Bird-Jones Telescope

A Bird-Jones (or Jones-Bird, depending on who you talk to) Telescope (aka Bird-Jones Newtonian, Bird-Jones Reflector, Jones-Bird), is a derivative of the Newtonian telescope design.

A true Newtonian has a primary mirror with a parabolic curvature, and a flat, angled secondary mirror. A Bird-Jones telescope uses a primary mirror with a spherical curvature. The primary reason for this is cost.

A parabolic curvature has a radius that varies, while the curvature of a spherical mirror has a constant radius. This makes mass-producing a spherical mirror much easier than a parabolic, which leads to cheaper production.

Due to the inherent spherical aberration caused by a spherical mirror, a correcting lens is added to the optical path, usually at the inner-end of the focuser draw-tube, to produce a corrected image. This correcting lens is usually nothing more than a Barlow lens which takes advantage of the principle of reducing aberration through increased focal length. Because the Bird-Jones design uses both mirrors and lenses, it is classified as a catadioptric telescope design.

While conceptually the Bird-Jones design can make sense, in actual practice, they do not. Almost all Bird-Jones telescopes are aimed at the budget/entry-level sector of the market. In fact, these types of telescopes are derisively referred to as "department store telescopes"; they are advertised on their boxes as being capable of incredibly high levels of magnification, such as 300x, 450x, even 675x, which simply are not possible to achieve in real world use. They typically use lower-standard optical components, both in terms of the mirrors as well as the eyepieces provided, which produce lower-quality views. Additionally, the telescope tube is placed on an extremely inexpensive mount, resulting in both shaking of the object that you're trying to view, as well as difficulty in finding the object in the first place. The manufacturers cut every possible corner to deliver the cheapest telescope possible.

Worse still, the existence of the correcting lens makes collimation significantly more difficult. When looking through the focuser to properly align components, the correcting lens prevents you from getting a focused view of the position of the secondary, and makes it exceedingly difficult to get proper adjustment. A Cheshire, collimation cap, or laser collimator are essentially useless due to the de-focusing accomplished by the corrector. Without a properly collimated telescope, the quality of the view will be poor. And since every reflector will eventually need to be collimated, whether that takes place in a matter of months or a year or two, what happens is that these Bird-Jones scopes will fall out of collimation, never to return to its initial state. The result is that the novice user will think that this is just what happens to all telescopes after a while, leading them to believe that the money they spent was wasted, and forever driving them out of the hobby.

Most manufacturers do not explicitly label their Bird-Jones scopes as such. The much-reviled Celestron PowerSeeker 127 EQ is one such example. On their website, Celestron lists the scope as a Newtonian Reflector, when, in fact, it is a Bird-Jones.

A good way to tell if a telescope is a Bird-Jones is to compare the focal length to the physical length of the telescope tube. Generally, a Bird-Jones telescope will have a tube length that is approximately half of the focal length. For example, the above-mentioned PowerSeeker has a focal length of 1,000 mm, but the physical tube size is 508 mm. This means that there must be something to double the effective focal length, which would be the correcting Barlow lens. Indeed, it is precisely this feature of halving the physical length of the tube that leads department stores to carry Bird-Jones scopes; they can stack more boxes of telescopes in less room on their shelves.

The following is a list of entry-level reflectors, under different brand names, some of which are regular newtonians, and others which are Bird-Jones telescopes.

Orion

Starblast 4.5 = Regular Newt

Starblast 114mm = Regular Newt

Spaceprobe 130ST = Regular Newt

Spaceprobe 3 EQ = Regular Newt – but with a spherical mirror. The negative effects of the spherical mirror are mostly counteracted by the very long f/9.2 focal ratio.

Celestron

Astromaster 114 = Bird-Jones

Astromaster 130 = Regular Newt – but with a spherical mirror that causes spherical aberration

Powerseeker 114 = Regular Newt

Powerseeker 127 = Bird-Jones

Omni XLT AZ 114 = Regular Newt

Omni XLT AZ 130 = Regular Newt

Computerized 114LCM = Bird-Jones

NexStar 114SLT = Bird-Jones

NexStar 130SLT = Regular Newt

Meade

114EQ-AST = Bird-Jones

StarNavigator 130 = Bird-Jones

Polaris 114 = Regular Newt

Polaris 127 = Bird-Jones

Polaris 130 = Regular Newt

Lightbridge Mini 114 = Regular Newt

Lightbridge Mini 130 = Regular Newt

Other brands

Astronomers Without Borders One Sky = Regular Newt

Skywatcher Explorer 130M = Regular Newt

iOptron N114 = Bird-Jones

Vixen R130SF = Regular Newt

Bresser Spica 130 = Regular Newt

Bresser Messier NT130 = Regular Newt

Konusmotor 130 = Bird-Jones

Bushnell Northstar 114 = Regular Newt

Seben Big Boss 150-1400mm = Bird-Jones

ProTelescope.com’s “6” GPS Computer Controlled Reflector Telescope Astrophotography Package” = Bird-Jones

Visionking 5.9 inches 150 – 1400mm = Bird-Jones

Baytronix 150/1400 = Bird-Jones

TS-Optics Megastar1550 – 150/1400 mm = Bird-Jones

AstroVenture 6″ Short Tube Reflector Telescope = Bird-Jones