Polar alignment with an OAG

Using SharpCap's Polar Alignment feature
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bluesilver
Posts: 80
Joined: Mon Oct 05, 2020 12:51 am

Polar alignment with an OAG

#1

Post by bluesilver »

Hi, I have been using SharpCap for a few years now doing a polar alignment and has been working a treat.
I have been doing this by a guide scope, but now i am going to be using an OAG

So my question is:
Can i still polar align with an OAG or am i better polar aligning via the main imaging camera?
My scope is a Skywatcher Esprit 150
Main imaging camera is ZWO ASI2600mc
Guide camera is a ZWO ASI290mm mini

The specs of the imaging camera and scope according to astronomy.tools are:
Focal Ratio: 7
Resolution: 0.74"x0.74" per pixel
Field of View: 1.28° x 0.86°
Dawes Limit: 0.77 arc/secs

The specs of the guide camera and scope are:
Focal Ratio: 7
Resolution: 0.57"x0.57" per pixel
Field of View: 0.31° x 0.17°
Dawes Limit: 0.77 arc/secs

I could be wrong here, but with the guide camera connected to the OAG, the field of view has become very narrow and i am thinking polar aligning with the main imaging camera would be the way to go here.

Any advice would be greatly appreciated.
Thanks.
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admin
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Re: Polar alignment with an OAG

#2

Post by admin »

Hi,

yes, the FOV for the OAG will almost certainly be too small to work for polar alignment, so you will be much better off using your main camera (and that will work fine).

cheers,

Robin
bluesilver
Posts: 80
Joined: Mon Oct 05, 2020 12:51 am

Re: Polar alignment with an OAG

#3

Post by bluesilver »

Sorry for the delay in replying,
Thanks for the reply, is very much appreciated.
Keen to put it all into practice as soon as i get a clear night.
Cheers.
zerolatitude
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Joined: Mon Mar 01, 2021 5:24 am

Re: Polar alignment with an OAG

#4

Post by zerolatitude »

Apart from the small FoV, OAG stars can get distorted, and Platesolving may not work as well as with a guidescope.
bluesilver
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Re: Polar alignment with an OAG

#5

Post by bluesilver »

Thanks for the information in regards to the OAG.
This will be the first time using one, i am interested to see how it compares to the 60mm guide scope that i have been using.
I will only be using the OAG for use with PHD2 multi star guiding.
All the plate solving, polar alignment, imaging will be done using the main camera ( ZWO ASI2600mc)
My theory was that an OAG will hold the starts when guiding better as there is less change of any movement like you can possibly get via a guide scope.
But that is only my theory, yet to have clear nights to put it all into practice though.
rkymtnbiker
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Joined: Tue Feb 09, 2021 5:48 pm

Re: Polar alignment with an OAG

#6

Post by rkymtnbiker »

I am also moving over to an OAG for my EdgeHD 925. At the F10 focal length for this scope, my OAG camera will have a FOV of 0.28 x 0.17 degrees and the imaging camera will have a FOV of 0.57 x 0.38 degrees.

I see the general requirements for SharpCap Polar Alignment are a FOV between 1 degree and 2.5 degrees. So, is a FOV of 0.57 x 0.38 degrees too small? I also have the option to use a PoleMaster camera. But I think it has an 11 x 6 degree FOV, so, that seems way too wide.

Maybe I'll have to pack an extra guidescope/camera just for Polar Alignment?

SharpCap Polar Alignment is my FAVORITE method, so any advice on how best to keep using it with this scope and OAG, I'd appreciate it!

Thanks,

Ted
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Re: Polar alignment with an OAG

#7

Post by admin »

Hi Ted,

the main imaging camera might just work - you can usually go down to about 0.5x0.5 degrees with some decent reliability, so the 0.57x0.38 might be OK. Certainly worth a try.

The Polemaster should work too - I have had plenty of reports of it being used with SharpCap's polar alignment routine, and although the field is a bit wide it should be OK. You could always try using a 640x480 ROI on the polemaster which would have the FOV.

I'd suggest giving both the main camera and the polemaster a try and see if either work to your satisfaction.

cheers,

Robin
rkymtnbiker
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Re: Polar alignment with an OAG

#8

Post by rkymtnbiker »

Thanks for the quick reply, Robin; I'll give both of those methods a try.

I attached my old guidescope/camera last night (1.52 x 1 degrees) and polar aligned meticulously because I'm setting up the new OAG and wanted to ensure the Polar Alignment wasn't in question. I've also discovered that when I setup with my EQ6-R Pro tripod, I get better Polar Alignment when I set the tripod on cement pavers to eliminate sinking of the tripod feet into the ground based upon which side the telescope is on.

SharpCap reported less than 20" of error. And, PHD2 Guiding Assistant reported less than 1' of PA error on all three runs I did.

Thanks for providing a great Polar Alignment routine!

Ted
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Re: Polar alignment with an OAG

#9

Post by admin »

Hi Ted,

glad that the PA is working well for you with the old guidescope and giving similar results to PHD. I'm not surprised that you do better on a hard surface - I'm sure that if you put the mount on grass/soil/gravel then it will probably slowly sink over the course of a few hours if the ground is at all soft, meaning that great polar alignment at the start of the evening might not be nearly so good an hour or two later. As you say, an uneven weight distribution on the tripod legs is only going to make this worse!

cheers,

Robin
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