View Full Version here: : 10" F4 build
27-02-2012, 05:55 PM
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".
27-02-2012, 11:17 PM
hi Alistair. Sounds like a great project.
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/Plop_optimized_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.
27-02-2012, 11:45 PM
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?
29-02-2012, 05:54 PM
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.
added comment: I just looked up Rolf's description of his scope - he used 3 silicone blobs to mount his 10 inch primary
02-03-2012, 02:57 PM
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?
02-03-2012, 11:07 PM
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.
06-03-2012, 06:44 PM
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.
07-03-2012, 09:13 AM
Hi , Are you using carbon fibre struts ?
07-03-2012, 10:05 AM
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.
07-03-2012, 10:57 AM
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.
07-03-2012, 11:12 AM
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.
07-03-2012, 11:23 AM
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.
07-03-2012, 06:50 PM
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.
not sure what the practical impact would be.
is the 51mm allowance for a camera nominal?
this is from the newt help page
"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."
07-03-2012, 08:39 PM
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.
07-03-2012, 09:07 PM
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.
10-04-2012, 04:56 PM
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?
10-04-2012, 05:49 PM
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.
10-04-2012, 06:47 PM
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.
10-04-2012, 06:54 PM
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. :thumbsup:
11-04-2012, 03:38 PM
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.
11-04-2012, 04:19 PM
If you think of it like this.
With your focuser draw tube racked out till it clears the inside of the OTA
Needed Backfocus with thoughts into if I change my setup in the future can .... for your imaging plane aka
where the first flange/adaptor butts to draw tube ----> Image plane is your back focus.
As for your focuser, I put a 3" in for strength and to carry big loads It also allowed for a non obstructed OAG pick up. So a 2.5" would have been a minimum i would have thought. Keep in mind as prices keep dropping and new sensors come out the 11K chipset no longer is inhibitive to own... So design for a 11K chipset then you will have maximum flexibility without having to stuff around.
As for a 4 point adjustment you can do it but it isn't the best. other wise we would have 4 legs on our tripods. Adjustments become difficult with 4 points of support. So historically we have all used 3 points because geometrically its just easier to adjust
11-04-2012, 04:27 PM
are there any reasonably priced 3.5 or 3" focusers, doesn't have to be motorized, but low profile would be ideal as I can then keep the secondary optimally small and not too big.
I came across websites for diy spiders where they talk about deficiencies with the 3 point secondary adjustment vs the 4 point where adjustment is in x,y,z and the 3 point isn't? not sure which is accurate though.
11-04-2012, 05:15 PM
Well you can always get a cheap GSO varient. but they are just that Cheap... if your planning to do astrophotography then you need a decent focuser.
As for slimline, you start looking at FLI PDF or Atlas focusers other wise you are looking at about 100mm for most 3" focusers and 3.5" even more.
At the end of the day its about what you want to do and how much you have to do it with and the last point how patient you are. I waited 3 years to upgrade my focuser... it was tough because i knew it wasn't good! but i did the jump in one foul swoop standard SW to FT.
I look at it in this light, if a 4 way adjustment was the best way then serveyers would use 4 legged tripods. The do not. you need left right back and forward if you use 3 adjustments it pivots off one pivot point, if you have 4 you may need to adjust 3 different points to make it sit properly or else it there will be different pressures on each of the 4 supports.
As for adjusting in X,Y,Z your only wanting to adjust in X (towards or away from the focuser) and Y (side to side) plane, Z is taken into consideration with the central bolt to bring the whole secondary hub and mirror up or down the tube.
Come to think of it everything pretty well uses 3 adjustments. Mirrors, secondarys, laser collimators etc. etc. why re invent the wheel?
11-04-2012, 06:08 PM
Thanks Brendan, I think the favour for the 4 point adjustment was in the way the 3 point affected the x axis when you adjust y whereas the 4 points didn't. I have no intention in re-inventing the wheel, just considering all possibilities since I'm building this from the ground up.
one other thing, with serrurier truss OTA's, are all the truss nodes strictly under compression or tension, or can there be radial forces as well?
