Periodic error in an eq3-2 mount on 60s exposures. #backlash


Matt
 

Hey guys, I recently ended up finishing my onstep board and it works amazingly, thank you for that! Anyways, I started getting some drifting and was not sure if it was polar alignment. I adjusted alignment as best as I could but as soon as I thought I fixed it the drift would begin again, only on the other side. I attached a quick clip of 10 60s exposures for you to analyze. I found that the 48 teeth pulley is not sitting properly and is wobbling when rotating. That might be it but still wanted to post this here as it could be something internal to the mount. I am genuinely grateful for all of your hard work and for allowing me to begin my astrophotography journey! Hope we could sort this out. Thank you!


Rafael Barberá Córdoba
 

Hi Matt, congrats for your working system. Do you have measured the period between maximum elongation? If it matches with the period of the RA worm, you can use the PEC option to reduce it. But you will need to guide for a full worm period (better multiple periods). Do you have auto guiding capabilities?. If not, you can always use the handhold controller (or the Android App) to send guide corrections while you are  looking though an eyepiece equipped with a retile.


Matt
 

Hi Rafael, I have not measured anything yet. Any idea on how to do it? It's rocket science currently for me. Thanks for your help!


On Wed, Jul 15, 2020, 3:42 PM Rafael Barberá Córdoba <rbarberac@...> wrote:
Hi Matt, congrats for your working system. Do you have measured the period between maximum elongation? If it matches with the period of the RA worm, you can use the PEC option to reduce it. But you will need to guide for a full worm period (better multiple periods). Do you have auto guiding capabilities?. If not, you can always use the handhold controller (or the Android App) to send guide corrections while you are  looking though an eyepiece equipped with a retile.


Khalid Baheyeldin
 

You can measure the periodic error roughly by doing a very long exposure, with the least ISO on an area that has dim stars (e.g. somewhere in the Milky Way).

The exposure can be double the worm period. The worm period depends on the GR2 reduction (main worm wheel) number of teeth. It is basically the number of minutes for a full worm gear rotation.

For example, for an EQ5 with 144 teeth, it is 600 seconds. For a mount with 180 teeth, it is 480 seconds, and so on.

When you do the long exposure, it should show two sine waves (approximately).

See an example of how I did it in this thread, with images.


Matt
 

Alright, I'll look into it. Thank you!


On Wed, Jul 15, 2020, 7:03 PM Khalid Baheyeldin <kbahey@...> wrote:
You can measure the periodic error roughly by doing a very long exposure, with the least ISO on an area that has dim stars (e.g. somewhere in the Milky Way).

The exposure can be double the worm period. The worm period depends on the GR2 reduction (main worm wheel) number of teeth. It is basically the number of minutes for a full worm gear rotation.

For example, for an EQ5 with 144 teeth, it is 600 seconds. For a mount with 180 teeth, it is 480 seconds, and so on.

When you do the long exposure, it should show two sine waves (approximately).

See an example of how I did it in this thread, with images.


Howard Dutton
 

On Wed, Jul 15, 2020 at 02:23 AM, Matt wrote:
I found that the 48 teeth pulley is not sitting properly and is wobbling when rotating.
This is a big deal.  If you can see it wobbling, it isn't going to track well.


Mike Ahner
 
Edited

On Wed, Jul 15, 2020 at 05:50 PM, Howard Dutton wrote:
On Wed, Jul 15, 2020 at 02:23 AM, Matt wrote:
I found that the 48 teeth pulley is not sitting properly and is wobbling when rotating.
This is a big deal.  If you can see it wobbling, it isn't going to track well.
Matt, I wonder if you have the wrong size bore hole for that pulley? It often seems the motor shafts & worm shafts are different sizes. That will likely cause an increase in periodic error but shouldn't make tracking slow.  (Sorry, I confused the other thread with this one) Perhaps the pulley bore is so big that it's not tightening down and slipping?

It could also mean the worm gear is bent or damaged somehow an so isn't straight.
-Mike


Graeme
 

I had wobbly pulleys also.
Turns out the shaft was 6mm but a lot of pulleys seem to be 6.3mm bore.
I found some on eBay that were 6mm and all good now.
Definitely check the bore if they're wobbling.


Matt
 

Will do thanks guys!

čet, 16. srp 2020. u 01:22 Graeme <grmwebb70@...> napisao je:

I had wobbly pulleys also.
Turns out the shaft was 6mm but a lot of pulleys seem to be 6.3mm bore.
I found some on eBay that were 6mm and all good now.
Definitely check the bore if they're wobbling.


BJ Ranson
 

I had the same issue with my worm gear shafts being slightly smaller than my pulleys. I was able to fix the issue by cutting small strips of thin sheet plastic and inserting it between pulley and shaft. It seemed to improve things, but I'm still having big issues with a periodic error on the RA shaft, as witnessed by PHD2 drifting. I may have to start going down the PEC rabbit hole...


Matt
 

Thanks for the tip! How long of exposures are you able to achieve and at which focal length?


