leveling the RAPAS on Mach2


weihaowang
 

Hi,

I understand that Mach2 can be used without leveling the tripod. This is great for us, since our tripod cannot be adjusted and therefore cannot be leveled.  However, this will create a problem for the polar alignment.  We purchased an RAPAS, and it's my understanding that using it requires the tripod to be leveled.  I can of course use a PoleMaster or drift alignment without worrying about leveling, but I like the quickness of a polar scope in the field.  So I want to find a solution.

A question here is whether I can assume the top surface of Mach2's RA motor box to be perfectly aligned to RAPAS?  

If the answer is yes, then a simple workaround would be to use my iPhone's inclinometer on the top surface of the RA motor box to measure the tilt, then do the necessary compensation in RAPAS.  I think iPhone's inclinometer is good to a degree.  Given the distance between Polaris and the pole, a 1-degree error in leveling would translate to 1 arc-minute of polar alignment error, which should be good enough for my imaging.

How do you think about the method above?  Or do you have any other suggestions on using Mach2+RAPAS on a unleveled tripod?

Cheers,
Wei-Hao

--

Homepage:

http://www.asiaa.sinica.edu.tw/~whwang/

Astrobin gallery:
http://www.astrobin.com/users/whwang/


Christopher Erickson
 

I would worry more about load instability on an unlevel tripod than polar alignment accuracy. Having the RAPAS rotated ever-so-slightly will still give you a rather precise polar alignment. And if you are imaging, you probably want to follow up with drift alignment anyway.

I hope this helps.

-Christopher Erickson
Observatory engineer
Waikoloa, HI 96738
www.summitkinetics.com
   

On Tue, Dec 22, 2020, 8:01 AM Terri Zittritsch <theresamarie11@...> wrote:

Wei Hao,
I don't believe you need to have a leveled tripod to use RAPAS.  It's only aligning the RA axis to the pole.    So as long as you have the adjustability to do this, with your altitude and azimuth adjusters...      I hope I'm not misunderstanding your question.

I have a RAPAS and polemaster.   I used my polemaster alignment (which is very good) to adjust my rapas.  I purchased the rapas to use in the field when I may not have a computer.    


best,

Terri


weihaowang
 

Hi Christopher,

Thank you!  My tripod is the TAK wood tripod, which cannot be adjusted.  In the past few years, I put heavy TAK stuff on it and there were no stability issues.  Of course I don't put it on a very tilted ground to start with.  So here I am just looking for ways to overcome a tilt of a couple degrees.  (The TAK polar scope comes with a ring level on itself, so the tripod does not need to be leveled.)

I was never a fan of drift alignment, primarily because I am doing portable imaging and every minute under dark sky is precious. Every drift alignment method I had tried took too long.   (I know polar alignment can be done during twilight.  Unfortunately I live in the tropical region, where twilight is very short.)  On the other hand, in the past, a good alignment with a polar scope worked quite well and it took just one or two minutes each time.  Very often I see long term drift during guiding, but it's quite manageable as long as the mount can perform the guiding correction.  So here I am trying to see if I can do the same with Mach2 (unadjustable tripod, polar scope alignment, no drift).

I saw you live in Waikoloa.  Nice place.  I once lived in the Waikoloa village when I was a visiting astronomer at CFHT.  When I had time, I drove up to MKVIS and imaged there. I no longer live in Hawaii, but I came back to conduct MK summit observations from time to time. I really miss the sky there. When the pandemic is gone and when we are free to travel again, I want to come back to Hawaii and see the stars there, perhaps with our Mach2.

Cheers,
Wei-Hao

--

Homepage:

http://www.asiaa.sinica.edu.tw/~whwang/

Astrobin gallery:
http://www.astrobin.com/users/whwang/


ernie.mastroianni@...
 

My thinking has been that if the RAPAS is properly calibrated to the Mach2's RA axis, then a non-leveled tripod won't make a difference. Once the polar axis is precisely aligned, it does not care what is going on below. In the past, the difficulty from a non-level mount presents itself when trying to drift align. Adjustments in alt and az are intertwined. You'd really need a level tripod to calibrate the RAPAS, but perhaps once that is done, you should be fine.

