per mm of field
stop diameter.  To convert the image scale to degrees per mm, divide by
60 - there are 60 arc minutes per degree.

Example:

6 inch f/8 scope, focal length = 48 inches or 48 x 25.4 mm = 1,219 mm.

Image scale = 3,437 / 1,219 = 2.82 arc minutes per mm. 

OR: 0.047 degrees per mm.

Now consider a 22mm Panoptic, with field stop 25mm in diameter.

The true field of view is: 25 x 0.047 = 1.17 degrees.

field stop data available at:

http://www.rahul.net/resource/regular/products/tele-vue-optics/eyepiece.htm

....................

Summary:

Try thinking of it this way:

Assume a given field stop diameter in an eyepiece.  The design of the
eyepiece [or type] we will vary - at our whim.

Consider the field stop to be a circular window.  If we sit quite far
back from the window, we are using a Plossl.  Up quite close, we are
using a Nagler.  Somewhere in between, we are using Panoptic.  When we
sit closer to the window, we can se things appearing larger - but we
don’t see any more sky.  The window diameter limits our actual view of
the sky.

Note:  The analogy isn’t perfect here, because everyone knows that if
you do sit closer to a window, you will always see more of what’s beyond
the window.

Now, if we need to see more sky, we need to install a bigger window -
our field stop diameter.  Up to a point, we can increase the field stop
diameter and stay within 1.25 inch barrels, after that we change to 2
inch barrels.

Some justification for the high cost of fancy eyepieces:

Assume three eyepieces of the same focal length:_ 15 mm.   Each will
show any particular object at the same apparent size.  The more
expensive ones will show more of each object or more objects at the same
time.

Type 1 has 50 degree apparent field of view, and costs $150.00.

Type 2 has 68 degree apparent field of view, and costs $300.00.

Type 3 has 82 degree apparent field of view, and costs $450.00.

Note that the true area of view is proportional to the square of the
apparent field of view.  We are considering areas, not diameters of
circles for sky area - which is critical for finding objects while
sweeping or star hopping.

Type 2 has 1.85 times the sky area visible as type 1.  Its cost per unit
area of sky is only 1.08 that of type 1.

Type 3 has 2.69 times the sky area visible as type 1.  Its cost per unit
area of sky is only 1.12 that of type 1.

Now consider that the Types 2 and 3 are well corrected at the edge of
field of view at f/4, and the Type 1 exhibits gross astigmatism of star
images at the edge of the field at even f/5.  We begin to see why, for
some people, the exotic eyepieces are not a bad value [while still
expensive].

I hope this helps.

Larry.


-- 
lynneandlarry.manuel@sympatico.ca