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rnobleeddy

How close to PA can I get with a polar scope?

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EQ5 polar scope. I thought it was relatively well aligned when I set it up, pointing at a far off lamp post and rotating around completely.

I've had 3 goes at lining up the mount using the polar scope + a polaris time app. My clock on the polar scope isn't rotated to the right point, but even so, I'd imagine I'm getting getting to within 1/2 an hour on the clock of where I'm supposed to be aiming.

I then use PHD2 to check alignment and I've been 55", 60" and 150" arc minutes away from PA, and so start the process of making adjustments to the mount until I get within 10". 

Should I be doing better with the polar scope? Is there anything I might be missing?

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So perhaps if you *actually* point your polar scope at polaris you'll get better PA.

But now I'm not sure how to. I made the mistake of assuming the only possible star in the FOV in the polar scope was polaris. I thought I'd pointed roughly at NCP but I guess not. I can see polaris in the sky, but if I move the mount a little each way, I tend to see > 1 star in the FOV of the polar scope and then I have no idea how to choose polaris?

Any tips?

 

 

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Polaris is the brightest one there abouts, having said which I managed to choose completely the wrong one at a dark sky site - too many 'bright' ones!

One thing I find makes it a bit easier is to set up quite soon after sunset - before the end of civil twilight, if possible - fewer stars are visible so making a mistake is less likely.

Using the polar scope on EQ5 or AltAzEQ6 I can usually get within 10' (as confirmed by PHD2 guiding assistant) and on lucky occasions within 5, but this only matters for imaging.  For visual 'near enough is good enough'. To this end marking the spots where the tripod legs go can help.  I sunk some small paving blocks in the grass, made a small dimple for the north leg and now Polaris is always in the polar scope field on initial set up.

There are a variety of apps if you are imaging, Sharpcap 2.9 is free and has a polar alignment routine, PHD2 (also free) has no less than 3 and none of these 4 agree with each other, which makes one wonder how exact a science Polar Alignment is?

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23 hours ago, rnobleeddy said:

I then use PHD2 to check alignment and I've been 55", 60" and 150" arc minutes away from PA, and so start the process of making adjustments to the mount until I get within 10". 

There is ambiguity in the way you show the PHD2 results. 55” normally means arc seconds not arc minutes. I generally aim to be less than 30” from the NCP which I usually achieve in two iterations and fine adjustments from my guidescope and camera through a PA routine. Your visual adjustments made via the polarscope all seem excellent to me. There are arguments that some (small) offset from the NCP can help with dithering the target on the imaging sensor although this will not be random.

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7 hours ago, almcl said:

Polaris is the brightest one there abouts, having said which I managed to choose completely the wrong one at a dark sky site - too many 'bright' ones!

One thing I find makes it a bit easier is to set up quite soon after sunset - before the end of civil twilight, if possible - fewer stars are visible so making a mistake is less likely.

Using the polar scope on EQ5 or AltAzEQ6 I can usually get within 10' (as confirmed by PHD2 guiding assistant) and on lucky occasions within 5, but this only matters for imaging.  For visual 'near enough is good enough'. To this end marking the spots where the tripod legs go can help.  I sunk some small paving blocks in the grass, made a small dimple for the north leg and now Polaris is always in the polar scope field on initial set up.

There are a variety of apps if you are imaging, Sharpcap 2.9 is free and has a polar alignment routine, PHD2 (also free) has no less than 3 and none of these 4 agree with each other, which makes one wonder how exact a science Polar Alignment is?

Thanks. Based on what I've experienced I think I must have just done a bad job of lining the scope up in the daylight, and then when I realized I had the wrong star, found it hard to recover because at that point it was much darker.

It'll help me to rotate/calibrate my guide camera to make sure I know which way is which in terms of changing alt/az. 

Definitely agree on the definition changing - I've seen that PHD2 is very reliable if I use polar drift alignment one the same star more than once, but can vary a lot if I choose a start on the other side of the FOV of the guide scope image.

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6 hours ago, Avocette said:

There is ambiguity in the way you show the PHD2 results. 55” normally means arc seconds not arc minutes. I generally aim to be less than 30” from the NCP which I usually achieve in two iterations and fine adjustments from my guidescope and camera through a PA routine. Your visual adjustments made via the polarscope all seem excellent to me. There are arguments that some (small) offset from the NCP can help with dithering the target on the imaging sensor although this will not be random.

Sorry, brain fade late at night. I meant arc minutes.  The one time I got good PA I was about 10  arc minutes away and guiding was good. I've learnt the hard way that at guiding at 60 arc minutes isn't as good, but still not a complete loss. I had to be up early-ish this morning so didn't stay up too late last night, but I'll hopefully get my routine sorted after a few more light nights.

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Don’t forget that Polaris is about 0.65° or 39 (= 60 x 0.65) arc minutes from the North Celestial Pole so if you point a (collimated) polar scope centre on Polaris you are already no more than 39’ away from the NCP. So to find that you are 150’ away means Polaris would not probably not be visible in the polar scope.

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Check your polar scope for alignment as well, i discovered mine was miles out, it takes a bit of fiddling to sort but is fairly easy, there are loads of tutorials out there on how to do it.  I now use sharpcap (you need the pro version) and it takes a couple of minutes, before i used the pole star and polar scope just after sunset then have to wait till dark.

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27 minutes ago, Astrosharkey said:

Check your polar scope for alignment as well, i discovered mine was miles out, it takes a bit of fiddling to sort but is fairly easy, there are loads of tutorials out there on how to do it.  I now use sharpcap (you need the pro version) and it takes a couple of minutes, before i used the pole star and polar scope just after sunset then have to wait till dark.

Coincidence, but I used sharpcap for PA for the first time last night. Was really easy to get good PA and I won't go back to the other methods now. 

For whatever reason by the time I got to guiding (about an hour later, due to some other issues) PHD2 thought I was 50 arc minutes away, but I'll put that down to either the fact that I had the mount directly on softish grass, and/or I'd left one of the bolts too loose, or the fact that the fov on my guide cam only had one particularly dodgy guide star.

Hoping for good results tonight!

 

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