Jgroub: Adding additional negative - BUT TRUTHFUL - information on B-J scopes to hopefully keep people from buying them.

2022-01-18T10:00:28Z

Adding additional negative - BUT TRUTHFUL - information on B-J scopes to hopefully keep people from buying them.

Jmh921: Jmh921 moved page Bird-Jones to Bird-Jones Telescope

2019-05-15T20:51:04Z

Jmh921 moved page Bird-Jones to Bird-Jones Telescope

Jmh921: Created page with "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 telesco..."

2019-02-01T22:09:52Z

Created page with "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 telesco..."

New page

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 mirror|parabolic]] curvature, and a flat, angled secondary mirror. A Bird-Jones telescope uses a primary mirror with a [[spherical mirror|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, the majority of Bird-Jones telescopes are aimed at the budget/entry-level sector of the market. As such, they typically use lower-standard optical components which produce lower-quality views.

Worse still, they make 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.

Most manufacturers do not explicitly label their Bird-Jones scopes as such. For example, the much-reviled [https://www.celestron.com/products/powerseeker-127eq-telescope 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. 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.

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 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, the majority of Bird-Jones telescopes are aimed at the budget/entry-level sector of the market.  As such, they typically use lower-standard optical components which produce lower-quality views.
+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, they make 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.
+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.  For example, the much-reviled [https://www.celestron.com/products/powerseeker-127eq-telescope 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.
+Most manufacturers do not explicitly label their Bird-Jones scopes as such.  The much-reviled [https://www.celestron.com/products/powerseeker-127eq-telescope 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.  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.
+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.

← Older revision	Revision as of 20:51, 15 May 2019
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Bird-Jones Telescope - Revision history

Jgroub: Adding additional negative - BUT TRUTHFUL - information on B-J scopes to hopefully keep people from buying them.

Jmh921: Jmh921 moved page Bird-Jones to Bird-Jones Telescope

Jmh921: Created page with "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 telesco..."