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Old 27-03-2017, 12:35 AM
glend (Glen)
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Can i turn off DEC guiding?

Maybe this is a stupid question, I am wondering about my DEC guiding and if i need it at all? I hear occassional references to ' turning off DEC'. My CGX mount, on a pier, is pretty closely aligned to SCP, about 8" now, mostly RA. I use Metaguide and it offers me the option of RA only guiding. So can i turn off DEC, are there any consequences?
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Old 27-03-2017, 12:36 PM
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PRejto (Peter)
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I think that if you carefully drift align in the region of your target you might get away with no dec guiding. But, if you pick another target further away there will be drift in declination, but only in one direction. A better solution might be to only apply dec corrections in one direction if that option is available. Clearly corrections in an opposite direction would be due to random variations in seeing.

However, this is probably not entirely true given that all telescope systems flex to some degree. This is where software like T-Point can come to the rescue as long as the flex is predictable.

Another problem is that if you turn of declination corrections you probably cannot dither - at least automatically. Even guiding in one direction results in this problem.

Peter
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Old 27-03-2017, 12:51 PM
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Shiraz (Ray)
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yep, the biggest issue is dither. You can do RA dither only, but then noisy pixels will trace out discernable lines in the dither direction - our eyes seem to be very good at detection this form of correlated noise and it is almost impossible to process out.
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Old 27-03-2017, 01:07 PM
glend (Glen)
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Thanks Gents, I have been doing some further research on this and found some information from Frank F, the Metaguide author, which outlines how he sets up DEC guiding on his CGE mount. The CGE and CGX use near identical spring loaded worms, so I am going to try Franks recommendations.
He says that RA bearing noise can be transmitted to DEC as wiggles and those need to be corrected. Great bearings minimise this of course. Even with great PA, there maybe a slow up and down movement to DEC on a scale of a 50 second time scale.
Frank recommends setting up a small non-zero value for DEC Reverse in Meatguide Setup, use about 0.3" . The DEC error needs to reach that point in order for DEC corrections to reverse direction. Add abit (200ms) of DEC lash in the Setup page, which is added to whatever amount the mount uses. Offset the balance in N/S direction so the mount is not floating on the gear. Most of the time the mount will not be moving the DEC, but it will be gradually chasing the error in small steps and DEC errors tend to be slower anyway. "A lot of the DEC peaks we see in the guide logs are not actually movements but just unwinding backlash. So it is feasible to see chasing behaviour on the guide plot, and think that you have a DEC problem, when in fact, it is not actual motion."

I need to try this out.
Thanks

Last edited by glend; 27-03-2017 at 01:29 PM.
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