24 inch telescope Drive System

Declination Axis Drive

A microstepped stepper motor drive incorporating a worm and wheel has been fitted to the declination axis replacing the “temporary” flexure brake system. The flexure brake system was originally intended to operate for a few months but was actually used for several years.

Right Ascension Axis Drive

A microstepped stepper motor drive incorporating a low backlash gear speed reduction system and a friction drive on the RA axis has been installed replacing the synchronous motor drive.

Encoder System

A differential encoder readout system has been fitted to both the declination and right ascension axes of the 24 inch Cassegrain telescope. Once initialised on at least 2 known stars this system makes it easier to locate objects in the sky. Eventually we hope to incorporate this system in a computerised goto drive system

24 Inch telescope Optical tests

Interferometric Hartmann test

The interferometric modification of the classical Hartmann test devised by Tapio Korhonnen in 1984 uses the same equipment as the classical Hartmann test, that is an aperture screen with a regular array of apertures (usually on either a square or a hexagonal grid) and a camera. However the interferometric modification of the test is at least 5 times more accurate than the classical test. The camera is used without a lens and is placed either inside or outside focus at a distance where the spots produced by adjacent apertures in the screen overlap. The defocus distance is smaller than for a classical Hartmann test where the spots from adjacent apertures in the screen do not overlap. Consequently the size of the spot pattern is smaller than that for a classical Hartmann test. Thus for a given image sensor size more apertures can be used. This technique has usually been used in the near infrared (700 – 1000 nanometers) on large professional telescopes such as the VATT, NOT and the MMT. However it works just as well in the visible for amateur sized telescopes. If the seeing disk is too large the spot pattern will be washed out, the solution is to use a greater number of apertures. For large telescopes the number of apertures can become unmanageably large with poor seeing however this will rarely be a problem with amateur telescopes.

ds = 

where

ds is the defocus distance

F is the focal length of the telescope

d is the centre to centre separation of adjacent apertures in the screen

λ is the centre wavelength of the passband

Spot pattern diameter = 

where N = is the number of apertures across aperture.

where fwhm is the full width half maximum seeing disk diameter

A DSLR camera is a good choice for the image sensor as the built in filter array on the image sensor can be conveniently used to define the passband which should be reasonably wide. However a CCD camera with a filter can also be used as long as the image sensor is large enough.

24 Inch classical Cassegrain Korhonnen Hartmann Test Parameters

Parameter Value
Telescope 24”(610 mm) Classical Cassegrain F ~ 8m
Defocus distance 130 mm
Wavelength 600 nm (red pixels)
Effective aperture diameter 9 mm
Effective aperture spacing 16.8 mm
Aperture pattern Square array
Camera Canon EOS 20Da
Exposure time 30 seconds
ISO speed 800
Object Spica

Ronchi Test using a star

When the seeing isn’t perfect, a Ronchi test using a star and a Ronchi grating produces a set of shimmering wavy bands that are difficult to interpret when viewed by eye. It is necessary to average out the effects of seeing by taking an image of the Ronchi bands with an exposure time of several seconds. A camera focused on the aperture stop (usually the primary mirror) through the Ronchi grating must be used to capture the Ronchigram image. With the Ronchi grating placed inside focus a simple cemented doublet with a suitable focal length, located near the focal plane will produce an Ronchigram of the desired size on the image sensor.

24 Inch classical Cassegrain Ronchi Test Parameters

Parameter Value
Telescope 24”(610 mm) Classical Cassegrain F ~ 5m
Camera lens focal length 150 mm
Wavelength Visible
Effective aperture diameter 9 mm
Effective aperture spacing 16.8 mm
Ronchi grating 150 lpi
Camera Canon EOS 20Da
Exposure time 4 seconds
ISO speed 1600
Object Sirius

 

24 inch telescope Optomechanical

Auxiliary Instrument Mount System

A more secure and flexible system for mounting auxiliary instruments like finders, guide telescopes and cameras has been designed and will be fitted to the telescope fibreglass structure. The new system which employs 6mm ISO coarse threaded brass inserts bonded to the fibreglass telescope Optical tube structure will allow temporary and permanent auxiliary instruments to be easily, quickly and securely mounted on or removed from the telescope.

Primary Mirror Mount System

It is thought that the current primary mirror support system has excessive friction between the edge support system and the mirror. The mirror moves off its rear supports when the mirror is tilted too far forward and stays off the rear supports until the axial component of the mirror weight overcomes the friction in the edge supports. This hypothesis will be checked by reducing the friction between the edge support system and the mirror. If this alleviates the problem an improved edge support system will be fitted. An investigation into the significance of friction between the rear support points and the mirror will also be undertaken.
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