LX85 Declination Slewing Problem (diagnosed, but not solved)

I’ve been trying to figure out an issue where the Meade LX85 mount will slew way past, or in the case of a target below the celestial equator, way short of a target in declination. The RA axis on the other hand is just fine. To get around the issue, I’ve been performing a star alignment prior to polar alignment. After a star alignment alignment, everything works just fine, and goto’s are great. However, in astrophotography, there isn’t much need for a two or three star alignment. I think most prefer to just plop down the rig, do a polar alignment and slew to the target. Other mounts I’ve owned, including those from Meade operate just fine with this setup.

This past weekend I tore down the mount to change out the gluey grease with Super Lube and make adjustments. Since I’ve heard this from other owners and I’ve had the issue with two LX85 mounts myself, after reassembling I decided to try to figure out what was going on with the slewing in Dec and see if I could fix it. I found the issue, but unfortunately I can’t fix it. It looks to be in the firmware.

So what is the diagnosis?

First I thought the mount was remembering the last position for Dec when it was shut down, since I never park the scope. So I parked the scope before switching it off and found no change in slewing behavior.

Next I though maybe it was the driver. I noticed a similar behavior using just the hand controller which should rule out a driver issue, but I tried a different one anyway. No change again, as expected. The problem persisted.

Lastly, I started the scope, set the date and time and connected it to the PC. Then I imported the mount’s position into Astrophotography Tool and bingo! The Dec was reading -89:52:00 and Alt was -41:50:46

Initial coordinates read from the mount using APT

Here’s where that is in the sky at 8AM in February… 41 degrees below the horizon, which would be great if I was in southern Chile or Argentina, but I’m in Chicago.

Stellarium view of where the mount thinks it’s pointing at powerup.

So why is the Dec thinking I should be pointing to the Southern Hemisphere when I start the mount? Well, it doesn’t, at least not exactly. What I’m gathering is that there is a – (minus) somewhere in the firmware where there shouldn’t be. I initially thought it was a location issue, but it knows I’m in the northern hemisphere since the RA functions properly and only slews to northern hemisphere targets. What I found out through testing and importing what the mount was reading is that if I slewed to a target without a star alignment, the RA drive was dead-on, but dec was slewing to the spot equidistant to the opposite side of the celestial equator. So, if I’m slewing to a target +13, it will slew to -13. If I slew to a target that’s -10, it will slew to +10.

Then why does it work after a star alignment? I’ll start that answer with another question. What’s the first thing that pops up on the handbox screen when you start a star alignment? “Put the scope in the polar home position”. When you hit enter after that screen, the first thing the handbox does is reset the position properly to RA 00:00:00 and Dec 90:00:00. The second thing it does is magically slew right to the first star selected for the alignment. The beginning process for an alignment fixes the errant declination minus sign.

What I’ve been searching around for now is a way to just reset it without going through the star alignment process. The process can’t be stopped once it once it starts. I haven’t found a way to reset the home position to the correct position without initiating a star alignment. I have confirmed that a One Star alignment triggers the reset, and it only takes a minute, but it’s still something I’d like to see fixed. For now, I’ll just stick to the one star alignment, manually returning to “home” and performing a polar alignment.

Clear Skies, Bleary Eyes – KA

I do have some connection with Meade, so I’ll reach out, but I don’t expect something like this to be high on a priority list. Like many other companies (including my day job) they have been hit hard by the pandemic. When you can’t get inventory from China, you can’t sell it. When you have little to sell, you have to trim where you can and that usually throws development to the side. That’s not just a Meade problem, it’s currently an industry problem. Just take a look at OPT, Highpoint Scientific or Agena and how little of anything is in stock. Let’s take it easy on manufacturers and resellers. I’ve seen some nasty comments lately about shipping times and out of stock items that are unwarranted.

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