20+inch+F4+present+telescope+design

Image below shows the telescope as it is installed at the moment. Images below this show the development of the telescope during it's rebuild.

More images of the telescope can be seen here:- http://picasaweb.google.com/daveowen1956/Telescopes#



USER GUIDE (20 inch F4)

20 inch F4 Instruments

20 inch F4 Software systems

20 inch F4 installing the new servo system drive

Design and construction of the Observatory and Telescope can also be seen here:- 20 inch Observatory and Telescope documents

Dew heater calculations spearedsheet:

=Mirror testing of the 20 inch F4=

The 20 inch F4 mirror was not made by me, but by Oldham optics. As can be seen from the images it has taken, and the optical test results we have done, it is a good mirror.

What is interesting is the divergence from the ML graph of the Figure XP and knife edge readings. These appear to agree somwhat. Hence the good wave error given by Figure XP (and good images when you look through it). So it appears that the ML method leaves the mirror 'undercorrected' in particular increasingly at the edge!

[|20_inch_F4_Oldham_optics.xls]





=Construction images=

Image below shows the telescope as it was with the 18 inch mirror insalled just before it was disassembled to be redesigned for the dome.

The run off shed was originally built for the 30 inch TRO telescope. The shed no longer exsits, as it was removed after the the dome was built.

Image below shows the original azimuth bearing set up. This was radically changed in the new design as this was flexing and quite 'sloppy'. The thin aluminium plate was removed and replaced with a 20mm thick machined steel plate (see images later on). The new central bearing and boss was drilled into place. This has a 100mm central hole to allow for cable to pass through to the mount. It is better in design than the original large running suface and lateral bearings in the abovve image.

Image belwo shows how the final bearing was installed. A new stronger drive plate for the tappered drive wheel was insatlled also. One of the main issues of the old design was that the plate would flex and was not that well aligned too. Great care was put into aligning the new bearing. The bottom of the fork was refaced and made flat to accept the new steel drive surface. It was sealed with fibre glass resin to preventwater ingress over time.



Image belwo shows the new azimuth steel bearing (very heavy!)

Altitude and azimuth stepper motors and gears installed. These have now been replaced (2008) with a new set of gears and pullys for the servo system. The original stepper set up worked ok, but had quite poor PE. Only very short exposures could be done unguided. The new servo system is much better, and faster!

The new spider is very stiff. Adjustment is by the 3 fixings on the outer cage ring. The secondary is heated with a resistor ring 24 hours a day.

Completed telescope



The new de-rotator installed below. Only the motor is to be installed. The focuser is a converted Astro System focuser with home made DC drive and limit switches. Works well with Cartes via Mels board. The system will be updated in 2009 with a servo motor and SiTech system for Autofocus control. Being stripped for installation into the dome.



Image below shows me leveling the new azimuth bearing in the dome with a precsion level. There are 3 points in the floor of the dome for the domed feet to sit in. They were formed with epoxy resin to allow the feet to sit firmly yet allow the feet to rotate when adjusted.



Images of the new 20 inch mirror (after the 18 inch was dropped) back in 2003. It is installed in the telescope and has not needed a recoat as of December 2008.





The 20 inch F4 secondary is larger than the old 18 inch F5 secondary.