MX RA Backlash!

[PRejto]

I finally may have figured out what has been wrong with my MX since day one. It's quite simple actually, but has taken me forever to get to this conclusion, no doubt due to the fact that I started off with this gear knowing essentially nothing. It's been reasonable for others, and for me, to assume that I simply have done something wrong. It's not an invalid assupmtion because I have made plenty of mistakes. But, underlying all of this is the fact that the mount just has not performed as advertised. Typical issues have been that no matter how well I am polar aligned, no matter how good my model, Protack simply has not worked reliably. Sometimes an unguided image might work only to have the next attempt completely fail. I don't mean just a little non-round, I mean totally smeared. And I've had guiding issues galore. Not just in guiding, but in even getting a guide star to show up in the center of a guiding graph once it has been selected!

In the process of using PEMpro to measure my PE I noticed in a very clear way what seems to be happening. If I slew to a star by moving the mount in a Westward direction I will get a beautifully straight line demonstrating perfect tracking in RA. However, if I slew to a star by moving the mount Eastward, all hell breaks loose and I will get drift of up to 10 arcsec over the next worm cycle. After 1 cycle the mount stabilizes and tracks beautifully, but not necessarily where one initiated the tracking. It might be 5+ arcsec away. This is pretty clearly demonstrated in the attached graphs from PEMpro. (PEC on, Protact off, +15 dec on meridian)

Does anyone reading this forum know what adjustment(s) might be off in my MX to cause this behavior? I've asked the question at SB but so far there is no response from the powers that be. The only things I've done to my mount are to replace the belts and adjust the cam-stop. The belt is not loose, and I believe not too tight either.

Thanks,

Peter

[alocky]

Nice to finally discover the root cause, I'll bet.
I had a similar experience last night when I re-adjusted the backlash on the RA of my G11, and suddenly the guiding settled down and I finally started to see round stars in my subs. I've still got to get to the bottom of my guide camera drop-outs and some odd autofocus results though.
Fun, this astro-imaging, isn't it?
Cheers,
Andrew.

[cfranks]

Have you put any cables through the mount in addition to those from the manufacturer? I did and had some problems in one rotational direction. They seemed to be acting as a spring, winding up going, say, West and helping to drive the mount in the opposite direction. (I'm not sure which direction was which and the effect was small.) I didn't notice any guiding errors like you describe but I removed them anyway.

Charles

[PRejto]

Hi Charles,

That's an interesting thought. I've run several cables through my mount that I can check out further. In balance mode I cannot feel any resistance in RA, but who knows? I would be very sorry to lose through the mount cabling as it was a strong selling point and is a great feature!!

Thanks,

Peter

[PRejto]

You bet! A real barrel of laughs...

P

[frolinmod]

One of the big remote telescope hosting operators warns against running cables through the mount. He suggests it is not a good idea. Nothing but trouble in the long run. Hard won experience. Anecdotal evidence.

[Logieberra]

Through-the-mount cabling gave me some grief last year. My homemade dew heater cables were binding inside the mount, too taught and resulted in RA motor strain and odd noises...

Attached is a 15 minute test image of mine, unguided with Protrack and PEC ON. You'll get there...

[Mighty_oz]

Some do some don't , have u a recent pic of your setup Logieberra so i can see the cabling ?
Never been able to get even 10 mins unguided looking like that on mine. Would like to see how it looks.
Thanks Marcus.

[PRejto]

Sometimes I think I will NEVER get anywhere! Logan, are you saying that you do not run any cables through your mount? Or, did you just do a better job running cables?

I'm concerned that all the posts here are about cables. Couldn't it be something mechanical in the worm, or the way the worm is mounted? I don't know what the plungers do for example, and isn't there an adjusment that provides back/forth adjustment of the worm? Sure, the comments about cabling might be the cause and I certainly will investigate it as best I can.

Peter

[rally]

Peter,

If you physically apply varying pressures to your RA axis in both directions, Do you get physical movement showing up at the camera ?
ie are the stars moving in your images - could be done by looking at elliptical stars in a finished image or by blinking and seeing the movement between a sub with RA load and no load.
You would need to try both directions.

If you start back at the camera and work your way forward to just the OTA/Mount you may also discover where that movement may be coming from, including from within the mount if that is where it is.

You would expect to get some movement, the issue is if its actually backlash which would usually show up as movement with two fairly finite end stops or if its gradual based just on the strain which just gets larger as the load gets greater.
You would still get both of course if you have backlash.

If you are concerned about your cables you could check to make sure the cable loop around both axis is sufficiently large enough (as per the original loops) and that the cables are not bound up.
Adding too many extra cables could be a real issue.
Some silicon spray may help, but if its tight start by removing one cable.

I will assume that you have left all cables loose and not zip tied or interwound.

Debugging this needs to be done by a process of either introduction or elimination of all possible causes - otherwise you are just guessing.

Rally

[Logieberra]

Agreed, we might be going off track with cabling.

Peter, the plunger settings are critical, as is engagement of the worm via adjustment of the large bolts on either side of the worm. There are PDFs on the bisque site under Downloads.

