Weasner's 8" LX90-ACF Report #3

27 April 2008

8" LX90-ACF Second "First Light" Report

[25 April 2008]
Received the replacement telescope on Friday, 25 April 2008. Mounted the OTA and fork arms on the tripod before installing the accessories. I then inserted the 8 C-cell batteries into their boxes. I wanted to check for the drive noise before I proceeded with the setup. I did notice one thing that was different from the first LX90 I received. The new one did not have the battery box wiring connected to the plugs whereas the first one did have them connected. It makes me wonder if the first one was a previously returned one. When I had unpacked the first one I had noticed that one of the accessory boxes appeared to have been previously opened but I didn't think anything of that until I discovered the difference with the battery boxes. Hum...

Anyway, once I got the new telescope mounted and the batteries inserted I powered on the telescope, did a CALIBRATE MOTOR, and selected TARGETS-->ASTRONOMICAL to start the drive tracking. There was no sound similar to the problem sound on the previous LX90. The drives appeared to be running normally. At least I could not hear any obvious unusual sound from the mount.

Unlike what is typical when you get a new telescope and what occurred when I received the first LX90, this first night was clear and there was no wind! I took the telescope outside an hour before sunset to align the finderscopes and TRAIN DRIVES. After that I waited for it to get dark.

Once I could see the alignment stars I powered on the LX90 and did an Auto Align. The LNT and GPS modules did their thing and the telescope was slewed to the first alignment star (Sirius). It was about 15 degrees off. I centered it and the telescope slewed to the second star (Arcturus), which was also about 15 degrees off. I figured there was a time zone error. But alignment was successful so I did a GOTO Saturn and it was put in the 26mm eyepiece field-of-view (FOV). I switched to a 9.7mm eyepiece and watched for any sign of mount vibration. And it occurred. It was not as bad nor as frequent as on the first LX90, but every 10-15 seconds Saturn would vibrate in altitude about the diameter of the planet's disk size. And I could hear a distinct vibrating sound from the right fork arm when the vibrations were seen. The sound was less obvious than on the previous LX90 but it still sounded wrong.

I checked the Site; it was correct from the GPS and the Time Zone was correct. And the Daylight Saving setting was correct. I powered off the LX90, then powered it back on, and did an Easy Align (which doesn't use the LNT but does use the GPS). The first and second alignment stars were nearly centered in the finderscope FOV. I centered them in the 9.7mm eyepiece and did a GOTO Mars. It was placed near the center of the 9.7mm FOV. But as I viewed it there was some slight vibration. Not as bad as it had been on Saturn but still enough to be distracting. I then did a GOTO the star Capella, which was placed in the 9.7mm FOV. I watched Capella for several minutes. The vibrations were more noticeable. I timed them as occurring every 10-15 seconds and they were now occurring in both altitude and azimuth and I could hear separate unusual sounds from the base and the right fork arm when the vibration was occurring. I have never seen such vibrations on any of my ETX telescopes, nor did I ever see it on the LXD55 or LXD75 mounts that I had. And I don't recall ever seeing it in LX90s that I looked through at star parties. Visual observations are impacted by these vibrations and certainly astrophotography will be impossible. And since the vibrations occur at inconsistent intervals and in inconsistent amounts, correcting for Periodic Error would not help. Maybe my expectations are too high but I find it unacceptable that it occurred on two LX90 telescopes.

As to the initial pointing errors during Auto Align, I suspect that I need to do a CALIBRATE SENSOR on the LNT but since this telescope will likely be going back to Meade, I decided to not waste my time on it.

I will be contacting the dealer again. Stay tuned.

[27 April 2008]
More on LX90 vibrations

The weather was still clear on Saturday, 26 April, so I took the LX90 back outside after dark to do some more testing in Alt/Az based on some suggestions from Dick Seymour. Here's what I did and the results:

Did Easy Align. Following the successful alignment I stayed on the second alignment star, Arcturus with a 9.7mm eyepiece (206X). Arcturus was in the east at about an altitude of 30 degrees. I heard the odd noise only 3 times over several minutes but they were short with no noticeable visual vibrations. After about 5 minutes of excellent tracking there was one noticeable vibration in Altitude. But then none for several more minutes. I then slewed up to the zenith (a slew of about 60 degrees) and back down to Arcturus. I had to make a slight adjustment in azimuth to put the star back in the eyepiece FOV. There was one slight vibration within a few seconds of completing the slew to center the star and then none for several minutes. I then slewed northerly (to the left) in azimuth about 90 degrees and then back to Arcturus. I had to make a slight adjustment in altitude to put the star in the FOV. Vibrations in altitude started in about 5 seconds after completing the slew and repeated every 5-10 seconds.

I then did a GOTO Saturn, which was near the zenith. The AutoStar put Saturn dead center in the 9.7mm FOV. But immediately as the slewing stopped Saturn began vibrating up and down in the eyepiece (in altitude) every 2-5 seconds. The movement was more than the diameter of the Rings! Saturn was at an altitude of 70 degrees and about 90 degrees in azimuth westerly from Arcturus.

I then did a GOTO Sirius, which was also placed in the center of the FOV. About every minute I would hear the odd thumping sound from the right fork arm but I could not see any vibrations. Sirius was about 22 degrees in altitude and about 45 degrees west in azimuth from Saturn.

GOTO Capella. Dead center. Vibrations in altitude and azimuth began immediately after slewing stopped and repeated every 5-10 seconds. Capella would make large oval patterns (as large as the disk of Saturn appeared in the same eyepiece). Capella was at 32 degrees altitude and about 45 degrees north in azimuth from Sirius.

