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1. Heliometer and Solarimeter
Solar Radiation

2.  Heliometers
a refracting telescope with a split objective lens, used for finding the
angular distance between two stars.
A heliometer (from Greek ἥλιος hḗlios "sun" and measure) is an
instrument originally designed for measuring the variation of the sun's
diameter at different seasons of the year, but applied now to the
modern form of the instrument which is capable of much wider use.
The basic concept is to introduce a split element into a telescope's
optical path so as to produce a double image. If one element is moved
using a screw micrometer, precise angle measurements can be made.
The simplest arrangement is to split the object lens in half, with one
half fixed and the other attached to the micrometer screw and slid
along the cut diameter. To measure the diameter of the sun, for
example, the micrometer is first adjusted so that the two images of
the solar disk coincide (the "zero" position where the split elements
form essentially a single element). The micrometer is then adjusted so
that diametrically opposite sides of the two images of the solar disk
just touch each other. The difference in the two micrometer readings
so obtained is the (angular) diameter of the sun. Similarly, a precise
measurement of the apparent separation between two nearby
stars, A and B, is made by first superimposing the two images of the
stars and then adjusting the double image so that star A in one image
coincides with star B in the other. The difference in the two
micrometer readings so obtained is the apparent separation or angular
distance between the two stars.

3. The first application of the divided object-glass and the employment of
double images in astronomical measures is due to Servington Savary from
Exeter in 1743. Pierre Bouguer, in 1748, originated the true conception of
measurement by double image without the auxiliary aid of
a filarmicrometer, that is by changing the distance between two object-
glasses of equal focus. John Dollond, in 1754, combined Savary's idea of
the divided object-glass with Bouguer's method of measurement,
resulting in the construction of the first really practical heliometers. As
far as we can ascertain, Joseph von Fraunhofer, some time not long
before 1820, constructed the first heliometer with an achromatic divided
object-glass, i.e. the first heliometer of the modern type.
The first successful measurements of stellar parallax (to determine the
distance to a star) were made by Friedrich Bessel in 1838 for the
star 61 Cygni using a Fraunhofer heliometer. This was the 6.2-inch
(157.5 mm) aperture Fraunhofer heliometer at Königsberg Observatory
built by Joseph von Fraunhofer's firm, though he did not live to see it
delivered to Bessel. Although the heliometer was difficult to use, it had
certain advantages for Bessel including a wider field of view compared to
other great refractors of the period, and overcame atmospheric
turbulence in measurements compared to a filar micrometer.

4. Heliometer, astronomical instrument often used to measure the Sun’s diameter and, more generally, angular
distances on the sky The heliometer consists of a telescope in which the objective lens is cut along its
diameter into two halves that can be moved independently. This produces two separate images of an object.
In the case of two stars, the distance the lenses must be moved in order to superimpose the two images
together can be used to derive their angular separation. In the case of the Sun, the distance at which the
two images of the Sun touch can be used to derive its diameter.
 Solarimeter
A solarimeter is a pyranometer, a type of measuring device used to
measure combined direct and diffuse solar radiation. An integrating
solarimeter measures energy developed from solar radiation based on
the absorption of heat by a black body. The principle this instrument
was designed on was first developed by the Italian priest,
Father Angelo Bellani. He invented the actinometric method which is
based on physical and chemical techniques.
A modern pyranometer, shown here is model SR20
The instrument consists of a solarimeter fitted with a hemisphere of
heat-absorbing glass. By this means, a well-tried instrument, already
widely used in meteorological stations for the measurement of total
incident radiation, is converted into an instrument suitable for
measuring photosynthetically active radiation. The good angular
response is preserved by using a hemispherical filter.

5. With a commercial electronic microvoltmeter the instrument is sensitive enough for
use in phytotrons, and if photosynthetically active radiation is assumed to be
bounded by the wavelengths 0.4 and 0.7μ, calculations show that it compares
irradiances of common light sources with daylight, in these units, with an error of
not more than 20%, reducible to 10% with an improved filter glass. Arguments are
presented for specifying and measuring light directly in such units, here called
“plantwatts/m2”, rather than using illumination units and a conversion factor for
each light source. Daylight measurements with a filtered and an unfiltered
solarimeter over several months have shown that the number of plantwatts/m2 is
not a constant proportion of the total irradiance but varies (in one locality) from 48
to 65%. Moreover, the variation is systematic, the highest proportions occurring
during the dullest weather, presumably because water vapour is the chief absorber
involved. A systematic error would, therefore, be introduced by measuring with an
unfiltered solarimeter and assuming that the proportion of photosynthetically active
radiation in daylight is constant.

6. Thank you
By
Ravi Pramod Surisetty
319106101024