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DIY ET-8 GOTO Mount

astrophotography eq mount
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#76 photo444

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Posted 29 January 2015 - 11:50 AM

Steve,

 

That is great. It will be OK for those components. Is the power supply input for stepper drivers separate from the supply for the Mega board?  



#77 ssagerian

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Posted 29 January 2015 - 02:02 PM

Unfortunately, we could have added a switch mode power supply wired-or into the 5V side, to simplify the number of power supplies, but we didn't.

 

If you use 12V power supply for your motors you can wire the 12V directly into the power plug (anything up to 20 V), other wise you will need a supply for your motor and a supply for the MEGA.

 

Of course if your using the USB as a serial communications to drive your mount (such as Stellarium ), the USB 5V provides power to the MEGA.



#78 orlyandico

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Posted 30 January 2015 - 01:00 AM

Some of you may find this surprising, but..

 

I made up an RA-only drive using an Arduino motor shield (the official one, not the Adafruit) and a regular Uno.  Driving a Vexta PK243 hybrid stepper.

 

The thing runs off the USB (alone). No separate power.  And it can run at higher than sidereal speed. Of course that's only one axis.

 

But it seems that you can power the entire mount off USB so long as the slewing rates aren't too high and the mount is balanced.



#79 ssagerian

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Posted 30 January 2015 - 09:01 AM

I think that is great, and I am not actually too surprised, since from my own experience with LX200 motors, they will run on 6V (are often characterized as "toy" motors")  and given the high gear ratios, they have enough torque for a limited load. But in actuality, I doubt you can draw enough current (power) via USB to drive the higher torque motors. The OnStep drivers are capable of delivering 2.5 A at 24V (60Watts),  a standard USB is capable of delivering 5V at 0.5 A (in general) 2.5W. An of course the over all goal is to match the big boys with our design..



#80 pbunn

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Posted 30 January 2015 - 09:35 AM

I bought one of the Byers drives on ebay. Very nice worm and wheel supplied with an AC synchronous motor. I plan to add a stepper in its place and use this system to make RA axis camera drive.

 

 

Pat



#81 photo444

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Posted 30 January 2015 - 10:11 AM

Unfortunately, we could have added a switch mode power supply wired-or into the 5V side, to simplify the number of power supplies, but we didn't.

 

If you use 12V power supply for your motors you can wire the 12V directly into the power plug (anything up to 20 V), other wise you will need a supply for your motor and a supply for the MEGA.

 

Of course if your using the USB as a serial communications to drive your mount (such as Stellarium ), the USB 5V provides power to the MEGA.

Steve,

 

At this time I am using 12V and it is sufficient for my application.  When I was using the prototype with Mega board, the unregulated input (12V) was shared with the stepper drivers. In my Teensy3.1 board I am using USB suppy for 5V and all others are separated for 12V or higher.

 

Thanks,

Paul



#82 Geo.

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Posted 31 January 2015 - 11:15 AM

I'm planning to use this to drive a Celestron C14 Compustar mount. I've been fooling with replacements for the 1987 goto system for a few years and the biggest hurdle has been the draw of the original steppers (Vexta 2 phase stepper, 1.8° Step DC 1.4V, 3.8 A Model # PH265-05). The OEM power supply was rated at 12VDC @15A! There's a photo of the Compustar stepper driver attached. Note the cooling fan and heat sinks. the enclosure was about 6x6x16". Hard to believe that this is going to be replaced by two DRV8825 IC's. To make it easier on them I've sourced a couple Vexta 2 phase steppers, 0.9° Step DC 6V, 0.85A (1.2A in bipolar) Model # PH265M-31. These are bolt on replacements. 

 

So a couple questions. Beyond the power supply(s), the Mega PCB and a device capable device running the Android app, what will be needed? What soft/firm ware is required?

Attached Thumbnails

  • Celestron Stepper Controlador.jpg


#83 hjd1964

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Posted 31 January 2015 - 09:00 PM

I'd guess you're looking at two H-Bridges there, eight transistors, so it can switch power/reverse the polarity.  A bunch of diodes to block the fly-back from the motors.  Some big resistors so it can run at higher-than-rated voltages without overheating the motors.  Lots of wasted power, the DRV8825 is way more efficient.

