Olly and Tom's EQ8.

[ollypenrice]

Olly will need a larger scope to use the Gaia CCD, it has a total of 106 CCD's with a size of 6cm x 4.7cm each, resolution is 4500x1966 pixels.

Total size of the array is around 0.5m x 1m with a total of 938 MP.

Total area of the array is 2.8m2

Damn!

If you are going to use that GAIA CCD, maybe you could get computing time on Zeus, my nifty little compute server (nothing too fancy, just a 2U rack server with 64 cores and 512 GB RAM (and 6 TB disk). Chews through a 870 Mpixel image in 11 seconds for multi-scale processing)

I might need you to hold my hand through some of this, Michael! My IT skills are at the level of an abacus user. In fact that is pretty unfair on abacus users, thinking about it...

Olly

Edited December 21, 2013 by ollypenrice

[ollypenrice]

OK, regarding the Dec backlash I've sent off enquiries to OVL and SW. The issue seems to be well established on the net and there's a PDF document discussing a fix to the clutches which I still haven't successfully downloaded, but I'm hoping to get there!

Olly

[perfrej]

Olly, that thing has low resolution absolute encoders! What happened to your principles? It is even suitable for automation! (sorry, shouldn't use that word)

/per

[ollypenrice]

Olly, that thing has low resolution absolute encoders! What happened to your principles? It is even suitable for automation! (sorry, shouldn't use that word)

/per

The modern world is creeping up on me. I've even started replacing some of my wooden lenses with glass ones...

Olly

[Zakalwe]

This thread suggests that the author has a PDF detailing the routine to adjust backlash:

http://stargazerslounge.com/topic/200569-eq8/

[ollypenrice]

I now have a copy. Many thanks to Ray for that. The process is quite complex but later mounts have three large access screws to reduce the complexity of the adjustment. If you have these access holes for the Dec adjustment (the one that really matters) it will be easier. At the end of the PDF there's mention of a verison of the document covering the version with the access holes. I don't know where to find this version of the instructions.  Be aware that the allen bolts blanking the holes are actually just grub screws and if screwed inwards will fall into the worm housing. Why no heads on these blanking bolts? No idea.

Olly

Edited December 22, 2013 by ollypenrice

[Xplode]

My mount is probably from the same batch as yours Olly so i think i have those 3 access screws. Will take a look at the adjustment later today if i get that PDF from Ray.

[ollypenrice]

OK, the bad news.  After a lot of net research I've found out how to adjust the mesh of the worms. However...

Stage one is to rotate the dec drive a little at a time using the motors and to feel the amount of backlash at each point. The purpose of this is to determine the tightest point of mesh between worm and wheel before tightening the mesh any further. 

Stage two is no longer relevant in my case because on one half of the wheel's rotation the mesh is tight (no backlash) and on the other half it is very slack. (Lots of backlash.) Even if I felt like trying it there would be no point in adjusting the mesh because the problem, presumably, is that the wheel is not concentric with the Dec axis. I found an Australian owner who has precisely this problem and was in contact with Tasco who seemed to be trying to fob him off with a software fix. This has to be a waste of time because the problem is mechanical. Adjusting the dec guiding response speed is not the way to sort it. The way to sort it is to have a concentric (or round) worm wheel.

Oh dear, oh dear. Not very happy.

The most basic QC would have spotted this, especially since they obviously know to look out for it. (Hence the modification to the dec motor housing to assist in mesh adjustment.) Unless I've completely misread the symptoms I suppose the mount will be going back.

Olly

[libraryman]

Hi all, it seems that there are two camps in the EQ8 backlash saga, those who swear that they have none and those like myself and Olly who appear to have backlash in both axis!

from my perspective i have reasoned that the Dec backlash (the only one that matters) can be dealt with by un balancing the scope, i have found that doing this does sort out the issue and my dec guiding is quite good.

Not an answer, i admit but i am waiting to get some driving under the belt, i am hoping that some bedding in will take place and the backlash can be adjusted correctly in due course.

