How Much RAPAS Misalignment is Too Much ?


M Hambrick
 

I have been getting a lot of what I suspect is field rotation in my guided images so I thought I would check my polar alignment using the RA Correction method as described in the manuals. I have a portable setup with an 1100 GTO mount that I set up and take down every night, and I set my polar alignment using the RAPAS. 

At the end of the RA Correction process I checked the position of Polaris in the RAPAS and compared it to what the Polar Align App said it should be. The results are in the sketches below.

I am assuming that if I am going to adjust the RAPAS, I will need to adjust the push pull screws in the plate at the base of the RA axis where the RAPAS attaches, but before I do this I wanted to make sure that this is not an irreversible process. I am worried that if I suddenly start adjusting the push-pull screws that I will do more harm than good.

Is the RA Correction procedure reliable enough that I should trust these results ? I think that at a minimum I will repeat the alignment procedure before I start adjusting anything.

The offset as seen seems in the sketches seems excessive, and I am also wondering what could cause the RAPAS alignment to get so far off, assuming that it was correct when I first got the mount. To my knowledge I have never dropped or damaged the RAPAS or the mount.

Any comments or advice would be appreciated.

Mike


ap@CaptivePhotons.com
 

FWIW I checked mine once after doing a fairly careful align photographically and mine was off about the same amount, maybe a tad less (different directions).

 

Of course, no alignment is perfect so it’s hard to know that the RAPAS was the one off, but just as a data point.

 

I put mine back in the case and just do imaging alignment.  It’s on my list to one day try to align properly. 

 

What is “normal” for an out of the box setup?

 

Linwood

 

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of M Hambrick via groups.io
Sent: Friday, September 24, 2021 12:11 PM
To: main@ap-gto.groups.io
Subject: [ap-gto] How Much RAPAS Misalignment is Too Much ?

 

I have been getting a lot of what I suspect is field rotation in my guided images so I thought I would check my polar alignment using the RA Correction method as described in the manuals. I have a portable setup with an 1100 GTO mount that I set up and take down every night, and I set my polar alignment using the RAPAS. 

At the end of the RA Correction process I checked the position of Polaris in the RAPAS and compared it to what the Polar Align App said it should be. The results are in the sketches below.

I am assuming that if I am going to adjust the RAPAS, I will need to adjust the push pull screws in the plate at the base of the RA axis where the RAPAS attaches, but before I do this I wanted to make sure that this is not an irreversible process. I am worried that if I suddenly start adjusting the push-pull screws that I will do more harm than good.

Is the RA Correction procedure reliable enough that I should trust these results ? I think that at a minimum I will repeat the alignment procedure before I start adjusting anything.

The offset as seen seems in the sketches seems excessive, and I am also wondering what could cause the RAPAS alignment to get so far off, assuming that it was correct when I first got the mount. To my knowledge I have never dropped or damaged the RAPAS or the mount.

Any comments or advice would be appreciated.

Mike


George
 

Mike,

 

Yes, use the push-pull screws to adjust the rear plate.    You can always restore the plate to flush and do it again.

 

Regards,

 

George

 

George Whitney

Astro-Physics, Inc.

Phone:  815-222-6538 (direct line)

Phone:  815-282-1513 (office)

Email:  george@...

 

From: main@ap-gto.groups.io <main@ap-gto.groups.io> On Behalf Of M Hambrick
Sent: Friday, September 24, 2021 11:11 AM
To: main@ap-gto.groups.io
Subject: [ap-gto] How Much RAPAS Misalignment is Too Much ?

 

I have been getting a lot of what I suspect is field rotation in my guided images so I thought I would check my polar alignment using the RA Correction method as described in the manuals. I have a portable setup with an 1100 GTO mount that I set up and take down every night, and I set my polar alignment using the RAPAS. 

At the end of the RA Correction process I checked the position of Polaris in the RAPAS and compared it to what the Polar Align App said it should be. The results are in the sketches below.

I am assuming that if I am going to adjust the RAPAS, I will need to adjust the push pull screws in the plate at the base of the RA axis where the RAPAS attaches, but before I do this I wanted to make sure that this is not an irreversible process. I am worried that if I suddenly start adjusting the push-pull screws that I will do more harm than good.

Is the RA Correction procedure reliable enough that I should trust these results ? I think that at a minimum I will repeat the alignment procedure before I start adjusting anything.

The offset as seen seems in the sketches seems excessive, and I am also wondering what could cause the RAPAS alignment to get so far off, assuming that it was correct when I first got the mount. To my knowledge I have never dropped or damaged the RAPAS or the mount.

Any comments or advice would be appreciated.

Mike


Roland Christen
 


Is the RA Correction procedure reliable enough that I should trust these results ?
It is not a reliable process for precision polar alignment. It was meant for quick setups when no other means are available.If you have any non-orthogonality in your telescope optics, then any method using your telescope will not be accurate.

When you say field rotation, over how many hours are you experiencing this? Also, how do you determine that it is field rotation? If you are seeing field rotation over a long time period, say 6 hours from beginning to end in a wide field image, that is natural because the skies do not follow an exact sidereal path due to atmospheric refraction.

Roland

-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Fri, Sep 24, 2021 11:10 am
Subject: [ap-gto] How Much RAPAS Misalignment is Too Much ?

I have been getting a lot of what I suspect is field rotation in my guided images so I thought I would check my polar alignment using the RA Correction method as described in the manuals. I have a portable setup with an 1100 GTO mount that I set up and take down every night, and I set my polar alignment using the RAPAS. 

At the end of the RA Correction process I checked the position of Polaris in the RAPAS and compared it to what the Polar Align App said it should be. The results are in the sketches below.

