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ArcticWolf
member
Reged: 09/05/09
Loc: South Africa
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Flexure with Orion XT10i/EQ6 setup
#5231966 - 05/21/12 06:09 AM Attachment (28 downloads)
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I seem to be having flexure issues with my current setup. My setup is as follows:
Telescope: Orion XT10i
Mount: SkyWatcher EQ6 Pro
Camera: Canon 550D
Guiding: PHD Guiding + Orion magnificent mini autoguider
The guiding graph in PHD Guiding is mostly good. I've attached a screenshot of my graph.
In the main image though, the stars are trailing. Sometimes there are double stars. Like it guided fine and then jumped to a new position and guided fine further. Then there are subs with oval stars, the trail being in the direction of the "jumps" I've described. When going through the subs, the stars are drifting from sub to sub. All in the same general direction.
Now I know this could be due to bad polar alignment, but won't the star being guided on not move? The guidescope is aligned with the main scope so the star being guided on should not be moving due to field rotation. That's why I'm suspecting flexure somewhere in the setup, but I don't know where. The following are some ideas:
Guidescope: It seems to be quite sturdy, so I don't think it's this.
Imaging camera + focuser: The focuser is the Orion 2" low-profile focuser. I've noticed that the camera can rock in one axis very slightly due to there being only one screw to hold the camera. The focuser itself seems to be attached securely to the scope.
Secondary spider vanes: They are quite thin and bends easily when twisting the secondary mirror, even after turning the screws to tighten them fully.
OTA: The tube is mounted using the stock Vixen style saddle and a 13" bar with rings. Could the tube be bending? If so, wouldn't the guidescope "bend" with it, moving the star the same way as through the imaging camera?
I've noticed that some people use the same setup as mine and doesn't seem to be having any issues. Could anyone with a similar setup help?
Thanks
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tejasdragon
sage
Reged: 09/08/11
Loc: Garland, Texas
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Re: Flexure with Orion XT10i/EQ6 setup
[Re: ArcticWolf]
#5232113 - 05/21/12 09:12 AM
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Just my opinion but I would not be imaging
at 1200mm and guiding at a 160mm I don't
care what anyone says will work. I own a
similar setup see sig. I guide with the
st80 FL 500mm but I 2x barlow it so it
puts me at 1000mm Fl. This would be my
first point of concern, the second is wind
this type of setup is pushing our mounts
past the limits for what a lot would say
is possible for good AP, any breeze will
make the ota quiver, I always image behind
a wind screen ( well maybe I should say an
OBs) but the point is try a wind screen.
First I'd try a wind screen, good balance ( just
a little heavy to the east and I run it a little
heavy to the mirror end), every thing is
snug but not over tight ether. If this did
not fix it I'd get a decent guide scope.
And did I say keep it out of the WIND.
Just my nickles worth
Henry
Edited by tejasdragon (05/21/12 09:19 AM)
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AlBoning
sage
Reged: 03/06/11
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Re: Flexure with Orion XT10i/EQ6 setup
[Re: tejasdragon]
#5232240 - 05/21/12 10:44 AM
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I ran into problems with my Orion 8" f/4.9 newt because its rolled steel tube wasn't quite round. Measured across the vanes the internal diameter differed by 4 mm.
One night I thoughtlessly rotated the tube with the laser in and on. I noticed as I loosened the tube ring bolts the laser spot on the faceplate moved several millimeters, and when tightened again the laser spot moved back but was no longer collimated.
I realized then what was happening was a changing of the tension on the vanes. The change in tension results from a not quite round tube in tube rings that are not quite round themselves, and that this difference leads to a change in shape of the OTA which each adjustment of the ring bolts/tube rings and with each rotation of the tube.
After trying some other solutions (adjusting the vanes to hold the tube round led to deflection of the tube changing the tilt of the focuser) my solution was a second pair of tube rings (which also act as Wilcox Rings). The second pair are always tight, and by pre-loading the stresses on the OTA hold the tube in a consistent shape during rotation. In addition, I believe, they stiffen the tube thus reducing sag, which was further combated by going to a longer dovetail mounting plate. While the first pair remain loose (a necessary condition for the other pair of rings to achieve the desired effect) allowing easy rotation. I use a rubber wedge between the OTA and dovetail to hold the tube when in use.
As a result I now see what most people report, i.e. the secondary remains in collimation from one set up to the next, and collimation is no longer effected by rotation or altitude changes during use.
I'm of the opinion a rolled steel OTA isn't up to handling the stresses of EQ mounting (and perhaps Dobsonian mounting as well) and therefore requires some assistance, and that problems related to this are more prevalent than the number of posts regarding same would indicate.
