Motorising the dob - first baby steps.

[davidpretorius]

Hi All,

It is amazing how a hobby can so completely consume your brain.

How to motorize a dob.....here are some initial thoughts!


Obviously two axis to be done, RA and DEC
Stepper motors controlled by my computer thru the usb port seems a neat way to go if the printer port is taken up by long exposure software. I should be able to drive four motors with stepper motor controller ie RA, DEC, the focuser and.....spoon to stir coffee.
Where to drive DEC axis? - at the pivot point or with rails on bottom of tube.
Question 1: at maximum magnification, what would the angle of the normal field of view be? say at 6mm ep with 2X or 3X or even 5X. I want to know how fine in terms of degrees i need to be to be so i can get fine movements ie .1 of a degree. I may have to gear it!
Question 2: Assuming everything works in driving accurately, getting the computer to adjust for the rotation of the earth should be possible. ie point at a know location, calibrate the machine and of you go. Do anyone know of references for calculating the drift at different spaces from the celestrial equator?
I have on purpose not searched on the internet just yet, and I know lots of people have probabley tried this already. I am enjoying the mental exercise dreaming up stuff. It may not be useful and feel like reinventing the wheel, but it is fun.

[asimov]

Now THIS will be an interesting thread! :D

[Starkler]

I dont have any links, but try googling "Mel bartels". He is the guru for motorising dobs and even sells kits to make it easier.

[elusiver]

Ok.. anything i say here may be corrected by Rajah.. cause i know it will be. I know very little about these sorts of things as I'm still learning myself :D

to approach this in a complete DIY approach I'd start off with setting up DSC's first. As the encoders would be a major part of the motorising project. I doubt u'd be able to do it with steppers alone, as i'd assume there'd be some leeway in the steppers so you'd have to put encoders and a system in place to feed the current position of the scope to your software.

U'd also definately need to gear it. Getting fine movement out of a stepper of size to move the dob around would mean that even a half step would be too much.

I too have thought about it.. and basically you'd have to be or know someone with the skills and machinery to machine a few parts.

I started with the theory of X & Y, which is very simple. To get to Point A step X motor 5 steps and Y motor 2 steps sort of thing. But there's a few more variables in it that can mean you'd be way off by that method, which is why u'd need encoders. All in all a tough project, and you'd have to have ALOT of cloudy nights to work on it(which means it's a great project for kiwi's ;) :P ). But it is possible, and it'd be very rewarding once accomplished.

my motorised focuser's keeping me going for now. Pretty much one major hurdle thats holding me back on the software stage (does anyone know how to input values from a text file in VB6? i can output.. but can't input to a variable) then i can start working on the mount.

el :)

[acropolite]

A good starting point would be something like Mikes EQ platform, maybe using stepper motors with friction drive, but it's noteworthy that the industry in general uses normal motors, gearing and encoders and not stepper motors....:confuse3:

[davidpretorius]

i have had a look at the DSC and also the dob mount that the guy makes in wood to order. It would be nice for it to be simpler. I would hate to have to have different mounts for different places as my latitude changes. ie when i go to snake valley in nov 2005

At this stage i will keep pursuing the holy grail of stepper motors moving the scope and software knowing where it is and allowing for drift as it changes ra and dec after an initial callibration to a known point.

Is a projects area on the site worth while so great ideas like lucifers's focuser dont get lost in the forums???

[RAJAH235]

David, Why not PM Mike re; that request?? There is a 'Projects' section, in the main menu. Might even get our own section > 'IISAAP's'. For all us 'builders'. :computer:
ELLEX, Why pick on me? :P I only state the facts m'am. (old 'Dragnet' fan). :D L.

[ballaratdragons]

I really can't be bothered GoTo'ing or motorising my Dobbie base. I would rather motorise/computerise an EQ or Fork and mount my 12" in it.

But that's just me! :D

[davidpretorius]

shall pm mike and see if this naughty corner boy can redeem himself and get a section under the projects!!

ken i would like to have both! a motorized goto on the dob base and also the eq's as well. I just like to tinker!

[Orion]

One thing to remember is that if you wan to motorize your telescope so it's slews to a destination and then tracks on the object, you will get field rotation.
Now there is nothing wrong with this when do visual work but there is if you want to do astrophotography! If imaging or astrophotography is what you want to do I would go with a equatorial platform.