But I may be mistaken in this notion as I don't fully understand how the absolute encoders work. Perhaps an azimuth tilt in an otherwise precisely aligned polar axis would affect how the Mach2 perceives its position. Would the ultra precise Home Position then be off?

Ernie


W Hilmo
 

In terms of making the adjustments to polar align the mount, being level matters very little.  The only effect is that an adjustment to one axis might slightly affect the other, but as long as you put Polaris in the right spot, you are correctly polar aligned.

 

The possible issue with being unlevel in east/west, is that the clock angle of the reticule might be slightly off.  My thought off the top of my head, is that you would need to be very unlevel for this to make a meaningful difference.  In thinking it through, I think that the actual effect of this is latitude dependent.  For example, if you are at the equator, if your mount is tilted two degrees towards the west, the reticule would also be rotated two degrees west.  But if you are at the pole, the reticule would be at the correct clock angle, no matter how much the mount is tilted.  You could use some trigonometry to calculate this for other latitudes.

 

In practice, I maintain that this is simply not an issue.  I tend to image between 40 and 48 degrees north, depending on which site I’m using.  When I set up the mount, I do use a carpenter’s level to level the pier before I put the mount on it.  My polar alignment routine is to do the daytime polar alignment when I set up the mount, and then use the RAPAS to touch up the polar alignment at dusk.  The purpose of the daytime polar alignment is to get close enough that Polaris is unmistakable in the RAPAS reticule.  It also gives me a chance to sanity check my compass work that I have sufficient azimuth adjustment to complete the polar alignment (if I have to turn the entire setup a few degrees, I would rather do it during the day).  If you add up the total time that I spend on polar alignment, including both the daytime work and using the RAPAS, it’s probably around two to three minutes.

 

When I first got the RAPAS, I did a drift alignment and adjusted the RAPAS accordingly.  It was already very close, so it was a tiny adjustment.  In terms of performance, I always image from dusk to dawn.  As a normal part of my processing, I always blink through my stack of sub exposures.  In doing this, any field rotation through the night would be pretty obvious.  With the above routine, I never see any rotation, even if I blink the first and last exposures against each other.  I don’t know of a better validation of the polar alignment.

 

That’s why I say that the RAPAS works fine and is easily sufficient for any imaging that you want to do.  Don’t worry about it.

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of ernie.mastroianni@...
Sent: Wednesday, December 23, 2020 5:46 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] leveling the RAPAS on Mach2

 

My thinking has been that if the RAPAS is properly calibrated to the Mach2's RA axis, then a non-leveled tripod won't make a difference. Once the polar axis is precisely aligned, it does not care what is going on below. In the past, the difficulty from a non-level mount presents itself when trying to drift align. Adjustments in alt and az are intertwined. You'd really need a level tripod to calibrate the RAPAS, but perhaps once that is done, you should be fine.

But I may be mistaken in this notion as I don't fully understand how the absolute encoders work. Perhaps an azimuth tilt in an otherwise precisely aligned polar axis would affect how the Mach2 perceives its position. Would the ultra precise Home Position then be off?

Ernie


weihaowang
 

Hi,

Perhaps as you said, it doesn't matter that much.  But I really want to make things right, and I hope someone can answer me about the squareness between the top surface of the RA motor box and the polar scope.  If this is the case, then an inclinometer in a cell phone can easily solve the problem.

And as Hilmo said, it is exactly the wrong clock angle that concerns me.  Polaris is roughly 0.8 degree from the pole.  An error of 2 degrees (or 8 minutes) in the clock angle (a 2 degree tilt of the tripod) translates to 1.7 arcmin of polar alignment error.  For guided exposure of less than 10 minutes with a guide-star inside the FoV of a few degrees, the 1.7 arcmin of polar alignment is probably OK and will not introduce observable field rotation within the single exposures.  However, the polar alignment error introduced by the tripod tilt is just one of the error budgets.  There can be other polar errors, and all the errors can add up. So I want to do my best to control the error associated with the tripod tilt.

