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weathermon
07-06-2016, 08:15 AM
Hi guys,

I've got a really annoying problem that I am struggling to solve so thought I'd ask the help of experts :) Ok let me explain my setup - I've got a GSO 10" F5 newtonian, it's been flocked and I've also installed a moonlite focuser with the high res stepper motor. I've also purchased a Howie-Glatter 2" collimation laser with the barlowed attachment. What I've noticed is that lately, when I am imaging, I am constantly having to adjust the focus (talking about adjusting about 20-40 steps on the focuser every 10 minutes), and also my stars never seem to be rounded anymore.

I decided to set the scope up inside on the mount, get collimation right and then tilt the mount on the RA axis - I was shocked to see just how much the collimation moved! I'm pretty sure from what I can see, that the primary mirror doesn't move as it's the little black circle that is *slightly* bigger than the hole in the collimation tube, but seems to be either the focuser or secondary movement as the rings move way off centre?

The mirror is done up quite firm (springs are tensioned almost all the way in and locked quite firmly). The focuser appears nice and tight with no slop, all screws and bolts appear to be done up tight, but I just cannot for the life of me figure out what could be causing this?

I'm using the tube rings that came with the GSO and also the skinny dovetail as well so not sure if that's the cause. I've uploaded a Youtube video below to show you what I mean.

https://www.youtube.com/watch?v=AhOKBi8ljrA

Cheers, Mike

glend
07-06-2016, 08:36 AM
Have you upgraded the primary mirror springs? I have had several GSO reflectors and all suffered from weak collimation springs - which allows collimation shift at different attitudes. The old Dob trick of collimating at a 45 degree angle might help, then observe the collimation shift as you change altitude.
Those steel tube scope will also be affected by temperature changes. If you allow proper time for it to adjust to ambient temperature your focus maybe more stable.finally, i would uggest testing with the stepper motor assembly off if you can, yo see if it is slipping. The weight of the imaging equipment maybe causing the focuser to slip. Try it manually with the Moonlight locked , assuming you have a shaft lock model.

astro_nutt
07-06-2016, 09:09 AM
Hi Mike. I had similar problems with my 12 inch dob. First thing I did was check the tube ends and make sure they are tight. Next came the spider holding the secondary mirror. Check the focuser/tube fastenings. Next the clips holding the primary mirror in it's cell. The collimation lock screws for the primary mirror are push against a steel plate which does move when you tilt the scope. I glued a piece of 6mm aluminium flat on the steel plate with a shallow 2mm recess using a 6mm drill under the lock screws. I then drilled the end of the lock screws with a 6mm drill just enough to hold a ball bearing. You will now be able to apply more locking force to the steel plate. I also find that you only need to adjust the locking screws when collimating. I hope this might help.
Cheers!

weathermon
07-06-2016, 10:04 AM
Hi Glen,

I have thought about upgrading the springs definitely - the circle for the primary mirror doesn't appear to be moving in this instance though when I rotate the scope on the mount, but I'll def be getting the upgrade kit sometime down the track as well. This test was just with the laser collimator in which weighs a few hundred grams. I had the focuser locked firmly in place when rotating the scope as well. I know what you mean about the cool down of the tube, it's amazing how much the temperature can affect them.

Cheers, Mike

weathermon
07-06-2016, 10:09 AM
Haha yeah that was the first thing I checked - I found a few loose screws on the secondary side and thought you beauty, this must be it! unfortunately not to be though. I might pull the secondary spider out again and check it all out to make sure nothing there is loose at all. Interesting idea re the aluminium flat! Something else to look into for sure. I did adjust the clips holding the mirror as well (might have tightened a little too much as I have almost triangle shaped stars - gotta love pinched optics).

Cheers, Mike

Shiraz
07-06-2016, 02:48 PM
I had a GSO in which the secondary was slightly loose in the plastic holder - they are stuck in with double sided tape and for whatever reason it had given way enough for the mirror to move ever so slightly - drove me mad until I found it. different scope when fixed.

weathermon
07-06-2016, 03:27 PM
Wow that's definitely one I'll be checking! thanks for this, will check that out when I get home.

weathermon
07-06-2016, 08:47 PM
I may have found the problem! I've got the Orion 80mm guidescope on top and it's only mounted on with the rings, no 2nd dovetail at the top to stop any flexure of the tube.. I had the tube on the ground and went to lift it by the open end and was surprised at how much the laser moved when I barely lifted it a few mm off the ground! I'm going to order a 40cm dovetail as well as the losmandy rings for the autoguider.. I checked everything else with a fine tooth comb tonight and can't see any other problems.

janoskiss
08-06-2016, 04:54 PM
Sounds like you may have found the culprit. But just a note re primary collimation springs:

Use the locking screws and don't rely on the springs. Tightening the locking screws is the final stage of the collimation process. You should tighten them bit by bit while keeping an eye on the barlowed laser reflection and making sure it does not shift when you're done. The locking screws done up firmly finger tight also help a lot with the scope keeping collimation in the long run. I still check every time I observe just in case but adjustment is needed only maybe once a year. And the scope gets around (100's kms between my home and fave spots with some rough terrain along the way).

DaveNZ
12-06-2016, 05:41 AM
I have a GSO 10' f4 also. I have had issues holding focus from the beginning too.

I have upgraded the scope with a bobs knobs kit and a carbon tube but it has made little difference.

The issue with my system seems to be temperature related. A 1 degree temperature change and I need to refocus the Moonlite.

An unusual weather pattern last weekend illustrated the issue, the temperature was oscillating back and forward 3C every 30 minutes or so which made imaging hopeless. The focuser had to refocus back and forward around 80 - 90 steps.

glend
12-06-2016, 08:05 AM
Look steel tube scopes like this GSO will always have some flex, even the dob models show collimation shift when the altitude is changed, so much so that collimation is recommended at 45 degrees so that there is minimal shift at higher altitudes. Basically, aside from springs to stop primary movement, and the use of a top mounted vixen bar to lock the rings in place, there is not much more you can do with that tube.
However, you can buy carbon fibre tubes that are direct replacements for the GSO steel tube. These carbon fibre tubes come from Germany, and are available from Teleskop- Express. Check their website. The problem of course is that the basic carbon fibre tube costs more the GSO scope did originally. Your present tube components transfer directly onto the carbon tube (focuser, secondary, primary cell and mirror, etc.

Link here:

https://www.teleskop-express.de/shop/product_info.php/info/p5242_Carbon-Tube-Upgrade-for-250mm---10--F5-GSO-Newtonian-Telescopes.html

The tube will cost you 538 Euros, or $821 AUD, then add shipping on top of that. (Note: that when Australians buy from Teleskop-Express you will not have to pay the Euro zone VAT tax, which is why the price is reduced to 538). You will get great collimation holding power and tube stiffness for that price. I doubt you would be able to sell of the old GSO tube for much, as a scope without mirrors etc is just scrap steel.


I should add that my 10" f5 imaging newt uses the same mirrors, as your scope, which I bought directly from Bintel (the mirrors), but I built my own carbon fibre strut tube to hold everything. If you can work with carbon fibre and epoxy it might be an option for you.
You can find my build thread here:
http://www.iceinspace.com.au/forum/showthread.php?t=126947&highlight=imaging+newt