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Star diagonal
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 Some points on Dielectric coated diagonals

It is doubtful that anyone would notice the 10% brightness difference with the low and highend diagonals (ie eye sensitivity). Dielectric 99% reflectivity is good, and 91% reflectivity in cheap diagonal seeming bad. However, the 10% difference may seem a lot, but the eyes 'sees' brightness in an almost logarithmic scale (power scale generally), so you don't actually see something 10% brighter with a Dielectric coated diagonal.

Super wave accuracy isn't that important for diagonals.
Better wave accuracy is better - but needs only to be matched to the accuracy of the main mirror.
Most good main mirrors have 1/8 wave accuracy (and very good handcrafted mirror could have 1/20 surface accuracy). But that accuracy is halved at the focus. A 1/10 wave, or better, accuracy diagonal is perhaps 'overkill' for a 1/8 wave mirror (which will have at best a 1/4 error at the wavefront).

The top coating of a Dielectric acts like a film or layer of glass over the mirror, (This is what makes them so durable).
However, light will bounce between the top and bottom surfaces of that glass layer and spread out - a dangerous laser test will show the 'fuzzing effect' (compared to non coated).
The scattering of light affects different wavelengths, and is related to the thickness of the coating (This is exactly the same physics as how certain specialist filters work, by bouncing light between layers).
The net effect is that 'points' become fuzzy. The effect is minor, but real.

But, the reflective coating on high end diagonals are less likely to be misapplied and consequently scatter less light than badly coated cheaper mirrors.

It should also be said that diagonals for most people are nonessential. The telescope will function just as well without a diagonal (tube extensions, such as an Barlow tube minus lens, may be needed for focus); and for astrophotography, it is better not to use anything that degrades the light path.



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Diagonal mirror
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 Diagonal Calculator

Software utility to calculate the correct size diagonal mirror for your Newtonian telescope 

Download (2.7mb, .exe)



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RE: Star diagonal
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Adding A Heater To Your Star Diagonal

Are you tired of having to remove and reinstall your eyepiece heater every time you change eyepieces? Simply heat the eyepiece holder on your Star Diagonal (or Paracorr) and you will never need an eyepiece heater again.
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A star diagonal is designed to place the eyepiece at comfortable viewing position on an astronomical telescope.
A star diagonal is either constructed from a prism or mirror, and reflects the path of the light at, usually, a 90-degree angle to the telescope's optical axis.
Newtonian-type reflecting telescopes already have a 'star diagonal' by design.

Standard star diagonals come in three main barrel sizes; 0.965", 1.25"(31.7mm) and 2" (50.8mm).
The 2" diagonal is likely to have an adaptor fitting, so that smaller 1.25" eyepieces can be used.
One feature to look out for is that the barrel of the diagonal is threaded to allow filters to be used.  48mm filters are the usual size fitted to 2" diagonals.
Most higher priced telescopes are adaptable to use 1.25" and 2" eyepiece diameters.

A larger 2" diagonal on a telescope will allow the use of lower power (lower than 32mm) eyepieces to give an apparent field of view unobstructed by the mounting barrel. 
With short focus refractors there is little reason to upgrade to a 2" diagonal (apart from the need of a wide field of view required by astrophotography - a rather mute point).
2" eyepieces are considerably more expensive and heavier compared to 1.25" eyepieces.
Diagonal5.jpg
Good quality diagonals will have a barrel that is recessed to stop the diagonal from slipping out of the focuser.
Another feature to look out for, but is not essential, is a compression ring which prevents the tip of the locking screw from scratching the surface of the eyepiece barrel.

Good quality mirror star diagonals will have a surface with 99% reflectivity, and lower priced diagonals with a seemingly poor 91% reflectivity; However, the reality is that most observers cannot tell the difference between the two.

Since the mirror or prism of the star diagonal is located nearly at the focal plane of the instrument, surface accuracy of greater that ¼ wave is more in the line of advertising than any increase in optical performance.

The major disadvantage of mirror diagonals is that unless the reflective coating is properly applied they can scatter light rendering lower image contrast compared to a 90-degree prism. Also they deteriorate with age as the reflective surface oxidizes. The newer Dielectric mirrors have largely solved the deterioration problem, and if properly made the Dielectric mirrors scatter less light compared to conventional mirrors. With short focal length instruments a mirror diagonal is preferred over a prism.

A well-made conventional 90-degree prism star diagonal can transmit as much or more light as a mirror, and do so with higher image contrast since there is no possibility of light scattering from a reflective metallic surface as in a mirror diagonal. Also a prism will never degrade over time as a mirror will since there is no reflective metal coating to degrade from oxidation. However prism diagonals may introduce chromatic aberration when used with short focal-length scopes although this isn't a problem with the popular Schmidt-Cassegrain and Maksutov Cassegrain telescopes, which have long focal lengths. On longer focal ratio telescopes a well-made 90-degree prism diagonal is the optimum choice to deliver the highest image contrast short of using the telescope without a diagonal entirely. However prisms seem to be falling out of favour probably due to marketing forces which have been favouring short focal length instruments which tend to function better with a mirror diagonal. In some special cases however, the colour dispersion effects of a prism diagonal can be used to advantage to improve the performance of undercorrected refractor objectives (regardless of focal length) by shifting the spherical and colour correction of the objective closer to the design optimum. The natural colour dispersion properties (overcorrection) of the prism works to lessen or nullify the undercorrection of the objective lens.

Source

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Collimate A Star Diagonal



Home made star diagonal
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