Questions for G11 users

JohnH · #1 03-08-2009, 07:54 AM

Hello,

I have a new G11G and am just learning my way around the mount - so far I like it very much - superb engineering and good capacity. Now I want to make sure I get the best I can out of this.

I have measured my PE and got the curve attached, it looked ok - not brilliant but comparable with other G11 traces I have seen and with my expectation.

The second attachment is a plot of the guide log from PHD now that is again ok but I am not sure what is causing the spikes.

I have tried the PEC feature in the Gemini but so far it has had no measureable impact on the guided results. I am also playing with PEMPro to try and get a good understanding of the mounts behaviour.

So a few questions :

Is my PE curve OK or is my mount in need of a tweak - in particular the RA worm meshing, I think it might be a little tight?

What is a good set of guide parameters for a G11 with PHD or Guidemaster or AA4?

My imaging scale is 1.4" /pixel (127mm F7.5 with Opticstar 145m) and I am guiding at 2.75" /pixel (WO66 SD with QHY5). I think this should be ok as the guide programs all do sub pixel centroids (some better than others).

How much balance offset do you use? Any tips or tricks to getting the best out of the PEC functions in the Gemini?

rsbfoto · #2 03-08-2009, 08:20 AM

Hi,

The PE curve looks quite normal and with +-7.5" PE you are in the normal range of this mounts.

How much unbalance ? Not more then 250g (~1/2 pound)

How do you know that you have too much imbalance ? Touch the motor and if you feel comfortable with the temperature of the motor then it is OK, if you think it is hot, then back off with the imbalance.

If you have the brass worm then I would recommend to swap RA and DEC cables, take off the DEC axis assembly and let the RA axis run in for about 2-3 hours in every direction. This laps the worm against the aluminum gear a bit and the RA tracking could become smoother. Be sure to check the temperature of the motor all the time. Brass is by far softer then the used Aluminum.

If you put the Gemini tracking speed into terrestrial then you will not get a Stalled message for the RA axis if you do not have a motor plugged on it. Remember you are using DEC controller for the RA axis. ¿ Why ? because then you do not need take care of the slewing limits

Your guiding curve, I assume the values are arcseconds, then what you see is maybe the seeing what you try to chase

If those values are pixels then you are guiding above your image scale. Have you dome some imaging ? Perhaps as the RA values are the same as DEC values you will get round stars, because the RA and DEC movement are equal and it avergaes out.

I always use as guide speed 0.8x, 500ms exposure time. Try using the default values of PHD until you get a feeling. Then you can start adjusting the values downwards.

Also open the graph in PHD and watch the behaviour of the RA/DEC axis movement. That should give you also a feeling of the behaviour of your mount.

JohnH · #3 03-08-2009, 09:30 AM

Thanks Rainer, good tip, my RA is now running in...and yes you are correct the PE curve avis is arc secs and the guided curve is pixes (1 pixel = 2.75") sorry to confuse.

I am not sure the spikes are seeing related - the reason I sat that is I would expect seeing related excursions to impact both axes equally over time - but there is more "noise" on the RA trace - and it seems to be periodic....

I will see if the run in has any impact on the noise - hope it does, if not I may be in for an Ovision worm I guess...but it a little early for me to make that call.

rsbfoto · #4 04-08-2009, 08:09 AM

Quote:

Originally Posted by JohnH

Thanks Rainer, good tip, my RA is now running in...and yes you are correct the PE curve avis is arc secs and the guided curve is pixes (1 pixel = 2.75") sorry to confuse.

I am not sure the spikes are seeing related - the reason I sat that is I would expect seeing related excursions to impact both axes equally over time - but there is more "noise" on the RA trace - and it seems to be periodic....

I will see if the run in has any impact on the noise - hope it does, if not I may be in for an Ovision worm I guess...but it a little early for me to make that call.

Good. Before you make a bigger investment in the OVISION wrom I would suggest to check the backlash in the RA axis and the alignment of the ball bearings located inside the 2 blocks.

You can easily check this by using a ruler and look if one of the blocks is twisted. You can see this by a possible gap between the block side and the ruler.

If you are technically handy then try to reduce as far as possible the RA backlash and let the worm run in. Use a good grease maybe with teflon ingredient.

I also have a more nervous RA axis compared to the dec axis but this os certainly due to the friction of the worm against the big gear.

