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Equipment Discussions >> Reflectors

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VĂ­ctor MartĂ­nez
sage


Reged: 02/05/07

Loc: Cádiz - Spain
Argo Navis alignment stars
      #6216675 - 11/26/13 08:54 AM

Hello,
I had always been understood that the Polar Star is a perfect star to align a Dobson telescope.The fact is that the last summer the pointing accuracy was quite high having used the Polar Star how first alignment star, This autum-winter changing the second alignment star (I can not remember which I used in summer), the accuracy was decreased enough. Last night I made an initial alignment with my 16 " f5 using the known parameters of ALT REF +090 000 Auto adjust on, and selecting Polaris and Altair as alignment stars (theoretically two stars meet the assumptions of separation and position in the sky) and the accuracy was mediocre to the east area, and acceptable to the west area ( Altair situation ). At 21.00 pm when Capella was high enough in the sky, I decided to align this time using this star and Deneb, and the situation changed dramatically. precision greatly improved, most of the objects were at midfield of a 12 mm Plössl reticle eyepiece (169x and 0.30ş), and from there everything went into the field of an Ethos 10mm (200X and 0'5ş), even with objects close enough to the zenith (sensitive area for Dobson) . Every time I have more clear that the stars used for the alignment are critical for the accurate pointing, whether other combinations may seem that meet the same requirements. Any more experience in this regard? .


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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6216688 - 11/26/13 09:02 AM

If you have an iPhone, have you tried Alt Az Align?

There's also Best Pair II.


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tomcody
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Reged: 07/06/08

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Re: Argo Navis alignment stars new [Re: Vic Menard]
      #6216736 - 11/26/13 09:29 AM

+1 for Alt-Az Align
Rex
P.S. its: ALTitude AZimuth ALIGNment when you search for it on the APP store.

Edited by tomcody (11/26/13 09:33 AM)


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VĂ­ctor MartĂ­nez
sage


Reged: 02/05/07

Loc: Cádiz - Spain
Re: Argo Navis alignment stars new [Re: tomcody]
      #6216801 - 11/26/13 09:57 AM

Thank you,
It's a pain, it looks an interesting software, but I use Android. The best pair II program already I knew and I have used it sometimes quite some time with equatorial mounts.


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6216845 - 11/26/13 10:18 AM

The RA of Polaris is 2h31m, close to the other fall/winter time stars you are choosing as a second alignment star. The more difference there is between RA's of the two alignments you choose, the better your alignment will be.

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Jeff Morgan
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Reged: 09/28/03

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Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6216903 - 11/26/13 10:45 AM

The last several months I have used Polaris and Alpheratz with good results, and last time (a couple of weeks back) I got good results using Polaris and Aldebaran.

Here are a few neat little tricks for ServoCAT users:

1) Chose your non-Polaris star FIRST. Reason: For 2-star alignment Argo will uses first-in first-out memory. So if you are not happy with the results from the first 2-star align, you can just pick a third star to align on and retain Polaris in the solution.

2) Defocus the star so that you are using the secondary shadow for alignment. Much easier!

3) Use a good illuminated reticle. I'm using the Celestron Micro Guide with excellent results.

4) Align the reticle east-west such that the star is drifting along the illuminated bar. Worth taking a few minutes for best precision.

5) Use the GOTO button on the ServoCAT hand controller for catching that exact moment the star is centered.

6) Move on to Polaris and take as much time as needed for precise centering.

Using a medium-powered eyepiece with a 18 arc minute field I am usually in the inner half of that field on GOTO jumps.


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VĂ­ctor MartĂ­nez
sage


Reged: 02/05/07

Loc: Cádiz - Spain
Re: Argo Navis alignment stars new [Re: Jeff Morgan]
      #6216961 - 11/26/13 11:13 AM

Peter:
On the first alignment, I used Polaris (2h 31 m) and Altair (19h 51m), are not far enough?.
Jeff,
I'm using a 12 mm plössl with illuminated reticle, and how you well say I do:
-Align the reticle east-west such that the star is drifting along the illuminated bar. Worth taking a few minutes for best precision.
-Use the GOTO button on the ServoCAT hand controller for catching that exact moment the star is centered.
-Move on to Polaris and take as much time as needed for precise centering. The next once I will do this, to take Polaris how second alignment star

Edited by Víctor Martínez (11/26/13 01:25 PM)


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Vic Menard
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Reged: 07/21/04

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Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6217295 - 11/26/13 02:05 PM

It's not so much the right ascension as the azimuth difference. Ideally, a line drawn between the two alignment stars should pass through the zenith (Az difference equals 180-degrees). You also want a good altitude difference, without using a star that's too close to the zenith (should be less than about 80-degrees Alt) or too close to the horizon (should be more than about 15-degrees Alt).

