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revans
Carpal Tunnel
Reged: 09/26/05
Posts: 1516
Loc: Fitchburg, MA
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At what focal length do you start to use an autoguider with a G11 mount? I did a test a couple of nights ago taking unguided 2, 3 and 5 minute subs of M27 at FL=500 mm, FL=750 mm and FL=1500 mm. Even at 5 minutes the FL=500 and FL=750 images showed no star trails at all. However, the 5 minute sub at FL=1500 did show slight trailing but that was bad enough to ruin those images. I'm guessing that maybe 1000 mm is about the limit for avoiding an autoguider, but I was wondering what other folks experience has been... I don't need to take any subs longer than 3 to 5 minutes because of light pollution which starts to wash out the images if I go longer (I live in a tri city area with a population just under 100K and there is plenty of light pollution).
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Rick Evans
http://www.freewebs.com/revans_01420/
"The universe is there for us to see, but it cannot be understood without learning its language -- mathematics." Galileo Galilei
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gnowellsct
professor emeritus
Reged: 06/24/09
Posts: 732
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You don't mention whether you have PEC turned on. Out of the box performance is variable because of the worm block design, anywhere from 5 to 12 arc seconds PE is normal, so your question has no firm answer. Moreover there are aftermarket options such as the Ovision where people report 1-2 arc second errors with PE turned on and sub arc second tracking with autoguiding and PE.
But your basic point clear: the longer your fl, the more you will need an autoguider. But the requirement won't magically start at 1350 or 1650mm fl, because the mounts themselves have different performance levels "as sold."
regards
GN
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"Aperture will get you through times of no money better than money will get you through times of no aperture."
featuring selected astrojunk:
bunch o' widefield eyepieces
bunch o' narrowfield eyepieces
couple o' Barlows
couple o' scopes
couple o' mounts
couple o' tripods
and a pier 'n' stuff
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Patrick
Postmaster
Reged: 05/16/03
Posts: 7807
Loc: Franklin, Ohio
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Quote:
At what focal length do you start to use an autoguider with a G11 mount?
The best way to find out is to measure your mount's periodic error. HERE's An Example where I ran tests on my CG5-GT. You can try running the numbers with and without PEC to determine how much PEC is helping.
Once you determine the average PE for your mount, you can plug the PE, camera, and scope data into Niel Noordhoek's Periodic Error Modeler. Adjust the scope focal length parameters (or the exposure parameters) so that the PE Curve Size values on the bottom of the window are less than one and you should be close to the max focal length/exposure parameter value you can take with your setup. I had to do a little playing to figure out how the program works, but it seems to be pretty accurate...at least it should get you in the ballpark. Let me know how it works out for you, or give me your data and I can try running the program for you.
Obviously, the better the PE and polar alignment, the longer the focal length/exposure you can take. But, your camera's sensor and pixel size are also important considerations along with the scopes focal length.
Patrick
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10" f/6 Truss Tube Newtonian
Celestron C6S-GT SCT
AT66ED Refractor
Oberwerk Ultra 15x70 Binocular
475B Geared Tripod & 501HDV Head
Celestron Regal 8x42 Binocular
Canon XSi; Meade DSI;SPC900-NC
Vixen GP2 Photo Guider Mount
My Astronomy Pages
Edited by Patrick (10/07/09 11:40 AM)
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