How do I confirm that RA tracks the sky properly and is not fast/slow? And how do I tune it?


Sam
 

Hi, I've built a MiniPCB v2 on a Vixen GP.

I have a rough polar alignment (Can't see Polaris) with a compass, some drilled holes in the ground, and latitude scale on the mount.
When I sync to a star, using a 114mm f/8 Newtonian with 20mm EP, I find that within 15 mins the star is out of view already.


Martin Chapman
 

If you you use SharpCap pro, (not free but very cheap), it has an excellent polar alignment feature built in that takes less than 20 minutes to achieve excellent PA.
Take care and stay safe.
Mart.


On Tue, Aug 24, 2021 at 4:07 AM Sam <sfac130@...> wrote:

Hi, I've built a MiniPCB v2 on a Vixen GP.

I have a rough polar alignment (Can't see Polaris) with a compass, some drilled holes in the ground, and latitude scale on the mount.
When I sync to a star, using a 114mm f/8 Newtonian with 20mm EP, I find that within 15 mins the star is out of view already.


Sam
 

Given that I have good alignment, how do I make sure that tracking in RA has the correct speed and how do I correct it?


Thepowersellersunion@...
 

I have found easiest way to test is hook up your camera and take a picture. Then read the star trails. I'd it's on all will be aligned
 


JOAN
 

Probably you mount has not an accurate pollar alignment.
I reccomend you NINA. It is an astrophotography software that runs on Windows 10. It is free. It reccently has a plugin  that permits to polar align by platesolving photos shot to any place of the sky.
You just need a camera. Either a DSRL works fine.
More information:
https://youtu.be/QnOjbJ0oWVE
Excuse me . My english is not good enough.


Khalid Baheyeldin
 

Try to do a 3 star alignment using OnStep, and center each star as best as you can.

After you do that, wait for a few minutes, then read the alignment errors in Alt and Az
(the Android app can show you these).

It could also be that your polar alignment is not the source of your troubles. It may be
that you have a mismatch of the mechanical parts (number of teeth, gear ratios, ..etc)
and what is in the spreadsheet and/or Config.h.


davel
 

Here's a polar alignment routine that a couple of astro imagers have recommended: https://www.cwjames.info/astro/howto/polar_alignment_ccd.htm


davel
 

And another idea: https://www.cloudynights.com/articles/cat/articles/new-approach-for-polar-alignment-r3204

Noto Bene: I'm NOT an imager, and I use the "Kochab Clock" method on my CGEM (6" Meade f/8 achromat): http://www.thestardeckobservatory.com/Polaralign.html It's good for an evening of observing, and could even be adequate for short (<5 min.) exposures; however, as I declaimed, I'm NOT an imager.


Khalid Baheyeldin
 

This is all good info, but before the original posted definitively concluding that polar alignment is where his problem is, we need to rule out a mismatch between the mechanical and firmware parameters.

For example, if the steps per degree (based on GR1, GR2, motor steps, microsteps) are different from what the gear reductions do, there will be some offset.


Sam
 

On Tue, Aug 24, 2021 at 04:30 PM, Khalid Baheyeldin wrote:
This is all good info, but before the original posted definitively concluding that polar alignment is where his problem is, we need to rule out a mismatch between the mechanical and firmware parameters.

For example, if the steps per degree (based on GR1, GR2, motor steps, microsteps) are different from what the gear reductions do, there will be some offset.
I attached my config.h. I followed the spread sheet as much as possible (144t RA and DEC on Vixen GP, 48t attached to worm, 16t attached to motor, 1.50 desired base slew rate.)


Khalid Baheyeldin
 

Your calculations are correct, assuming the motor is a 400 step motor.

What is the current rating of your motors? Usually, they are either 0.9A or 1.68A.

But the IRUN and IGOTO that you are using are not calculated for either.