It's also important to remember that any orthogonality issues with the scope's movement and pointing axes (Az/Alt/optical) will impact the scope's pointing accuracy as the scope is moved away from the alignment stars. It's also very common to have pointing accuracy issues near the zenith and horizon.

Finally, if you're using 4000 count encoders, your built-in accuracy is about 1/10th of a degree. So if you're consistently 1 or 2 tenths of a degree off-center, there's not much you can do that will guarantee better performance (there's always the possibility of a "lucky" alignment, where the errors add up in your favor and all your selected targets end up centered or nearly so).


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VĂ­ctor MartĂ­nez
sage


Reged: 02/05/07

Loc: Cádiz - Spain
Re: Argo Navis alignment stars new [Re: Vic Menard]
      #6217662 - 11/26/13 05:18 PM

Vic,
I am using 10000 count encoders. You say:
"It's also important to remember that any orthogonality issues with the scope's movement and pointing axes (Az/Alt/optical) will impact the scope's pointing accuracy as the scope is moved away from the alignment stars".Well, but these issues should be present regardless of the selected alignment stars, isn't it?.
"Ideally, a line drawn between the two alignment stars should pass through the zenith (Az difference equals 180-degrees)". According to this criterion the Polaris should not be an ideal alignment stars.
Thank you.


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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6217854 - 11/26/13 06:52 PM

Quote:

...I am using 10000 count encoders.



I should pay more attention to the title of the thread. Your Argo will utilize those 10,000 count encoders to extrapolate the best possible pointing accuracy.

Quote:

...these (orthogonality) issues should be present regardless of the selected alignment stars...



Yes, they are. My point was that there is more than just alignment star selection that can impact accuracy. Again, your Argo Navis is capable of modeling orthogonality issues utilizing a TPAS run, which can reduce the impact of the error.

Quote:

"Ideally, a line drawn between the two alignment stars should pass through the zenith (Az difference equals 180-degrees)". According to this criterion the Polaris should not be an ideal alignment stars.



Polaris is (approximately) due north, so any star that is due south will have an azimuth difference of 180-degrees.


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Argo Navis alignment stars new [Re: Vic Menard]
      #6217975 - 11/26/13 07:56 PM

One thing that has troubled me with regards to performing a two-star alignment with Dobs is the shift in altitude of the entire telescope as you quickly engage the ServoCAT's altitude drive to the altitude bearing after the second star is aligned at the eyepiece. At the eyepiece, I defocus the star in a 50° afov eyepiece at ~200X to center the star more accurately. Since the second star is moving (not Polaris) across the eyepiece fov, quickly engaging the altitude drive is important to keep alignment accuracy. With my past Starmaster Dob's and currently with my StarStructure Dob, engaging the altitude bearing to the drive lifts the entire telescope a bit so that at the eyepiece it moves the second aligned star almost out of the field of view - approx 1/4 degree at 200X. This effects all following go-to actions by positioning objects off center by that same 1/4 degree, in addition to what ever else went wrong with the two-star alignment. The pointing errors are all cumulative.

Quote:

It's not so much the right ascension as the azimuth difference. Ideally, a line drawn between the two alignment stars should pass through the zenith (Az difference equals 180-degrees). You also want a good altitude difference, without using a star that's too close to the zenith (should be less than about 80-degrees Alt) or too close to the horizon (should be more than about 15-degrees Alt).

It's also important to remember that any orthogonality issues with the scope's movement and pointing axes (Az/Alt/optical) will impact the scope's pointing accuracy as the scope is moved away from the alignment stars. It's also very common to have pointing accuracy issues near the zenith and horizon.

Finally, if you're using 4000 count encoders, your built-in accuracy is about 1/10th of a degree. So if you're consistently 1 or 2 tenths of a degree off-center, there's not much you can do that will guarantee better performance (there's always the possibility of a "lucky" alignment, where the errors add up in your favor and all your selected targets end up centered or nearly so).




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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6217995 - 11/26/13 08:07 PM

Over the summer. I was getting good results with my two-star alignment using Polaris and Altair with GoTo objects between these two stars.

