CLEANING
The best policy is not to let the lens get dirty and/or dusty in the first place. The regular use of the dust caps is highly recommended. However, no amount of preventative measures will keep your objective from eventually collecting dust and airborne pollutants on the first optical surface. We recommend that you do not clean the objective too often, no matter how frequently the urge to do so may hit you. A few specks of dust on the lens will not be visible in your images, as they are not in the focal plane and don’t block enough light to measure, let alone be seen.
Depending on how often you use your scope, and the amount of pollutants in your air, you may have to clean your scope as often as twice a year, but generally no more than that – and preferably no more than once a year. If the front lens surface becomes dusty, smeared, or shows fingerprints or any other surface build-up, and you find it absolutely necessary to clean the lens, use the following cleaning technique.
First, gently blow away any surface dust or particles with a clean air blower (a child’s ear syringe or a photographer’s camel’s hair brush with attached blower bulb, for example). The use of canned or compressed air should be avoided, if possible, as the propellant in the can may spit out and leave difficult-to-remove deposits on your lens. Also, the expanding compressed air drops in temperature as it leaves the can. The cold air coming out of the tiny tube that most compressed air cans use to direct the air flow has been known to chill a lens to the point of spalling chips off the lens if pointed at the same spot on the glass for too long.
If you want, or need, to use compressed air to remove stubborn particles, use a high quality compressed air duster (of the R-134 propellant type). ChemTronics sells a high-quality unit. Do not tip or shake the can. Blow any loose particles off the lens surface using short blasts at an angle to the glass, without getting too close to the lens surface or aiming directly at it.
Next, moisten a ball of USP grade pure cotton with a few drops of a photographic-quality optical cleaning solution designed for multicoated camera and binocular lenses. You can use Formula MC (available from many telescope dealers) or your own mixture of distilled water and a drop or two of mild soap. A well-worn 100% cotton handkerchief also works well and Zeiss and Kodak both make suitable cleaning fluids. Blot the entire surface with the dampened cotton ball or cloth to pick up any stubborn particles and to clean the surface. Exchange the cotton ball and/or turn the cloth frequently so you always have a clean portion of the cotton ball or cloth in contact with the lens.
Use a very small amount of liquid – not so much that the fluid could be wicked between the lenses by capillary action. Do not drip the cleaning fluid directly on the lens. Do not, at any stage, apply hard pressure. Using a fresh piece of cotton or a lint-free white facial tissue, carefully clean the surface of the lens by wiping across in a radial direction. Repeat the process with denatured alcohol, using a blower brush to clean off any dust that may fall on the lens as you are cleaning it.
If you want to take the ultimate step in cleaning, a final rinse with high-grade acetone will clean the surface to new condition. You may notice a few faint streaks from the dried solvent. They will not affect performance, but they can be removed with light pressure and a Q-Tip slightly moistened with a small amount of alcohol or acetone. Finally, a clean air blower bulb can be used to remove any remaining dust.
Avoid overcleaning your scope. The multicoatings on the lens are quite hard and durable. However, frequent overzealous cleaning can scratch the coatings if all the dust particles (which are often tiny flecks of windborne rock) are not removed before you start pushing a damp tissue around the lens surface. Clean your optics
only when absolutely necessary. If you take proper care of your scope, cleaning should rarely be needed.
THE STAR TEST
We strive to make the best apochromatic optical systems available. You may find your lens may not test “perfect” during an extended and stringently-graded “star test,” particularly if the scope has not fully reached thermal equilibrium before the test. This is not an indication of a poor optic, but is rather due to the test star’s complex wavefront of light being changed constantly by our living atmosphere as seeing conditions vary from moment to moment. No optic is perfect, and each will show some error under a detailed and extended star test. The sensitivity of the star test under perfect conditions is 1/20th wave P-V on the wavefront for third order aberrations, and 1/60th wave for sharp (fifth order) aberrations. It is highly unlikely that even the most ardent observer can see errors of this small a magnitude on an extended object, even under very good seeing conditions (which is when the atmosphere typically presents a 1/4th wave P-V wavefront to the instrument).
The refractor also presents another factor: the change in spherical aberration with a change in wavelength. This “sphero-chromatism” is found in all refractors. As a lens is tested in the longer (red) wavelengths, the lens becomes “under-corrected.” Tested in the shorter wavelengths (blue), the lens becomes “over-corrected.” These overlapping corrections at different wavelengths change the star test pattern from perfection.
However, TMB objectives are corrected at the peak visual wave- length centered around 560nm in the green-yellow portion of the visual spectrum. The eye sees over 80% of the visual detail at this wavelength. It is the correction at this visual peak that makes the difference between a merely good objective and a superb one. Our lenses are figured for the best possible wavefront at green-yellow wavelengths, for the sharpest images and highest contrast.
While star tests are interesting and useful, most observers spend their nights enjoying extended and detailed objects, not just examining sharply focused points of light. We feel that the proof of optical excellence is in the observing, not just in the testing.
A FINAL WORD
Thank you for your purchase of our TMB-80. We believe this apochromatic refractor will outperform any other telescope type, inch for inch, and is the most trouble-free telescope that you can buy. Its versatility for visual, photographic, and CCD work is unmatched. With a little care, this fine apochromatic refractor will last you a lifetime. Use it to enjoy the wonders of the night sky!
| Tom Back |
| BRIEF SPECIFICATIONS |
Aperture | 80mm (3.1”) |
Focal Length | 504mm |
Focal Ratio | f/6.3 |
Objective Type | triplet apochromatic, FPL-53 ED element |
Optical coatings | fully multicoated |
Resolving Power (Dawes’ Limit) | 1.45 arc seconds |
Visual Limiting Magnitude | 12.0 |
Light Grasp Versus the Eye | 131x |
Focuser | dual-speed Crayford with 11:1 ratio fine focus; |
2” and 1.25” compression ring eyepiece holders; |
| and 360° rotating camera angle adjuster |
Focuser Travel | 3.1” (80mm) |
Tube Diameter | 90mm (3.5”) o. d. |
Tube Length (lens shade retracted) | 15.25” (387mm) |
Tube Length (lens shade extended) | 17.75” (450mm) |
Optical Tube Weight | 6.5 lbs. (2.95 kg) |
Case Dimensions | ........................................... | 22.5” x 12.75” x 8” |