As rally pointed out, I'd examine all possibilities.

Sorry for curt replies, I reply over mobile. Not easy.

[Logieberra]

After reading this

'if I rotate the scope to DEC +15 on the meridian, grab hold of the counter weight shaft and apply modest pressure I can clearly rock the RA axis enough to visually see the camera on the end of the scope move back and forth relative to the ground'

From the SB document, Replacing the Paramount MX Worm Block Assembly:

Centering the Worm Block
To ensure the worm teeth are as centered as can be on the gear teeth, ensure the 1⁄4-20’s are tightened while lifting up and engaging worm block. You will need two 3/16 allen wrenches.
1. Ensure 1⁄4-20 screws on rotate axis are loose.
2. Lift up on worm block, and engage worm teeth with one hand, and rotate the gear
back and forth with other to ensure teeth are sitting inside each other and not on
top of one another.
3. There are two ways to tighten the 1⁄4-20’s. One way is to have someone help you
by one person pulling up on worm block with both hands while the other person tightens 1⁄4-20’s simultaneously. The second way is to pull up with both hands, then while holding worm block in place with one hand (not letting go of both at once) and tighten the right side first which will keep worm block engaged. While holding that screw tight with the wrench, tighten the left side simultaneously.
4. Make sure the 1⁄4-20’s are nice and tight. They can then be tightened a little more while the worm isn’t engaged.

Worm 'centering' is via 3/16-inch bolts #3 and #4.

P.S. There is slop when in lock mode. That's normal and intended. I assume you're in tracking mode here when you see the RA slop?

[PRejto]

OK, I put the scope/mount to ca +15 DEC/meridian and applied some pressure back and forth to the counter weight shaft. In that position the counter weight shaft is horizontal. The RA axis moves and the camera swings quite a bit relative the ground. Pushing on the end of the scope in the declination direction produces close to zero motion. So, there is quite a big difference between the two axis.

I removed the cover of the RA axis. I would expect to see movement in that situation since the cover locks the worm down in tracking mode. I applied upward pressure to the worm gear housing which I think engages the worm and the ring gear. It takes a very large amount of pressure to diminish obvious movement of the worm gear assembly. I don't have any idea if this test is valid to determine anything.

I guess the RA movement might be indicative of something since it seems so much more than the dec axis, however there is also a lot more weight moment on the RA axis. Perhaps that is responsible for the movement. I'm not sure.

Peter

[PRejto]

Logan,

Thanks very much for this!!

I've read the entire Worm Block Assembly document. Just to be completely certain (as the document doesn't identify the plungers) are they the 2 vertical screws on either side of the cam stop pin? And the 1/4-20 are the 2 hex nuts with the two allen wrenchs shown in the photo you attached?

Peter

[Logieberra]

I'll make a video this arvo. I'm very interested to hear about your current plunger settings. Fingers crossed that's it!

[Logieberra]

Yes, the plungers are the 2 vertical (small grub) screws on either side of the cam stop pin, no's 5 and 6.

The two 1/4-20s are the hex/allen bolts that you tighten with two allen wrenchs on either side of the block, no's 3 and 4, for horizontal worm spacing.

[PRejto]

Hi Logan,

Thanks for the video of your cables. Looks very neat!

Re the plungers and 1/4-20s, does it matter which adjustment is made first?

Thanks!

Peter

[Logieberra]

From SB: 'please be sure to do the 1/4-20 adjustment before doing the cam adjustment'.

Not sure when plungers should be set, before or after?

[PRejto]

Hi Logan,

Thanks a lot for the video! I've now made the adjustments and I think the RA axis feels much better. Now, just a clear night to confirm all is well. The mount homes fine so I couldn't have messed it up too much, hopefully.

Re the plungers. They were not too far out. In fact they were 2.25 turns out. I'm not sure if that means the worm on the gear is shallow or a bit deep. I decided to leave them at 2.25 for the time being. The firmness came entirely from adjusting the 1/4-20s. I didn't have a helper but think it's right now.

I had previously changed my belts months and months ago from black to grey. And, now that I think of it, that might be the genesis of this issue for me. I followed Chris' Youtube video on removing the motor/worm, but comparing what he shows to the pdf on doing this maintenance I notice that he skips over the 1/4-20 adjustment in the process. That missing step may have tripped me up. I think it's odd that SB recommends the video - perhaps not realizing that step is missing. I think Chris has done a great job with his video, but perhaps I should point this out to them. Would you?

Thanks again for your help!
Peter

[cfranks]

Peter,

I may be incorrect but I think the plungers are just springs to hold the worm in the wheel. They don't adjust the depth of the worm in the wheel which is, I believe, your problem. The Bisque pdf, Adjusting the MX Cam stop, probably should have a reference to the plungers but assumes they are set correctly and I believe adjusting them will only give a different pressure holding the teeth engaged.
The Bisque pdf, Paramount Worm Block assembly, IMO, is the one that you need to follow as that adjusts the depth of the Worm in the Wheel.

