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Bill gave me two prizmatic scopes for a weekend
to test. I decided to make experiments with IC
correction using various scopes and various filter combinations.
The scopes used were the following:

Scope A, prizmatic, apparently x6, 30mm
Scope B, prizmatic, apparently x7 or 8x, 30 mm

Invereting x6 or x7, 30mm
Galileo x3 or x4, 40mm (Called "Star scope" in the manual).
(These two are the two native SNO-T scopes of my own).

First I include some general remarks of the prizmatic scopes A and B
tested and then the table of results.

1. Scope B is completely out of collimation, by more than one degree.
Given that collimation on this type of scopes is not adjustable,
it can be hardly used for anything but index error tests.
(See 5 below).
2. Scopes A and B are both very heavy, B heavier than A.
My hand gets tired, and the stars tremble.
3. They are not equipped with rubber eyepiece covers, so there is
a lot of glare, especially in the scope B which has a larger diameter eyepiece.
4. The field of view of B is about the same as that of the invereting one,
but only very central part of it is usable: the picture at the
edge is very poor. The field of view of A is somewhat smaller, and the
picture is good at the edges.
5. Both A and B are poorly made. I can imagine such scopes
could be made on board of a yacht for emergency,
by cutting some cheap binocular into two pieces.
The worst part is the fork and its attachment. It is crudely attached
with screws of unappropriate size, with unappropriate heads,
protruding far inside the scope. It is actually quite surprizing
that collimation of the scope A is within the reasonable limit.
(See remark 1). The fork edges are covered with thick paint...

Now I come to the test results. I denote by R,Y,G the filters.
The index mirror has 4 filters: the darkest is Red, then
dark Yellow, light yellow, light Grey.
The horizon mirror has 3: dark Yellow, light yellow and light Grey.
In "filter combination" I write the horizon filter first.

April 14 IC from stars.
Scope     No. of obs. Sigma   IC

Invert.       7        0.2   -0.2
Invert.       8        0.2   -0.1
Scope A       8        0.2   -0.2
Scope A       7        0.2   -0.2
Scope B       8        0.1   -0.2
Scope B       5        0.1   -0.2
Galileo       5        0.1   -0.1

April 14 IC from Sun True 4SD=64.0
Scope   Filters   No obs       4SD   sigma  IC

Scope A   Y+Y        5        63.7    0.2   -0.6 (very low Sun)
Scope B   Y+Y        5        63.2    discarded
Invert    Y+Y        5        63.5    discarded

April 15 IC from Sun, True 4SD=64.0
Scope   Filters   No of obs    4SD   sigma  IC

Invert    Y+Y        5        64.4    0.4   -0.3 (?)
Invert    Y+Y        5        64.5    discard
Invert    Y+R        5        64.2    0.3   -0.1
Invert    YG+RG      5        64.4    0.1   -0.1 (?)
Invert    YG+YG      5        64.4    0.1   -0.1 (?)
Scope A   YG+YG      8        63.8    0.2   -0.3

April 16 IC from Sun True 4SD=64.0
Scope   Filters   No of obs.   4SD   sigma  IC

Invert  YG+RG         5        63.9   0.1   -0.2
ScopeA  YG+RG         5        63.8   0.1   -0.2
ScopeB  YG+RG         5        64.0   0.1   -0.1

Remarks. Observations with |4SD-true.4SD|>.4 are discarded
as SNO-T manual recommends. The boundary cases have question mark.
On April 14 I started to suspect that my Yellow Index filter has
some prizmaticity.

Conclusion about scopes. Scope B gives slightly better results
in IC because of its higher magnification. If I had to choose
between A and B, I would choose A. But fortunately, I don't
have to choose because I have a much superior inverting scope,
which is:
a) well made,
b) looks nice, the screws do not protrude everywhere,
   have appropriate shape and matching color.
c) is light, and
d) has collimation adjustment.
And the price of an old SNO-M with such scope soon will be
comparable to the price of a prizmatic scope:-)

When you buy a scope without collimation adjustment you always take
a bet that the fork will match your sextant exactly and the
collimation error bill be small. You can do nothing about it except
hummering the fork:-) With such crude forks as I see on these scopes
such precise match is unlikely.

Other conclusions: careful choice of filter combinations matters much.

Alex.

   

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