Shooting straight up in EQ mode highlights gear lash problem

Daemon42

The Seestar unit is two components. The main unit which rotates on its base (azimuth) and the secondary unit with the scope itself which tilts up and down (altitude). Both altitude and azimuth have some gear lash, but altitude is more than 1/2 degrees (more than the diameter of the moon).
The secondary unit is not perfectly balanced fore/aft so when shooting at lower angles it tends to rest on one side of the gear teeth, especially when you add a dew shield which makes it a little more nose heavy.
But what I've found is that the secondary unit is almost perfectly balanced when it's pointed close to straight up, and this causes it to just flop around between the teeth. I kept finding star trails in some of my EQ shots, and random stacking failures, and when I'd stop stacking the subject wasn't even close to centered. Would be way at the top or way at the bottom of the screen. Remember, it doesn't stop tracking when you stop stacking. Can point it up at something bright enough to see live (like M15 star cluster) and then just gently touch the secondary unit on one side of the other and feel and see it flop back and forth.

Anyway, I'm testing a workaround. I've taped a couple small lead weights on top of the scope in the hopes that it'll unbalance it enough that it'll stay resting lightly against the teeth on one side instead of wandering between them. In hindsight occurs to me that I should put em on the bottom, as the way I have it, at some point as I lower the altitude angle, it'll find a new balance point.

Note also that since the default calibration points for EQ mode are straight up, this could/should affect them as well.
It most definitely could seriously throw everything off if there's even a tiny bit of wind. I'm shooting in zero wind conditions right now, and still saw it in the live view though.

diegorodriguez

YYes, that's an issue, but this imbalance would only result in a missed shot in the rare case of a flip-flop between the teeth. It could be modeled and factored in using software. A bigger concern is the appearance of color blotching due to insufficient tethering in EQ mode, that affects all shots. Luckily, looks like also a sofware fix is feasible here.

Daemon42

diegorodriguez You can't model randomness, which is what you get when it is free to drift between the gear teeth with no resistance. After I added the weight as described above, the random vertical (and diagonal) star trails are gone, and stacking percentage went up. Subject stays centered in frame in the Live view when not stacking but still tracking.

I had hoped that this might make 30s and 60s exposures more usable, but about 50% of the longer exposures now have short horizontal trails. The scope as a whole is not balanced side to side, so azimuth gears always have a load on em in EQ mode, thus the presence of trails suggests that it's not actually tracking continuously or has an issue with the gear train itself. Sky moves at .25 degrees/minute so if it actually failed to track entirely you'd have trails spanning up to half the frame. These trails are only like 5-20pixels long, but it's enough to make em unusable for stacking. Worse yet, sometimes it legs one of the really short trail ones into the Live stacking, which ruins it. It suggests that the tracking is actually stopping/starting for just a second or two somewhere in the middle of the exposure. Hard to say why they're so much less common in the 10s and 20s exposures. Example attached of one of the worst..
Note, before I added the weight on altitude axis, these would often be diagonal. Now they're only horizontal.

P1x26Yb4

I measured my S30 backlash last weekend. Main mount was 0.44 degrees. Arm was 0.97 degrees. I don't think there is a factory spec for the mfr process in that seestar support indicates "you get what you get" or "that looks about right". In EQ mode, the backlash varies from the starting position and after 180 degrees of rotation.

I alientaped a USdollar coin to the arm but no clear, unspoken or unwind days in the forecast.

It's pretty easy to measure using scenery mode focused on door or wall 30-50 ft away and measuring the vertical and horizontal movement, then applying a bit of simple trigonometry.

Daemon42

P1x26Yb4 Ya. It's no better on the S50. As long as you're only driving in one direction and there is some load on the gear train either from friction or imbalance it's generally ok, which is probably why they don't seem to care about actually eliminating the lash. Doesn't get bad until you have to reposition or reverse directions as we do in Mosaic mode, or when it's trying to live track the sun/moon. If they were smart, they'd never try to reverse directions at all, but would slow or stop and let the sky go by to go one direction, and speed up to catch up to go the other. Until EQ mode came along we never had to worry about the arm because we couldn't shoot straight up, but it appears to be too well balanced in this orientation to handle all this lash. In theory horizonal tracking should be better in Mosaic mode because it is so imbalanced on the AZ axis when tilted back on the EQ mount, but my 30-60s exposures suggests there's some other issue as well. .