Charles

[Logieberra]

I think Charles is onto something there. I might need to re read documentation re vertical mesh. One thing, if plungers are too far out - the mount slips under load - so it does have some bearing on vertical mesh, how much, I don't know. The MX worm is a lovely bit of engineering, IMHO. Ever tried gear meshing a G11? Bloody hell...

Main thing, your plungers are correct.

Re 1/4 20's, as I pull 'up' on the block, I rotate the large silver cog with my finger and check whether it's smooth, back and forth. Then I lock both sides down.

[gregbradley]

You've been very patient. If I were you I would be sending it back for a replacement or a refund. We get this gear to image with not to tinker with for the next year.

A certain amount of tolerance is often required but the better manufacturers get it right straight out of the box. The PMX has been a disaster for SB with lots of QC problems. My one is probably one of the better ones but I still had to replace the worm and fix the cam adjustment.

You would have been better off with the new AP1100 or an older AP1200. You never hear about issues with AP gear.

SB has to be on a buyer beware status for their mounts. Just on this one small astro community here there have been at least 6 or more problem mounts over the last year. They have shown they are not in control of their quality and seem to find it acceptable that the buyer debugs/replaces parts on their brand new mount. Not even mentioning the question mark over PEC in the southern hemisphere.

Although when they do work though they are good. So its a quandary we find ourselves in there. SB obviously changed something from when they made their PME mounts. Possibly outsourcing components from China?? But whoever assembles them is clearly incompetent.

Greg.

[PRejto]

Hi Greg,

I certainly can relate to what you are writing, and I have found SB's customer service quite spotty at times. But I have to say that in this instance I'm not going to blame them (entirely) for this issue. The more I think about it the more I'm starting to feel that I may have caused this issue when I changed over the belts 6+ months ago. For some reason Chris's video showing worm /motor replacement leaves out the 1/4-20 adjustment that is in the pdf file from SB. Perhaps it wasn't in the pdf when he made the video. But, I suspect now that this step was critical (for me) and has resulted in months of trouble. I'm sure someone with more experience would have noticed what the problem was sooner, but I cannot blame them for my inexperience.

However, SB recommends Chris' video but doesn't mention the missing step and I followed it rather than the pdf. I think that needs fixing and I intend to bring it to their attention. And, I think some sort of trouble shooting matrix as part of the manual would be incredibly helpful. I know of another former MX user that had the exact same slew issue I have (which I've yet to confirm I've fixed), and I have seen another one on the forum at SB a while back. I think what is most annoying about customer service at SB is that there is a feeling (that I detect and feel first hand) that the end user has made the mistake and that there is nothing wrong with their product. Eventually if one is really persistant they might fix something and/or send parts out. In fact, I think they are quite generous in that respect. But, on more than a few occasions I have simply been told that I'm wrong and only later have I been proved to have been correct from the getgo.

I still believe this is a great mount and that I will eventually get there. Right now I think I am very very close to having this all figured out. Fingers crossed!

Peter

[Logieberra]

"Not even mentioning the question mark over PEC in the southern hemisphere"

I'm hoping that the upcoming automated PEC wizard will help here, scheduled for release in the next Windows daily build installer. Hopefully the Mac build isn't too far behind. Fingers crossed.

[gregbradley]

issue when I changed over the belts 6+ months ago. For some reason Chris's video showing worm /motor replacement leaves out the 1/4-20 adjustment that is in the pdf file from SB.
Peter[/QUOTE]


I am not sure what the 1/4-20 adjustment is you are referring to. Is this the threaded rod cam tightening adjustment or are you referring to the tension on the belt?

An automated PEC wizard would be handy. Kudos to SB if they get that one. PemPro is promising something similar soon.

Greg.

[PRejto]

I am not sure what the 1/4-20 adjustment is you are referring to. Is this the threaded rod cam tightening adjustment or are you referring to the tension on the belt?

Greg.[/QUOTE]


Greg, Have a look at Logan's post #12 on the first page of this thread. It's the 2nd picture.

I'm just now trying to see if anything has changed but clouds keep getting in the way and my camera is acting up too (naturally).

Peter

[PRejto]

Back to square one. The adjustments that I made have not changed anything even though the RA axis feels much firmer. I verified that the plungers are correct, redid the 1/4-20 adjustments, and reset the camstop.

Results are attached.

I think I now have very limited options.

1. Pull out all the through the mount cabling.

and/or

2. swap the dec/ra worms

3. ??????????????????? (cry!)

Anybody out there willing to see if their MX mount shows a difference in tracking depending on what direction the slew was in just prior? You wouldn't necessarily need PEMpro to do this. Just collect PEC data with the camera X=RA with PEC on and Protrack off, and compare the amount of drift for 4 min.

