For those who are interested I tried a simple Ronchi test on the mirror tonight. Just as well I have a long workbench in the garage cos I was twiddling around at 3.2 meters. I don't have a tester as such but taped a printed ronchi screen (200 lpi from Stellafane) over a small led torch ( don't laugh, it works ! ) and in the darkened garage had a look. Looks to be a very classic long FL Parabola. Central bulge and curves with a wee turn out near the edges. No deviations I could see. Mirror seems good.
>Arief : Nope, should be good on the EQ6 for weight. Construction is very lightweight so far. Using a 6 strut design (3 pairs) rather than 8 and UTA is in 12mm ply rings. My EQ6 can handle the 10" f5 no problem. Weighs about 18kg all up. This should be lighter although longer. Parking it on the mount might be more of an issue with roof height and distance to the Ob walls. OTA total length should be just over 1.5 meters.
I'll get some pix done tomorrow.
UTA pic as promised. Very lightweight. Note the nice clean working area . The 6 struts are 2 alum 10mm tubes plus 4 alum split extrusions with an extruded side plate seen in the second pic. The Focusser plate will be attached there and a light shield on the opposite side. Dowels in the tube and wall fitting plugs in the split extrusions hold the screws.
Funny how ideas come to you if you ponder long enough. The spider design has been bothering me. I like Rolfs wire model but wasn't comfortable with it's mechanics, seemed slightly complex. I didn't want to go with another 4 vane version as in my f5 10". Hunted around on www and found some Guitar machine heads in black. Then thought about how guitars guide their strings over a metal or plastic bar at each end. Which led to thinking about mounting the Machine heads through the UTA upper and lower rings with the capstan above and below each respectively and guiding the wire (.23mm steel E String ) over the outer edge with a metal guide. It extends the wires to the max angle with no chance of slipping off the capstan.
I like the idea so much I'm ordering 3 sets ( 6 heads in each set = 18 ) so I can modify the 10" UTA as well. I need 8 for each UTA and they're only $10 a set. E Strings are $3 for 5.
Watch here for updates
Cloudy weekend, lots done. Got the 12mm ply, build the centre cage. Drew up a cardboard template first so I could draw them out identical. Marked up the ply and routed the centre hole and then trimmed the Hex's out. Cut 3 pieces of 20mm finished timber and squared and sanded them to exactly equal length. The cage is the reference point for all other measurements so needs to be as square and parallel as possible. Checked assembly accuracy then glued and triple screwed every joint. Checked after completion and it came out within .25 of a mm for parallel between the two hex faces. Phew !
The dovetail will mount off one of the three side plates, a guide scope off one of the other two sides.
The round disk assembly on the last photo is a tentative idea for a back plate for the mirror cell, still cogitating on that and ideas for the truss tubes mounting hardware.
Also picked up some varnish for all the timber work and the three bolts and wingnuts for the mirror cell.
Machine heads and E strings are ordered.
It's been a wet and wild weekend, can you tell ? More progress with the centre cage. First a trip to Bunnings for a few bits then made up the brackets for the truss struts to attach to. All cut and filed to the same length then set up a jig on the drill press to drill the holes all accurately. I have worked out a neat and tidy attachment method you'll see later. Drill presses are very useful.
Then started the mirror cell. Basic design now sorted. Still a lot of weight to be removed from all the ply disks involved but again minimal hardware and complexity.
Update & Piccie Time, quite a bit been done. Centre cage finished, Mirror cell done with my mini 6 point cell design and UTA waiting for Machine heads to arrive. Also a look at a simple adjustable drilling jig for when you have maintain dimensions across componentry.
Mirror Cell assembled except for side retainers. I'll fit those when I place the mirror on so I can set them for minimum gap. Velcro mirror pads are in place now. I strip the white backing off the top and lower the mirror onto them and that's it.
Side on shot shows the rubber 'tyres' that are the adjustment springs for the cell. Entire cell is only just over 5.2 cm deep, very low.
Been busy again.