JohnH · #5 04-08-2009, 10:25 AM

Thanks Rainer - the nearly full moon and high cloulds gave me a chance to do some further tuning and the result was a better tracking log (attached) and test images (600s - bright star is Altair) so I am now close to the limit I think of what can be done usefully at my suburban location - tracking to +/- 2" for 20 mins means I am limited by seeing and skyglow....

If you don't mind though I do still have a question on the use of PEC.

I have had a few tries at this now and used both the native Gemini train routine and PEMpro to generate and upload a curve. Both methods failed to improve the PE, in fact it made it worse so the trace attached is with PEC off. I assume that it would be best to get PEC working as that will leave the guider less to do. Is there any advice as to the best way to use the Gemini for this? I think my problems will be the lag between the correction and the error and therefore I will need to advance the PE curve to compensate? Not really sure about this though and when I tried to enter an advance (a -ve offset of 54 steps - my exposures were at 2s intervals) the Gemini screen corrupted and the performance was no better.

I will go and read up on the Yahoo group as well.

Thanks again for the advice!

Added my before and after PEMpro curves - very odd it seems as if PEC is having no effect?

rsbfoto · #6 04-08-2009, 11:51 AM

Hi,

I do not want to take the wind out of your sails but I have never used PEC as it gave me worse results then without it.

The PEC function seems to have a problem and it is that it introduces a drift in the RA tracking. So far nobody knows why ¿?

If you are guding with PHD try to goe wodn with the guider exposure time as well as the allowed value until it should start to guide. I am guiding with a setting of about 0.15 to 0.25 pixel movement. The G11 having a decent imbalance and the guide speed setting at 0.8x does react very good to short guide impulse commands.

Another thing is you should try to guess the seeing and compare it with the guiding result in order to judge if you are not chasing seeing ...

I do not know for how long you are working with your G11 but that 10 minute exposure is by far nuch better the the results I did achieve after having the mount for 6 months.

Keep playing with the guide settings and get a feeling for your mount over many nights. You will be astonished as how much the climate changes will make you doubt about the mount and sometimes you will be amazed of how good the results can be.

The climate conditions do play a far bigger role then we want to admit ...

This you can only find out ocer a longer period of observing your results. Even better if it is permanently miunted on a pier under a Dome or a Roll Off Roof.

Happy Seeing Hunting

JohnH · #7 05-08-2009, 10:30 AM

Thanks Rainer. I have had the mount a week so far but I have prior experiance with another GEM so I am not totally new to tuning performance. I got +/- 2" both sides of the meridian last night - the key does seem to be imbalance and my tracking wass not assisted by a poor polar alignmnet - I will nail that tonight if it stays clear.

PEC really seems to be an issue - I think I must be doing something wrong (see above PEMpro screens) so I have asked the question on the Gemini Yahoo newsgroup perhaps someone there can help.

Bassnut (Fred) · #8 05-08-2009, 12:03 PM

I struggled with Pempro PEC for a long time on my G11 and just gave up. Sometimes it seemed to work at bit and sometimes made it worse. I asked Pempro long ago why I often got correction curves the mirror image of what they should have been (making the PE twice as bad), but they couldnt help me.

The last 2 curves you show indicate some other problem IMO, the PE shouldnt vary that much, 30" peak to peak is far too high, I got 10" P/P worse case. For a start, youve got wild oscilation between samples.

JohnH · #9 05-08-2009, 12:34 PM

Thanks Fred....I think I may well have my PE curve inverted, I will try it the other way up! I think the noise is a centroiding issue in AA combined with a saturated star - I will try a redo of PEC with my imaging CCD - this curve was with my guidecam (a QHY5) - might also be impacted by a poor polar alignment....or perhaps my RA worm is up a bit to tight or has a high spot, I do see similar levels of RA chatter when guiding.

multiweb (Marc) · #10 05-08-2009, 09:32 PM

Hi John, been using my G11 with a QHY5 on an ED80 for about 2 years now. Your RA and DEC curves are similar to what I'm getting in guiding, typically +/- 1 to 2 arcsec. As Rayner mentioned I don't use PEC either. Never did. Poor polar alignment will affect your guiding as RA and DEC are somehow related. PHD does a real good job at compensating though but the better your PA the better your tracking. Also disable any anti-backlash features on the mount. Finally I use 0.3 as the guide rate and guide every second. All PHD settings to the default.

