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Equipment Discussions >> Mounts

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Peter in Reno
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Reged: 07/15/08

Loc: Reno, NV
Re: PHD guiding defies logic. new [Re: alpal]
      #4508149 - 04/10/11 12:09 AM

I think it's flexure. The star trails are not curved. If they were curved then it would be bad polar alignment. I can count nine star steps in the cropped image.

I used to own an Atlas EQ-G and Celestron 11" SCT guided by Orion Short Tube 80. I was getting exactly what you are getting. I was getting image shifts in the same direction at every sub. I concluded mirror flop flexure due to moving primary mirror in C-11. I solved the problem by replacing guide scope with Off Axis Guider. OAG practically eliminated flexure.

OAG might be difficult for your scope due to very limited back focus. You would probably need the thinest OAG.

You said there may be some play in tube rings, start there. I have read Newtonian mirror mounts can cause the mirror to shift if either improperly mounted or low quality mount is used. Double check to make sure the mirror is well supported. What about secondary mirror? Check that as well.

Can you post a nice picture of your setup including guide scope, cameras, etc?

Peter


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4508179 - 04/10/11 12:29 AM Attachment (25 downloads)

Thanks Peter.
I just took a pic now.
here is a link: http://i262.photobucket.com/albums/ii119/alpal2002/IMG_2355_A_large.jpg

or a smaller version:


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: alpal]
      #4508244 - 04/10/11 01:21 AM

As for an OAG - Off Axis Guider.

I have tried to avoid that.
I would have to shift the mirror up a bit to allow
for it & also it's not always possible to find a guide star
using an OAG.
I tried to take the Tarantula Nebula the other night
which is inside the LMC & I couldn't find a guide
star even with the SSAG guider - on 1 second capture.
I would need to go to 5 second capture but my RA alignment
wasn't precise enough.
It's not an easy hobby!


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Peter in Reno
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Re: PHD guiding defies logic. new [Re: alpal]
      #4508274 - 04/10/11 01:45 AM

SSAG is probably the least sensitive autoguider in the market. I use to own one and never liked it so I replaced it with a high sensitivity Lodestar autoguider and I have never had to hunt for guide stars with OAG. I have guided to as low as 0.2 second.

Not only it's not an easy hobby but an expensive one!!!

I do not know enough about Newtonian scopes so I am not sure how much I can help you. I do know that in order to handle a large scope requires a strong mount with very good tracking and it's not going to be cheap. Try downsizing to smaller scope and go from there.

Good luck.

Peter


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4508288 - 04/10/11 02:08 AM

Quote:

SSAG is probably the least sensitive autoguider in the market. I use to own one and never liked it so I replaced it with a high sensitivity Lodestar autoguider and I have never had to hunt for guide stars with OAG. I have guided to as low as 0.2 second.

Not only it's not an easy hobby but an expensive one!!!

I do not know enough about Newtonian scopes so I am not sure how much I can help you. I do know that in order to handle a large scope requires a strong mount with very good tracking and it's not going to be cheap. Try downsizing to smaller scope and go from there.

Good luck.

Peter




Thanks for the advice Peter.
It's great to have the opinion of others.

I now wish I'd bought a Lodestar camera but still -
I am pushing the limits with 1220mm focal length &
magnifying the stars up so much as I have here.
I was going to try a carbon fiber tube to cut down the weight
but none are available for the size I need for an 8" mirror
and a 1 inch gap each side.
Making my own carbon fiber tube is too big a job.

I am thinking of making some better tube rings & that might help.

An OAG wouldn't work without a Lodestar camera/guiding system
& it all means a lot more money.

If I was using a smaller/lighter 500mm focal length telescope
with my present system I know it would work.
It would divide my error by more than a 1/2 & the
ligher weight would make it easier for my mount to guide.
(also less length means less leverage on the mount)
That's a pity because my 8" telescope is a fine instrument
when properly collimated.
It has a 1/20th wave primary P to V
& a 1/12th wave secondary P to V.
It also has a great focuser & Carl Zeiss flocking paper inside the whole telescope tube.