 

I have fairly extensive documentation on my site describing the process of setting one of these up:

http://www.stellarjo...quipment_onstep

 

The Arduino software is necessary to send the firmware up to the Mega2560, it also installs the serial port driver for it.

The firmware is on GitHub (search OnStep or use the link on my site.)  I recommend using the "Dev" branch.  Drop the OnStep folder in your Documents->Arduino folder.  See the Config.h file for firmware setup.

The Android App is in the Google Play Store - just download it like any other App.

The ASCOM Driver is on my site.

 

Either "Sky Planetarium" or my Android App is necessary to do an align and get going, then other software can take over from there.  You could skip the ASCOM driver and just point Stellarium, for example, at the Mega2560's serial port for LX200 control.  Some software can have problems when used this way though, nothing to do with OnStep... Google "mega2560 reset on connect" .



#84 orlyandico

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Posted 01 February 2015 - 09:15 AM

You can add a capacitor to the reset line on the Arduino to eliminate the "reset on connect" problem..

 

http://orlygoingthir...controller.html

 

IMG_8398.JPG


Edited by orlyandico, 01 February 2015 - 09:16 AM.


#85 hjd1964

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Posted 01 February 2015 - 09:55 AM

Even easier, http://arduino.cc/en...noBoardMega2560 :

 

"The Mega2560 contains a trace that can be cut to disable the auto-reset. The pads on either side of the trace can be soldered together to re-enable it. It's labeled "RESET-EN". You may also be able to disable the auto-reset by connecting a 110 ohm resistor from 5V to the reset line; see this forum thread for details."

 

Again, only an issue with LX200 across the USB/Serial interface with some software.  My ASCOM driver handles this correctly and Serial1 (Bluetooth) doesn't trigger this.


Edited by hjd1964, 01 February 2015 - 10:01 AM.


#86 hjd1964

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Posted 01 February 2015 - 10:32 AM

I'm planning to use this to drive a Celestron C14 Compustar mount. I've been fooling with replacements for the 1987 goto system for a few years and the biggest hurdle has been the draw of the original steppers (Vexta 2 phase stepper, 1.8° Step DC 1.4V, 3.8 A Model # PH265-05). The OEM power supply was rated at 12VDC @15A! There's a photo of the Compustar stepper driver attached. Note the cooling fan and heat sinks. the enclosure was about 6x6x16". Hard to believe that this is going to be replaced by two DRV8825 IC's. To make it easier on them I've sourced a couple Vexta 2 phase steppers, 0.9° Step DC 6V, 0.85A (1.2A in bipolar) Model # PH265M-31. These are bolt on replacements. 

 

So a couple questions. Beyond the power supply(s), the Mega PCB and a device capable device running the Android app, what will be needed? What soft/firm ware is required?

 

I just noticed that you're adapting a fork mount, be aware that OnStep is for GEM mounts only at this point.

 

ssagerian is also adapting a (LX50) fork mount and I'm planning to address OnStep's issues with handling this once we have some hardware to test on.  It all has to do with the logic of how OnStep moves the mount and when the controller decides to re-orientate it.  It's easy to disable GEM Meridian flips in OnStep which gets you half way there but there's more to it than that.

 

I've never personally owned a goto fork 'scope, but I have worked with manual ones in the past and should have this feature added shortly.



#87 Geo.

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Posted 01 February 2015 - 05:40 PM

GEM - schem, shouldn't make a difference, the Compustar is used in EQ not alt/az. A GEM just has one less fork. Aside from no meridian flip, I can see no difference.

 

Thanks, George



#88 hjd1964

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Posted 01 February 2015 - 07:54 PM

GEM - schem, shouldn't make a difference, the Compustar is used in EQ not alt/az. A GEM just has one less fork. Aside from no meridian flip, I can see no difference.