[cfranks]

Peter,

Can you remember, when you loosened the 1/4-20 bolts, did the (not sure how to describe it) 'unit they were screwed into' (UTWSI) move? It's this movement you use to bottom the Worm gear teeth into the Worm Wheel teeth, and then tighten the bolts up. The reason I ask is that I am going through the same adjustments (just to make sure mine is adjusted properly) and on my RA assembly, the bolts were already loose but the UTWSI is unmovable! If yours is/was unmovable, you would have had virtually no effect on the teeth mesh and therefore your backlash. Do you want to give it a quick try? Loosen the 1/4-20 bolts and see if the unit can move back and forth.

Charles

[gregbradley]

Buy yourself a big box, put the mount in it, give it a bit of a kick as you put it in the box! and ship it back with a big note saying your problem not mine.

Greg.

[PRejto]

Just did it again to be sure, and short answer: Yes. The RA axis gets a lot tighter, but I'd still say not as tight as dec. But, is that just because the RA is carrying the scope and counter weight?

Anyway, I spent the morning removing the 3 cables I worked so hard to install. If it's clear I will test again but I will be very surprised if my results are any different. After that, one more test of swapping ra/dec worms...really a job I don't want to do at all.

Greg, I'm really close to taking your advice!

Peter

[Logieberra]

Peter, from your recent SB post, it sounds like you're heading to the US shortly. I'd pay for excess baggage, put the MX in its original shipping box (CW shaft removed, no counterweights, no accessories etc, just the bare mount) and ship it via domestic in country to SB in Colorado. Or at least rip out the worm blocks, fly with them and save yourself some international shipping!

Let them test in house, service and return the components to you. They'll look after you, of that I'm certain :)

[PRejto]

Small negative update. Pulled all my "through the mount cables" but no relief from the drift after an East slew. Next step is to replace the worm.

Also tried adjusting the plungers (to as tight as 1.5 turns) with no positive effect.

Peter

[gregbradley]

I think you've arrived at the "box and kick solution".

You've been given a dud and you are doing their job for them even though you paid them to do it. Its not fair.

SB were quick to replace worms so I think the worm must have been the biggest area where they had trouble in QC. Of course the worm is a sensitive part of the mount adjustment-wise but I found it easy enough and it corrected a problem my mount started to exhibit.

I can see why you are perservering as its a good mount when it all goes. But I think SB rushed it out the door in the middle of a US recession and probably desparate to get some money back in to offset the cost and investment and they did not get it right.
Hopefully the latest deliveries have all this corrected. I feel they have lost a lot of their reputation built up from the excellent PME. But for all I know they also may have had a slew of issues when first released - it was before my time.

Greg.

[Logieberra]

Peter, can you post a few of your images? Unguided. Prorack on. Protrack off etc. I'm interested to see the end result, Pempro graphs aside. Perhaps you've already uploaded them to another forum post? We're running similar sized gear on the MX (140/150mm glass) and you've seen what's possible, unguided with my very bulky TOA-150...

[PRejto]

Sorry, Logan, I cannot. Just don't have any! My PEC is probably not ideal any longer since I've been messing with the worm, and the model is not current.

I will say this; sometimes I got a half way decent result with Protrack on but it was never repeatable and I think this slew issue is why not. SAme with guiding. Sometimes I will select a guide star and bang it's right dead center in the graph where you want it from the first point. Then I might slew away to focus, come back, and for several minutes all attempts to guide are impossible. The star shows up way off centre, or off the graph, because the mount is not keeping up with the star as it moves relentlessly westward during the time TSX takes to expose, dark expose, and plot.

Last night I used the jog controls while collecting PEMpro data. I would blip the mount in 1 arcsec increments. If I moved the telescope East, the plot on the PEMpro graph would move higher in a positive direction, meaning (I hope!) that the position in the positive zone of the graph represents the star drifting to the West (as expected if backlash is present). All of my PEMpro graphs show drift to the West if I slew first to the East. And when I slew West I pretty much get a straight line for 1 worm cycle.

Peter

[PRejto]

Greg, You are right, but I don't quit. I figure I've got 12 more days to get further along this road. If I don't I will pull both worms and when I get to Tucson the first thing I will do is mail them to Golden Colorado. I'm not ready to spend $1000+ to ship the whole mount, though if they tell me to, or bring it along on my flight, I certainly will. SB is at least communicating with me and has said "we will get to the bottom of this."

Peter

[Peter Ward]

Without detailed knowledge on assembly procedures and test gear on how to set-up the belts etc, I'd recommend you return the mount.

[frolinmod]

The how-to documents are all there on the Bisque website. Unlike the ME, the MX apparently doesn't need the belts to be adjusted using an exotic sonic meter.

Nevertheless, I agree.

[Peter Ward]

How can I put this delicately :question:

While I have the service manual for my car, I prefer to get my mechanic, who was trained at Maranello to do the servicing... his skilled eye often picks up stuff I miss.

[PRejto]

I don't see how changing the worm gears can make the situation worse, but might just fix the problem. The documentation is straight forward enough and more than a few guys have done this in Australia without issue and have fixed PEC problems in doing so. If a different worm fixes the backlash problem that will be great. If it doesn't then I agree there really isn't much more I can do or try. The mount will go back to the factory!

