DIY - Sidereal pan head

[Photosbykev]

I'm starting to spec up a home brew stepper driven pan head for some terrestial/astro work, it would only be used for DSLR timelapse work.

The stepper motor/gearbox is a 1.8 degree nema 17 motor interfaced to a planetary 100:1 gearhead rated at about 40Kg/cm.

One step of the motor is going to give me a 0.018 degree step on the gearbox output shaft. I'm wondering if this is small enough or should I consider adding a 10:1 belt driven gearing on the output shaft and mount the camera on that?

Has anyone else gone down this road and are willing to share their problems/insight?

[Horwig]

Nice project. If my maths is up to it, your motor will step every 4.32 seconds, which should be plenty fast enough for a reasonable focal length lens.

This is a good calculator I've used in the past:

http://www.sceneplanner.com/tool3.php

I used to have a Vixen Polarie for just such use, but the problem was that a 5Diii with lens was on the upper end of its payload capacity, and so I'd get slippage, so make sure you've got enough power and holback torque to lift the load to the meridian, and hold it on the way back down.

Best of luck, looking forward to see the pics.

Huw

[Lonestar70]

Post removed...  due to bad maths will re-do.

[Lonestar70]

Hi,

OK I have now got my maths cap on straight...  

I originally calculated 200 steps per motor revolution giving 3.6 deg rotation at the output shaft.

I had already calculated sidereal rate to be 3.98907 minutes per deg.

I then, for some daft reason, decided that 3.98907 / 3.6 gave a motor turns rate of 1.108 revolutions per minute... which was, of course, total rubbish... needless to say what followed became even more rubbish...    

I have now corrected my thinking and got me hat on straight so...

The step rate given by Huw is approx correct ...(I calculated it to be 4.308 sec per step based on 23hrs 56 minutes and 4 sec per sidereal day), however, there is another issue that you may need to consider.

0.018deg per step = 1.08 arc minutes per step... or 64.8 arcseconds per step.

This is a lot when compared to the arcseconds of image per pixel of the camera sensor (which will vary with focal length)

If your stepper controller is capable of micro stepping then it may be beneficial to use it.

at 1/8 micro stepping rate (1600 steps per motor revolution) the output would become 64.8 / 8  = 8.1 arc seconds per step... this would mean the input pulse rate would become 4.308 / 8 = 1 step every 0.5385 seconds (1 per 538.5 milliseconds)

Better still would be to use 1/16 step rate (if available = 3200 steps per motor revolution) which would result in 4.05 arc seconds per step and an input pulse rate of 4.308 / 16 = 1 step every 0.26925 seconds (1 per 269.25 milliseconds)

This may be more in keeping with modern high pixel cameras.

Hope this makes sense.

Best regards.

Sandy.

[Horwig]

The step rate given by Huw is approx correct ...(I calculated it to be 4.308 sec per step based on 23hrs 56 minutes and 4 sec per sidereal day), 

OOPS, I Stand corrected

   

And yes, microstepping at 1/8 step should indeed give you round stars up to about 150mm on a Canon 5diii (hope my maths is better this time, if not, Sandy, can I borrow your cap!) .

Huw

[Lonestar70]

 

Huw,

Yours was an acceptable approximation based on 24 hours...wherase, my original effort was just utter rubbish... you can borrow my cap any time, however, it is very fussy about how it is worn.  

Keep happy.

Best Regards.

Sandy.

[Photosbykev]

Thanks for the maths lesson I've copied it into a spreadsheet lol. A lot the time I'll be timelapsing and sometimes a slight star trail per exposure is beneficial as it make the final video output smoother. I was planning to use a move/exposure/move rather than shoot on the move. Using the Adafruit or Big easy driver I can microstep at 1/16 which I might implement during an exposure, certainly I'll programme the processor to give me the option so I can test the result.

[Photosbykev]

For information shooting with a Canon 5d3 and 24mm prime lens I think I have 46 arcsec/pixel. Obviously less than that as the focal length goes up

[Photosbykev]

Initial testing on the stepper gearbox with an arduino 2560 board and big easy stepper board is giving me a resolution of 884 microsteps / degree with more torque than I know what to do with lol. Just need to code it up properly now and add a LCD display, buttons, RTC and temp/humity sensor. Oh and get some metal cut to built the head up.

[Photosbykev]

Note to self: soldering and wiring on an old antique oak table constructed with large iron rivets is not recommended. The large rivets and the back of a stepper control board do not make good bed fellows !

[Lonestar70]

Note to self: soldering and wiring on an old antique oak table constructed with large iron rivets is not recommended. The large rivets and the back of a stepper control board do not make good bed fellows !

Hi Kev,

884 steps/deg sounds perfect... well done.

You need to get one of these for working on electronics boards.

http://www.ebay.co.uk/itm/490-x-600-Desktop-Anti-Static-ESD-Grounding-Mat-Cord-/370564096073?pt=UK_Computing_Other_Computing_Networking&hash=item5647576049#ht_738wt_723

What metalwork are you needing to get made?... I have a fully equipped engineering workshop if I can be of any help.

Best regards.

Sandy.

[Photosbykev]

Hi Kev,

884 steps/deg sounds perfect... well done.

You need to get one of these for working on electronics boards.

http://www.ebay.co.uk/itm/490-x-600-Desktop-Anti-Static-ESD-Grounding-Mat-Cord-/370564096073?pt=UK_Computing_Other_Computing_Networking&hash=item5647576049#ht_738wt_723

What metalwork are you needing to get made?... I have a fully equipped engineering workshop if I can be of any help.

Best regards.

Sandy.

I've got an anti-static setup but I was being lazy and got what I deserve lol. New driver board ordered

I've got access to a full cnc workshop but thank you for the offer it's much appreciated.

[IanWatkins]

Hi Kev,

Watching with interest as I've recently been playing with Arduinos and steppers. I've got a couple of NEMA17s and thought the same thing, homebrew Polarie.

Keep us up to date and let us know how you get on.

Cheers

Ian

[Photosbykev]

Hi Kev,

Watching with interest as I've recently been playing with Arduinos and steppers. I've got a couple of NEMA17s and thought the same thing, homebrew Polarie.

Keep us up to date and let us know how you get on.

Cheers

Ian

no problem

I've got all the components breadboarded together, my main focus will be on the code for camera control or more likely I'll use a Promote Control handset which can be used to talk to the camera and also the stepper drive.