Hi John. I think you have a number of mechanical and operational problems that are behind your difficulties. As you probably know, your limiting condition now is the poor response of your Dec axis. I think you’re probably on the right track by questioning the balance of the OTA – it may be quite poor. Here is a particularly bad guiding sequence when you were pointing at Dec=36 degrees:
The red arrows below the x-axis show that the mount is really not responding at all to the Dec guide commands. This could be caused by a couple of things:
1. A major imbalance of the OTA in declination that makes it nearly impossible to move the scope in the required direction
2. An external impediment like a binding or pulling cable that has the same effect
There’s no way for us to know the difference, it’s something you will need to track down. Balancing a scope on a fork mount can be a challenging process. If you loosen both the azimuth and altitude clutches, you should be able to point the scope in any sky position without having the OTA then move on its own. If you can’t do that, you are going to have guiding problems.
In contrast, here’s a guiding sequence that occurred right after the first one – but at a Dec of -18 degrees:
This has a very different appearance. The mount is still slow to react to guide commands but mostly when the direction of the corrections has changed. It isn’t nearly as unresponsive as the first graph. This looks more like the typical Dec backlash found in these mounts. The Guiding Assistant will measure that for you – have you done that? The big difference between these two sessions is what makes me thing you’ve got a large imbalance problem or cables that are interfering with the guiding assembly.
The upper arrow in the first graph shows an event where the OTA spontaneously moved by about 7 arc-sec. This wasn’t caused by guiding or by the mount drive system, it was caused by something external to the mount. Judging from your image scale, I’m guessing you’re using one of the flimsy finder-scope arrangements as a guide scope. These are notorious for not providing the rigidity necessary for good guiding and there are many places that can become slightly loose and allow the guide scope/camera to shift by a small amount. Or, as mentioned earlier, you might have a cable tugging on the guide camera. The amount of movement is tiny – probably less than 10 microns or 20% of the thickness of a human hair. This is one of the many problems with these types of guide scope arrangements –a perfect storm of flimsy mechanics and very tiny pixels in the guide camera. So a 1-pixel movement of the guide camera on your setup translates into a 6 arc-sec glitch. Finding and eliminating these problems is going to take some work, not something that you can do while imaging or just by hoping that things get better. But you can probably explore the balance problems during the day, especially if the current balance is way off the mark.
Finally, the Guiding Assistant keeps warning you to get a better focus on your guide camera – and I think you should, it’s probably pretty bad. Your typical star sizes are over 4 px in half-flux-diameter but I think with your setup you should be well below that, probably well below 3 px. You need to use a measurement tool to reach good focus – a Bahtinov mask, the PHD2 Star Profile tool, or an app like SharpCap. You can’t just look at the star image on the display and get a critical focus. While you’re tracking all this down, you *must* stop fooling around with the guiding parameters – these aren’t guiding problems. So restore all those settings to their default values and try to work through the mechanical issues. On a positive note, you can quit worrying about polar alignment – yours was plenty good enough and isn’t the cause of any of these problems.
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Hi John. This was what provoked my comments about fiddling around with the guiding parameters:
These aren’t big deals but I try to discourage this sort of thing when the user is just getting going and should be concentrating on equipment shortcomings. If the gear is working correctly and the new-profile-wizard has been run, the default guiding parameters in PHD2 will produce generally good results. At that point, changes should be made only with a clear understanding of the why and what and even then, only carefully. When the changes are being made blindly and in desperation, they usually just make things worse.
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