Onstep Goto Accuracy


Hemendra Rawat
 

Hi All,

 

I took my telescope out again for testing last night. The Wi-Fi connection is stable now and mount can now actually find the alignment stars with decent accuracy during the alignment process. However, after successful alignment process, DSOs were not in the field of view of telescope. So I need some guidance from you guys as this is my first time using a Goto mount.

 

Below is my setup procedure:

 

  1. Balance the tripod using bubble level and setup EQ5 mount on the tripod.
  2. Polar align the mount using the Polar scope. (Good enough alignment but not perfect).
  3. Align the finder scope with the telescope. I have two, Rigel finder scope and a 9x50 finder scope
  4. Move the telescope to Home position. Weights pointing down. Telescope facing towards Polaris.
  5. Turn on Onstep. Initialize Location, Time and start a 3-star align.
  6. Star 1 : Vega. Telescope slewed towards Vega and the star was in the finder scope view. I would say it was about 5 degrees off. Centered the star in the FOV using reticle eyepiece and then accepted the alignment.
  7. Star 2: Arcturus. Telescope slewed towards Arcturus and the star was in the finder scope view. But the error was more than the first star. I would say it was roughly 6-7 degree off. Centered the star in the FOV and then accepted the alignment.
  8. Start 3: Dubhe. Telescope slewed towards Dubhe and the star was in the finder scope view. But the star was again 6-7 degree off. Centered the star in the FOV and then accepted the alignment.

 

After this, I tried finding Andromeda galaxy, M13 Glob and Alberio double star but none of them were in the FOV of the telescope. Although the scope was pointing in the right direction of the sky. I tried 3-star alignment twice but no success in getting any DSOs or double star . I’m pretty sure that I used the correct stars for alignment. So I would like some suggestions as to how to improve the goto accuracy of my mount?

 

Telescope : 8 inch 1000mm Newtonian,  SkyView Pro EQ mount (EQ5)

Onstep: STM32 black pill board, LV8729 motor driver, 0.9 degree steppers

 

 

Regards,

Hemendra

 

Sent from Mail for Windows 10

 


Khalid Baheyeldin
 

There are many possible reasons for this.

Check the second question in the Frequently Asked Questions list.


d_petta@...
 

Hello, I made a mistake in the configuration
With a 32 step motor driver, I put 16 steps in Goto, that gave me Goto errors, left the stars too far away.
Greetings


Hemendra Rawat
 

I checked my configuration. It is set to 32 micro steps. How large the error was in your case? In my case the star comes near the outer circle of the finderscope. So I'm using the error is around 5-10 degrees.


On Wed, Jul 14, 2021, 7:43 AM d_petta via groups.io <d_petta=yahoo.com@groups.io> wrote:
Hello, I made a mistake in the configuration
With a 32 step motor driver, I put 16 steps in Goto, that gave me Goto errors, left the stars too far away.
Greetings


johnwashington1978@...
 

How are your motors connected to the worms.... the worm ratio is 144:1 I think on all variants, but the motors / spur gears have come in an array of different choices over the years and it sounds like you've added your own motors.

The other issue maybe payload, it sounds like a hell of a lot of scope for an EQ5, could it be skipping steps?  Are the steppers drivers / steppers burning hot?  I'd imagine you'd want a 4, 5 or 6:1 reduction between the motor and the worm.


Hemendra Rawat
 

I have connected motors to the worms via GT2 pulleys and timing belts and the reduction ratio is 2.5:1. The OTA is quite heavy at 16.5lbs and the mount is rated for 20lbs. How do I verify if it is skipping steps? Last time I checked the stepper motors after completing alignment, they were not hot at all.


On Wed, Jul 14, 2021, 3:02 PM <johnwashington1978@...> wrote:
How are your motors connected to the worms.... the worm ratio is 144:1 I think on all variants, but the motors / spur gears have come in an array of different choices over the years and it sounds like you've added your own motors.

The other issue maybe payload, it sounds like a hell of a lot of scope for an EQ5, could it be skipping steps?  Are the steppers drivers / steppers burning hot?  I'd imagine you'd want a 4, 5 or 6:1 reduction between the motor and the worm.


pettadavi70@...
 

hola, si estuviera en el borde del ocular.
Tengo wemos Esp32 con DRv8825, funciona muy bien.
En la próxima actualización del Esp32 es de doble núcleo, habría que darle más funciones o pensar en esas placas.
 


