An Astrotrac clone

[jimao22]

Hi,

I want to show you an ATM project on the run, started by a friend of mine and continued by me and another friend of mine.  

The initiator of this project have in mind to create a barn door for the beginning. The idea evolved and at some point he said is a better idea to try "clone" the Astrotrac - which was a new astro-item at that time (2013), but he goes further and made a mount operated by a stepper motor by friction. He made the project and he had access to a CNC machine and everything was made under his supervision. Initial drawings and final ones are down bellow.   

[jimao22]

Everything was installed on a photo tripod and the stepper was controlled by a 555 timer circuit. The speed of the motor could be adjusted very easy with a potentiometer. The accuracy was not an issue due to the fact he wanted to use a DSLR  with a short focal length. First pictures with this nice set-up are shown here.

[jimao22]

The project was forgotten for a long while, because of lack of time and lost of interest. As a result, this nice mount goes to a close friend of mine (the one who made the motor control with the 555 circuit) who had in mind to upgrade the controller with an ARDUINO system. But he agreed to give me the mount in a win-win trade we did.

So here begin the story of the mount for me. 

My friend's task was to start and finish the ARDUINO upgrade - a better way to control the mount and an easy way to change and upgrade the tracking. He did his job very well, according with my requirements - 2 forward speeds (sidereal and lunar) and one backward (very high speed back). Also, he installed in the electrical system 2 limit switches to avoid accidents when motor is at the end of the run.

My task was to improve the mechanical part of the project, with the goal to use on this mount a small telescope or a RFT and a camera - of course, obtaining exposures of  5 minutes at least. I wanted to use ready-made parts and components because of lack of time and because I was sure the result will be better. i found a good idea is to use parts from Skywatcher Star Adventure,a good mini-mount with parts that can be purchased separately. 

In order to have a functional mount, I needed a tripod, a wedge and a "L" shape adjustable support (see down bellow).

  

[jimao22]

So the first result looked that way...

 

Tomorrow I will show you the continuation...

Edited March 2, 2019 by jimao22

[jimao22]

The problem with that configuration was not so obvious at the first glance, but it was a big problem. 

Due to the fact the RA movement is only on a 30 degrees of an arch, I couldn't point to a huge part of the sky. For example - on the South, or to the Zenith. The RA should be changed accordingly.

So my idea was to buy another "L" shape support and a counter-weight to solve this issue. This way, I "doubled" the RA movement and kept the DEC as well. Both (new RA and old DEC) movements are like a spherical  joint and so I have access to all sky, without problem. Moreover, I can motorize both movements for auto-guiding independent from the tracking, which I consider is a big advantage.

The pictures are bellow. I intend in few days to install a finder on the set-up to have a better field of view.

This story will be continued... 

[jimao22]

The two buttons to adjust DEC and RA independent movements.

[jimao22]

A last minute update - as I said, the finder will be installed on the mount. I find a way to do this with minimum of damage or changes in the components and maximum of sturdiness.

Now I have to calibrate the sidereal and lunar speed by changing a variable in the ARDUINO controller program. I made some measurements on the tracking sector and the following days I will make the calculation and adjustments to this parameter to see how it works. Anyway, a new improvement is rising because is obvious I need another counter-wight to compensate the imbalance on the RA tracking motion. Tomorrow I will be in my workshop to try solve this problem. When the new counter-weight will arrive to me, I will show you what I did.    

[jimao22]

Last day I spent it on my workshop working for the changes to the mount to resolve that imbalance of the weight on RA axis. I tried to resolve this using a piece left from one of the "L" shape supports and by milling a new part from aluminium to connect this part with the mount itself. This took me a lot of time, even if the piece had a shape free of constrains. Anyway, the result was very nice and after a sandblasting treatment of the mount and the connector, I had a nice surface to be painted. The color I choose was close by the equatorial wedge (red) keeping the edge natural (aluminium color) - to have a bit of contrast.

Because of the fact I have only one counter-weight at this moment, the pictures bellow will show you how the mount should look like with the only weight I have installed on both position one at a time. I will have the new pieces these days and I will be close by the final result one step further. 