Cheers,
Wei-Hao

--

Homepage:

http://www.asiaa.sinica.edu.tw/~whwang/

Astrobin gallery:
http://www.astrobin.com/users/whwang/


W Hilmo
 

To be clear, when I am talking about blinking images, I am not talking about evaluating a single 10 minute exposure.  I am blinking the first exposure of the session with the last.  They are separated by many hours.  I am not seeing field rotation over the entire night, and I am imaging at about 0.5 arc seconds per pixel.

 

I understand and appreciate your interest in the theoretical aspects.  I wanted to comment that they are just that…theoretical.  In actual practice, the RAPAS gives excellent results.

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of weihaowang
Sent: Wednesday, December 23, 2020 7:36 AM
To: main@ap-gto.groups.io
Subject: Re: [ap-gto] leveling the RAPAS on Mach2

 

Hi,

Perhaps as you said, it doesn't matter that much.  But I really want to make things right, and I hope someone can answer me about the squareness between the top surface of the RA motor box and the polar scope.  If this is the case, then an inclinometer in a cell phone can easily solve the problem.

And as Hilmo said, it is exactly the wrong clock angle that concerns me.  Polaris is roughly 0.8 degree from the pole.  An error of 2 degrees (or 8 minutes) in the clock angle (a 2 degree tilt of the tripod) translates to 1.7 arcmin of polar alignment error.  For guided exposure of less than 10 minutes with a guide-star inside the FoV of a few degrees, the 1.7 arcmin of polar alignment is probably OK and will not introduce observable field rotation within the single exposures.  However, the polar alignment error introduced by the tripod tilt is just one of the error budgets.  There can be other polar errors, and all the errors can add up. So I want to do my best to control the error associated with the tripod tilt.

Cheers,
Wei-Hao

--

Homepage:

http://www.asiaa.sinica.edu.tw/~whwang/

Astrobin gallery:
http://www.astrobin.com/users/whwang/


Jeffc
 

Fwiw...  I use an AP portable pier which does not have level adjustment.   I’m more concerned with weight imbalance but I also prefer a level pier when imagining.   

So... I level the pier with a plastic bubble level (or iPhone measure app) and some thin hardwood “shims” under one (or two) of legs as needed.  

-jeff 


Roland Christen
 


But I may be mistaken in this notion as I don't fully understand how the absolute encoders work. Perhaps an azimuth tilt in an otherwise precisely aligned polar axis would affect how the Mach2 perceives its position. Would the ultra precise Home Position then be off?
Yes, the encoders do not have tilt sensors to compensate for an out of level setup. So for a 1 degree tilt E-W you will have a pointing error of 1 degree in RA when referencing any sky object to the absolute home position (this is normal for any mount that has an absolute Home). However, once you center and recal on any sky object, all other objects will be referenced from this recal position, not from the Home position. Therefore in reality the tilt error will be compensated for all subsequent slews to any object on either side of the meridian. That assumes you don't have an orthogonal error in your optical train - but that can also be compensated by using our ortho fix in the keypad, or a quick model in APCC Pro.-

Rolando


-----Original Message-----
From: ernie.mastroianni@...
To: main@ap-gto.groups.io
Sent: Wed, Dec 23, 2020 7:46 am
Subject: Re: [ap-gto] leveling the RAPAS on Mach2

My thinking has been that if the RAPAS is properly calibrated to the Mach2's RA axis, then a non-leveled tripod won't make a difference. Once the polar axis is precisely aligned, it does not care what is going on below. In the past, the difficulty from a non-level mount presents itself when trying to drift align. Adjustments in alt and az are intertwined. You'd really need a level tripod to calibrate the RAPAS, but perhaps once that is done, you should be fine.

But I may be mistaken in this notion as I don't fully understand how the absolute encoders work. Perhaps an azimuth tilt in an otherwise precisely aligned polar axis would affect how the Mach2 perceives its position. Would the ultra precise Home Position then be off?