This is what I have noticed about ServoCAT and Argo Navis one my current Dob: that the GoTo operation will be most accurate for finding objects 'between' the two alignment stars chosen and that the two stars shouldn't be more than ~100° apart nor any of them too close to the horizon. The more the telescope is asked to find an object away from the area between the two alignment stars, the more pointing errors pile up until you are better off using your optical finder. If you choose alignment stars within one quadrant of the sky (<90°x90°) and decide to look at objects outside of that area perhaps in another quadrant, then it's best to perform a new two-star alignment with new stars that are well-positioned in that area. Because if this, my procedure in observing for the evening is by GoTo'ing to objects in a constellation-by-constellation basis so that finding the objects are well 'within' the two alignment stars chosen.

Quote:

Peter:
On the first alignment, I used Polaris (2h 31 m) and Altair (19h 51m), are not far enough?.
Jeff,
I'm using a 12 mm plössl with illuminated reticle, and how you well say I do:
-Align the reticle east-west such that the star is drifting along the illuminated bar. Worth taking a few minutes for best precision.
-Use the GOTO button on the ServoCAT hand controller for catching that exact moment the star is centered.
-Move on to Polaris and take as much time as needed for precise centering. The next once I will do this, to take Polaris how second alignment star




Edited by Peter Natscher (11/26/13 08:13 PM)


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Vic Menard
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Reged: 07/21/04

Loc: Bradenton, FL
Re: Argo Navis alignment stars new [Re: Peter Natscher]
      #6218060 - 11/26/13 08:37 PM

Quote:

One thing that has troubled me with regards to performing a two-star alignment with Dobs is the shift in altitude of the entire telescope as you quickly engage the ServoCAT's altitude drive to the altitude bearing after the second star is aligned at the eyepiece. At the eyepiece, I defocus the star in a 50° afov eyepiece at ~200X to center the star more accurately.



For 50-degree AFOV at 200X, TFOV = 0.25-degree (I normally use a 100-degree AFOV at 200X with TFOV = 0.5-degree).

Quote:

Since the second star is moving (not Polaris) across the eyepiece fov, quickly engaging the altitude drive is important to keep alignment accuracy.



It's not necessary to engage the altitude drive to select the second alignment star--you can even enter from the hand pad. I normally do a quick "manual" goto on a known object to verify the two-star alignment before I engage the ServoCAT clutches.

Quote:

With my past Starmaster Dob's and currently with my StarStructure Dob, engaging the altitude bearing to the drive lifts the entire telescope a bit so that at the eyepiece it moves the second aligned star almost out of the field of view - approx 1/4 degree at 200X.



That should be a little less than 1/8 degree (about 0.1-degree) for a 1/4-degree TFOV eyepiece. I see some similar, small movement with my StarStructure (it could be 0.1-degree), but after I've verified the target centering manually (as I described above), when I press the GoTo button on my hand pad the target always moves back to the center of the FOV as the Argo Navis read out re-zeroes on the target (this is all part of my normal Argo Navis/ServoCAT "setup and verify" procedure). I don't understand why your scope behaves differently, perhaps it's because you're engaging the altitude drive before you press enter on the Argo Navis? Or have I misunderstood your actual procedure?


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Vic Menard
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Re: Argo Navis alignment stars new [Re: Peter Natscher]
      #6218095 - 11/26/13 08:56 PM

Quote:

...the GoTo operation will be most accurate for finding objects 'between' the two alignment stars chosen and that the two stars shouldn't be more than ~100° apart nor any of them too close to the horizon.



My experience has been a little different from yours. If I can get an "optimal" two-star alignment (optimal as described by Alt Az Align), I get "good" performance (usually within +/-0.1-degree) for most of the sky except near the zenith (usually +/-0.2- to 0.3-degree). This makes sense to me, since my secondary mirror isn't mechanically offset away from the focuser, which generates a 0.2-degree error close to the zenith (and a 0.4-degree "hole" at the zenith). If I get a "good" two-star alignment, I also find that 6 hours later targets in the "new" half hemisphere of sky are still +/-0.1- to 0.2-degrees from center.

I rarely realign on a third star after I have a "good" two-star alignment, but I do use the local sync to recenter on objects located in heavily populated areas (galaxy clusters, etc.). If my alignment suddenly loses accuracy (as it sometimes does after hours of multi-target observing)--I just reinitialize with a new two-star alignment.


