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Argo Navis alignment stars

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#26 Jeff Morgan

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Posted 27 November 2013 - 04:19 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.


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.

#27 ausastronomer

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Posted 27 November 2013 - 05:18 PM

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,

#28 Peter Natscher

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Posted 27 November 2013 - 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.

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.



#29 Víctor Martínez

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Posted 27 November 2013 - 06:05 PM

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.

#30 Jeff Morgan

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Posted 28 November 2013 - 01:34 AM

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.

#31 a__l

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Posted 28 November 2013 - 05:32 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.


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.

#32 stacpa17

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Posted 28 November 2013 - 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.

#33 Vic Menard

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Posted 29 November 2013 - 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?).

#34 a__l

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Posted 29 November 2013 - 05:50 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.


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.

#35 Víctor Martínez

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Posted 30 November 2013 - 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.

#36 Jeff Morgan

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Posted 01 December 2013 - 01:19 AM

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?

#37 Vic Menard

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Posted 01 December 2013 - 04:14 PM

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).

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).

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.

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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