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Promising new way to align iOptron without seeing Polaris

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#1 Michael Covington

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Posted 03 July 2020 - 09:22 AM

I've been experimenting with my new iOptron mount (GEM45) and have some up with something handy -- if it continues to work well, I'll write more about it letter.

 

It's a better way to polar-align without being able to see Polaris.

 

I'm 95% sure iOptron's Polar Iterate Align [note: not iPolar, but rather their method of sighting on 2 stars alternately] is not very accurate because it assumes a perfectly accurate starting position in declination, and if there is an error in the starting position (telescope not perfectly parallel to polar axis), that error will never be corrected.  So it's only good for getting you within a degree or two.

 

Here's my method. 

 

(1) Calibrate your azimuth and altitude adjustments so you know how far the axis moves for each turn of the screw.  This is simple geometry, thread pitch and distance from center.  I'm going to write about it at greater length.  For the GEM45, in azimuth, the axis moves toward the ****'re tightening, 20' per quarter-turn of the screw.  In altitude, it moves up as you move the screw to (your) right, 12' per 1/6 turn (which is the spacing of the wrench holes).

 

(2) Do a 3-star alignment.

 

(3) Look at the reported error and try to correct it.  For example, if the axis is 60' high, move the altitude screw to the left 5x the wrench hole spacing (that is, move a wrench hole to where the previous one was, 5 times), and if it is 40' west, then loosen the west screw and tighten the east screw two quarter-turns.

 

(4) Do another 3-star alignment and see how well you did.

 

In a quick experiment, doing two cycles of adjustment (and not adjusting after the 3rd alignment), I got within 10' of the pole.  I'm sure one more cycle would get me within 5', which is as good as Celestron's ASPA.  And this may actually be faster than ASPA.

 

More about this later.  I wanted to share it and encourage others to try the same thing.


Edited by Michael Covington, 03 July 2020 - 10:26 AM.

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#2 mborland

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Posted 03 July 2020 - 10:21 AM

On my CEM40EC and CEM60 at least, I think iPolar is quite accurate, based on drift alignment reports from PHD2's Guiding Assistant. I regularly get <2 arc minutes, and sometimes less than 1 arc minute. At least for my purposes, that's very good.

 

You calibrate out the non-alignment of the axes when you first use iPolar, by rotating to three positions about the RA axis while iPolar is viewing the sky.

 

--Michael



#3 Michael Covington

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Posted 03 July 2020 - 10:25 AM

Yes, nobody denies that iPolar is extremely accurate.  I'm talking about Polar Iterate Align, which is a two-star method that doesn't involve Polaris or a computer.  Obviously, with a computer on hand, iPolar is the way to go.



#4 mborland

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Posted 03 July 2020 - 12:41 PM

Yes, nobody denies that iPolar is extremely accurate.  I'm talking about Polar Iterate Align, which is a two-star method that doesn't involve Polaris or a computer.  Obviously, with a computer on hand, iPolar is the way to go.

Sorry, I misunderstood.

 

--Michael




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