I have spoken to one owner who did the adjustment and adjusted it just a little too much..the motors complained and the adjustment was backed off to suit.

It appears also that Skywatcher do not consider this an issue at all,   i have read one reply to a request for info that  "polar alignment, Balance etc" should be correct and no issues will then arise from small backlash differences.

This owner also had a tight spot @ 180degs, suggesting that the worm wheel was off centre or something worse.

I still feel that the EQ8 will live up to it's reputation (call me gullible) as a heavy duty mount with excellent tracking ability for half the cost of an AP 1200

I have forwarded at least 10 Backlash adjustment PDF's so it's not a rare occurrence at all. 

Ray

[Xplode]

I can confirm that i have the same problem as Olly with uneven backlash. Will contact the Norwegian dealer about it see what they say. I'll try the mount for a while to see if there's any issues from my mount when imaging.

[libraryman]

It might be prudent to do a number of checks to ensure that:

A: the dec backlash is completely dec backlash and not some RA creeping in..it can be difficult to separate to two

B: to ensure that the tight spot is there all the time?? i found inconclusive results that made me wonder!  

Ray

[perfrej]

I find it hard to picture what is wrong when a worm gear has uneven "tightness".  The only thing that comes to mind is that the worm wheel is not centered, as Olly suggests. How the heck do you fix that???

/per

Edited December 22, 2013 by perfrej

[ollypenrice]

I find it hard to picture what is wrong when a worm gear has uneven "tightness".  The only thing that comes to mind is that the worm wheel is not centered, as Olly suggests. How the heck do you fix that???

/per

In my opinion you won't be able to. Either the wheel isn't centred or it isn't round. Or both. Ruddy nuisance. It would be so much easier for everyone if Skywatcher spent a couple of quid on QC and saved us and themselves an incredible amount of wasted time.

Ray has a point about repeatability. I'll also give the drives some run time to see if anything changes.

Olly

[libraryman]

Not sure that you can! However; i also wonder about the possibility of this; i'm making big assumptions here, but it's likely that they are produced on a CNC machine in one operation, that being the case, the  centre would be concentric though it would be possible for a dull tool to produce the effect on the gear cutting, i am also assuming that the worm wheel is Alloy.

i do not think it's the worm that's out, if there are any machining wizards out there they might have a skill view, though it's all irrelevant if the tightness at 180 degs is real.  

Ray

[Tom OD]

Hmm this is such a shame. I don't have that PDF file, but does anyone have any pics of the worm gear and meshing gears?

Well lets see the guiding goes Olly, will you try it out before returning.

From a visual point of view it looks very impressive, and there is lots of room to shift those weights down more to allow for that extra Tak. Should I continue to search for a Robin Cassidy plate. I e mailed him a few weeks ago but he didn't reply. I'm surprised no one else has taken up the slack and starting making a similar product.

Tom

[Zakalwe]

Ive had a play with mine (seeing that there's near constant cloud and rain hhere there's little else to do!). Am pleased to report that mine seems to be OK. With a 15" dovetail in the saddle (all overhang to one side) there is a tiny amount of backlash that can be felt. I'm talking about way less than 0.5mm as measured at the end of the dovetail. This is consistant right through the DEC rotation. Using the handest to measure the backlash it comes in at roughly 1-1.5 arc minute. I don't have enough experience to deem this acceptable or not. It feels OK to me, but what says you?

[Peter Drew]

I made a large number of worm and wormwheel sets before retirement, concentricity was paramount and each set was tested for mesh and smoothness. You can't adjust out eccentricity with a rigidly fixed worm assembly, you could get away with a small amount with either a spring loaded or rubber mounted unit. Unfortunately you can no more judge the performance of a gear set than that of optics just by looking at them. I hope it's not the dec/ra  shafts that are running eccentric.

