VintageBigBlue.org

 

Re: PEMPro backlash test


May 8, 2016

 


----------------------------

#57405 May 8, 2016

I've been fighting with my G-11 for several months now, finally decided to bite the bullet and see if PEMPro can help me out. ��My issue has been erratic guiding with PhD and a piggyback autoguider. ��PEMPro tells me PE is less the 4 arcsec P-P which I find find hard to believe but maybe it is. So, on to other things...I'll work on the guider later.

In the meantime, I thought I'd look at declination backlash - with Polemaster my polar alignment is now, finally, very good so I get little Dec drift. ��When I do the PEMPro backlash test, I can easily see insufficient TVC vs excessive TVC - the overshoot is clearly significant if set too high. ��I added a photo to my test file directory to see if anyone else has observed a similar Dec exercise. ��Each leg of the sawtooth is 10 seconds, total length 60s. �� N is left. ��TVC was set to 010.

What I recorded is the expected sawtooth image but the declination motion seems to hesitate a few seconds into the directed motion, more so moving north (star going to the right in the image) than when the mount is reversed to move south. ��



I would never expect any Dec corrections to be as long as a few seconds so am I making a mountain out of a molehill or is this indicative of something I should consider correcting? ��I haven't adjusted the Dec worm since I last greased the mount a couple of years ago, haven't seen any binding or other issues. ��The Dec worm adjustment has almost no play that I can manually feel so maybe there's binding but Gemini never detects it. ��Gemini-I, by the way.



John

xa.yimg.com/df/Losmandy_users/Backlash010.jpg?token=t-s_k95MxnI68ErRsXiXDt3_6t8QHxxjpMFS_IL6P4QNd66eQ4Ie6_XtKHKg8ZfKR92fUZKiA08-7BFyw_JEatubWEomWkHP3CpCcDISN5_YUa4&type=download



xa.yimg.com/df/Losmandy_users/Backlash010.jpg?to...



----------------------------

#57406 May 8, 2016

Hi John!Very nice description of your problem.There are several possible links that could cause backlash.. I think they are: worm to ring gear spacing (the feeler gauge adjustment), backlash in the gear system, backlash in the Oldham coupler, and, finally, worm movement if it slides along its axis between the bearing blocks.The middle 2 causes are outside of adjustment...I think the time backlash would be very short...I'd ignore those.The first and last might interact...in that trying to compensate for a worm sliding movement, you'd be tempted to overtighten the worm to ring space... But overtighten its leads to binding, so you could get stuck with a backlash and time delay there.Usually the manuals recommend only snuggling up the worm to ring space, with feeler gauges to help make the adjustment.. I'd start there...easy enough to adjust...very accessible.I always wonder if possible side to side motion of the worm in the worm bearing blocks could have an adverse effect on guiding.. People generally feel that putting a weight cord on RA, or similarly forcing the worm to one side of the RA worm solves this, but you have raised the other axis problem: how to handle Dec backlash and the time delay it causes autoguiding.You could eliminate any worm sliding by adding in a round Belleville type spring washer to one end of the worm.. That would put up to 7 lbs on the worm axis, forcing it against the opposing bearing block.. It does the same as adding a weight cord in a very compact way.. Then the only adjustable element would be the worm to ring spacing.. I have some from McMaster -Carr...contact me if interested.Best of luck,

Michael

On May 8, 2016 7:42 AM, "hazyskyobservatory@... [Losmandy_users]" Losmandy_users@yahoogroups.com> wrote:

.I've been fighting with my G-11 for several months now, finally decided to bite the bullet and see if PEMPro can help me out.. My issue has been erratic guiding with PhD and a piggyback autoguider.. PEMPro tells me PE is less the 4 arcsec P-P which I find find hard to believe but maybe it is. So, on to other things...I'll work on the guider later.

In the meantime, I thought I'd look at declination backlash - with Polemaster my polar alignment is now, finally, very good so I get little Dec drift.. When I do the PEMPro backlash test, I can easily see insufficient TVC vs excessive TVC - the overshoot is clearly significant if set too high.. I added a photo to my test file directory to see if anyone else has observed a similar Dec exercise.. Each leg of the sawtooth is 10 seconds, total length 60s. . N is left.. TVC was set to 010.

What I recorded is the expected sawtooth image but the declination motion seems to hesitate a few seconds into the directed motion, more so moving north (star going to the right in the image) than when the mount is reversed to move south. .



