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Scroll down for - CELESTRON EDGE HD TELESCOPE OPTICAL REVIEW



Celestron CGE Pro HD 9.25 Inch EdgeHD Optics Telescope
Price $12197.95
Sale $6899

[OPTICAL REVIEW]
[Instruction Manual]

Celestron CGE Pro HD 11 Inch EdgeHD Optics Telescope
Price $13997.95
Sale $7999

[OPTICAL REVIEW]
[Instruction Manual]

Celestron CGE Pro HD 14 Inch EdgeHD Optics Telescope
Price $17797.95
Sale $9999

Customer Score:5.0 Stars
[OPTICAL REVIEW]
[Instruction Manual]

Celestron 8 Inch EdgeHD OTA Optical Tube
Price $2197.95
Sale $1299

Customer Score:5.0 Stars
[OPTICAL REVIEW]
[Instruction Manual]

Celestron 9.25 Inch EdgeHD OTA Optical Tube
Price $3997.95
Sale $2299

[OPTICAL REVIEW]
[Instruction Manual]

Celestron 11 Inch EdgeHD OTA Optical Tube
Price $5897.95
Sale $3399

[OPTICAL REVIEW]
[Instruction Manual]

Celestron 14 Inch EdgeHD OTA Optical Tube
Price $10197.95
Sale $5799

[OPTICAL REVIEW]
[Instruction Manual]

Celestron T-Adapter for Celestron EdgeHD 8 Inch
Price $34.95
Sale $27.95

Customer Score:5.0 Stars




CELESTRON EDGEHD OPTICAL TUBES 800, 925, 1100, 1400

CELESTRON EDGEHD OPTICAL TUBES

EdgeHD 800 EdgeHD 925 EdgeHD 11 EdgeHD 14
Aperture 203mm (8in) 235mm (9.25in) 279mm (11in) 356mm (14in)
Focal Length 2032mm 2350mm 2800mm 3910mm
Focal Ratio 10 10 10 11
Tube Length 432mm (17in) 559mm (22in) 610mm (24in) 787mm (31in)
Tube Weight 6.4kg (14 lbs) 9.5kg (21 lbs) 13kg (28 lbs) 21kg (46 lbs)
Central Obstruction (dia) 69mm (34%) 85mm (36%) 95mm (34%) 114mm (32%)
Strehl Ratio of Central Obstruction 0.81 0.78 0.81 0.83
Visual Back EdgeHD 800 EdgeHD 9.25 EdgeHD 11 EdgeHD 14
Star Diagonal 1.25" 2" with 1.25" adapter 2" with 1.25" adapter 2" with 1.25" adapter
Included Eyepiece 1.25", 40mm focus 2", 23mm focus 2", 23mm focus 2", 23mm focus
Next thing EdgeHD 800 EdgeHD 925 EdgeHD 11 EdgeHD 14
from



FEATURES COMMON TO ALL EDGEHD TELESCOPES -

Celestrons Premium Starbright XLT optical coatings

Mirror Support Knobs hold the mirror in place and reduces image shift during imaging.

Celeston Focal Reducer will soon be available

Fastar Compatible: all optical tubes are equipped with a removable secondary mirror for fast f/2 ccd imaging

9x50 Finderscope

Aluminum Tube

Dovetail compatible with CGE Mount

2-year Telescope Warranty




BEHIND THE REVIEWS

The Edge HD Celestron range has been designed with aim of correcting the off-axis aberration normally associated with the Schmidt-Cassegrain design. Optical aberrations are the optical "facts of life" which tend to degrade images produced by optical instuments. Aberrations are inherent in the optical design itself. They can be eliminated, or reduced, only by a change in design.

The main mirror in SCTs is normally spherical, as this is easier and cheaper to produce. This does introduce spherical aberration, but the front corrector lens corrects for this. So the main aberration inherent in standard SCT designs is Coma. Coma causes blurring of star images off-axis, with the blurring increasing with off-axis distance. One possible solution would have been to position the front corrector plate further from the main mirror - this could have reduced coma, but it would have had the undesirable effect of nearly doubling the length of the telescope tube.

So standard SCTs tolerate a degree of coma in their design. Centre of the field of view is fine, and therefore, for visual use, coma does not cause much of a problem. Similarly, coma is not really an issue in imaging with small video cameras (or webcams), where the field of view is small. For these reasons, standard SCT designs retain the aberration of Coma, with the trade-off of low cost and ease of manufacture.

However, amateur astronomers are increasingly using large-format CCD cameras, and DSLR cameras with their telescopes. In these cases, the field of view is much wider, and the effects of coma are readily apparent, showing as blurred images towards the edge of the field. So the Edge HD series sets out to eliminate coma from its design. Celestron have done this by placing two small corrector lenses near the point of focus.

Celestron have been particularly successful in these aims. As a result, the Edge HD telescopes are capable of wonderful wide-field viewing with modern wide-angle eyepiece designs, and produce sharp-to-the-edge images with both DSLR cameras and large-sensor CCD cameras. We can say here that, for any astronomer interested in imaging with DSLR cameras or large CCDs, the Celestron EdgeHD telescopes represent a significant improvement over classic SCT designs and are the equal of anything available to the amateur before now.

