From time to time I have a night where whatever I do I just cannot get my autoguiding to behave. I guess I am not alone in this. Normally when this happens I do a drift align and try again and it seems to make things a lot better so I put it down to poor polar alignment - normally I just do a carry/plonk (tripod leg poistions marked on the patio) but I guess this will only get me to 1 deg or so.
Now the thought occurs to me that if my alignment is off and I then ALIGN the controller will then try to compensate for the misaligment when tracking (ie the RA motor will run at not quite siderial rate and the DEC motor may run also).
Is this correct? Will a goto GE controller try to run the DEC motor to compensate for a poor polar alignment? If yes then clearly I am better off not doing an alignment and just allowing the RA motor to run at default (siderial) rate while letting the autoguider make DEC corrections?
Am I getting muddled up here? What is your method for AP - do you use the align feature on your controller or not, obviously ALIGN makes your GOTOs better but can it make autoguiding harder? What is your experiance?
You are getting muddled? Me too John.
At the expense of sounding blunt, forget the goto at this stage, and tell us what you mount tracks like, and guides like.
I would be possibly the slackest auto-guider round, really. I have a goto mount, and most times it is "on the chip" but of late this has been annoying me (after all it is a decent mount), so I have drift aligned some more, and also hit a modeling session with T Point in TheSky. My setup is carry out, and fit to a pier, so alignment is pretty close, but not as good as a fixture.
OK, what I am saying is that regardless of the inaccuracies of your alignment, your guiding shouldn't really mis-behave that much.
From your sig, is it safe to assume the Vixen/20D is the imaging scope/camera, the WO/QHY5 the guide-scope? If so then you should be having no problems.
What software etc? I still favour both axes guiding/correcting, I certainly do it this way.
Gary
What sort of changes do you do to your mount between imaging sessions John? Do you pull it down each time, do you swap imaging gear around, how goods your balance each time, what are you using to autoguide with? You know all the little stuff?
John what is guiding your Sphinx - a starbook (I don't know if this corrects tracking for polar mis-alignment) whereas the SkySensor2000-PC certainly does on a 3 star alignment if you put it into polar unaligned mode.
I suggest its doing more than just running DEC, its true King displacement - air's refractive index changes with elevation, its adjust both motors to compensate I believe - and it does it very well.
Gary, perhaps I was a little unclear in my original post.
I normally guide with the QHY5 on the W066 and image with the Canon20d on the VC200L. The setup rides on a Sphinx SXW. I guide with Guidemaster v 2.0.25 using the outputs on the QHY5 to drive the ST4 inputs on the Starbook.
This setup is built up and torn down each session but I do make sure I balance correctly...
I normally use exposures in the 2-5s range and RA guiding is always painless typically +/- 1" or so. DEC however can drive me nuts. Mostly I get what I expect - a slow unidirectional drift in DEC with corrections off, I set GM to apply corrections only in one direction (opposite the drift) and all is well. At other times I find that after I get a periodic signal on the DEC axis which can go from -ve to +ve. I have not been able to determin a pattern to this behaviour.
Now IF the starbook behaves the same way as the SkySensor2000 then an alignment of 3 or more stars will start to compensate for polar alignment errors and run the DEC motor. The Starbook does not have a polar unaligned mode so I do not have visibility of this behaviour or control over it.
I think this means that I should restrict myself to a single alignment point in the starbook and drift align the mount.
I am not certain that this IS the cause if my problem. It does seem to make sense that for a session where autoguiding is to be used the mount should not be allowed to perform DEC corrections, some mounts seem to have a special AP mode that (I assume) would do this, the starbook does not (at least to my knowledge).
I get the opposite. Dec is pretty good (<1 pixel), RA is erratic (2, sometimes 3 pixels). The funny thing about the dec is that there are both N and S corrections. I would have expected only one.
Geoff
Just a suggestion : maybe the problem is how the autoguider software interprets the drift or maybe there is something iffy in your wiring to the Starbook. (Toggled correctly ??)
Just a suggestion : maybe the problem is how the autoguider software interprets the drift or maybe there is something iffy in your wiring to the Starbook. (Toggled correctly ??)
What version of starbook software have you got ?
Perhaps GM can send incorrect signals sometimes - that would be difficult to prevent. The wiring seems fine as the manual NSEW buttons in GM all operate as expected and normally guiding is fine.
