LENSES IN GENERAL
OPTICAL BENCH KITS
Several brands of telescopes are labeled as "50 mm objective lens" when the actual working objective lens is stopped down to less than half the label diameter. The problem is more prevalent on the larger lenses (50mm, 2 inches) but occurs on even 25 and 30 mm lenses. All are Chinese made and the iris may not have been in the original design or even specified by the importer. Chinese manufacturers are famous for unilaterally skimping on quality after the contract for manufacture and delivery is signed.
Look into the objective (large) end for a an opaque disk with a rather small hole placed just behind the objective lens. Obviously, only the smaller section of the lens is functional. Sometimes the disk is further inside the telescope tube and harder to see. A simple test is to look into the objective (large) lens and observe the disk of light at the eyepiece. Swing the telescope and notice whether the light disk can be seen at the edge of the objective lens. If not, that is, it is occluded part way along a radius, then there is a capability robbing diaphragm somewhere in the optical path. This is done to be able to use a piece of junk for the objective lens.
Aperture disks may be in the telescope for very good functional reasons without obscuring any of the image rays. Disks are properly placed to block reflections and scatter from the sides of the tube. These rays are of no use anyway and their removal is an enhancement. They always have an opening that is most of the disk diameter and are never placed near the objective lens.
There are some surprisingly good inexpensive telescopes available. One adequate 15 power scope is only $10. Usually, and for a very few dollars more, one can get a very good scope from camera, nautical, birder, hobby, science specialty, etc stores. Take care in the "upscale toy stores". They try to buy the best, but rarely have the expertise to know the difference. If you find one of these frauds, bring it to the attention of the store manager. Better yet, also to the store's home office, your State Attorney General's and/or the Federal Trade Commission's attention..
Remember high power is not truly required. Light gathering power is, and the junk described has an image dimmer than seen by the unaided eye. Galileo made his discoveries with telescopes of 3, 8, and 30 power.
Telescopes have two purposes. 1. to make objects appear closer than they actually are, thus improving the view of details, and 2. to make objects as bright or brighter than seen with the unaided eye.
The large objective lens is to gather more light than the eye, and the magnifier ocular lens(easy) are to produce an image larger than direct viewing. These two work at cross purposes. Magnification spreads the available light over a larger area than unmagnified, thus, the brightness is diminished with increasing magnification. A small objective lens (actual working diameter) immediately precludes a large magnification. Yet many small diameter telescopes are sold with magnifications of 30 to 50 power. The approximate formula for the brightness utility improvement criteria is (D*D/P*P)*(D*D/P*P)/16 > 1 [diameter in mm]. This says that the magnification of a telescope with a working 50 mm lens should be no more than 12. More simply, the lower limit ratio of the objective lens in Millimeters to the magnification (no units for magnification) should be more than four. . Of course, brightness loss is quite tolerable in most daylight terrestrial uses and for bright celestial objects like the moon, Venus, Saturn, etc.
A simple in-store test is to focus the telescope on some distant object with distinct outline and good contrast and compare the telescope image with the unaided eye image. If it looks better unaided, the telescope is junk. If possible look at the object simultaneously directly with one eye, and through the telescope with the other. If there is some kind of regular ruling on the object, the power of the telescope can also be evaluated for veracity of claim.
Always test a telescope before buying. Some makers have the audacity to sell inverting telescopes for children. Ok for astronomical use, but what kid looks only at the stars.??
The two most offensive brands of telescopes are one from an old line Japanese brand once known for quality binoculars and cameras; The tube is red; and the other carries the brand name of a nation-wide ecology do-good organization; the tube is white. Someone else imports and distributes it while sucking off the good name of the organization. Even so, it makes one wonder about the integrity or reliability of the organization and/or its opinions on anything. Especially so when the environmental movement is the center of unquestioning, no-think, accepting and promulgation of junk science, faulty reasoning, and vast conclusions made with half-vast information; AND any questioning of method, finding, or conclusion results in career destroying personal attack on the critic.
Except for cheep, flimsy microscope bodies and inappropriate illumination-lamp selection, microscopes are not so gregariously bad as to be fraudulent. Watch for the power. A beginner's microscope should have powers of 10-150. Any more is of no value. The lower powers, about 50 can see all kinds of interesting things, especially the wigglies in pond water, aquarium filters, or hay infusions. Any more power and the adjustment is too difficult for beginners, lighting requirements are too difficult, and staining may be required. Staining is a skill for older children, but is wonderful when they can do it. I have never seen a 900 power setting that worked adequately.
A 50-100 power microscope should be able to be used in room lighting or window day light. If possible test it. Also test it for top lighting. Some have such small objective lenses that they cannot get enough light for top light.
One Chinese fraud is to put a 1 or 2 volt lamp into the lamp socket, but use
two batteries totaling 3 volts. Lamps last about 10 minutes, a little longer
if rechargables are used as they are lower voltage than dry cells, 1.2 volts
each. I doubt that the importer knows this. Again it was in the famous
Japanese optics company's microscopes where I first discovered this
Note -- this same company had been producing a fine line of telescopes and microscopes until something went wrong. Not sure now, but I suspect they stopped making their product where there is some integrity and they could monitor the process (like in Japan) and went to China (el cheapo??). The products I am describing herein are the same model numbers as the good ones, and were on the shelves at the same time.
LENSES IN GENERAL:
Hand lenses come in superb to junk quality. Even some low cost toys have almost perfect lenses.
It is easy to test individual lenses.
Simply hold the lens at about 18-24 inches from your eye, and look at anything with a straight line. Adjust the distances so the line is in focus.
Move the lens so the image of the line swings from one side of the lens to the other. If the line bends, the lens is a reject. If the line breaks, the lens is so bad the store manager should be shown what crap he is selling.
He should thank you.
A similar test can be done in a telescope by watching the light disk described above. Also, sometimes the surfaces of the objective lens reflect enough to make a similar test. Best if you can remove the lens and test it separately.
OPTICAL BENCH KIT: There is an optical bench kit on the market titled "Shiney". It contains a set of lenses, some filters and other optical elements, a strand of fiber optics, a flashlight, a very good instruction manual, lens holder and bench assembly.
At first, it appears very attractive as a teaching kit, and it would be if the lenses were not such CHINESE CRAP. The lenses have no useful, functioning portion. Extreme distortion. In other words, the kit is very limited in its use, certainly not as intended or what the buyer expects.
The companion kits "Slimey" and "Shocking", are very good, and I suspect
"Stinkey" is also.
Slimey and Stinkey are not a chemistry sets. They are the equipment and instructions to go around the house and yard collecting specimens and testing for tackiness, viscosity, and odor.
OCT 9, 1998
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Last Revision -- June 30, 1998
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