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PostPosted: Mon Jan 17, 2011 11:16 pm 
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Location: Queenstown, New Zealand
Nice shots popo, the horsehead is one of my favourite parts of the sky - and thankfully quite visible here in NZ right now, although it's currently the height of Summer, so the sky isn't particularly dark.

PS - if you think polar alignment is a fuss, try it down here! At least you have a star near to the pole!


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PostPosted: Mon Jan 17, 2011 11:34 pm 
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While it is dark here, clear nights are still few and far between. And its getting lighter already so the chances are going to get slimmer going ahead.

On polar alignment, I personally think given today's technology, there must be a better way to do it than optically. Until then it is only a dream.

My other plan would be a movable fixed mount, if that makes sense. Something I can take indoors for storage, but can move out for use into exactly the same position repeatably. Then I could set it once and forget about it in future. But as I found out, thermal expansion was significant so I'm not sure how reliable this could be...

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PostPosted: Tue Jan 18, 2011 1:51 am 
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Weeeell, there's those top-end Mead and Celestron alt-azimuth mounts which thanks to a builtin GPS only need to be pointed at two or three known objects in order to work - although obviously an alt-az mount has field rotation issues for photography. Then again they do motorised field-de-rotators too!

I recently bought an Orion auto-guider with a Borg 50mm scope, so it'll be interesting to see how well that corrects for my poor alignment!


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PostPosted: Tue Jan 18, 2011 12:44 pm 
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Well... my Nexstar 4SE has the alt-az star alignment tracking option, and a configuration option for equatorial too, but I find it hard to get a good alignment even on that. Perhaps I need to get a reticle eyepiece and guider for that too. From memory I wasn't getting rotational issues at 30s on that, but I don't know how much I can get away with.

I saw that Meade have a self-setting unit with level/directional sensors and GPS, and also a built in CCD saving you the effort of using your own eyes. I think that is alt-az too.

The idea I had was if a variation on GPS configuration might give enough accuracy for directional alignment. Here I'm not so interested in absolute positional accuracy, which as we know isn't good on this scale with GPS. I wonder if two synchronised detectors spaced a small distance apart (each end of scope?) would cancel out the positional error and leave an accurate enough relative position output. But I'm not familiar enough with the details in GPS errors to know if that would work...

I think at this point it might be easier if I moved house to one with a big garden in the middle of nowhere :)

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PostPosted: Wed Feb 09, 2011 6:40 pm 
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You can buy a GPS dongle which can feed data to your scope via EQMOD and ASCOM connections, but as you say you are relying on the accuracy of GPS systems.

http://cpc.farnell.com/1/1/62842-usb-gps-unit-bn-903s-bn-903s-bluenext.html

I found the long and lat of my site using this website:

http://www.satsig.net/maps/lat-long-finder.htm

You can put in your postcode, allow the map to zoom in then find your exact location, double click and you get the long and lat. I would imagine it is more accurate than GPS.

Moreover, you should only need to add this information once into the mount. My mount, at least, remembers my Long and Lat. The difficulty will be when you use several sites, having to change the long and lat. I suppose this is where the GPS dongle comes in handy?

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PostPosted: Wed Feb 09, 2011 7:34 pm 
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Ok, two parts there.

One, the GPS system is "accurate enough" I think for simple positioning of a scope. If you're even a few meters out I doubt that'll be really noticeable.

My thought for using GPS for alignment wasn't for your local positioning, but for directional alignment. That is, polar alignment. That is the bit I'm not sure if it would be accurate enough for.

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PostPosted: Wed Feb 09, 2011 7:39 pm 
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Ah ok I get it.

Im not sure whether a GPS would help in that respect?

A polar scope is the best bet. You use a compass to get yourself facing north (roughly) and then the polar scope allows you nail the alignment by using polaris, and the date and time.

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PostPosted: Wed Feb 09, 2011 7:54 pm 
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I might be solving a very niche problem, but I'm an urban "back yard" astrophotographer. My garden isn't big and the view is very obstructed. Depending on what I want to image, I might move around my garden to pick which gap to use. Light pollution aside, low in the south is particularly challenging, and requires me to go as close to my house as I can. The problem there is I then lose sight of Polaris. I really wish there was a non-trial and error way to set it without visible means. GPS seems to be the best technological platform I can think of to base such a solution on, with the concept outlined previously based on the differential position of two receivers.