I would never expect any Dec corrections to be as long as a few seconds so am I making a mountain out of a molehill or is this indicative of something I should consider correcting?. I haven't adjusted the Dec worm since I last greased the mount a couple of years ago, haven't seen any binding or other issues.. The Dec worm adjustment has almost no play that I can manually feel so maybe there's binding but Gemini never detects it.. Gemini-I, by the way.



John

xa.yimg.com/df/Losmandy_users/Backlash010.jpg?token=t-s_k95MxnI68ErRsXiXDt3_6t8QHxxjpMFS_IL6P4QNd66eQ4Ie6_XtKHKg8ZfKR92fUZKiA08-7BFyw_JEatubWEomWkHP3CpCcDISN5_YUa4&type=download



xa.yimg.com/df/Losmandy_users/Backlash010.jpg?to...



----------------------------

#57407 May 8, 2016

Michael

I did the belleville washer/higher quality bearings mod to my RA worm so I think I have that problem more or less resolved. ��4 arc-sec P-P in RA is about as good as I'd ever expect from a G-11 but that's a number I want to verify again...seems to good to be true.

But I haven't touched the Dec worm in quite a while and my first guess is it's too tight. Last time I checked I didn't feel any side-to-side motion on the Dec worm but I think that's the place to check.

Re:��McMaster -Carr: ��only after I ordered the parts from them by mail did I discover they have an over-the-counter pickup window 10 miles away from me!



John��







----------------------------

#57408 May 8, 2016

I believe PemPro reports the size of the��periodic��error. This doesn't include random, or other errors that do not repeat regularly (periodically). Take a look at the PemPro curve captured from the mount. What are the maximum excursions of this curve? Also, what is the RMS error after PemPro curve fit? If large, this will tell you that the periodic curve Pempro constructed does not fit the original error curve well.

Regards,

�� ��-Paul



----------------------------

#57409 May 8, 2016

John,



There is one more source of movement. The DEC shaft in the bearings. See if you can detect any side

movement in the entire DEC assembly. I bought my G11 in 1996 and when I measured both shafts they

were 1.247". That is 0.003" undersized. The bearings have a 1.250" ID, so there was detectable side

to side movement on both axes. I solved the problem by having both shafts chrome plated and

centerless ground down to 1.2495".



Don

----- Original Message -----

From: "hazyskyobservatory@... [Losmandy_users]" Losmandy_users@yahoogroups.com>

To: Losmandy_users@yahoogroups.com>

Sent: Sunday, May 08, 2016 10:42 AM

Subject: [Losmandy_users] PEMPro backlash test





> I've been fighting with my G-11 for several months now, finally decided to bite the bullet and see

> if PEMPro can help me out. My issue has been erratic guiding with PhD and a piggyback autoguider.

> PEMPro tells me PE is less the 4 arcsec P-P which I find find hard to believe but maybe it is. So,

> on to other things...I'll work on the guider later.

>

>

> In the meantime, I thought I'd look at declination backlash - with Polemaster my polar alignment

> is now, finally, very good so I get little Dec drift. When I do the PEMPro backlash test, I can

> easily see insufficient TVC vs excessive TVC - the overshoot is clearly significant if set too

> high. I added a photo to my test file directory to see if anyone else has observed a similar Dec

> exercise. Each leg of the sawtooth is 10 seconds, total length 60s. N is left. TVC was set to

> 010.

>

>

> What I recorded is the expected sawtooth image but the declination motion seems to hesitate a few

> seconds into the directed motion, more so moving north (star going to the right in the image) than

> when the mount is reversed to move south.

>

>

>

> I would never expect any Dec corrections to be as long as a few seconds so am I making a mountain

> out of a molehill or is this indicative of something I should consider correcting? I haven't

> adjusted the Dec worm since I last greased the mount a couple of years ago, haven't seen any

> binding or other issues. The Dec worm adjustment has almost no play that I can manually feel so

> maybe there's binding but Gemini never detects it. Gemini-I, by the way.

>

>

>

>

> John

>

>

> xa.yimg.com/df/Losmandy_users/Backlash010.jpg?token=t-s_k95MxnI68ErRsXiXDt3_6t8QHxxjpMFS_IL6P4QNd66eQ4Ie6_XtKHKg8ZfKR92fUZKiA08-7BFyw_JEatubWEomWkHP3CpCcDISN5_YUa4&type=download

> xa.yimg.com/df/Losmandy_users/Backlash010.jpg?token=t-s_k95MxnI68ErRsXiXDt3_6t8QHxxjpMFS_IL6P4QNd66eQ4Ie6_XtKHKg8ZfKR92fUZKiA08-7BFyw_JEatubWEomWkHP3CpCcDISN5_YUa4&type=download

>

>

>

> xa.yimg.com/df/Losmandy_users/Backlash010.jpg?token=t-s_k95MxnI68ErRsXiXDt3_6t8QHxxjpMFS_IL6P4QNd66eQ4Ie6_XtKHKg8ZfKR92fUZKiA08-7BFyw_JEatubWEomWkHP3CpCcDISN5_YUa4&type=download

>

> xa.yimg.com/df/Losmandy_users/Backlash010.jpg?to...

