I've seen Prestons 20" RC thread on CN, That is one MAMMOTH under taking... It is amazing what some people can achieve when they set their minds to it...
Tell me, Do you lose any contrast by having the truss tube vs a closed OTA with some baffles? I'd think that it would, although if that were the case, the most high end scopes available wouldn't be designed in this way.. Planewave and RCOS obviously looked into this, and decided the loss of contrast if any was negligable...
I have to ask this too.. If it doesnt lower the weight of the OTA, what is the benifit over the standard VC200L OTA? wind buffet would probably affect the ota less during imaging, perhaps faster cool down times of the mirrors (This was the main reason I originally considered converting my C11 OTA to an open truss design)
Overall, I guess my question what were the planned benifets of this before you started the build, and has the modification achieved these goals?
Hi Alex,
I am yet to notice contrast issues - but they would likely be there if you have a backyard situation with stray light hitting the mirror. I haven't had any contrast issues with the RC or the VC from my back yard - there are a few neighbors lights that reflect into the yard at times. The VC baffles seem very effective. There are many pro reflector telescopes out there which produce exceptional contrast without a conventional tube or outer shield.
I did initially pursue a solid CF tube replacement for the VC - this was way too expensive by commercial suppliers. I know that a couple of CN users offer a service for solid tubes.
The steel tubed version rode on my Paramount for a while while building the 12 RCs, with a heavy camera rig at the rear and a heavy telephoto and camera piggybacked, connected via over and under Losmandy dovetails (probably a bit overcooked). The tube and dovetail connections flexed noticably and eventually visibly with this arrangement. Heavy duty saddles can alleviate this.
I am confident that I can reduce the VC truss weight further by additional milling and use of aluminium for the truss connectors - these are currently cast stainless. The original rear casting assembly is surprisingly heavy as well.
The truss appears to be extremely rigid and the upper dovetail can support a very significant piggyback weight like an FSQ with STL or similar without challenge. I'd say it would support much more if you have the mount to suit. If you carry an STL equivalent camera at prime focus as well - like many do - a decent capacity mount is required (obviously). Of course, a solid CF tube with a cage stlye saddle mount like the RCOS would have good rigidity I'm sure.
The main weakness is now reduced to the R&P focuser if you use a heavy camera rig. I know people who happily use the vc R&P with an SBIG STL - others report frustrating shift issues with the focuser. If you don't need all of the backfocus available an intermediate motorised focuser like an FLI PDF or DF2 works very well, since you just lock the R&P up and use the FLI to focus precisely. I have used a an ST8 with filter wheel, pyxis rotator and a DF2 attached to the locked R&P focuser - I also have a custom DF2 to Vixen 62mm adpater which helps a lot.
I'm hoping there is a way of bolting on a replacment focuser which preserves optical characteristics and removes flexure or weight carrying issues - possibly with a VSI.
Guy, great work. One of the often mentioned issues with VC200L is the think secondary spider vanes, which can cause Square stars in images. In your design are you replacing this with thiner vanes?
Thanks Fahim.
No.
I milled a few mm off the original vanes.
The square stars do not affect all users of the VCs - even with thick vanes according to some reports. However, logic would tell you vane thickness must make a difference to performance.
What a wonderful work!
