Astronomy and Sky Website of Martin Lewis

Fossil Light is a great scope to use but is a cumbersome to set-up for short periods of observing. To enable me to observe more often at home in St Albans, UK, in 2001 I started building a 222mm (8.75″) Dobsonian which I have jokingly named ‘Mr.Orb Inspector’ in recognition of its mainly planetary usage. 

The scope uses an excellent set of f5.9 Pyrex optics made by David Hinds shortly before he ground his last mirror and became a UK Celestron importer/seller. The scope is pictured sitting on my equatorial platform after a long cold session imaging Mars in early 2010. 

This is the scope I used for all my webcam images up until 2013, when I started using Fossil Light, my 444mm Dob. I now primarily use Fossil Light at night due to its greater resolution and light grasp which really does make a marked difference to the level of detail captured on the planets. Mr.Orb Inspector is not neglected though, and I still use it for daytime Mercury and Venus imaging (see below), ISS imaging and for quick imaging sessions as required. It is easier to set up and smaller which makes in an advantage to use in some circumstances. This is the scope I usually take to public imaging sessions with WOLAS (West of London AS) at Ruislip Lido as it is easy to transport and low enough for kids to easily reach the eyepiece with a small set of steps.

Imaging Venus with Mr.Ob Inspector when close to the Sun using a custom portable sunshade – do not try this at home!

The structural components of the scope are mainly made of 3/8″ and 1/2″ Baltic plywood but it also uses 3/4″ aluminium truss tubes for lightness and good rigidity. Unlike the conventional truss tube Dobsonian design, the truss tubes are left fixed in place during storage and transportation.

Other features that may be of interest to readers are;

  • Corex (fluted plastic) cylinder as an internal main tube – sitting hard up against the inside the truss poles. This keeps out extraneous light and blocks convection currents from my body disturbing the light path
  • Corex tube coated with black self-adhesive flocking on inside to cut down on scattered light
  • Door at base of Corex cylinder to access dust cover placed over primary mirror
  • Radiator foil all over outside of tube cylinder and top half of upper tube assembly to reduce severity of inverse tube currents where the skin of the telescope is chilled below ambient temperature by exposure to the cold night sky
  • DC cooling fan behind primary sucking air past back of primary (with special baffle) to aid rapid cool down of mirror. Fan is speed controllable
  • Elevated 3 legged base to rocker box to raise up box and make observing more comfortable.  One leg has screw adjustable foot to enable fine control in altitude when centering objects in webcam field whilst scope sits on the equatorial platform
  • Home adapted electric focuser with slip clutch for manual override.  Focus able to be change in discrete steps of 0.015mm
  • Positional numerical readout of focus position to 0.015mm
  • On-board 12v battery for fan and anti-dew measures
  • Digital encoders fitted for Argo Navis digital setting circles
  • Lots of carrying handles -you can never have enough!
  • 9×50 RACI Celestron finder with right-angle Amici prism for correct orientation views and illuminated double cross-hair reticle
  • Green laser pointer