motorizing my mount.


Barry
 

Khalid has been giving me tons of info and I bought one of his Kits to help him out but i get far more benefit than he.... Thanks Khalid

Now onto the problem at hand.
I have the Sky-watcher 300p collapsible Dobsonian. NOT the synscan version.

it comes with standard mount etc.
I need to mount motors to it.
the bottom is as simple as can be (i think)
i'll put a belt around the base and attach a motor to it so motor gear engages directly to the belt that is around the base plate that is about 20" diameter.
this swivels very easily since its on ball bearings.
the elevation is the one i need a bit of advice on. the scope can be balanced pretty closely and swivels pretty easily as well. it has a 5" or so disk on the side that rides on some nylon knobs but i could very easily replace those with skateboard bearings. 

so my thoughts are:
use a toothed belt and pulleys 
100 tooth is available easily
but 60 tooth is very common 

i'm wondering if anyone here has some insight into gearing and belt width recomendations. 
i can easily calculate ratios.
can easily put double pulleys to gear down lower or even 3d print any size pulley.
is 8mm belt strong enough? that 100tooth pulley takes an 11mm belt 

Thoughts?


Khalid Baheyeldin
 

A few points ...

Since this is a Dobsonian, then scroll down on the Showcase page to the Dob section, and follow the links. Lots of examples there of how people did it. Then come back with two or so of these designs that are practical for your mount, and hopefully those who did them will help.

If your goal is imaging, then an Alt-Az is not suitable for that. You need either a German Equatorial (GEM) or a Fork mount.

A large GEM can be made from pipes and pillow blocks, but to have good precision, you need to couple it to a high quality work gear and work wheel, e.g. Ed Byers.

A fork mount has the advantage of not needing a counter weight and no meridian flip.

But those are future considerations for when you will be moving from Alt-Az to equatorial.


Barry
 

thanks. 
didn't see that before

the orion Plus XT10 has all i need i think.
thats awesome!


On Mon, Oct 4, 2021 at 11:55 AM Khalid Baheyeldin <kbahey@...> wrote:
A few points ...

Since this is a Dobsonian, then scroll down on the Showcase page to the Dob section, and follow the links. Lots of examples there of how people did it. Then come back with two or so of these designs that are practical for your mount, and hopefully those who did them will help.

If your goal is imaging, then an Alt-Az is not suitable for that. You need either a German Equatorial (GEM) or a Fork mount.

A large GEM can be made from pipes and pillow blocks, but to have good precision, you need to couple it to a high quality work gear and work wheel, e.g. Ed Byers.

A fork mount has the advantage of not needing a counter weight and no meridian flip.

But those are future considerations for when you will be moving from Alt-Az to equatorial.


Khalid Baheyeldin
 

Here is another conversion of a Dob to OnStep, by Pinak.
But he made the mount himself, and I think it was a 12" mirror.
So quite close to yours.

The challenge with Dobs is often the lack of a worm wheel, and
the ease of adding pulleys and belts.

So you can get some ideas from that, and the motors.

Check his video, and his photo gallery for details.

P.S. He is using an S6 as well, but really any board can run a basic
OnStep setup.


Oscar Lithgow
 

I have succesfully motorized a few Dobs now.. The most dificult part is the ALT drive. I suggest you to instal a "tangent arm" in one of the ALT bearings of the OTA, with suficient radius to be able attach a high presicion planetary stepper motor on the rocker box close to the tangent arm so you it can drive it with cable or belts. Some advantages of having a larde radius tangent arm in ALT is the better movement control and higer reduction ratio

The one in the picture I used the cable drive method wish worked very well


Barry
 

beautiful.
for all intents and purposes thats a pulley with a radius from motor to axle of scope pivot point.
brilliant.
zero backlash too.


On Wed, Oct 6, 2021 at 10:23 AM Oscar Lithgow <oalithgow@...> wrote:
I have succesfully motorized a few Dobs now.. The most dificult part is the ALT drive. I suggest you to instal a "tangent arm" in one of the ALT bearings of the OTA, with suficient radius to be able attach a high presicion planetary stepper motor on the rocker box close to the tangent arm so you it can drive it with cable or belts. Some advantages of having a larde radius tangent arm in ALT is the better movement control and higer reduction ratio

The one in the picture I used the cable drive method wish worked very well


Oscar Lithgow
 

Thats correct. The only backlash you will have is in the stepper motor transmision (Because you will need extra reduction here)  I recomend you:

One of this NEMA 17 steppers:
Nema 17 Bipolar 0.9deg 46Ncm
In combination with one of this planetary gears:
PLE Series Planetary Gearbox Gear Ratio 10:1 Backlash 15 arc-min


Barry
 

planetary gearbox....hadn't considered that. thanks for the suggestion.
can you take a picture of the motor mounted on the other  side?