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jmiele
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Re: PHD guiding defies logic. new [Re: alpal]
      #4508796 - 04/10/11 10:50 AM

My SBIG 402 is super sensitive if you are looking for something else. It is however, expensive as guiders go. Joe

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bluedandelion
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Reged: 08/17/07

Loc: Hazy Hollow, Western WA
Re: PHD guiding defies logic. new [Re: jmiele]
      #4509140 - 04/10/11 01:35 PM

Alpal,

Is your imaging scope the 8" F/6 reflector? If so I think the problem might stem from the long moment arm. From the image I can see where the tube rings are usually located. You are loading up the front of the scope with the ST80 and guide camera (3 lbs at least from my experience) plus your DSLR. I would suspect that the main OTA would flex quite a bit with such loading.

One solution might be to move your guidescope back to locate it directly over the saddle plate of your mount.

Ajay


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Peter in Reno
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Loc: Reno, NV
Re: PHD guiding defies logic. new [Re: bluedandelion]
      #4509210 - 04/10/11 02:11 PM

To do process of elimination, test your mount with your smaller scope to make sure your mount is behaving well and also make sure there is no flexure anywhere. Take 10 minute subs and analyze the images.

Like I said before, I had problems with Atlas EQ-G, C-11 and guide scope. So I tested with 80mm F/6 (or F/6.5) EON guided by ST80 using the same Atlas EQ-G and the guiding was perfect. I didn't drift align. I used polar scope. The guide scope was supported by Orion clam shell rings and they were rock solid.

Are your guide scope rings supported by plastic screws? Looking at Orion web site showing ST80 guide scope package, the screws look like nylon or plastic. If that's the case, the screws are too soft and can cause flexure.

Peter


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Peter in Reno
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Reged: 07/15/08

Loc: Reno, NV
Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4509758 - 04/10/11 07:44 PM

I found this link from another CN thread. He uses 8" Newt on the same mount as yours. Take a look at his pictures from his web site:

http://webpages.charter.net/paastroman/

I think his scope is F/4 which is quite a bit shorter than yours.

You might want to contact him and you might get better advice than me.

Peter


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4509941 - 04/10/11 09:29 PM

Quote:

I found this link from another CN thread. He uses 8" Newt on the same mount as yours. Take a look at his pictures from his web site:

http://webpages.charter.net/paastroman/

I think his scope is F/4 which is quite a bit shorter than yours.

You might want to contact him and you might get better advice than me.

Peter




Hi Peter,
I am at work so can't type much now.
His stars are nice & round & he does have the
guide scope on the tube rings.
I'll look into this later.
cheers
Alpal


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Nils_Lars
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Loc: Santa Cruz Mountains , CA
Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4509963 - 04/10/11 09:42 PM

I would guess the ST-80 is causing the flexure , that focuser tube with an extension is notorious.

I would put a block and a strap between the SSAG and the Newt so it cant move , once the focus is set it wont need to move anyway.


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: Nils_Lars]
      #4510082 - 04/10/11 10:54 PM

Quote:

I would guess the ST-80 is causing the flexure , that focuser tube with an extension is notorious.

I would put a block and a strap between the SSAG and the Newt so it cant move , once the focus is set it wont need to move anyway.




Thanks Nils,
The extension tube is rubbish.
It wobbles inside the focuser until you do it up tight.


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BlueGrass
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Reged: 07/25/09

Loc: Wasatch Front, UT
Re: PHD guiding defies logic. new [Re: alpal]
      #4510163 - 04/10/11 11:46 PM

Alpal,
Moving to a 9x50 finder / guider setup I think would help alot. Once the SSAG and finder is put in focus, there is no flex. This is the 'home brew' version of the Kwiq guider package. Many of us have made them and they perform very well. I mount mine atop all my imaging setups using a V clamp with a V plate. This allows a very rigid assembly that can be moved front to back to help in balancing. You've probably seen the threads that cover making one. Phd has no problems guiding at the 9x50 FL.

