PHD guiding - my graph is a sine wave :(

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Hi guys - Im pulling my hair out over this one. Im having lots of trouble getting my CGEM to guide properly with my new skywatcher Mak newt.

With guiding enabled the PHD graph is a nice sine wave - my images are useless :(

Ive balanced very slightly to the east, and have also checked and re-checked that there is no binding happening, but I simply cannot get a smooth graph.

The polar alignment seems to be ok - Ive done multiple iterations of the All star polar alignment routine.

Any ideas? Im using a finder guider, and have tried tweaking the settings for shorter focal lengths.

I seemed to get better results with my little 90mm refractor, so Im thinking that the skywatcher might be too heavy for the CGEM...

[PRejto]

What is your image scale for the guider and for your main OTA?

I take it that you are able to calibrate OK? How much PE do you have? Guiding a 90mm refractor is going to be a lot easier than a larger scope with I assume a long focal length. In fact you may need to guide through your imaging scope with an OAG to expect a decent result and to avoid flexure between your guide scope and OT.

Peter

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Calibration seems to work fine - no idea what the PE is but i suspect it will be nasty :)

last night I went back to using the 90mm refractor, and the results where the same - sine wave RA pattern when guiding, and poor results.

Im starting to think I may have damaged the mount..

btw, the skywatcher scope is around 1000 mm, so I thought a finder guider would work...

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I should add that I went though a process of adjusting the worm, because there was a quite a bit of play in both DEC and RA, and tried to find the balance between the mount binding (motor was making a struggling sound) and too much play.

[Shiraz]

maybe try loosening the RA worm a tad to ensure that it is not binding slightly and upsetting the guiding - a little bit of RA play should be OK.

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Here is a screenshot of my PHD guiding. I tried to find out my PEC, but according to PECPrep, its around 5, so Im not sure what's going on...

[White Rabbit]

A pe of five is good so it's not that. Can you post your phd settings? Without that we are shooting in the dark.
Good thing is that you dec guiding is excellent.
Looking at your graph, You may need to increase your aggressiveness and max RA move. Looks to me like move commands are being issued but not for long enough.

Thanks

[White Rabbit]

One other thing to try to ensure your polar alignment is good is to take an unguided sub. If you can't take a 60 sec sub with no movement then you need toe readdress your polar alignment. Start with 60 secs and see how long you can go fe fore stars start to trail.

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I redid the PE calculation since I had the fl wrong. It's around 30 ish, which seems typical for the cgem.

The longest I can go without a star trail is around 10-15 seconds with my mount - that's with multiple iterations of the polar alignment routine.

[White Rabbit]

PE of 30...ouch. I'd be looking at getting that down with pec training before I did anything, but I don't think that that is your issue.

Have you ever had good guiding with this mount? When I got my G11 my PE was around 25 and I couldn't guide the errors out. I needed the worm replaced.

Here is a few things to try.

Download the trial version of Pempro from ccdware and verify that your polar alignment is good. 15 secs subs before trailing suggests to me that your polar alignment is out by a lot. The PE of 30 could account for that but I don't think it would make your graph trail off like that. There is a way to use phd to drift align, at the very least I'd try that. http://njstargazer.org/PolarAlignment.asp


Run the pe training module in pem pro and see how much pe you can correct with training. It's an excellent piece of software.

Increase your aggression settings in phd.

Post your settings in phd

Cheers

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Thanks for the help! I will see if I can improve the Polar alignment - I suspect that there is a major issue with the RA in that Ive noticed that it is rather stiff and difficult to balance when the scope is on the left side.

Settings for PHD are all defaults, but I changed the Calibration step to 2200, and min. motion to 0.05.

I've tried changing the RA Aggressiveness, but it made no difference.

Ive occasionally gotten some reasonable results - I once got a 5 minute track with PHD.

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btw, just about everything I've read says that the average CGEM can't track for more than 15-30 seconds unguided, because of the problems with the gears.

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Is there any point using PEC if I'm going to be guiding? I thought they kind of fought against each other.

[Screwdriverone]

That PHD graph looks like there is slippage in the RA Tim.

Either the RA guide settings are too low or the scope is too heavy or not balanced so the mount seems to be struggling to "hold" the scope up when it is guiding and then slipping down after it is pushed back by PHD in RA.

Is the Meade 12" ACF too heavy for the mount? Do you have enough counterweights to balance the tube in RA?

I had to buy an extra 5kg counterweight to add to the two that came with my HEQ5Pro to balance the 8" newt tube and camera, guidescope etc, and even then, my mount struggles sometimes when pushing "up the hill" when the tube is east of the mount, no matter how aggressive I make PHD.

Of course, there could also be some issue with polar alignment, you mentioned the polar alignment "routine" but I think you meant the initial star alignment?

I would check that the mount is level, the Altitude is set correctly and that you are pointing fairly close to the SCP, then check your balance in RA and DEC with everything loaded and the clutches loosened (if your mount has these) and then tighten it up and perform the 1-3 star alignment routine if needed. I don't bother aligning anymore as I sync and drive mine through Stellarium Scope so I just fire it up, click on a star and hit GOTO.

I may be off track but I thought it might be worth going and checking the basics before looking deeper into other things?

Cheers

Chris

[White Rabbit]

I think Chris could be right about slippage but the thing that has me stumped is that your dec graph is excellent. I would think that if it were slippage or polar alignment dec guiding would be all over the place.
I guess if you double check everything to eliminate from the equation what isn't causing the problem you'll be left with the thing that is.

Definitely do some PEC training on your mount. The less guiding phd has to do the better. With a low pe and good polar alignment only minor corrections are needed from PHD.

Try changing you min motion to something ridiculous like one second. At the moment with 0.05 phd is only issuing a pulse command for 0.05 of a second for every correction phd issues, and if your guide exposure is about 2 secs your RA motors only receive a correction of 0.05 seconds every 2 secs. If you increase that 1.00 and see if if phd can move the RA line above the centre line then work backwards from 1.00 until the RA graph settles off.

Read this, http://www.rosecityastronomers.org/resources/pdf/GuideToGuiding.pdf
It explains what all the setting in phd do, it helped me a lot.

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Thanks guys - lots of stuff to try :). I also suspect that slippage could be a problem, especially when the scope is flipped over to the left and pointed north. I have a hypertune kit for the cgem arriving in a few days, so that should help.

[Rod771]

What is your total payload weight Tim? The CGEM max is 18kg , so you'd want to be way under that.

You'll have very sensitive balance after you hyper tune the mount. I've been following a CGEM DX users thread on another site.

Click here for thread (http://www.astronomyforum.net/telescope-mounts-forum/131925-cgem-dx-review-10.html) You may need to sign up to see the pictures

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I'm probably pushing the payload with the 7.5 inch MAK newt, but I've been having the same issues with my smaller refractor. Thanks for the link :)

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Just another question - I wonder if the issue is that the clutches are slipping? I didn't think it was a good idea to tighten the clutch all the way, but it might explain the behaviour...