Peter

[gregbradley]

You've got nothing to lose at this point. I changed my worm (single not both) over in less than an hour. It worked fine after I played with the tensioning rods and get them set correctly. They are 2.25 to 2.5 turns out from fully screwed in. You test afterwards and if too tight the mount will stall. If too loose you get the dreaded slipping crunching gears - ouch! It took about 20 minutes to get it right.

Greg.

[PRejto]

"New" worm is in. Now just waiting for dark with fingers crossed!

Peter

[PRejto]

I'm sorry to report that I have identical results following the RA worm change. I find this almost impossible to believe because the two worms are really different. The "new" RA worm has much greater uncorreced PE than the original (7.1 arcsec vs 2.3). Nontheless, I ran PEMpro and quickly was able to reduce the "new" worm to +.9 -1.5 arcsec. Not a great result but I just wanted to smooth out the unguided tracking so that I could distinguish it from drift.

I tried numerous times on both sides of the meridian, and with east heavy balance. I always get the exact same result. Perfect tracking after a west slew, and at least 5 arcsec or more of drift after an easterly slew. When I balanced the scope east heavy the results were by far the worst of the night with 8-10 arcsec drift.

At this pojnt I am perplexed. Perhaps this is "normal?" I suppose no worm can be perfect in backlash, but is 5 arcsec too much for an MX? I've posted the question at SB. It takes about 1 min all up for the drift to stop after a slew (so no longer a full worm cycle, but this improved also with my original worm after I adjusted the 1/4-20s). I can no longer feel or see any free motion in RA compared to what I found earlier in the thread and led to adjusting the 1/4-20s, so things are better.

If I chase this any further it seems I have few options. 1. I can assume that I still have a worm problem as both worms came from an early batch and this new worm is actually just out of spec. I can ask SB to swap out this worm. 2. My methodology is crook and PEMpro is giving a false sense about what is actually happening. I don't know why this would be, but I've posted the question to Ray Garalak. 3. TSX is causing this? Can't see how or why with Protrack off, pointing corrections off, but I suppose I could try running the mount without TSX at all, slew with the hand controller and collect logs using CCDSoft. (I don't want to do this experiment.)

Just a few days left before my trip. I hope SB responds with something to try, or an offer of a worm exchange.

Peter

[PRejto]

A friend loaned me yet another worm which I tested last night. No change. So, that is 3 different RA worms and I get identical drift after a slew to east, but no drift after a slew to west.

This time I took photos. 1 min exposures after east or west slew at .63 arcsec. PEC on, Protrack off. If I slew east, wait 1 min, then I get good tracking.

Some have suggested that I'm seeing telescope sag or camera sag, but I fail to see how that is possible. The geometry of imaging on the meridian at 0 dec would mean gravity would be acting on the y axis. All the motion I see is in RA.

Software Bisque has not responded to any of my posts since sometime last week. That alone is very frustrating!

Peter

[cfranks]

Peter,

Having tried 3 different worms would suggest (to me at least) that your drift is not worm related. Do you still have that 'slop' in the RA axis?.

May I ask the MX fraternity for a consensus on what direction you lift the RA Worm Assembly before tightening the 1/4-20 bolts. When I held/lifted it towards the worm gear I got the 'slop'. Holding/lifting it parallel to the Cam Stop pin fixed the 'slop'. My thoughts are that the latter allows the worm to self-centre in bottom of the worm wheel teeth and the Spring Plungers then hold it there with a specific pressure. I may have missed it, of course, but I couldn't find an unambiguous reference to the direction.

Charles

[PRejto]

Hi Charles,

No, no slop at all in RA. Sorry, I just can't quite follow your other question about "lifting up." I think it's pretty clear that if you press "down" you disengage the worm, so I've just lifted up in the opposite direction. No slop at all so I think it must be correct.

Yes, I agree, it cannot be the worm if 3 worms give the identical results. I now have a theory that perhaps the motor is taking longer than it should to reverse direction, a bit like a car trying to accelerate in too high a gear. I thought I might test this by slewing west and then turning RA off. Then turn RA on and see if there is drift similar to reversing direction after an east slew where the motor must momentarily stop to change direction. Perhaps the momentum of the east slew is contributing to the lag. Who knows, but I can at least try this test and see. I think one fact from a previous test lends some evidence that this might be correct. When I loaded the mount heavy on the east side (pointing west) the drift error nearly doubled. A lagging motor might do that. I should try loading it heavy to the west to see if it accelerates faster and reduces drift.

Peter

[cfranks]

Hi Peter,

One other thought passed through; assuming you use TSX, then by using its Closed Loop Slew to go to a target, you get very accurate pointing and could have used enough time to avoid the drift problem. CLS uses 2 slews, each followed by an image + download + imagelink. Sorry if I've talked a lot of nonsense but I've just about finished! :thumbsup: :)

Charles

[gregbradley]

What scope and camera are you using on the mount? What focuser does it have?

Greg.

[PRejto]

No, please don't apologise! It's a fine idea. Right now though I want to find out the root of the problem. I'm sure there are work arounds but I don't see others needing to do that and I didn't buy a mount of this quality to have these issues. Pretty soon though I''m going to give up!

