Red dot finder

What is a red dot finder and how does it work?
Is it any good?
Would you recommend one?

Many thanks

Tony Pagett posted:

What is a red dot finder and how does it work?
Is it any good?
Would you recommend one?

Red-Dot finders merely put a red dot of light on a piece of glass. Here is
some information on finders in general:

FINDERS FOR ASTRONOMICAL TELESCOPES

The use of some sort of sighting instrument, or "finder" is required in
order to put celestial objects in the main telescope's relatively narrow
field of view. There are two basic types of finders: finderscopes, and
unity-power "sight" finders.
FINDERSCOPES are small low-power wide-field secondary telescopes mounted
alongside the main instrument in adjustable mounting rings for aligment with
the main telescope. They usually have an eyepiece with crosshairs for object
centering, and a wide field of view which is several times that of the lowest
power field of the main telescope. Once the finderscope properly aligned
with the main telescope, the user can put an object in the telescope's field
by merely moving the main scope around until the object of interest appears
centered in the finderscope's crosshairs. It should then be in the field of
the main telescope. Some finderscopes have a light source for low-level
illumination of the crosshairs, which can be useful on a dark sky, as long as
the illumination does not wash out the target stars. Many finderscopes use
a star diagonal to put the viewing eyepiece in a more convenient position,
although it does reverse the field, making reference to star atlases more
difficult. Some equipment manufacturers have introduced right-angle "correct
image" finderscopes with amici prisms inside their star diagonals. This
produces an upright and correct right-to-left field of view in a very
convenient viewing location.
The minimum aperture for a finderscope is a subject of some debate. The
tiny finders provided with smaller department store telescopes are almost
useless, since they have only a single-element objective lens followed by a
tiny aperture stop, cutting the effective aperture to under 10mm. Many
observers have difficulty seeing many fainter stars through finderscopes with
objectives smaller than 30mm, and for a "standard" finder, a 50mm aperture is
often recommended. As for magnification, the finder should have at least 7
power and at least a 5 degree field of view, so that objects may be located
easily. Some seasoned observers who go for very faint targets with large
telescopes will even recommend using 60mm to 80mm finders in the 8 to 15
power range, although with the larger sizes, the number of faint stars
visible with these large finders may end up confusing the inexperienced
amateur.
1X FINDERS: These are just what the name implies: finders which do not
magnify or enhance the view. They range from simple sighting tubes or
gunsight-like devices, to illuminated reticle-type systems. The simple tube
sights will get you in the ballpark, but in dark skies they can be a little
hard to use. The "reflex" sights with illumination put a small red dot on a
tilted glass plate, allowing the user to look through the plate at the sky,
see the dot and place objects on it for finding. However, the object should
be visible to the unaided eye for this to work well. Many amateur
astronomers have built their own inexpensive dot-reflex finders from simple
ones found on some gunsight systems. In some cases, these simple reflex
finders can be a little easier for the beginner to use initally than an
optical one.
One of the best known commercial 1x finders is the TELRAD, a reflex-type
finder which puts an unique three-ring reticle of adjustable brightness in
the viewer's field. These rings are 0.5 degrees, 2 degrees, and 4 degrees in
diameter on the sky, and are most useful, not just as a simple bulls-eye, but
as a "pattern maker", for star hopping. By using overlays which resemble the
Telrad's reticle pattern on star atlases, and centering the target object's
chart location in the overlay, star patterns and alignments with the Telrad
rings in the area around deep-sky objects can be worked out which can then be
duplicated by looking through the Telrad at the night sky. The Telrad rings
can also be used as measured "steps" in extended star hops. This can make
finding even faint non-naked eye objects much easier than with the simple
"dot" finders. Those people who insist on downplaying the Telrad's
usefulness based on merely using the unit only as a "bull's-eye" centering
finder like the "dot" sights do not really understand how to use a Telrad!
It is the pattern making or stepping-off field size which makes it a truly
valuable finding device for the telescope.
The Telrad is a bit bulkier than some of the simpler "dot" sights, but its
low cost (less than many telescopic finders) and simplicity have made it very
popular, especially among Dobsonian telescope users. For smaller scopes,
another popular reflex finder is the Rigel Quickfinder, which is smaller and
lighter than the Telrad. However, it only produces two rings in its window
instead of the Telrad's three, and is a bit more fragile physically.
As for which is better (Telrad Vs. Finderscopes), it really depends on
the individual's preference. Many amateur astronomers use both a finderscope
and a Telrad on their instruments for locating objects in the night sky.

David Knisely, Prairie Astronomy Club, Inc.


--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

David Knisely wrote:

Tony Pagett posted:

What is a red dot finder and how does it work?
Is it any good?
Would you recommend one?


Red-Dot finders merely put a red dot of light on a piece of glass. Here
is some information on finders in general:

[text omitted]



David Knisely:

You've answered an important question.

Your replies are rewarding of knowledge, well written,
and they are interesting to read.

Thank you.

Ralph Hertle

"Ralph Hertle" wrote in message
...
David Knisely wrote:

Tony Pagett posted:

What is a red dot finder and how does it work?
Is it any good?
Would you recommend one?


