Re: Can't get less than 1" total RMS

Mark Christensen


There are two variables you didn't consider: Your skies and the sheer weight (not to mention the moment of inertia) of your rig.

I used my G11 for several years with a 12" f/5 and it was hit or miss for imaging - sometimes I couldn't shoot longer than 120seconds, while other times I could go for 300sec. And of course it made a great sail in any breeze.

When I pulled in my horns and went back to my 8" f/5.5 everything was much more reliable, even if I stick a Barlow on it.

As to guide rate, 0.5X is the max I'd go. The only time I guide at 1X is when I am doing something like chasing a comet head.

Personally I shot 1 to 2 second guiding exposures. If I have done decent job of polar alignment and the skies are stable I get between 0.25 and 0.33". The reason why polar alignment is important is because the fewer corrections the mount makes in either axis the less chance there is that it will impact the other axis. But I don't do a perfect job of polar alignment since that way all corrections (aside from chasing seeing) will be in one direction on DEC. So backlash isn't a player there, and of course it isn't a player in RA either, if you bias the RA balance slightly eastward - that way the gear faces always stay in contact.

Stock G-11, high precision worms, old fashioned worm blocks carefully adjusted. And I don't try to get backlash out! That is overdone by most people and even the factories.

Mark Christensen

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