In the case of double mirrors in the market, buyers can only pass careful examination and testing to compare the quality and performance of various products. Here are a few tips to buy.
Shan quality products are usually shown in detail. For example, painting quality, shell processing accuracy, fine seam of the degree of anastomosis and so on. But don't be fooled by the frills (such as veneer, sequins, boxes and useless little accessories). Gently twist the two-mirror tube, should not feel the combination of the department or other parts have a slight loosening or shaking. When the two mirrors are separated and closed, the feel resistance is even and smooth. The two sets of focusing systems of some early two-tube mirrors are linked by a transverse plate, and if you have different binocular vision, it is not advisable to use these two-cylinder mirrors. No matter what kind of telescope, gently push its eyepiece and focus knob, should not feel any small front and rear sliding.
Shan with bright light from the end of the objective lens to look at the lens of each optical surface should not have abrasions, streaks, stains or mildew spots (thin white filiform), the objective of the interface is particularly easy to mold. Some of the objective surfaces reflect purple or amber light, indicating that the antireflection film is coated (also known as the antireflection film), which is advantageous to reduce the loss of the incident starlight. The lens that reflects bright white light has not been coated, the performance is worse. The side rotating mirror tube can check whether the internal prism is coated, then the eyepiece should be directed towards an incandescent lamp. The other optical surfaces can also be tested from the eyepiece side for silver film.
Shan the telescope toward the sky or white bright walls, so that the eyes from the eyepiece about 10 centimeters, the small spot on the eyepiece is the beam pupil. High-quality binoculars with a uniform brightness, the surrounding is very dark. If there is bright light and shadow around the pupil, as if it were square, the prism of the double mirror is not good, the more light reflected in the wall of the mirror tube, resulting in the stray light around the pupil.
Shan with binoculars to observe the distant scenery, the more vivid image. The better its optical quality. If the image looks hazy. It seems to be covered with mist, and the quality is poor. This phenomenon is relatively high in the high magnification binoculars.
Shan if you see it in a double mirror-a staggered image, as if there were ghosting on the TV screen, it shows that the optical axis of the two mirrors is not parallel. However, if this defect is not very serious, your eyes will gradually adapt to this image dislocation, so we must constantly change the target to detect. You can align the two mirrors to the target and adjust the focal length. Slowly move it away from the eye about 12 centimeters, repeatedly closed and open the eyes, should always see the same scene completely coincide. The optical axis is mostly caused by the prism offset. The prism fixed bracket of the inferior double mirror often cuts corners. The prism of vibration will shift.
Shan with binoculars to see the target of the light and dark line, such as the blue sky against the tower, canopy and the moon in the night sky, bright edges should not have red or blue edge, this is a serious performance. The degree of color difference between two mirrors is different, and the color edge is better.
The Shan view star is the most intuitive test method. If the condition is not permitted, use a distant electric light or a parked car chrome to reflect the sunlight right when the "man-made star". Use each eye with a microscopic observation and focusing to see if you can pull out a clearest image (the light star is tuned to a very fine point). When the focus knob is rotated, the stars should shrink gradually evenly from disk to point. Some binoculars are first contracted in one direction and spread upward on the other side. Telescopes with this "astigmatism" defect have trouble observing comets, nebulae, and clusters. Let a star shift from the center of the field to at least the L 2 radius, there should be no dispersion, elongation and other coma.