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A sudden thought: why must I polar align every time?


BrendanC

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I leave my mount in the garden, covered up with a fairly substantial tent which keeps it nice and dry. I bring the OTA indoors. 

Since doing this, I've been polar aligning before each and every session. I use Sharpcap so it's not so onerous, and it's generally not far out - but I suddenly thought: if I don't touch the mount between sessions, why does it go out of polar alignment at all? It's never far out, but it's also never as good as when I last did it.

Shouldn't I just be able to get it as close as possible, and then leave it?

Or is it really down to those minuscule differences between the mount when I left it, and the mount when I come back to it?

I mean, it's not awful having to polar align every time but I'd rather save a precious few minutes.

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In theory if you don't move the mount at all then you don't need to re align every time. 

But in the real world, unless you use a pier mount then I think you would be moving the mount slightly every time you remove and put the OTA back on.  It might not be moved noticeably, especially if you are at short focal lengths and are guiding.  Try not polar aligning and see what happens.

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Here's an idea. Use Sharpcap to polar align until you're happy. Then, shut everything down but don't move it. Turn it all back on again and do another polar alignment. You'll probably notice that it isn't exactly the same as when you turned it off. 

When you go back to it and do it again, how far out of PA is it? If it's less than 1' then leave it. You're sky limited at that point anyway so trying to get closer than that is pretty much pointless, especially if you're guiding.

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Once the polar axis is aligned satisfactorily that should be it unless it is subsequently disturbed.  Slight differences in assembling the telescope shouldn't matter, it's the polar axis of the mount that's aligned, not the telescope.    🙂

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Interesting stuff, thanks everyone. I like the idea of aligning, switching off, then back on again and repeating, just to get a sense of what's going on.

Then, I'll spend a bit of time getting it as correct as possible. And then I REALLY like the idea of seeing how I go without it altogether.

It just never occurred to me over the past, what, six months since I started using the NEQ6 (my first EQ mount), that I didn't have to do it every time! Doh...

Tell you what though: I'm really good at it now... :)

Edited by BrendanC
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33 minutes ago, david_taurus83 said:

I used to carry my AZEQ6 in and out of the house with OTA attached each session. I have 3 indentations drilled into the slabs where I setup. I eventually didn't bother polar aligning each time as it guided out fine.

Same here except for lifting everything in and out of the shed onto concrete with indentations (and white painted rings).  I check the pa with PoleMaster every time for astrophotography but the amount of adjustment is so small as to be pointless.

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I could be wrong (I normally am and if I am I'm sure someone will correct me) but but given a particular (single measurement) error in RA and Dec drift there are an infinite number of possibilities for the error in Alt and Az. I suspect that the Alt - Az error is therefore determined by an iterative process rather than a 'fixed' formula so that the final result will always depend upon the chosen starting conditions, which would give rise to slightly different answers even if the mount was perfectly aligned.

 

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My EQ6R is permanently mounted and i don't do PA for weeks/months at a time, guiding in Phd2 is always around 0,5"/pix, but when i do check PA with my polemaster, there's always some adjustment to do, like mentioned above, even when i repeat PA 2 or 3 times in a row.

But if guiding is around 0,5"/pix i'm not complaining.

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  • 4 weeks later...

Just thought I'd pop up, seeing as someone had 'liked' my earlier comment, to say that everything's been going perfectly fine since stopping polar alignment! I might check it from time to time, but it's been working well with good guiding and results...

Autosave5.thumb.jpg.6f3c09534f1102d8211708e570b8ee0f.jpg

Cheers!
Brendan

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  • 8 months later...

  I found out that my polar alignment is off by a small amount between times I put my scope on  and off my mount inside my observatory even though the mount is on a permanent pier. Maybe it is due to expansion and contraction of the pier from temp changes. I am in South Carolina and it get to 100% then down to  72% or lower at nigh at this time. Makes sense to me that this might be the cause of a small change in polar alignment, so I check each time to make sure.

  I have also attached a pic of my mount in my homemade observatory  where I used a 12.35 ft old satellite dish as a dome, it works great and cost me less than $500 to make and its larger than one I could have bought from SkyShed or others. It has a concrete floor with carpeting now and later I might put up nicer inside walls.

IMG_0492.JPG

IMG_0458.JPG

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