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Old 20-06-2014, 03:26 PM
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SamD (Sam)
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HEQ5/NEQ6 guiding RMS vs typical seeing

Been using Skywatcher EQ5/6 mounts for over a year now, and been guiding happily, getting RMS in PhD (RA+Dec combined) of about 0.5 to 1.0''. Images have mostly round stars when imaging with an ED80 at about 2''/px. I'm now chasing more resolution with a longer focal length RC8 at about 0.9''/px, and considering if any of the mount overhaul/belt mods around might help.

Does anyone have any experience in how easy it is to get tracking RMS on these SW mounts down to say less than 0.5'' in RA with typical mods, or in fact, any experience to what extent mount mods actually help in getting tighter stars and better resolution (under typical seeing) ?

In theory, a RMS tracking error of 1.0'' on a normal distribution equates to a FWHM of 2.355''. Even with this largish tracking error, it seems that a typical 3'' seeing disk FWHM will still be the dominant factor in overall star FWHM, so why bother ? I suppose with very good seeing of say 2.0'', it would make sense to try and lower the tracking RMS on the mount. So, theory seems to say it's only worth mount mods to benefit from occasionally good seeing. Does this make sense ?
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Old 20-06-2014, 07:01 PM
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Shiraz (Ray)
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Hi Sam. that makes perfect sense to me.

I use my EQ6 at 0.9 arcsec and its OK under typical good seeing here of 2.5 arcsec. Usually looking at 0.4-0.7 arcsecRMS, which adds to the star size, but is amenable to deconvolution.

However, it is no good in wind and the other night I got some seeing that was well below 2 arcsec - the mount wobbles in RA were then quite noticeable, but that was a once a year event.

I reckon that you should give it a try at 0.9arcsec - the EQ6 is better than often reported, or at least later model ones seem to be. Haven't tried tuning it up, apart from the normal careful adjustment of backlash and bearing preload.

Found with my earlier HEQ5 that a pier helped a lot with tracking precision - haven't been able to do that with the EQ6 yet. If you want to do some mods and don't have a pier, maybe try that first.

Last edited by Shiraz; 21-06-2014 at 09:26 AM.
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Old 21-06-2014, 08:47 AM
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Merlin66 (Ken)
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I use the NEQ6pro/ CdC/ PHD2 combo with a C11 (at f10) for spectroscopy.
PHD usually reports 0.2 pixel RMS guiding - this certainly maintains the target star on my 29 micron slit gap...
Using the lodestar guide camera (2 x 2, = 16micron pixel) this equates to RMS around 3.2 micron.....
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