Guiding with Raspberry PI/Lin_guider and Economy RA Drive

[bobro]

My itch to try astrophotography was 'scratched' earlier this year with the purchase of a Meade Polaris 130 reflector, complete with economy ('clockwork') RA motor drive. It has been fun trying to image with a Canon EOS 1000D - 30 second images are generally OK, plus the occasional 60 second success. The RA economy motor is a drawback to longer exposures due to the difficulty in setting the speed with a tiny dial and no real speed control as the dc motor is simply driven by a variable voltage.

Rather than upgrade the motor drive, I decided to see if it could be made to guide, adding to the Meade scope an Orion 50mm mini guide scope, Microsoft Cinema webcam (as a guide camera) and a Raspberry PI running Lin_guider software. The PI drives a relay board, sending faster/slower signals to the RA motor drive, with just a single resistor mod to the motor drive for remote control.

It wasn't the best evening for a test of the setup, but the clouds finally parted, allowing the test to go ahead. The initial result - yes it worked! The relays clicked away, keeping the RA on track. The amplitude of the guiding swings looked quite large, especially when dew clouded the guide camera! There's a lot of tinkering to do to understand and adjust the various guide parameters.

The load is at the limit of what the EQ2 mount can carry (4kg), so it might struggle a bit. It looks as though the scope doesn't have a parabolic mirror, so star shapes aren't great towards the image edges. I need to work on polar alignment as there was Dec drift.

Nevertheless, an interesting (and very low cost) start into guided imaging - can't wait for more clear skies.

Below is the first guided image - 12 stacked lights  of 2 minutes at 1600ASA. I expected to see a bit more colour - overdid it in the processing to make it 'interesting'. Can someone tell me why so little colour? Any suggestions on (simple) improvements to the setup or the best way to achieve polar alignment would be helpful (I wondered about using SharpCap 2.9 for polar alignment but I don't think the Microsoft Cinema webcam will be sensitive enough).

Many thanks.

 

[Demonperformer]

Nice start.
(1) Make a small 'dew shield' for the guider - rolled-up thin cardboard should suffice.
(2) Stability can be added to a mount by hanging something (bag of sugar?) directly underneath it (it pulls the centre of gravity downwards).
(3) Try sharpcap - it will either work or not with your camera and costs nothing. After struggling for years with PA, I am a great advocate of the polemaster, but that is not cheap. You would also need to consider that, if you really get bitten by the AP bug, you are going to upgrade from the EQ2 at some point (really, you are - and on that basis, IMO don't even think about upgrading the motor on the EQ2). You would then need a new adapter for it to fit the new mount.
HTH

[D4N]

Well done getting an eq2 to guide, I like the ingenuity

 

It looks to me like that is a Parabolic mirror, the strange shape on the stars is coma.  This is quite normal on a reflector and a corrector lens needs to be used to get rid of it.

A non parabolic mirror would manifest itself as differing focal positions for stars in the centre compared to the edges, all of your stars appear to be at the same focus.

I would suggest a bahnitov mask (you can make these yourself) to make sure the focus is actually right.

 

DSS can kill colour, it has been a while since I have used it though so a bit hard to remember the best way to stop that from happening.

 

In addition to the weight mentioned by DP above you can fill the tripod legs with sand as this dampens vibrations.

 

It s very difficult to PA an eq2 as there is no polar scope, I guess you could drift align but it won't be easy.

[bobro]

Thanks DP and D4N for the useful advice. I can see now that my Meade 130 suffers from coma, though no plan to deal with that as yet. It's taken me a while to respond as I've been trying out what you suggested amongst getting to know my setup better. The scope is better balanced and the RA motor faster/slower directions more equal than they were before.

Unfortunately my guidescope/webcam combination couldn't capture enough stars for Sharpcap to help with polar alignment. Instead I've been using drift alignment, with the camera  outputting bi-directional star trails for viewing on my laptop. It's a bit fiddly to adjust azimuth on my EQ2 - but it works in the end.

So far, my guiding setup is working better than I had hoped. Below is an example of a 5 minute single image (IMG_0007 - captured at 400 ASA as it was early and the sky was bright).

My experience with DSS is going downhill - very bright final stacked image and very little colour, so I decided to try manual stacking with GIMP - see the Pleiades image below (2 minute exposures so not too difficult as high in the sky where RA movements are slower). GIMP is labour intensive though, so I will possibly need to look for another program if I don't have much success with DSS (have tried both raw and jpeg images - same results). Any suggestions anyone?

When guiding, DEC gently drifts north and south before finally going off the guiding screen as errors mount up - this can show up on the captured image. So I plan to add DEC guiding to the mount - using a 2nd (modified) economy RA motor connected in place of the DEC cable control, with the Raspberry PI guiding.  Hopefully I will then be able to see what the setup can do as far as imaging does, before adding suggested weight to the mount as necessary for stability. Do you think the DEC motor guiding will improve the image, or is backlash likely to be a problem?

Many thanks.

 

[adder001]

Wow that is brilliant. I love tinkering and I am in the process of making a ioptron type startracker using a raspberry pi zero and a stepper motor to attach my dslr to.

