Re: Frustrating guiding problem

Roland Christen

There are several things that you can check and tighten up on the Dec gearbox. First make sure that the spur gear on the end of the worm shaft is tight and this requires that the setscrew is tight on the flat part of the worm shaft. Remove the gearbox cover plate to get to the spur geartrain. If the setscrew is even small amount loose, it will definitely cause your problem.
Second, and quite important if the above did not solve the problem, is the end load bearings which hold the worm gear in place. There is a threaded washer at the end of the worm gear which can be tightened so that the worm is held more firmly in place. To do this requires that you remove the gearbox cover plate, then remove the two spur gears ( the large one and then the one which is attached to the end of the worm). Then with a spanner wrench, or two small pins (ends of small diameter drill bits work great for this), you can tighten this threaded washer. Check afterwards that the worm still turns easily by placing the spur gear back on and turning it with your fingers. It should turn smoothly with only small amount of lumpy feel.
Lastly, there are two setscrews on the inside of the gearbox which press against the two end load bearings. To access these, you will need to remove the entire gearbox from the mount and look for these setscrews. If they are loose, the bearings can move sideways, which will not only produce this delay effect during Dec guiding, but will also produce a very small but noticeable RA motion to the telescope. These two setscrews should be hand tightened, but be careful to not tighten so much that it impact the smoothness of the bearings. You can feel this by turning the gears by hand. You will notice a definite lumpy feel when these are too tight.
Once all three of the above are adjusted, your mount will guide very smoothly and accurately in Dec. If you have any problem understanding what I have written, then please contact George at AP, and he can talk you thru these adjustments ( I am in Hawaii on vacation right now, but I did forward this to him, so he is aware of your problem).
It may also help to put some fresh light lithium grease on the Dec worm and run it back and forth a few times at 1200x. Less friction in Dec means easier reversals.
Rolando ( on Kauai at the moment)

-----Original Message-----
From: Leonardo Priami
To: ap-gto <ap-gto@...>
Sent: Wed, Sep 4, 2013 3:04 pm
Subject: Re: [ap-gto] Re: Frustrating guiding problem

Hi Peter,
thanks for your help.
About your suggestion:
- I see (below the PHD screen) that the bigger correction are around 150/300 ms, but only when after a few attempt the mount still remain out.

- About the exposure time I set to 3 or 4 seconds. I agree. But still I see some jumps as you can see in the pictures I show.

- I can't use the OAG in the main scope because I have only 4 mm from the 3"Wynne corrector and the filter wheel flange.

- The FSQ is piggybacked but with very strong system. I would exclude flexure.

What I suspect is some "elasticity" somewhere in the gear train that bring the motor to rotate a bit and no rotation to the worm gear.
I think that may occur some stick-slip phenomena between the worm gear and the wheel. After have  accumulated sufficient force, than the movement will take place, realizing too much movement than requested and not when requested.

What do you think about?



Il 04/09/2013 17.13, topboxman2000 ha scritto:

Hi Leonardo,

Your Max Dec Duration of 600msec may be a little too high. Check the log if Dec correction ever hit 600msec. A-P mounts are too smooth and normally have very little to no backlash so Max Dec Duration should not have to be this high. If the seeing is not good and setting Max Dec Duration a little high could cause overshooting. Try set it to as low as 75msec.

Also the Min Motion might be a little too small. Small Min Motion can send too many corrections to the mount.

Because your mount is high end, I would set the guide exposure times to at least 3 seconds to balance out the seeing and improve guide star image (higher SNR).

Can you use OAG for your bigger scope? You can get differential flexure between guide and main scope. You said the guide scope is in parallel with main scope, do you mean side by side or piggyback?


--- In ap-gto@..., Leonardo Priami wrote:
> Sorry, here the link to see the screen shots:
> Thanks for help!
> Leonardo
> Il 04/09/2013 11.37, Leonardo Priami ha scritto:
> >
> > Hi,
> > sorry to being here only when I'm frustrated about something.
> >
> > Here about the guiding of my AP1200 CP2.
> > The handpad has the 4.17 v firmware and the last V2 driver.
> >
> > - I have a Newton-Wynne 300 mm f/3,8 (1140 mm of focal length) with an
> > Apogee U16M and in parallel a FSQ106 (530 mm) with a Apogee F9 (both
> > camera with 9 micron pixel).
> > - On the main telescope I have a FWHM star about 1,3/1,5 pixel (2,1/2,5
> > "), On the FSQ I have a FWHM about 1 pixel.
> > - The guiding is performed by a Starlight Lodestar using a MMoag
> > off-axis on the FSQ. (3,3/3,4 "/pixel).
> > With this parameters I see that I need to perform a guiding with a P-V
> > error less than 0,5 pixel (+/- 0,25 pixel). If I found oval stars when
> > the error goes to 0,5 pixel.
> > - The cable connects directly the Lodestar with the mount.
> > - The Lodestar doesn't work under MaximDL, neither with its driver,
> > neither with its Ascom driver (it perform a too short calibration even
> > if I set 40 sec on setting...)
> > -The guiding is performed with PHD
> > -The DEC balancing is perfect (may be necessary a little un-balancing?)
> > -Polar alignment is very good (I have a Dec Drift of about 1 pixel on
> > guider after 10/15 min)
> > -I tried more time to adjust the Dec backlash as suggested on the AP
> > site. The Dec warm gear shaft can be turn by hand from the gear side
> > (dismounting the first gear).
> > - Moving manually the gears I see a back-lash of about 1 tooth on the
> > first motor gear, moving with handpad I see about of less than 1 sec at
> > 0,5X to compensate a "1 tooth" backlash (so about 7 arcsec to set the V2
> > driver?)
> > -I tried with 0,5X and 0,25X without improvement
> > -I tried 2s, 3s, and 4 sec, the seeing is 2,3" FWHM, with 4 s I see
> > better results, but still I have some Dec jumps
> >
> > Principally I tried to set "Auto" on the Dec guide setting on PHD, but I
> > see a lot of jumps.
> > Selecting only one direction the performance is better, but I see some
> > jumps every 2-3 min. Also RA seems some times to be correlated.
> > I attached three shot that show the graphs. The red line is the Dec.
> > Some suggestion to solve?
> > Thanks a lot!
> >
> > Regards,
> >
> > Leonardo Priami
> >
> > [Non-text portions of this message have been removed]
> >
> >

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