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Old 15-11-2013, 08:22 AM
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pmrid (Peter)
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"drift alignment " for a Polarie

Polaries don't have any ST4 input, don't track in Dec and have only a couple of fixed rates in RA. So how can you modify the usual drift alignment techniques to get around these limitations? Any ideas anyone?

The Polarie has a front plate that is attached to the drive mechanism via 2 pressure screws. The camera attaches to that plate. The Polarie also has a removable rear plate which (a) contains a small coarse compass and (b) allows a proprietary polarscope to be inserted through the body of the device and aligned with the asterism of the SCP's Octans group via a reticle. That is certainly going to give you a pretty good approximation of alignment. I'm not sure what sort of error would be the optimal result of that process.

But if you wanted to do exposures somewhat longer than 60-90 seconds at FLs of 200-300 or so, then you're going to want to pare back that error as much as possible. So how can it be done?

To start, you need the ability to take an image that is precisely on the RA axis - i.e. up that centre hole. I'm wondering whether it might be possible to butcher an old finder scope to fit over the RA drive spindle where the camera plate would usually be fastened. And if that is possible, whether it is also possible to engineer a simple holder for a guide camera at an appropriate distance back from the objective of the finder scope - but inserted through or into the back of the Polarie.

Assuming all this can be done, how then can you use it to make some meaningful measurement of error in both RA and Dec? If you can persuade Maxim to see your guide camera as Camera 1, it should be possible to take exposures of any length - say 5-10 minutes - over which time, a misalignment should show up as an extended line - the longer the line the greater the error. This would, of course be a single line and interpreting the length and angle of that line would be a bit of a trick because it would contain elements of error in both axes. But I'm sure it could be done so that over time, you could get the length of that line to the minimum possible - meaning the best alignment of which that system is capable.

Does anyone have any other hairyarsed ideas?

Peter
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Old 15-11-2013, 08:38 AM
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alocky (Andrew lockwood)
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I've been wondering the same thing myself. There's two challenges, one is to be able to resolve drift into RA and dec vectors. The other is apply a sufficiently small adjustment.
I use a geared head and the vixen polar scope, but hang the camera off the front of the polarie during the process via its strap, as there is a surprising amount of flex in the RA drive.
I'm getting good 120s subs at 180mm focal length, I might try longer and see how good it is.
Cheers,
Andrew.
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Old 15-11-2013, 08:56 AM
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2stroke (Jay)
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Get the
Vixen Polarie Polar Scope

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Old 15-11-2013, 09:34 AM
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pmrid (Peter)
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Originally Posted by 2stroke View Post
Get the
Vixen Polarie Polar Scope

As I said:
"(b) allows a proprietary polarscope to be inserted through the body of the device and aligned with the asterism of the SCP's Octans group via a reticle. That is certainly going to give you a pretty good approximation of alignment. I'm not sure what sort of error would be the optimal result of that process."

I have one but want to go past the limitations it imposes.
Peter
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Old 15-11-2013, 09:46 AM
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pmrid (Peter)
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Originally Posted by alocky View Post
I've been wondering the same thing myself. There's two challenges, one is to be able to resolve drift into RA and dec vectors. The other is apply a sufficiently small adjustment.
I use a geared head and the vixen polar scope, but hang the camera off the front of the polarie during the process via its strap, as there is a surprising amount of flex in the RA drive.
I'm getting good 120s subs at 180mm focal length, I might try longer and see how good it is.
Cheers,
Andrew.
I just had a closer look - the rear cell that removes to admit to Vixen Polar scope device has a strange thread that defies my entire junk drawer full of "bits" . But because it is threaded, means that it would be a simple matter to cut a suitable length of a finder scope - about 100mm I'm guessing, and lathe up a collar to connect that to the back of the Polarie. I have already modified an old finder by adding a 1 1/4 inch removable eyepiece holder so attaching a QHY5 or similar to that would be a doddle.

At the front, there will need to be another collar turned up to allow the 50mm ID of a finder scope to sit over the 44mm OD of the RA plate - adding a few locating screws to hold it in place - so there is a relatively easy pathway to attaching (and removing) a guidescope to a Polarie. An afternoon on the lathe would do the job.

Assuming all that could be done and a guidescope could then be atached along the RA axis of the Polarie, the next question would be how to make best use of it.

If you just leave the power to the Polarie off and allow the camera to run for say 10 minutes you'd get some star trails which would be a coarse guide but because of low-elevation atmospheric distortion, you couldn't use that to ID the SCP because you'd be out by whatever that error was on the night.

Going to the east and placing yourself on a star and allowing a 10-minute exposure with the RA drive operating would give you an elongated star that drifted either north or south to give you an altitude error and the same near the zenith would give you your azimuth error.

So you could tell which way the image was drifting, I'd do a 10 minute exposure but put a bit of black card across the front after 60 seconds and hold it there for about the same - so that you'd still get a single image but with a distinct break marking the starting point.

Peter
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Old 15-11-2013, 10:50 AM
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nebulosity. (Jo)
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When I align my mount I take a four and a half minute subs like this: for the fist 30 sec I leave the mount doing normal tracking, and switch the mount to 2x RA for 2 min, and then the last 2 minutes is with no tracking. The resulting image shows the start position of the star and a trail heading to one side of the image and back again in a V shape, I adjust the mount accordingly until I get a single line.