GOTO Polaris. Dead center. About every 30-60 seconds I could hear the thumping sound briefly from the right fork arm but could see no vibrations. About every two minutes there would be the sound from the azimuth drive and Polaris would vibrate a little bit left and right. Polaris was 31 degrees in altitude and about 30 degrees to the north in azimuth from Capella.

I then manually slewed to Merak in the Big Dipper, altitude 64 degrees. Within a few seconds the sound and vibrations began in altitude and they would occur every 2-3 seconds, pause for 5-10 seconds, and then repeat. (I then had one other oddity occur; the AutoStar stopped responding to slewing commands from the arrow keys. I would even try to change the slewing speed but the display would show the last setting for speed and not change to a new setting. None of the slew arrows worked. Then after many presses of the MODE and number keys, things went back to normal. I have never had this occur before on any of my AutoStar controlled telescopes.)

I then slewed manually back to Arcturus. It stayed rock steady for about two minutes. Then it began vibrating in altitude every 10-15 seconds. I gave up.

On Sunday during the day I did some tests indoors based on inputs from Dr. Clay Sherrod and Dick Seymour. Here's what I discovered:

The OTA is very rear heavy. With the OTA horizontal, when the altitude axis is unlocked the OTA will move to the vertical (front rises) within less than a second. Dr. Clay had wondered if I had a dew shield attached (which might have made the OTA front heavy). I had not but that reminded me to test my white LXD 8"SC Astrozap dew shield (which was not stolen in the Dec 07 theft) with the 8" LX90. It does attach fine; the three screws hold the dew shield in place even though the 8" LX90 OTA does not have the same groove on the OTA as did the 8" LXD OTA. But the white Astrozap looks odd on the purple LX90 OTA but it will still be functional.

Dr. Clay asked about movement when the OTA is locked in altitude. There is about 1/8" movement when the OTA corrector plate end is pushed up or down. This seems normal.

Dr. Clay then asked about the wiring in the fork arm. I removed the altitude locking knob and reattached it without the large disk so that I could watch the movement of the gear and worm. I could not see any wires so that seemed to be ruled out as a source of interference with the gear or worm movement. With the OTA pointed at the zenith and TARGET set to ASTRONOMICAL I could hear the "thumping" sound every 10-15 seconds but I could not see any movement in the mechanism. I could feel a definite vibration on the worm housing at the left end where the worm shaft is attached at the times when I could hear the thumping. There was no detectable vibration on the other end. I switched the AutoStar to Polar mode and the sound went away. When I set it back to Alt/Az and TARGETS to ASTRONOMICAL, the sound returned. Also, I did not notice too much grease on the gear and worm. (I did not open up the base to check on the azimuth gearing.)

Dr. Clay noted another possibility: that the right fork arm trunion plate could be loose. I checked as best I could and both tube adapters on the OTA and the attachments to the fork arms seem secure. I could not detect any play in them.

Dick asked about the RA/AZ and DEC/ALT percentages. Both were 10%. He suggested setting both to 1%, which I did but the vibrations continued as before. I reset them both back to 10%.

Dick then suggested pointing the telescope to "Capella", which had obvious thumping last night, and testing at slow slew speeds. I did a fake alignment indoors and did a GOTO Capella. Almost immediately the thumping started. I set the slew speed to "1" (slowest) and held down the right slew key for about a minute. The thumping continued during the slew but less frequently than occurred during tracking. I then set the slew speed to "2" and slewed to the right, with the same result. I then set the speed back to "1" and slewed left. Again the thumping was less frequent but still there. As soon as I would stop slewing in either direction at either speed the thumping would occur every 5-10 seconds, which was more frequent than it was during the slews.

Dick then suggested a GOTO Saturn and do some slewing tests there. This pointed the OTA near the zenith. I set the slew speed to "1" and slewed right, up, left, and down for about a minute each. There were no thumps when slewing right, up, or left. But when slewing down (altitude decreasing) there were definite thumps from the right fork arm.

Dick wondered if I could feel any vibrations at the corrector lens retaining ring. I could feel a slight vibration during the thumps.

Lastly, Dick asked me to track "Capella" and unplug the AutoStar. He thought that the motors should continue to run at their last commanded speed. Within two seconds of disconnecting the AutoStar all sound stopped. With my ear on the base I could hear a very faint drive sound from the azimuth drive. With my ear on the right fork arm I could hear nothing. I powered off the LX90, reattached the AutoStar, and powered on. I left the telescope pointed at "Capella". I set the AutoStar for astronomical targets and almost immediately the altitude drive thumping began occurring every 3-4 seconds. Yes, this was faster than what occurred earlier in my tests when pointing at "Capella".

Dr. Clay responded to the above tests. At first he suspected the AutoStar. But since I have two complete LX90 systems, with their own AutoStar, and both experienced the same problem, I doubt the AutoStar is at fault. Alternatively he suggested a binding worm gear train. This sounds exactly like the culprit, as I could feel the vibrations on the altitude worm gear housing. (Since I didn't open up the base so as to not invalidate any warranty, I don't know if the same culprit is affecting the azimuth gear.)

I did connect the AutoStar from my ETX-125 (version 4.3Eg, same as the LX90 AutoStars) and almost had a surprise! I pointed the OTA upwards about 75 degrees and then powered on the LX90, set the AutoStar to astronomical targets and let it run for a couple of minutes. No odd sounds from the drives! All I could hear was a normal tracking sound from both drives. No thumps! I then began to slew manually up and down and left and right at the fastest speed and in a few seconds after that slewing the thumps in the right fork arm started. I did not CALIBRATE MOTOR, TRAIN DRIVES, or make any changes in the ETX AutoStar except to set TARGETS. So the problem seems independent of the AutoStar handbox although maybe 4.3Eg could be at fault, but then I would suspect more LX90 owners would be reporting this problem.

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Copyright ©2008 Michael L. Weasner / mweasner@mac.com
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