Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,329
Hi Leon,
Your image is hard to interpret. I presume the 60mmm poly pipe is either the polar axis on your tracker or somehow aligned with it.
There are two ways of interpreting your photo. If the centre of the rectangular camera frame is the tracker axis, by my assessment, you are pointing about 1 degree from the pole. If the centre of the circular vignette of the poly pipe is the tracker axis, you are about 30' off the pole.
It all depends what you want to use it for? I find that pointing a tracker within ≤5 minutes of the pole gives me dec drift free imaging with the sort of lenses I use with a tracker over several minutes. When within a few minutes of the pole, I can do exposures like the one attached, 4 mins unguided with a 135mm lens. As your polar alignment gets worse, the combination of usable focal length and exposure length both become shorter.
Do you have or can you attach, an optical polar finder?
If the device can have a small optical polar finder attached, there are two stars, mag 6.8 and 7.8 that straddle the SCP by just a few minutes. Shown in the attached chart. These can just be seen in a 10mm diameter polar finder in a small tracker and are an easy reference for accurate optical alignment. I've been using these for many years with great success.
If the device can have a small optical polar finder attached, there are two stars, mag 6.8 and 7.8 that straddle the SCP by just a few minutes. Shown in the attached chart. These can just be seen in a 10mm diameter polar finder in a small tracker and are an easy reference for accurate optical alignment. I've been using these for many years with great success.
cheers
Joe
Me too
About two years ago, I aligned my AP1600 with it's right angle polar finder scope, on the SCP location on your chart, this took just a few minutes and it meant subsequent gotos placed my targets on my chip or occasionally just off (FOV=38'X30') at FL=1120mm, so mostly within about 10 arc min all over the sky and more than accurate enough for good tracking and superb autoguiding.
Joe makes some good points and I would say that for camera lens work Leon, your PA looks good enough for exposures of at least several minutes, depending on the FL of the lens.
Hi Joe thanks for your response, as I posted earlier here some time ago I have built a tracking device that is at the correct latitude to where i live and it was Solar Noon Time aligned with the Suns Shadow.
I then fixed the tube to the polar axis and shot a photo through that tube and hence the final rotation image and although a little off center I was wondering if it was close enough.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,329
I like the way it's referenced to the wall.
So if you've set it off solar noon and exact elevation for your latitude, it's possible that the device is well polar aligned but the camera was not looking straight up the centre given the circle isn't centred in the field.
I'd just try taking an astrophotograph with it and see if there's any dec drift.
Joe the Camera will not stay there it is mounted on the polar shaft which in turn rotates at 1 revolution per/minute in accordance to the equation of the curved bolt tracking device.
This Machine was built with a curved ¼” threaded rod arrangement with a pitch of 24 to the inch, and set at exactly 242mm from the center of the main angled shaft supported through a bearing set-up, and one RPM small electric motor, I found this in an American Astronomy Magazine many years ago.
Radius = 1436.5 x ( n/2 pie t ) where n = 1 and t = 24 Radius = 1436.5 x ( 1/2 22/7 x 24 ) Radius = 1436.5 x ( 1/6.285714 x 24 ) Radius = 1436.5 x ( 1/150.85713 ) Radius = 1436.5 x ( 0.0066287 ) Radius = 9.52212 inches, (241.86184 mm)
Apparently this set up according to others works very well for reasonably long exposures.
Location: '34 South' Young Hilltops LGA, Australia
Posts: 1,329
Yes, providing the threaded rod is accurately curved, it should track just a tiny bit slower than the King rate. (15.041 "/s).
Your device:
Tracking displacement angle per min = ATAN {Pitch(mm) / [Radius(mm) x 60] }
Tracking rate of your device in arc sec per second = 206265 * ATAN{ (25.4÷24) ÷ (242*60) } = 15.034 "/s
So assuming a perfectly accurate construction and perfect 1rpm drive rate, you'll accumulate 0.007 "/s lag error. This accumulates to about 2" in a 5 min exposure + any construction & motor error.
Note: It's impossible to build these things perfectly accurately.
For example, if the radius is 243mm instead of 242, then the drive error is 10x bigger 0.07"/s or 21 arc sec over a 5 min exposure.
21 arc sec over 5 mins corresponds to approximately 1.5 pixel drift using a 50mm lens and 8 pixels drift with a 300mm lens.
If the tracker is accurately built, then the drive error is about 1 pixel in 5 mins with a 300mm lens.
Wow Joe thank you for that very informative super mathematical calculations, I am pretty good at math equations but that was a little beyond me, thank you.
I have some super fast lenses and if it get 30 seconds to a minute I will be happy.
I think some experimentation is in order, and thank you again for your help.
even if perhaps a tad retro now 
and amazing work
