The 4” telescope is one of the most popular size telescopes in the world. Every amateur astronomer has owned or currently uses a 4” reflector or refractor.
For many beginning backyard astronomers, a 114 mm (or 4.5”) Newtonian reflector is the entry-level telescope of choice. Whether on a Dobsonian, alt-az, equatorial, or GoTo mount, this telescope is readily available at local shopping malls, warehouse stores, sporting goods stores, mail-order, or on-line stores. Some are well made. Some, not so much. But the 4.5” Newtonian is a very popular telescope, and is a very powerful optical tool in the right hands.
For many advanced amateur astronomers, a 102 mm (or 4”) achromat or apochromat refractor is the workhorse instrument in their collection of telescopes. The versatile 4” refractoris a light, portable, and low maintenance telescope that offers sharp contrasty images of the Moon, planets, bright deep sky objects, and are a favorite for astrophotography. Many notable astronomy writers, such as the late Walter Scott Houston, Stephen James O’Meara, and John H. Mallas, have based their writings on their observations through a 4” refractor. Every telescope manufacturer features a bread-and-butter 4” refractor in their catalogs. With aperture variations ranging from 100 mm to 105 mm, these telescopes represent the elbow of the price curve for refractors, the maximum aperture available for a refractor without busting the budget. Especially when comparing the cost of apochromat refractors, going past the 4” aperture to larger refractors represents an enormous increase in cost for the telescope and the larger mounting required to support such a beast. For example, a typical 102 mm apochromat refractor can cost $2,500 for the optical tube assembly, or OTA. For a typical 130 mm apochromat refractor, the OTA cost doubles to the $5,000 to $6,000 range. A 152 mm apochromat refractor easily exceeds the $10,000 to $12,000 range and anything of larger aperture represents the cost of a new car or SUV. The cost of mounting larger refractors often represents an investment equal to or greater than the OTA!
I have three refractors in this size range: a 102mm doublet apochromat mounted on a modern computerized German mount, a 102mm achromat mounted on a vintage 60 year old weight driven German mount, and a slightly smaller 94mm triplet apochromat on a mid-1970’s/early 1980”s single axis RA driven German mount.
The 102mm apochromat and the 94mm apochromat are unequivocally the most used telescopes in my collection., with 75% of my viewing and observing done with these two telescopes. The 94mm is my favorite telescope for public outreach. They are both easily carried out from my living room to my deck, easy to set up, and easy to use. The optics never need re-alignment. Both show exquisite planetary views. Deep sky views are comparable to reflectors 6” or more. Because they are refractors, there is no secondary mirror present in the optical system to block and scatter incoming light and affect contrast. As a result of the higher contrast, the background space is darker and allowing the fainter light of DSO’s to be seen.
When people ask me what telescope they should buy, I give them two pieces of advice. The first is to buy their second telescope first. Never buy a “beginner” telescope. It is false economy to buy a cheap telescope with the false the interest in astronomy will build from there. A cheap telescope just ends up in a closet or a yard sale. A quality telescope provides quality images that truly builds interest and knowledge in astronomy.
The second piece of advice is to buy a telescope that will be used often, rather than a larger telescope that is difficult to move, setup, and use. The smaller will “see” more because of its frequency of use. A 102mm refractor is the perfect size that will maximize frequency of use.. The 102mm telescope never disappoints and will always be used, even if larger aperture telescopes are acquired in the future. A 102mm refractor can always take the role of a grab-and-go scope, easy to transport, easy to setup, and easy to use!