I have to agree with Bojan, I am concerned about the steps taken to 'productise' virtually all aspects of life that can possibly be covered by a broad stroke Patent. Much like 'open system' software projects like ASCOM and others, there is no reason that an open system alignment routine that can work with any sort of webcam could not be developed. Many imagers already have guide cameras that woud be suitable, and while app features like DARV are great for drift alignment there is perhaps a need for a quick dark site field tool that allows tripod setup with minimal time and effort. So Open Source this please.
To find the celestial pole, however, it is better to use couple of stars, located around the pole (it is easy to recognize the asterism around sigma Octantis, and all those stars should be used, perhaps others at the other side of the pole as well).
With my camera with its small sensor and the 105mm FL tube I find the FOV is only 2x1.5 degrees so Sigma Oct is out of view. But BQ Oct and HIP42708 (HD 90105) form a nice little asterism along with some other fainter stars as you can see in the photo I posted earlier
At last a clear enough night for testing. ! was getting good images that plate solved quickly. My spreadsheet showed I was a few degrees out initially so I stared by manhandling the tripod into position based on the coordinates from a single image- that is uncorrected for any collimation error.
Once within 1 degree of the pole I started to rotate the scope 180 degrees to get two images. Once I started to adjust in the right direction (a small spreadsheet error reversed the azimuth so 179 was showing as 181) I started to get quite close. Ultimately I got to within a couple of minutes on each axis. To put that in perspective, on the SP mount that was about an 80th of a turn on the alt bolt and a 32nd of a turn on the azimuth bolts.
Not bad for just plonking the tripod down on the ground unlevelled.
I wasn't set up to do a drift alignment to confirm o that's for another day.
Attached are the pics taken 180 degrees apart plus the composite of the two showing the circle traced by the scope as it rotates. Sigma Octans is the bright star to the left of the composite.
Well, I have a Vixen SP with an excellent polar scope.
Polar alignment is very simple as long I photograph subs of max. 4min at 600mm FL.
I just center BQ Oct (NOT Sigma called 'Polaris Australis' (Australian Pole star ?) !) in the polar scope image. It is only 10' apart from the true pole.
When I need more accuracy then the drift method is the only option as the template is expired.
In the North I put Alpha UMi (they should call it 'Polaris Europeanis') 40' (not 50' as stated on the outdated template) at RA 3:0 apart from the true pole using the template as there is no closer star visible in the polar scope.
This looks very promising!
Both my transportable mounts (EQ3 and LDX75) have sensitive bubble levels and provisions for attaching the compass (mounted on a 1m aluminium 12x12mm "L" profile), so the initial setup always brings the pole close enough for fine adjustment with polar finder… which is a real pain (literary!).
As for the mechanical side of the procedure, if the mount is moved very often, to prevent the inevitable damage to screw threads it is a good idea to go further and replace the screws with ones with larger diameter and finer pitch (I used M10 with hex head instead of original M8, see here).
Some people even modified the elevation screw like this (with double pitch, coarse and fine), to smooth-up the process. http://www.3ashop.it/articolo.asp?ID=4030
It is promising indeed. I like your idea of a template showing the position of the asterism when you are properly aligned. I started to work on it but the geometry is doing my head in.
I put together an animated GIF showing the position of the asterism at HA of 6, 12 and 18h when close to polar alignment, with the collimation error included. It was certainly not intuitive to me. The red line shows the BQ Oct to HD90105 orientation and the yellow line the BQ Oct to SCP orientation.
What I'll need to do next is to take a series of images at intervals of 30deg around the RA axis after polar aligning just to see what they look like. That might help me work out the maths needed to calculate the orientation.
I'm still somewhat surprised at the magnitude of the collimation error. The centre of the camera FOV is 172 pixels from the RA axis at a scale of 11 arc-sec per pixel: that's half a degree!
Also attaching the spreadsheet I use to do the calculations - uncommented of course it converts coords coming from plate solving into Alt-Az
Well, I have a Vixen SP with an excellent polar scope.
Polar alignment is very simple as long I photograph subs of max. 4min at 600mm FL.
I just center BQ Oct (NOT Sigma called 'Polaris Australis' (Australian Pole star ?) !) in the polar scope image. It is only 10' apart from the true pole.
When I need more accuracy then the drift method is the only option as the template is expired.
In the North I put Alpha UMi (they should call it 'Polaris Europeanis') 40' (not 50' as stated on the outdated template) at RA 3:0 apart from the true pole using the template as there is no closer star visible in the polar scope.
Me too. I kept the Vixen polar scope as it is much better made than the Skywatcher one. Only problem is that the reticule is for 1985 or thereabouts. You can get closer to the pole by centering halfway between BQ Oct and the faint pair of stars between it and HD90105.
