Voltage regulator to slow down fast RA drive?

[John K]

Hi guys,

I may have an inherently slightly fast RA drive with my scope which is only visible at very high magnifications - ie. like when I am imaging Jupiter at 9 meters focal length.

Have spoken to JMI who made the telescope and sure, although I could get them to make some adjustments to the RA bearing (ie. a new bearing), I was thinking that the simplest solution could be to simply reduce the amount of current that is being fed into the RA/Dec drive.

The scope gets fed with a 240v to 12v adaptor from a 240v household plug.

Is it simply possible to get something as simple as a dimmer type switch that allows me to reduce the amount of current that is being fed into the 240v to 12v power adaptor and then find the perfect point at which the RA drive slows a little to the right level?

Is this solution too simplistic?

This is an image of my scope, and the drive is made my Losmandy.

https://www.flickr.com/photos/johnka...57594183210993

Thanks,

John K.

[John K]

Or perhaps to simply add this to the 12v cable that feeds the telescope:

http://www.jaycar.com.au/productView.asp?ID=MP3209

[wasyoungonce (Brendan)]

Originally Posted by John K

...
This is an image of my scope, and the drive is made my Losmandy.JohnK.

John can you identify the drive...is it a Losmandy Digital drive system or Gemini.

I cannot tell from the pic. I didn't know JMI were using Losmandy drive systems?

Brendan

edit:
After a little search I found JMI has a unit fitted that looks like the Losmandy Digital Drive stepper system. Different front cover though.

So if it is running fast in RA then lowering the voltage in will not slow it at all. The system uses a crystal oscillator and microcontroller to determine the RA rate. A 9 metre FL is very unforgiving it's possible the scope is biased too heavy to one side perhaps. The other thing possible is that the drive system is fast in RA, the probable cause would likely be the crystal or the RA stepper motor. These can "cog" and fail to step (depending upon type of stepper motor used).

Maybe swap RA & DEC motors to see how this goes as first step.

[John K]

Originally Posted by wasyoungonce

John can you identify the drive...is it a Losmandy Digital drive system or Gemini.

I cannot tell from the pic. I didn't know JMI were using Losmandy drive systems?

Brendan

edit:
After a little search I found JMI has a unit fitted that looks like the Losmandy Digital Drive stepper system. Different front cover though.

So if it is running fast in RA then lowering the voltage in will not slow it at all. The system uses a crystal oscillator and microcontroller to determine the RA rate. A 9 metre FL is very unforgiving it's possible the scope is biased too heavy to one side perhaps. The other thing possible is that the drive system is fast in RA, the probable cause would likely be the crystal or the RA stepper motor. These can "cog" and fail to step (depending upon type of stepper motor used).

Maybe swap RA & DEC motors to see how this goes as first step.

Thanks Brendan - I remember the discussion with Losmandy via JMI mentioning a crystal osciallator / micro controller.

Changing the RA and Dec drives over could be a real pain. I don't even know 100% if they are actually inter-changeable.

Have been thinking about the bias factor. Perhaps I should try this by adding weight on the East side of the scope to "pull" the scope back on the RA side.

mmm....??

[wasyoungonce (Brendan)]

John..a pic of the drive controller would help identify it. Also a pc of the drive system. ...are their belts? or just plain gears and worm?

You should be able to swap just the motors, they will be interchangeable.

Brendan

[Wavytone]

At 9m focal length you're going to visibly see the effects of:

A) atmospheric refraction, which varies as a function of altitude and is neither constant nor linear, and it varies by location (altitude) and weather conditions.

B) flexure in the telescope assembly which will also vary according to its direction (both on altitude and azimuth) if it's an equatorial or fork mount. The flexure problem includes flexure in all parts from the mirror supports, the tube assembly, the focusser, as well as the saddle, equatorial and equatorial head and tripod.

And you'd be surprised how much the scope flops and sags under its own weight - for example just try adding to the OTA the weight of a second eyepiece and watch the image shift. Then consider the mass of the OTA and what this implies for the sag arising from that.

You can roughly compensate for these by tweaking the drive rate (eg the King rate is an approximation to allow for refraction) but if you are after zero drift on this scale the only solution is to use an auto guider.

[raymo]

240 to 12v adapters frequently put out more than 12v. Mine puts out 13.7v. I don't know if that would matter with your rig.
raymo

[John K]

Originally Posted by wasyoungonce

John..a pic of the drive controller would help identify it. Also a pc of the drive system. ...are their belts? or just plain gears and worm?

You should be able to swap just the motors, they will be interchangeable.

Brendan

The motors are using a steel roller system and are different in that the RA motor has a counter acting element on top of the motor unit that allows you to wind up so to speak the East or West direction of the drive and eliminate any slop that the motors have.

Here is a video of me rocking the scope in RA.

https://www.flickr.com/photos/johnka...57594183210993

I'll take a photo of the drive system and post.

Pretty much identical to this:

http://www.astromart.com/classifieds...fied_id=807974

Some more photos of the scope which show some drive shots.

http://www.optcorp.com/jm-ngt12-5-ng...telescope.html

Originally Posted by Wavytone

At 9m focal length you're going to visibly see the effects of:

flexure in the telescope assembly which will also vary according to its direction (both on altitude and azimuth) if it's an equatorial or fork mount.


