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Scary guiding, but does it matter?


MattJenko
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Hi,

I have moved to guiding with an ASI120MM rather than a Meade DSI Pro and all is not well in the PHD Graph world. Below is a graph from one of my recent sessions. I haven't changed anything in my setup process, and the graphs with the DSI were much better:

Phd graph

Is this something others have faced when changing guide cams, or something with the ZWO? This result has now been replicated over a few nights, so I am sure I haven't coincidentally gotten much worse at setting up. I use a Mac, so the driver is the one built into PHD2. Could it be that the images downloaded form the ZWO are a little jumpy themselves?

The graph above did however result in the image below, which has slightly egg shaped stars, but an image I am really happy with, so should I even care? (I do, as I want to get better, but the question still retains a degree of validity).

b843a006e47ff4471e7e72ccb9814b70.1824x0_

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Your basic leveling, alignment and balance of the mount is most likely the culprit here. Remember to balance your mount slightly heavy to the right of your pole star when setting up.This ensures the gears meshing correctly.  Also are you pointed to your true North/South position ( depending on your location) and NOT magnetic? That's over 15 degrees difference in most cases. So should you care? Always strive for perfection and look up!

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i'd say that phd graph isn't bad - looking at the scales, you've got a Dec error of less than one arc-second and RA error of about 1.5", both of which are pretty small.  What were the scale units on the graph when you were using the Meade ?

And the resulting pic looks fantactic, well done.

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The scale was the same for the DSI and the errors were regularly under 1" for both RA and Dec. Thinking about this some more, the evenings have been quite windy lately (and this particular evening was very windy, especially at the start of the session), so it is very possible this has impacted me, which is not something I had previously considered. The ED80/ST80 are pretty rock solid on the mount, but wind is wind!

I didn't realise that this was seen as pretty good guiding results, so thanks for that : I guess I thought otherwise because I am comparing to some of the semi-pros on here with their 0.03" error, flat as a pancake graphs!

Thanks for feedback and positive comments.

Edited by MattJenko
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As long as the stars are sharp who cares what the graph looks like? After all, the graph is showing that the guiding is making corrections, as is it's job. If the graph was flat as a pancake then either:

  • You wouldn't need guiding as the mount is tracking perfectly
  • The settings are too "smooth". After all, you can tweak the aggressiveness and hysteresis settings to flatten the graph. That doesn't guarantee a good final image though.

I've had some horrible looking graphs with decent images and some dead smooth graphs with horrible stars. And the stars in your image look good...I'd be happy with that result.

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Hi

You should display the star profile so that you can check it has a nice shape and isn't 'cut off' at the top and also to see what the seeing is like.

The cluster of points that are off-target might be symptomatic of the wind but could also be flex. I would also move your display slider to the left so you get a less contrasty view. One reason for doing that is to make sure there isn't another star too close to your chosen guide star. A close star would also show up on the star profile pane image :).

Very nice pic though!

Louise

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  • 2 weeks later...

Hi Ole,

The guide camera settings are all entered, as are the correct focal length of the guide scope. I now think that all this guide graph concern is irrational, and in reality it is all fine, the graph is evidence that PHD is actually guiding and working well as my results are coming out good, which is all that I want the system to do! It does however indicate that I might well have trouble if I decided to increase my focal length with this setup though, but that is a way off for me anyway.

Thanks

Matt

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