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tejasdragon
sage
Reged: 09/08/11
Loc: Garland, Texas
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Re: Flexure with Orion XT10i/EQ6 setup
[Re: AlBoning]
#5232283 - 05/21/12 11:14 AM
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Quote:
One night I thoughtlessly rotated the tube with the laser in and on. I noticed as I loosened the tube ring bolts the laser spot on the faceplate moved several millimeters, and when tightened again the laser spot moved back but was no longer collimated.
I agree completely with your steel tube thoughts, but
we are talking imaging there is never a need to loosen
and rotate the tube, I collimate after all things
are on the mount, and have checked it through out
the night and it is good, I have had a primary slip
after a big slew but that was because I forgot to
tighten the lock bolts.
I have not had a eyepiece in my scope for at least
6 months, I setup put the cam on and the telrad
has not been tweaked in that long as well, and
gotos always put the object just about in center
of both every time. If my steel tube got the type
of movement you describe that would not be possible.
Henry
Edited by tejasdragon (05/21/12 11:40 AM)
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Eddgie
Postmaster
Reged: 02/01/06
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Re: Flexure with Orion XT10i/EQ6 setup
[Re: ArcticWolf]
#5232343 - 05/21/12 11:49 AM
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Are you sure the flex is not coming from the tripod?
I may be off base, but this is only intended as a possibility.
Does the top leg casting on your Tripod legs have two little set screws in them?
If so, tighten them.
I have a Celestron CGE that has this system, and I was having fits with flexure. I looked and looked for the problem, and tightend everyting way to tight.
Everything except these screws.. LOL.
When I found them and tigntened them, the mount became rock solid.
I don't know if your mount has the same kind of setup, but if it does, it could be the cause of your problems. Again, feel the back side of the top leg castings for two small holes with hex head screws in them. These hold the steel tubes into the top casting. If they are present, ensure that they are tight.
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ArcticWolf
member
Reged: 09/05/09
Loc: South Africa
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Re: Flexure with Orion XT10i/EQ6 setup
[Re: Eddgie]
#5232486 - 05/21/12 01:13 PM
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If the problem lies with the tube or the tripod, shouldn't the guidescope and imaging camera have the same amount of movement? In other words, if the guidescope flex with the tube or mount, the imaging camera should also move with the same amount seeing as they are connected together.
Also, there was no wind on this particular night. It was dead quiet.
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tejasdragon
sage
Reged: 09/08/11
Loc: Garland, Texas
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Re: Flexure with Orion XT10i/EQ6 setup
[Re: ArcticWolf]
#5232534 - 05/21/12 01:52 PM
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Just curious are you walking around the scope?
on my ground I don't darn walk around the scope
it will make every thing move. Once up and running
5' min. distance to walk around. When I am under my
cover I stay put in the chair, if I need to go out
I do it in the pause cycle of my imaging routine.
Henry
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Refractor6
Post Laureate
Reged: 10/20/04
Loc: Vancouver B.C. , Canada
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Re: Flexure with Orion XT10i/EQ6 setup
[Re: Eddgie]
#5232586 - 05/21/12 02:19 PM
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Quote:
Are you sure the flex is not coming from the tripod?
I may be off base, but this is only intended as a possibility.
Does the top leg casting on your Tripod legs have two little set screws in them?
If so, tighten them.
I have a Celestron CGE that has this system, and I was having fits with flexure. I looked and looked for the problem, and tightend everyting way to tight.
Everything except these screws.. LOL.
When I found them and tigntened them, the mount became rock solid.
I don't know if your mount has the same kind of setup, but if it does, it could be the cause of your problems. Again, feel the back side of the top leg castings for two small holes with hex head screws in them. These hold the steel tubes into the top casting. If they are present, ensure that they are tight.
I know this is unrelated to the scope in question but coming across this post NAILED DOWN the last of the flex I was having with my 23lbs 6" f/8 refractor OTA mounted on a set of 2" steel legs/hub I recently aquired for my old EQ5 head I use with the scope.
Found those 2 sets of set screws on the top of each leg then found my container of all my small allan wrenches to find a match. What I found was each one required a few turns to snug them up tight to the inner leg. Checked the OTA for flex and it's much more solid in feel and the pressure needed put on the tube to achieve any movement now. You can FEEL the change when you pull on the mounted tube now after the tightening of just those little screws 
Thanks Eddgie!!... i'm down a few more secs of dampening time  ....and hope it helps in some small way with the other members current problem.
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