Yet another DIY star tracker and auto focuser

[rsarwar]

Hi - firstly, it is a long post, so I apologize for any typos - I am dyslexic but i will fix them as i find them.   

I have always loved taking night-time and landscape photographs. So, when I saw a picture of the Orion nebula, i was intrigued. and after a month of researching last year and realizing that it is probably not be very smart investing 2000+ pounds towards something that i did not know if i would find interesting in the medium/long run. So, I decided instead of buying ready-made stuff, I just make one.  

Step 1: I made myself a DIY tracker using an FPGA, two the software needed for it to integrate with Indi Eqmod. Step 2: Once i could see the motors spinning, I ordered an old Nikon D7000, took it apart and removed the IR cut filter using these instructions and checked to make sure it worked. Step 3: once confirmed, I got myself a EQ5 non-goto and a SW 72ED and integrated everything and the first night I tried, at NGC7000, I was amazed. quickly realized in need an auto guider and so I got me a sv305 + a Orion 50mm f3.5 guide scope and there I was taking 30-minute subs.  

I decided that I should write down what I did so that anyone else wanting to do so, can do it as well. all the source codes for the software and fpga are available online. knowledge of Linux would be desirable to carry out the instructions to remake it. 

Hardware needed:

1. PCB Prototype Board - £10
2. 2 x Nema17 1.7A 0.44Nm motors (RA/DA axis) - £20
3. 1 x Nema17  1A slim motor (focuser) - £8
4. CmodA7 FPGA - £80  (would have used cheaper Xilinx Artix boards from banggoods for £35 pounds if i knew about its existence - it is essentially the same thing but slightly bigger profile)
5. 4 pin Aviator plugs + enclosure + leds + optocoupler - £20 max
6. 3 x TMC2226 stepper motor controller - £21 pounds (could use cheaper DRV8825 as well but that would mean lower resolution for £7 but i dont like them as they run hot and burn out within a year or two. Plus the TMC2226 runs very cool and very very silent, absolutely no noise or vibration on my mount when slewing)
7. 2 x 60 teeth 6 mm bore and 2 x 12 teeth 5mm bore timing pullies - £20 max
8.  2 x 154 mm timing belt - £ 10 max.
9. Raspberry Pi4 2GB + SD card - £35 (but this is used for everything - connecting cameras, focuser, eqmod, filter wheel, guide cam-  so it is not fair to add the cost of it in this.)
10. Old 20v@4Amp laptop charger - £0 - we all have old chargers?.

that is a total of £224 or £179 when using the cheaper fpga. but you are also getting a ASIair with it as we will be running astroberry distribution on it.

And the gear ratio is such that the resolution is comparable to that of an eq6-r.

Mounting the motors:

mount - using the brackets that came with the motors:

Focuser - 3d printed a mount i found on thingiverse.com:

Hardware interconnect:

FPGA - FPGA contains the hardware logic needed to accept commands from the RPi and decode them. And according to commands it can issued.

1. GoTo/SlewTo commands 
2. tracking at a given rate,
3. take auto guiding inputs,
4. handle acceleration and deacceleration. 
5. Nikon D7000 cannot do exposures more than 30 sec over the USB, so it also controls the camera trigger - similar to SNAP port i guess.
6. SPI Save module
7. display IP (cant be turned off)
8. a noctua fan i had in the house somewhere and a 12 v converter for it.

Here is the picture of the board made. Notice that there is also a temp sensor connected to the 1wire bus of the Rpi

Here is all the connection from the controllers to the outside

 

Driver: The FPGA is controlled from the RPi using a 30 MHz SPI bus. The main driver for it is written in C++-14 using Koheron Framework, and runs as root at boottime runs a RAW TCP socket on the device. Any python script or C++ program with the right headers can send commands to the fpga over the spi bus to make it do things.

Software interconnect:

The question then becomes how to integrate it into one of the many control programs, e.g. kstar, nina, app, etc etc etc. I picked Kstar as it depends on indiserver which is open source and also has an elaborate eqmod implementation. but mostly because the code is beautifully written and easy to follow. Hence this is what the software stack looks like

1. Kstar runs on my desktop on ubuntu (I think they can run on windows as well. It runs on raspbery pi so that is also possible)
2. RPi is running astroberry distribution, so it comes packed with indiserver. on it the indiserver would run
3. the indiserver would then communicate its commands to the main_driver deamon (lets call in "serverd") which then sends the commands to the fpga.