The reason I'm doing this project is that I find using the polar scope a pain in the back and other parts of my anatomy, it is hard to see the Sigma Oct asterism in light polluted skies, it can be useful if the East and West horizons are not easily visible (made worse by LP) and most of all its a bit of fun and seems to be getting some folks quite animated.
Just had an AHA! moment when I realised (after drawing up some sketches) that when the mount is rotated in RA then the image will rotate about some point offset from the image centre when it is not collimated. So the trick is to find that point first with the polar scope and camera assembled. It could be done in daylight or with a bright star. e.g. Take a long exposure while rotating to scribe an arc with the star. Use the arc to determine the centre of rotation. I'm hoping that with PHD I can then set a bookmark on that point.
All you need to do then is to position the SCP on that point. Or somehow fashion an overlay that shows the circles that BQ Oct and HD90105 would trace when properly polar aligned. Those circles would be centred on the cente of rotation already found. Place the stars on their respective circles and bingo!
I made up an image that illustrates how it works. It overlays the images taken 180 degrees apart. One with white stars, the other with yellow stars.
The red dot is the centre of rotation of the camera which is also the RA Axis. It just happens to be close to the SCP in this case.
So to establish the centre of rotation one could start by placing BQ Oct such that when the mount is rotated, its image does not move. It could be any star but best to choose one near the pole or at least near the right altitude so you don't need to put the mount far out of alignment.
The green circles show where BQ Oct and HD90105 would be located if the mount is polar aligned. They are centred on the red dot. The radius of each circle is simply the distance of each from the SCP.
Hopefully, it is apparent from the graphic that no matter the orientation of the mount, if BQ Oct and HD90105 are placed on their green circle then the centre of rotation of both image and RA axis are aligned to the SCP.
Now the trick is to find some software that allows a custom overlay on the image. PHD2 allows bookmarks and SharpCap has a movable, rotatable reticule. Any other possibilities?
My camera is WDM compatible so I can view it in VLC and add and position an overlay. However the overlay must be wholly within the video boundaries so I would need a cropped one to fit when it needs to be near the edge
So last I tried with an overlay in VLC. It would have worked had I sized the overlay to have circles with diameter TWICE the distance of each star from the pole.
Also, finding the centre of rotation was a little tricky. It's important to pick a star near the pole so it doesn't drift away.
Attached snapshot from VLC shows the rotation of BQ oct.You can see a faint semicircle
Another snapshot with the (correct) alignment overlay. This is a mockup as VLC snapshots exclude the overlay
........
Now the trick is to find some software that allows a custom overlay on the image. PHD2 allows bookmarks and SharpCap has a movable, rotatable reticule. Any other possibilities?
I found couple of apps, could be very useful for markings on screen:
Thanks Bojan I'll look into those.
Another thought occurs to me that using the method of finding the centre of rotation, one could position the camera anywhere including the normal guide camera position. The camera would need to be pointing close to the pole and the dec axis could be used to collimate, so the centre of rotation is in the centre of the camera FOV. Cone error adjustment may also be required.
The the overlay can be centred on the FOV and the BQ Oct asterism used to align as before.
Not required, ( but it can be tweaked by the same method ).
Ie even if there is a little cone error, "If you dont move the DEC axle" the circle described on the sensor is still purely a factor of where the RA axle is pointing .
I had originally used this method for setting the orthogonality of my LX200 OTA ( by only using 2 spots 180deg apart ), ref http://www.cloudynights.com/topic/51...oto/?p=6894272
In this case, i used a terrestrial target to find the centre of rotation,
then tweaked the OTA to get the "circle" centred on the sensor.
Then i thought it could also be used in reverse as a polar aligning method by simply using the mount controls to centre circles on the sensor ( no matter where they were ) with the SCP asterism.
Thanks Andrew. I was mainly thinking of how to align the centre of rotation with the centre of the image, specifically along the "y-axis".
Funny how I started out looking to do something similar to a Polemaster using a polar scope and am now ending up not needing any special device at all, other than something that can draw and place circles.
Interestingly, the icmeasure tool from TheImagingSource looks very suitable for that purpose and obviates any need to align the centre of rotation, other than to bring it into the FOV.
Thanks Andrew. I was mainly thinking of how to align the centre of rotation with the centre of the image,
Yep, but then you realise that for purely polar aligning, that's not reqd.
As long as you can get stars to form circles "somewhere" on your sensor ( ie you are close to DEC = +90, and your cone error is small )
all you need to do is centre the circle over the pole astrism by using the mount controls.
Indeed it is unnecessary - it's just that I just like symmetry I had a go just now getting the centre of rotation well within the FOV and it was remarkably easy. I was focusing on an insulator on the pole outside and adjusting declination till the circle was as small as I could make it.
I used icmeasure to first mark the start point of the feature I wanted to measure and the end point after rotating 180 degrees. With icmeasure it's easy to draw a circle to fit the two points and find its centre.
it converts coords coming from plate solving into Alt-Az
).
