DIY Autoguider for Long Exposure Astrophotography (Part 3)

If you are new to this post thread it might be a good idea to check back on a few posts to see where this project started and how it has evolved:

 I managed to drag myself out last night to test the new tracking mount under a clear (suburban light poluted) sky. It felt like there was a million different settings to get right for this to work. I used the PDH ( software (version 1) to interpret the image from my webcam to decide how to adjust the tracking system in right ascention (RA). I do not have any way to adjust declination (Dec) and therefore that part is ignored.

Tracker 300914-6190

After I set the tracker up on the Meade tripod and made a very mediocre attempt at polar alignment I started up the tracking/guiding system and got nowhere. I could not figure out why the guidestar was drifting west no matter what I do. Agter a while I discovered that my Arduino sotware still had the step directions in the same way as my previous mount used and that it was driving the wrong way!  After a quick edit to correct this I tried again and at first thought that something was not working because it looked like my webcam was not sending updates to the PHD software. I opened the tracking graph on PHD and saw that the tracker was actually tracking very well for a system that was not optimised at all. I got a correction graph that was ±3 pixels accurate. The guiding was very jumpy and this is probably due to my asymmetrical Arduino software stepping rates for the +RA and -RA ST-4 interfaces. Should be easy to fix this for the next try. I will try to smooth out the tracking to control the sudden RA events. Here is an image of the RA tracking graph from PHD. The grid is spaced at 1-pixel increments on the y-axis.

PHD Graph

I downloaded the log file and used Excel to create a graph of the RA data:

RA Graph

These two graphs are not from the same tracking period but shows the typical results I achieved last night.

The proof of the pudding lies in the results. I took a five minute (302 seconds) with a Canon 600D, Canon EF-S 60mm lens at ISO200 and f/2.8. The image shows a sudden jump in the mount and this might have been me or one of the overcorrections that I have to smooth out in the guiding settings. (Please ignore the rest of the image as I was only interested in the tracking performance.) Click the image for a larger version.

Tracker 300914-6188

I was surprised to see that I captured a small galaxy in the image!  It turns out that the galaxy is the Sculptor Galaxy or NGC253.


8 thoughts on “DIY Autoguider for Long Exposure Astrophotography (Part 3)

  1. Dastro

    Great work on this project. So To understand your approach , your tracker works assuming you line up to the southern pole and then rotates at the same speed as the sky ? So how does the guide work ? Does it only adjust the polar (sky rotation) speed?

    1. Theo Post author

      Hi, sorry for replying so late…
      Yes, you are correct. I spend some time aligning the RA shaft to the southern celestial pole and then the tracking will only work to track and make corrections on the RA axis. To add the DEC axis is another project for the future…
      The webcam locks onto a guidestar and measures the guiding error. Software in a laptop and a small microprocessor will then work together to change the stepping rate in the stepper motor that drives the RA axis. In this way there is a closed control loops that, if set up correctly, will be able to keep the camera locked onto a specific section of the sky over a relatively long period (perhaps an hour?). This allows you to take a series of longer exposures that you can use to develop a great image.
      Its not easy, its a lot of work but it is very satisfying to make your own gear and get reasonable images from it.

  2. Mohammed Amr Abdelfattah

    Hello Theo!

    I have a quick question for you. Have you ever found it difficult to find suitable guidestars using your Lifecam/finderscope combination? I’m considering buying this camera to use with a 9*50 finderscope.


    1. Theo Post author

      Hi Mohammed,
      YES!!! Just last weekend I was trying out a new DEC axis on the tracker and was battling like crazy to get a guide star in view and in focus. It would really be a good idea to buy a proper guide camera to do the job properly. The webcam is ok to start with but it’s frustrating when you are trying to change to a new target quickly. The sensitivity of the webcam is not great. In a dark sky it works reasonably well but in a polluted sky it’s no good. If you can afford to spend a few hundred dollars on a dedicated autoguider camera with a ST-4 port, do it!
      Good luck

  3. Dasphys

    Hi Theo ,
    So at this stage alignment of southern pole must be very acurate in order for the star not to drift . Have you blogged how you find the southern pole alignment ?

  4. onedich

    Hi Theo,
    What an amazing job. Congratulations.
    One question: Can you show me the pin conections from ST4 to paralel port?
    Thank you very much

    1. Theo Post author

      Thank you for your message. It’s great to see that people are finding my posts useful!
      The ST4 standard is based on a RJ45 plug as follows:
      Pin 1: –
      Pin 2: Gnd
      Pin 3: RA+
      Pin 4: Dec+
      Pin 5: Dec-
      Pin 6: RA-

      With my design I don’t use a standard RJ45 plug as I get it out of the parallel port and then into the opto-coupler circuit. I only used the GND, RA+ and RA- signals for my current design.
      The laptop parallel port pinout is as follows:
      Pin 6: RA+
      Pin 7: RA-
      Pin 24: GND

      This might be different depending on the tracking software that you are using.

      If your implementation work the wrong way around, you can swap the pin 6 and 7 around or the function can be reversed easily in the software.
      Good luck!


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