I've been looking at the CF tubes, and the common ones are pultruded where the carbon is laid axially, whereas twill weave tubes are multidirectional so have higher axial and radial stiffness.
I got a sample of a pultruded tube and while its very stiff under compression and tension, they're not good if loads are non-axial.
My understanding of truss structures is that all members are always under tension or compression. if that's the case pultruded tubes would do as they're cheaper. I was advised against roll wrapped tubes.
in an exaggerated example, if one were to twist the upper cage by hand like turning a wheel, would the loads on any of the 8 nodes be non-axial?
the real world scenario would be the torque induced by 2kg of camera gear hanging off one side which I presume is very low. haven't calculated that though.
I got a source for 2mm wall 20mm OD twill weave tubes for a decent price but they're about twice the price compared to pultruded.
12-04-2012, 01:23 AM
You are right in your assumption of how truss members work they are axial force members and hence why as a engineer they are a joy to design especially when you can get them to work in tension only! (no buckling)
Trying to calculate the force induced into the 8 members can be done but its not easy and would require a bit of thought and free body diagrams. needless to say 2kg of equipment will not cause the member to come any where near failure nor flexure in my honest opinion.
2mm Aluminium tube with a Youngs modulus of approximately 80 GPa has nothing on CF tubing which is generally anywhere from 250-400 GPa depending on quality in the axis of the strand and about 10-15 GPa perpendicular to the strand. :)
Im personally still not convinced that a 4 post will give you a better outcome just due to geometry its a personal thing really but just for your piece of mind if you have a piece of scrap wood/steel put 4 screws/bolts though it. and try get each point touching the surface at any point in time you will find as you move one at least two will come free so you need to move two to get one adjustment. Do the same test with 3 and see the difference.:thumbsup:
12-04-2012, 09:28 AM
Yep, but the velcro has that wee bit of give in the hook\wool joint that makes it irrelevant. Theoretically you could make a disk covered in velcro completely the size of the mirror and have an 'infinite' number of support points which would be perfect .... theoretically. As far as I can figure out there is virtually no stress between the mirror and the suppports because of this slightly flexible no-strain connection.
And it is stronger in a supported weight position.
That's my story anyway ... :thumbsup:
12-04-2012, 01:25 PM
When I build my dobby, not being an optics expert, I am going to have a sliding wooden panel for the focuser and secondary (with routed hole grooves under them) so the actual best focus for the primary and end use of the focuser is achieved. Can also use a primary with a slightly different focal length (should a perfect mirror ever become available in the FL range). Sliding panels saves drilling holes in wrong place for us rank beginners. (and have seen several of them with focuser holes drilled in wrong place.) Even rank learners can make a useful dobby. Screw panels into place when the optic path suits.
12-04-2012, 03:15 PM
Sliding focuser plate sounds good but the tricky bit would be to manage tilt.
you could use two steel rods through the plate like a printer carriage head to avoid tilt.
I"m trying to get my measurements as precise as possible to reduce errors.
So I'll first assemble the three sections using just straight pipes where the three sections can slide and I'll carry out a focus test. once I determine the correct spacing between the three sections, I'll then cut the CF tubes to match the spacing.
I just finished the upper cage and mirror cell cuts. now I need to add the 6 point support to the trifoil, springs, silicone the primary, assemble the secondary and then test segment spacing.
12-04-2012, 05:49 PM
Your project is obviously all figured out and looking great. Quite an advanced project for a home builder. Light pathway to the optimal part of the focuser would be my hurdle to get exactly right. Many websites suggest with focuser closed inwards to stop, then exactly half of the travel outwards is the point. Anyway following your project with interest.
12-04-2012, 10:32 PM
It's looking good Alistair, you must be having fun! :)
13-04-2012, 11:12 AM
made some more progress. Finished the 6 point mirror cell.
I used nyloc nuts for the pivot bolt on each segment and washers in between so I can adjust the pressure and control how easily the segment see saws. it'll be just so it doesn't move on its own but doesn't offer much resistance to movement either.
