I'm starting off with my build of a 10" F4 OTA targeted at AP. I've ordered the primary and secondary mirrors from Orion Optics in the UK and should have them this week.
I was looking for ideas with building a 6 point mirror cell and secondary holder and had a few questions
in threads here, velcro is mentioned as a good material to secure the primary to the mirror cell. I don't want to fix it permanently so velcro sounds good, but which type of velcro do you use and how do you stick it to the underside of the mirror and where do you get them?
second, as this will be used for imaging, is there a possibility of the mirror shifting ever so slightly if velcro is used?
For the secondary holder, I might use a solid wooden rod with a 45 degree cut. what type of silicon do you use and can you get them at bunnings or similar hardware stores?
I will be building a wire spider similar to Rolf serrurier truss ota.
I still have to figure out how the secondary holder will be held by the wire spider.
For the OTA itself, it will be a truss, just finalizing the designs now.
I had plans to build a 12" F4, but since I need to keep it portable enough, I decided on a 10".
re mirror support, if you want to image over longish periods, would suggest a rethink of velcro - any movement of the mirror will mess up your subs. Velcro is probably fine for visual use, but my gut feeling (admittedly not backed by experience) is that it might be a bit too flexible as the scope moves around the sky.
assuming that you will be using an EQ mount, you also need to consider an all-angle edge support as well, so maybe you could consider glueing the mirror onto a ring at 0.4 of the mirror radius with three equally spaced dabs of silicon - I did that with an 8 inch f4 and it provides good stable support with no noticeable surface distortion in a very simple mount (no edge support needed). According to Mark Holm's PLOP analysis, 3 point mounting should also work fine for reasonably thick 10 inch mirrors. (http://www.atmsite.org/contrib/Holm/...timized_cells/). What sort of mounting did you use for your 8 inch?
You dont want a permanent mounting and silicon could be removed fairly easily with a scalpel if need be (not as easily as velcro, but still possible).
I used an acid cure silicon adhesive from the local hardware store - 3 dabs about 1cm across with matchstick spacers for thickness control.
I also built a secondary holder using plastic waterpipe fittings and it works well.
would you have any pics of what you've done?
I ran plop and a 3 point cell came up with pretty marked distortion, hence I'd decided on the 6 point cell with centre pivots.
I might either use wood or aluminium plate as the mirror bottom support. So if I use silicon, can it be removed or will it damage the mirror?
I'll give the matchstick spacers a go.
how long does it take to dry.
I was thinking of using three side supports with a 1mm pad to the mirror with a slight overhang of the side support which won't actually touch the mirror.
as for the secondary, how do you ensure that the major axis is perfectly centred with the axis of the holder?
if a heater strip is used for the secondary, will the heat affect the silicon bond?
Attached a pic of the secondary holder. The adhesive took a few days to cure fully and I would be very surprised if a heater could affect it. The silicone is pretty tough and you would need to be able to get at it with a scapel to free up a mirror - a solid back plate would be a problem (but that will also produce cooling issues). It would also be difficult to clean it off completely from a mirror, but it should be possible to get most of it off with a blade (maybe try some out on window glass).
I was also concerned as to how I could align the major axes, but found that it was possible to get good alignment by eye - I just marked the desired offset longitudinal position of the support on the back of the mirror and then moved the support around until everything looked symmetrical when it matched the mark. maybe I got lucky, but it really wasn't difficult at all.
I guess your primary mirror is fairly thin if PLOP shows significant errors on a 3 point mount. Even with 6 point, glueing onto the support points might be worth considering to get around the edge support issue. The standard Dob edge supports are fine if "down" always has the same alignment to the OTA - but of course it doesn't with an EQ mount and you will always have residual (and varying) astigmatism from non-optimal edge mounting. I found out how bad an imperfect edge support can be when I accidentally set up my 12 inch Newt with an edge support directly under the CofG and taking much of the weight of the mirror - the astigmatism was horrendous and way worse than anything arising from a glued mounting (which is its normal situation). The discussion on http://www.cruxis.com/scope/mirroredgecalculator.htm seems to be on the money and well worth a read if you haven't already come across it.
regards Ray
added comment: I just looked up Rolf's description of his scope - he used 3 silicone blobs to mount his 10 inch primary
Last edited by Shiraz; 02-03-2012 at 07:44 AM.
