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Here comes a description of the hardware and settings i have used to successfully Autoguide with my ServoCat driven Starstructure:
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1. Hardware:
- Generic 50mm/180mm Guidescope, i ordered this one:
it's very light and stiff, comes with mounting options so i even could use one of the threads for the Telrad base to mount it.?
It is almost a crime in imaging to guide with 180mm focal length on 2725mm FL that i am using, but it is enough to provide corrections for and verify guiding down to 0.8" arc seconds total RMS.
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- 5 meter ST4 cable, if you can find the right length RJ12 6P6C in a local store it will be much cheaper, at least here 3 meters was in stock.
- ZWO ASI 120 MM Mini Guidecamera (comes with a 2 meter ST4 and USB cable). Touptek or a similar brand could have a cheaper alternative.
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The above hardware was selected because it was the lowest cost alternative that i could find which did not require any modification to my telescope.
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1. Software?
- PHD2
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After mounting the Guidescope, connecting the ST4 cable from the Guidecamera to the back of the ServoCAT AUX/AG port and connecting the USB cable from the Guidecamera to your PC you are ready to go from a hardware standpoint except aligning the guidescope with the main scope.
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In ServoCAT sky i use 6 arc seconds per second for the GUIDE 2 value (0.40 x Sidereal).
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Download PHD2 and the camera driver, start PHD2 and follow the instructions. When choosing mount select "On camera" and for the Mount guide speed enter 0.30 x Sidereal since the measured guidespeed at least in my case has been lower.
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The instructions will end with creating a dark library where it asks you to cover the scope, after that the setup is done!
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Click on the brain icon and choose Algorithms at the top of the window. Change the declination Algorithm to "Hysteresis" like is already selected on RA and make sure dec mode is on auto. In our case RA is AZ and DEC is ALT. Now click on Global and uncheck "use multiple stars" and uncheck Star mass detection.
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I then pointed the main scope at the moon with a 28mm eyepiece and the guidecamera looping exposures connected to SharpCap. I then adjusted the Guidescope untill the moon was centered in the guidescope as well. You could align the guidescope off center with regards to the main scope as long as the main scope is pointing within the guidescope field of view, and as long as the guidestar you choose when imaging is as close to that point as possible since the field will appear to rotate around the guidestar. PHD2 has a crosshair overlay that can be used.
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You can now focus the guidecamera on the moon by slightly moving it in or out before locking the lock screws. Personally i reached better focus later on by doing the same on stars (the guide assistant will tell you if you need to focus), be aware that the movements required to reach focus are very small.
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Make sure that the "Guide" button on the ServoCAT handpad is selected or PHD2 will not be able to send pulse commands.
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Slew the scope to the South slightly east of the meridian about 35 degrees above the horizon, PHD2 recommends that the calibration should be done close to the Celestial Equator. Begin looping 1s exposures, now slew the scope UP in Alt untill it moves to resolve backlash. Make Sure you are on the east side of the Meridian since Alt will change directions and move down after the meridian.
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At the bottom of the window hit Guide, now the calibration will start. PHD2 will move the mount East, Weast, North, and South in order to understand how the mount behaves. When the calibration is done click on tools at the top of the window then "review" calibration data.? Verify that between 8-12 steps where used on either axis during the calibration.
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If much more than 12 steps where used the guidespeed entered when creating the PHD2 profile is faster than the actual guidespeed, that will result in PHD2 not moving the mount enough, the reason being PHD2 sending short guide pulses assuming a fast guidespeed.
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If less than 8 steps where used that means the entered guidespeed when creating the PHD2 profile is slower than the actual guidespeed, causing PHD2 to send long guidepulses moving the mount to much = same distance covered but with fewer steps.
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If the guidespeed is incorrect PHD2 advises not to change that manually, but to create a new profile. In the new profile enter the measured guidespeed you got from the calibration.
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The scope will already have begun guiding after the calibration, and don't worry if the guiding is bad, since the scope is only 35 degrees above the horizon the atmosphere will cause the guidestar to bounce around causing the scope to chase the seeing if it is bad with the 1s exposures from the calibration.
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Stop guiding and slew to the object, loop 2s exposures and begin guiding and wait a minute or so, then click tools on the top of the window then choose "Guiding assistant". PHD2 will stop guiding and measure the mounts unguided tracking performance/behaviour. Uncheck "Measure declination backlash" in the lower left corner.
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PHD2 recommends waiting 2 minutes, click on "STOP" and follow the recommendations, you don't have to go and change the settings, you simply press on "apply" for the suggestions PHD2 comes up with, with the exception of improving guidescope focus which it may suggest, or polar alignment which is not relevant for us.
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If you after all this see very large guidestar deflections especially after corrections, the PHD2 documentation recommends that you should not wildly begin changing settings, you should however take a closer look at how the guidescope is mounted.
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Just a few microns of flexure is capable of throwing of the guidestar by 6 arc seconds, several star diameters, a human hair is 50 microns thick.
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I followed that recommendation and tightened the adjustments screws as best i could on the Guidescope, it did change the pointing of the guidescope slighlty but my main scope is still pointing in the lower right corner of the Guidescope field of view so i just select guidestars there instead. It helped a lot taming the deflections.
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Besides that seeing plays a huge roll, if the seeing is really bad guiding will be bad since the stars will move even if the scope is not moving.
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If the seeing is good, guiding should work very well, If the guiding looks bad but the exposures look good, don't worry about the guidegraph, if the guidegraph looks bad and the exposures are bad then it's worth taking a look at.
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I tested this 2 days ago for 1 hour on M106 and it stayed locked with M106 exactly in the center for 1 hour with 1-1.3 arc seconds total RMS guiding with round stars.
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I did not mention the following above to not complicate things but it is possible to connect the Meade Generic or ServoCAT? driver to PHD2 in order to provide PHD2 with pointing information so that it can use dec compensation. If you use "On Camera" like i have suggested above you will have to enter the DEC (ALT) manually before calibration and before guiding on a new object.
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Clear skies!
Gabriel
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