Ernie

--
Roland Christen
Astro-Physics


Roland Christen
 


But I really want to make things right, and I hope someone can answer me about the squareness between the top surface of the RA motor box and the polar scope.
The top surface of the motor box and the polar scope are machined to be square with respect to the bottom surface of the mount.

Rolando


-----Original Message-----
From: weihaowang <whwang@...>
To: main@ap-gto.groups.io
Sent: Wed, Dec 23, 2020 9:35 am
Subject: Re: [ap-gto] leveling the RAPAS on Mach2

Hi,

Perhaps as you said, it doesn't matter that much.  But I really want to make things right, and I hope someone can answer me about the squareness between the top surface of the RA motor box and the polar scope.  If this is the case, then an inclinometer in a cell phone can easily solve the problem.

And as Hilmo said, it is exactly the wrong clock angle that concerns me.  Polaris is roughly 0.8 degree from the pole.  An error of 2 degrees (or 8 minutes) in the clock angle (a 2 degree tilt of the tripod) translates to 1.7 arcmin of polar alignment error.  For guided exposure of less than 10 minutes with a guide-star inside the FoV of a few degrees, the 1.7 arcmin of polar alignment is probably OK and will not introduce observable field rotation within the single exposures.  However, the polar alignment error introduced by the tripod tilt is just one of the error budgets.  There can be other polar errors, and all the errors can add up. So I want to do my best to control the error associated with the tripod tilt.

Cheers,
Wei-Hao

--
Homepage:

--
Roland Christen
Astro-Physics


Harley Davidson
 

Jeff

Could you try this:
Leveling a Telescope Tripod by Shimming it (HOW TO)
https://www.youtube.com/watch?v=AjHrB34E0Rs

This is my video. I use this technique all the time. I just find it easier even though I can adjust the legs position. And I only concern myself with East West level.

tony

On 12/23/2020 12:00 PM, Jeffc wrote:
Fwiw...  I use an AP portable pier which does not have level adjustment.   I’m more concerned with weight imbalance but I also prefer a level pier when imagining.   

So... I level the pier with a plastic bubble level (or iPhone measure app) and some thin hardwood “shims” under one (or two) of legs as needed.  

-jeff 


Don Anderson
 

The AP portable pier has quite a bit of adjustment range using the turnbuckles for leveling the top plate.

Don Anderson


On Wednesday, December 23, 2020, 10:00:40 a.m. MST, Jeffc <jeffcrilly@...> wrote:


Fwiw...  I use an AP portable pier which does not have level adjustment.   I’m more concerned with weight imbalance but I also prefer a level pier when imagining.   

So... I level the pier with a plastic bubble level (or iPhone measure app) and some thin hardwood “shims” under one (or two) of legs as needed.  

-jeff 


weihaowang
 

Thank you, Roland.  This solves my problem.


Jeffc
 



On Dec 23, 2020, at 10:21 AM, Don Anderson via groups.io <jockey_ca@...> wrote:


The AP portable pier has quite a bit of adjustment range using the turnbuckles for leveling the top plate.


Don - I was going to mention the slight leveling capability, but it is not relevant to the OP (fixed wood Tak tripod) and is tangential to the topic.

Fwiw, I find using the turnbuckles only gives me a tiny amount of movement.  
In the 15 years I’ve been setting up the portable pier w/ AP1200 at a location for public outreach the pavement is never flat enough and I need a shim.   I’ve given up on the turnbuckles, and just shim it.  Much faster setup time. 

Don Anderson


On Wednesday, December 23, 2020, 10:00:40 a.m. MST, Jeffc <jeffcrilly@...> wrote:


Fwiw...  I use an AP portable pier which does not have level adjustment.   I’m more concerned with weight imbalance but I also prefer a level pier when imagining.   

So... I level the pier with a plastic bubble level (or iPhone measure app) and some thin hardwood “shims” under one (or two) of legs as needed.  

-jeff