Tonight I tried a few things. I stopped the motor, then started it up and took an immediate photo. There was no trailing. Then I tried changing settings in the Bisque TCS. I found that if I reduced the acceleration/decelleration setting to 300 (a very low values with very slow slew speed) there was a clear improvement in drift. Usually I see 5+ arcsec in the first min. At this setting the drift averaged about 2-3 arcsec. I don't know what that might mean but I'm sure it is a clue that to someone at SB might ring a bell. I also disconnected TSX and ran the mount only from the hand control. I get round stars if I slew west, and trails if I slew east.

Peter

[PRejto]

Hi Greg,

It's a Feathertouch on a TEC140 + Moravian G2-8300. But, before you start telling me about flexure the geometry isn't correct for this be be flexure. Observing at 0 dec on the meridian puts the force of gravity on the Y axis (dec) and there is o movement I can see there. All the trailing is in RA both in the photographs and in the PEMpro graphs.

A friend has repeated my tests with his MX/refractor combination and has had similar results! He gets good tracking after west slews, and trailing following east slews! I expect he will be posting his results in my thread at SB very soon.

Also, I have determined that the trailing is sensitive to the acceleration/deceleration parameters in the TCS. Putting the value to minimum decreases the problem to zero if slews are confined to 2 degrees or less. Anything on the order of 5 degrees slew produces the drift after an east slew. Interestingly I have been told that Astrophysics mounts compensate for backlash by making a second small slew that is adjustable. I can find no such setting in TSX.

Meanwhile, SB has not responded to any of my posts on their forum for more than a week! I'm starting to think they don't like what I'm showing and maybe they don't know what to do about it. It's not just in my mount. I'm fairly positive about this.

Peter

[gregbradley]

Just curious, I am not implying its flexure but its not mentioned in your posts what you exact setup is so it leaves it open. Do you have a photo of your setup.

Just so you know Yuri did test the FT3545 focuser and found minor flex under certain loads. It has less flex when it is upside down (focusing knobs up). Not that it flexes much. I have not seen focuser flex on my TEC180 and Proline, MMOAG and filter wheel.

What autoguiding system do you use? A guide scope or the ONAG?
Do you use all screwed adapters or eyepiece holders and extension tubes?

I have no experience with an ONAG. Is it reliable? Have you used it on other scopes/mounts?

I take it you've double checked your connections and everything is seated properly, the pick off prism is firm and does not wiggle etc etc.

Greg.

[SpaceNoob]

I have sometimes had issues guiding after a slew, usually assumed operator error. Once the weather improves I will see if this is present with my PMX.

Overall I have found my mount to be very stable, any failed subs have usually been caused by the guider getting a mind of its own, sometimes correcting backwards, yes calibration was correct and the guider functioned fine for a few worm cycles but it would suddenly send reverse corrections resulting in a drift on one axis. When this happens I just turn the damn thing off and get much better subs without the guider. I'm not saying you have a guider issue at all, just that I have found SB software to really bug out and randomly malfunction, not just the guider stuff. It wouldn't surprise me if there is underlying issues with what is driving the mount. Mechanical behaviours with motors etc could easily be compensated via software, tbh I would expect this and can understand why AP mounts compensate for it.

[PRejto]

Hi Greg,

Sorry I jumped the gun on your question, but I thought I knew where it might be heading. Of course I'm not saying my system wouldn't have any flexure. Everything does for sure, and so would my adapters and focuser, etc. It's just that the way I'm doing the tests I don't believe it is possible for ordinary flexure to explain the consistent results of drift after a slew east. For all these tests I've completely eliminated the ONAG. The camera is fitted to a Baader Click Lock that is clamped into the 2" Feathertouch. No, not ideal & not screwed together, but flex in this would show up in declination since I've got X=RA. And, it sure would be odd that flex would only exist after a slew in a particular direction, wouldn't it? I'd reason that something loose would move in both directions. Here is a quick view of the business end (camera is not RA=X in this photo!):

http://www.pbase.com/prejto/image/149159999

I certainly cannot feel anything moving in this setup!

The issues I've dealt with mostly concern two problems that I've never been able to properly rectify. One is the issue of Protrack and unguided. My supermodels have told me that I've got excellent PA, and I've used large models with 100s of points, yet unguided is all over the place. One time it will seem good but the next run terrible. I think I now realize that this has been slew dependent. Usually I'd just slew to a different part of the sky and start an exposure. From what I've been demonstrating I now think that if I slewed west I would get a good result, but that if I'd slewed east, and not waited at least a full minute, I would get a lousy result. The mount would be drifting between 5-10 arcsec over that first min and Protrack has no way to deal with that at all. Same thing with guiding I believe. If I slewed west, took a guide camera photo, select star, etc, the star will show up reasonably centered on the guide graph. But if I've done an east slew, the mount is "running away" during that first minute while selecting a guide star and plotting the first point on the graph. Often the first point is completely off the guide graph. Of course, if I wait a minute+ everything will be fine! Unfortunately I didn't understand this and thought I must be picking the wrong aggression settings, etc. You see the guide star escaping and think you need a higher setting to get it back centered. Then of course the drift ends and the setting is too aggressive. It also explains an issue that has made me nuts. I might have perfect guiding established. I finish with one filter and slew away to refocus with a different filter, slew back to the target and not be able to reestablish guiding with settings that worked just minutes before....all because I probably slewed east returning to the target.

