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Old 26-10-2020, 07:28 PM
BKruger (Bert)
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Nexstar 11 GPS - Conversion to single arm

I have de-forked my 2005 C11 carbon fibre scope which is now equipped with a Hyperstar V3 + ASI294 camera and housed in my roll off roof observatory (on a CGX-L mount) - I am just using it for EAA and needless to say I am very spoiled with it.

However I still had the Alt-Az GPS mount which cost me a fortune to fix because of "No Response errors" - replaced boards, hand controllers, etc. With the last error I decided That’s It! - I've had it. I finally converted the mount now to a single arm Alt-Az mount to carry my C9.25 Ultima ( with which I spent many hours/nights star hopping). The C9.25 is 22 years old and has fantastic optics - couldn't let it go.

I have used Onstep with a STM32 board for the GOTO conversion and to motorise the focuser. The motors used are:
- 0.9 degree per step, 1.68 A Nema 17 motors for the ALT and AZM axes – total reduction is 400 steps per revolution x 32 microsteps x 2.4 reduction with pulleys) x 180 final gear ratio) = 15360 Steps per degree.
- 1.8 degree per step Nema 11 motor for the focuser

My 3D printer was used to print the following items for this conversion:
- GT2-48T pulleys for the both the Dec and AZ axes.
- Focuser pulley (GT2 - 40T)
- Focuser cover (at this stage I am only using an elastic band on the focuser pulleys and it is doing the job for the interim)
- Cover for the carry handle where the one fork was removed (makes it very easy to carry the mount) which comprises 3 parts.

No view finder is used – just doing a plate solve to centre targets.

The first image shows the converted mount with the Ultima 9.25 scope.

The second image shows the changes that were required in the remaining arm of the mount. I had to cut a bit in the arm to accommodate and fit the Nema 17 motor. The 48Tooth pulley is 3D printed to fit over the 6.34mm shaft of the worm drive. It also had to be modified slightly to run next to the 180Tooth worm gear (it was chamfered off on the one side).

The third image shows the carry handle (that used to be recessed in the arm that was removed) added on the open side of the mount. Covered up around it with 3D printed covers. This makes carrying the mount very easy.

The forth image shows the 3D printed cover for the focuser to cover the gears (where the micro-meter dial gauge used to be). The 40T pulley was 3D printed to fit over the 19mm shaft of the focuser mechanism but it also accommodates the original focus knob. At this stage it is driven with a double elastic band over the pulleys until the closed loop belt and 16T pulley arrives - it actually works surprisingly well this way!

The last two images are examples of the EAA results (with Sharpcap Pro) on the first night out under Bortle 4 skies. A 0.5 GSO reducer was used with no filters. No dark subtraction or flats were used:
- Trifid Nebula = 104 seconds total exposure (53 x 2) at Gain 350
- Sculptor Galaxy = 250 seconds in total - (121 x 2sec + 2 x4 sec ) at gain 250. The greenish colour is because of light pollution and lack of filter (aimed towards the sky glow). Drifting is also clearly visible in this image but that does not concern me right now because this can be fixed. It is not because of poor tracking but rather poor setup and alignment from the start.

I should mention that I got the idea from the CloudyNights website.

The basics are working now - just fine tuning required. I have definitely managed to give this telescope and mount a new lease of life and will decide whether I am going to add more bells and whistles at a later stage (as would be possible because ONSTEP provide so many options but for me at this stage it is a big achievement just to get the basics working).

This configuration is much more portable and easier to handle now because the scope and mount can be separated. The Onstep conversion also provides a maintainable solution at very low cost is any of the electronic parts should fail in the future.
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Click for full-size image (Convertes N11 GPS mount with 9.25 Ultima.jpg)
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Click for full-size image (Arm conversion.jpg)
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Click for full-size image (Mount with extra carry handle and 3D printed cover.jpg)
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Click for full-size image (Focuser conversion on  Ultima 9.25.jpg)
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Click for full-size image (First night out - Trifid Nebula.jpg)
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Click for full-size image (First Night out - Sculptor Galaxy.jpg)
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  #2  
Old 30-10-2020, 09:19 AM
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mental4astro (Alexander)
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Excellent job, Bert!

