Introduction: Auto Guiding Sky Tracker
Have made a sky tracker before. The function is quite OK.
But I'm frustrated by the polar alignment. To get accurate polar alignment, it is very time consuming.
That is why I build a new one with auto guiding capability. With this new sky tracker,
the set up time can be reduced to below 10 minutes.
It is not the normal auto guiding sky tracker that guides only the west-east direction.
It guides both the east-west and north-south directions.
On the other hand, it is not the traditional equatorial or Alt-Az mount.
It is in fact a normal sky tracker on the top of a seesaw.
Anyway, it still can be used as a normal sky tracker that does not have auto guiding capability.
That is no computer or guiding scope is required if you want to reduce the loading of my backpack :)
Step 1: Moving Mechanism 1
The key materials are two 1:100 harmonic geared NEMA17 steppermotors,
three camera quick release adapters, some aluminum bars,.......
No additional gear, timing belt or else are required.
It makes the fabrication works very simple.
Precise workmanship is not required basically.
A bench drill and some fitter's tools will be OK.
Of course, a bench milling machine and lathe will help a lot.
The dimensions are not critical.
Just make sure the angle of the seesaw matches with the latitude of your location will be OK.
Step 2: Moving Mechanism 2
On the top, it is the normal sky tracker that control the west-east direction.
And on the bottom, it is the seesaw that controls the north-south direction.
Nearby the bearing on the side of the seesaw, there is a screw.
It is used to limit the movement of the seesaw.
It is used when auto guiding is not used.
That is the normal sky tracking mode.
On the other hand, under the seesaw, there is a mini turn table.
It is used for polar alignment by drift method.
Step 3: Moving Mechanism 3
The whole mechanism is mounted on the camera tripod by using a camera quick release adapter.
That is it can be taken down from the camera tripod quickly and easily.
Step 4: Moving Mechanism 4
Not full 360 degrees of the west-east motor will be used.
Only the region in red will be used.
The aim is to reduce the length of the moment arm, and thus reduce the torque being applied to the motor.
On the other hand, in this region, the weight of the camera will help to reduce the effect of the gear's backlash.
Step 5: Moving Mechanism 5
It can be split into two portions, so that they can be put into the backpack easily.
Step 6: Moving Mechanism 6
Owing to the design limitation, some sky trackers get difficulty in pointing the camera to the zenith.
And it is not easy to maintain the balance of the system.
A L-bracket and a tilting platform have been built to resolve these problems.
The L-bracket lets the camera mounted right over the motors.
Thus reduces the torque being applied on the motors.
More stable and less electricity consumption.
Step 7: Moving Mechanism 7
The tilting platform let the camera point to the zenith easily.
Step 8: Moving Mechanism 8
The L-bracket can also be mounted in opposite direction.
It helps the camera to point to the low latitude southward sky.
Step 9: Moving Mechanism 9
This tilting platform can be rotated in 360 degrees.
It helps the camera pointing to low latitude sky in whatever directions easily.
Step 10: Moving Mechanism 10
On the left hand side, it is the polar scope.
To make sure it is parallel with the motor axis,
a aluminum tube with a hole the same size as the polar scope is turned by a lathe.
And both this tube and the motor are fixed on the same aluminum plate perpendicularly.
So the parallelism of the scope and the motor axis can be guaranteed.
Step 11: Moving Mechanism 11
A nylon stopper is put on the camera quick release plate.
It helps to avoid the camera from moving.
It is very essential for long exposure time photo taking.
Step 12: Guide Scope and Guide Cam
I used a old 50mm lens as the guide scope.
And used a cheap electronic eye piece as the guide cam.
A rear lens cap and a 3D printed adapter are used to put the lens and eye piece together.
Step 13: Control Electronics 1
A very old palm size computer that runs XP is used.
And a free software "GuideMaster" has been installed
-- to receive the image signals from the guide cam through a USB cable.
-- to process the image signals and then generate the sky tracker control signals.
Software download : http://guidemaster.software.informer.com/
Step 14: Control Electronics 2
The sky tracker control signals need to be converted into the signals that can be understood by the sky tracker controller.
Thus a "signal converter" has been built.
For the detail, please refer to :
Step 15: Control Electronics 3
Sky Tracker Controller.
This is in fact an Arduino based controller for panorama and timelapse photo taking.
I modified the circuitry and program to include the sky tracking function.
BTW, this controller can control the sky tracker to perform the normal star tracking without the guide cam and computer.
Step 16: Power Supply
The internal batteries of the computer and the sky tracker controller can last for about 3 hours.
External power supply is also available, just to make sure you can enjoy the whole night without worry :)
A 10000mAH power bank for the computer.
A 12V battery pack for the sky tracker controller.
Step 17: Result
Of course, polar alignment is still required.
But the time consuming drift method is not a must now.
The set up time has been reduced to under 10 minutes.
And for wide angle lens, the normal sky tracker mode is OK.
That is no need to use the computer, guide scope and guide cam.
Less weight :)
We have a be nice policy.
Please be positive and constructive.
I really like what you have done here making this a versatile compact design.
I'm wondering if you are relying on micro-stepping to get sufficiently small increments in RA? Is the Harmonic Drive gearing, with its very small backlash, essential, or would a less expensive planetary gear work as well?
Your result with 10 minute exposures @400mm has nice round stars!
Thx misharetzer, I think the micro stepping is not that reliable in positional accuracy. But it helps a bit in smooth movement.
And I guess the planetary gear will just work fine as long as the zenith position is avoided.
Thank you. I believe you are correct on both points.
I'm fairly new to this, and have been studying what other's have done and looking at commercial products before building something myself. I like several things about your design. Your approach is very compact, making it easy to transport while also reducing many of the practical problems with drive trains and material flexing. The only down-side seems to be that the increments in RA tracking are somewhat coarse, with fairly large jumps:
I do not really know what is "good enough," and this may very well be a good tradeoff within your overall error budget.
Not know the "good enough" figure neither :)
The commercial sky trackers use worm gear usually. And I saw the gear ratios are from 120:1 to 180:1.
There are some DIY sky trackers, I saw in the internet, use 100:1 harmonic gear drive also.
I guess it is the biggest gear ratio that is available for the DIYer.
BTW, if no micro-stepping is used, a bit jumping can be felt.
I used 128 micro-steps, no this feeling anyway.
Your results using the 400mm lens look really good. My guess is that the micro-steps are helping a lot to smooth out the jumps. I think I'll change my design strategy to look more like yours.