Flexure probem with newt astrograph

[Garyh]

Hello IIS guys and Gals,....
I hope someone can help me here with my problem..Its driving me nuts!!!
Ever since I made my OTA up I have had problems with flexure..(all homemade).
For normal visual use this isn`t a problem but as I made this up basically for deepsky imaging its
rather anoying...
I have attached some pictures of the tube mounting, mirror cell and spider assemblly.
I have rebuilt the secondary since and I don`t think this is a problem as its soo solid. All made

from solid aluminium (has been hollowed out to reduce the weight). Vanes are 20x3mm flat bar while

the centre is 50x20mm round bar. Secondary is siliconed to a similar sized bar cut at 45*.
Tube is rolled 1.6mm aluminium about 20cm dia and welded.
Primary cell is made from 6mm plate aluminium and I have put collimating lock bolts in as well to

stop any movement. Mirror is attached with silicone to 3 polycarbonate disks 2.5mm thick, 30mm Dia

at 40% from centre of mirror. I had soft neoprene blocks to start with but I could really notice

mirror movement in any image over 1 min. so replaced with hard poly.
Tube rings and the support bar have both been reinforced with aluminium ribs and I can`t see them

causing any issues.
Flexure doesn`t seem much af a problem near the meridian like 70* elevation but lower down like 50*

above the horizon I can get only get a few minutes before I get star trailing..
I know it`s not the guidescope setup as well as I use that with my Tv genesis with no problems..
Any suggestions from any of you ATM`ers or imagers would be greatly appreciated and you might save

some of my hair on my head!!!....
Cheers Gary

[avandonk]

My guess is that it is the three bolts holding the mirror cell to the tube flexing as the elevation is decreased due to changing load forces. Reinforcing the tube inside and outside might help. Increasing the diameter of these bolts will also help. Spacers that fill in the gap around the bolt so the three corners of the cell are held firmly against the tube would solve the movement as well.

Bert

[gbeal]

My first thought as I read your post was it is over-engineered, LOL.
The primary floaters perhaps, but as you suggest they are hard now. How is the primary cell made up, I can't see what holds the mirror proper, is it clipped at the edges, or can it "slide" slightly?
Also, no slop in the focuser is there, as everything else looks tight as.
Gary

[Garyh]

Thanks Guys,
You might be on to something there Bert, might make up some spacers to go from the cell to the tube wall and see if that helps?
Gary, the focuser is very rock solid so thats no issue. the primary has 3 side clips but they actually don`t touch the mirror as I left like .5mm clearance top and side. More of a safety thing I suppose incase the silicone lets go the mirror won``t go too far. With the primary cell the plate behind the mirror is the same as what is visable. Just a smaller triangle with 3 springs in between them. To this the 3 polycarbonate disks are araldited to the top then the mirror is siliconed to this. The 3 locking bolts lock the two plates together so there can`t be any movement with the collimating screws and bolts.. but I suppose there could be some slight sidways movement possible?
Would it be better to support the mirror closer to the outer edge to stop flex?
I positioned the points similar to the program plop, but maybe thats not as suitable for a imaging system?
Thanks so far guys..

[gbeal]

Gary,
I am at a loss, apart from the lead that Bert has you on, and he could be onto something here as the majority of the weight will likely be at the rear.
If he is right, perhaps some spacers to fit between the triangle, and the inner edge of the tube.
Could it be a too healthy dose of silicone, rather than a smaller amount. And this "blob" is flexing? I too used silicone to affix my 8" f5 recently, and after placing the blobs I put a series of tin cardboard next to them to give me a "measured" thickness that was also consistant for the three of them, just a thought.
Gary

[fringe_dweller]

I agree with bert, those lengthy 3 support bolts/attatchment to thin ally tube look suspiciously like the problem, too flimsy?. Solid Bracing/spacers ect. as already said would fix it prolly.
nice looking set up mate well done!

[Garyh]

Thanks Gary and Kearn,
Just finished putting some aluminium spacers in over the bolts..(12 x 2 mm tubing) between the mirror cell and the tube and done up very tight..I will give it a test tonight weather permitting. Hopefully I shall see a improvement?
If I still get flexing I shall look at the silicone and see if thats a problem like you mention, Gary. Maybe a little too thick. As I remember it would only be around 1mm thick..
Fingers crossed.... and thanks Guys..

[bojan]

I have one possibly stupid question: do you guide on some star in the field of interest, or the guiding star is (relatively) far away ?
If the polar axis is not pointed accurately enough to the pole, and with guiding star away from the camera FOV, you will have that problem.
Your mount looks pretty solid, so I am just suggesting other posibilities.
B

[skeltz (Rob)]

DONT know gary, could be maybe your aluminium tube?not thick enought maybe?
Or could be your 6 mill mirror cell could be thicker,that might be something to think about ,look mate i am no expert but im thinking the mirror cell .....cheers ROB

[Garyh]

Hi Bojan,
Not a silly question at all. Done some drift aligning last night to find my elevation was too high. So that wouldn`t help at all. I try to guide on a star in the field I am imaging but sometimes the guidestar is a few degrees away from the target.. On my flexure tests but, I did guide on a star in the middle of the image and still see the trailing uniform over the image..But thanks for the suggestion and I am better polar aligned!...

ROB, I agree with you, I think its something to do with the mirror cell.
Next big cloudout I might remove it and rebuild it and see if I have fixed it.
Nothing else has made a difference except when I work on the miror cell.
By the way I only have a few pics of my setup. Still a work in progress....pier is just about done then a nice shed next..etc..but heres a pic from last month on a hot summers evening getting it all ready..
Cheers....