I was thinking of adding the silicone blobs on the ends of the flat, but not sure how strong the adhesion to Aluminium is. I might roughen the surface a bit.
I'll also add side clips that don't touch the mirror and have about 1mm clearance that way the mirror can never fall out even if the Al-Silicone bond gives.
Collimation adjustment is very smooth and with the wing nuts, I don't need any tools. My secondary adjustment will be very similar but with a carbon fibre flat.
the weight is exactly 1kg, my mirror is 2kg, so the bottom assembly should weigh around 3 and upper around 3.
I also got a heavy duty vixen dovetail plate that has a flat base like the losmandy ones. that way contact area in the mid section is maximized.
my biggest challenge is to keep the optics centred in the whole assembly while marking and fixing clamps.
Rolf, I am having fun, but this is inspired by yours and Brent's truss ota's.
can't wait for my first star test, but still heaps of work to be done.
Paul, that focuser calculation sounds about right. thanks
Is this for photography? If so, you won't need what I would define as "low profile" - the distance from the OTA wall to the CCD chip will be determined by the length of the corrector. If you start with this measurement, the others will fall into place. Ideally you want the end of the corrector at the OTA wall. If it's too low profile, the corrector will protrude inside the OTA.
Hope this make sense.
13-04-2012, 02:11 PM
Thanks James, yes this is for photography and what you've mentioned makes sense.
But I will need a decent amount of clearance between the CCD and the top of the focuser to allow for an OAG, filter wheel or a different camera, so the corrector would not be near the ota edge, most likely at the top end of the focuser.
if I use a standard focuser, that height plus the gap I need would increase the distance to ccd, bigger secondary, broader focuser. if its low profile, the cone size at the focuser base won't be as big so I won't need too big a secondary.
The baader one looks good but is only 2". TS have the 3inch Orion focuser for 249euro, but direct price is a lot higher. still looking around.
Have a look at the last picture on this page
In this case the item labelled 'coma corrector' goes all the way through the full length of the focuser and ends in line with the OTA wall.
If you were to add an OAG, it would need to go in the space labelled "adaptor"
The filters and guide chip are inside the camera housing :-)
Does that make sense?
13-04-2012, 10:51 PM
So the glass of your corrector is near the OTA? that's a fair way off from the ccd. I'm guessing the 3" corrector you're using needs this spacing?
I use the baader mpcc which needs 55mm and the corrector itself is only around 25mm wide, so that's 75mm from the ccd.
I'll check a bit more. Not sure which focuser I'll end up with though.
Yep. The 3 KORRW (http://www.astrosysteme.at/images/Corr_3Zoll_Wynne.pdf) is 137mm long and needs about 58mm back focus. I had to add a small spacer between the corrector and the camera to get the correct back focus as per the spec.
Although the sizes are different to the MPCC, the principle is the same. You want to minimise the distance from the secondary to the CCD chip, so the ideal is to have the corrector glass in line with the OTA wall.
15-04-2012, 07:18 PM
Hmmm, you have just given me an idea Alistair. I need to move my primary up about 20-30 mm to get my DSLR into the focal plane. I might replace my flat alum supports with box section as you are using in your cell. Be a heck of a lot easier than cutting the truss tubes or making recessed spacer blocks as I was planning. And reversible. Excellent !!
Many thanks :thumbsup:
15-04-2012, 08:59 PM
Good to know Brent. The box is easy cause it doesn't shift sideways.
I just need to find the right silicone that won't corrode the aluminum.
Not sure why the silicone sealant is used more than silicone adhesive
Selleys has a metal and glass adhesive.
You might have to check your secy size and vignetting if you're moving it up by 20mm
16-04-2012, 10:25 AM
2" Focusser and the secondary is a generous one. I only need about another 3-4 mm but that is already stacked up on the adjusters so I will pull some of the neoprene supports out and use the box section to get it into position.
I think the sealant is better because it is designed for some thickness and setting as a volume whereas the adhesive works better as a thin film. The thickness is part of the flexible no-stress support structure. You would lose that with a thin film I'd think.
