Make Telescopes: The Perfect Machine
August 13th, 2009 Subscribe To Our Feed
The development of building a telescope was greatly aided by the construction of the achromatic lens.
In 1733, the achromatic lens was invented by Chester Moore Hall, an English barrister. This was accomplished by combining a convex crown and a concave flint lens in such a way that their focal lengths were inversely proportional to their dispersions.
Dollond’s efforts led to a demand for clearer glasses of more varied densities and of less equal dispersions, needed to improve achromatism, and chemists pursued experiments in learning how to control the refractive indices of melts, and in the pouring of large disks of limpid, homogeneous glass.
Dollond was making refractors (spyglasses) with single-lens objectives as early as 1742, his price for a 2-foot telescope then being 7s 6d. In comparison, in 1762 he sold a 2-foot telescope with a two-lens objective (achromat) for 2 guineas. The lens diameters in each case were just under 2″.
In 1783, with a view to combining the benefits of the wide field of Huygens’ eyepiece with a means of making micrometric measurements of an image in the focal plane, Jesse Ramsden, an English optician, designed the compound eyepiece. Therefore, the process to make a telescope was becoming more like the process undertaken today. It can be seen that a measuring device, such as adjustable parallel wires, set in the focal plane would be magnified along with the image. With the advent of the achromatic lens, the erecting or terrestrial eyepiece assumed considerable importance. In the early part of the 19th century, small achromatic refractors were being manufactured by several concerns. For those not having the means to buy achromats, telescopes with single-lens objectives continued to be made. Enterprising opticians were also offering lens sets that could be assembled into simple refractors.
The Modern Era
The method of chemically depositing silver on glass discovered about 1840 by Justus von Liebig, of Nuremberg, was successfully applied to a small glass telescope mirror in 1856 by Karl Steinheil, a German physicist, and independently in the following year by Jean Foucault, the famous French physicist.
Various processes of plating glass with metal for the make telescopes had been known and practiced for centuries, but for one reason or another, the coatings were unsuited for front-surface reflection.
Then, in 1858, Foucault announced the development of his amazingly delicate and simple test for a concave reflecting surface, using an illuminated pinhole and a straightedge placed in the vicinity of the center of curvature of the mirror. The pinhole and straightedge were the outgrowth of earlier experiments in which simultaneous microscopic comparison was made of a pin point, likewise placed at the center of curvature of a mirror, and its reflected image, which was caused to fall alongside.
The last speculum of note to be constructed was one four feet in diameter, made by Grubb in 1870 for the Melbourne Observatory. Silver-on-glass mirrors replaced the more expensive and difficult-to-work speculum.
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Christmas Gifts - A Beginner Telescope
November 30th, 2008 Subscribe To Our Feed
Just a word of advice - do your Christmas shopping early this year for the best selection of beginner telescopes. With a surprisingly strong showing at the retail stores on “Black” Friday, the more popular beginner and kid telescopes will be selling out soon.
Check out the recommended beginner telescopes from Amazon — they do have a great selection of name brand telescopes and their shipping costs are generally quite reasonable. In most cases, they have lower shipping costs than most of the specialty optics stores. In fact, many of the recommended telescopes qualify for their FREE Super Saver shipping, but you must buy early to assure these Christmas gifts arrive before December 25.
So, don’t disappoint your young astronomer because you waited too long to purchase their Christmas present - a brand new telescope. Shop early and make sure everyone has a Happy Holiday!
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Learning To Buy A Good Telescope
September 27th, 2008 Subscribe To Our Feed
For those interested in buying a telescope then it is important to know how buy a good telescope. Since telescopes do cost quite lot, then it is important to ensure that the money being invested is well spent and this would mean putting a great deal of time and effort in the buying process.
If you want to know how to buy a good telescope, there are a few issues in particular that you are going to want to be aware of.
Know Your Budget
You will first need to know what your budget is when you want to buy a good telescope. To do this you will need to first take some time to look around and familiarize yourself with the average prices of the telescopes available, so that you have an idea of the amount of money that you may have to spend in buying a good telescope.
You should be aware and keep in mind that telescopes come with many different functions and features, so you will need to figure out your needs and what you want on the types of models, then this will assist you in finding one within your budget which meets the standards you set.
The cost of telescopes will increase depending on the features that come with the telescope. In general one of the better ideas for deciding on how to buy a telescope is based on your level of experience.
Where to Buy
If you have your mind set in buying a good telescope then another important step to buying for you to be able to decide on the ideal retailer for you to shop. It is an important step since your goal is to buy a good telescope and you will want to ensure that you will be buying from a retailer that knows the product well and is experienced in telescopes. They should be capable of providing you with exactly what you need and be able to cater to your specific requirements.
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