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Old 27-04-2012, 02:39 PM
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allan gould
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How to guide at 2500mm+

Last night I went out and started up my equipment to image M83 since it seems to be the flavour of the month. I didnt have too much time as rain was predicted for last night and I was lucky to get 90 min in 15 min subs before it all clouded over.
I was imaging a f10 with my 10"SCT at 2500mm and I thought that I should share some experiences of how I manage to do this when some others find difficulty. I use a 72mm WO DDG scope that sits atop either of my 10" scopes on a Losmandy rail. It is securely bolted to this rail to eliminate any flex in this area. I tried side by side which is a disaster as no matter how well it was made there were too many points where flex could creep in.
The next thing I did was to eliminate the long length associated with the guide camera that is necessary to achieve focus. Originally I started out with extension tubes but they always flexed until I placed a bolt through the Losmandy rail underneath the camera, screwed it up until it supported the weight of the camera which stopped the long length of focuser, tubes plus camera from flexing. Even screwed in extension tubes still flexed until I tied it down to the Losmandy rail using this bolt arrangement. This was a bit of a pain so I eventually hit on my present arrangement where I put a 2" diagonal in place of extending the focuser and an extension tube before the camera. And to adda little cream to the system introduced an x/y shifter just before the camera, a DSI II Pro. This gives a flex free guidescope that I can guide at 2500mm for up to 40 min (done just to see if I could do it). See attached photo guiding.
Ive also attached the shot of 90 min on M83 and also a screen grab of the guiding on M65 through PhDguiding. I should state that that I used PemPro2 to get a very, very good polar alignment on my mount and pier. That also is very important.
Hope this helps someone else.
Allan
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  #2  
Old 27-04-2012, 03:39 PM
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I must give that a go some time, Allan. I tried a guidescope with the RC10 once and didn't get good results so I went straight to OAG and now AO.

Cheers,
Rick.
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Old 27-04-2012, 03:44 PM
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Good work on getting out the noticable flexure, but an OAG system works far better far more elegant and just easier to get working perfectly!

Look forward to the end photo though
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Old 27-04-2012, 04:28 PM
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Nice detail in the core Allan. It will be good to see what you can do with that rig with a few more subs and some colour.

Can't wait for some clear skies now Summer has passed.
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Old 27-04-2012, 05:06 PM
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Quote:
Originally Posted by bmitchell82 View Post
Good work on getting out the noticable flexure, but an OAG system works far better far more elegant and just easier to get working perfectly!

Look forward to the end photo though
I keep hearing what you have stated Brendan re an OAG, however although I have a QSI583wsg and a separate OAG for other systems I've never been able to get decent stars or consistently find find a star to guide on. I find the separate system far easier and I've never had to alter a composition to find a guide star yet. To each their own but I find a separate guide scope far easier and simple to use. The only secret is to eliminate ALL flex in the system.
I only put this up to help others going down this path that it is doable and how I've done it.
Allan
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Old 27-04-2012, 05:34 PM
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I'd just like to back up what Allan says about flex, I had issues with flex until I did much the same thing, supported the camera on the rail and problems disappeared.

These days I tend to use the OAG for most things, but with Hyperstar I have to use a separate guidescope.

Which mount are you using Allan?
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Old 27-04-2012, 05:35 PM
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Very tight guiding at 2500mm. I like it!
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Old 27-04-2012, 05:44 PM
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Quote:
Originally Posted by Poita View Post
I'd just like to back up what Allan says about flex, I had issues with flex until I did much the same thing, supported the camera on the rail and problems disappeared.

These days I tend to use the OAG for most things, but with Hyperstar I have to use a separate guidescope.

Which mount are you using Allan?
I'm using a Losmandy G11 mount. I've trained the PEC with PemPro but this guiding was done with PEC off.
I also have an EQ6 that has been hyper tuned and fiddled with extensively and I can also guide at the same FL with this. Using focal reducers on the 10" SCT or GSO RC makes this a doddle. But if an OAG floats your boat then use it.

