Ok so I'm looking at making my own equatorial mount, and I'm in the brainstorming stage. I'm not especially worried about go-to, only tracking, and I want this to be a long term investment, so I would like it to be able to hold a good amount of weight (probably around 50kg equipment min (100kg including weights)) and be able to track smoothly.
I'll just ramble off a bunch of questions I have, and if anyone can provide any extra info or links it will be greatly appreciated
- What's better/smoother, stepper motors, or servo motors?
- If I'm correct, the RA axis motor (directly driven) will need to turn at about 0.0007RPM to track objects, is it best to use gears, a DC drive/speed controller (potentiometer and IC's), or a combination of both to reduce the speed of the motor?
- I've noticed that a lot of equatorial mounts use a worm gear system. Is there any advantage or reason for this?
- About how much torque should I be looking at to easily move this amount of weight?
- What material is best to construct the mount out of?
- Are there any instructions around for building a mount an using a manufacturers go-to/tracking system (eg Losmandy Gemini)?
- Is it worth building a large mount myself, or am I better of waiting until I can easily afford to buy a mount such as one from Mathis Instruments?
Blake I'll get the ball rolling with my take on this. I'm not talking from a great spread of practical experience but am working on something in the shed.
- Servo's are reported as better then steppers. I've not yet used servo's but from what I've read steppers mostly win on price (motor and controller).
- Direct Drive seems to be very cutting edge. I suspect that for most mortals a well controlled motor and lot's of reduction is where we end up.
- Worm gears provide a lot of reduction in one step with limited backlash. They also provide some holding capacity (they don't run backwards under load). Kinetic (Steve) has done some great work on home made worm gears and given some great info about it on other threads.
- Over to others, I like aluminium because it's light, easy to machine and does not rust. Stainless shafts make a lot of sense.
- I've not seen anything on fitting manufacturers go-to tracking on home built EQ mounts. Mel Bartels servo drive system might be useful http://www.bbastrodesigns.com/BBAstroDesigns.html The Argo Navis http://www.wildcard-innovations.com.au/ can be fitted to a wide variety of scopes and if you are building one allowing for encoders should be a given. It can interface with the Servo Cat (and some other drive systems).
- I suspect that will depend on what you hope to achieve and what skill's budget and equipment you have. You could throw a lot of money at a home built mount and never get the result's you want but if you have the skills and equipment then you might get something that's hard to buy both in hardware and satisfaction.
I can say that taking on such a project is a huge undertaking in (mostly) time and effort. Only you can decide if you wish to invest that time and effort into building a mount. Or is it better to invest the time in say a second job and use those $$$ to purchase the mount of your dreams.
For me, it was something I had to do - and I'm very happy with it as it performs the astronomy tasks that I currently ask of it. But it's not perfect (it has a wicked PE) and it is restricting a type of astronomy tasks that I want to explore in the future, so I'm currently contemplating an upgrade to the drive method to allow longer exposures which means more work - sigh... or I might purchase something to do this new task. It's unresolved at the moment...
Another aspect is - how do you put a value on the finished mount - for insurance purposes? There is no receipt that you will have for a purchased mount that you can wave at an assessor. So in the end you have something that does have a value (your time and effort) but no monetary value. Does that worry you?
Anyway, IMHO there are some physological aspects to consider first and foremost, before worring about steppers "V" servos, ect ect - and only you can answer those questions...
I use stepper motor on RA, controlled by Mel Bartels' programme Scope.exe running under DOS. Using 20 microsteps (on a 200 step per rev motor) I get 4000 m/steps per rev - smooth as you could wish for.
I've done some more research, and instead of direct drive, I've decided to go with a friction drive system with servo motors.
I'm thinking of rubber, urethane or stainless steel rollers. Does anyone have any opinions about which material is best?
Also, I'm going to use 2 motors for the RA. One for tracking, and one for slewing the scope. The tracking motor will be at 700RPM with a series of friction rollers with a ratio of 1000000:1 to reduce the speed to 0.0007RPM. I will then have a second motor further up the friction roller system at 1000RPM which will be reduced to 1RPM. In order to stop the motor that is not in use from generating voltage when the shaft is turned by the motor in use, I will have a switch on each motor to create an open circuit when it's not in use. The issue now is how can I stop the scope from wanting to drop/rotate under it's own weight? If the scope and counterweights are balanced correctly I don't see there being a huge problem, but the last thing I want is to build a scope and have it destroyed because the mount wasn't designed properly.