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Argo Navis alignment stars new [Re: Vic Menard]
      #6218272 - 11/26/13 10:24 PM

To clarify my two-star align procedure, with both drives 'disengaged' I hand slew the Dob to each alignment star pushing the ENTER button in the Argo after each star is centered in the eyepiece. I use a higher power 1-1/4" 6-8mm eyepiece with a 50° narrower-field (i.e, Plossl or Ortho) to view these stars fully racked out and defocused until the star's shape practically touches the fov edge. You can do this with a bright 1st or 2nd mag star. It's after the second star is manually centered at the eyepiece and entered in the Argo, I engage both drives, choose an object, actuate a Go-To and confirm the alignment accuracy. BTW: it's Mike Zammit of StarStructure that advised me to use this defocused star at high power for centering the alignment stars at the eyepiece. It's an easy way to accurately center a bright star.

Quote:

Quote:

One thing that has troubled me with regards to performing a two-star alignment with Dobs is the shift in altitude of the entire telescope as you quickly engage the ServoCAT's altitude drive to the altitude bearing after the second star is aligned at the eyepiece. At the eyepiece, I defocus the star in a 50° afov eyepiece at ~200X to center the star more accurately.



For 50-degree AFOV at 200X, TFOV = 0.25-degree (I normally use a 100-degree AFOV at 200X with TFOV = 0.5-degree).

Quote:

Since the second star is moving (not Polaris) across the eyepiece fov, quickly engaging the altitude drive is important to keep alignment accuracy.



It's not necessary to engage the altitude drive to select the second alignment star--you can even enter from the hand pad. I normally do a quick "manual" goto on a known object to verify the two-star alignment before I engage the ServoCAT clutches.

Quote:

With my past Starmaster Dob's and currently with my StarStructure Dob, engaging the altitude bearing to the drive lifts the entire telescope a bit so that at the eyepiece it moves the second aligned star almost out of the field of view - approx 1/4 degree at 200X.



That should be a little less than 1/8 degree (about 0.1-degree) for a 1/4-degree TFOV eyepiece. I see some similar, small movement with my StarStructure (it could be 0.1-degree), but after I've verified the target centering manually (as I described above), when I press the GoTo button on my hand pad the target always moves back to the center of the FOV as the Argo Navis read out re-zeroes on the target (this is all part of my normal Argo Navis/ServoCAT "setup and verify" procedure). I don't understand why your scope behaves differently, perhaps it's because you're engaging the altitude drive before you press enter on the Argo Navis? Or have I misunderstood your actual procedure?




Edited by Peter Natscher (11/26/13 10:29 PM)


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a__l
professor emeritus


Reged: 11/24/07

Re: Argo Navis alignment stars new [Re: Peter Natscher]
      #6218588 - 11/27/13 03:07 AM

Victor,
Gary recommends about 30 degrees or more separation for the alignment stars (Az). Make sure they have different Altitudes (>10 degrees Alt). So better work algorithm Auto adjust On.
Although I have questions about the stars in the Polaris region. For some stars (Align 2 stars region Polaris Az>30 Alt>10, mode Az Alt) I get RAW RMS up to 1.5 degrees(!) some values with FIT RMS 3.8-4.0' (based on a sample of 30 stars).
I do not understand why.


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VĂ­ctor MartĂ­nez
sage


Reged: 02/05/07

Loc: Cádiz - Spain
Re: Argo Navis alignment stars new [Re: a__l]
      #6218592 - 11/27/13 03:29 AM

"Polaris is (approximately) due north, so any star that is due south will have an azimuth difference of 180-degrees". Yes but the line drawn between the two alignment stars hasn`t to go necessarily through the zenith to meet this requirement. Both could fall in either hemisphere.
Vic, please, could you say me for this geographic coordinates 36°27'42"N and 5°43'26"W, to the 18'45h local time, the program Alt Az Align, what alignment stars recommended?
Peter, it is recommended to use ServoCAT goto button after each alignment star is centered in the eyepiece.

Edited by Víctor Martínez (11/27/13 04:02 AM)


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a__l
professor emeritus


Reged: 11/24/07

Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6218597 - 11/27/13 03:37 AM

I do not know how to make software for Auto adjust ON.
This knows Gary. He recommends Az>30 Alt>10.
In my part of the sample of stars, I took all possible steps to good results (FIT RMS 3.8 minutes!)


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VĂ­ctor MartĂ­nez
sage


Reged: 02/05/07

Loc: Cádiz - Spain
Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6218603 - 11/27/13 03:44 AM

a___l,
In my first post, I say that I used how alignment stars Altair and Polaris, both are separated by 134° in azimuth and 17° in altitude, theoretically meet the requirements.