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Very tight guiding at 2500mm. I like it!
Me too Marc.
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Old 27-04-2012, 08:34 PM
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That looks pretty god guiding. I don't believe you can get all the flexure out of a side by side or saddle system (unless you use Robin Cassidy gear). I'd say opt ofr on or off axis guiding.
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Old 27-04-2012, 09:02 PM
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so how did you use pempro Allan? g1 or g2 system
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Old 27-04-2012, 09:13 PM
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Quote:
Originally Posted by allan gould View Post
I keep hearing what you have stated Brendan re an OAG, however although I have a QSI583wsg and a separate OAG for other systems I've never been able to get decent stars or consistently find find a star to guide on. I find the separate system far easier and I've never had to alter a composition to find a guide star yet. To each their own but I find a separate guide scope far easier and simple to use. The only secret is to eliminate ALL flex in the system.
I only put this up to help others going down this path that it is doable and how I've done it.
Allan
I suspect you'll find things different with a Lodestar (or similar). I had little or no success with a SSAG on my QSI WSG when trying to image a 2200mm - it was just too dim, even with 8-10sec exposures. The lodestar could pickup guide stars with 1-2 sec exposures. Only pain with swapping to a lodestar is that you will need to get the C-mount adapter from QSI, instead of the T-mount adapter for the OAG.

DT
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Old 27-04-2012, 09:16 PM
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Originally Posted by DavidTrap View Post
I suspect you'll find things different with a Lodestar (or similar). I had little or no success with a SSAG on my QSI WSG when trying to image a 2200mm - it was just too dim, even with 8-10sec exposures. The lodestar could pickup guide stars with 1-2 sec exposures. Only pain with swapping to a lodestar is that you will need to get the C-mount adapter from QSI, instead of the T-mount adapter for the OAG.

DT
David
I purchased that at the same time so Im all set, also have the 5mm spacer as well as the acrylic disc. I used to be a boy scout
Allan
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Old 27-04-2012, 09:32 PM
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That was going to be my next thing that i said but i have been beaten to the chase! You have to use a decent guide camera, Load star, SBig402 and the like work well. Qhy5's and what not need a lot of aperture to pick stars up.

There is a reason why i spruke the OAG, because no matter the focal length it is 100% accurate and unless you are a dork and have no idea they are nigh on infallible! the star moves in your oag, it moves in your image plane! the star doesn't move in your OAG it hasn't moved on your imaging circle Trust me i'm no stranger to flex I have a 1.2m long Newt with 40cm worth of imaging kit hanging off the OTA.

But yes i see your point that you have gotten acceptable results with a guide scope! aka it can be done
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Old 28-04-2012, 06:44 AM
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Quote:
Originally Posted by allan gould View Post
David
I purchased that at the same time so Im all set, also have the 5mm spacer as well as the acrylic disc. I used to be a boy scout
Allan
Good to hear! I haven't needed to spacer or disc with mine.

DT
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Old 28-04-2012, 11:16 PM
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Allan,

Very interesting points there. I read with interest because I have a similar setup from a guiding point of view.

1. You mention side by side is a "disaster". So reading this, you mount your guide scope on top of the imaging scope. I assume this works on the principle if the imaging scope flexes, it takes the guide scope with it?
2. You've gone to a 2" diagonal. Again assuming this provides more rigidity to the guiding system instead of extension tubes.
3. How do you find the XY finder? I got one a few weeks ago and yet to try it out.

Cheers,
Darrin...
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Old 29-04-2012, 10:23 AM
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Quote:
Originally Posted by h0ughy View Post
so how did you use pempro Allan? g1 or g2 system
David I used it with G1 and found it very easy to use. Sorry I missed seeing your post.
Quote:
Originally Posted by midnight View Post
Allan,

Very interesting points there. I read with interest because I have a similar setup from a guiding point of view.

1. You mention side by side is a "disaster". So reading this, you mount your guide scope on top of the imaging scope. I assume this works on the principle if the imaging scope flexes, it takes the guide scope with it?
2. You've gone to a 2" diagonal. Again assuming this provides more rigidity to the guiding system instead of extension tubes.
3. How do you find the XY finder? I got one a few weeks ago and yet to try it out.

Cheers,
Darrin...
Darrin
1. No I mount it on top of the main scope with Losmandy rail and it's tied down tightly so there is no flex. If ,the mirror flops then I get bad guiding but I use a mirror bolt so this doesn't happen. No flex is the mantra.
2. I use a 2" diagonal so that the weight is close to the focuser and so the focuser has less pivotal motion to deal with than with extension tubes.
3. X/Y finder is most excellent. Usually there is always a GUidestar with the 72mm scope but if it's too faint or I want a better one then a few shifts with the xy finder and I have a better star. Highly recommended
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