GSO 10" Truss Newt issue (SOLVED!!!!!)

[spiezzy]

hi all
I was wondering if any one can throw some wood into the fire with this issue I am having .
I have a GSO 10" Truss F4 newt I have fitted a Moonlite focuser to it its a great scope for imaging but last night i noticed when collimating with my Hotech SC
collimator when i move the scope after getting the laser spot on the laser dot would move out of the centre dot on the main mirror then I would move it back to the position that I did the collimation in and it would be centred again
this is telling me that when I move the scope to different areas of the sky it will be out of collimation .
I have checked that every thing is tight but it has me puzzled
any suggestions ??
cheers Pete :thanx:

[Slawomir]

I hope it is not the main tube flexing...

[rrussell1962]

I suspect that either your trusses are shifting or the secondary holder is shifting. Try checking that the spider is nice and tight. I get a bit of collimation shift on my Obsession that I have never quite nailed down. My cats eye auto collimator suggests it is in the secondary, but within reasonable tolerances.

[spiezzy]

Hi Suavi
I had thought of that but there is not that much weight on it and feels solid if you apply pressure to the front section of the scope it does move but you have to push fairly hardish .
the truss design is carbon fibre trusses

[spiezzy]

hi Russell
I will have a look at that tomorrow good suggestion the secondary is fairly big and heavy being quartz glass I'm just wondering if this is the issue how do you go about correcting it ?

[glend]

The old advice with GSO dobs was to collimate with the scope at 45 degrees, as collimation can vary slightly with attitude. With your newt, are you collimating in park position ( say aiming at the south celestrial pole)? I would suggest you look at the lock push bolts for the primary, if your just sitting on the springs the collimation will change as you move the scope. Another thing, GSO dobs and newts, always seem to need a spring upgrade; perhaps they have addressed this old issue in new models but i have owned several GSO reflectors, right up to a 16 " Strut Dob and they all needed stronger springs.
The larger Bunnings store stock springs that are suitable, check in the cabinet fittings section of the store. You can buy springs that are too long but can cut them down with a hacksaw or Dremel tool. The important thing is that the diameter matches the stock spring and that they are stiffer.

[spiezzy]

thanks Glen
I collimate in park postion to remove the mirror cell looks quite complicated and requires removing the back struts i am just a bit worried about when I put it all back together again with the struts be aligned or are they self locating
but would be good to check the springs you mentioned

[rrussell1962]

As Glen said it is worth checking the primary cell. Years ago I had a GSO Dob and collimation was much better when I upgraded the springs. In terms of the secondary holder I found that moving the secondary as close to the spider as I could and using the collimation screws to centre the secondary under the focuser minimised the flex as the scope moved in altitude. Does that make sense?

[glend]

These are recently released scopes from GSO, right? The truss newts have been promised for some time, like last year, but have just hit the market. Did you buy this scope from Andrews or Bintel? If so it is under warranty. If your having these sort of problems, send it back.

[spiezzy]

well Spoke to Don at Bintel today and he was very helpful but as confused as I am he is sending me some replacement springs which are a lot more heavy duty so I guess I start there .
I purchased the scope this time last year and as I have replaced the original focuser with a moonlite with small mod to the focus holder plate it might void the warranty it was one of the first one in the country
I demounted the OTA today and found out it is quite easy to remove the mirror cell I will add some pics soon
cheers Pete

[DaveNZ]

Hi Pete,

The main issue with my gso newt (with a carbon tube) was secondary movement. I had to almost over tighten the three adjustment screws.

There was also some occasional mirror movement as well. This was with Bobs Knobs and springs.

I hope the new cells have improvements made to the design.

I still waiting for my 12inch truss GSO newt.

[spiezzy]

Well after much studying and net searching I have solved the Issue with the collimation moving in different locations of the sky it ended up being flexure in the front secondary mirror section .
there are 2 plates separated by a metal tube and then there are 3 x 8mm bolts which clamp it all together and the plate that holds the focuser now I would never have picked it but I read a thread on Cloudy Nights on the same scope and he was having issues as well but it was him that pointed me in the right direction.
the front plate is quite heavy around 2kgs and if the scope was pointing to the side it would twist a little and put the collimation out so the solution was to beef up the front ring mounting .
I used 8mm threaded rod with nyloc nuts and 12mm aluminium tube I might have over engineered it a bit but boy does it work and at a cost of $40 bucks it was so worth it I can now point the scope anywhere in the sky and the little red dot does not move at all and that has made me very happy because the scope it self is of very industrial build and quite solid but being a first generation of this type from GSO this maybe just something over looked .
I also replaced the primary mirror springs and the locking grub screws were replaced with longer hex head screws that I can do up with my fingers .
I am now looking forward to imaging again with the scope staying in collimation
I would like to say thank you to all those that replied with very helpful information
cheers Pete

[DaveNZ]

Nice Job Pete. The secondary section did look rather flimsy. If you look at other expensive truss newt designs the secondary section have rods similar to your mod.

If you wanted to go carbon fibre you could check out dragonplate who sell end connectors for carbon tubes.
https://dragonplate.com/ecart/categories.asp?cID=264

Cheers Dave

[spiezzy]

Thanks Dave
those carbon fibre connectors look great the extra weight I have introduced with this Mod is only 870 grams which is not that much considering the strength it add to the front cell I can even flip it upside down and every which way and the collimation does not budge .
I will send Don from Bintel some pics and maybe he will mention this to GSO
tried it out last night and the images where nice and sharp anywhere I pointed the scope a huge difference from before I did this as I thought it was focus problem when you get your new scope I would check that it is not doing the same thing but GSO might have addressed this issue already
cheers Pete

[spiezzy]

Well got to try out the new mods on the scope last night on M20 and all seemed to work great no collimation issues this is a stack of 60 x 60 sec exposures with the ASI1600MC cooling -15 captured in Nebulosity 4 with a CLS filter also at the bottom towards the middle there is what I suspected is a Asteroid trail but have not had chance to see if there was one in the area or not and M5 which is a stack of 100 x 30s possibly should have re focused on this :thumbsup::thanx:
cheers Pete