View Single Post
  #1  
Old 17-05-2018, 07:30 PM
Paul Haese's Avatar
Paul Haese
Registered User

Paul Haese is offline
 
Join Date: Jan 2009
Location: Adelaide
Posts: 9,489
GSO Carbon Truss Newtonian Review

I have had a long association with GSO since the first sales of the RC8 in Australia. There were some teething problems with the RC's along the way which have subsequently been resolved. Problems like the need for longer and better primary baffles, carbon trusses rather than steel tubes, better primary mirror cells and focuser centring rings to name a few.
So when GSO announced it would be producing a new fast Newtonian with a truss system I immediately thought it would be on my list of purchases, given that a lot of experience would be put into these scopes from previous developments, or so I thought.
Last year I took delivery of the 12” version and began commissioning of the OTA into one of my remote observatories. I removed the stock focuser and bought a focuser from a reputable manufacturer (which I have had to also replace with an Atlas focuser I had already because the new focuser was flexing too!).
What I like about the Newtonian:
· Conical mirror which aids in fast cooling.
· Primary mirror cell holds the mirror on a large thick plate, which prevents pinching of the optics.
· Optics test quite well with no apparent aberrations.
· Carbon truss OTA, makes the telescope lighter than a steel tube and probably more economical than a carbon tube.
· Images show sharp detail of both terrestrial and astronomical objects. I have produced quite a few images which show good detail despite the stiffness issues in the OTA.
· Fast imaging speeds with an f4 telescope. Subexposures are greatly reduced with a fast f ratio and a responsive camera.
What I found was problematic with the Newtonian:
My first imaging run showed the presence of astigmatism in images. Nice neat little crosses appeared where every star was on the first download. Rotating the primary showed it was the secondary that was at fault. With consultation it was suspected that the glue holding the primary onto the secondary stalk was at fault. Removing the silastic which had hardened on the outside and was still sticky on the inside; replacing it with aquarium grade silicone solved that problem. The silastic’s uneven cure was causing the secondary to distort and produce astigmatism.
Next I found like others that the secondary cage was flexing a lot and this was causing huge distortions to stars shapes when pointing at varying parts of the sky. I installed aluminium tubes with threaded rods (8 of them) and locking nuts. Star shapes had improved and star shape changes were now less severe.
With the help of a friend who is a machinist in the Astronomical Society of South Australia (Alan) we installed a counter weight to the secondary assembly. The thinking here was that the secondary was rotating perpendicular to the optical line and needed to be balanced. Again the modification helped a little but still there was movement in the shape of the stars from one side of the meridian to the other.
My next move was to glue the secondary shroud to the secondary assembly rings with Loctite. This created further stiffness but has sadly, did not remove the several pixels of movement left I was seeing in star shapes.
The final resort will be to rebuild the secondary cage and assembly. I think the focuser board needs to be a lot bigger and there needs to be another board on the other side of the secondary assembly too. The secondary spider also needs to be replaced with a far sturdier one than the one that is provided. The current vanes are too flimsy and have no way of tightening them either. Gravity is therefore causing the secondary to rotate and that is causing star shapes to change across the sky.
So would I recommend this OTA as an imaging scope? I am unhappy to say the answer at present is no. Whilst the optics do produce sharp images, the OTA cannot produce consistent results. The secondary cage, spider and focuser plate are way under engineered. It all needs beefing up to be usable as an astrograph or to be sold as an astrograph. When GSO fix the current stiffness problems, it should be a great astrograph for the price.
Reply With Quote