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a__l
professor emeritus


Reged: 11/24/07

Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6218612 - 11/27/13 04:02 AM

I can only say that I have the two stars were the difference in Alt=0 (Polaris and Altair in October). I used for Align Polaris and Alioth.

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a__l
professor emeritus


Reged: 11/24/07

Re: Argo Navis alignment stars new [Re: a__l]
      #6218619 - 11/27/13 04:19 AM

"ServoCAT goto button after each alignment star is centered in the eyepiece"
______________________

I often do not work GoTo ServoCat -> signal Align Argo. I have two box ServoCat. Wired and Wi-Fi. Collisions? I do not use goto button for Align ArgoNavis.
Ps. Yellow buttons Up, Down etc work. Green (GoTo for Align) 50/50 work/not work. GoTo (for GoTo) - works (on both box).

Edited by a__l (11/27/13 04:43 AM)


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Vic Menard
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Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6218971 - 11/27/13 10:28 AM

Quote:

Vic, please, could you say me for this geographic coordinates 36°27'42"N and 5°43'26"W, to the 18'45h local time, the program Alt Az Align, what alignment stars recommended?



Looks like...
Vega - Alpheratz (18:57)
And a few others later in the night...
Capella - Alpheratz (21:09)
Rigel - Navi (23:31)
Aldebaran - Polaris (00:29)
Navi - Sirius (00:31)

(I think these are correct. I had to recalculate your local time, could be a bug with Alt Az Align and iOS7?)


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Peter Natscher
professor emeritus


Reged: 03/28/06

Loc: Central Coast California
Re: Argo Navis alignment stars new [Re: a__l]
      #6219237 - 11/27/13 12:36 PM

Ditto for me. I've never heard of pushing the GOTO button on my Hand Controller after each align star is centered in the eyepiece and entered on the Argo. Is this a new direction to follow? If you push the GOTO button on the HC at that point in time, then the disengaged drives will start to spin up out of control.

Quote:

"ServoCAT goto button after each alignment star is centered in the eyepiece"
______________________

I often do not work GoTo ServoCat -> signal Align Argo. I have two box ServoCat. Wired and Wi-Fi. Collisions? I do not use goto button for Align ArgoNavis.
Ps. Yellow buttons Up, Down etc work. Green (GoTo for Align) 50/50 work/not work. GoTo (for GoTo) - works (on both box).




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Vic Menard
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Loc: Bradenton, FL
Re: Argo Navis alignment stars new [Re: Peter Natscher]
      #6219464 - 11/27/13 02:13 PM

If your Argo Navis is using v2.0 or higher firmware, the GoTo button on the hand pad sends an "ENTER" command during the 2-star alignment. (See here, bottom of page 10.)

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VĂ­ctor MartĂ­nez
sage


Reged: 02/05/07

Loc: Cádiz - Spain
Re: Argo Navis alignment stars new [Re: Vic Menard]
      #6219704 - 11/27/13 03:56 PM

Quote:

Quote:

Vic, please, could you say me for this geographic coordinates 36°27'42"N and 5°43'26"W, to the 18'45h local time, the program Alt Az Align, what alignment stars recommended?



Looks like...

Vega - Alpheratz (18:57)
And a few others later in the night...
Capella - Alpheratz (21:09)
Rigel - Navi (23:31)
Aldebaran - Polaris (00:29)
Navi - Sirius (00:31)

(I think these are correct. I had to recalculate your local time, could be a bug with Alt Az Align and iOS7?)




Thank you


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Jeff Morgan
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Re: Argo Navis alignment stars new [Re: Peter Natscher]
      #6219752 - 11/27/13 04:19 PM

Quote:

Ditto for me. I've never heard of pushing the GOTO button on my Hand Controller after each align star is centered in the eyepiece and entered on the Argo. Is this a new direction to follow? If you push the GOTO button on the HC at that point in time, then the disengaged drives will start to spin up out of control.




It's in the ServoCAT documentation (Gen 3), and a practice highly recommended by Gary Myers.

The GOTO button(s) only register as "enter" buttons until 2-star alignment is complete. Once Argo has a valid solution the ServoCAT GOTO button(s) return to their normal function.

In actual use this is a wonderful feature using the wireless hand controller. My scope is long (2800 mm) and the reticle eyepiece provides about 240x. The align star is moving across the field very fast, no way I could reach back to the Argo to hit the enter button and get an accurate fix.

If you don't have the wireless HC, it would be worthwhile to get a long cord from StellarCAT so you can get the wired HC to the eyepiece.

