DAVID LUKEHURST

ASTRONOMICAL TELESCOPE MAKER

Astronomical Telescopes for the Discerning Visual Observer

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Telescope Options

Mirrors

The standard mirrors used for my range of Dobsonians are of an accuracy of 1/4-wave peak-to-valley on the reflected wavefront. This accuracy satisfies the "Rayleigh Criterion" and is what is sometimes referred to as "diffraction limited". The Rayleigh Criterion states that in an optical imaging system where the maximum peak-to-valley wavefront error through the full light-path is no more than 1/4-wavelength light, the image will be "sensibly perfect".

Under common observing conditions, a telescope with good 1/4-wave optics will deliver most of the resolution that is possible with a telescope of its size. Atmospheric turbulence is frequently a major limiting factor determining image sharpness and detail. However, on nights of exceptional atmospheric stability, when observing objects with fine, resolvable detail, such as the moon, planets or close double stars, benefit can be seen when using higher accuracy mirrors.

As standard, mirrors of up to 16-inches diameter are made from 1-inch thick fine annealed plate glass. Larger mirrors are made from low-expansion Schott "Suprax" or Corning "Pyrex" borosilicate glasses or BVC glass /ceramic

Mirrors are also available at the higher accuracy of 1/10th-wave PTV and low-expansion glass is available for all sizes, as are a variety of focal ratios. Please see the mirror page for details of the full range.

The mirror quality is guaranteed by the maker and a full Certificate of Conformity is available for a small charge. This certificate is supplied free of charge with mirrors of 20-inches and larger. A summary sheet giving the mirror specification is supplied with all telescopes.

Mirror Cells

Main Mirror Cells

All telescopes are fitted with multi-point flotation cells for the main mirror. Telescopes of 8.75-inches up to 16-inches are fitted with 9-point flotation cells. Telescopes of 18-inches and larger are fitted with 18-point flotation cells. These cells provide excellent support for the high accuracy mirrors used in all of my telescopes. Cells can optionally be upgraded to 18-point or 27-point.

Secondary "Spider"

I have two styles of "spider", both of which have their own advantages. On smaller telescopes (up to 14-inches), I normally fit a spider consisting of a single curved vane. This has several useful advantages. It is easy to make, stiff and also, due to the curved profile, it does not cause the sharp diffraction spikes seen on stellar images with straight vanes. When viewing planets, the diffraction "spikes" are actually bands of diffracted light which also cross the image of the planet itself and result in somewhat reduced contrast and generally less "clean" images.

For larger telescope (16-inches and larger) I use the more conventional 4-vane spider as this is necessary when supporting the larger, heavier secondary mirror assemblies. 4-vane spiders can be fitted to any size of telescope on request. 

      

Single curved vane              Classical 4-vane

Focusers

Telescopes are normally supplied with a good quality 2-inch rack & pinion focuser. I can also supply the telescopes with MoonLite 2-inch low-profile Crayford focusers. All focusers are supplied with 2-inch to 1.25-inch adapters.

       

             2-inch Rack & Pinion (Standard)

                      

                                                MoonLite Crayford (CR1 +140, CR2 +230)

 

Finderscopes

As standard, the telescopes are fitted with 10 x 50mm straight-through finderscopes with fully adjustable mounting rings. 60mm, 70mm, right-angled and erect image versions are also available. Prices: Upgraded to 60mm +30, 70mm +45, 100mm +100, right-angled +35, erect image +40.

 

Telescope Finishes

All wooden components are fully sealed with a two part lacquer and then undercoated and top coated to give an attractive and durable finish.

The "default" colours are black or dark blue, but telescopes can be finished in any colour of your choice if this is specified with the order. There is no extra charge for any standard paint colour.

The truss-tubes are normally finished in plain aluminium. As they are not in the light-path, this works fine. They can be etch-primed and spray painted to any readily available colour for an additional 30. This finish looks good but is relatively easily damaged.

Alternatively, the truss-tubes can be black satin anodised. This finish is attractive and very durable. The approximate cost is 125. Other colours than black are also available. Please enquire when ordering if you are interested in this finish.

 

"Wheelbarrow Handles"

Telescopes up to 14 or 16-inches aperture are moveable by one person (I have moved and set-up a 24-inch telescope by myself, but with some difficulty!) and if separated into its main components, larger sizes. However, moving larger telescopes can be made much easier by the addition of removable "wheelbarrow handles" that are attached to each side of the assembled telescope and allow the telescope to be wheeled to its observing position. These are available as an option for all sizes of telescope. Price: For up to 16-inch +80. 18 to 24-inch +100.

"Wheelbarrow-handles" on dismantled 16-inch f/4.4 Dobsonian

Digital Setting Circles & Drives

I have fitted many sets of "Sky Commander Flash-4" DSC's. These consist of two optical encoders, one fitted to each axis, and a hand set with the co-ordinate display. Please see: http://www.skyeng.com/. Please enquire for a price as this is depended on the /$ exchange rate. As of August 2016, it would be approximately 550. Fitting is best done when the telescope is constructed as mountings for the encoders need to be fabricated and fitted. However, retro-fitting is usually possible. ArgoNavis DSC's are also available for 625 fitted. Please confirm prices before ordering as they can fluctuate with the exchange rates.

In addition, I can fit the ServoCat dual-axis drive system which integrates with the ArgoNavis or SkyCommander DSC's. I have recently fitted several  of these systems  to  20", 22" and 24" telescopes.

 

Shrouds & Truss-Tube Carry Cases

I have a good supplier for custom made shrouds and truss-tube carrying cases. Please enquire if you are interested in having either of these. (See: http://www.scopeshrouds.com/)

 

Other options are also possible. I am happy to customise telescopes to individual requirements whenever possible. Please contact me to discuss your ideal specification.

 

David Lukehurst

30th July 2016