Welcome to the Single Arm Conversion Club! A couple of other IIS'ers have also done the same mod.

http://www.iceinspace.com.au/forum/s...d.php?t=171320
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Old 30-10-2020, 03:24 PM
BKruger (Bert)
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Thanks Alex

Your conversion looks great as well. I like the way that you have filled the gap with the plywood mod. It's a pitty I haven't seen it prior to my mod because that would have been so much easier than the trial and error approach that I have used with 3D printing (to fit all the curves and to make it strong enough around the handle).

I love this mount and scope combination now. It will be my first choice to use for visual because the AltAz mount is just so much more convenient to use when I get visitors that want to take a peek through the eyepiece. A GEM sometimes require a very awkward viewing position and for older people it is all but impossible to achieve Many of them are honest enough to refuse the invitation and I don't blame them.
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Old 31-10-2020, 02:53 PM
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mental4astro (Alexander)
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The plywood "plug" not only serves to fill the hollow and act as a handle but the GPS receiver is also inside it under the laminate cover.
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Old 14-11-2021, 09:59 AM
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Hi Bert, Alex

I am converting Nexstar 11 to OnStep for a friend (Ritchie-I) because the electronics is fried (previous owner reversed the power, then someone was swapping cables on motor board and managed to force 12V onto 5V VCC rail.. so the whole thing is damaged beyond easy/low cost repair).

Could you please share some more mechanical details of your conversion/implementation?

I can see to some extent what you did about arm motor, but it would be nice to see how you did the Az axle as well.
I am thinking about using 16/80, GT2 belt transmission, I think it will fit (with short shaft extension).
Thank you in advance!
Bojan

Last edited by bojan; 14-11-2021 at 10:32 AM.
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Old 14-11-2021, 01:40 PM
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I think this is the good way to fit stepper for Az axis (12/50).

Motor can be mounted with help of standard bracket, using existing holes on the work assembly.
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Old 15-11-2021, 12:01 PM
BKruger (Bert)
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Altitude arm changes and drive:
I had to do a bit of surgery to fit the Nema 17 motor. Three small cuts as indicated in the following two photos. One cut at the back of the stepper motor (which I may have to redo to allow a little more room) and two cuts on the front side of the stepper motor. The cuts are not weakening the structure of the arm assembly. The crooked red lines indicate where the cuts were made.

A 50x50x3 Ali angle is used as a stepper motor bracket. Two holes were drilled and tapped into the existing worm drive assembly for fixing the stepper motor bracket. Two countersunk M3 screws were used to fix the bracket to the worm drive assembly). The 180 tooth gear and worm drive assembly were removed and taped up with masking tape to prevent any dirt or metal shavings from getting into the teeth.

I have used 3D printed GT2 pulleys for the final reduction (48T/20T) mainly because I didn’t want to wait for the Ali-Express order. An idler gear also had to be used to tension the belt (again I had a 160T GT2 belt that had to be used). Stepper motors are 1.68A, 400 steps per rotation. So the steps per degree works out as 400 x 32 microsteps x 180 worm reduction x (48/20 final reduction) / 360 degrees = 15360.

The GT2 pulley of 48T was a little too big for the job and as a result I had to chamfer the side facing the worm drive off to prevent it from touching the main gear of the worm drive. My recommendation is to use a smaller 12T or 16T pulley on the stepper and a 30T to 36T pulley on the worm drive. This will give steps per degree figures of between 14400 and 16000 which is good (as long as it is greater than 12800 and smaller than about 60000 then tracking should be good. Even a 40T/20T combo will do (it works out as 12800 steps per degree). These gears will also allow you to offset the larger pulley away from the worm drive to get a good clearance (my 3D version didn't allow that).