Red-Dot finders merely put a red dot of light on a piece of glass. Here
is some information on finders in general:

[text omitted]



David Knisely:

You've answered an important question.

Your replies are rewarding of knowledge, well written,
and they are interesting to read.

Thank you.

Ralph Hertle
Worth (perhaps) just adding to David's good reply (in case some people get
the wrong idea from the comment about a 'red dot on a piece of glass'), that
in general these finders don't generate the 'dot', on the bit of glass you
look though. The 'key' is that this piece of glass is acting as a partial
mirror, allowing you to see two images 'over' one another. If you look
diagonally out through a window, at a scene in the distance, when there is
something else in the room also brightly lit, you can see both objects at
once. Generally, the problem with this, is that if one object is close, and
the other a long way away, both will not be in focus at once. However you
can take a source, like a red LED, and place it spaced at the focal point of
a small lens, and if you then look into the lens, you see the dot as if it
is placed at infinity. Put this assembly below the partial mirror, and you
have the normal 'red dot' sight. The alternative is to put a glass reticule
at the same point, and then illuminate this. This is the basis of the Telrad
system. Unfortunately, to get a good focus across the larger field, tends to
require a longer working focal length, and this is why this system is not
available in the smaller sights.
In both systems, the accuracy with which the lens is positioned, affects how
well the light is focussed when you look through to partially reflective
mirror it at a far object, and also the how much 'shift' is displayed as you
move your head. The longer focal length of the Telrad, gives it more
latitude in this regard.
These systems, have moved on massively from their state a few years ago, due
largely to the popularity of this design, in shooting, where 'optical
collimator' sights, allowing both eyes to be kept open while seeing an
accurately placed target reticule, are used on both civilian and military
weapons.
As David says, the real 'power' of the Telrad, is that the calibrated rings
on the sight, allow it to be used for positioning, and even 'offset
placement', as well as simple 'aiming'.

As a seperate comment David, I look forward to hearing how you get on with
the Cape Newise scope!. A review of this system will be very interesting. I
was talking with the designer before the weekend, and he says they do use a
reasonably expensive glass combination for the corrector system, and that
they have a new 16" coming, which potentially has a slightly better overall
result, and he has an alternative design for the corrector system on the 8",
which (on paper) is better, but the difference is so small that the cost
makes it not worthwhile. I was looking at the available focus range (which
does seem to be excellent), and an initial report from a friend, says that
the stars do show really good images across a quite suprising field.
It'll be interesting to hear from somebody with a better chance of some
reasonable observing weather, how the design 'holds up'. :-)

Best Wishes

Roger Hamlett asked:

As a seperate comment David, I look forward to hearing how you get on with
the Cape Newise scope!. A review of this system will be very interesting.

The first instrument (the 8 inch version) unfortunately arrived with some real
problems due to the "kind" efforts of UPS (jolted *wildly* out of
collimation). Despite some extensive attemps at realignment (on both sides of
the Atlantic), mechanically, things were just too far gone to do much. The
"gorillas" of UPS had won the battle, so I had to send the instrument back
(via FedEx this time). Peter Wise is sending me a new one soon (via a
different courier), so hopefully I will be able to get a review of it done
sometime this spring (monitor the CloudyNights web site, as that is where the
review will appear). Right now, I am reviewing Celestron's C9.25GPS, and that
will be taking up most of my time. Clear skies to you.
--
David W. Knisely
Prairie Astronomy Club: http://www.prairieastronomyclub.org
Hyde Memorial Observatory: http://www.hydeobservatory.info/

**********************************************
* Attend the 11th Annual NEBRASKA STAR PARTY *
* July 18-23, 2004, Merritt Reservoir *
* http://www.NebraskaStarParty.org *
**********************************************

"David Knisely" wrote in message
.. .
Roger Hamlett asked:

As a seperate comment David, I look forward to hearing how you get on
with
the Cape Newise scope!. A review of this system will be very
interesting.

The first instrument (the 8 inch version) unfortunately arrived with some
real
problems due to the "kind" efforts of UPS (jolted *wildly* out of
collimation). Despite some extensive attemps at realignment (on both
sides of
the Atlantic), mechanically, things were just too far gone to do much.
The
"gorillas" of UPS had won the battle, so I had to send the instrument back
(via FedEx this time). Peter Wise is sending me a new one soon (via a
different courier), so hopefully I will be able to get a review of it done
sometime this spring (monitor the CloudyNights web site, as that is where
the
review will appear). Right now, I am reviewing Celestron's C9.25GPS, and
that
will be taking up most of my time. Clear skies to you.
Curses on the courier!.
Given the difference in seeing opportunities compared to the UK (sky, that's
that grey thing isn't it mummy...), it'll be interesting to see the review
when done. :-)
I'll hope that FedEx does better, and look forward to the review.

Best Wishes

Right now, I am reviewing Celestron's C9.25GPS, and that will be
taking up most of my time.

Let me know if you need any help with that!

DaveL

caution should be used, with any red dot finder when traveling to India.

deactivate the heads up display feature, prior to powering up



I kill me