EQ6 Dec backlash

[nightster]

I have a sloppy Dec axis and I believe it's causing issues. Not a lot but I can feel the movement in that axis. I will try yet again to tune the gap in the gears but I was thinking about another solution. I've read that other gear driven mounts use a spring to preload the worm and keep lash to a minimum. Has anyone tried simply using a light spring pressure on one side of the DEC axis to keep it from bouncing back and forth in its gear lash. We all know to unbalance RA east side heavy to keep the gear mess against the push side of the gear. This works well on my RA axis. But Dec, even if you unbalance either front or back heavy will teeter when your imaging near zenith, for example. My thought is to add either a bungie strap or maybe a spring to put just enough pressure on the Dec axis trying to keep the gear contacted.

Is this a crazy thought?

[ollypenrice]

Not so crazy that I haven't tried it myself, and I know of an Astro Physics mount user who has resorted to the same trick. Crazy minds must also think alike! An alternative to using elastic would be to contrive a pull system which raised a small weight. This could be organised to lift a weight centrally under the mount.

However, it is rather a desperate measure! Have you tried instructing the guider to correct only in one direction? (That will be the direction needed to correct for polar misalignment however slight. In fact going for a deliberate misalignment might help with this fix.) You can only find this out by trial and error so disable first one Dec guiding direction and then the other to see which one you don't need. Many people find that this cures oscillation across the backlash in Dec. It certainly solved my own problems - which the elastic didn't.

Olly

[nightster]

Here is the error I'm talking about.  I can start to see it in the 600sec subs and by 1200s the subs are unusable.  I've never imaged this long before and my first attempt at NB has shown this error in my rig.  It looks like I had the PHD2 Dec set at AUTO, (I take screenshots as I begin a sequence.)  FYI ADM Side by side mounted refractors, no adjustable guider rings.    I had tried guiding in one direction but the graph was erratic, I should probably disregard the graph and try again.

600s

1200s

PHDlog from the 1200s sub

[Ibbo!]

I am also a fan of the slight ( or inevitable)  misalignment and guiding in dec in only one dirrection but I have a mount that is known for it, strangely I never thought of doing the same with my eq6 :icon_scratch:

next time out

[ollypenrice]

Looking at the 1200 second sub the stars are significantly better in the centre, to my eye, than in the corners. This simply points to polar alignment and agrees with the way that it gets worse with longer subs. (If it were a conventional guiding or backlash issue this wouldn't be likely to happen since one rotation of the wheel - in 8 minutes or so - should be much the same as another.)

When polar alignment is out and the guidescope is perfectly on-axis polar misalignment shows very much as an elongation of stars in the corners. When the guidescope is not perfectly on axis the elongation may be visible across the whole image.

One simple check to verify whether or not it really is Dec backlash is to determine the direction of the trailing relative to RA and Dec. Can you identify the orientation of the camera in this image? If the Dec axis is not orientated along the line of the trailing then it isn't the culprit.

Since the graph also looks OK my money is on PA. If you are using a software based polar alignment routine I'd drop it and try the classical drift method. This is the best explanation I've ever found. You don't need to do it with an EP, you can use the camera and crosshairs on the capture screen, which are far more accurate as well as easier. To set the camera orthogonal to RA and Dec before beginning just slew slowly on one axis during a multi second exposure. The trails show the present camera angle. http://www.andysshotglass.com/DriftAlignment.html

Olly

[nightster]

This is much better news than a mechanical issue. I do PA with the polar scope following the EQMOD guide. I've tried those software solutions and never really trusted them. Up till now this was fine for 5-6 minute subs. I will have to learn how to drift align and spend the time during setup to get good PA.

I use a Side by Side mount, and the weight of my Imager is much heavier than the guider. To get balance I have my Guider farther east or right of the mounts saddle. Should this be centered? Not sure if should of set it up that way now.