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Apochromatic Refractors We Build
Color free refractor objectives from small to large for over 35 years
Polishing a block of windows
Built for you
Centering your lens
Apochromatic refractor telescopes and how they are made to give you images that they are designed for.
- Virtually color free with low chromatic and spherical aberrations.
- We start with raw glass either in the form of a blank round disk or what is called "strip glass" just as the name implies it's a strip of glass that we cut into a disk. Either way both of the disks are oversized in both diameter and thickness.
- Once we have a glass disk it's held in place using hot wax or special blocking pitch. The use of this is for a couple of reasons, first while processing the soon to be lens, there should not be any stress introduced by holding or clamping the lens while it's being worked on.
- Unlike metal or wood, the glass cannot usually be mechanically held in place.
- Next a machine called a curve generator grinds both surfaces of the blank with the correct spherical curve that will make up the design parameters of the completed apochromatic lens objective assembly.
- After the curve is generated it's very rough and then it's moved into the grinding department where the curve is precisely ground down to 9 microns or finer and the thickness of the lens is now maintained at the design tolerance.
- After this grinding process the lens moves into polishing, where it's blocked up again with pitch so it doesn't move and is held virtually stress free. Then another type of pitch polishes the lens surface so there are no marks or pits left from the grinding process. At this point the lens gets tested during the different polishing processes. This assures that the lens being made conforms to the design parameters.
- After the lens meets the final polishing parameters, it gets removed from the holding blocker and is completely cleaned and inspected. It's also tested and all parameters are kept in a log for each individual lens being made.
- Now, something is done here that a lot of shops don't or cannot perform. We center the optical axis to the mechanical axis of the lens to within 2 microns and edge the diameter down to the final size. This step makes centering the assembly of lenses into the objective and performing the collimation (alignment) to the mechanical cell very easy. Sometimes you may hear of rotating lenses to bring them into alignment, this is only taking into account half of what the centration we accomplish with our edging. All our cells are made to be adjustable by the user but we prefer you don't touch this. If you require an adjustment we will perform this for you at our factory for a nominal charge.
- Now the lens is almost done, special optical coatings are applied to all polished surfaces. This coating makes it so the individual lenses transmit almost 100% of the light through the lens.
- There is marketing hype about about optical coatings. Get straight answers from someone you trust, we have been making and coating optics since 1983. You can have plain old single layer MgF2 (magnesium fluoride) coating which will increase the transmission of a single lens if it's made of BK-7 glass to 97% in the visible light spectrum. If you have a multi-layer coating which will usually consist of 3 layers you can get transmission close or over 99% in the visual where your eye is most sensitive. Our coatings are done by us after we edge and center the lens. Depending on the type of glass used the design is modified to achieve maximum transmission.
- I hope you enjoyed this information and it was helpful of how we make apochromatic objective lenses here in our shop.
Precision Applied Products 140 mm F-6
Apochromatic tube assembly. Made entirely in our shop with meticulous details. Our main focus is on how well we center the lenses compared to the mechanical axis. This is done to assure perfect collimation. The lenses are centered to within 2 microns of each other and the cell.
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