[davidpretorius]

thanks orion,

not sure what you mean by field rotation, if the image is being tracked, then are you saying that a object taking up the width of a image ie nebula etc will rotate around on the screen, some of it will be fixed, but maybe the outer edges will rotate?

whilst with a eq mount the whole image remains tracked and not rotating

ssshhh, don't tell asmiov about this advantage of the eq, we all had converted asimov to believing that dobs are the best and eq's are sh1t that it is much better to push it two directions than one!!!! now where is that little smiley guy with a stick? argh here it is...:poke:

[Astroman]

Field rotation is exactly that, it's like walking in a circle around your EP the view seems to rotate around. Not noticable much due east and west, terrible at zenith. It's okay if you do really short exposures of the planets but not good at all for DSO. I tried bartelising my dob but I really had no idea what I was doing with the mechanics so gave it away, might take it up again one day. I tried making my own worm gear using Selleys Kneadit and a threaded rod, work to a degree, but I think I rushed it a bit. I think I will try friction drive next time.

I remember when the scope did move on it's own a great feeling of satisfaction came over me, until it stopped and stripped the gears.

Good luck with it anyway.

mel's website - http://www.bbastrodesigns.com

[davidpretorius]

more software to write..... if you can computer knows where the dob scope is pointing and it is tracking, it will also need to know how much to adjust the rotation depending on where it is pointing.

this is going to be fun to play with on the dob initially. PLease do not think I am being stubborn about attempting this with a dob, I fully realise and understand that the eq mount is the way to go straight up and it would be cheaper at up to $1500 than what i will spend playing, but I am more interested in the journey than the end destination. I will learn heaps!

When asimov goes to usa to live, does he need a passport to use the forum? Will he understand our lingo anymore?

[netwolf]

There are two issues here.

1. Goto an object and track it.
2. Countering the Earth's rotation by Polar alignment.

What you need to do is duplicate Meades Fork mount Autostar system.
With the meades you can work in Alt/Az with Goto and track an object for viewing. You can also do short exposures of planets. However if you want to do DSO's then you have to add a Wedge to the Alt/Az mount.

Imagine tilting your Dob base so that it is aligned with the South Celestial Pole (about 11 degrees of from Earth's magnetic pole). The CP is the pole on which the Earth rotates. But just puting it on a wedge is no good you need to then rotate your dob on this wedge to counter the earth rotation. With the Meade fork mounts, you just tell your Alt/Az software that your polar aligned, and it makes the Alt/Az movement such that it counters the polar rotation.

The concept of the SCP or NCP (north celestial pole), and alignment to these seems mind bogling to the begineer. But there is a lot of good websites out there that show you and explain to you what this is better than i have tried to do..

Regards
Netwolf.

[davidpretorius]

thanks

[gd253]

Or build a field derotator. I think you can even buy these in the USA and it'll link up to the Bartels system. Check the Yahoo scopedrive forum. NB I don't own one & have never used one. I'm in the process of making the gears to drive a 12" Bartels dob.

--GD

[davidpretorius]

thanks GD, we have another thread going on this as well and would love to have your thoughts and input and updates on your progress.

i will be taking some shots of the current setup and ideas for a wedge setup today hopefully.

[mch62]

When I made my mount I gave serious consideration to going Alt Az instead of EQ as Mel Bartel's system can do this with field de-rotation but this required another servo controller and servo to do this plus modifying a focuser to move rotate meant more weight at the end of the OTA.
One of Mels servo controllers does 2 motors so if using 2 controllers you have both drives and focuser and focuser rotation covered.
I went EQ instead for the simplicity , but if i were to do it again with a larger scope I would go Alt Az due to the smaller foot print inside an observatory.
Some thing to consider.

Have included a few pics of Mel's Servo system.
Waiting on drive and idler rollers to be made for the friction drive .

In my procrastinating about making a mount I toyed with the idea of making a stepper system but my patience is not up to that task and hence I used Mel's ready made servo system with the excellent software he has written for it.
My other consideration was integration into charting software and ultra fine tracking with out PE and fast slew speeds.
His stepper system can be bought in various stages for price reduction.

As far as drive reduction goes you need at least a 1/2 to say 1/10 arcsecond tick size or smaller to get pointing accuracy and tracking.
The sky is 12960000arcseconds

My system works out around 1/3.with a total friction drive reduction of 2200:1 and enocder ticks of 4 500 000
I can still slew at around 6 degs /sec with the servo's which is more than fast enough..


The tick size is the encoder on the motor or shaft that is used for positional info.

The smaller you go the more accurate but the slower the slew speed is going to be with a given motor .

If your not worried about positional encoding but just driving then you could get away with less reduction as long as there is sufficient to smooth the steeper pulses.

Hope you have some fun with the project. :thumbsup:

[davidpretorius]

thanks for this info, i was going to be doing the maths today, but you have helped me heaps. I really appreciate it.