For sure one can live with this if its understood. But, it's far from ideal and I sure didn't plonk down $10k for a mount that needs to be pampered in this way. These issues have all come into focus and demand resolution when I upped my imaging scale to .63 from 1.14. Probably for most people using the MX and doing wide field, or imaging scales greater than 1 arcsec you wouldn't really see this issue as much, but it gets amplified at long focal lengths for sure.

If I'm correct about this I bet there is a software fix that could deal with this by adjust how slews are done. Perhaps the mount needs to apply the brakes sooner and/or more gradually, or overshoot the target and slew back (west) by a couple of degrees. It could all happen automatically and transparently.

Peter

[PRejto]

Hi Chris,

I think it would be terrific if you could confirm or refute what I'm seeing!

My tests were all done at .63 arcsec using my imaging camera near 0 dec, near the meridian. However, day before yesterday I ran the same quick test over wide areas of the sky and got identical results.

I too believe that software could fix this. At the moment I think SB thinks I'm just a big pest. A few confirming responses my kick a potential fix into high gear so any input you could offer would be appreciated! Thus far I have one other person confirming this, and one person no longer with an MX but saying he had a similar issue. He dealt with it by waiting an entire worm cycle after slewing in order to get a good Protrack result.

http://www.bisque.com/sc/forums/t/19985.aspx


Thanks!

Peter

[gregbradley]

Thanks for posting that.

It sounds like something mechanical that is either too loose or too tight.

So the problem is after an eastern slew it will not guide for a minute as it drifts a lot.

I notice the PME is designed to slew past the object and then come back to it. I seem to remember some AP mounts do that. Its probably a way to get all the slop in the gears out so the gears are fully in contact and enmeshed nicely for accurate tracking.

Perhaps SB don't know what to do next without the physical mount in their hands to check it over.

I'd be asking them to take it back and replace it with another unit and shipping at their expense at this point. Its probably something hard for us to see and know like a bad ball bearing or even shipping damage. Who knows if it a part got dropped in the factory and installed anyway??

Greg.

[PRejto]

Greg, I agree with you 100%. But I cannot even get a reply to my posts now for nearly 2 weeks. I think I should be phoning SB on Tuesday and demanding some help. I leave for the USA on Wednesday and if they even read my posts they know this.

However, as others begin to confirm this it may be something in some MX mounts and not just in mine. The most recent post on the SB thread is from a friend in Australia showing possibly the same behavior with his MX.

Your comment about how the ME slews past and comes back is most interesting. I wonder why SB felt that this wasn't necessary in the MX? Perhaps they made an error in judgement on this.

Peter

[cfranks]

Hi Peter,

I've a feeling that this is what the Closed Loop Slew does although I'm not sure if the first slew overshoots or falls short. The target is always visible in that cr@ppy Fits Window, and the second slew drops it dead centre.

[PRejto]

Charles,

Thanks for pointing this out. Have I missed something basic? Is there a recommendation that the CLS should be used instead of just slewing to an object? Why would one use this type of slew? Is it just for centering an object if it isn't in the center after a slew?

Peter

[cfranks]

Peter,

It adds far more precision to your slews. I did a meridian flip followed by CLS and got with 2 pixels of my image. I mentioned it because of your comment about why this feature wasn't built into the MX. I felt it might give you time for your Drift problem to overcome what might be causing it. Search the SB site for Closed Loop Slew, there is a fair amount of traffic that might be informative. I use it all the time where I can.

Charles

[gregbradley]

Hi Charles,

Is CLS an option somewhere in the Sky X? Where is it located in the menus?

Greg.

[frolinmod]

It's in FIND, but you'll need a recent build:

[cfranks]

You can also put the CLS icon on the Toolbar if you wish. Tools/Preferences/Toolbars/Customize/All Other Commands

Charles

[gregbradley]

Thanks for the tips. I bought the update yesterday as that should be a very handy feature. Especially at 1am doing a meridian flip and getting everything to line up. That can take some time.

Greg.

[PRejto]

Thanks to all for participating in this discussion. I'm off to Tucson tomorrow am where amongst other things I will be doing an imaging workshop and then attending the Science and Astronomy Expo. It should be very interesting with lots of goodies to view. I also pick up my Trius! After the fun I actually have some work to do in Tucson...