Bassnut (Fred) · #11 05-08-2009, 10:33 PM

That oscillation is more likely due to a loose worm, not a tight one (backlash), but leave the backlash comp off as Marc says, and just fiddle with the worm. A "lag" error is symptomatic of a binding worm, I assume you dont get this error?. Assuming the 4th curve is one run, and the green curve at the top is the start of the next one, your polar alignment is indeed way off.

rsbfoto · #12 06-08-2009, 02:22 AM

Hi Guys,

I started with a Losmandy G11 about 5 and a half years ago and have been doing a lot of testing until maybe 5 months ago when I decided too give up in trying to find more solutions in regard to mechanical issues of the Losmandy G11 mounts. At the moment I have 2 G11 and one GM-8 and I have found out that the resukts I get vary from day to day as well as from month to month and year to year

So I decided 3-4 months ago after tryinf to do a perfect Polar alignment of my mounts using all sort of Aids like the Polar align routine from Guidemaster, Drift alignment, Aligning using the Gemini GoTo values etc. etc. etc.

After I had a more or less good alignment I said to me next day let me get it better and guess what, absolutely impossible to get a good Polar Alignment and this went on and on until I started to realize that atmospherical variables do influence a lot in this matter.

Now I just left it at a more or less good alignment and do live with the little issues which come out in my imging.

The PEC activation from Gemini is quite simple and if you did train Gemini with a 1 second exposure then you should set the offset or delay advance at a little bit more then a second. On the other hand after fighting with PEC for some weeks and always getting the RA drift I just forgot about what the PEC funtion in Gemini is and now I just use the autoguider.

By the way. I am getting the best tracking results for sidereal objects using the " King rate "

Sorry for the long story but what I try to say is that having reached a certain point it is nearly useless to further try to get rid of what we see as problems and just start imaging. The result of the image you have shown if I would have got that 2 years ago I would have been totally satisfied.

Use the mount over a longer period and, as I already wrote in one of my messages before, compare the day by day results ...

Starting tomorrow i will be for about 2 weeks in the Observatory and if I feel like doing it I could try the PEC function again and post the results here

Here http://rainerehlert.com/ObsReal14/cons/OReal14-05.jpg you can see my 2 G11 and here the first one piece womblock for a G11 http://rainerehlert.com/astro/RS-Wormblock-00.jpg and here how it is built inside http://rainerehlert.com/astro/worm/PPWB01.jpg

As you see this is so far the only one piece wormblock with a thrust bearing. The OVISION and the Martin Farmer do not have the thrust bearing inside.

multiweb (Marc) · #13 06-08-2009, 07:49 AM

That looks great Rayner!

Would you have the manufacturing drawings for download for the worm blocks and the bearing specs anywhere?

JohnH · #14 06-08-2009, 11:02 AM

Thanks for all the responses!

I confess I did have the PE curve inverted - so last night I downloaded it and then inverted and uploaded again - result is PE is working now - a bit - my peak to peak error is down to about 10" from 16" so there is still more room for improvement. I think that will be due to the phase (time) lag between the error and its detection/correction.

Rainer - I really value your advice - thanks - and I will shut up and get on with imaging again soon but as the moon is about full so I did want to take advantage it to learn the Gemini system and make sure my G11 is performing as it should before the dark nights return...

Fred - re the spikes in RA can that really be a loose worm? I thought the unbalance in RA would compensate for backlash - do you think I should increase the unbalance based on the RA spikes? And you are correct I do not get the lagging message and I have backlash compensation off. I intend to do a measurement with tracking off and see if the spikes are seeing or mechanical in origin.

And yes polar align is giving me some issues I must say - I did a drift align last night and got it dialled in pretty well I thought (< 1" drift/min residual not sure if you would call that good or not but as my setup is not permanent I am hoping it is good enough) but then when I slewed to my test target, M8, I noticed far more rapid DEC drift I suppose my walking around the scope and bumping the finder etc when getting M8 centred could do this as it in not on a pier. I suppose I could re-adjust AZ for 0 drift when on target (which would loose my pointing accuracy but so what once the target is tracking).

I will try guiding with King rate selected also - I did wonder about that.

John.