0.9 x 1.41 x 0.35 = 444 mA
1.68 x 1.41 x 0.35 = 829 mA

That is usually for tracking. IGOTO may need to be more, e.g. 0.5 or 0.55 instead of 0.35.

Do you see the motors stopping and making a high pitch noise during a slew? If so, that may
be your issue.

Are you sure all the pulleys and belts are GT2, and not MXL? Mixing types causes real issues.


Sam
 

On Wed, Aug 25, 2021 at 01:57 PM, Khalid Baheyeldin wrote:
Your calculations are correct, assuming the motor is a 400 step motor.
Its the recommended 400 step 17HM15-0904S

What is the current rating of your motors? Usually, they are either 0.9A or 1.68A.
0.9A

But the IRUN and IGOTO that you are using are not calculated for either.

0.9 x 1.41 x 0.35 = 444 mA
1.68 x 1.41 x 0.35 = 829 mA

That is usually for tracking. IGOTO may need to be more, e.g. 0.5 or 0.55 instead of 0.35.
I started with 400mA and settled with 600mA.

Do you see the motors stopping and making a high pitch noise during a slew? If so, that may
be your issue.
I don't experience that issue.

Are you sure all the pulleys and belts are GT2, and not MXL? Mixing types causes real issues.it
The marking is "146-2GT", bought it from AliExpress on the same store many here recommend/have used.


Khalid Baheyeldin
 

Then, it looks like none of my suggestions are your underlying problem.

The next step is polar alignment. Try to do a 3 star align and tell us
what the Alt an Az errors reported in the Android app.

There is also the Refine PA function in the Wiki which can help you
correct the errors in Alt and Az after a 3 star align.


Khalid Baheyeldin
 

Another guess: are you sure that your tracking rate is Sidereal or Refraction?
Sometimes a person will set it to Lunar, then forget to set it back. Or sometimes
adjust the tracking rate by the +/- buttons and forget to reset it back.


Sam
 

On Wed, Aug 25, 2021 at 02:28 PM, Khalid Baheyeldin wrote:
Another guess: are you sure that your tracking rate is Sidereal or Refraction?
Sometimes a person will set it to Lunar, then forget to set it back. Or sometimes
adjust the tracking rate by the +/- buttons and forget to reset it back.

As a default, should it be sidereal or refraction?
I haven't fiddled with either the Lunar, or +/- buttons. I just do 2 star alignment and synced to a star.

 

I'll do the 3 star alignment later, clouds allowing, as it is currently 0500 in the morning where I am in.


Khalid Baheyeldin
 

On Wed, Aug 25, 2021 at 05:43 PM, Sam wrote:

As a default, should it be sidereal or refraction?

Default is Sidereal. Refraction provides a bit better tracking, but if you are autoguiding, there is not much noticeable difference.

I haven't fiddled with either the Lunar, or +/- buttons. I just do 2 star alignment and synced to a star.

I'll do the 3 star alignment later, clouds allowing, as it is currently 0500 in the morning where I am in.

Two is not enough. Three are needed to correct gotos for alignment errors.
By doing 3 and letting us know the errors we can tell if polar alignment could be the source of the problem or something else.

There is also scope balance, both in RA and DEC. Bad balance can speed or slow tracking.


Sam
 

On Wed, Aug 25, 2021 at 02:49 PM, Khalid Baheyeldin wrote:
morning where I am in.Two is not enough. Three are needed to correct gotos for alignment errors.
By doing 3 and letting us know the errors we can tell if polar alignment could be the source of the problem or something else.
I have a somewhat limited view and selection of stars (Bortle 8). I'll try reporting later what the error numbers are.

There is also scope balance, both in RA and DEC. Bad balance can speed or slow tracking.
Everything is well-balanced as far as I know.


HenkSB
 

If you have an EQ mount, any errors with RA tracking will result in star trails in the same direction, most easily seen on a camera image.  If the error is due to polar misalignment the trails will be in different directions for different areas in the sky.