Stunning stuff indeed. How are going to focus it?. RCOS move the secondary. If you add a focuser at the back, dont you lose the necessary constant within-a-mm BF distance?.
No matter how you focus, the distance between the front of the primary and the imaging plane does not move. you're either adjusting the camera position to place the sensor within the focus zone, or moving the secondary to bring the focal plane to the camera. the physical distance between the primary and the imaging plain remains the same...
I remember reading a while back on the RC design, the distance between the rear of the primary and the focal plane is critical, I don't quite understand how however..
RCOS, who sell a scope that costs so much money are in theory making the image quality suffer for convenience. The important thing for an RC is the mirror spacing. This paper talks about it a bit
So for an RC to work at it's best the spacing between the primary an d secondary should be a specific fixed length. A moving secondary obviously goes against this even though it is most convenient for a user.
In mine the spacing will remain constant and an extension tube is on the back of the scope bringing the camera to within ~4mm of focus. The FLI PDF focuser has 8mm of travel which will bring it to focus at the correct BF length..
Ahh.. thats it.. I must have confused the details (it was a good 8 months ago that I read the info) I knew there was some measurement that was critical.. Distance from primary to secondary.. In which case, yes, Jason's design is better...
I don't see how a moving secondary could be seen as more convenient to the user however.. whether the camera moves to achieve focus, or the secondary moves to reach focus is of little consequence to the user at the end of the day. as long as its computer controlled, its all the same...
Jason - that's a bargain! You know, you could probably double the cost (say AU 10k), and take this up as a full time business, and you'd probably get a tonne of orders, enough to keep you in business I think. Of course, delivery times would probably be slow, if it's a one man, labour intensive team. I'd honestly consider 10k for something like this in a few years time when I actually have the money lol!
when you can have so many accessories like OAG, filter wheels, rotators, reducers etc then moving the secondary will get you to focus with all sorts of imaging trains.. Much more convenient then having to add the right extension tube for every conceivable set up.
I see.. yours is designed specifically for your imaging train where as RCOS build for the multitude of options available to us.. If RCOS did things as you did, they would have to provide extension tubes at increments of 4 mm or there abouts to accommodate different external focusers and imaging trains...
Yes, I dont understand either, what you say makes sense. But here is a quote from the RCOS site.
Secondary Focuser
"Cassegrains have a two mirror, amplified optical system. Mirror spacing and backfocus are directly related to each other. If you change one, it affects the other. Different accessories or cameras will change the backfocus, which will change the required mirror spacing.
Since an Ritchey-Chrétien is part of the Cassegrain family, a secondary mirror focuser is required for optimum optical performance. All of our telescopes are shipped with a secondary focuser as standard equipment."
It doesnt explain why here really, shame. I wouldve thought an external focuser at prime would be easier and cheaper to make, so I guess they thought it necessary.
I see.. yours is designed specifically for your imaging train where as RCOS build for the multitude of options available to us.. If RCOS did things as you did, they would have to provide extension tubes at increments of 4 mm or there abouts to accommodate different external focusers and imaging trains...
Yes Fred, spacing and back focus are related and spacing will change the other but there is only one optimal spacing and that will give a specific back focus.
Now, that back focus may be 13" but not a lot of people end up with having enough extension tubes to achieve that, especially if they dont have a long imaging train.
It's wrong for them to say that different accessories will change the back focus. The back focus is constant for the optimal mirror spacing. What they are saying is that it's more convenient to bring the back focus to the camera by changing the spacing then it is to get the camera to the proper back focus which is determined by the correct mirror spacing..
So I understand, in a commercial world its the easiest way to sell stuff, but there is no theory that says what they are doing is optimal..
Having said that, as long as you don't change the backfocus too much there probably isn't too much of an issue. If you look at the RCOS scopes that the best imagers have they are certainly using the right amount of extension tubes so that the mirror spacing is as close to perfect as it should be..
The design is a certain pitfall for people who dont know and make radical focus shifts using the secondary..
