Christiani Hugenii Const. F. Astroscopia Compendiaria, Tubi Optici molimine liberata

Having thus removed this inconvenience, I noticed that something else remained which might make this method of ours for observing more perfect. Although neglecting it would generally do no harm, the curious examiner of the stars should by no means overlook it. Namely, when I was searching more diligently for those Cassinian discovered by Giovanni Domenico Cassini companions of Saturn, and having difficulty tracking them, especially on nights that were not entirely dark, I understood the cause to be a certain faint light emanating from the air toward the eye; not the light that enters through the larger lens, but that which slips in from outside around the sides. I knew that it would certainly contribute somewhat to excluding this annoying little light if I were to place that circular piece of paper, which I used for observing the moon, around the larger lens here as well. But it seemed better to add another more effective remedy to this, namely, that the pupil be narrowed by the opposition of a perforated plate. As soon as I experimented with this, I clearly beheld the three little moons of Saturn, whereas, with the small hole removed, only that middle one of ours could be discerned. Nor should it be thought that the view is obscured even in the least by this contraction of the pupil, if the method is applied as will now be told.

Because the intended star is less easily investigated with a reduced pupil than when it is fully open, we have attached that perforated disk—half an inch wide—by means of a small movable arm, similar to the Greek letter A Alpha, to the bottom of the small tube through which the ocular lens is viewed, and which is open through a wider hole, so that the other, narrower one is not brought over it until the star has been found through this hole. These things would be clearer with the described figures, which we do not have engraved now; yet they will be understood by the industrious reader even without them. Moreover, if the diameter of the small hole holds the same ratio to the diameter of the aperture of the larger lens as the distances of their respective focal points have to each other, it is necessary that everything be seen no less clearly through such a telescope than if the eye were left open and free. But it is better to double this small width, or even increase it a little more, so that the search for the object to be viewed becomes less difficult.

In our telescope, 34 feet long, the diameter of the small hole is about one-sixteenth of an inch; it is located two and a half inches from the ocular lens, which is precisely the focal distance in this lens. This must be diligently attended to, because otherwise a large space cannot be encompassed in one view, as is customary. A circle surrounding the large lens should be made double or triple the size of the aperture of the lens itself.

CHRI