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Subject: Re: [M]: Maximum field
From: Ric L Ecker
Reply To: mapug@shore.net
Date: Mon Jun 23 00:04:45 1997

The Orion Giant Easy Guider???? should of said: Lumicon Giant Easy
guider..............I had one and sold it to a friend, its a great
reducer for non-vignetting photography......works on f6.3 and f10 scopes,
10" and larger.

ric ecker co-pres. of OPTAS
OPTAS homepage and picture of Hale-Bopp
http://members.aol.com/rlecker/ric.html

On Sun, 22 Jun 1997 23:17:49 -0400 steve sanacore <steves@flinet.com>
writes:
>Very well said Doc;
>
>I think some of the confusion may be with the Orion Giant Easy Guider
>that fits on the larger opening on the 10" and larger SCT's. Those
>reducers will in fact increase the field of view a bit over a standard
>2" focuser. But only, as you explain, because you will be using the
>larger opening in the rear of your scope.
>
>Steve
>
>
>
>R. A. Greiner wrote:
>>
>> RP asks about maximum field. I wrote a brief note on this topic
>which
>> is attached.
>>
>> ---------------------------------------------------------------
>> Field of View - Basic Optics
>>
>> I am sure most MAPUG members already know what I am about to say.
>> However, there are some who seem to be confused about Field of View
>> and the relation of it to Eyepiece Power, the Eyepiece field of view
>> and the use of so called field reducers. I present here a way of
>> thinking about these issues that I hope will be useful.
>>
>> The actual field of view of the sky is determined only by the
>> focal length of the telescope and the diameter of the most
>restrictive
>> field stop in the optical system. The field stop is usually at or
>near
>> the real aerial image position in the system. For a telescope this
>is
>> just outside the back plate of the telescope at a position called
>> the the back focus focal plane.
>>
>> The ultimate field stop is the hole in the back plane of the
>telescope.
>> The practical field stop is that of the eyepiece. The size of this
>> field stop is limited by the diameter of the eyepiece tube.
>> On a given telescope a 2 inch eyepiece has greater maximum field of
>> view of the sky than a 1 1/4 inch because it can intercept more of
>the
>> real aerial image that the telescope objective produces.
>>
>> The purpose of the eyepiece is similar to that of a fancy magnifying
>> glass. It enables the human eye to inspect the real aerial image in
>> fine detail. The focal length of the eyepiece determines the
>> magnifying power of the whole system. A so called wide field
>eyepiece
>> is designed in such a way that for a given focal length of the
>> eyepiece the field stop can be made larger. The field stop can
>never
>> be larger than the diameter of the eyepiece tube. (obviously) The
>> larger diameter field stop of the wide field eyepiece actually
>> intercepts a larger portion of the real aerial image and thus gives
>a
>> view of a larger circle of the sky. It also gives what is called a
>> larger apparent field of view. This is the size of the circle seen
>> when one looks into the eyepiece.
>>
>> Now what this has to do with using field reducers is as follows.
>> The field reducer is placed inside the focal point of the real
>aerial
>> image and not too far from it. Thus the reducer becomes the
>effective
>> field stop and limits the size of the actual fild of view of the
>sky.
>>
>> The reducer actually moves the real aerial image forward and reduces
>> its size. This means that the field of view as observable by the
>> eyepiece is confined to the size of the now reduced real aerial
>image
>> even if the field stop of the eyepiece is much larger. So the actual
>> field of view of the sky is still limited by the focal length of the
>> telescope and the diameter, in this case, that of the focal reducer.
>>
>> Thus if you can see a certain amount of sky with an eyepiece with a
>> field stop 2 inches in diameter, you will be able to see the mearly
>> the same amount of sky with the 2 inch diameter field reducer.
>> The real arial image will simply be smaller by the amount of field
>> reduction. This is the purpose of the field reducer. It reduces
>the
>> image size in order fit more of it on the imager chip.
>>
>> There is no point of having an optical element close to the opening
>> in the rear of the telescope that is much larger than that opening.
>> You can't squeeze ten pounds of image through a five pound hole.
>>
>> Similarly, once the field of view is limited by a field stop, no
>matter
>> where it is in the optical path, you can't view more of the sky.
>That
>> is why people who make field reducers say that their reducer will
>only
>> cover a more limited field size than the original image. Note that
>when
>> the most limiting field stop is not exactly at the focal point of
>the
>> real aerial image, the will appear out of focus. The intensity will
>> fall off rather than being cut off abruptly. This effect is called
>> vingnetting.
>>
>> When you think about it, the name field reducer is not very precise.
>> It should be called an image reducer. The reducer only gives an
>> apparent shortening of the focal length and increase in the speed of
>> the telescope because it concentrates the image, makes it smaller,
>as
>> a shorter focal length would do, and at the same time puts the same
>> amount of light energy in the smaller image making it brighter (of
>> lower f number). The actual amount of energy you intercept from the
>> object (star) is limited only by the size of the objective.
>>
>> I hope this discussion helps when thinking about viewing the image
>> that the objective creates at the eyepiece end of the telescope.
>> Again, I apologize for the wordyness but I can't help trying to be
>> clear and precise. (among professors this is known as verbal
>overkill)
>>
>> Sincerely - Doc G
>