I've seen no corrosion problems with Alum and Sealant but if you want to be sure of a hold drill a hole where the sealant will be and squeeze a 'button' through to lock it on the far side. I did this with my lateral side supports. In fact I drilled the hole and set the support about 2 mm from the side and then squeezed the sealant through the hole to the mirror edge tofill the gap, very tidy looking.
16-04-2012, 10:44 AM
yes I noticed the side sealant, but did you use spacers while it cured? else won't it tend to pull the mirror sideways?
I found this sealant that works well on glass and aluminium
specs mention it doesn't shrink while curing. But does mention its not suitable for roofs and gutters as it will cause corrosion. should be fine for Al.
16-04-2012, 12:57 PM
just received the CF focuser plate. I went with 3mm and is very stiff, strong and light as expected but I could've gotten away with 2 or 2.5mm.
it'll be held by sandwiching it between two pieces fixed to the frame with epoxy glue, unless I can drill a 3mm wide slot on the UTA so it slots in. Not sure if there are 3mm router bits available.
16-04-2012, 06:26 PM
Nup, no spacers, the velcro held it solid. I really didn't need the side supports I reckoned but was just playing safe.
If it will cause corrosion on gutters and roofs which are galvanised then it may not be great for alum either. Silastic cures by absorbing atmospheric moisture using acetic acid as a hygroscopic medium. Alum is not keen on Alkali's ( Try it with NaOH, Caustic Soda and see !! ) but it ain't great with acids either. Galvanising is zinc plate on steel and acids and alkalis attack zinc ( think the old batteries ) thus exposing the steel underneath to rust.
The sealant version cures differently, not sure how but my Aquarium fish appreciate the fact that it is a neutral ph when I fix their tank joins. Hence the sealant version.
27-04-2012, 03:52 PM
Bit more progress made..
I came up with a secondary holder that has its bolts inverted so adjustments are pull rather than push and this makes adjustments very simple and tool less.
(Brendan, I stuck to the 3 point adjustment rather than 4. )
There is no central bolt that holds the mirror holder, the central bolt on the upper holder block instead is the locking bolt. so tightening the bolt locks the assembly and it doesn't move.
I got tired of the old holders where if you loosen two push bolts, the whole secondary is free to rotate.
with this one, the mirror holder is tightly held to a plate which in turn is supported on the upper block.
I drilled a hole through the mirror holder and embedded a nylock nut in the holder, then tightened a bolt through the plate so it doesn't rotate freely.
The bolt holds the lower holder with sufficient compression such that the mirror no longer freely rotates, but if I do want to rotate it, I simply rotate it by hand and it holds.
I decided to go with a wire spider similar to Rolf's design after a few rudimentary tests.
I slotted the carbon fibre focuser plate between the two upper sections. I bought a 3mm Router bit off ebay, cut a straight slot with a 6mm depth using a simple jig and its a tight fit and doesn't move.
I managed to align upper and lower slots so the focuser plate is square to the upper assembly. the carbon fibre plate is very stiff and light, but once I cut out the focuser hole, I expect it will become a lot weaker. so I'll add two vertical supports along the sides to reinforce.
Primary 10" mirror is now siliconed to the 6 point mirror cell. I used Selleys Wet silicone which is neutral cure. I tested it with some offcuts and its very strong. I will add some side supports though.
I will be ordering a 3" orion optics focuser from TS in germany. until then will have to use a 2". once I get the new focuser, I'll just cut up a new carbon fibre plate for the bigger focuser.
Hope to string the secondary, assemble the whole OTA with temporary Al trusses to check measurements and do a star test this weekend.
Once that's completed, I'll dissassemble, and then sand, finish and coat with marine grade black stain to make it waterproof. Will then order the carbon fibre truss tubes and epoxy it to the tube ends.
The total weight so far as per my calculations with a QHY8, St80guidescope, (Till I get an OAG), qhy5, focuser, heavy duty dovetails, carbon fibre truss tubes, etc is around 8 to 9 Kg. One of my major objectives is to keep it under 10kg with everything.