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PostPosted: Wed Feb 09, 2011 7:58 pm 
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How about drift alignment? I don't know the ins and outs but I think this is a way of obtaining accurate alignment without necessarily using Polaris, as long as you roughly point north, which a compass will aid it should work.

Try this:

http://www.andysshotglass.com/DriftAlignment.html

or

http://www.iankingimaging.com/show_article.php?id=11

I am interested to see if a GPS system would work though.

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PostPosted: Wed Feb 09, 2011 9:00 pm 
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That's the trial and error method I was thinking about, which I want to avoid.

The GPS method I outlined can work in theory, but in practice I don't know what the limits of accuracy are. In short, the longer the separation, the more accurate it gets. I don't know if that separation will be practical or not!

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PostPosted: Thu Feb 10, 2011 5:35 pm 
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Ok I see what your aiming for, but surely using a compass is far more simplistic and practical? We know what the magnetic variance is for a given location therefore we can accurately find where North is.

Small, portable and easy to use.

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PostPosted: Thu Feb 10, 2011 5:47 pm 
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Define accurate? A compass is too easily deflected by simply having it near a large metal object, like a scope and mount...

How accurate is accurate enough anyway? For example, I typically place Polaris in the polar scope pretty much exactly where it says I should, and being generous my error would be less than 1/10th the distance between Polaris and the actual pole. And that still leads to visible trailing at not that long focal lengths (to 400mm on crop) and durations (minutes).

I very much doubt I'll even get that close with a compass.

My only practical workaround I think is to have movable fixed mounts, if that makes sense. Then I can calibrate each mount position exactly using the drift method. But that means gardening first... maybe it'll get done in time for next winter but I doubt it :D

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PostPosted: Thu Feb 10, 2011 6:09 pm 
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Quote:
Define accurate? A compass is too easily deflected by simply having it near a large metal object, like a scope and mount...


Indeed it is, so this should be used before the mount is positioned. It will then obviously need marking etc so you can place the legs accurately.

Quote:
How accurate is accurate enough anyway? For example, I typically place Polaris in the polar scope pretty much exactly where it says I should, and being generous my error would be less than 1/10th the distance between Polaris and the actual pole. And that still leads to visible trailing at not that long focal lengths (to 400mm on crop) and durations (minutes).


Remind me what mount you have again? If you are accurately lining your mount this way then you should be able to get at least between 2-3mins per sub with no trailing.

Quote:
I very much doubt I'll even get that close with a compass.
I think, as you say, the same is for using GPS.

Quote:
My only practical workaround I think is to have movable fixed mounts, if that makes sense. Then I can calibrate each mount position exactly using the drift method.


Maybe not moveable fixed mounts but several piers with each pier being callibrated is the best bet.....or you could just move house, lol ;)

I hate to say it but it looks like drift aligning is going to be the best bet (or forking out for an autoguider.....)

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PostPosted: Thu Feb 10, 2011 6:42 pm 
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I'm using an Astrotrac on tripod with gear head. Only recently did I get a timer remote so I can do longer exposures. Without reviewing the images again, some trailing is visible but if I'm really honest that isn't my biggest limiting factor anyway...

Moving house is a very tempting option, but where to?... nowhere in the South of England is far from anywhere and I need a plan B before I quit my day job.

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Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.


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PostPosted: Thu Feb 10, 2011 6:54 pm 
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Ah yes, of course, silly me I forgot :)

Im guessing the view through the polar scope looks like this? (Top Image)

http://astrograph.wordpress.com/2009/09/14/collimating-the-astrotrack-polar-scope/

If so this is where some of the error is creeping in. The polar scope on my NEQ6 Pro looks like this:

http://www.astro-forum.cz/cgi-bin/yabb/YaBB.pl?num=1253607595

Basically I know for a given date and time the position of polaris, I set this on my mount and then fine tune my alignment so that polaris sits within the circle, labelled polaris. The cross hairs in the centre of the circle should now sit over the NCP. It looks like this isnt possible with the astrotrac polar scope?

I guess you better start playing the lottery as your plan B ;)

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