Vixen VC200L CF truss conversion

[Doomsayer]

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.

guy

[Dennis]

Wow – that looks just fabulous! Well done – a fine design and implementation indeed!

Cheers

Dennis

[Doomsayer]

thanks Dennis.

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.

guy

[mental4astro]

Truely, truely stunning work Guy. Workmanship to proud of, mate.

Top stuff, :thumbsup:.

Mental.

[Doomsayer]

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.


guy

[Doomsayer]

thanks.

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.

guy

[Dennis]

An excellent series of photographs showing the components and assembly – a really fine piece of work, thanks for posting the additional photos and details!

Cheers

Dennis

[iceman]

Lovely work Guy, and congrats on the ATM award at SPSP. Well deserved.

[Doomsayer]

thanks.
hopefully I'll get to use this a bit more seriuosly soon.
guy

[AlexN]

Very impressive work indeed mate! Where were you when I was considering converting my C11 to an open truss design??!! :D

[bratislav]

Could you please share the source of carbon trusses ? I guess you didn't make them yourself (possible of course but very messy).

[Doomsayer]

I got my CF tube supplied from New Zealand

http://www.c-tech.co.nz (http://www.c-tech.co.nz)

[gbeal]

Yep, great crowd to deal with too, I got some from there recently.
Gary

[leinad]

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 ?

[Wavytone]

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 ?

[AlexN]

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!!

Alex.

[Doomsayer]

I'm pleased that most people like the results.

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.


cheers
guy

[allan gould]

I'm always impressed by talent and good ideas. Such a beautiful scope - I'm in awe.

[Dennis]

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.

Cheers

Dennis

[Doomsayer]

Thanks again...

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.

guy

[AlexN]

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?

Alex.

[Doomsayer]

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.

hope the long winded reply suffices.
guy

[AlexN]

Thanks for the extra info Guy..

[Emanuele]

Soomsayer... you have a PM! :) What a wonderful work!

[netwolf]

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?

Regards
Fahim

[Doomsayer]

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.

guy