Motor for RA axis on modified Dobsonian mount?

[sploo]

I've recently picked up a used Skywatcher 300P; the truss tube version on a non-GOTO Dobsonian mount.

For astrophotography I'm considering building an equatorial fork mount - in this instance, quite simply a Dobsonian mount, but tilted at an angle; such that the Az axis now becomes RA.

I'd leave the Alt (now Dec) axis manually operated, but I'd need to motorise the RA axis. Given that it would need to rotate at sidereal rate (once every 86164s) that's going to need some hefty gearing reduction. I see that worm gears (with a large worm wheel) are a common solution; but even with a very large worm wheel (several hundred to one reduction ratio) that's still nowhere near enough, even for a slowly spinning stepper motor.

E.g.

One rotation every 86164s is one rotation every 1436.066 minutes (0.0006963 rpm)

A stepper running at 200rpm needs a 200 / 0.0006963 = 287213:1 reduction

Even with two stages (two sets of worm gears and wheels) that'd require 535:1 reduction per gearing pair; still pretty large.

Three or four stages (66:1 or 23:1) would be more practical in terms of worm wheel sizes, but then I'd worry that the smaller gears wouldn't be strong enough to support the 300P tube (risk of stripping teeth).

What are others using for this sort of application? Perhaps an off the shelf heavily geared (e.g. planetary) stepper motor, plus a couple of smaller worm wheel stages, or maybe a combination of smaller and larger worm wheels (with the larger wheel at the output to carry the torque)?

[vlaiv]

Why would you have your stepper running at 200rpm?

I think you are approaching this the wrong way. If you want to determine good reduction ratio for a stepper motor to be driving RA - you need to think in terms of resolution rather than speed.

Stepper motors have about 200 steps (1.8 degree per step) and each of those steps has certain number of micro steps - let's say you will do 64 micro steps per step.

You also want good resolution of about 0.1 arc second per step.

This means that full circle will have 360 x 60 x 60 x 10 = 12960000 steps - that is you need 12960000 steps, 0.1" each to make full revolution.

With 200 steps per circle and 64 micro steps - one revolution of stepper motor will have 200 x 64 steps = 12800 steps.

Reduction that you will need will be - 1012.5 : 1 reduction

In fact, HEQ5 equatorial mount has something like 0.14" per step and worm gear system having 705:1 reduction.

If you are going to use dob mount, base that enables azimuth movement has base with diameter of at least half a meter. That means that circumference of it will be at least meter and a half.

You will have no problem getting 1000 worm teeth on such diameter and using simple screw that has 1mm pitch to drive it for reduction of about 1000:1

In fact - this is what is commercially available for dob mounts in terms of EQ platforms, and I would recommend you looking at one to see if that will satisfy your photographic needs. You did not mention what sort of photography you want to do.

EQ platform will be more than enough for EEVA and planetary, and even some short exposure, lucky type DSO imaging. Of course, if you are into DIY than this fork mount thing could be nice project, but so would EQ platform.

 

[Dr_Ju_ju]

As Vlaiv, has stated, a decent stepper motor with appropriate drivers, combined with micro-stepping, will easily do what you want.... 

If you want further info on a 'home-built' system, then have a look at Tom Carpenter's excellent AstroEq system (https://astroeq.co.uk/tutorials.php). I use this system to convert a non-goto EQ5 system to a fully remote controlled system, with all adjustments to rotation etc., being done in software\configuration. 

[sploo]

2 hours ago, vlaiv said:

I think you are approaching this the wrong way. If you want to determine good reduction ratio for a stepper motor to be driving RA - you need to think in terms of resolution rather than speed.

...

Understood - many thanks. At ultra low rpm I was thinking that the stepper movement would not be sufficiently smooth, but with a 1012.5:1 reduction that still means (I think) somewhere in the region of 150 microsteps being done per second (with 200 steps per circle and 64 micro steps per step).

The large diameter of the Dob style mount had occurred to me (from the point of view of being able to fit in a big worm wheel. That should also result in plenty of tooth engagement, and therefore reduced risk of stripping the teeth.

Photography: initial planetary, and hopefully some DSO. I currently only have DSLR gear, so exposure times would be limited by the sensors.

PS A quick Google didn't provide the meaning of "EEVA", so I'm afraid that one's lost of me.

[sploo]

1 hour ago, Dr_Ju_ju said:

As Vlaiv, has stated, a decent stepper motor with appropriate drivers, combined with micro-stepping, will easily do what you want.... 

If you want further info on a 'home-built' system, then have a look at Tom Carpenter's excellent AstroEq system (https://astroeq.co.uk/tutorials.php). I use this system to convert a non-goto EQ5 system to a fully remote controlled system, with all adjustments to rotation etc., being done in software\configuration. 

Interesting; thanks.

I probably wouldn't try to make a full goto system, but that's an interesting site. It may well be that some of his existing PCB parts would be suitable, and given the low cost, probably more sensible than making one myself.

[vlaiv]

21 minutes ago, sploo said:

Understood - many thanks. At ultra low rpm I was thinking that the stepper movement would not be sufficiently smooth, but with a 1012.5:1 reduction that still means (I think) somewhere in the region of 150 microsteps being done per second (with 200 steps per circle and 64 micro steps per step).

The large diameter of the Dob style mount had occurred to me (from the point of view of being able to fit in a big worm wheel. That should also result in plenty of tooth engagement, and therefore reduced risk of stripping the teeth.

Photography: initial planetary, and hopefully some DSO. I currently only have DSLR gear, so exposure times would be limited by the sensors.