On Wed, Oct 6, 2021 at 11:57 AM Oscar Lithgow <oalithgow@...> wrote:
Thats correct. The only backlash you will have is in the stepper motor transmision (Because you will need extra reduction here)  I recomend you:

One of this NEMA 17 steppers:
Nema 17 Bipolar 0.9deg 46Ncm
In combination with one of this planetary gears:
PLE Series Planetary Gearbox Gear Ratio 10:1 Backlash 15 arc-min


Khalid Baheyeldin
 

The majority of dobs that are converted to OnStep use geared
steppers, because there is no worm wheel.

The Showcase page I linked to have many conversions, and
pictures on how it is done.


Barry
 

yes saw several, but Oscars seems best.
i'm thinking we may not even need a gearbox if its balanced well unless more steps is needed for smoother tracking


On Wed, Oct 6, 2021 at 6:13 PM Khalid Baheyeldin <kbahey@...> wrote:
The majority of dobs that are converted to OnStep use geared
steppers, because there is no worm wheel.

The Showcase page I linked to have many conversions, and
pictures on how it is done.


Khalid Baheyeldin
 

Yes, more reduction is for more steps, so better resolution, as well as more torque.

For imaging, we recommend a minimum of 12,800 steps per degree.
For visual, it can be lower. Even 6,400 may work.


Barry
 

well i will want to take photos so gearbox it is.



On Wed, Oct 6, 2021 at 6:36 PM Khalid Baheyeldin <kbahey@...> wrote:
Yes, more reduction is for more steps, so better resolution, as well as more torque.

For imaging, we recommend a minimum of 12,800 steps per degree.
For visual, it can be lower. Even 6,400 may work.


Khalid Baheyeldin
 

As I said before, an alt-az mount (like a Dob) is not suited for imaging.

You still can get good results using short exposures (a few seconds),
but that means a huge number of images have to be taken vs. much
less from an equatorial.

There is also field rotation which is inherent in alt-az mounts. This can
be mitigated somewhat in post processing, or you add a field de-rotator
(which the S6 supports, but you need an additional motor and mechanical
parts for it).

So, don't get your expectations high with an alt-az for imaging.


Barry
 

is that because onstep cannot manage two motors for tracking properly?


On Wed, Oct 6, 2021 at 6:52 PM Khalid Baheyeldin <kbahey@...> wrote:
As I said before, an alt-az mount (like a Dob) is not suited for imaging.

You still can get good results using short exposures (a few seconds),
but that means a huge number of images have to be taken vs. much
less from an equatorial.

There is also field rotation which is inherent in alt-az mounts. This can
be mitigated somewhat in post processing, or you add a field de-rotator
(which the S6 supports, but you need an additional motor and mechanical
parts for it).

So, don't get your expectations high with an alt-az for imaging.


Khalid Baheyeldin
 

No, it is not OnStep.

It applies to any mount and any controller (for amateur
equipment).

The mechanical complexities of having two axes moving all the
time, plus the imprecision inherent in amateur equipment.


Barry
 

thats just mathematics...figure it out in software and source a more powerful CPU :)


On Wed, Oct 6, 2021 at 6:57 PM Khalid Baheyeldin <kbahey@...> wrote:
No, it is not OnStep.

It applies to any mount and any controller (for amateur
equipment).

The mechanical complexities of having two axes moving all the
time, plus the imprecision inherent in amateur equipment.


Oscar Lithgow
 

The alt az tracking in onstep its quite good, it depends more on the motorizing quiality and base leveling. The field de-rotator is a must, for longer exposure astrofotography (onstep supports), also autoguiding can help a lot (onstep also supports)


George Cushing
 

General Alt/Az can't give you image resolution that an EQ mount can. At least for the same money. The very large observatories that have been built lately are often Alt/Az as EQ mounts are not practical. I have to assume they've found some way of dealing with the drive issues.


davel
 

Large observatories use something like this: https://www.newscientist.com/article/dn8770-enormous-laser-beam-produces-artificial-star/ The laser excites the sodium ions in the (of all places ;-) ionosphere. Then drive correction is applied, using something very much like PHD2. BTW, there's a photo here showing one of the 2 altitude bearings: https://www.eso.org/public/news/eso9725/


Dave Schwartz
 

The laser artificial star is for adaptive optics correction, not guiding.

All large earth-bound telescopes have, for many decades now, been alt/az. They all use derotators for their imagers. The math isn't that complicated and I'm sure that OnStep on a fast processor could run the three motors needed on any telescope where you would even consider OnStep for a controller.

On 2021-10-07 4:24 p.m., davel wrote:

Large observatories use something like this: https://www.newscientist.com/article/dn8770-enormous-laser-beam-produces-artificial-star/ The laser excites the sodium ions in the (of all places ;-) ionosphere. Then drive correction is applied, using something very much like PHD2. BTW, there's a photo here showing one of the 2 altitude bearings: https://www.eso.org/public/news/eso9725/

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