On Tue, Jul 21, 2020, 7:20 PM BJ Ranson <bjr21185@...> wrote:
I had the same issue with my worm gear shafts being slightly smaller than my pulleys. I was able to fix the issue by cutting small strips of thin sheet plastic and inserting it between pulley and shaft. It seemed to improve things, but I'm still having big issues with a periodic error on the RA shaft, as witnessed by PHD2 drifting. I may have to start going down the PEC rabbit hole...


Rafael Barberá Córdoba
 
Edited

If your mount is as "bad" as my EQ5, expect an amplitude around 60 arc sec for the PE. As the period of the worm is 600 sec, you will go to maximum positive deviation to maximum negative deviation on half a period (300 sec), so, you will have around 0.2 arc sec / sec for the PE speed. Use your focal length and pixel size to compute the maximum allowed exposure time.



For example, I'm using a TV85 at primary focus. This means a FL on 600mm. My camera is a Canon 40D with 5.70µm pixels. So my image scale is 206 * 5.70 / 600 = 1.95 arc sec / pixel. So, if I want to keep the star images inside a pixel my maximum exposure time, unguided will be around 10 sec.

You can activate the PEC in your mount and guide visually for a full worm period. I've made some test (guiding automatically with a CCD) and I can reduce de PE about 1:3, with a peak of only 10 arc sec (first period PEC is ON, second is OFF)


I've also a very apparent wobble on my RA pulley. You can "see" it on the shape of my PE: it's not sinusoidal, it has a lot of strange curvatures. If we look at the frequency spectra, we can see that apart for the main 600 sec period, there is a lot of armonics at 300, 200 and 150 sec, surely caused by the interaction between the main PE and the wobble of the 4:1 pulley


BJ Ranson
 
Edited

@Matt - I haven't yet done the PEC recording, so my "cliffs" on the RA auto-guiding are pretty scary. That said, I did manage to pull this out of M13 the other night at 60 x 60s guided exposures (1000mm f/5 Skywatcher 8" Newtonian):


And here is some of the plastic poking out from between pulley and RA shaft:


@Rafael - very interesting data. I also have an EQ5, and have spent the last 4-5 sessions (widely interspersed because of truly shocking cloudy summertime weather here in Britain) since I bought my guide camera, scratching my head to try to understand what is going on. I've been making gradual progress, and made a particular breakthrough the other night by setting the Backlash on my Dec in the OnStep Android App to 180 arc-sec. This has now stabilised my Dec excursions in PHD2 to within 1 arc-sec, which is amazing.

My problem now is calming down the RA. I had no idea the Periodic Error could be so high on the EQ5! - almost an arc-minute! In my next session I will do a PEC recording with auto-guiding on per Howard's instructions in the wiki, and then see if I can eliminate the "cliffs" when it's played back.

What software did you use to get the harmonic data?


Graeme
 

Eq5 here also but no guiding yet.
Focal length is 650mm and if I take time polar aligning I can get 50secs without throwing hardly any.



Rafael Barberá Córdoba
 
Edited

If you have used PHD2, you already have all the information needed :) Download https://openphdguiding.org/phd2-log-viewer/ and open the any guide session. From this data you can generate all those graphs and analysis.

You can follow this document to learn how to use this utility and interpret your guiding data https://openphdguiding.org/tutorial-analyzing-phd2-guiding-results/


Rafael Barberá Córdoba
 

Eq5 here also but no guiding yet.
Focal length is 650mm and if I take time polar aligning I can get 50secs without throwing hardly any.
Nice shot!. And very uniform tracking :)


Rafael Barberá Córdoba
 

On Wed, Jul 22, 2020 at 03:12 AM, BJ Ranson wrote:
That said, I did manage to pull this out of M13 the other night at 60 x 60s guided exposures (1000mm f/5 Skywatcher 8" Newtonian):
Nice shot!


BJ Ranson
 

@Graeme - very impressive, especially considering you have no guiding. Is this the stock tin-can motor with gears, or a new motor + conversion to belts?

@Rafael - thanks - I will check out those links. This hobby requires a lot of persistence, diligence and patience to get the best results. I guess that's what makes it so rewarding when you do get them.


Matt
 

Hey guys, thank you for the replies! I recorded PEC while guiding manually last night and got 7 HFR stars on a 10 minute shot without guiding. I can certainly achieve 60s exposures with good enough stars. I'll try guiding again tonight because I used a camera with huge pixels and it was hard to center the star I was guiding on. Thanks again for the replies!


On Thu, Jul 23, 2020, 11:11 AM BJ Ranson <bjr21185@...> wrote:
@Graeme - very impressive, especially considering you have no guiding. Is this the stock tin-can motor with gears, or a new motor + conversion to belts?

@Rafael - thanks - I will check out those links. This hobby requires a lot of persistence, diligence and patience to get the best results. I guess that's what makes it so rewarding when you do get them.


 


Wow!  That is a wonder shot!
 
On Wed, 22 Jul 2020 03:52:02 -0700 "Graeme" <grmwebb70@...> writes:

Eq5 here also but no guiding yet.
Focal length is 650mm and if I take time polar aligning I can get 50secs without throwing hardly any.