BTW, if you finish alignment and forget to engage the drives, the motors will spin when you attempt the first GOTO. No problem though, either let it run out, or do an emergency stop. Engage the drives and hit the GOTO button again and the unit reads the encoder values and re-figures where it needs to go. You do not lose alignment.


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Re: Argo Navis alignment stars new [Re: VĂ­ctor MartĂ­nez]
      #6219870 - 11/27/13 05:18 PM

Quote:

a___l,
In my first post, I say that I used how alignment stars Altair and Polaris, both are separated by 134° in azimuth and 17° in altitude, theoretically meet the requirements.




There's your problem. Gary from Wildcard is a good friend of mine and I do some beta testing for him. The critical thing when choosing your alignment stars to ensure good pointing accuracy is to have the scope move more than 30 degrees in both axes. ie select 2 alignment stars separated by > 30 degrees in both altitude and azimuth. Those 2 alignment stars are only moving the scope 17 degrees in altitude. In addition it is important to select stars that are higher than about 10 degrees in altitude and lower than 80 degrees in altitude. Atmospheric refraction affects pointing accuracy if you use stars lower than 10 degrees and above about 80 degrees the "dobsons hole" effect comes into play where large rotational movements in azimuth cause only a small positional change of the telescope. In addition to this orthagonality issues with your mount can have an effect on pointing accuracy. As others have said the Argo Navis TPAS can compensate very well for any mount errors.

Cheers,


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Re: Argo Navis alignment stars new [Re: Jeff Morgan]
      #6219916 - 11/27/13 05:40 PM

Thanks for the clarification! I do use a wireless HC and can't say how much more enjoyable it is to use vs. the short-cable wired HC. In my 70" f.l 24" Dob, it's pretty much necessary for slewing the scope while observing through the eyepiece.

Quote:

Quote:

Ditto for me. I've never heard of pushing the GOTO button on my Hand Controller after each align star is centered in the eyepiece and entered on the Argo. Is this a new direction to follow? If you push the GOTO button on the HC at that point in time, then the disengaged drives will start to spin up out of control.




It's in the ServoCAT documentation (Gen 3), and a practice highly recommended by Gary Myers.

The GOTO button(s) only register as "enter" buttons until 2-star alignment is complete. Once Argo has a valid solution the ServoCAT GOTO button(s) return to their normal function.

In actual use this is a wonderful feature using the wireless hand controller. My scope is long (2800 mm) and the reticle eyepiece provides about 240x. The align star is moving across the field very fast, no way I could reach back to the Argo to hit the enter button and get an accurate fix.

If you don't have the wireless HC, it would be worthwhile to get a long cord from StellarCAT so you can get the wired HC to the eyepiece.

BTW, if you finish alignment and forget to engage the drives, the motors will spin when you attempt the first GOTO. No problem though, either let it run out, or do an emergency stop. Engage the drives and hit the GOTO button again and the unit reads the encoder values and re-figures where it needs to go. You do not lose alignment.




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VĂ­ctor MartĂ­nez
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Re: Argo Navis alignment stars new [Re: ausastronomer]
      #6219973 - 11/27/13 06:05 PM

Quote:

Quote:

a___l,
In my first post, I say that I used how alignment stars Altair and Polaris, both are separated by 134° in azimuth and 17° in altitude, theoretically meet the requirements.




There's your problem. Gary from Wildcard is a good friend of mine and I do some beta testing for him. The critical thing when choosing your alignment stars to ensure good pointing accuracy is to have the scope move more than 30 degrees in both axes. ie select 2 alignment stars separated by > 30 degrees in both altitude and azimuth. Those 2 alignment stars are only moving the scope 17 degrees in altitude. In addition it is important to select stars that are higher than about 10 degrees in altitude and lower than 80 degrees in altitude. Atmospheric refraction affects pointing accuracy if you use stars lower than 10 degrees and above about 80 degrees the "dobsons hole" effect comes into play where large rotational movements in azimuth cause only a small positional change of the telescope. In addition to this orthagonality issues with your mount can have an effect on pointing accuracy. As others have said the Argo Navis TPAS can compensate very well for any mount errors.

Cheers,




Ok John, thank you, I will put into in practice what you have discussed in relation to the parameters to consider when selecting alignment stars.


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Jeff Morgan
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Re: Argo Navis alignment stars new [Re: Jeff Morgan]
      #6220645 - 11/28/13 01:34 AM

Quote:

No problem though, either let it run out




Mistake on part - I don't think it will "run out".