27-04-2012, 06:35 PM
I don't think mirrors really float once they are glued down like that . You might as well mount the mirror on a single ring of silastic . I'd lose the big square bars and get the profile of the cell down and beef up the thickness of the backplate.
27-04-2012, 07:29 PM
Thanks for the comments Mark. I guess I was a bit generous with the silicone.
Most of the 6 point ATM cells I found mentioned the silicone blob to be the size of a dime, mine is around a 20c piece.
what I don't understand with the commercial ones is how the mirrors are secured without pinching at the sides, this one for example, http://www.orionoptics.co.uk/OPTICS/opticsmirrorcell.html
I'm guessing the side clips prevent the mirror falling off so there's no silicone at the floatation points.
I was trying to secure the mirror without side clips or having the primary support at the sides.
if I used smaller blobs, would the mirror be secure enough?
I went with the big square sections to get the contact area the size of a coin, this one's 25mm. I could use smaller ones and that would lower the profile.
But do you recommend lowering the profile to prevent lateral movement?
the mechanics of the 6 point actually prevent sideways movement and any movement would be more due to the elasticity of the silicone.
anyways, I will research more and redesign or might just get the one in the link above.
27-04-2012, 09:06 PM
just found this page that shows astigmatism when a 10inch mirror is glued onto a 6 point cell and pointed at the horizon.
That calculation is for a 3/4" mirror (i.e. plate), and scope pointing horizontal.
28-04-2012, 03:47 PM
My mirror is 26mm thick at the edges and is the Suprax glass (3/4" is 19mm), not sure how different its density is from plate glass so if that calculation is correct, mine could suffer similar effects.
anyways, I've already redesigned the mirror cell and will go with a combination of smaller silicone blobs for the 6 points, around 10mm and edge supports as well.
Is there any way of visually verifying cell induced errors with star tests or is it not worth the hassle cause I will test the mirror visually the way it is before moving it to a different cell.
29-04-2012, 09:15 PM
Agree with Mark - that amount of adhesive will probably negate the advantage of 6 point mounting - your redesign should help.
The analysis that you found of the glued mirror includes a cubed scaling term, so you can divide the results by half to get something applicable to your system - and then ask yourself how many times you will be viewing at the horizon.
Suggest that you leave it as is and try it out - change it only if you can detect any real effects. I have a glued 12 inch GSO mirror in a 9 point cell and can just see the effects of adhesive mounting in out-of-focus star testing. However, there have been two occasions where the mirror crept sideways enough to run into side supports that were put there as a fail safe - then the resulting astigmatism was well and truly visible - even in focused images. Equatorial mounting will rule out most edge support systems and glueing will probably be better than anything else you can do. regards Ray
29-04-2012, 11:29 PM
Had a few questions
How did you use glue in your nine point cell, just small blobs around the 9 supports?
When you say crept sideways, was that due to the silicone's elasticity?
Any harm in adding silicone at the sides?
What should I look for to see effects of adhesive mounting?
Could you explain the relation between eq mounting and edge supports and why it won't work?
30-04-2012, 01:51 PM
Mine sits on the 9 point support and then i have the silicone blobs holding the mirror so its fully supported, and locked in so the mirror doesn't slide around. I have no issues.
30-04-2012, 02:23 PM
Yes , I thought the high profile puts more load on the rear of the cell which looks fairly thin. Experiments with silicon always have to be carefully monitored. If you are not getting nice round extra focal rings at high power you may be compromising resolution on your images.
30-04-2012, 02:26 PM
are your silicone blobs around the 9 support points or at the sides?
30-04-2012, 02:31 PM
I initially started off with an 18mm backplate, but then changed to the 12mm as the load is widely distributed and not concentrated. Moreover, the mirror weighs only 2kg's so the backplate remains stiff even at 12mm.
with the redesigned cell however, I'll be using 18mm ply and the support points will only be around 5mm from the backplate against the current 30mm or so.
since my primary interest is DSO imaging, would it make much of a difference as opposed to high power planetary imaging?
