A number of poeple have expressed interest in my recent project which replaces the original steel tube in a Vixen VC200L with a very rigid carbon fibre truss assembly optimised for imaging - hence this post. I showed one of these to a number of people at SPSP - feedback seemed very favourable. It broadly resembles a little RCOS carbon fibre truss OTA with over and under dovetail plates and black anodised milled aluminium plate.
As with many other VC200L users, for some years I have been keen to enhance the performance of the out of the box VC200L Having recently constructed two 12.5" CF truss tube ritchey chretien telescopes I had some useful design & build experience to draw on.
The main aims were:
1. Retain the existing primary & secondary mirror assemblies
2. Maximise thermal stability, rigidity and optical alignment of the OTA and dovetail mounts
3. Reduce the thickness of the secondary spider vanes (originally about 5mm thick in mine)
4. Use off the shelf components where applicable.
I'll add a few posts here which give an idea of what it looks like and how it is done. I am very happy with it so far. I've made two conversions so far.
I have been talking to Paul Van Slyke of VSI about replacing the existing R&P assembly with something more hefty.
It should be noted that the original steel tube now acts as a light shield - it does not have any structural function.
I also decided to use a Losmandy 8" SCT dovetail rail kit in the build to save work - I had a couple already. The two Losmandy radius blocks are carefully bolted to the primary mirror casting, after removing the primary mirror. This requires drilling 2 or 3 3/16" holes in the casting with the radius block firmly held in place by small bolts.
Two ~100x100x25 mm blocks are bolted firmly to these radius blocks - and are also secured to the rear baffle flange (which carries the truss poles). This provide the rigidity to the rear cage - if made carefully it is extremely rigid and makes for a concentric and square truss. The secret to such trusses is accurate and repeatable placement of attachment points and consistent component sizes.
I have also accurately centre spotted the secondary mirror, which makes laser and other collimation techniques far easier, especially since you can see the laser dot from outside the truss.
I had an old VC200L ota which has been subject to a number of tortures over the years. This is shown in these photos as the one with the black painted rear casting. Once I knew the first build worked, I then converted the more original looking one here.
I accurately milled two fitted slots in the 25mm thick block - this holds the radius blocks and rear flange plate firmly and squarely in registration. A couple of photos here without the shortened steel tube show the design idea well.
To round this off, a few more pics of the MK2 ota.
This time showing the front end. The existing spider casting sits snugly in the front truss ring, held in place by grub screws. I milled 2 or 3mm from the vanes, plus I was very careful to keep the mirror spacing constant and in register during this project.
Weather has not been good to say the least - some quick tests with a 5D only. No secondary dewing issues as yet.
Another two of these conversions are yet to be completed for some friends.
That is fantastic work! Would weight does it come in at now?
Theres been talk on the VC200L Yahoo Group forums that Starlight instruments are working on a new focuser now. JMI and Moonlite have apparently halted their plans on a new focuser due to design limitations.
I'd be eager to do this one day also. Would you be looking at placing orders for this in the future ?
Seems to me you're trying to turn a sows ear into a silk purse...
Taking the optics from a VC200L (and little else...) build a whole new scope... I very much understand the effort involved to get it right - but what I don't understand is was it really cost effective compared to buying a better (and possibly bigger) scope from the outset ? Was it really worth that effort for a measly 20 cm aperture ?
I dont know hey.. The VC200L optics are very highly regarded... I think to be honest, price to performance for an imaging scope they are hard to beat really... spending a bit of money to convert a nice scope into a very nice scope may well be worth it. And for an imaging scope, in most cases 8" is more than enough aperture to capture an absolutely stunning image... Look at what some people are producing with 4"~6" APO's. Sure, you will get more detail on a perfect night with a 12" scope than an 8", but then you're talking about going to a more stable mount as well.. I'd really love a truss converted VC200L, if nothing else, it TRULY looks the business!!
I admit there is some 'why would you bother' element to this.
Not sure about the sow's ear comment. I left both the rear end primary casting assembly and the front secondary cage intact, apart from a few carefully placed holes.
I had the tools and access to materials to do the conversion pretty easily, provided I didn't get bogged down in making mirror cells, lens mounts, baffle tubes etc - this stuff is difficult and would make the conversion tedious and possibly pointless.
Although I like the idea of having something bigger than a 12.5" RC, I doubt that I have the seeing locally to justify a conventional 16" or 20" RC, CDK et al on most nights.
This reinvented VC OTA is quite a bit heavier than the original steel tubed one. Weight was not a significant consideration in this case for my needs.
If you outsource the milling & drilling of plate for parts here I'd guess you would be looking at $800 to 1000, maybe more (for small quantities). Other parts such as the truss connectors and CF tube add another $200 -300. Plus the Losmandy SCT dovetail and radius blocks. If you but the VC OTA cheaply as a used item, then the expense is not so huge.
Yes – it is inspirational to see what a skilled person can design and build when they are in their element. I don’t have metal working skills and thus these projects often appear quite daunting to me. But, I have seen some of the ATM guys at the ATMQ fabricating small pieces with their (under the house) lathes and milling machines and they make it look quite effortless; such is their mastery of tools, techniques and materials.
My machining work is adequate for the task and certtainly looks the part in this case. I am not a professional machinist. The 20" CF truss Ritchey-Chretien build ATM thread by Preston on Cloudy Nights is an example of the 'excellent' end of the serious ATM spectrum - worth a look and inspirational if not daunting. I'm sure there are more skilled astro-oriented machinists like Preston still lurking on this forum and elsewhere.
I am considering doing very small runs of these VC truss conversions for people - or am happy to advise people. This was never my intention when I started the project however - just a curiosity project for me - more a response to personal feedback off forum. There is about a week of milling & drilling required for the two prototypes so turnaround would not be immediate if this went anywhere.