At that kind of OTA weight. A DIYS fork mount may be a better option.
There are two companies (both in the UK who can help out with high quality wormwheels to suit your needs for the RA and DEC axes. Look up BEAKON HILL TELESCOPES. http://beaconhilltelescopes.org.uk/ or Tel: ++44 (0)1507 363 381.
And someone else who I've come across who I highly recommend who can help you out with complete drive systems to suit , look up AWR Technology in the UK. http://www.awrtech.co.uk/ email alan@awrtech.co.uk
At that kind of OTA weight. A DIYS fork mount may be a better option.
There are two companies (both in the UK who can help out with high quality wormwheels to suit your needs for the RA and DEC axes. Look up BEAKON HILL TELESCOPES. http://beaconhilltelescopes.org.uk/ or Tel: ++44 (0)1507 363 381.
And someone else who I've come across who I highly recommend who can help you out with complete drive systems to suit , look up AWR Technology in the UK. http://www.awrtech.co.uk/ email alan@awrtech.co.uk
Thanks for the links! I'm not sure about the weight entirely. I was planning to build my own cassegrain OTA. How much would you expect a 20-30 inch pyrex glass mirror to weigh?
Thanks for the links! I'm not sure about the weight entirely. I was planning to build my own cassegrain OTA. How much would you expect a 20-30 inch pyrex glass mirror to weigh?
I would say it would be a bucket load heavier than that Bojan, my 10" mirror weighs just on 5 kg. if your designing a cassigrain at 30 inch that is a MASSIVE undertaking that requires a lot of skill and knowledge to get it right. I would suggest a fork style mount over a GEM as a GEM would have to be at least as big as a paramount maybe bigger for strength.
have you got an observatory to put this bohemith in? because I would say that once its placed down it will not be moving any time quickly.
I just did some quick calcs,
borosilicate glass (pyrex) is approximately 2.44g per cm^3
30" = 76.2 cm divide by 2 gives your radius 38.1cm
2*Pi*R = 239.3 cm^2
estimating the thickness to be at least 5 cm possibly more due to size
239.3 * 5 = 1190.5 cm^3
1190.5 * 0.0244 = 29.19 kg
I believe that you should be constructing the mount to carry at least 80 - 90kg if this kind of set up is your target.
Last edited by bmitchell82; 10-07-2010 at 11:29 AM.
Yep, Brendan, you are quite right. My very rough guesstimate was based on my 10" dia, 2" thick mirror, which, if I remember correctly, weights about 3kg.. but my memory might be corrupted here, it was a long time ago when I was building my newt.
And I forgot about corrector plate..
This weight might be a hard nut to crack even for a professional telescope designer..
it wont be that hard, it just wont be moving at all
You wont have to worry about a corrector plate because of the cassigrain style.
look up on you tube there was a gent who made a 30" relay cassigrain its a 7 part presentation but well worth the hour and a bit. It will give you a big idea on what is required and the amount of effort needed to undergo this type of construction as he made the whole shabam from telescope to mount to observatory!
I've changed my plan now after getting some feedback and doing some more in depth research. The plan was to build a large telescope, firstly planning it, and then building a small version. I still plan on doing this, but I was wondering about some other telescopes and mount designs I came across.
I am now looking at the split ring style equatorial, and either a cassegrain or nasmyth OTA. Any opinions about either?
What I don't understand is if you look at the light path diagram of a newtonian, it simply reflects the light from the parabolic mirror out the side of the tube from a flat mirror. Why can't you use a flat mirror to reflect the light back down the tube for a cassegrain or nasmyth style scope? Does the convex mirror increase the focal distance and allow a smaller obstruction? I plan on having the mirrors made by someone else, but are there any tips for the OTA?
You could , but the secondary would be a big circular flat mirror with a diameter at least 1/2 that of the main mirror, so the performance of the telescope would be pretty bad compared with other newtonians and cassegrains of the same size.