Khalid Baheyeldin
 

On Wed, Jul 14, 2021 at 08:08 PM, Hemendra Rawat wrote:
How do I verify if it is skipping steps?
Can you try using only the mount without an OTA, nor counter weights?
For example, with only a finder scope attached with rubber bands or velcro?

At least that will rule out the excessive weight as a cause.


Hemendra Rawat
 

Sure. I have access to a lightweight 120mm OTA. I will give it a try.


On Wed, Jul 14, 2021, 6:40 PM Khalid Baheyeldin <kbahey@...> wrote:
On Wed, Jul 14, 2021 at 08:08 PM, Hemendra Rawat wrote:
How do I verify if it is skipping steps?
Can you try using only the mount without an OTA, nor counter weights?
For example, with only a finder scope attached with rubber bands or velcro?

At least that will rule out the excessive weight as a cause.


Khalid Baheyeldin
 

Excessive weight is not the only reason for losing steps.
There are many others, but at least this way, you rule out weight as the reason.
For example, overtaxing the gears, ...etc.


johnwashington1978@...
 

This is a sensible plan, the other thing to check is the stepper driver vref - I'm a bit surprised by the not hot at all comment - the RA should be warm as its always driving and thus sinking 12W (or more), unless you live somewhere very cold.

The clutches in the EQ5's aren't up to much they can slip (after sometime you will need to take the clutch knob off rotate it 90deg and screw it back on to get enough holding force) and the "payloads" skywatcher put on the EQ5 are I believe in reference to the point at which the stock motors can no longer drive the mount - anything more than about 5kg would make AP impossible - visual is a bit more forgiving so you are right up to the mount capability I would imagine.


Hemendra Rawat
 

Hi John, Khalid,

When I said stepper motors were not hot, I meant they were slightly warmish to touch. Nothing significantly hot. My stepper motors are 0.9 degree Nema 17, 0.9A motors with an LV8729 (2x R220 resistor version) stepper driver. The Vref is set at 0.577 V.   

Last night, I tested the mount again with a lighter OTA (AWB Onesky, 130mm) which is around 7lbs. The performance was significantly better, I used Deneb, Arcturus and Dubhe as the alignment stars and the pointing error got reduced after every star alignment. After the alignment was completed, I tried goto for M13 and Alberio and the object was within the FOV of the scope (at one corner of my 25mm eyepiece). So it looks like reducing the weight helped a lot in reducing the pointing error.

I would like to get the mount to work for the larger scope (8 inch, 17lbs) as well. What are my options? My mount is around 6 years old and hasn't been lubricated. Will it help if I disassemble it and reapply grease? 

Regards,
Hemendra



On Thu, Jul 15, 2021 at 6:04 AM <johnwashington1978@...> wrote:
This is a sensible plan, the other thing to check is the stepper driver vref - I'm a bit surprised by the not hot at all comment - the RA should be warm as its always driving and thus sinking 12W (or more), unless you live somewhere very cold.

The clutches in the EQ5's aren't up to much they can slip (after sometime you will need to take the clutch knob off rotate it 90deg and screw it back on to get enough holding force) and the "payloads" skywatcher put on the EQ5 are I believe in reference to the point at which the stock motors can no longer drive the mount - anything more than about 5kg would make AP impossible - visual is a bit more forgiving so you are right up to the mount capability I would imagine.


Khalid Baheyeldin
 

First, is the mount smooth enough with nothing on it? If you disengage the clutch and push it gently with one finger, does it move smoothly? If it does, then lubrication is unlikely to be the issue. If it is not smooth, then the motors may be fighting to move them adequately, and that mechanical issue should be rectified before you try anything else.

Second, you say your LV8279 are at Vref 0.557V, but there are two versions of them that have different resistors, as explained in this message. For your motors, the starting current to try is 0.9 X 1.41 X 0.4 = 507mA. You can also try 0.5 (50%) instead of 0.4 (40%), so 635mA, and see if there is any difference between the two.

Third, there is a point at which one has to live with the mechanical capabilities of a given mount (i.e. not exceed the practical capacity limit, which may not be the advertised limit, but significantly less). To get over this, one may need to upgrade to a larger, often more expensive and more precise, mount.


