In astrophotography related mainly to large bodies of solar system: sun, moon, planets, it is useful to have an accessory, the Barlow, extremely useful, because you can significantly increase the size of the object on the CCD detector or other sensor.
The Barlow lens is used to increase the focal length of primary education, is a diverging lens. Its use, however it is generally closely related to three basic conditions: 1) well-collimated optics
2) Adequate resolution of the sensor.
3) Good transparency atmospheric conditions (seeing).
For the first two points, we can always intervene direttamente per migliorare, mentre per il terzo punto, purtroppo, dobbiamo affidarci a quelle che sono le condizioni di turbolenza del nostro sito di osservazione, ed attendere il momento in cui l'oggetto e la trasparenza atmosferica sono nelle migliori condizioni per le riprese.
Devo dire che nel mio caso, molto vicino al livello del mare, tali condizioni concomitanti sono piuttosto difficili da realizzare, ed ho stimato, sulla base di ricerche specifiche, che tali situazioni possono verificarsi non più di 15 - 20 volte ogni anno. E' facile intuire che riuscire a riprendere dettagli fini ad esempio dei pianeti maggiori, non è una impresa facile.
Per evitare tali incovenienti è possibile Download all car sites and reach the highest altitude (700-900 meters), but this is not always possible in case of heavy equipment and in the absence of someone who can give us a help.
So you will need to obtain an adequate patience and in most of the comments seek a compromise between weather conditions and the size of the object you want to achieve.
The first experiences in 2009 I made predominantly predominantly with the planet Jupiter
visible almost throughout the summer in the south-southeast . The results were encouraging, but I think we can still improve .
Let's see how a Barlow .
The left figure presents a schematic.
goal
A = B = barlow
Po = original distance
Pn = new distance
Two equations describe the geometrical optics:
1/Fb = 1/Po - 1/Pn (1)
Pn Po Fb = (Pn-Po) (2)
where Fb = focal length of Barlow
The multiplicative factor of R Barlow lens is expressed by:
R = Pn / Po (3)
by (2) gives:
Pn = Fb (1 - R) (4)
from which we get the multiplication factor R as a function of distance and the focal length of the PN Barlow:
R = 1 + Pn / Fb (5)
Without further complications, we have the necessary elements for our calculations. We recognize that if the two distances Pn and Po are the same, we are the primary focus, then no optical element and without any focal aggiutivo result. Therefore, an essential condition is that R must be greater than one (R> 1)
Now suppose you have a telescope with a focal length of 2000 mm. to F10 and a Barlow lens whose focal length is 45 mm. and you want to obtain a multiplication factor equal to 3x the distance at which Pn we have to ask for this factor?
From (5) we obtain: 3 = 1 + Pn/45 where Pn = 2 * 45 = 90 mm. Or our positioning sensor (focal plane) to 90 mm. barlow back of the lens will increase the focus of our primary instrument of 3 times, then you have 2000 * 3 = 6000 mm. to F30.
I use a good Barlow lens, the Baader Planetarium, with click-clock system, a perfect centering on-axis, modular, with accessories that allow you to vary the R-factor of 2x to 4x, depending on the configuration.
Its focal length is 66.67 mm.
In the standard configuration as shown, we get an R factor equal to 2x. In practice, when attached to the frog, for example 31.8 of the CCD, the system click-clock, should be taken into account that the sensor plane turns out to be a few mm further. In the calculations, as far as the sensor II DSI Pro B / W, which I make good use, you must add 25.5 mm. And, therefore, in its standard configuration that Barlow, is have an R factor equal to 2.38X.
With extension tubes with different lengths you can get all the intermediate factors R up to 4x. In this way, everyone can make best use of this barlow according to his needs and in relation to optical instruments in use.
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