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  #21  
Old 11-04-2012, 04:19 PM
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If you think of it like this.

With your focuser draw tube racked out till it clears the inside of the OTA
Plus
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

Brendan
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  #22  
Old 11-04-2012, 04:27 PM
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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.
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  #23  
Old 11-04-2012, 05:15 PM
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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?
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  #24  
Old 11-04-2012, 06:08 PM
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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.
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  #25  
Old 12-04-2012, 01:23 AM
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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.
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  #26  
Old 12-04-2012, 09:28 AM
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Quote:
Originally Posted by alistairsam View Post
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?
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 ...
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  #27  
Old 12-04-2012, 01:25 PM
Matt Starwave (Paul)
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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.
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  #28  
Old 12-04-2012, 03:15 PM
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Hi Paul,

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.
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  #29  
Old 12-04-2012, 05:49 PM
Matt Starwave (Paul)
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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.
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  #30  
Old 12-04-2012, 10:32 PM
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It's looking good Alistair, you must be having fun!
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  #31  
Old 13-04-2012, 11:12 AM
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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
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  #32  
Old 13-04-2012, 01:56 PM
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Quote:
but low profile would be ideal as I can then keep the secondary optimally small and not too big.
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.
James
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  #33  
Old 13-04-2012, 02:11 PM
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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.
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  #34  
Old 13-04-2012, 04:00 PM
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Have a look at the last picture on this page
http://deepspaceplace.com/at12in.php
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?

James
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  #35  
Old 13-04-2012, 10:51 PM
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Hi James,
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.
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  #36  
Old 13-04-2012, 11:19 PM
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Quote:
So the glass of your corrector is near the OTA?
Yep. The 3 KORRW 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.
James
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  #37  
Old 15-04-2012, 07:18 PM
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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
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  #38  
Old 15-04-2012, 08:59 PM
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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
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  #39  
Old 16-04-2012, 10:25 AM
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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.

Last edited by ZeroID; 16-04-2012 at 10:28 AM. Reason: Added info
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  #40  
Old 16-04-2012, 10:44 AM
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Quote:
Originally Posted by ZeroID View Post
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.
Hi Brent,

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
http://www.selleys.com.au/sealants/silicone/glass
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.
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