Hemendra Rawat
 

Hi Khalid,

The RA and DEC axis move smoothly without the 8 inch OTA. When I put
on the 8 inch scope, the RA axis slow motion knob becomes a bit stiff
but I never expected the stepper motor would lose steps. What are the
benefits of increasing the motor current to 60% ? Will it improve the
torque? Also, do you think reducing the micro-steps from 32 to 16 will
help? I have a 8x40mm finderscope on my scope. So, even if the Onstep
goto can bring the object in the FOV of the finderscope, I should be
able to center it in the FOV of the scope.

Regards,
Hemendra

On Thu, Jul 15, 2021 at 11:39 AM Khalid Baheyeldin <kbahey@gmail.com> wrote:

First, is the mount smooth enough with nothing on it? If you disengage the clutch and push it gently with one finger, does it move smoothly? If it does, then lubrication is unlikely to be the issue. If it is not smooth, then the motors may be fighting to move them adequately, and that mechanical issue should be rectified before you try anything else.

Second, you say your LV8279 are at Vref 0.557V, but there are two versions of them that have different resistors, as explained in this message. For your motors, the starting current to try is 0.9 X 1.41 X 0.4 = 507mA. You can also try 0.5 (50%) instead of 0.4 (40%), so 635mA, and see if there is any difference between the two.

Third, there is a point at which one has to live with the mechanical capabilities of a given mount (i.e. not exceed the practical capacity limit, which may not be the advertised limit, but significantly less). To get over this, one may need to upgrade to a larger, often more expensive and more precise, mount.


Khalid Baheyeldin
 

On Thu, Jul 15, 2021 at 04:27 PM, Hemendra Rawat wrote:
The RA and DEC axis move smoothly without the 8 inch OTA. When I put
on the 8 inch scope, the RA axis slow motion knob becomes a bit stiff
Same for me with a C8 + DSLR + reducer + heavy guide scope (60mm F/4).
But for me, it the DEC that becomes stiff, not RA.

But I can still get fairly accurate GOTOs, autoguide and get good results.

but I never expected the stepper motor would lose steps. What are the
benefits of increasing the motor current to 60% ? Will it improve the
torque?
That is what I am hoping, but I don't have concrete information here. It is
just an experiment which may or may not work.

Note that good torque is good up to a point. You don't want the scope to
be physically blocked by something, and the high torque still pushing
the gears and possibly damaging them in the process.

Also, do you think reducing the micro-steps from 32 to 16 will
help?
I don't think this matters.

What is your pulley ratio?


Hemendra Rawat
 

Also, do you think reducing the micro-steps from 32 to 16 will
help?

I don't think this matters.

What is your pulley ratio?


Hemendra Rawat
 

My pulley ratio is 2.5 : 1 (40 GT2 pulley on mount drive and 16 GT2
pulley on motor axis).


On Thu, Jul 15, 2021, 1:44 PM Hemendra Rawat via groups.io <hkr1990=gmail.com@groups.io> wrote:
> Also, do you think reducing the micro-steps from 32 to 16 will
> help?
>
> I don't think this matters.
>
> What is your pulley ratio?
>






Khalid Baheyeldin
 

On Thu, Jul 15, 2021 at 04:55 PM, Hemendra Rawat wrote:
My pulley ratio is 2.5 : 1
That is a common enough configuration for EQ5 mounts, so you are not
alone. There are a few hundreds of these conversions around.

So it should work, though not sure how much people load EQ5, and
whether that is actually your underlying issue, or not.


Hemendra Rawat
 

How accurate does the polar alignment need to be for the goto? I'm
just doing visual astronomy.

On Thu, Jul 15, 2021 at 2:16 PM Khalid Baheyeldin <kbahey@gmail.com> wrote:

On Thu, Jul 15, 2021 at 04:55 PM, Hemendra Rawat wrote:

My pulley ratio is 2.5 : 1

That is a common enough configuration for EQ5 mounts, so you are not
alone. There are a few hundreds of these conversions around.

So it should work, though not sure how much people load EQ5, and
whether that is actually your underlying issue, or not.


johnwashington1978@...
 

For visual +/- 1deg really, assuming your target falls within the triangle drawn by the 3 star alignment.

Where are your clutch knobs in relation to the hard stops, are they hitting the stops before the clutch really bites?  With the clutches done up it should take an uncomfortable amount of force (tripod moving, tube flexing) to get them to slip.

The other thing that's really critical is balance, on both axis, given the weight of your scope it'll need to be close to perfect on an EQ5 - with both clutches undone no matter which direction you orientate your scope, either side of the Meridien it should not move under gravity - there are a number of good youtube videos on how to achieve this.