Reason: addition
Thanks for that.
So did you use silicone all round the edge of the pipe that contacts the mirror? did you use spacers?
is there a specific type of silicone that is to be used?
here is a drawing of the mirror cell I have in mind. it'll be a combination of wood and aluminium.
the 3 bars will be on a pivot. I'll have to use silicone on the 6 pads so that'll hold the mirror. I've added edge supports that don't contact the primary, maybe a mm off just in case it dislodges during transport.
will add a fan in the centre as well.
Got the specs from orion optics, the mirror has an edge thickness of 26mm and weighs 2.7kg. secondary is 75mm minor axis and is 10mm thick.
I have an 8" F4 at the moment, so I'll try out the mirror cell, spider and truss design on that first and then build the 10" with carbon fibre truss tubes.
do you have pics of your secondary holder support fixed to the spider and your mirror cell?
thanks
the silicone is just 3 dabs - if you look carefully, you can make out the gaps between them (where the bench top can be seen between the mirror and the support). I used matchsticks as spacers until the silicone was partially set and then pulled them out. It is just standard acid cure Selleys clear adhesive/sealant - nothing exotic.
your mirror cell looks the goods and the mounting philosophy seems sound to me for the relatively thin mirror. Your design would also allow you to easily try out a 3 point mounting if you wanted to reduce the complexity - if it didn't work to your satisfaction, add the 6 point components?
I do not have access to my scope where I am at present, but will post a couple of pictures when I can. Nothing unusual though - the spider is a bog standard GSO with 4 vanes and a central hub. the secondary holder is attached to the hub with a central bolt and alignment is taken care of by 3 screws near the perimeter of the hub, that push against the secondary holder. nothing as sophisticated as you will be building. Also worth considering - Anthony Wesley came up with a very solid secondary support system that you could look at if you haven't already - it's on his website.
My mirrors are in transit via Germany and should arrive thursday.
I've drawn up some plans on two options, one a serrurier truss similar to Brent and Rolf's designs with changes, and the other similar to an RC truss OTA.
Since its an F4 and overall length will be short, I could get away with the RC design as midpoint would be quite close to the mirror and sag not that pronounced, that'll reduce stress on the central section as well and make the lower section very rigid. That will in turn keep the upper section centred rigidly.
I'll have to calculate the mid point and then decide.
Haven't decided on the secondary supports as yet.
But will be starting build of the mirror cell and upper section this week.
I might also change the central support from a square to an octagon like the officiane stellare ultraRC series. that changes the angle of the truss to less than 90deg from the light path and should reduce flexure of the upper and lower sections as it resembles a cone. but issue with that is the trusses need to have a ball and socket end due to the varying angles.
Yes I am, that's the truss poles though.
I'm ordering a few samples to determine the ideal diameter and stiffness.
The sheets are a bit expensive so I may use Aluminium although I don't have access to CNC machining.
Hi Alistair, bit late to the discussion but Velcro works great IMHO. I used 3 strips 77mm(3") long, 25mm (1") wide Industrial Grade onto 3 x 25mm alum flats that were my 6 point support built very similarly to the diagram previously posted.
The Velcro will support about 500 gms (1 lb) per square inch of surface area suspended upside down.
I use three edge supports, 1 mm gap with a dob of silicon through to the side of the mirror to ensure it doesn't move laterally but they are not really necessary.
As far as I can tell there is zero stress on the mirror due to the spread surface area and characteristics of velcro and it has certainly held collimation without any adjustment required since build.
I'll post a pic from home later if you want.
6 point was best support design, the 9 and 18 looked apparently worse and involved heaps more work. 6 point as you have drawn is easy and works well.
The Serrurier truss is a bit of a fiddle to build, accuracy in build is the key issue but I'm very happy with its stabilty, strength and lightnes. Not to mention it's easy to access parts for modifying as I did last night.
I wouldn't bother with an octagon centre cage. Too hard to build without welding gear etc and at F4 you build will be plenty strong and stable enough. Rolf makes a good case for the design parameters. I also mounted mine to a long dovetail bar. Allows me to slide it in the mount head to balance any changes in accessories and eliminates the need to add weight.