Last session I had a bit of slack in my altitude cable and (naturally) the 31 Nagler in the focuser. The scope was low to the horizon and was attempting to GOTO a target of higher altitude. The motor ran, slowed down, speed up, slowed down, and tried mightily to get to the desired encoder position. Finally I stopped it with the e-stop and re-tensioned my cable. From there forward it was all fine.

Point is, ServoCAT appears to keep driving the motors until the desired scope (encoder) position is achieved.


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a__l
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Reged: 11/24/07

Re: Argo Navis alignment stars new [Re: ausastronomer]
      #6220754 - 11/28/13 05:32 AM

Quote:

Quote:

a___l,
In my first post, I say that I used how alignment stars Altair and Polaris, both are separated by 134° in azimuth and 17° in altitude, theoretically meet the requirements.




There's your problem. Gary from Wildcard is a good friend of mine and I do some beta testing for him. The critical thing when choosing your alignment stars to ensure good pointing accuracy is to have the scope move more than 30 degrees in both axes. ie select 2 alignment stars separated by > 30 degrees in both altitude and azimuth. Those 2 alignment stars are only moving the scope 17 degrees in altitude. In addition it is important to select stars that are higher than about 10 degrees in altitude and lower than 80 degrees in altitude. Atmospheric refraction affects pointing accuracy if you use stars lower than 10 degrees and above about 80 degrees the "dobsons hole" effect comes into play where large rotational movements in azimuth cause only a small positional change of the telescope. In addition to this orthagonality issues with your mount can have an effect on pointing accuracy. As others have said the Argo Navis TPAS can compensate very well for any mount errors.

Cheers,




>30 degree difference Az and >10 degree difference Alt, this recomended Gary from Wildcard (for me with e-mail correspondence).
find two bright stars Alt>30 Az>30 horizon>10 zenith<80 difficult. Sometimes, such a configuration is not. See table Align Star (manual Argo). Only use the Align Star + Re-Align (method FIFO).
For example >30 degree Alt. For the second Align star limits Alt 11 ... 14 or 76...79 degrees (for Polaris on my latitude). It is very difficult to estimate by eye. Hmm...

Ps. I spent about eight full nights with navigation Argo and TPAS. Eight full nights only navigation and improvement of TPAS/modernization my 24" telescope for use ServoCat+ArgoNavis. It is a half of the summer and 2/3 of the autumn season (this year). I still have questions ...
I will continue to the end of winter. The winter is very cold.

Edited by a__l (11/28/13 07:17 AM)


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stacpa17
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Re: Argo Navis alignment stars new [Re: a__l]
      #6221305 - 11/28/13 12:08 PM

I have never used Polaris but have the last time I was out I used Deneb and Vega and had a warp of .1 and was set for the rest of the night with the 10k encoders. I usually choose a zenith star and a horizon star but was in a valley and never get as low a warp. This is of course after I remembered to change my location in the Argo Navis from my previous star party.

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Vic Menard
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Re: Argo Navis alignment stars new [Re: stacpa17]
      #6223746 - 11/29/13 04:47 PM

I've been reading the concurrent alignment star discussion between Victor and Gary (on the Argo Navis Yahoo eGroup) and Gary seems to be unimpressed with the star selection generated by Alt Az Align (which apparently works better with the alignment models used by Meade and Celestron DSCs).

But Gary does point out that while a wider separation won't eliminate orthogonality error, it can help to "average" the error between the two points. Because the typical orthogonality error is an offset pointing axis (the result of a mechanically "centered" secondary mirror with an offset alignment) which primarily impacts accuracy near the zenith, it would seem to make sense to pick two stars to enhance the error averaging near the zenith.

After using the Argo Navis (with AUTO ADJUST "ON") with my StarStructure Dobsonian (secondary mirror is mechanically centered in the OTA, alignment is offset which generates a 0.2-degree pointing/orthogonality error) for the past six or seven years, I do seem to get consistently better pointing accuracy using stars with an altitude difference of 15- to 50-degrees and an azimuth difference of 180-degrees (+/- a few degrees). FWIW, while Alt Az Align generates star pairs close to 180-degree azimuth separation, it doesn't exclude stars with small altitude separations. This means the user needs to pick the pairs with better altitude separation (perhaps in a future update Alt Az Align will fix this for Argo Navis users?).


