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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.
Attached Thumbnails
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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Old 30-10-2020, 09:19 AM
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mental4astro (Alexander)
kids+wife+scopes=happyman

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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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