I am assuming that if I am going to adjust the RAPAS, I will need to adjust the push pull screws in the plate at the base of the RA axis where the RAPAS attaches, but before I do this I wanted to make sure that this is not an irreversible process. I am worried that if I suddenly start adjusting the push-pull screws that I will do more harm than good.

Is the RA Correction procedure reliable enough that I should trust these results ? I think that at a minimum I will repeat the alignment procedure before I start adjusting anything.

The offset as seen seems in the sketches seems excessive, and I am also wondering what could cause the RAPAS alignment to get so far off, assuming that it was correct when I first got the mount. To my knowledge I have never dropped or damaged the RAPAS or the mount.

Any comments or advice would be appreciated.

Mike


--
Roland Christen
Astro-Physics


Robert Berta
 

I used PEM PRO to polar align my newer 1100 and older 900 mounts. I than insert the RAPAS and zero it to agree by placing Polaris on the correct location on the RAPAS dial (from the app) by adjusting the three tlt points on the RAPAS adaptor. Once I have done that it seems that should than be more than accurate enough for polar alignment to just trust the RAPAS.....or is it still better to do a PEM PRO polar alignment each time?  I do get excellent auto guiding so suspect that I may be getting a bit to anal about this whole process ;-) 


Roland Christen
 

My mount is not very well polar aligned and I image for hours with no problems.

Rolando



-----Original Message-----
From: Robert Berta <biker123@...>
To: main@ap-gto.groups.io
Sent: Fri, Sep 24, 2021 2:23 pm
Subject: Re: [ap-gto] How Much RAPAS Misalignment is Too Much ?

I used PEM PRO to polar align my newer 1100 and older 900 mounts. I than insert the RAPAS and zero it to agree by placing Polaris on the correct location on the RAPAS dial (from the app) by adjusting the three tlt points on the RAPAS adaptor. Once I have done that it seems that should than be more than accurate enough for polar alignment to just trust the RAPAS.....or is it still better to do a PEM PRO polar alignment each time?  I do get excellent auto guiding so suspect that I may be getting a bit to anal about this whole process ;-) 

--
Roland Christen
Astro-Physics


M Hambrick
 

Hi Roland

I guess I am not sure what it is that I am seeing that I am calling field rotation. I have noticed that when I take a series of images, there is a shift in the position of the stars from the first to the last image. I decided to measure this shift using the information window in MaxIm DL to track the centroid location of the same star in successive images from first to last. The example below shows what I am seeing. I plotted the centroid coordinates over time for a series of images that I took.

In this particular example I was imaging M13. I took four 120 second subframes each of L, R, G, and B. The gaps in data are where I checked the focus whenever I switched filters, but I had the guider running continuously the whole time. You can see a relatively constant rate of change in each axis. The camera is oriented so that RA corresponds to the X-axis, and DEC corresponds to the Y-axis.

FYI I do not see any elongation in the stars in my guided images, even for 600 second subframes.

I would be grateful for any insight that you can give on this. I do not want to start messing around with the alignment screws on my RAPAS if it is not necessary. 

Mike


Roland Christen
 

This simple polar misalignment coupled with atmospheric refraction effects. It's not field rotation, just simple drift and the amount of drift you are getting is quite normal, between 25 and 50 arc sec per hour. I get this amount also, and it will be variable depending on where the object is with respect to the zenith. The further from the zenith, the more the drift, regardless of how well polar aligned you are.

You can certainly model and compensate for it. That's what i do, but it really doesn't affect the final images in any way, except to make guiding a bit better. In fact, I have a simple model for this area of the sky which reduces the drift from 80 arc sec per hour to about 2, which lets me image 20 minutes unguided.

Rolando



-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Fri, Sep 24, 2021 3:51 pm
Subject: Re: [ap-gto] How Much RAPAS Misalignment is Too Much ?

Hi Roland

I guess I am not sure what it is that I am seeing that I am calling field rotation. I have noticed that when I take a series of images, there is a shift in the position of the stars from the first to the last image. I decided to measure this shift using the information window in MaxIm DL to track the centroid location of the same star in successive images from first to last. The example below shows what I am seeing. I plotted the centroid coordinates over time for a series of images that I took.

In this particular example I was imaging M13. I took four 120 second subframes each of L, R, G, and B. The gaps in data are where I checked the focus whenever I switched filters, but I had the guider running continuously the whole time. You can see a relatively constant rate of change in each axis. The camera is oriented so that RA corresponds to the X-axis, and DEC corresponds to the Y-axis.

FYI I do not see any elongation in the stars in my guided images, even for 600 second subframes.

I would be grateful for any insight that you can give on this. I do not want to start messing around with the alignment screws on my RAPAS if it is not necessary. 

Mike


--
Roland Christen
Astro-Physics


M Hambrick
 

Thanks Roland.

It does not sound like I need to start messing around with the alignment screws on my RAPAS until I do a more rigorous drift alignment procedure.

Mike


Roland Christen
 

Do your drift alignment once when the skies are hazy and not good for imaging. Then adjust the Rapas. Use it for subsequent nights and don't obsess over polar alignment.

Rolando

-----Original Message-----
From: M Hambrick <mhambrick563@...>
To: main@ap-gto.groups.io
Sent: Fri, Sep 24, 2021 4:47 pm
Subject: Re: [ap-gto] How Much RAPAS Misalignment is Too Much ?

Thanks Roland.

It does not sound like I need to start messing around with the alignment screws on my RAPAS until I do a more rigorous drift alignment procedure.

Mike

--
Roland Christen
Astro-Physics