Indiserver does not have any driver, but rather has loads them "dynamically" so we have to point to some drivers to control the mount, guide scope and the Nikon camera. we are not interested in the guide cam, as that is a standard driver. for the nikon camera indi_nikon_ccd works is well suited. and for the mount, indi_eqmod_telescope. However there are few  of problems:

1. indi_nikon_ccd: it has two problems, firstly, it has not temperature sensor. secondly, Nikon D7000 cannot do an exposure more than 30 secoond. There is a DSUB thing that can be used - but that does not do mirror lock up - so that's useless. Even if it could - i could make one far cheaper using an optocoupler from the fpga/RPi GPIO. hence i forced the source code and implemented a call to the fpga to send trigger and get temp from the temp sensor and store them in one of the FITs parm. I also made a peltier cooler and stuck it to the underside of the camera which is also controlled by the same driver using a different branch. However that experiment failed due to uncontrolled level of condensation.
2. indi_eqmod_telescope: This is the eqmod compatible driver for most skywatcher mounts. I am not entirely sure how it connects to them but i know that is it's intention. However, skywatcher uses some ancient ASCII based serial communication operated at a few khz (still? its 2021!). And i am using SPI@30MHz through the serverd operated on the localhost at port 36000. so i made some changes and swapped out all the calls made to the skywatcher mount, with equivalent calls to the serverd. and voila. we have a full implementation of GoTo. The screenshot below shows extra configs added to the program inoder to control the calibrations
3. indi_fpga_focuser: there is no good implementation of a focuser that came close to what i was trying to achieve. there is a close match, cannot recall what it is called at the moment, astroberry_focuser maybe, so i used that as a template to write myself as module that does the job. screenshot shows the main control page.

 

Performance:

Max slew Rate - 700x when using TMC2206 or 1200x when using DRV8825. running at 20 V.

Guide: here are some screenshots that may be of interest. My mount has not been mechnically optimized, i.e. it is a stock eq5. Total payload is about 7kgs with dew shield (quite a bit over limit) which stays on all the time to help with neighbour's bathroom habits. Here is one of my setups:

1. SW 130pds 1x CC
2. Nikon D7000 (same as ASI071MC) @ 4.76 um pixel size
3. Orion 50mm f3.5 175 mm scope
4. sv305 (same as ASI290MC) @ 2.9 um pixel size

so my main scope is at 1.19 a-s. Hence my theoretical max guide performance is about 0.6 a-s RMS. T On a vey good day, i say between 0.6-0.7. on a bad day 0.9-1. but mostly between 0.7-0.9. I should not, that I almost never do a precice polar alignment - but i do use static polar allignment before starting and happy with results less than 45 sec of deviation

Now I have no frame of reference as to what an eq5 goto can do, or heq5/eq6. reponses on forum varies from post to post - probably due to the level of maintanence. Please feel free to comment on the comparison.

 

 

I started this project as a way to prevent boredom during lockdown and so that my wife does not kick me out. while i am not bored, my wife may still kick me out as i seem to be hooked in this o-so-expensive hobby and spending a lot on it as is evident by my latest qhy268m + 2inch filters/wheel.

If you wish to make one, all the source code is there, and i have a pcd design that I will be happy to share with you. All you need is some skills with a soldering iron. the software codes are straightforward to compile - just follow the instructions. As is the FPGA code, but i can provide the binary firmware for it if you are interests  - all you have to do is flash it. fpga pinouts can be found here if you wish to make the pcb yourself. 

 

I am hitting my mounts load capacity so my next target is to find myself a old eq6 or maybe even a heq5 and do the same. if anyone has got one that they wish to get rid off at a price, i be happy to oblige . I found a AVX on the sales forum, but i am reading bad things about it so i am in two minds about that. 

 

 

 

 

 

 

 

 

 

 

Edited May 4, 2021 by rsarwar

[rsarwar]

new cleaner implementation of with proper PCB. this time using the cheaper chinese board.

 

been working on a belt-modification for CGEM.

Edited June 9, 2021 by rsarwar

[rsarwar]

finally managed to iron out all the issues i was having with the belt mod. mostly with lateral movement the the worm gear bearing - just got new ones.

obviously i donot have PEC on my DIY tracker so i use PPEC on phd2

 

this is the performance before the belt mod - very spiky and thos is one of the better nights

new one:

Edited June 22, 2021 by rsarwar

[Mach13]

Mind blown .🤯 Amazing piece of work...  If you have any tips for using the D7000, i'd appreciate them...Mine arrives tomorrow  👍

[rsarwar]

Thanks.

D7000 is a fine camera. astro modding is very straightforward with no real pit fall. 

https://www.lifepixel.com/tutorials/infrared-diy-tutorials/nikon-d7000-ir

 

One thing to keep in mind is that if you are controlling it from a laptor/rpi through a USB, your exposure will not go beyond 30 sec as it does not have full bulb support through usb. You can use a remote release, or get yourself a DSUB. but DSUBs are too expensive for what they actually are, so you can make yourself one using Esp32 and control it over wifi, or something like this: https://www.cloudynights.com/topic/456774-usb-serial-ftdi-and-ir-remote-shutter-control/

Also get an power adapter - about £25 on amazon.