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a__l
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Reged: 11/24/07

Re: Argo Navis alignment stars new [Re: stacpa17]
      #6223877 - 11/29/13 05:50 PM

Quote:

I have never used Polaris but have the last time I was out I used Deneb and Vega and had a warp of .1 and was set for the rest of the night with the 10k encoders. I usually choose a zenith star and a horizon star but was in a valley and never get as low a warp. This is of course after I remembered to change my location in the Argo Navis from my previous star party.




Polaris - "stable" star in the northern hemisphere. In any period and anytime.
Polaris is always the desire to use - this desire to always get a stable result Align.


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Re: Argo Navis alignment stars new [Re: a__l]
      #6224647 - 11/30/13 06:24 AM

I only know that after the fist alignment using Polaris and Altair, the accuracy point was mediocre, especially in the eastern sky hemisphere, and later, after a second aligned using Deneb and Capella, the accuracy was very good in both hemispheres and even nearby points to the zenith. Something will have to see the stars that are chosen.

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Jeff Morgan
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Re: Argo Navis alignment stars new [Re: ausastronomer]
      #6226259 - 12/01/13 01:19 AM

Quote:

The critical thing when choosing your alignment stars to ensure good pointing accuracy is to have the scope move more than 30 degrees in both axes. ie select 2 alignment stars separated by > 30 degrees in both altitude and azimuth.




I wonder if there is a frame of reference issue here.

Firstly, azimuth. Not to be too northern hemisphere biased, but all of us up here want to use Polaris for the obvious reason. So, Polaris has an RA of 2 hours 49 minutes. Since one hour of RA = 15 degrees (at the equator), does this mean that any second alignment star between 00h49m and 4h29 minutes is useless?

Secondly, altitude. One tends to think of 0 degrees through 90 degrees - basically, a quadrant of the sky. If this is the case, Polaris sits nearly in the middle of the altitude band, placing more limits on a suitable second star.

Or perhaps what is really more important is not altitude and azimuth per se, but linear separation between Polaris and the second star? In this case, one could draw a circle of say, 30 degrees around Polaris and any second star outside of this circle would be a good choice?

Edited by Jeff Morgan (12/01/13 01:20 AM)


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Vic Menard
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Re: Argo Navis alignment stars new [Re: Jeff Morgan]
      #6227401 - 12/01/13 04:14 PM

Quote:

Quote:

The critical thing when choosing your alignment stars to ensure good pointing accuracy is to have the scope move more than 30 degrees in both axes. ie select 2 alignment stars separated by > 30 degrees in both altitude and azimuth.




I wonder if there is a frame of reference issue here.



The frame of reference is stated above--movement in both axes (a Dobsonian, for example, moves in altitude and azimuth, and an equatorially mounted scope, in right ascension and declination).

Quote:

Firstly, azimuth. Not to be too northern hemisphere biased, but all of us up here want to use Polaris for the obvious reason. So, Polaris has an RA of 2 hours 49 minutes. Since one hour of RA = 15 degrees (at the equator), does this mean that any second alignment star between 00h49m and 4h29 minutes is useless?



It would depend on the altitude and azimuth of the star. For example, a star with RA 00h49m will be the same altitude as Polaris twice each day, and the same azimuth as Polaris twice each day--either of these particular alignments would not be good (Alt diff = 0 or Az diff = 0).

Quote:

Secondly, altitude. One tends to think of 0 degrees through 90 degrees - basically, a quadrant of the sky. If this is the case, Polaris sits nearly in the middle of the altitude band, placing more limits on a suitable second star.



Unfortunately, this behavior is true for any quadrant of the sky. Picking a star that sits midpoint of the 15- to 80-degree suggested range (near 47.5-degrees altitude) is going to make finding star with a 30-degree altitude difference a bit more tedious, no matter where it is in the current hemisphere above the horizon.

Quote:

Or perhaps what is really more important is not altitude and azimuth per se, but linear separation between Polaris and the second star? In this case, one could draw a circle of say, 30 degrees around Polaris and any second star outside of this circle would be a good choice?



Nope. On your circle, the best positions relative to Polaris would be at the midpoints between up and down and right and left--which makes the 30-degree radius the hypotenuse of a triangle with sides of 21-degrees elevation and 21-degrees azimuth.

As an example, Alt Az Align will sometimes pair Polaris with another star due south (180-degrees azimuth differential) at the same altitude as Polaris (0-degrees altitude differential). The linear difference is over 90-degrees for me, but the star is not a good alignment pair for the Argo Navis.

If the mounting is Alt/Az, the encoders are Alt/Az and that's what needs to move 30-degrees--both encoders--both axes. For an Alt/Az mounting, the right ascension and declination of the alignment stars is not important.


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