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#1 Teddythefinger

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Posted 23 July 2019 - 07:38 PM

I've been doing research on this for a while and can't for the life of me find an answer! I'll apologize in advance if this sounds dumb....
I'm looking into getting a polemaster because of astrobackyard's video review. I get pretty good exposures on my star adventurer of about 90 seconds already. But I'd like to get 3 or 4 minutes. I also like being able to monitor my pa while I'm imaging.
So my question what's the difference between using a polemaster vs a guide scope? If I can get 4 or 5 minutes with the PM why would I need the guide scope? If I go with a guide scope can I still monitor my PA?
Ok thanks in advance, thanks for any help you can give!!

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#2 Garthid

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Posted 23 July 2019 - 08:14 PM

Greetings,

Polemaster is used to accurately align your scope with the celestial pole. An accurately aligned scope has a better chance of having usable exposures. After you go through the setup procedure, the program will tell you how far off it thinks you are from the pole. As far as I know, you can accept that number, refine or try again, but once you accept it the program does not actively track polar alignment. With good alignment you may be able to get 3 or 4 minute exposures depending on the focal length you use but without active guiding via a camera your chances will diminish. Study the features of both Polemaster and PHD2 to get an idea of the capabilities of each. Hope this helps



#3 Teddythefinger

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Posted 23 July 2019 - 08:22 PM

Greetings,
Polemaster is used to accurately align your scope with the celestial pole. An accurately aligned scope has a better chance of having usable exposures. After you go through the setup procedure, the program will tell you how far off it thinks you are from the pole. As far as I know, you can accept that number, refine or try again, but once you accept it the program does not actively track polar alignment. With good alignment you may be able to get 3 or 4 minute exposures depending on the focal length you use but without active guiding via a camera your chances will diminish. Study the features of both Polemaster and PHD2 to get an idea of the capabilities of each. Hope this helps

Hey Garhid, what do you mean my chances will diminish? Like I'll only get a few exposures at 4 or 5 minutes?

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#4 FlankerOneTwo

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Posted 23 July 2019 - 09:44 PM

The PoleMaster can monitor PA after you click the "Start Monitor" button. It will show you calculated and actual center of rotation in real time. In practice though, unless you bump the mount or think you've disturbed it changing equipment or whatever, there is little reason to monitor it.

Every mount will exhibit some degree of tracking error due to mechanical tolerances of parts, imperfect polar alignment (even polemaster will get you only typically within 30 arcsec at best), atmospheric refraction leading to change in apparent star location closer to horizon, etc. This will lead to trailing of stars with longer exposures. Whether the trailing is noticeable depends on your pixel scale.Tests here https://www.cloudyni...-and-test-pics/ suggest to me that there is enough drift over the span of 3-5 minutes that I would think it would be noticeable unless you are only using a wide angle lens. Guiding is used to correct for these errors.

 

The PoleMaster, if placed aligned with your mount RA axis instead of the scope, will be continuously looking at the NCP (North Celestial Pole). A guide scope of course will be looking where your imaging setup is looking and will therefore not be of any use monitoring your PA.

 

So you can get plenty of exposures at 4-5 minutes, but unguided chances diminish that you will want to keep any of them.


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#5 Pauls72

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Posted 23 July 2019 - 10:00 PM

I use a PoleMaster, plus I guide. I can get 30 minute exposures with guiding. I am sure I could get longer too.

The majority of mass produced mounts will not track well enough for long exposures without guiding.


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#6 starman876

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Posted 23 July 2019 - 10:03 PM

Great information provided here.  I was also wondering about a polemaster.  Thanks



#7 Teddythefinger

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Posted 24 July 2019 - 07:46 AM

The PoleMaster can monitor PA after you click the "Start Monitor" button. It will show you calculated and actual center of rotation in real time. In practice though, unless you bump the mount or think you've disturbed it changing equipment or whatever, there is little reason to monitor it.
Every mount will exhibit some degree of tracking error due to mechanical tolerances of parts, imperfect polar alignment (even polemaster will get you only typically within 30 arcsec at best), atmospheric refraction leading to change in apparent star location closer to horizon, etc. This will lead to trailing of stars with longer exposures. Whether the trailing is noticeable depends on your pixel scale.Tests here https://www.cloudyni...-and-test-pics/ suggest to me that there is enough drift over the span of 3-5 minutes that I would think it would be noticeable unless you are only using a wide angle lens. Guiding is used to correct for these errors.