I also have a eq2 mount and suffer the polar alignent probs. I would be reallly interested in a few more details of your setup maybe some pictures or diagrams etc, I would love to do something similar.

Thanks for sharing anyway

[bobro]

Glad you find the PI guiding interesting adder. Of course I am more than happy to share the mods and setup with anyone interested. Here is a diagram and also a photo of the just added DEC motor (actually a modified Economy RA motor) waiting for a PI relay board to arrive for test.

You can find more photos at http://guiding.web.fastmail.co.uk/ and I can post any other info required to this web location. 

Thanks for the interest.

Bob

 

[adder001]

Thanks for sharing that info. That is a very nice idea It never occurred to me to try the Pi as a guider also to drive the dec axis with a motor also. I am mainly using my EQ2 mount now solely for my DSLR I hardly ever put the scope on it as I find the images are far more rewarding and I can see more by using a camera than with my scope.

I am definitely going to use some of your techniques there especially driving the dec axis.   I am very familiar with using python and the raspberry Pi but I am not not familiar with any guiding software so I would have to learn that first.

Just a question though - I am understanding this correctly that the PI is running the guide software not the laptop?

I think my version would be probably using stepper motors attached to RA and DEC drives and perhaps use python to control them and some kind of guide camera from the Pi. It would be nice if it could be guided using python but I am not sure how that could be achieved (maybe I am getting ahead of myself here ).  I think that I now have another project on the cards even before my current one is finished, The wife is gonna kill me!!

Anyway the pictures below are something that I made a while ago using a raspberry Pi B+ and a touchscreen LCD and the Raspberry Pi camera.

It gave surprising results, obviously only good enough for Lunar and planets etc but amazing for a cheap basic camera and was good fun to make too.

 

[bobro]

That's correct - the guiding software (lin_guider) runs on the PI. The laptop is purely a remote desktop to the PI in my setup.

Guiding normally requires some pretty heavy maths e.g. https://en.wikipedia.org/wiki/PID_controller

You can find out more about lin_guider at : http://rpm.pbone.net/index.php3/stat/45/idpl/18820081/numer/1/nazwa/lin_guider

Nice use of the PI camera to capture the moon (that's how I initially tested out my Microsoft Cinema webcam). Perhaps it could be a useful guide camera?

Good luck with the projects!

[adder001]

Thanks for that info. See what you mean about the guiding maths, I think I will leave that to the software already written for it, way too complicated for my programming skills. Good to know that the Pi can handle it though.

Cheers

[ozcarlosa]

Hi 

I am new in this topic and I found that the website guiding.web.fastmail.co.uk is down and I would like check your mod for DEC motor drive. Please, can you repost that information? 

[bobro]

4 hours ago, ozcarlosa said:

Hi 

I am new in this topic and I found that the website guiding.web.fastmail.co.uk is down and I would like check your mod for DEC motor drive. Please, can you repost that information? 

Hi,

The website has been put back. Only the motor was used from the DEC motor drive (you can see it in one of the photos) as, for guiding purposes, it is only switched on briefly, with the polarity used to determine guiding direction. Guiding works pretty well in the DEC direction as there isn't significant backlash in the drive.

This was my first attempt at guiding an EQ2. My second attempt used an Arduino for the hardware interface connected to PHD2 for guiding on the same laptop as for capturing images.

[AstroMuni]

2 hours ago, bobro said:

This was my first attempt at guiding an EQ2. My second attempt used an Arduino for the hardware interface connected to PHD2 for guiding on the same laptop as for capturing images.

Its amazing what you have achieved with this simple setup. Great work!

 

On 05/01/2017 at 21:23, bobro said:

When guiding, DEC gently drifts north and south before finally going off the guiding screen as errors mount up - this can show up on the captured image. So I plan to add DEC guiding to the mount - using a 2nd (modified) economy RA motor connected in place of the DEC cable control, with the Raspberry PI guiding.  Hopefully I will then be able to see what the setup can do as far as imaging does, before adding suggested weight to the mount as necessary for stability. Do you think the DEC motor guiding will improve the image, or is backlash likely to be a problem?

If you get a DEC motor, then you might be able to take the next step to platesolve too. That seems an obvious step for me. That would help with Polar aligning too.

I use Kstars/Ekos on a Pi to control my mount (its a Go-to), and it could work for you too. It comes with its own guiding software too.

[rickwayne]

Great bit of maker-ism!

What's translating between lin_guider and the faster/slower commands to the board? If it isn't an INDI driver, you might consider writing one -- you can use an existing driver as a skeleton and replace the business end with your code. That would give you many software choices, including PHD for guiding and Ekos for a SharpCap-style polar alignment routine. The imaging camera will undoubtedly work with it, and you might be able to find or write a driver for the guide cam. I mean, kudos to you for building it from the bottom up, but with those drivers you'd  also have access to a  complete observatory system if you want it.

C'mon, you KNOW you're going to want to put your rig in a shed with a motorized roof controlled by the observatory scheduler...

Or have you moved on  in the interim?

Edited July 10, 2021 by rickwayne