Built and assembled the LTA and trusses. Again the critical factor is accuracy so the distance between the bolt holes on the trusses has to be identical, 380mm in this case. The wooden insert is to give the alum tube end support and held the shape under compression of the screws. Finished up weighing 2.9 kg and the back plane for the mirror cell is parallel within 1mm of the front element of the centre cage.
Guitar machine heads and the strings have still not arrived yet so UTA was untouched for now.
Once it is all assembled it will all come apart again and all shiny metallic surfaces will be sprayed black matte. Was looking for a protective cap and light shield for the mirror once installed and I think it may end up being a 230mm cake oven tin. Should sit down over the mirror onto the brackets nicely. We'll see.
Dave > If I use a blob of silicon at each end then I have a six point cell. I chose to use the Velcro because it is a heck of a lot easier to remove and reposition. I understand what you are getting at but if you look at the alum bar as a lever then the ends are the levelling points as it rocks to evenly support the mirror. The other nice thing about the Velcro is it has a minute amount of give vertically and horizontally in the flex of the hook and chain surfaces so minimal stress.
Guitar Machine Heads and 'E' strings arrived and it was a wet weekend so workshop time. Other than attaching the mirror and centering the mirror support that is completed. That will let me preset the upper truss lengths so they are last on the build list. I'll grab the dovetail off the 10F5 for now and I have a spare finder bracket.
The wires are only .23mm diameter, almost invisible and I will blacken them later. The scope is designed as an astrograph specifically, it is not a visual scope.
The wires are only .23mm diameter, almost invisible and I will blacken them later. The scope is designed as an astrograph specifically, it is not a visual scope.
They maybe thin but they will not be invisible. You could end up 16 spikes if you're lucky. If each side is not lined up that makes them .46mm and if one side is not parallel to the other you will get two spikes per side.
Design and accuracy in build, I've spent quite a lot of time working on this idea ... we'll soon see. This is my third reflector scope build, I seem to get better each time and I am only emulating Skyvikings (Rolf ) work.
Blackening them will actually increase the diffraction spikes by making the wires slightly thicker.
Very nice build, looking good. .
Dave
Thanks Dave,
Black Marker pen, it will stain the wire surface, virtually no thickening.
Rac > Yes, my last build followed Rolfs model, the 10' F5. This build I've picked up the wire spider ( I may modify the 10" as well, I bought extra Machine Heads and 'E' strings)
If you are referring to his swivelling pivots for the machine heads, yes, I was aware of them. But another comment he made was how stable and rigid the secondary was in the wire spider. It holds collimation and under normal use shows no movement at all. By coming through the UTA rings with the spindles and matching the alignment and wire clear side on the spindles of the machine heads they all align and free float without the need for any more mounting hardware. My UTA is therefore shorter and lighter. Also this is a 6 strut build rather than 8. Saves weight again and aesthetically it just looks better matching with the mirror cell bolts which always bugs me with visual alignment of all the nice hardware. Yeah, I know, a bit OCD ..
I may need to adopt his method if I convert the 10" as the build specs are different for the UTA.
Hopefully this weekend I'll get the upper trusses done and assembled and you can see the whole beast. I'm kind of impresssed with how nice it looks and how well it has come together myself. A rather satisfying project.
And if this version of the spider doesn't work as expected I can easily build a conventional 4 vane, more fun workshop time.
Another (extremely) wet weekend and OTA is fully trussed. Axially it is about 2mm out of alignment end to end but that is easily sorted. Not bad result I thought. I'll tweak that right first and do a simple collimation with some spare mirrors I've got before I add the secondary. Then it comes apart for blackening all the shiny surfaces, light shields for UTA opposite the focuser and around the primary and the dovetail attached.
at some point, you might want to consider using Carbon Fibre Tubes instead of Aluminium even if just for the UTA.
you won't believe the rigidity a broad diameter CF twill tube offers and the weight it reduces and costs around $30. But don't get it from ebay. they're hopeless. https://flic.kr/p/o6hNnU
. The 6 struts are 2 alum 10mm tubes plus 4 alum split extrusions with an extruded side plate seen in the second pic. The Focusser plate will be attached there and a light shield on the opposite side. Dowels in the tube and wall fitting plugs in the split extrusions hold the screws.