Edited June 26, 2021 by rsarwar

[BCN_Sean]

That's a beautiful project, something that I'll keep in mind as occasionally unpowered eq3-2's and the equivalent pop up around here for not a lot!

21 minutes ago, rsarwar said:

One thing to keep in mind is that if you are controlling it from a laptor/rpi through a USB, your exposure will not go beyond 30 sec as it does not have full bulb support through usb. You can use a remote release, or get yourself a DSUB. but DSUBs are too expensive for what they actually are, so you can make yourself one using Esp32 and control it over wifi, or something like this: https://www.cloudynights.com/topic/456774-usb-serial-ftdi-and-ir-remote-shutter-control/

 

If the mount has the capability to trigger the camera, then it's possible to leverage that instead of using something like a dusb; it does mean another connection between the camera/mount/controller though and writing a bit of script to read a virtual serial port.  It's something I've been thinking of doing, picking up an old D7000 and using that as an imager.  Here's a link to the indilib post where it's being described https://www.indilib.org/forum/ccds-dslrs/8485-setting-snap-port-as-external-shutter-in-indi/65152.html

[rsarwar]

17 hours ago, BCN_Sean said:

That's a beautiful project, something that I'll keep in mind as occasionally unpowered eq3-2's and the equivalent pop up around here for not a lot!

My next plan is to make a real-time PEC for the mount. The idea is have a 2+2m multi-pole magnetic strip and use a AS53111 chip measure the rotation. it has a resolution of 0.5 micrometer. so should be able to run a feedback loop to the fpga to correct for periodic error. Only thing is i need to work out how to mount the chip on to the mount and also if i should run the feedback loop from the fpga or from the rpi. the correction latancy for running it on fpga is about 20 ms (5 ms if i dont use any smoothing filter), and latency for running it from Rpi will be about 0.5-0.6 sec which i fear is too slow but a lot simpler to implement.

That is about 30-40 pounds worth of hardware and to imagine that a similar instument is sold by explore scientific for £1250  is mind boggling. 

Edited June 27, 2021 by rsarwar

[BCN_Sean]

7 minutes ago, rsarwar said:

That is about 30-40 pounds worth of hardware and to imagine that a similar instument is sold by explore scientific for £1250  is mind boggling. 

To someone who has knowledge in building things it does seem quite costly, but to an ordinary person maybe not.  It's something I have to remind folk when they come to me for things that it's not the materials that cost what they are paying is the price of experience.

[Mach13]

20 hours ago, rsarwar said:

 

One thing to keep in mind is that if you are controlling it from a laptor/rpi through a USB, your exposure will not go beyond 30 sec as it does not have full bulb support through usb.

What now?  😮 can be controlled via USB? 😁 Amazing  I did not know that...  

What level of control can you have?  🤔

[rsarwar]

On 27/06/2021 at 18:07, Mach13 said:

What now?  😮 can be controlled via USB? 😁 Amazing  I did not know that...  

What level of control can you have?  🤔

you cannot really do anything much with the USB unless you get yourself a DSUB, use your snap port if you have one or make something to accopmish  remote release control

 

can i ask how much you paid for the camera and if it was astro modded?

[Mach13]

On 28/06/2021 at 23:35, rsarwar said:

you cannot really do anything much with the USB unless you get yourself a DSUB, use your snap port if you have one or make something to accopmish  remote release control

 

can i ask how much you paid for the camera and if it was astro modded?

Thanks rsarwar,  it seems you can actually do quite a lot... 👏  I've found (through the forum) that there are a couple of apps that work quite well Controlmynikon and backyardnikon 👍 the latter being the one I'm trialling see the YT demo.  🥇 You can create a live view from your camera to a laptop, adjust all sorts of settings, with some cool focus aiding tools too.. and take  images with a variety of options 🤯 Still a learning curve but well worth a look if you haven't seem them.  

 

As for my camera  I ended up buying a used but in good condition Nikon D7000 and Nikon Nikkor 28-100mm f3.5-5.6 AF lens £250 inc delivery.  I'm still trying to get used to it but its thrilling to get such quality terrestrial shots... 🤩 I got a T2 ring for it which attaches it directly to the OTA and give a good picture... Just need some clear skies to try it all out... When will that be? who knows 🙄

Its not astro modded as far as I know, but i will look into that when its out of warranty

Edited July 2, 2021 by Mach13
Correction

[markse68]

Really nice work there Rsarwar 👏

Mark

[old_eyes]

That is a pretty impressive project @rsarwar. All I managed to do in lockdown was fix a focuser problem and do some gardening!

[Mr Spock]

Excellent project and very well engineered 👍

[SaberJeff]

nice