Hello
I've just completed construction of two near identical 12.5" f6.7 Ritchey Chretien astrographs. Yesterday, I put #one onto my Paramount for a the first time - certainly looks the part - see pics below. They are based on 'budget' pyrex optics from Star Instruments and include a 2.7" field flattener doublet and a big 6" secondary mirror. They resemble the RCOS carbon fibre truss 'scope configuration, but are basically my own build design, based on sparse information and advice from diverse sources and a few ideas of my own. Anyone who has considered an RC ATM project would be aware of the limited 'how to' info out there. Early inspiration came from another ATMer in NZ who has the same 12" optics, plus the bloke who has posted detailed pics of his impressive looking 20"RC on Cloudy Nights.
If people are interested, I can post some more pics of the various stages of the process.
They combine off the shelf parts such as a VSI secondary focuser (massive!) and spider, plus A-P 2.7" barrels. I have a conical secondary baffle and an unusual conical profile primary baffle tube of my own design which has stepped rings on the outside of the baffle to mimic a conical baffle. There are several knife stop baffles inside the main baffle throat, plus flocked paper elsewhere. The rest is CNC or hand machined 6061 plate, custom anodised, plus some custom made threaded fittings. I had the advantage of CAD plus 1:1 print layouts to check the design as I went - this allowed me to avoid or solve problems. No significant mistakes as yet.
If I did it again I would use more CNC to lighten up the Al-plate a lot more and maybe look at building a smaller secondary focuser. The two OTAs are beefy at close to 30kg each. I have carefully tested squareness and concentricty of the OTAs with a laser and paper targets - all basically bang on - which is a relief!
I am now of course waiting for the clouds to part so I can start the tuning & testing process with my ST8. The other OTA should get some testing with a bigger chip at the end of the month. I expect that these will deliver a large flat field - to be confirmed.
Thanks Doug, they look great let us know how they perform, and I for one would like to see construction pics as well. Many of us have thought about doing what you've done.
Cleared up last night here. I fired up the the new RC on the Paramount last night. The out of focus stars looked well collimated - the earlier daytime adjustments did the trick. After a quick resync and a mapping run, some slews to a few DSOs revealed tight stars across the field of the ST8xme chip - as expected. Now to getting all of the hardware and software running, including the pyxis rotator, the robofocus unit on the VSI and the guider - all through CCD autopilot. Might take a little while to get it all talking - need to update few old drivers.
I will sift through my archive of photos of the build process and post a series of pics today or tomorrow which should give some idea of the contruction.
I've attached a simple artists impression of the rear cage. This is the crucial part of the OTA. The upper and lower dovetails are keyed into the front and rear plates via a rebate and a slot.
mate what an awesome project and they certainly look the goods. You are indeed a clever and talented ATMer. Will be expecting a test image (when it does eventually clear and you have the time)
Like Houghy says "Awesome!" job there. They look beautiful.
I would love to make a little 8" RC in the future if I ever get a little spare money. I did some research some time back and like you say...very little info, especially with baffling and spacing..
more pics please!
Guy, these are awesome creations and they look so professionally made. Excellent work. Hmm you have two there could one get lost in transit and delivered to the wrong address perhaps ... lol...
OK - thanks for the compliments & kind words - It has been worth the large effort for me.
I should say also that I am not a hugely experienced machinist, hence the simplicity of the design, but I am good at building things generally. So apologies to any expert machinists on the forum - I do have a couple of friends who I have learnt much from regarding precision machining over the years.
I will now start posting a few low res pics in some sort of sequential order as promised.
A rough estimate of cost is at under 10K each - difficult to measure now with the shift in the US dollar. Remember since these are sort of protoypes and has some quite expensive off the shelf parts it is difficult to economise.
First pics:
1.The two mirror sets fresh from Star Instruments USA with Spectrum USA enhanced coating. Paul Jones was very helpful at Star Instruments.
2.The raw 6061 Al-plate for the rear cage after CNC routing. This was the minimum CNC required to ensure accuracy, concentricity and squareness etc of the rear cage components. With hindsight, I would have done more CNC at this stage - but it was good to take it slow with an 'analog' milling machine - this allowed me to be more sure of the design effectiveness as I went.
3. A preliminary assembly of the rear cage after drilling & threading holes etc. The next step from here is to 'honeycomb' mill the plate to reduce weight. A lot of work for me here.
1. rear flange and barrel assembly - this holds the primary mirror and the field flattener lenses - took a while to get the design thinking right here.
2. the primary cell is a simple 9point system with push-pull bolt collimation. A flange at the base of the primary baffle tube holds the mirroe down with a cork washer in between.
3. the secondary conical baffle was TIG welded from aluminium
More pics approaching the final stages:
1. The primary baffle is shown mounted but unfinished with the conical profile step rings on the outside.
2. assembly of the two OTAs shown side by side. still more finishing work at this stage to do.
3. All of the milled marts polished and ready for anodising. 'Anodise This' in SA were very cooperative and reasonable to deal with - they were happy to do small jobs.
Now time to do a visual optical test to assess if there are problems with collimation or pinching of the optics etc.
The visual star test was good. no obvious optical problems and the simple collimation system works. Baffling seems effective as well.
The picture shows the raw milled OTA on the Paramount. Note that 25mm aluminium tube is being used for the struts here - carbon fibre tube comes later.
A few more posts to come later showing the finishing stages.
1. red anodised rear plate - anodising by AnodiseThis (SA).
2. 2.7" two element field flattener mounted in the barrel. The placement of these lenses relative to the mirror surface is critical.
3. two anodised OTAs assembled prior to fit out
The other OTA is for a friend of mine - we have been sharing the costs.
Pics of:
1. two assembled OTAs without optics, showing fan wiring
2. anodised red rear plate showing collimation screws. It is possible to level both the rear 2.7" collar and the primary baffle from here. Note a 'riser block' added to the lower dovetail mount - this raises the OTA struts above the level of the Paramount versaplate - probably not needed for other mounts.
3. benchtop collimation check with sight tube and a good 2" laser. At first light with this OTA on the PME, out of focus CCD star images appeared textbook - relatively easy to collimate. The field flattener does not appear to distort the path of the laser in this case.
I will likely add a Kendrick Premier dew controller - this will allow me to control the 12v fans, plus heatstrips and monitor optic temperature. At the moment the RC just has a simple on off switch for the rear & side fans.
Now to concentrate on getting the thing going with imaging - road testing and evaluation will take a while yet.
Certainly beautiful workmanship! More pictures... Yes please.