Azimuth drive:
No cutting required here. A 50x50x3 Ali angle is used and fixed to the worm drive assembly in the same manner as described above. The final reduction is also 48T/20T and the same stepper motor was used. Thus, steps per degree is also 400 x 32 microsteps x 180 worm reduction x (48/20 final reduction) / 360 degrees = 15360. The same recommendation applies to use a 30T/12T, 36T/16T or 40T/20T combo for this drive butt a 50T/20T will also do the job well.
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Click for full-size image (Nexstar C11 - Alt arm cuts with 3D pulleys.JPG)
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Old 15-11-2021, 12:27 PM
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Bert,

Thank you, you just confirmed my way of thinking.
I will use 16/50 transmission, so final number will be 10000 for 200s/rev motors.

Cheers,

Bojan
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Old 29-03-2022, 09:12 AM
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Hi Bert,
My friend (Ritchie-I) reported issues with tracking, he indicated tracking is too slow.

It seems I have similar problem with my Dobson after upgrade from Bartels system (which was running perfectly OK for the last 20 years on Fujitsu Stylistic 1000) - I decided to change because OnStep is somewhat simpler to operate (using mobile as handpad)... tracking speed looks like 1/2 of what is required, objects (Sirius) is visibly moving westwards.. I suspected wrong local time was set during init, but my phone says correct time was sent (GMT+10.. and we still have Dls).
Did you do something else we should be aware of?
Thanks in advance for your help
Bojan
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Old 29-03-2022, 07:20 PM
BKruger (Bert)
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Hi Bojan

I have not used the 2 Altaz mounts for months now mainly because of bad weather. However, for starters I remember the following are important for setup the Altaz mounts for alignment and tracking:
  • UTC time offset in the smart webserver (SWS) of Onstep should be -10. Note that Skysafari indicates +10. Not sure if you use wifi at all to connect to Onstep’s SWS as well or do you just use Skysafari?
  • Longitude and latitude values should be negative for Australia or East for longitude, South for latitude.
  • The scope should be levelled and pointed North before alignment starts. Thus not to the SCP.

Could you verify the above before we explore other possibilities?

Regards
Bert
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Old 30-03-2022, 06:11 AM
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Hi Bert,

Thank you for quick reply.
Mostly I am using mobile phone and OnCue app, connection is via BT (web server doesn't work reliably for some reason, it keeps dropping connection so I gave up on that).
When initializing, I noticed app send the correct time to OnStep (+10, despite daylight saving). Or CdC running on lap/comp via USB (BT doesn't work on comps for another reason..)

Everything else you mentioned seems to be the same as for Bartels, so I think there should be no issue there.. and I am pretty sure the transmission ratio used in Config.h is also correct (in my case I am using the same numbers as used with Bartels (but converted to units used by OnStep)). Perhaps I need to check that again as well..
As to Ritchie's Nexstar mount - when testing, GoTo seemed to be OK, but because I didn't have scope when doing modification, I didn't test tracking rate - it seemed to be OK, judging by shadow of the the mount.

At the moment mu dobson is dismantled (I am sorting out some mechanical issues (lack of vertical shaft rigidity), so I cant verify anything before this was sorted.
Anyway.. thank again for your feedback :-)
Bojan
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Old 22-10-2023, 01:10 PM
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After quite some time (almost a year!!) and a bit of fiddling, this weekend I figured how to make my (and Ritchie's, hopefully) dob to track correctly:

The problem was in absence of RTC from the system.

Once I have it connected, the tracking is quite precise (same as with my old Bartel's system).
Interestingly, I checked the available documentation, buy this detail was not mentioned anywhere (or I didn't see it) .. Maybe because noone attempted to do it without RTC?

There is a note on MEGA being more precise with RTC.. but I never had any issue with tracking in EQ mode, without or with RTC.
Anyway.. maybe this piece of info might save someone considerable effort testing their OnStep system.
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