[jimao22]

And the last updates, before getting into the sidereal and lunar speed calibration (1) and motors for auto-guiding (2). Today the second counter-weight arrived to me. Mean while I made 2 lids for the objective and the eyepiece of the polar scope. A last problem to be solved at this stage - the shaft of the tracking motor is intersecting the rod of the secondary counterweight when working. I have some options but I need to think properly before to act.

[jimao22]

Problem with the shaft was solved by grinding it (the fastest method). 

The remaining problem before goes under the stars with the mount and the telescope, was to adjust the sidereal and lunar tracking speed. This was not an easy task. The first time I had to figure out what is the current speed and precisely determine the major angles of the circle sector on which the engagement is made. That's why we used a special protractor. I found that for a full bow stroke we have 50.6 degrees and the angle between the arms of the arms is 35 degrees. This was the one I used for the following calculation. 

 

[jimao22]

A friend suggested using the tracking speeds used by ASCOM, so that I can no longer compute them (I only started calculating them and it was a beautiful exercise of theoretical astronomy). So I used the "Sidereal tracking rate" = 15.041 arcseconds / second and "Lunar tracking rate" = 14,685 arcseconds / second ( https://ascom-standards.org/Help/Platform/...eRates.htm). After two attempts to reduce the errors to lower values, the third one came out perfectly (or almost) so that the 35 degree circle sector of the mount was moved by the engine in 2 hours and 19 minutes vs. 2 hours, 19 minutes and 37.2 seconds as the theory was. Approximation was good enough, I said ...
The last step was the star test. So, Sunday night - March 18, I pulled out the mount outside the observer, pole aligned not very accurate, and looked at a brighter star from the southwest sky that was at my fingertips (star turned out to be Procyon of the Canis Minoris). I made 30 'and 60' exposures, obviously unguided, with the ASI 174 mono camera, using FireCapture as an acquisition software. The .fits images I've uploaded to MaximDL to analyze them there. The results were more than my expectations.

The roundness of the stars was between 0.162 and 0.25 (the latter eliminated by the software) for the 30 'and 0.170 and 0.252 exposures for the 60'. For unguided images, at first try, I think is amazing! I am so pleased!...

Edited March 19, 2019 by jimao22

[bitnick]

What a nice build! Beautiful!

Can you tell me more about the friction drive? It looks like it is metal-to-metal contact - steel-to-alu to be exact. Does this work well? Is there some flexibility somewhere in the construction or do you rely on very precise machining of the arc (concentric to the RA axis) to keep the contact force constant?

Also, I'm trying to figure out how the RA bearing is constructed. What kind of bearing do you use (radial, single/double angular contact etc) and how is it mounted?

Thanks for presenting a very interesting build!

[jimao22]

Hi,

As you can see in the pictures with the star shape analysis, the mount is working great, so the concept is very good. Moreover, I am sure the results will be better than usual commercial mounts same class because the friction drive is very stiff an robust and allow greater weights to be worn.  The bearing is a radial SKF one and is mounted with a press, like the ones used in the car maintenance. And the machining was very precise being done on a CNC machine wit a high accuracy.

[jimao22]

An update to this project. After few runs for testing the pros and cons for this mount, I made some technical adjustments. 

1. I had some problems to polar align the mount, due to the fact the polar scope was not illuminated. So I bought a device for this purpose, a part of the modular system of Star Adventurer mount. Problem solved.

2. I had problems to obtain a good focus for the stars. So I made a Bahtinov mask who fit perfect to my Omegon 66 telescope. The Bahtinov mask is a wired one - a mask with some good advantages over a regular one due to the fact an increased quantity of light is getting inside the camera sensor. As a result, you can focus with very short exposures (1), or on dim stars (2) or with narrowband filters (3).

3. I made a huge mistake thinking that the finder is installed on the right position on the horizontal bar (see the pictures from previous posts). The finder is aligned with the main telescope only on RA, but not on DEC. So, to resolve this problem I put a shoe on the main scope and use a lighter finder scope - in this case a green laser or a red dot finder.

