Yes, this is very long, but that’s how important the mount is.
I didn’t choose the LXD-75, it was just something I already had from my original attempt at Astro-imaging. This mount was Meade’s second attempt at a mid-level GOTO German Equatorial mount – the first being the lighter weight LXD-55. For 2005 it was a step forward in computer controlled mounts. While the tolerances were poor, and the adjustment out of the factory essentially required a rebuild, one could get some 60-second exposures with a really good polar alignment. Software based guiding correction really didn’t exist in 2005, so your only option for truly long exposure captures was a mount from manufacturers like Software Bisque, starting at around $10,000 back then if I remember correctly. They are amazing mounts, but also way out of reach for the beginner.
In 2005, I didn’t know how to adjust or use this mount. I lacked a real understanding of polar alignment and had no idea how important it was. Because of the complete ignorance, and the shoddily assembled mount, my maximum exposure time was about 10 seconds.
In 2017, Guiding solutions and my increased patience after having three daughters have made this mount useable for astrophotography. With a proper polar alignment and the use of PHD2 for guiding, I could obtain useable 300 second (5 minute) exposures about 80% of the time. That means that 8 out of 10 exposures are useable to stack, which is very good considering this mount had stayed in a bin untouched and out of adjustment for about a decade.
Some people are obsessed with the math behind the exact periodic error numbers, etc, and point to them as proof that this mount cannot be used for imaging. To an extent, and not a huge extent, the doesn’t matter if you are very well polar aligned and using an auto-guiding solution. Through software aided polar alignment and the tuning I’ve done to the mount, I can usually get just as flat of a guide line that people with “legitimate” AP mounts.
Just recently, I put the time in to see what this mount was actually capable of. Over the span of about 8 hours on a Sunday, I toiled with gears and tolerances and poor assembly.
- Polar Scope Alignment
- I replaced the setscrews for this adjustment with thumbscrews to speed up this process. There is no magic trick here, and the idea is to get the cross marking the celestial pole to remain in the center without moving off the center while rotating the RA gear. See here (create hyperlink) on this process.
- That said, I don’t pay too much attention to this. I use the polar scope as a rough estimate to get within a couple degrees, and then I use PHD Drift Alignment to get the mount very accurately aligned.
- Clean and Regrease the Worm Gears and Worm Wheels for the Right Ascension and Declination motors
- Doing this requires a complete breakdown of the mount and you need a certain level of confidence to take this on. Any mistakes could be costly. I’ve seen posts time and again where otherwise mechanically inclined people made a seemingly small mistake only to realize they destroyed their mount to the point where buying new was less expensive. However, with an older mount, this maintenance is necessary to get the most out of it. Newer mounts have much tighter tolerances and better lubricants that make all of this much less critical and I would advise against it.
- I used a low-temperature bicycle grease on the gears. It will not become more viscus at cold temperatures, and being partially wax-based is less prone to attracting dirt and grit. I already had this from my cycling passion.
Adjusting the RA and Dec worm gears is it’s own challenge!
- Adjusting the gears to make the mount rock steady took over an hour. Getting it close is easy and takes a few minutes, but I was left with movement about equal to about the thickness of a hair in the Declination drive. This doesn’t sound like much, but when that few microns of movement at the drive is extended out to the ends of the OTA at magnification the backlash and guiding correction is unmanageable. When the temperature drops, any existing play is then magnified.
- The fine adjustment of the worm gears is a delicate balance. If you adjust it too tight, you’ll put too much strain on the motor and get faults – not to mention flexing and even bending the gears. Don’t go tight enough and you’ll never get rid of the movement. About 1/32 of a turn on the screws is the difference, and there are three screws for each worm gear that need to be adjusted in sync.
- In addition to the tension of the worm gear against the drive gear is the adjustment of the worm gear retention shaft itself. This is accomplished by first adjusting a barrel set screw and then the corresponding locking nut. Of course, when you tighten down the lock nut, the set screw has a reaction and you need to readjust. Getting rid of unnecessary movement, and inability to turn the gear and all here was also determined by about a 1/32 turn of the set screw prior to tightening the locknut.
Other Required Adjustments found after disassembly
- The declination drive cover/dovetail holder was found to be rubbing against the main body of the mount, causing strain on the motor. I added thin washers between the cover and the attachment point to create the necessary gap. This also further leveled the head.
- The declination motor gear was not fully meshed with the worm drive gear so a shim was added to the motor attachment point.
- The locknut for altitude rotation point required about 1/8 of a turn to really solidify the mount while still allowing for rotation. This one was hard to find as I was looking to the gears as the source of movement.
- After several fun geometry exercises, I found that the Parked position for both the RA and Dec were off… a lot. RA was off about 1 degree and DEC off about 3. That doesn’t sound like much, until you try to find something in the sky.
- The Dec motor would also be way off on GoTo commands, and about 10 degrees off when returning to the “Park/Home” position after slewing. I found one window on the encoder that was crusted over and cleaned that out, performed a reset, calibration and train for the motors and now Parking the scope returns it to less than 1 degree off. I can live with that.
All of these tiny adjustments are absolutely necessary for an older mount like this. The time spent allows for about 90% usable 10-minute guided exposures, vs. the previous 80% usable 5-minute exposure. Given the light pollution in my area, a 5-minute exposure is about all my camera can handle, but now pretty much all of those exposures are usable, and I can stretch it out a bit more on nights of really good seeing. On a newer mount manufactured in the last few years – such as the Celestron Advance VX or iOptrion iEQ30 you’ll get some pretty good performance without having to make these adjustments, as long as you’re very well polar aligned and auto-guiding. Disassembling the mount, cleaning, lubricating and adjusting really helped me to understand what was going on and see the real reasons behind the issues I was having trying to get consistent auto-guiding.