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BQ Octantis for Quick-and-Dirty Southern Polar Alilgnment

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#1 BQ Octantis

BQ Octantis

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Posted 15 January 2018 - 03:11 AM

The skies in the Outback were clear for the first time in weeks last night, so I went out to shoot the sky. I used my EQ2 mount, which is lightweight but has no polar alignment features. But alignment is pretty quick with a 50mm lens. Through a 50mm lens, the width of an APS-C sensor subtends a 25˚ angle—so you can reference Stellarium with a 25˚ FOV aimed at the southern celestial pole (SCP). The handiest reference from there is the 4-star asterism used in southern polar alignment scopes, from which you can find BQ Octantis, a magnitude 6.2 star less than 15 arcminutes from the SCP.

 

How to Find the SCP at 25˚ FOV

 

By spinning my setup through an unlocked right ascension axis with the aperture open (5 seconds at ISO 12800), I can see where the RA axis is pointing. I then just use BQ Oct align to the SCP:

 

SCP Polar Alignment at 50mm

 

For higher zooms, the handiest reference is an asterism of an obtuse isosceles triangle formed by BQ Oct, HIP 48752, and the SCP at the apex. There is a pair of unnamed stars fairly close to the SCP for reference.

 

How to Find the SCP at 1˚ FOV

 

Here's a spin through a 200mm lens with the alignment about as close as I could get (the azimuth adjustment on the EQ2 is horribly coarse). Note the considerable offset between the center of the sensor and the RA axis.

 

SCP Polar Alignment at 200mm

 

This gave me 90 seconds of streak-free subs close to the pole, allowing me to use full sensor resolution; the ecliptic was less forgiving, and I could only use a scaled full-frame image.

 

Full crop of the Tarantula Nebula region of the LMC:

Full Crop of LMC

 

Full sensor scale of the nebula region of Orion:

20180114 1172

 

Cheers.

 

BQ


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#2 happylimpet

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Posted 15 January 2018 - 05:37 AM

I think this is basically the principle of the polemaster. Nice work!



#3 BQ Octantis

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Posted 18 January 2018 - 02:58 AM

I think this is basically the principle of the polemaster. Nice work!

Thanks, mate! I hadn't heard about the Polemaster—I assume you mean this? If so, 30" pixel resolution is pretty coarse—a 50mm lens on an APS-C sensor gets you under 18"/pixel, and a 200mm will get you under 5".

 

I tried the method on my SkyViewPro mount two nights ago. In three tries and 4 minutes I eyeballed a ≤9-pixel (≤2.6') alignment with the 50mm.

 

Here's a full crop of the two final alignment images:

 

IMG_1197.gif

 

This gave me ≥6 minutes of tracking on the celestial equator. Here's a full crop of HIP 25976 and HIP 26149 (the two bright orange stars at the bottom of the image; the celestial equator is a horizontal line that passes right between them) after 6 minutes:

 

IMG_1227a.jpg

 

This tells me that the EQ2 tracking accuracy is fairly poor.

 

BQ


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#4 happylimpet

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Posted 18 January 2018 - 06:39 AM

 

I think this is basically the principle of the polemaster. Nice work!

Thanks, mate! I hadn't heard about the Polemaster—I assume you mean this? If so, 30" pixel resolution is pretty coarse—a 50mm lens on an APS-C sensor gets you under 18"/pixel, and a 200mm will get you under 5".

 


 

Yeah thats the fella. People are going crazy over them. Sure theyre ace but theyre not free!

 

I know the pixel resolution isnt ace, but sub-pixel accuracy is always possible with centroiding, so a few arcseconds alignment is surely possible.



#5 BQ Octantis

BQ Octantis

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Posted 19 January 2018 - 06:22 AM

Yeah thats the fella. People are going crazy over them. Sure theyre ace but theyre not free!

 

I know the pixel resolution isnt ace, but sub-pixel accuracy is always possible with centroiding, so a few arcseconds alignment is surely possible.

 

Sounds like another tool in the tool bag. Not perfect, but useful.

 

What are the kids aiming for these days on polar alignment accuracy? (Or do they even care anymore?)

 

BQ




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