As explained elsewhere on this website (click here), in any Schmidt-Cassegrain design, the diffraction effect caused by the large central obstruction, is the major factor affecting optical performance. For that reason, the optical components must be better than "diffraction limited" in order that the telescope should perform as "diffraction-limited" as a whole system.

By means of ray-tracing spot diagrams, it has been demonstrated that the Celestron Edge HD optics are "diffraction-limited". (See The White Paper by the Celestron Engineering team here). Ray-tracing spot diagrams do not take account of the effects of diffraction, and so are a measure of the optical quality of the components. Celestron's results would suggest that, at least in the case of the EdgeHD 925, the optical compoents are of sufficient quality to result in an overall "diffraction-limited" performance.

COMPARISON WITH OTHER TELESCOPE DESIGNS

Celestron claims that the EdgeHD telescopes produce coma-free images at up to 20mm off-axis. That is, the diameter of the coma-free image is 40mm.

Comparison with standatrd SCTs: Tables elsewhere on this website (HERE) show that, the size of the coma-free image in standard SCT designs is barely 5mm diameter, compared with the claimed 40mm diameter of the EdgeHD images.

Comparison with Newtonian Telescopes: The same page (HERE ) shows that the diameter of the coma free image with Newtonians varies with f number as follows: f4 has 1.42mm diameter, f5 2.78mm diameter, f6 4.8mm diameter, and f8 11.38mm diameter. All these are, of course, very inferior to the edgeHD's 40mm diameter coma free field.

WHY BUY CELESTRON EDGEHD?

The Celestron EdgeHD telescopes cost significantly more than normal Classic SCTs. But the extra cost is justified if you can appreciate the brilliant off-axis resolution. So, whether you are imaging with a large CCD or DSLR camera. or you want to use wide field superior short-focus eyepieces for wide-angle views, then the Celestron Edge HD telescopes make sense.

BEST BUY CELESTRON EDGE

OK, so which is the best Celesctron EdgeHD telesope to buy?

Celestron results show that, in terms of central resolution, the Celestron EdgeHD 925 beats the other Celestrons very clearly, producing the best diffraction-limited performance (ie the smallest airy disk diameters across the visual spectrum). Importantly, the difference is most marked for green light (predominant in visual observations). Celestron's own Ray-tracing spot diagrams show that, alone among the range, the EdgeHD 925 is diffraction limited, across the spectrum and up to 15mm off-axis. Central resolution is described as "near perfect".

The EdgeHD 800 doesn't quite match the in overall performance of the EdgeHD 925, but the differences are small, and will be difficult to notice under any but the very best seeing conditions.

If your primary aim is the imaging/viewing of the very faintest deep sky objects, the EdgeHD 1100 or EdgeHD 1400 are the perfect choice. That assumes that expense is no object and that a permanent site is available for the telescope. But rarely will seeing conditions permit these telescopes to achieve their theoretical resolution. they would not be a sensible choice if you want to view bright objects from a site with poor seeing, ie a urban, suburban or town site.

For most amateurs, portability will be important, and the choice is the EdgeHD 800 or EdgeHD 925. Both telescopes have excellent on-axis performance, and good edge-to-edge sharpness for wide views and images. In terms of the wide angle viewing and edge-to-edge sharpness, most amateurs, under the most common seeing conditions, will not be able to distinguish between the wide views achieved by these two telescopes. On the other hand, if your normal local seeing conditons are very good, the superlative resolution of the EdgeHD 925 will give better, sharper, views of the planets and other bright objects. Both the EdgeHD 800 and EdgeHD 925 are superb telescopes - but under the very best seeing conditions, the superior quality of the EdgeHD 925 will shine through.

SUMMARY

For imaging with DSLR, or large CCD cameras, the Celestron edgeHD telescopes have no equal. Of the four telescopes, the star of the show is the EdgeHD 925, in my opinion. It is the most accomplished telescope of the four, and offers the best combination of surperb performance, reasonable cost and portability. It represents excellent value-for-money.


SCHMIDT-CASSEGRAIN TELESCOPES - FOR AND AGAINST

Schmidt-Cassegrain telescopes are wonderful system telescopes. The major manufacturers, particularly Celestron and Meade, provide a wide range of accessories for the telescopes.

Other advantages are the closed tube, which reduces air currents and prevents dust and corrosion of the optical elements; the folded optical design which makes the scopes compact for their aperture and focal length, and so good portability and ease of mounting.

The main disadvantage of the schmidt-cassegrain is the size of the central obstruction, which is typically 34% to 36% of the diameter. This is not just a case of some light loss; as explained elsewhere
(click here) central obstuctions of that size can have the same effect as if an additional, less-than-perfect, optical element were introduced into the optical path. So, the Schmidt-Cassegrain would not be the first choice for high-resolution planetary viewing at high magnification under the best seeing conditions. But it makes sense for just about everything else.


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