> xa.yimg.com/df/Losmandy_users/Backlash010.jpg?token=t-s_k95MxnI68ErRsXiXDt3_6t8QHxxjpMFS_IL6P4QNd66eQ4Ie6_XtKHKg8ZfKR92fUZKiA08-7BFyw_JEatubWEomWkHP3CpCcDISN5_YUa4&type=download

>

>

> View on xa.yimg.com

> xa.yimg.com/df/Losmandy_users/Backlash010.jpg?token=t-s_k95MxnI68ErRsXiXDt3_6t8QHxxjpMFS_IL6P4QNd66eQ4Ie6_XtKHKg8ZfKR92fUZKiA08-7BFyw_JEatubWEomWkHP3CpCcDISN5_YUa4&type=download

> Preview by Yahoo

>

>

>

>

>



----------------------------

#57410 May 8, 2016

Hi John!Sounds like you have things well understood for the Dec axis...let us know your progress, as autoguiding is so critical to our imaging work..You really are lucky to have McMaster so close... shipping costs me a bundle from there, so I order rarely.All the best,

Michael.On May 8, 2016 11:38 AM, "hazyskyobservatory@... [Losmandy_users]" Losmandy_users@yahoogroups.com> wrote:

.Michael

I did the belleville washer/higher quality bearings mod to my RA worm so I think I have that problem more or less resolved. .4 arc-sec P-P in RA is about as good as I'd ever expect from a G-11 but that's a number I want to verify again...seems to good to be true.

But I haven't touched the Dec worm in quite a while and my first guess is it's too tight. Last time I checked I didn't feel any side-to-side motion on the Dec worm but I think that's the place to check.

Re:.McMaster -Carr: .only after I ordered the parts from them by mail did I discover they have an over-the-counter pickup window 10 miles away from me!



John.



----------------------------

#57411 May 8, 2016

Hi gang,

I thank John for asking about the DEC... and also for whomever asked about the feeler gauge measurements...

After that discussion...I was wondering if my own DEC could be faulty, and I discovered that my DEC was indeed binding up.. I had forgotten that on my last imaging session I was getting "DEC LAGS" warnings and I couldn't fathom what the problem was...too busy trying to get planetary videos... but my DEC worm was set too tight against the ring gear, and in some rotation angles it did lag, and then stopped altogether, with the Gemini-1 reporting the DEC had stalled.. It is a smart system engineering wise, but the hand controller does not clearly spell out what to do. .My particular G11 DEC axis has the original steel worm held by 2 worm blocks.. When I first measured the gap with a feeler gauge, the gap was 0.009 inch (stalling at that setting).. Now that it is backed off and not binding or lagging, the feeler gauge that slips in is 0.011 inch and (step feeler gauge) 0.013 inch will not go in.. I ran the DEC around +/- 90 degrees from CWD to assure it is now working correctly.. Maybe those readings will help you at least get the approximate range for the feeler gauge, as worm block dimensions, worm diameter, and bearing tolerances can vary. .



Best regards,Michael







----------------------------

#57432 May 12, 2016

I've convinced myself (fingers crossed) my DEC problem is a too tight worm adjustment. ��When I checked it out, I had zero side-to-side motion of the worm (which is good...) but I also had no detectable play between the worm and worm gear, absolutely none, which probably accounts for my TVC settings close to 0, anything above 012 and PEMPro shows me overshoot. ��I've loosened the worm so when the monsoons of April/May give way to the drought of summer here in Ohio, I'l test it for real instead of in the garage.

But another thought occurred to me when I was adjusting the worm: the garage temp was around 75F and I believe the last time I had adjusted the worm was during the winter a couple of years ago so the garage was closer to 40 or a bit lower. ��Since most of my imaging is with air temps 40 or greater (getting wimpy in my old age), is there an optimal temperature to adjust the worm? ��If my previous adjustment had been under 40F, I would expect the worm to tighten as the mount's temperature rose. ��Enough to consider or am I over-thinking this?





John



Contact Us
This Site's Privacy Policy
Google's privacy policies

S
e
n
i
o
r
T
u
b
e
.
o
r
g