30-04-2012, 02:47 PM
mine are around the perimeter of the mirror, and just resting on the 9 point support
30-04-2012, 03:27 PM
I presume this is with the 9 point Orion optics cell and you added the silicone in on the 3 bottom corners? is it primarily an edge support cell when shipped from the factory?
30-04-2012, 07:25 PM
i have used a variety of adhesives - the current brew is double sided adhesive tape, since the silicone de-adhered from the GSO supports (which turned out to be thermoplastic). While it was attached, the blobs were about 1cm across and about 3mm thick - it worked well from an aberration viewpoint. the sideways creep was due to an earlier version of double sided tape that softened in the heat - silicone should not have that trouble.
I haven't tried silicone on the mirror edge, but the astigmatism from a wrongly placed hard edge mount was pretty bad.
The optical effect that I interpret as being due to adhesive mounting is a vertical gradation of the appearance of an out of focus star image - the image remains nice and round, but the pattern looks a bit different top to bottom.
All of the edge mounting techniques that I have seen are aimed at Dob mounts, where gravity has a fixed orientation wrt the OTA. on an EQ mount, the direction of "down" varies with where the scope is looking, so it is not possible, for example, to put supports at the recommended plus and minus 45 degrees from the bottom of the mirror - as soon as you move to another part of the sky you have to rotate the OTA to get the supports back in the right orientation. And the last thing you want is a hard edge support in the wrong place.
01-05-2012, 10:08 AM
Yep thats the one I have, though :D me being me it isn't as from factory. When i recieved it the poor thing was in a bit of a state semi modified from somebody else it didn't fit the requirements.
I have three 3mm flat ally plates that are about 20mm wide that come up the sides much the same as what is there now. but they come upwithin a few mm of the actual mirror. there are blobs of silcone on those. So basically what i did was, set the mirror up so the 9 point support was sitting flat and everything was honky dory, then put the silicone in and waited for a few hours before i put it all back together.
My collimation holds after long trips with minor tweaking from the cats eye equipment and holds perfeclty in a eq setting.
you cant really see what im talking about but where the blue feet are is where the mirror side supports are.
07-05-2012, 09:56 PM
Progress has been slow, but I've completed the string spider. I'm really surprised with how stable it is. Thanks to Rolf for his advise.
I did incorporate a 4mm offset, hope I got it right.
I avoid over tightening by testing the note made by each string!
I used two carbon fibre off cuts to terminate the strings. they're fixed with two bolts that run through the upper holder, that way they remain stiff. the CF provides stiffness with hardly any weight.
As rolf suggested, its possible to collimate the secondary mirror with the string adjustments alone, but I prefer to keep their tensions fixed and make minor adjustments with the 3 spring loaded central bolts.
Central brace turned out pretty strong as well. I will add two more bolts through the dovetail and wood that way both pieces are held firmly to the dovetail.
I will have to test the strength with the camera and mirror sections, but it should stay sufficiently stiff for the 6kg odd load.
I will add one more dovetail to the opposite end so it can fit in my fork mount as well.
07-05-2012, 10:33 PM
That looks great Alistair! You probably found that the strings don't have to be particularly tight in order to hold the secondary stable, which is nice. I was surprised with how little tension they need. People often get the impression that it's pure tension that holds a wire spider in place, where in reality it's the geometry (at least when you make the geometry right!)
I see you used the same system of pivoting guitar machine heads as I made for mine. Have you adjusted it so the wires are squared yet or is this just a test? It can be a little tricky to adjust exactly but you only have to do it once ;)
07-05-2012, 10:38 PM
wow I wouldn't want to fall thought that :D you would be diced up like jullien carrots
Ide hate to have to remove the secondary hub for any reason!
Good work in any case
07-05-2012, 11:44 PM
Thanks Rolf, yep it still is sort of a test as I have to redo the bottom part of the UTA due to that incorrect slot I made for the focuser.
So will square it up once assembled. I was thinking of using butter paper and light through the focuser to do a basic alignment and will then use star tests.