Found the thread from last year, that shows some of these 9x50 units and also some posts with in-depth discussion of guiding performance and results...
http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/3929567/page/0/view/collapsed/sb/5/o/all/fpart/1/vc/1


Edited by BlueGrass (04/11/11 12:06 AM)


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: BlueGrass]
      #4510389 - 04/11/11 05:06 AM

Thanks guys for all the tips.

I can't do them all right now but what I can do quickly & easily
is mount the Guide Scope directly onto the tube rings.
OK - the telescope itself will flex slightly but it should
have a resonance that won't last longer than 1/2 a second
& that would be minimal compared to a 5 minute sub-frame.
That way the quidescope is at the closest possible point
to the mount itself & I'll see if that improves things
& get back to you all here.

I think that will make a considerable improvement.
After that I can change the 6 plastic screws on the guide scope rings to metal.

Also - I have a friend who is working on an idea to make some better
main tube rings out of aluminium.
There are no standard tube rings available for the Sonotube size
which has an outside diameter of about 10.5"

After that I'll try the finder scope option.


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: alpal]
      #4510495 - 04/11/11 06:57 AM Attachment (22 downloads)

OK - I changed it over.
There is one pic below & some more here on photobucket:

http://i262.photobucket.com/albums/ii119/alpal2002/IMG_2360_A.jpg

http://i262.photobucket.com/albums/ii119/alpal2002/IMG_2359_A.jpg

http://i262.photobucket.com/albums/ii119/alpal2002/IMG_2362_A.jpg

I just need a clear night to test it.


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Peter in Reno
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Loc: Reno, NV
Re: PHD guiding defies logic. new [Re: alpal]
      #4510756 - 04/11/11 10:10 AM

For the screws in tube rings, I wouldn't recommend metal screws touching the guide scope. I suggest screws with special tip material like Delrin so they won't scratch the guide scope. Here is a sample:

http://www.admaccessories.com/Miscellaneous_DTTS.htm

Peter


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bluedandelion
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Reged: 08/17/07

Loc: Hazy Hollow, Western WA
Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4511246 - 04/11/11 01:38 PM

Alpal,

That looks much better. The dovetail plate for the ST80 will give the tube rings an extra bit of stability. With the setscrews on the ST80 extender tightened down, see what you get.

Await your results curiously.

Ajay


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Peter in Reno
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Reged: 07/15/08

Loc: Reno, NV
Re: PHD guiding defies logic. new [Re: bluedandelion]
      #4511303 - 04/11/11 02:01 PM

What kind of dovetail are you using? Standard NEQ6 (Vixen style) or Losmandy "D" style? If you are using Vixen style, I highly recommend getting "D" style since it's quite a bit wider and will provide much greater stability for a heavy tube.

http://www.admaccessories.com/D_Series_Universal.htm

You will also need "D" saddle.

http://www.admaccessories.com/D_Series_Saddles.htm

I am a little concerned of your tube rings. They look a little thin. They may flex differently depending on where the scope is pointing to.

Parallax makes great tube rings. They have 10.3" and 11".

http://www.parallaxinstruments.com/ring.htm

Peter


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: Peter in Reno]
      #4511662 - 04/11/11 05:11 PM

Quote:

For the screws in tube rings, I wouldn't recommend metal screws touching the guide scope. I suggest screws with special tip material like Delrin so they won't scratch the guide scope. Here is a sample:

http://www.admaccessories.com/Miscellaneous_DTTS.htm

Peter




Thanks Peter -I'll organise some of those.


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alpal
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Reged: 06/15/09

Loc: Melbourne Australia.
Re: PHD guiding defies logic. new [Re: bluedandelion]
      #4511666 - 04/11/11 05:12 PM

Quote:

Alpal,

That looks much better. The dovetail plate for the ST80 will give the tube rings an extra bit of stability. With the setscrews on the ST80 extender tightened down, see what you get.

Await your results curiously.

Ajay




So do I - I am looking for an improvement.


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