Re the mount the issue stands unresolved and I was recommended by Bisques not to bring anything. They basically remain unconvinced by the data I've thus far provided and believe the problem may be in the pier or the focuser/camera mounting. When I return I will take up a friend's offer to put my mount on his pier, and after that I will put a different scope on my mount (unless my pier proves the guilty party). If the problem remains after all of this I think it just has to be in the mount! If any of you MX users would care to repeat any of the experiments I've demonstrated in this post I would be very grateful! At the very least it would demonstrate to me that the problem is isolated to my system, or is an unnoticed issue with the MX. I have no ax to grind one way or the other; I just want it solved.

Thanks!

Peter

[PRejto]

I'm in Tucson at the moment but thought I'd just post a little update to this thread. At the Arizona Science and Astronomy Expo over the weekend I had the opportunity to speak directly with Dan Bisque about the issue I've been reporting in this thread. It was great to meet him and I must say that meeting him in person was completely different to any preconceived expectation I may have carried in my mind based on experiences at the SB forum. The upshot was a very productive discussion and hopefully a resolution. SB is mailing me a new RA motor which should arrive in Los Angeles shortly. I was promised a software TCS solution if somehow the motor fails to resolve the issue. I have come away from this experience very satisfied!!

BTW the Expo was fantastic and it is rapidly growing into a major astronomy event in the USA. The quality of exhibits and speakers left nothing to be desired. My own desire to buy everything that appealed however went sadly unfulfilled!

[rat156]

Hi Peter,

Good to hear that you have a resolution hopefully.

I too have had some success with SB after a long wait. I too was mailed a new RA motor after many many "mount hit something" messages. Finally I have run an entire night without an error, hopefully it is permanent.

Mine turned out to be a faulty encoder on the RA motor, which eventually failed completely so I couldn't even initialise the mount.

Cheers
Stuart

[PRejto]

Hi Stuart,

Thanks for your update. I fear had I not traveled to Tucson I would be in for a long wait for a resolution. It's interesting that you mention the encoder as this is Dan's theory about my mount as well. I heard a rumour that SB might be about to change the design of the MX motor due to these sorts of issues.

Peter

[Logieberra]

That is good news, but not an uncommon thing with EQ mounts. I remember when Losmandy moved to the Maxons. The same is true for AP. Their later 2010 models of the 1200GTO moved to the same motors as used on the 3600GTO (El Capitan!). It's the natural course of things I guess. These companies don't sit idle, which is good to see :)

[gregbradley]

Well done for your persistence. It seems like you are finally getting to the bottom of this elusive issue.

Still, I bet you want to get an AP mount next time! Think of how many images you missed out on taking because of all this.

Greg.

[rat156]

Hi Greg,

I live in Melbourne. So I haven't missed that many nights. What makes you think I want another mount? This WAS supposed to be my last mount ever!! I'm sure that, one day, something new will spark my interest, so you're probably right.

Other than the frustration of having paid for something which should be the duck's nuts and ending up with another mount which needs mucking about with, I haven't lost too much imaging time. A few half nights here and there, and a couple of full nights over the last week or two. Luckily I'm a persistent mongrel and could usually coax the mount to life and get some imaging done.

I'm looking forward to some trouble free imaging now. I have also learned heaps about the internals of the mount, I reckon I could service the PMXs in Australia now!

Cheers
Stu

[PRejto]

Greg,

I don't know what you mean by saying I've missed out on imaging. For example I took this image of the Crepe Nebula just yesterday and I didn't even need a mount or a telescope!!

[frolinmod]

Outstanding image. You must have spent hours processing it. :lol:

[PRejto]

Yes!! and I used a Narrowpan filter!!

[gregbradley]

Tremendous resolution there.

Is that some vignetting or did you eat part of it?

Greg.:rofl:

[PRejto]

Yes, the nebula no longer exists. As I understand it the light has reached us first but a massive supernova shock wave has destroyed the crepe nebula and it can no longer be imaged. You are viewing the one and only picture of it in history!!!

[PRejto]

An update. After another month + of experimenting, and a second 5 page thread at the Software Bisque forum, my mount is finally going back to SB under warranty.

The culprit may be some hysteresis I was able to identify visually by sighting on a far away target during the day and rocking the polar axis back and forth. With everything adjusted to spec as best I can the mount doesn't spring return to the same position when moved west or east. A friend tested his mount in the same way and it did not show this hysteresis. So far that is the one and only working theory. The hysteresis I measured was ca. 30 arc-sec. That equates to an extremely small mechanical motion on the order of 25 microns. Even if I pulled my mount entirely apart I highly doubt my ability to detect such a small motion.

Thanks to all that have contributed to this thread! It will be most interesting to find out what is discovered during servicing.

Peter

[gregbradley]

It would be a nice gesture of good will if once they fix it they gave you an extra counter weight or some software to allow for the annoyance factor.

Greg.

[PRejto]

Perhaps. Whilst I feel confident in what I'm measuring and experiencing, it feels a bit like sending the car to the mechanic and having the mechanic say "I don't see any problem!" Of course that hasn't happened and I hope it doesn't but the feeling of apprehension is certainly there. It's costing them a lot of money to ship the mount!! That's good enough for me, especially if it turns out I have somehow failed to make the correct adjustments.

Peter