PS A quick Google didn't provide the meaning of "EEVA", so I'm afraid that one's lost of me.

EEVA section exists here on SGL, but it is often called EAA - Electronically assisted astronomy. EEVA stands for electronically enhanced visual astronomy (or similar, I'm not 100% sure).

EEVA is a bit broader term than EAA as it includes night vision devices, where original usage of EAA term was using of video cameras and recently cmos cameras and viewing recording on monitor / computer screen.

It is very close to planetary style / lucky DSO imaging. In planetary style lucky imaging exposures are very short - order of 5 to 10 ms. For Lucky DSO imaging - exposures are kept short at about 1-2s, while EEVA / EAA or "Live Stacking" as it is sometimes called uses exposures longer than that, but still shorter than regular DSO imaging - from few seconds up to dozen or so seconds (sometimes people use half a minute exposures).

Point with EEVA is to watch image of target build up in (near) real time - so you observe for few minutes (and stack up to 30-40 short exposures) and the move on to different target. This requires goto and computer control to locate next target, but in principle you can move scope by hand.

In any case - do search for EQ platform as that is going to be by far easiest solution to either purchase or DIY. It will let you do most of things mentioned here - Planetary for certain and Lucky DSO imaging. Depending on tracking accuracy of EQ platform you might even be able to do EEVA.

Another solution that you might want to try is friction gear instead of worm. That one has both advantages and disadvantages compared to worm.

 

[sploo]

11 minutes ago, vlaiv said:

EEVA section exists here on SGL, but it is often called EAA - Electronically assisted astronomy. EEVA stands for electronically enhanced visual astronomy (or similar, I'm not 100% sure).

EEVA is a bit broader term than EAA as it includes night vision devices, where original usage of EAA term was using of video cameras and recently cmos cameras and viewing recording on monitor / computer screen.

It is very close to planetary style / lucky DSO imaging. In planetary style lucky imaging exposures are very short - order of 5 to 10 ms. For Lucky DSO imaging - exposures are kept short at about 1-2s, while EEVA / EAA or "Live Stacking" as it is sometimes called uses exposures longer than that, but still shorter than regular DSO imaging - from few seconds up to dozen or so seconds (sometimes people use half a minute exposures).

Point with EEVA is to watch image of target build up in (near) real time - so you observe for few minutes (and stack up to 30-40 short exposures) and the move on to different target. This requires goto and computer control to locate next target, but in principle you can move scope by hand.

In any case - do search for EQ platform as that is going to be by far easiest solution to either purchase or DIY. It will let you do most of things mentioned here - Planetary for certain and Lucky DSO imaging. Depending on tracking accuracy of EQ platform you might even be able to do EEVA.

Another solution that you might want to try is friction gear instead of worm. That one has both advantages and disadvantages compared to worm.

 

Interesting - thanks again.

I've done lucky imaging of the moon (though only currently with a DSLR + lens + SkyWatcher Star Adventurer mount), and some limited DSO shooting with the same (30 second exposures max).

The little Star Adventurer mount is obviously not going to carry the 300P (somewhere in the 20kg region I think), and commercial EQ mounts capable of tracking such a tube tend to be expensive.

I have some basic engineering gear (lathe), plus a CNC machine (though the latter would not handle steel), so DIY is attractive.

[vlaiv]

9 minutes ago, sploo said:

Interesting - thanks again.

I've done lucky imaging of the moon (though only currently with a DSLR + lens + SkyWatcher Star Adventurer mount), and some limited DSO shooting with the same (30 second exposures max).

The little Star Adventurer mount is obviously not going to carry the 300P (somewhere in the 20kg region I think), and commercial EQ mounts capable of tracking such a tube tend to be expensive.

I have some basic engineering gear (lathe), plus a CNC machine (though the latter would not handle steel), so DIY is attractive.

Then have a look at this:

It is not fork mounted and needs to be rewind for each use - you get about 30mins to 1h in one go, but it is much easier to make (and cheaper).

[sploo]

4 minutes ago, vlaiv said:

Then have a look at this:

It is not fork mounted and needs to be rewind for each use - you get about 30mins to 1h in one go, but it is much easier to make (and cheaper).

I thought about one of those (a sort of Poncet Platform I believe) but I really like the idea of making a fork mount. I'm probably making life hard for myself aren't I 😉

[vlaiv]

2 minutes ago, sploo said:

I thought about one of those (a sort of Poncet Platform I believe) but I really like the idea of making a fork mount. I'm probably making life hard for myself aren't I 😉

Probably

I mean - as DIY then sure, if that is a challenge for you and you like that sort of challenge - sure. What I'm trying to say is that you should not go for it based solely on expectation that it will provide good tracking for such a large scope because it is better design than EQ platform. Unless you are very very skilled at building things - odds are that you will have large PE and only planetary and lucky DSO imaging will only be possible anyway.

[sploo]

30 minutes ago, vlaiv said:

Probably

I mean - as DIY then sure, if that is a challenge for you and you like that sort of challenge - sure. What I'm trying to say is that you should not go for it based solely on expectation that it will provide good tracking for such a large scope because it is better design than EQ platform. Unless you are very very skilled at building things - odds are that you will have large PE and only planetary and lucky DSO imaging will only be possible anyway.

I'm limited at the moment in terms of exposure time due to the DSLR (I've been advised anything between 30s and 2 minutes max per exposure), but certainly I won't be able to attempt multi-minute single exposures, so it may be ok. Maybe...