Brendan, the secondary assembly comes off easily by just removing the 3 wing nuts because its actually 2 parts, the mirror with the holder comes off. So I have access to the secondary if needed without disturbing the wires.
Only bits left now are to measure the upper and lower truss lengths.
Trevor here has kindly offered to machine the carbon fibre truss ends for me. still in 2 minds of whether to use 16mm pultruded or 20mm woven CF tubes.
08-05-2012, 12:15 AM
that question is easy to answer :) protruded, as woven is better at bending.
carbon strands have the most strength when pulled aka put into tension they are incredibly strong. but normal to the grain they are flimsy. Hence why woven twill is used on "cf plates" because it allows far more strength when loads that act normal to the face are applied.
the loads that your telescope will see the protruded rods will work just fine in tension and compression
08-05-2012, 12:22 AM
I guess that's inherent to trusses in that all members are under tension or compression and no radial loads.
only problem with the pultruded rods is the diameter. the max dia available is 16mm with a 2mm wall.
correct me if I'm wrong, but I think even a 1.5mm wall but 20mm diam would be better than the 16mm as wider pipes tend to be more stiff.
I've got a sample 16mm, so will test.
I wonder why most professional scopes like planewave, officina stellare and other RC's use woven tubes.
I've seen some ATM builds with roll wrapped tubes that are actually bad at axial loads.
08-05-2012, 01:40 PM
you nailed the issue there mate. ATM Roll Wrapped! This is why im hesitant to construct my own CF tube as the CF actually reinforces the epoxy resin. get the mixture right and its pretty damn good. get it wrong they are woeful... your basically using a Resin tube.
Planewave and the like use Woven tubes for one reason.... they look damn good, couple that with extruded rods :P they are just grey/black
I would say a 16mm rod depending on your length between cages is possibly a bit to thin. Ide be looking more like 20mm x 2mm as you cannot get protruded rod then woven rod will be fine.
07-01-2013, 03:50 PM
Its been a few months, but I finally finished my 10" F4 serrurrier truss with CF tubes. I went with woven 1mm x 16mm CF tubes rather than pultruded as they were half the weight of the pultruded, 72g for a 1metre x 16mm OD tube.
I've tested collimation with a cats eye AC and its holding well at different orientations, there still is some flex but very minor.
The guitar string spider is working a treat, hardly any spikes and very rigid.
I need to get a proper shroud done though.
All up, the OTA weighs 11.5Kg with a QHY8 ccd and the guidescope/guidecamera and my dew heater box.
I plan to replace the wooden dowels in the UTA with CF as well, so should end up at 11kg.
The truss nodes are terminated on Aluminium elbow joints which is basically two U channels inserted into each other so they are free to swivel to the required angle.
The Octagonal shape causes the trusses to form a cone which is what I wanted as that has better rigidity than the cylindrical truss configuration.
I used a 1inch long 16mm wooden dowel as the CF tube insert to connect to the Aluminium elbow joint, but since the standard size is 16mm OD, and the CF tube ID is 14mm, I had to convert my drill press to a wood lathe to turn it down to 13.5mm. I then used the 6minute Epoxy glue available from hobby shops to glue it inside the CF tube. They're only 1 inch long so doesn't add up too much weight.
I also got a belkin 4 port powered hub which supports the qhy8, qhy5 and canon1000d. I finally have no more USB issues after changing about 3 different hubs.
My HEQ5 pro seems to handle it okay, but i'm guessing its borderline for this mount.
still, I'm happy with the way its turned out. comments, suggestions, welcome.
07-01-2013, 04:33 PM
A lot of work in that ...well done ..!! :thumbsup:
07-01-2013, 05:51 PM
A beautiful job Alistair. You mentioned CF tubing at the SVAA camp in November, and that spider is a work of art. Glad to see it all coming together.
07-01-2013, 06:48 PM
That's one nice looking telescope!
08-01-2013, 12:01 AM
Thanks Roland, Larry.
yes Flash, it was quite a bit of work, sometimes I wondered if I over complicated it, but being F4, I was set on improving rigidity as much as possible, and it just added on. Moreover, I had no access to Al milling, so had to work with wood.