4. I encountered some problems when went on the field at night and tried to power the mount from my car battery. The battery had around 13 V when the mount wasn't working but the voltage droped down to 10 V when mount worked. As a result, the mount was blocked after few seconds. So I made a 12 V voltage stabilizer. This device is inserted between the power supply and the control module of the mount and the tension never drop under 11 V no matter what. So now the mount is working in the field too.

 To be continued with pictures of deep sky objects... When weather will allow.

[bitnick]

On 07/04/2019 at 15:21, jimao22 said:

As you can see in the pictures with the star shape analysis, the mount is working great, so the concept is very good. Moreover, I am sure the results will be better than usual commercial mounts same class because the friction drive is very stiff an robust and allow greater weights to be worn.  The bearing is a radial SKF one and is mounted with a press, like the ones used in the car maintenance. And the machining was very precise being done on a CNC machine wit a high accuracy.

Thanks! Yeah I'm sure it's better than most commercial mounts of the same type; it looks very well made!

How do you preload the friction drive? Is it enough to push the motor axle onto the aluminium arc and tighten the screws holding the motor?

[jimao22]

The friction drive can be adjusted with a screw.

You cannot preload by pushing the shaft other than using  some kind of lever operated by a screw.

So is adjustable.

[bitnick]

I've looked very carefully at the pictures in the thread but I cannot see the adjustment mechanism. Can you post a detail image of this? DIY friction drives are rare - even more so DIY friction drives that work well! - and it would be very interesting to see how you've solved it.

I've been sketching a similar but somewhat simpler astrophotography mount, using a couple of angular contact bearings (single row would be more fitting but turns out to be considerably more expensive in this size and with rubber seals) on a long 3/8 inch bolt that goes directly into the tripod mount thread on a ball head. Under the ball head would be a plate similar to the "arc plate" in your mount driven by a friction drive using a small stepper motor and gear box (these are made to fit together). But I'm a bit stuck with the friction drive; I find it diffcult to come up with a mechanically simple preloading mechanism.

[jimao22]

You cannot see it because is on the back side of the mount, where the motor is.

Is not rocket science, is just a screw that make a lever to push the motor axis on the sector. Is not big deal. 

[jimao22]

Due to the very bad weather this spring time, it was very hard to find a good night for testing properly the new mount. But somehow I find a proper night being with one of my astro buddies in my observatory.  I made a lot of exposures with Sharpcap but the darks and flats made with MaximDL didn't work to calibrate these images. I suppose is some kind of incompatibility in acquiring the data. Even if I tried to make new darks and flats few day later using the same Sharpcap, I didn't obtain better results. The best results I obtained when acquisitions were made with MaximDL - lights / darks / flats and biases. But I had only 5 exposures like this, 90 sec each (lights). 

The result is great in my opinion, roundness of the stars is 0.084 and I am sure I can increase the exposure time somewhere between 2 and 3 minutes without problems - unguided subs with same great figures. The most important thing to obtain these results is the proper polar alignment.  I think a PoleMaster it will be the best improvement ever for this nice mount I have.

[jimao22]

Finally I had some decent weather and I made some exposures with my new rig: the home-made mount, the Omegon 66/400 and the ASI 174 camera. All images have the roundness less than 0.1 (measured in MaximDL) and I found that is no need for long exposures to have very good results. The final result is a balance between the shortest exposure you can do it with a good enough SNR  (1) and the longest exposure you can afford  with a good roundness (2). 

(1) depends on the local light polution, the contrast between the deep-sky object and the  sky background

(2) depends on the mount accuracy

On my site I found that I need exposures between 30 and 60 sec for the best results with this set-up.

Next tests - with the ATIK 460EX CCD camera. Much bigger... 

M92 - 9 x 40sec

 

M101 - 20 x 60sec

M51 102 x 40 sec

 

M51 - detail

Edited June 10, 2019 by jimao22

[jimao22]

These days I was at some star-party into the mountains and I finally made the tests with the ATIK 460EX. The field is the bigger I can obtain with what I have and due to this fact, I made some exposures to Amdromeda and Pleiades.

Andromeda is 27x60s+30*x20s+30x5s. M45 is 47x60s. All procesed in PI without calibration, no filters at all and a Moon at 35%. 

Edited August 27, 2019 by jimao22