Ignore my 'Rotating UTA' system. Works well enough for visual but when I go serious AP I'll lock it down. It is handy to be able to pop the whole UTA off though to work on although not needed much now, maybe later I might get mod happy.
Last edited by ZeroID; 07-03-2012 at 11:18 AM.
Reason: More info...
Thanks Brent, that would be great.
Where did you get the velcro from?
Also, I've seen pics of your al flat on the mirror cell but how does it pivot as you have a bolt in the centre.
I guess you'd need a see-saw type motion for the 6 point cell which is the motion I'm aiming for.
I've seen pics of an Al U channel pivoting on an Al square tube.
the dovetail bar is the bit that worries me as mine is the vixen type for the heq5 which is quite narrow.
if you attach the mount on the bar section that's outside the central brace, won't that bar flex as the bar is only fixed to the central brace by bolts along its axis and not like a wide flat that Rolf or Doomsayer uses that has bolts side by side covering a larger area.
That would distribute the shearing forces a bit wider than the narrow dovetail bar.
in my case, I'll be mounting an ST80 on the opposite side which weighs around 2kg's all up.
what I'd planned to do was to bolt a wide plate in the central section and then bolt the dovetail bar on that as then the shear would be across a broader section of the aluminium square and the plate would shear less than the section of Al square tube where the dovetail bar is fixed.
I'm also looking at Carbon fibre sheets I found on EBay.
They're 2.75mm and I'm wondering if I can sandwich a few layers. Just a thought.
after a few calculations with an octagonal central and distance to primary from newt.
I know the octagonal will be more difficult to construct given truss angle to optical axis will no longer be 90 deg, but my hunch is that the truss arranged along the surface of a cone would provide better rigidity than along a cylinder.
Moonlite has ball and socket truss connectors, might have to look at alternatives as trusses will be fibre.
newt output says there will be vignetting at focuser of 75% ray if I use a standard 2" focuser with a 51mm ID and 51mm for eyepiece focal plane to camera sensor. I need a minimum of 57mm ID to correct this.
"If you will be using a camera, enter the amount of additional height for the focal plane above the focal plane used for eyepieces. This is usually about 2 inches for 35 mm Single Lens Reflex cameras. You should also add any height required for an off-axis guider. This is usually three quarters to one inch."
Hi Alistair, as I mentioned in the PM you can acieve the cone truss effect with the square brace also (without using a larger brace), by attaching the truss poles 45 degrees from where I put them. See the attached pic.
So you wouldn't have to construct an octagonal brace.
Thanks, you're right I could use a square central and circular upper brace and achieve the same cone.
But have a look at the attached which roughly shows the direction in which I see forces being transferred to the dovetail bar along the square and along the octagon.
if this is considered, then either a circular or octagonal central would appear to transfer the forces better than a square as corners aren't involved.
My engineering background is electronics and not structural so I could be wrong.
In your case, the dovetail bar is quite wide and may not be such an issue, but with my thin bar and the heavy guide scope it could be an issue.
I might be over thinking this, but its better to consider these factors at this stage rather than later.
My objective is to build as light and rigid as possible as it will be transported very frequently and not fixed permanently.
I'll also be using this on a fork mount as well as on a heq5pro.
the fork mount can easily take heavy ota's, but it'd be on the limits of the heq5pro, hence the obsession to reduce weight.
Last edited by alistairsam; 07-03-2012 at 09:36 PM.
Hi,
I've finally started build of the 10" F4 truss OTA.
But I need some clarification with newt calculations to work out ota length.
Attached are values I've entered in newt.
What I don't understand is the "Focuser camera travel". from newt help "enter the amount of additional height for the focal plane above the focal plane used for eyepieces"
I've specified the focal point for eyepieces to 20mm from the tip of the focuser. I don't get why I need to add additional distance to a ccd plane.
I thought the CCD would need to be at the focal point.
other bit is illumination field.
with the specs provided, I get a 100% illuminated dia of 15mm.
the qhy8 sensor is 28.4mm diagonal.
should the 100% illuminated field be as close to the sensor diagonal as possible? what should I change to get this right?
its to do with your draw tube length and what you need to make it clear the inside of the OTA. if your draw tube is say 100mm and you specify your imaging plane at say 90mm from ota it will drop 10mm into the light cone.