The PoleMaster, if placed aligned with your mount RA axis instead of the scope, will be continuously looking at the NCP (North Celestial Pole). A guide scope of course will be looking where your imaging setup is looking and will therefore not be of any use monitoring your PA.

So you can get plenty of exposures at 4-5 minutes, but unguided chances diminish that you will want to keep any of them.

Ok thanks! This helped me out a ton. So if I use the guide scope to polar align, can this be done after I have acquired my target so I don't have to touch anything, other than rotating the ra, or does if have to be done before?

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#8 Pauls72

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Posted 24 July 2019 - 10:00 AM

When you polar align, you are setting the RA axis of your EQ mount to be pointed at the north or south celestial pole. If done correctly your mount will only have to move on the RA axis when tracking. Now, because of mechanical tolerances of most mass produced mounts your mount will still have to move slightly in the DEC and correct issues in the RA.

 

Polar alignment should always be the first alignment step done. You can do it by any of the following methods.

Polar Scope on/in the mount.

PoleMaster or iPolar

Drift Alignment

Software using your main imaging scope and camera. I have never used a guide scope and guide camera for polar alignment, but I suppose you could. I don't think the accuracy would be as good as your main scope.

Some mounts even have a polar alignment routine in their hand controller.

Once polar aligned, you do not touch the Altitude and Azimuth adjusting bolts on your mount.

 

At this point your can do a 1,2,or 3 star alignment through your hand controller or computer software or you can platesolve.You are matching the sky model in the hand controller or software to the sky.  Any of these methods gets the mount aligned to the sky so your GoTo's are reasonably accurate.

 

Then you can do your GoTo, to the target and start imaging or guiding and then imaging.


Edited by Pauls72, 24 July 2019 - 02:44 PM.


#9 FlankerOneTwo

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Posted 24 July 2019 - 05:40 PM

Ok thanks! This helped me out a ton. So if I use the guide scope to polar align, can this be done after I have acquired my target so I don't have to touch anything, other than rotating the ra, or does if have to be done before?

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When you polar align you set the coordinate system for the entire mount, so polar alignment is normally the first step done. Polar alignment will involve pointing the RA axis at the NCP, typically by setting altitude and azimuth of the axis, not rotating about RA. If you locate your target first, you will have to change the orientation of the mount when you align, and therefore lose your target pointing and will have to reacquire it. If you have a GoTo system, it will point waay off if our mount is not relatively accurately aligned prior to slewing.

 

Guide scopes not frequently used for polar alignment, because any error in alignment of the guide scope to the RA axis will induce an error. This can come from non-orthogonality of guide to main scope, non-orthogonality of main scope to mount, imprecise zeroing of the Dec axis, etc.

You can use SharpCap to polar align with either the guide or main scope because it uses a computational algorithm similar to PoleMaster, that does in fact work quite well. It does have to point at NCP when you align, so again you have to do before target acquisition. But once you slew to your imaging target, you will not be able to monitor PA with this setup.



#10 NuclearRoy

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Posted 25 July 2019 - 10:32 AM

I use the PoleMaster and guide with PHD2.

As said above, good Polar Alignment will get you longer exposures, but there is a limit.

Good PA also makes it 'easier' to guide (Less corrections needed)

 

The selling point for PoleMaster, for me, is just how quick and easy it is. It mounts directly to the RA shaft centerline. Click Polaris, rotate on RA a couple times. Align Polaris with the software mark by moving the mount adjustment bolts. Align the crosshairs for the true pole positions with the mount bolts. Done.

And, I never had to lay on the ground and try to look through a tiny PA scope. (I'm old)

 

I'd give the PoleMaster a try and see how long your subs can go. If you consistently get the length you want, forget about guiding until you need more exposure time. I just had to extend to 10 minutes last night on NGC 6888 in H-alpha and O-III. Longest subs yet for me. PHD2 worked like a charm :)



#11 Teddythefinger

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Posted 25 July 2019 - 07:21 PM

Thanks everyone! I think I'm pretty well convinced to invest in an auto guide setup. Thanks so much!

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