Quote:
Originally Posted by ghsmith45
This is the sort of result that you'd expect, but my GM8 has about 3 times this sort of error. Is the PEMPro worth the money?. Also, is there any sort of problem with the autoguider and the PEC correction fighting with each other?
Geoff
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Hi Geoff,
Before you start autoguiding, try to eliminating as many possible sources of bad tracking. Balance, polar alignment, Periodic Error (PE).
Balance is a relatively easy one to gauge and resolve. Polar alignment is a critical one, which of course can be mitigated through good drift alignment techniques. If you find that your autoguided images have a central star that all other stars rotate around, you polar alignment is off - that is an extreme case however.
Try to reduce your raw PE through a combination of mechanical tuning and software (if possible). For instance, after many tests I found my DEC worm provided a slightly less rate of error than the RA, so I swapped them. The RA error was my main concern as with excellent polar alignment, your RA is the only axis that will move. After doing so, I noticed a reasonable improvement to tracking. Now I know not all mounts will allow you to do this, but if you can, its worth investigating to determine if there a plausible gains. Tweak backlash between by optimising the space between the teeth and worm if possible. Are you experience erratic vibrations (perhap high frequency)?
If your mount supports PEC, program it. Any form of PEC is usually better than none. Again, keep in mind what you are trying to do is improve your tracking accuracy. Record PEC on a single guiding run or use software such as PEMPro to average multiple worm runs (averages seeing and random errors). PEMPro is a great piece of software in my opinion, but others have had mixed results. Part of the reason could be experience. You may need to inverse the PEC curve PEMPro creates before programming your mount and there are a few other "gotchas".
Once you feel you've nailed all of the above to the best of your ability and equipment capability, commence autoguiding. Autoguiding has the ability to mask many of the above errors, hence the reason for this pragmatic approach. If you find your autoguider corrections are working over time, stop and think about why this is occuring. The less autoguiding corrections made, the better your images will be. I look at autoguiding as a fail safe or "patch" to hide all your mounts limitations. It can only do so much, hence prevention is better than a cure. As everyone says, sink all your money into a quality mount, optics and cameras can come later.