A CF rolled tube would have been perfect if I could afford it, but since I couldn't, I had to make my own and Rolf and Brent's trusses provided the inspiration.
I'm currently doing a PEC train on the HEQ5 as the PE is around 45arc sec and with the weight of the OTA being borderline for the mount, I need to use PEC to smooth out the guiding as much as possible.
I wanted to fix a cylindrical shield on the inside rather than cover the trusses, just for aesthetics, but not sure where I can get plastic sheets.
Else I might have to use a shroud with velcro.
At snake valley, the primary was completely dewed up when I had no shroud.
08-01-2013, 06:08 AM
That looks absolutely gorgeous Alistair, congratulations on completing your build! :)
I'm in the process of building a 12.5" version of my Serrurier truss design, over here: http://www.iceinspace.com.au/forum/showthread.php?t=98823
I'm trying to optimise every piece of it in terms of weight savings, and at this stage overall it looks like I will end up with a 12.5" OTA weighing only the same as my current 10" :D
08-01-2013, 10:36 AM
Thanks Rolf, I had to do the same to save every gram.
I wanted to mention that the UTA might need reinforcement. I found that the four supports aren't sufficient to keep the secondary rigid at different orientations.
the weight of the secondary mirror actually warps the 18mm ply by a few microns, and that's sufficient to throw a laser off the primary at different orientations, and this is even though I terminated the strings very close to the 4 supports.
what is the weight of your 10inch truss?
with the new 3inch focuser and stepper motor, along with a bigger mirror, won't the weight be quite a bit more than the 10inch?
what stage are you at?
08-01-2013, 11:12 AM
The total weight for the old 10", including optics is 11.85 kg, without optics it's 7.85 kg.
For the new 12.5" so far I've calculated the expected weight based on the exact volume and density of the different materials (birch plywood, aluminium, steel nuts & bolts). OTA alone excl. optics should be around 6 kg, and with optics ~11 kg.
I'll be using a conical mirror which will save quite a bit of weight. The weight of the conical 12.5" primary is pretty much the same as that of my current traditional shaped 10" primary. The conical is also mounted using a single bolt through the back plate of the mirror cell, so that saves some hardware too. :thumbsup:
I'm also using lighter birch ply this time (the old one is made from pine). Also, the 3 long steel bolts that hold my mirror cell are now only 10mm thick this time, the old ones were 12mm but that is unnecessary.
I've done various other tweaks here and there (the anchors with the guitar tuners are now as slim as possible etc).
I have excluded my whopping 1.3 kg 80mm finderscope from these calculations because this time I intend to make it detachable. I only use it initially to locate the target, so it's silly to have that extra weight sitting the OTA while imaging. Instead I want to create some sort of clip-on holder for it. Alternatively I'll just put a Telrad (~300g) on the OTA, but I really prefer using the big finderscope.
08-01-2013, 12:09 PM
That is very light. What about using pine instead of ply? once they're varnished, they shouldn't warp. they're softer, but lighter.
08-01-2013, 12:21 PM
Alistair, marvelous work, my friend. I am in awe of this project of yours. I've been following in it quietly, but closely. I am so impressed.
08-01-2013, 12:31 PM
Hi Alistair, I found that the pine ply I used for the old 10" was a tad too soft for my taste and also the finish of the beech ply is much finer.
With all the various bits and pieces that end up being attached to the ply rings I preferred using beech this time. The beech rings also appear very stiff compared with the pine.
I think the weight difference all up is about 200 grams, so I'll live with that. :P
08-01-2013, 12:43 PM
Thanks Alex, it has been quite a journey. your work has been an inspiration as well.
This is why DIY is so rewarding. you learn so much and see the fruits of your labour.
I built this 10inch as this will stay with me for a long time. I don't see myself splurging on a new mount anytime soon, especially with a new baby en route, so I tinkered away as much as I could to get close to what I wanted.
I've added some build pics here and will keep adding more.
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