On second thoughts, could I just buy 2 parabolic mirrors and build a gregorian style telescope combined with a nasmyth? Or do the mirrors need to be made to suit each other?
I built this EQ mount for a 12" Newtonian on a very low budget-
(a few hundred$) http://www.planillustrator.com/astro.html
The mount has worked great for 100's of hours so far, the camera
is what really needs upgrading someday, but some pics are included.
Ok so I'm looking at making my own equatorial mount, and I'm in the brainstorming stage. I'm not especially worried about go-to, only tracking, and I want this to be a long term investment, so I would like it to be able to hold a good amount of weight (probably around 50kg equipment min (100kg including weights)) and be able to track smoothly.
I'll just ramble off a bunch of questions I have, and if anyone can provide any extra info or links it will be greatly appreciated
- What's better/smoother, stepper motors, or servo motors?
- If I'm correct, the RA axis motor (directly driven) will need to turn at about 0.0007RPM to track objects, is it best to use gears, a DC drive/speed controller (potentiometer and IC's), or a combination of both to reduce the speed of the motor?
- I've noticed that a lot of equatorial mounts use a worm gear system. Is there any advantage or reason for this?
- About how much torque should I be looking at to easily move this amount of weight?
- What material is best to construct the mount out of?
- Are there any instructions around for building a mount an using a manufacturers go-to/tracking system (eg Losmandy Gemini)?
- Is it worth building a large mount myself, or am I better of waiting until I can easily afford to buy a mount such as one from Mathis Instruments?
Thanks in advance!
Hello Blake,
A word to the wise the design of these mounts is not trivial - the devil is in the (considerable ) detail.
Of course one can put together something that sought of works but when one looks to using things like servo mechanisms to slew loads of the order of 100KG one needs to have a very good understanding of the feedback systems that control the servo motors.
The use of direct drive (torque motors) is nothing new having designed the stable platforms (controlled via the ships inertial platform) on naval vessels. The use of torque motors? Way to go!!
Ok so I'm looking at making my own equatorial mount, and I'm in the brainstorming stage. I'm not especially worried about go-to, only tracking, and I want this to be a long term investment, so I would like it to be able to hold a good amount of weight (probably around 50kg equipment min (100kg including weights)) and be able to track smoothly.
I'll just ramble off a bunch of questions I have, and if anyone can provide any extra info or links it will be greatly appreciated
- What's better/smoother, stepper motors, or servo motors?
- If I'm correct, the RA axis motor (directly driven) will need to turn at about 0.0007RPM to track objects, is it best to use gears, a DC drive/speed controller (potentiometer and IC's), or a combination of both to reduce the speed of the motor?
- I've noticed that a lot of equatorial mounts use a worm gear system. Is there any advantage or reason for this?
- About how much torque should I be looking at to easily move this amount of weight?
- What material is best to construct the mount out of?
- Are there any instructions around for building a mount an using a manufacturers go-to/tracking system (eg Losmandy Gemini)?
- Is it worth building a large mount myself, or am I better of waiting until I can easily afford to buy a mount such as one from Mathis Instruments?
Thanks in advance!
Hello Blake,
A word to the wise the design of these mounts is not trivial - the devil is in the (considerable ) detail.
Of course one can put together something that sought of works but when one looks to using things like servo mechanisms to slew loads of the order of 100KG one needs to have a very good understanding of the feedback systems that control the servo motors.
The use of direct drive (torque motors) is nothing new having designed the stable platforms (controlled via the ships inertial platform) on naval vessels. The use of torque motors? Way to go!!
Blake, here is a thread about my home-made (sort of) HUGE mount.
Home-made but made in the workshops of the CSIRO by Bert (avandonk) and his CSIRO buddies.
I didn't make it but I restored it, replaced the bearings with new ones, etc etc.
It may not suit what you want to make, but you might be able to pinch some ideas of it, like the Holden Torana wheels and bearings.
If a car can carry a ton of weight, then this mount will carry lots too.
It was originally designed to carry a heavy tubed 16" Newtonian, but I use my 12" on it and it isn't even breaking a sweat.
Have a look and see if you like any of the ideas built into it. You may be able to adopt them into your project:
