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Polar alignment near tropics

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#1 swap_saraf

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Posted 23 February 2020 - 01:22 AM

Dear astrophotographers,
I am a beginner, living near 18 north 73 east tropical belt. I recently upgraded to a skywatcher eq6r mount from a skytracker. I have hard time getting a good polar alignnent on phd2.
I use PHD2 polar drift align after rough polar alignment. The PA error gets around 40-50 arc mins when I point on a star near Polaris. If I get slightly away, I see a big difference. Question: Why do I have to be so close to polaris? I get couple of stars in FOV. The phd2 polar align circle ('lollipop') shows up just few times and then goes out of screen. Question: What could I be doing wrong, not to get the phd2 circle inside screen?
Question: Since sharpcap need more number of stars, do you guys think Polemaster can help me with PA?

Can Fellow phd2 PA experienced folks please help.

Edited by swap_saraf, 23 February 2020 - 01:28 AM.


#2 Chuckwagon

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Posted 23 February 2020 - 03:07 AM

If you can see Polaris, and you have a scope/camera combo that results in a field of view in the range of around 2 degrees down to about 1/2 degree, then Sharpcap should be able to work for you.  (I've succeeded with it using fields of view as large and as small as that.)  You may need to use longer exposures to see more stars so it will work, but it does seem to work.  You can also input you location info so it can account for refraction.

 

The Polemaster should also work for you, and can correct for refraction as well.

 

I don't use the PHD2 drift align, but I have done plenty of drift aligning in general, with eyepiece or a camera with cross-hairs.  Here is a good website to get a handle on drift aligning.  It even has a simulator so you can practice.  It may be a little tedious to learn how to manually do it, but you'll know it's correct and it may help with the software method as well.  http://www.grnbrg.org/polar_align/

 

Good luck.



#3 Dynan

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Posted 23 February 2020 - 05:44 AM

Here is another good method usable from any latitude:

 

http://www.cwjames.i...ignment_ccd.htm

 

+1 for SharpCap PA!



#4 swap_saraf

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Posted 23 February 2020 - 12:22 PM

Thank you. Yes, Polaris is visible. Sharpcap says 10-15 stars. Hence I was slightly apprehensive as I see just 2- 3 stars. Does polemaster need those many stars? I will also try with the increased exposure. Since polaris is just 18 degree above horizon, if I increase the exposure I get lot of light pollution. I can still manage 2 secs at max. Thanks for these links. Will try.

#5 Michael Covington

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Posted 23 February 2020 - 12:24 PM

18 north is not so close to the equator as to be problematic.  I would approach the problem the same way as someone in temperate latitudes.



#6 Dynan

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Posted 23 February 2020 - 03:30 PM

You'd be surprised at what SharpCap can see, especially if you use your main camera for PA. For only $11 a year, it may be worth a try. One of the best tools in my 'astronomy shed'!!!



#7 1DegreeN

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Posted 23 February 2020 - 10:42 PM

I'm a bit further south, at 14 degrees latitude. If you can see Polaris then Polemaster is certainly a quick and easy way to get a good PA - but its not cheap. For myself, Polaris has now been hidden by a bamboo grove and I can no longer use Polemaster. I too highly recommend SharpCap and in fact prefer it to Polemaster: it usually doesn't take much longer and it is a bit more accurate. However at this time of year there is so much dust and smoke haze around that I can't see enough stars to complete the SharpCap procedure. So I use regular PHD2 drift alignment. I'm using an EQ6R-Pro too. The first time I did drift alignment to determine both altitude and azimuth and that took around 30 minutes. But then I marked the position of the tripod on the ground and take care when carrying the mount in and out of the house not to change the altitude setting, so now when I set the mount up I put the tripod legs back on their marks and only need to do a drift alignment for azimuth which typically takes maybe 10 minutes and once or twice I've been lucky and got it good enough first time. I always check the PHD logs afterwards and it reports my PA error at typically around 4 minutes or less which is more than good enough for my imaging. Another advantage for me of this azimuth only drift alignment is that I can begin the procedure earlier in the evening when the zenith is dark but the northern horizon is still too light to use SharpCap. 


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#8 swap_saraf

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Posted 24 February 2020 - 02:02 AM

I am definitely trying sharpcap pro this week.

@1degree This is a good tip. I keep the tripod roughly at the same position. However, I too will make those markings to keep it as precise for drift alignment. I point in a region near orion when it is near meridian to perform the drift alignment. I see a big PA error of around 700 arc mins. Something is way off.

Edited by swap_saraf, 24 February 2020 - 02:41 AM.


#9 swap_saraf

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Posted 24 February 2020 - 02:36 AM

@Micheal, there is lot of light pollution near that area. Let me try though by adjusting the params.

#10 Chuckwagon

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Posted 24 February 2020 - 11:04 AM

I see a big PA error of around 700 arc mins. Something is way off.

Yep, that's a big problem.  If you point the mount directly at Polaris and don't even bother with PA you will only be around 30 arc-min off of the pole.  So either something isn't reading correctly, maybe a parameter has been entered incorrectly, and the software is giving an erroneous reading, or the star you are using as Polaris isn't actually Polaris.  (Which I've somehow managed to do in the past, and it makes for a really messed up PA.)  :)
 


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#11 swap_saraf

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Posted 25 February 2020 - 12:39 AM

Yep, that's a big problem.  If you point the mount directly at Polaris and don't even bother with PA you will only be around 30 arc-min off of the pole.  So either something isn't reading correctly, maybe a parameter has been entered incorrectly, and the software is giving an erroneous reading, or the star you are using as Polaris isn't actually Polaris.  (Which I've somehow managed to do in the past, and it makes for a really messed up PA.)  smile.gifW

That 700 arc mins error is shown during drift alignment and not polar alignment. I thought, not to use Polaris and tried with drift alignment directly. It used to guide for some 5 - 6 secs and then I used to get Star Lost. I am trying things!



#12 Chuckwagon

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Posted 25 February 2020 - 03:09 AM

That 700 arc mins error is shown during drift alignment and not polar alignment. I thought, not to use Polaris and tried with drift alignment directly. It used to guide for some 5 - 6 secs and then I used to get Star Lost. I am trying things!

Ah, I see.  Even if you are going to drift align, you should at least start by pointing the mount at Polaris.  Then you are at least close, and the drift alignment will go more quickly.  You'll still need to track a star near the Celestial Equator and the meridian for adjusting your mount's azimuth, and then choose a star near the Celestial Equator that is 15 - 20 degrees above the East or West horizon to adjust the mount's altitude, and iterate back and forth adjusting azimuth then altitude until you zero in on a good PA.  It's all well explained on the website I linked.  And even if you are using PHD2 to simplify the process, it won't hurt to have a good understanding of the drift align process and what's happening.  I'm sure you'll be able to get it licked in short order.


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