 

Thanks, George

 

Sure should be usable, just not ideal.  I've even messed around with it without hardware attached and it appears to do what it should as far as I tested.

 

The values for minutesPastMeridianE/W (in Config.h) each need to be set to 720 (12 hours.)  That should allow you to do a goto anywhere in the sky without triggering a meridian flip.

 

The polar home position would be with the forks on their sides one over the other.  To support fork mounts I'd adjust this to be side by side.

 

A one-star align should work.  A two/three star align will not work since it requires that you do a meridian flip so it can measure the cone error and then move on to polar align error measurements too.  To support fork mounts I'd need to drop the meridian flip requirement, ignore cone error, and proceed - I think.

 

There's some special code that handles under the pole gotos.  Again it works by forcing a meridian flip based on minutesPastMeridian and should be ok I think.

 

Then there's an underPoleLimit that protects GEM mounts from running the 'scope into the pier. But it also triggers a meridian flip when doing a goto under the pole and past this limit.  Not 100% sure but setting it to 12 should allow it to work, just when you cross between +12 to -12 (HA) under the pole the 'scope will move 24h in RA to get you there.  Might leave it that way too.

 

The above should have it working I think, now:

 

Do commercial goto systems flip the 'scope in RA to manage the eyepiece position?  I bet they do. 

Do they have settings to keep you from crashing a camera into the base?

Any other behavior tweaks that I haven't thought of?

 

Comments from all are welcome, Thanks.



#89 Geo.

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Posted 02 February 2015 - 11:08 AM

I really don't have much experience with forked EQ mounts. Alt/az systems have cord wrap, below the horizon limiters as well as Alt limits. These can be turned off or customized and I presume will function if the system permits EQ operation.  

 

The CGEM has cone value calibration, options for OTA orientation for tandem bars, favored meridian and meridian limits.

 

One of the CGEM users on CN says he just uses Quick align and Re-align with one known star near the meridian and fins his gotos quite accurate. I've found that Quick align, which models the sky from GPS time and location inputs will put bright objects in the finder's field.

 

The C14 is going in the observatory, so hopefully polar alignment will be a one time affair.



#90 hjd1964

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Posted 02 February 2015 - 01:16 PM

OnStep has overhead limits and horizon limits too, they should work as intended with fork mounts.

 

Meridian flip behavior is designed to help with cord-wrap, it's always on.

 

Alignment operations for fork mounts beyond one-star will require time and thought but I'll get there.  Current pointing performance with GEM mounts is documented on my site:  http://www.stellarjo...step_notes.html

 

My Sky Planetarium software contains a similar pointing system (Goto Assist) and I intend to eventually integrate the two so the more advanced n-star alignment capabilities of my PC software can be used to derive pointing models that are significantly better than a 3-star align can provide.  These pointing models could then be uploaded to OnStep.  This is really only useful in an observatory setting but would still be nice to have and I'm already 90% of the way there.  Goto Assist has provision for fork flexure terms too, it's just not coded in yet.  I've been so busy with OnStep that I haven't touched it in almost two years.

 

With just a one-star align my GEM mounts will get close to target, especially if cone error has been dealt with by shimming.  Objects were always in a 1° FOV, but that's with a good polar alignment.  Now, with a three star align it'll put them near the center of that FOV even if the GEM isn't well polar aligned and a large amount of cone-error exists.



#91 ssagerian

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Posted 09 February 2015 - 09:16 PM

Hello Everyone,

 I am happy (relieved?) to announce that the boards are here.

Its late for me (8PM chicago time) and I have just arrived home so I will start on these tomorrow evening.

I have the parts, the solder stencil and now the boards so I can begin the SMT assembly phase.

I am still waiting on the shield Pin Headers (to mate to the MEGA shield). Hopefully they will arrive in the next day or so.

If so, I would expect that I will begin mailing them out by the weekend. Thank you for your patience,

 

Here are a couple of pics..

inpackage.jpg

barepcb.jpg



#92 photo444

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Posted 14 February 2015 - 09:06 AM

Steve,

 

Looking good! When can I get the finished one?