Not sure if something like this could be done with the Polarie?

cheers
Jo
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Old 15-11-2013, 11:41 AM
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pmrid (Peter)
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Quote:
Originally Posted by nebulosity. View Post
When I align my mount I take a four and a half minute subs like this: for the fist 30 sec I leave the mount doing normal tracking, and switch the mount to 2x RA for 2 min, and then the last 2 minutes is with no tracking. The resulting image shows the start position of the star and a trail heading to one side of the image and back again in a V shape, I adjust the mount accordingly until I get a single line.

Not sure if something like this could be done with the Polarie?

cheers
Jo
Jo maybe a slight variation on the theme - since the Polarie does not have a x2 tracking rate (although it does have a 1/2 rate). Perhaps 1 minute with 1/2 speed guiding, 4 minutes at full speed and 5 minutes unguided? What do you think?
Peter
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Old 15-11-2013, 02:27 PM
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renormalised (Carl)
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Why go about even trying. These type of tracking platforms aren't designed for long exposure astropics. They're designed for wide field pics of sub lengths up to a minute or two in length at FL's of around 18-200mm. If you want better peformance, then you should be prepared to pay more money for a more robust and better tracker, such as an astrotrac or starlapse. Better yet, buy an inexpensive EQ mount with dual axis motor drive like the little iOptron SmartEQ Pro, EQ3 or something similar.

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Old 15-11-2013, 02:42 PM
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pmrid (Peter)
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Originally Posted by renormalised View Post
Why go about even trying. These type of tracking platforms aren't designed for long exposure astropics. They're designed for wide field pics of sub lengths up to a minute or two in length at FL's of around 18-200mm. If you want better peformance, then you should be prepared to pay more money for a more robust and better tracker, such as an astrotrac or starlapse. Better yet, buy an inexpensive EQ mount with dual axis motor drive like the little iOptron SmartEQ Pro, EQ3 or something similar.
Hello Carl. normally I'd say you're right. But I am setting up a traveling rig that I can manage in the confined spaces of a caravan for an extended period and have already decided that carrying around a HEQ5Pro with associated bits and pieces was no what I wanted - having done it for some time already.

And secondly - why bother - because I have a lathe, time and skills to do it for my own amusement. Capice?

Peter
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Old 15-11-2013, 03:12 PM
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Quote:
Originally Posted by pmrid View Post
Hello Carl. normally I'd say you're right. But I am setting up a traveling rig that I can manage in the confined spaces of a caravan for an extended period and have already decided that carrying around a HEQ5Pro with associated bits and pieces was no what I wanted - having done it for some time already.
Peter, I've got a StarLapse and the complete unit including polar scope fits in one of those Craftsman cases you can get at Bunnings.
It does need a more study tripod since it will carry a bigger load. I use a Manfrotto 475B and 405 head. No problems with lenses up to 500mm.
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Old 15-11-2013, 03:13 PM
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renormalised (Carl)
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Well, with a lathe and milling machine, etc, why not make your own mount

A HEQ5 is a little too much to be lugging about, that's why I suggested the smaller mounts. The iOptron mount isn't all that much larger than the polarie and tripod itself. But the polarie has less "fiddly bits" to lug about, like hand controller etc.

You may find that adding to the polarie and/or modding it might overtax the tracker completely and render it useless for what you want. To be able to drift align it, you're going to have to be able to correct in both axes and these trackers just aren't designed to do so. The only way to correct in Dec would be to have a dec motor/drive in the tripod head and sync it with the RA motor in the tracker.
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Old 15-11-2013, 04:28 PM
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Polar scope is easy to use assuming you have darkish skies and a view to the south. It will get better results than drift aligning.

If you want to drift align then this is what worked for me:

1. I used a digital inclinometer to set the angle of latitude (about $25 off ebay).

2. I used a compass to set south fairly close allowing for my latitude offset (12 degrees).

3. I set up a 180mm lens on my full frame camera. I took a 30 second shot and zoomed in to check the stars. If elongated I moved the mount east/west a bit and took another shot. If it was better I moved it the right way and moved it a bit more if stars were not perfect. If worse I went the wrong way so moved it back further than where I started from.

Exposure, check the stars, move east/west as per the above. As long as you set your latitude angle right then this should get round stars in 90 second shots at 180mm.

Expecting 5-10 mins round stars at 300mm is in my opinion not possible with this unit, its not meant for that level of accuracy.

15 minutes at 21mm and round stars I have achieved with a polar scope.

Greg.
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Old 15-11-2013, 04:34 PM
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Peter
I also used an inclinometer and compass to get my bearings with the Ioptron mount When i came up to your place but also see the pm that I sent you. It may be of assistance but Greg's post above certainly is a very good method.
Allan
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Old 15-11-2013, 04:39 PM
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What we need is to convince NASA to launch a special "polar alignment" satellite to hover above the sth pole rotational axis of the planet and have it shine at around mag 1 or 2...not too bright, not too dim. That would make things so much easier
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