Periodic error of the Meade LXD75 mount

[Jairo]

I have a periodic error of ~2 arcmin during in ~1 minute. The stated full cycle of the worm gear is about 12 min. I'm about to start using the PEC function, but I still have hopes to reduce it without computer help.

It appeared when I opened my LXD75 mount to reduce the backlash, adjusting the three screws on the worm gear box. I only turned them less tha 1/4th of turn and checked it wasn't too tight to turn the RA axis, so I hope I didn't damage the worm gear. In fact, I don't even know whether this have to do with the worm gear, as this periodic error has a much smaller cycle. Could it be in the interface between the motor box and the mount gears?

I followed others' instructions to correct the backlash and I was very successfull, but I still don't know what is happening inside the box. For example, why do I have to loosen the middle screw when I tighten the right and left ones?

Do you have any advice?

Thanks in advance.

[RickJ]

Not using that mount I can't offer specific help.

Backlash in the RA axis is not much of an issue for DSO imaging. I'd not worry about it. Instead, use a slight weight on one side to unbalance the scope so it is forced against one side of the worm at all times. Since any guiding correction is made by only slowing or speeding up the drive gear, never reversing it, backlash is eliminated entirely. For declination this is a different story. If your polar alignment is dead on then you might find you are reversing the direction and thus backlash becomes a major issue. This is solved by intentionally misaligning by a couple minutes of arc from the pole (often the case even without trying). This will cause the declination error to work in only one direction. Most guiding programs allow you to force declination corrections in only one direction. Thus it doesn't try to chase seeing the wrong way and reintroduce backlash already removed. I've been doing this since the 50's. Mounts back then were horrid for backlash. Far worse than the LX75 which, of more modern mounts has a bad reputation in this regard. I dealt with it as explained above quite successfully. When misaligned for dec backlash purposes be sure to guide using a star very near or even inside the field of the imaging scope. Otherwise rotation of field can become an issue if you carry the misalignment too far. Something that's easy to do.

Periodic error however is error that repeats predictably every worm gear rotation. So if the worm takes 12 minutes to turn then any error that is not 12 minutes in duration is NOT periodic error from the worm. Periodic error is usually caused by the worm not being perfectly centered and by its surface not being absolutely smooth and of the exact right shape at all times. These are manufacturing issues when it is machined and not something you can adjust out. PEC does a good job of reducing it but not eliminating it. Unfortunately LX200 and LX75 mounts are rather famous for also having other unpredictable errors that can't be fully eliminated. Some sell upgrade kits for various models of the LX200 mount that help reduce these but don't eliminate them. The LX75 is far worse in this regard and I know of no kits for it to help. Most DSO imagers avoid this mount as it isn't up to the task without lots of work. Most put the scope on a higher quality GEM mounting rather than fight the LX75 issues. At least all I know have done this. The rest of my comments are really aimed at the LX200 mount those users I know have tamed but with a lot of work. Some might apply to the LX75. Maybe a LX75 user can help further though they are rare for this mode from my experience.

For long exposure deep sky work you will need to guide with a guide camera using short exposures (1 second or so) to follow these random errors. This often has the problem of causing you to chase seeing which degrades the image as you are always pointing to where the star was rather than where it is. This can increase the diameter of a star by 1" FWHM or more some nights. But they will usually be rather round. The expensive solution is an AO unit (cheaper than a new mount however). Most I know that use the LX200 mount have finally resorted to using an AO unit. Others report having tamed their LX200 mounts but none I know personally have managed this feat without A0.

If the SCT on the mount has no mirror lock then another issue will be "mirror flop". The way the mirror is moved to focus the scope there's of necessity a bit of play in that sliding bearing to allow it to slide. As the scope tracks gravity is constantly changing where the mirror sleeve contacts the shaft it slides on. You see this as a shift in the objects position as you first focus in then out. Same happens, more slowly, while guiding a long exposure. Sometimes the motion is in jerks other times rather smooth. The former can be better as you only lose the frame it moved in rather than it moving in all. If you guide using an off axis guider or using a camera with an internal guide chip the guider will follow this motion, even the sudden variety, rather well and you'll get a good image but if you use a separate guide scope the guider won't know the mirror moved. This ruins the exposure. So unless you have a model that locks the mirror don't use a separate guide scope unless you are willing to throw out a lot of frames due to mirror flop. The very latest production models (some not yet released this is so new) have changed to an internal Craford style focuser to move the main mirror. This, they claim, fully eliminates mirror flop without need of a mirror lock. Their mirror locks sometimes weren't all that "locked" users report though mine appears to be but I had to take the whole back end apart and work with mine to get it to work well. As it came from the factory it didn't lock at all, in fact.

With the LX75 you may be limited to using many 30" to 60" exposures and throwing out the bad ones. If so be sure to use a very low noise camera. Some ATIK cameras have a read noise of about 4 e- which would be about as high of read noise as is acceptable for good imaging at such short exposures.

Rick

[Jairo]

I heard that the LXD75 worm gear structure, including the three screws for regulation, is a chinese copy of better models like Vixen. I suppose they work the same way, only with less precision.

I didn't know that the PEC would influence declination too. In my attempt to track Centaurus A with 1 min exposures during half an hour, there was a drift in every axis, but I got consecutive frames with good tracking, so I feel I could even go 2 min if it weren't for the periodic error in RA.

[RickJ]

Pec has no effect on declination. It only tries to correct machining errors in the worm gear.

Drift in declination means your polar alignment is off.

Pec will reduce -- not eliminate periodic error. Nor will it alter other RA errors that are random in the drive train. That mount requires either very short exposures or for best results fast guiding an AO unit provides. This removes drive issues as the optics of the AO unit provide nearly all the corrections except when the mount needs to be "bumped". Even then it will correct for any error in the bump caused by backlash or other issues. AO is a great way to get decent performance from an inexpensive mount like the LX75.

Rick