Thats why they provide a set extention tubes to get within a mm or so of BF with a mid range secondary focus to start with for optimum mirror spacing. Then there are tables of distance values for common image train devices including FRs so that BF distance can be calculated and the correct extention tubes added. Its a pain in the bum to sort out, but as you know, each scope is provided with its own mirror set BF so that it can be worked out for the image train and spacers provided.
I myself cant see any reason why final minor focus cant be achieved by secondary or external at prime devices, ive tried both and cant find any realworld difference. Its a bit unfair to trash the way RCOS approach this, seems to me either way would give acceptable results.
My scope has the standard 12" BF, and with all the gear I have in the image train, I have no trouble reaching the optimum with spacers provided.
Hi Fred, I don't mean to trash the way RCOS do things.
There is always theory vs practice and there are many, many, RCOS scopes that show their way works fine. I sure would be happy to have one of there scopes so I don't mean to bash it by any means.
I think what got me going was the description you posted from their literature which said 'a secondary mirror focuser is required for optimum optical performance'.
I just don't see how when they are actually using it to adjust the BF to the focal plane,not move the focal plane to the mirror sets real BF.
It's academic really because if you follow their instructions your not going to complain about the results, I wouldn't and it seems neither would you
Hi Jason
Good to see you're nearly at the end. These are not easy things to build!
As you know I built two similar RC truss scopes using the same optics (shown on a thread here some time ago).
My designs are quite different and use a very beefy Van Slyke secondary mirror focuser & spider. I started my RCs at about the time you got your mirrors as I recall (I have been in touch with Jason quite a bit during the throes of construction off forum and on CN). Both scopes are performing well now - I had a few teething problems with alignment of some components which took me a while to sort out. It is a very 'solid' scope - which really needs a PME or an AP1200 to carry it with extra gear.
Paul Jones (optician at Star Instruments USA) told me that mirror separation should be within a few mm (not super critical) but that concentricity and relative 'squareness' of the two mirrors is more critical (plus any subsequent collimation). So placing the image plane at the right position for optimum mirror spacing is a bit tricky in these short backfocus RCs.
Because these f6.7 RC optics have a relatively short backfocus of around 9 inches from the front surface of the primary, I felt that a secondary focuser is essential - especially if the rear plate of the OTA and the primary mirror cell eats up a few inches of backfocus - and also if you want to have a large camera rotator and an off axis guider in front of the camera.
The two RCs have a doublet field flattener which reduces the backfocus to 8.5 inches. I plan to upgrade my Pyxis 2" camera rotator to the 3" one before long. Still saving pennies for a bigger camera - likely an FLI 16803. I have the loan of an STL6303 as of yesterday for a short while - hopefully I'll get the chance to use it with weather and other to-do's that get in the way.
I visited our local powder coater today and brought the scope to get a quote. I haven't powder coated anything before so I wasn't sure what I was in for.
He said $150NZD so I no longer have any plans to paint the thing myself. I will clean it up a little and hopefully drop it in tomorrow. Should have it back by mid next week.
I finished off one of the counterweights today. I copied how the paramount one works with a bolt pushing a spring loaded pin on to the counterweight shaft.
I will get the weight powder coated as well. The big difference between mine and the paramount weight is that mine coast ~$45nzd and a paramount weight would cost between 400-500nzd to get one landed here.
I like to see precision workmanship and this looks to be the case with your OTA construction. Very impressive work. I hope that everything works as it should for you. I am looking forward to seeing the images you get from it later in the year or when you get a camera on the back.
Thanks Paul and Ryan, it's always nice to get (positive) feedback
I certainly hope that first light pics wont be too far away and that there wont be weeks worth of tweaking after a camera is attached but who knows, it's my first venture in to this type of telescope.
I am so jealous .... 14-15" is my next step up, after I sort out my 10" OTA issues and have had some fun with it and imaged everything I want that's in a 10" 's capabilities.
How heavy did your assembled OTA windup being (with optics installed) ?
You're a bloody legend! 
... and it looks like a million dollars too.