You can roughly compensate for these by tweaking the drive rate (eg the King rate is an approximation to allow for refraction) but if you are after zero drift on this scale the only solution is to use an auto guider.

I'll try a different rate tonight but also get some weight to create an East bias and see if this helps. Firecapture allows me to guide, but at the moment only does corrections every 1 second. Have written to the author and ideally want 0.1 seconds - so then I can capture and I can guide simultaneously but without a back and forth motion.

Just a thought, if a voltage regulator does not work the way I imagined it, what about placing this between the drive and the RA motor it self? So the drive will think it's putting out the right level of voltage but the motor gets less??? The only issue is that it's a telephone jack type chord. Just a thought.

[wasyoungonce (Brendan)]

Hi John

Wavytone was correct, 9m FL is very unforgiving. The drive is definitely a Losmandy Digital Drive (DD) but it appears to use belt from the stepper to the RA drive.

I can see the rocking in RA. I don't really think this is a major problem unless directly over head. Either side will bias the drive with mass of the scope thus it is either driving against this slop or riding it down. Either way when guiding this shouldn't be a major issue in the same manner bias weight is used on a GEM to hold the worm against the RA gear.

I also very much doubt the Digital Drive system is the issue. Its rare to impossible for crystals wander off their designated frequencies these days.

I suspect King rate will help apart from that it might be slipping around the belt (is this belt toothed?)?

Putting a voltage regulator between the DD or between the DD and RA motor will achieve nothing. In fact it will interfere. The DD needs at least 12V to operate changing this voltage up down achieves nothing as there is an onboard voltage regulator.

The RA motor is a stepper. It works by receiving a series of 12V pulses that makes it "step" fwd (or back). The DD will count these pulses so retain positional accuracy. Reducing this voltage will make the motor "cog" that is misstep or stall.

So back to the issue...differential flexure as Wavytone stated. More than likely it is this. The mass of the truss at the secondary and focuser twists the structure when pointing one side (east) and twists the opposite way when pointing the other side (west).

This flexure is small but at 9m FL......is amplified greatly.

You best bet is image capture when the object is at zenith which will minimise flexure errors.

[John K]

Thanks for additional comments.

The drive actually uses a friction clutch that is tightened to drive the steel roller which rives the horseshoe ring. So no belts involved in the RA drive.

I actually sent the drive back to Losmandy soon after I got the scope because the crystal they had installed caused a rocking back and forth motion to the RA drive.

I actually now think that JMI have not been able to get the steel roller to be 100% accurate in size to allow the RA drive to operate accurately at such a high focal length.

So another solution would be to have a slightly smaller RA roller that turns the RA horseshoe ring slightly slower - BUT, that might cause it to go too slow as well.

Last night I did image at a different rate setting and added some weight on the East side of the scope and this has definitely made a difference and eliminated 50% of the issue. As the scope is not permanently mounted and is wheeled in and out to a pre dedicated pad, this also contributes.

I think the best solution is when Firecapture allows me to guide and capture at the same time - I think even at 1 second corrections, I will play with this, and should get quite close, although I would like this to be shorter perhaps when a Firecapture update occurs.

Thanks again for suggestions.

[John K]

Well, looks like the writer of Firecapture took my request on board from a few months back and now the latest version has 0.1 second guiding. Try out tonight and this could be the simple solution.

[wasyoungonce (Brendan)]

Originally Posted by John K

The drive actually uses a friction clutch that is tightened to drive the steel roller which rives the horseshoe ring. So no belts involved in the RA drive.

Thanks I was unsure from the pics.

Originally Posted by John K

I actually sent the drive back to Losmandy soon after I got the scope because the crystal they had installed caused a rocking back and forth motion to the RA drive.

Quite odd...me thinks this would be a stepper drive IC issue. Anyway.

Originally Posted by John K

I actually now think that JMI have not been able to get the steel roller to be 100% accurate in size to allow the RA drive to operate accurately at such a high focal length.

So another solution would be to have a slightly smaller RA roller that turns the RA horseshoe ring slightly slower - BUT, that might cause it to go too slow as well.

looks as if they have it pretty right though.

Originally Posted by John K

Last night I did image at a different rate setting and added some weight on the East side of the scope and this has definitely made a difference and eliminated 50% of the issue. As the scope is not permanently mounted and is wheeled in and out to a pre dedicated pad, this also contributes.

Excellent. This sounds as if your on the right track

[John K]

Just an update. With the latest version of Firecapture this issue is now solved.

Even with slightly out polar alignment at 8750mm focal length, Jupiter stays nailed in the centre of the screen on a 640 x 560 ROI CCD chip. I can get myself a cup of coffee and relax whilst I wait for the seeing to improve. Then simply click on the mouse and start the RGB imaging process effortlessly!

Thanks for all the advice.

John K.

[wasyoungonce (Brendan)]

Your welcome John...although I don't think I gave any advice..Waveytone did and you sorted it yourself!

Well done indeed!