As for your fully illuminated circle yes you are correct. the main modifications that will increase your 100% is the secondary and focal point distance from the OTA. Don't worry about the 75% rays they don't matter if your illuminated circle is nigh on 100%
Just for your info my f4.7 uses a 70mm secondary and sits close to 300mm out of the tube for the focal plane i see as measured in CCDisp approximately a 6% light drop off to the outside.
Ide expect if you had about the same back focus then your secondary will be around the 80-85mm
Im not an expert in Newt, but these are the things i have come to agree on for myself and it hasn't let me down yet.
Thanks Brent, that would be great.
Where did you get the velcro from?
Also, I've seen pics of your al flat on the mirror cell but how does it pivot as you have a bolt in the centre.
I guess you'd need a see-saw type motion for the 6 point cell which is the motion I'm aiming for.
I've seen pics of an Al U channel pivoting on an Al square tube.
Sorry, haven't been back here for a while.
Bunnnings had the Velcro, about $30 IIRC for 50 meter double roll ( hook and wool)
The alum flat has a countersunk hole in the middle. It sits on, wait for it ...., a neoprene rubber roller out of a photocopier with a longish screw down through the middle, longer than the roller is deep. This sits in a hole done with a spade bit in the support ring and the screw end goes down into the centre hole the spade bit left. Drill press is useful here to set the depth the same for all three holes. Fill this all up with silastic goo and drop the roller\flat\screw combination into it with a heavy flat weight on top of all three. Let it set. The neoprene roller and silastic allow the alum to flex a little but they will not move sideways. Add the velcro and drop the mirror on. I have been able to separate them when necessary but it will hold the mirror suspended safely.
Here's some pix from the first build, Skeletope I. I have used the same method in Skeletope II.
First pic, the components, ignore the nylon washers, not used. And I was lucky with clear neoprene rollers.
Second pic put together.
Third pic all sprayed balck and velcro added.
Fourth pic, side shot with mirror mounted.
the dovetail bar is the bit that worries me as mine is the vixen type for the heq5 which is quite narrow.
if you attach the mount on the bar section that's outside the central brace, won't that bar flex as the bar is only fixed to the central brace by bolts along its axis and not like a wide flat that Rolf or Doomsayer uses that has bolts side by side covering a larger area.
That would distribute the shearing forces a bit wider than the narrow dovetail bar.
in my case, I'll be mounting an ST80 on the opposite side which weighs around 2kg's all up.
what I'd planned to do was to bolt a wide plate in the central section and then bolt the dovetail bar on that as then the shear would be across a broader section of the aluminium square and the plate would shear less than the section of Al square tube where the dovetail bar is fixed.
I'm also looking at Carbon fibre sheets I found on EBay.
They're 2.75mm and I'm wondering if I can sandwich a few layers. Just a thought.
My centre cage has been widened to 120mm by use of extra square section on the lower side plus two right angle alum brackets outside that. All held by a couple of 100 mm bolts through the whole assembly.
Heres a pic, not a great pic but you should be able to see how it goes together..
Dovetail is bolted to this directly with about 110mm gap between two bolts. It doesn't move.
its to do with your draw tube length and what you need to make it clear the inside of the OTA. if your draw tube is say 100mm and you specify your imaging plane at say 90mm from ota it will drop 10mm into the light cone.
Thanks, so does this height need to be the length of the drawtube inside the ota when retracted as a minimum, plus whatever you need for filter wheels, oag, etc?
so the focal point is the same whether you use an eyepiece or camera.
Looks like I'll need a 2.5 inch focuser as the minimum focuser ID without vignetting for my mirror shows 58mm.
I'm going with a 4 way push pull secondary adjustment similar to the primary mirror where i'll have two sheets separated by 15mm with 4 spring loaded bolts facing upward. one sheet close to the 45 deg mirror holder, and the other closer to the end of the OTA.
so for collimation, i'll be tightening or loosening the nut on top of the second sheet.
are there any disadvantages to a 4 bolt secondary vs 3 bolt traditional ones?
Thanks for the pics Brent. that is simple but effective, but have you thought of using velcro just at the ends of the flat rather than through the middle as then you'd have the proper 6 point support?
Just ordered a 3mm CF sheet for the focuser plate, and carbon fibre tow to try out a string spider.