Try this - set up about 10 or even 20 degrees off polar alignment - say due West - align on 3 stars (if you can) and see if tracking is great. If it is - the mount is compensating. If stars disappear in a few minutes - then it isn't compensating.
I don't think the Starbook has anywhere near the smarts of the older SkySensor2000 unfortunately.
Try this - set up about 10 or even 20 degrees off polar alignment - say due West - align on 3 stars (if you can) and see if tracking is great. If it is - the mount is compensating. If stars disappear in a few minutes - then it isn't compensating.
I don't think the Starbook has anywhere near the smarts of the older SkySensor2000 unfortunately.
The more balanced the axis are, the more likely they will attempt to correct both ways, this is not good if your mount has backlash. Slight out of balance will make corrections allways push or pull.
If Polar align is accurate, RA will still always drive during tracking/guiding but DEC will need far less correctng, and will also then tend to need correcting both ways. It only drives on guiding corrections, they can be iether way, hard to avoide.
Thanks all, now I just need the clouds to move over.
I have both PHD and GM but find GM more accurate as it is easier to see what is going on and tune the configuration.
My conclusion is that some mounts can put our DEC corrections when tracking - dependant on the number of align stars chosen and mode selected. It seems unlikely that this is a useful feature though except for visual use with a poorly aligned mount (eg starparty setup in daylight). For imaging such corrections are insufficient to avoid the use of guiding (you would get field rotation anyway) and when guiding they can make things worse. The Skysensor2k, Gemini and latest Meade autostar, all apparently can make DEC corrections while tracking, I will know if the starbook does so soon.
Notwithstanding all of that my DEC problems may have anouther cause altogether - most likely a combination of balance and alignment. I will do some more testing to see if I can get a more reliable setup and post my feedback here.
The clouds allowed me to do a quick test last night.
For this test I re-aligned my mount (not very well as you will see) but did not align in the controller so only the RA motor was running during tracking.
The chart is as you would expect. I seem to have had backlash issues in DEC as the corrections take a long time to become effective and then (mostly) cause overshoot, I assume this was due to a the balance of the scope in DEC not keeping the gears loaded. Not quite sure how to cure that however the excursions were not too bad and a 600s exposure at 1.2 arc" per pixel has reasonable looking stars....
...Next test...offset the mount and align in the controller....
John. On your graph, it looks like DEC is only correcting one way, the blue line below zero doesnt show any coresponding brown line corrections above zero. As soon as the blue line goes above zero, corrections occur to bring it back to zero, but overshoots to -2 odd, then the blue line drifts back to zero before corrections occur again. I would have expected + corrections also as soon as the blue line dipped below zero.
And, yes your right, DEC correcting is done during tracking based on the alignment map (well at least the G11 does, I guess most if not all mounts do that too). What I ment was, if the mount is "perfectly" aligned, then no dec corrections will occur during tracking, so when guiding, only guiding corrections will occur.
Quite right Fred - I only allow DEC correction in one direction to avoid backlash issues (does not seem to have worked this time!). My SXW has fairly large backlash on DEC if I correct at siderial type rates, eg 10s for a reversal. Correcting DEC cin both directions usually makes things worse. BTW is it normal practice to correct DEC slowly? I know RA corrections are kept below siderial to avoaid unloading the gears but what is best practice for DEC ? I use 0.5 for RA and 0.8 for DEC but really have no reason for the DEC selection?
well, I set RA to 0.5 but have never set the DEC guiding rate, I would guess 0.5 for DEC too. Whats more of a worry is the big leap to minus. This looks like backlash, have you adjusted the worm gear closer to the DEC gear?, is the DEC axis balanced biased a bit?, is there a backlash setting in the controller?. I dont think a lower "agressiveness" setting would help, since its guiding one way anyway. If DEC cant take guiding both ways, theres a serious backlash or other mechanical problem that you need to sort 1st. If that graph is scaled in arcsecs, thats pretty good anyway.
Follow up from the test last night. Clouds rolled in an termniated my session but the tracking was much improved.
I did a multiple alignment in the starbook and got +/- DEC behaviour gain. This is a strng indication that the starbook does indeed send DEC corrections during tracking to compensate for poor polar alignment.
My original thinking was to correct this I would have to re-set the starbook and run unaligned but before I did so I adjusted the backlash compensation in the starbook. (I have been advised not to use this in the past).
To my happy surprise the mount proceeded to deal happily with DEC corrections in both senses with no overshoot. I think I have my problem solved....
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