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Bad Guiding Graph


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Does anybody have any ideas how to improve my guiding? I am using an EQ6 with a Orion ST 80 with an Orion SSAG. I am using 2.5 second exposures on a moderately bright star.The settings are at default. If I forgot any information, let me know, I am posting pretty late...

Jacob

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Define 'bad' please? Dec looks like it is probably fine, you are pretty close to the centre line all the way through. You definitely have a periodic error in RA; I'd hazard a guess it is one of the smaller gears from the graph scale, but you do not have a problem with oscilaltion looking at the OSC index figure (the probability that the next correction will be in the same direction as the previous one). A mount with no PE would have a figure of 0.5, i.e. equally likely to have to correct in either direction, but a figure between 0.25 and 0.4 for a real mount is generally considered okay.

Do you have a close up of the centre of one of the subs as well so we can see the star shape?

Also you've covered up one of the most important bits of info, which is the RMS figure (in guider pixels), just below the OSC Index on the left of the graph.

Depending on the relative pixel scales between your guider and the imaging scope/camera, and the actual RMS error, you can have a pretty ugly looking guiding graph that produces little or no visible effect on the actual image. The opposite is also true of course!

I'm assuming you are using the Equnox and the 314 with no reducer? If so your imaging pixel scale is about 2.66 arc seconds per pixel. (in RA at the equator)

If the ST80 and SSAG are being used with no Barlow then your guiding pixel scale is about 3.43 arc seconds per pixel. (Ditto).

So a guider error of 0.76 pixels translates to an imager error of 1 pixel, i.e. 1 / (3.43 / 2.66). Bear in mind the RMS and OSC-index only apply to the RA error (fine since your Dec graph looks great).

If you had a guider RMS of 0.39 pixels (0.76 / 2) I'd say you would be getting great results since there would be no noticeable elongation of stars on the imager. Not that a RMS that is more than 0.39 is bad; it depends on your requirements and the effort/ability to tune your setup.

The best I've managed (on a similar setup) on a NEQ6 is:

Orion ST80 + QHY5: 2.64 arc seconds/pixel guider

SW ED80 + Canon 500D: 1.45 arc seconds/pixel imager

0.2 RMS, which is less than the 0.27 RMS I'd need for pixel perfect guiding.

OSC index was 0.54, which seems odd as that would imply the mount was not being successfully guided in RA (over time being guided more in one direction than the other) but the results matched the RMS, so who knows?

Tips:

- Your results will vary according to the Alt and Az of the target. I consistently get better results on higher targets nearer to the meridian. Targets lower in the east produce rougher guide graphs. Whether that is due to atmospherics affecting the guide stars or something mechanical I know not.

- Keep doing whatever you are doing with your balance on the declination axis as it looks to be working.

- Try changing the balance on the RA axis. The usual guidance is east side heavy, so that the mount is working to lift the weights (move them down the shaft) if they are on the east side of the mount, or working to lift the OTA (move the weights up the shaft if they are on the west side of the mount).

- Personally I find that so long as I have an bit of imbalance it is fine, and normally I leave the counterweights down the shaft a bit from the balance point regardless of which side they are on the mount.

Edited by IanL
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Some very useful info there Ian - thank you :) From what you say it would seem that my guiding is working very well (as long as it doesn't lose the star) and that is borne out bt the images. Gratifying to know :)

Edited by Gina
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Hi,

I am using an Atik 314L+ with my Equinox at 400mm. The image is significantly streaked. I can't post an sample anytime soon but its fair enough to say that in the dec axis, the stars are tight but in the RA, they are elognated quite a few pixels.

Jacob

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Ouch, yes, RA is horrible but it is also spectaularly systematic which means it should surrender to logical investigation. I don't use PHD other than on a mount which hardly needs guiding anyway. However, it is making a sudden and excessive lurch on a regular basis. This is too short to be a PE thing so must be a hit from the guider, I'd have thought. I'm sure the mount would do better without it, so maybe try reducing aggressiveness? What about altering the max movement? I think the guider is sending an over correction regularly. I don't know how to alter that in PHD since I guide in Astro Art on our in house rigs but I dare say someone will come along. At least that kind of trace is usually sortable. It's the ones without pattern which are the real devils.

Olly

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I can't see that PHD would be the cause of the problem per se, something is happening to require the guide corrections in the first place and given it is so regular it is almost certainly a gear/motor-related issue not flexure or random slippage in the rig.

Maybe take a look at PECPrep, which can analyse PHD guiding logs (best if you record a session using PHD to track a star but not issue guide commands). Software and instructions here:

http://eq-mod.sourceforge.net/pecprep/

You'd want to try and identify a periodic error that matches (or is a harmonic of) your error when guiding. That would tell you which gear it is (or not as the case may be).

Looking at the graph again, if it was a 2.5 second guiding period, your error seems to be in the range of 60 seconds or so (2.5 x 100 / 4 and a bit peaks = 60ish). Don't know if it is significant, but the EQ6 stepper gear has a period of 122.5 seconds, pretty much exactly double your error, so maybe something in the RA drive is going clonk every half revolution of the RA stepper - just a guess and you'd need to do a proper analysis to be sure.

Whatever it is, PHD may be taking a long time to catch up with it and overshooting at the end. You might well be able to address that by playing with the guiding parameters, but if it is such a big error you'd be best to find the underlying cause.

Edited by IanL
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Great post Ian. Could do with all that info consolidating into one post for a sticky I should think. If you fancy writing that post let me know and i'll get a bit of glue applied ;)

Cheers

Tim

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Great post Ian. Could do with all that info consolidating into one post for a sticky I should think. If you fancy writing that post let me know and i'll get a bit of glue applied ;)

Cheers

Tim

Tim, I'll try to put something together this week if I get a chance. Bit busy with another project which is nearly ready for prime time and want to get that done asap.

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Could it potentially be the balance? My balance was pretty good, for some reason I didn't bias it to the East side like I have read to do. I usually do this and even thought about doing this when balancing, but for whatever reason I didn't...

Jacob

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