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Old 31-10-2008, 12:23 PM
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allan gould
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Guidescope FL for 10"SCT?

What focal length would I have to use for guidescope for a 10"SCT at 2500mm?
I have a WO Megrez 80mm 560mm FL f7 guidescope. Would I have to put a 2x barlow for 2500mm fl imaging scope?
I assume the guidescope would be ok if a used a f6.3 converter on the main scope.
Experienced help would be appreciated
Thanks, Allan
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Old 31-10-2008, 01:01 PM
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The common sense is telling me that resolution of the gui9de scope should be the same or better than the main instrument (arcs sec per pixel)
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Old 31-10-2008, 01:17 PM
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Correct me if I'm wrong but I assume it's irrelevant what you use as a guidscope especially if you are using a CCD Guider as you can use any star in the field of view or not to guide on.

Cheers
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Old 31-10-2008, 01:46 PM
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If you are autoguiding your f7 WO will be plenty long enough. If you are manually guiding then aim for 1.25x the FL of your imaging scope at least.
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Old 31-10-2008, 01:49 PM
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Allan,

I've successfully guided my C11 @ F/6.3 with an ST80 (400mm). At F10 I've never tried, however I've seen a few guys in the states using 200mm focal length camera lenses, and also 9x50 finderscopes to guide 12" LX200's...

I dont think its overly critical, just as long as your guiding is accurate. (given the TV Guider, I reckon you'll be right as rain with a 560mm guide scope)

Bojan makes a valid point though. for guiding to have sub-pixel accuracy in the final image, the guidescope and camera should have same or better resolution.
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Old 31-10-2008, 01:51 PM
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Thanks for that information guys - its sort of what I had thought but I wanted someone to comment that had some experience in this area.
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Old 31-10-2008, 02:19 PM
ozstockman (Mike)
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Allan,

From my experience I wouldn't recommend anything more than 1000mm for a guide scope. I used to have 8" SCT reduced to 1260mm as a guide scope and it worked very bad with PHD. It did strange things during calibration, sometimes it was loosing a guide star or didn't want to accept it at all displaying "low SNR" message.

Now I'm back to a short FL guide scope(3.5" f/5.3 telephoto lens) for my VC200L with reducer as an imaging scope and it works much better.

cheers,

Mike
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Old 31-10-2008, 03:16 PM
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Well, any guider is basically a close loop system.
In order for such a system to be unconditionally stable, the open loop gain must be lower that 1.
Also, there are some other loop parameters, like phase lag and loop damping, and first of all, the type of the system (does it contain PID element or not etc.. in other words: open loop transfer function)
They all must be set properly otherwise the system will either tend to oscillate (show overshots) or the tracking will not be adequate (sluggish, not reactive enough or with ever present error).
They will be different from case to case, and they depend on the detector gain (guide camera resolution - arcsec/pixel), mass of the telescope, the way speed of the motor is changing (time delay, the presence of acceleration etc).
I think the proof is in the pudding here.. Also I imagine various softwares will behave differently on different mounts and setups.
All that are just my deliberations.. before really biting a bullet :-)

Last edited by bojan; 31-10-2008 at 03:35 PM.
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Old 31-10-2008, 07:36 PM
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Depends on the pixcel resolution of the autoguider and the how the autoguider detects error signals (full pixcel, 1/2 pixcel, 1/3 pixcel , 1/4 pixcel or less or more) and if you are using the big scope only at prime focus or via a barlow or eyepiece too.

I am only guessing , but I would expect the guidescope needs a focal length not less than 1/2 that of the main scope.
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Old 31-10-2008, 07:41 PM
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Quote:
Originally Posted by bojan View Post
Well, any guider is basically a close loop system.
In order for such a system to be unconditionally stable, the open loop gain must be lower that 1.
Also, there are some other loop parameters, like phase lag and loop damping, and first of all, the type of the system (does it contain PID element or not etc.. in other words: open loop transfer function)
They all must be set properly otherwise the system will either tend to oscillate (show overshots) or the tracking will not be adequate (sluggish, not reactive enough or with ever present error).
They will be different from case to case, and they depend on the detector gain (guide camera resolution - arcsec/pixel), mass of the telescope, the way speed of the motor is changing (time delay, the presence of acceleration etc).
I think the proof is in the pudding here.. Also I imagine various softwares will behave differently on different mounts and setups.
All that are just my deliberations.. before really biting a bullet :-)
True .

But I doubt however many amateur astronomers have the skills to determine the laplace transform and examine the controllability and stability of their autoguiding setup.
Also a back of the envelope doodle reveals it is actually a highly coupled 2D MIMO system and may well have a non-linear response.
Ooooo !!! messy.
Would be a great honours year thesis project for a electrical engineering or computer engineering or mechatronics engineering student .... I might suggest it to Newcastle Uni.
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Old 02-11-2008, 07:00 AM
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Quote:
Originally Posted by Ian Robinson View Post
Would be a great honours year thesis project for a electrical engineering or computer engineering or mechatronics engineering student .... I might suggest it to Newcastle Uni.
This may be a good idea..
And, if the paper is made available to some Chinese company , excellent!

Because, the cost of hardware for project like this is not that much... $200 max for prototyping, including controller evaluation boards. In my particular case, since boards are already available, I have to buy only some parts, and of course, mechanics...
Still way better than PC.
BTW, TvGuider has its website, here: http://foto.astronomy.cz/TVGuider.htm

On system properties, you are right, it is not linear, because of the mechanical backlash, for the start.
Then, the gain will be variable because of PE...
But I am sure it can be treated as two independent 1-dimensional loops.
The challenge will be to design the response such that the stability is adequate for variety of mounts (some have steppers, some have DC motors coupled in their own control loops).. or perhaps not because the response of those loops will be relatively fast compared to to our guiding loop, so maybe it can be ignored for the start.
I think I'd better have a good look at Niemi's souce code :-)

Last edited by bojan; 02-11-2008 at 08:10 AM.
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