 

Thanks,

 

Paul



#93 ssagerian

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Posted 14 February 2015 - 11:37 AM

Hello Paul, Everyone!,

 

I completed placing all the SMT parts last night.

 

I still have the following tasks to complete before I can ship them out..

 

  • check continuity  on all the IC connections to the shield header pins (make sure the is a good connection from the solder pad to the ICs)
  • Solder the thur-hole parts (motor connectors, power connector, current adjustment pots, and shield headers)
  • power up the boards and connect to the bluetooth module and change its baud rate, security pin and name for OnStep
  • attach each device to a MEGA and verify the motor operation.

I am still wating for the single row Shield headers to arrive, which keeps me from finishing the second bullet, but hopefully they will arrive today.

 

Assuming the headers arrive today, I would expect to start shipping Tuesday 2/17/2015

 

Here is an image of the boards in their current state of completion, you can see all the SMT parts are in place, (motor ICs, bluetooth, big bad capacitor)..

and the thur hole parts in the background..minus the slow boat from china headers..

 

smtComplete.jpg



#94 ssagerian

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Posted 18 February 2015 - 07:05 AM

The boards have been completed. If you paid for a board, send me you mailing address (off forum) and I will respond back with the cost of mailing. If you have a favorite form of shipping , please let me know in the email, as well as if you want any insureance over whats provided by shipping/mailing entity.



#95 photo444

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Posted 18 February 2015 - 08:18 AM

Steve,

 

Wow, you finished that quick!

I have PMed my address to you.

 

Thanks,

Paul



#96 photo444

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Posted 23 February 2015 - 01:15 PM

Steve,

 

I just opened the box which was delivered at the door a few minutes ago. It looks exactly the same as the picture shown on the post #57.  Thank you for your good work. I will test as soon as possible.

 

Best regards,

Paul



#97 ssagerian

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Posted 23 February 2015 - 01:35 PM

Hi Paul,

 

hopefully, it should have "Rev B" in the upper right corner (oriented in the same manner as post #57). The silk screen (white lettering) along the edges of the board should indicate "what goes where", if you are in doubt about the polarity of the power supply connections (or anything else pertaining to the board) let me know, the ground connection is on the left, the power is on the right. The motor wiring is also marked on the left edge of the board, be mindful of which motor is which..DEC and RA.

The Bluetooth module should slowly blink when first powered up and go solid blue when connected via Howard's niffy Android app.

If your using the serial comms to the MEGA, then its bypassed, and should blink at the slow constant rate.

 

You are welcome, it was a fun project, working with Howard, learning about how my mount works and (currently) converting my LX50 to be a GOTO mount.

 

Steve



#98 photo444

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Posted 23 February 2015 - 04:22 PM

Steve,

 

My board says 'B 10' on the green circular sticker.  The power terminals are labeled so I do not concern. 

 

Since it has connections for HALL and LIMIT switches, could you please tell me how to connect them?

 

have fun with LX50.

 

Paul



#99 hjd1964

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Posted 23 February 2015 - 05:10 PM

The limit switch pins simply need to be shorted together by switch(s) to stop any gotos and tracking.  A stop isn't immediate, OnStep needs up to a second to stop the mount (plus de-accelleration time if a goto is active.)

 

The hall connector has +5V, Gnd, and Signal pins (in order, look at the schematic Steve posted in this thread.)  The +5V/Gnd are provided to power a sensor of some sort, HALL effect for example, but an optical switch etc. can work too.  Several varieties of these devices exist with the idea being that we detect the rising edge of the signal (+5V) when the index is present.  The Signal needs to be held high for a minimum of 10ms as currently implemented (but this requirement can easily be eliminated.)  Once the index is detected, a repeat detection isn't allowed for a minute so the index has plenty of time to pass by.

 

I used Sparkfun.com cables, P/N: PRT-08670 and SEN-08733 for these connections.



#100 photo444

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Posted 23 February 2015 - 07:15 PM

Howard,

 

It's been a while since last time we talked.  Thank you for the information. I am looking for a housing for the board all together. Stay warm!

 

Paul




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