[ollypenrice]

Ive had a play with mine (seeing that there's near constant cloud and rain hhere there's little else to do!). Am pleased to report that mine seems to be OK. With a 15" dovetail in the saddle (all overhang to one side) there is a tiny amount of backlash that can be felt. I'm talking about way less than 0.5mm as measured at the end of the dovetail. This is consistant right through the DEC rotation. Using the handest to measure the backlash it comes in at roughly 1-1.5 arc minute. I don't have enough experience to deem this acceptable or not. It feels OK to me, but what says you?

How did you do this measurement with the handset? I can see how to do with a guidescope calibration. If you let me know I'll give it a go. 

Olly

[ollypenrice]

I made a large number of worm and wormwheel sets before retirement, concentricity was paramount and each set was tested for mesh and smoothness. You can't adjust out eccentricity with a rigidly fixed worm assembly, you could get away with a small amount with either a spring loaded or rubber mounted unit. Unfortunately you can no more judge the performance of a gear set than that of optics just by looking at them. I hope it's not the dec/ra  shafts that are running eccentric.

Thanks Peter. This has been mooted in Australia and speculatively linked to the delay in the initial production run. I don't have a view on that, given no known evidence.

Olly

[Zakalwe]

How did you do this measurement with the handset? I can see how to do with a guidescope calibration. If you let me know I'll give it a go. 

Olly

Hi Olly,

I powered up the mount, entered the date and time as normal and unparked. I then went to Utility Func. >Show Position. You can select RA/DEC, Alt/Az or Axis 1 & 2 readouts by pressing the selection arrows (not the 4-way directional arrows). I slewed to various points around the DEC axis and watched the readout as I pushed and pulled on the dovetail. This will also show if the movement is in RA or DEC. Obviously, this method is pretty crude and is only as accurate as the resolution of the encoders, but it does give a way of measuring the backlash.

Rgds

Steve J

[stephen_usher]

I can't test mine at the moment as I'm about 300 miles away from the mount. I'll have to wait until the new year when I get back home.

[ollypenrice]

Hi Olly,

I powered up the mount, entered the date and time as normal and unparked. I then went to Utility Func. >Show Position. You can select RA/DEC, Alt/Az or Axis 1 & 2 readouts by pressing the selection arrows (not the 4-way directional arrows). I slewed to various points around the DEC axis and watched the readout as I pushed and pulled on the dovetail. This will also show if the movement is in RA or DEC. Obviously, this method is pretty crude and is only as accurate as the resolution of the encoders, but it does give a way of measuring the backlash.

Rgds

Steve J

Good idea. I'll try that. I was forgetting the encoders.

Olly

[opticalpath]

If weather permits, I'd test it through the scope, looking at a distant object (or star) at high power (or better, image it with crosswires view).  Set speed to low guide rate (say 0.1 x sidereal : 1.5 arcsec per sec) and move the joystick in one direction.  Count how long it takes before the object starts moving in the FOV, then try in reverse direction. 

Adrian

[libraryman]

Just an idea!! find the tight spot on the dec axis then loosen the clutch and rotate the saddle into the polar position arrow up, relock the clutch and test for backlash, is it exists rotate the saddle back to the tight spot location and re-tighten the clutch, test for backlash..is backlash still zero?

Yes?

Undo clutch rotate saddle again to polar and  test again..if backlash is there it is either Ra or a clutch issue? possibly not an out of round worm wheel?

comments/ critique  welcome

Ray

[ollypenrice]

Just an idea!! find the tight spot on the dec axis then loosen the clutch and rotate the saddle into the polar position arrow up, relock the clutch and test for backlash, is it exists rotate the saddle back to the tight spot location and re-tighten the clutch, test for backlash..is backlash still zero?

Yes?

Undo clutch rotate saddle again to polar and  test again..if backlash is there it is either Ra or a clutch issue? possibly not an out of round worm wheel?

comments/ critique  welcome

Ray

The clutches lock the puck to the worm wheel. If I move the puck via the clutches the amount of play doesn't change; It only changes if the worm wheel is driven round to the other side.

Olly