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dshankar
member
Reged: 02/23/08
Posts: 27
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Hello,
Would it be possible to build a wedge and build an alt-az mount with drives and potentially add autoguiding?
Here's what I was thinking:
An alt-az mount appears to be very simple to construct and has very simple drives (basically, vertical/alt drive and horizontal/azimuth drive).
I'm not sure but I would think a wedge would be very easy to construct as well? I don't think wedges have drives since they are just aligned with one's latitude, right?
DIY autoguiding, in my opinion, is possible by controlling the drives using USB connections and regulating the voltage supplied to the drives. I think it would be possible to write some software to determine the error correction (using an attached guidescope) and then regulate the drives appropriately.
I would expect to use this 'mount' to do long exposure astrophotography (so no field rotation allowed) for about 1-4 hours.
It would be better than a barn-door mount w/ drives right? I'm not sure if it would be easier to DIY autoguiding for barn door mounts...?
Thoughts?
Thanks,
-D. Shankar
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Rob Willett
professor emeritus
Reged: 02/07/05
Posts: 588
Loc: London, UK.
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I think your plans would be considered somewhat on the ambitious side  However I have seen some highly successful ATM projects that would shame many serious manufacturers. None of mine are in that category 
A couple of issues to consider.
USB is a digital interface, so you can't just vary the voltage to control the motors. There are a number of projects that allow you to control a clock based motor circuit, I'd search for 555 on Cloudy Nights and look at any threads that brings up. USB interfaces are not trivial to build either.
You can also build a small microcontroller to drive a stepper motor, these aren't that difficult (really) to build. Have a look at the EasyPic controller (which I use).
Writing the software to guide isn't trivial to do, e.g. you can write software to read a webcam in real time, but then generating the appropriate offsets, writing commands out to the USB/Parallel/Serial/Bluetooth port is more tricky. You then need to build the receiving end (see previous) on your mount.
So yes you can do it, at the end of the day are you going to get better results than spending a couple of 100 dollars, I don't know but it will take you a lot of time to get this far.
I did look at this and came to the conclusion that it was many hundreds of hours work. The programming side is quite tricky to do and I would not underestimate the time taken to do this. Of course you could use something like PhD guiding and that takes away a lot of the hard work, then all you have to do is design, develop and build an ASCOM compliant mount.
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Thanks,
Rob
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walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2193
Loc: Doylestown, PA
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A very well thought out response Rob. From one who has built and programmed pieces of this type of system I agree with your assessment.
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Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
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Luigi
Carpal Tunnel
Reged: 07/03/07
Posts: 1554
Loc: Massachusetts
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There's really no difference between what you're suggesting a german equatorial mount or a fork mount, depending on the implementation you choose.
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17.5" f/5 Discovery Truss
IM715 7" f/15 MCT (due July!)
Eon-120ED refractor
CG5A coffee grinder, Orion Skyview Alt-AZ
35,19,15 Pans.9 Nag. Meade 24.5 4kSWA, 4.7 5kUWA.
BO-TMB 7mm planetary.
Zeiss Diascope 85
Zeiss, Leica, Canon IS, Fujinon, Nikon binos
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Owen
sage
Reged: 06/21/07
Posts: 314
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This is a job for Lego.
Specifically the mindstorms NXT brick.
Full robotic control, easy to develop interface, and now with Blue-Tooth conectivity... That takes care of many of the headaches...
Quote:
One of the Flickr groups led me to a page for what looks like an astronomy conference (http://www.cacr.caltech.edu/hotwired/). From there, I read through the Agenda to find the presentation by the telescope's creator (http://www.cacr.caltech.edu/hotwired/program/presentations/HTU_threadsafe.pdf). I also had to do a little sleuthing, but it also looks like the presenter is tied to a Virtual Observatory project (http://nvo.noao.edu/).
from
http://thenxtstep.blogspot.com/2007/06/nxt-telescope.html
PDF of the concept, with better pictures
http://www.noao.edu/noao/noaonews/mar08/pdf/93dpp.pdf
Thats enough food for thought.
Owen
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dshankar
member
Reged: 02/23/08
Posts: 27
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One problem, imo, with the NXT is power. I'm not sure if it will be able to handle heavy cameras and lenses  .
I'm going to have to go through what you said very carefully, Rob! I'm trying to look up ASCOM mounts but it seems to be quite the challenge...
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dshankar
member
Reged: 02/23/08
Posts: 27
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So maybe building a fully autoguided system for a barndoor/altaz mount is too much...
As a first step, how does the following sound:
{
In order to achieve basic guiding (not auto):
Mount a webcam, connected to a computer via usb and
calculate offsets.
Manually change these offsets by rotating the nut.
}
This way, I get reliable guiding..to some extent, right?
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Owen
sage
Reged: 06/21/07
Posts: 314
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The key part of NXT is just the controller - it can be used to 'drive' very large motors if needed
But then your scope will be well balanced - so its only stiction that will be the big hinderance...
Owen
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Rob Willett
professor emeritus
Reged: 02/07/05
Posts: 588
Loc: London, UK.
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Quote:
So maybe building a fully autoguided system for a barndoor/altaz mount is too much...
As a first step, how does the following sound:
{
In order to achieve basic guiding (not auto):
Mount a webcam, connected to a computer via usb and
calculate offsets.
Manually change these offsets by rotating the nut.
}
This way, I get reliable guiding..to some extent, right?
I'm not sure what you now gain? You can attach a webcam to a laptop, but what are you looking for that you can't see by manual guiding, as that is really what you are doing.
You can use PhD (or similar) to get most of what you want, it can show you the offsets on a laptop, but you're not going to gain anything. You can't manually adjust the scope based on laptop numbers with the accuracy you need. You would be far better off using an off-axis guider and manually guiding.
If you consider that most people are very happy with 5 mins exposure on reasonable scopes, aiming for 1-4 hours is very, very hopeful.
I speak from bitter experience here, I've tried the DIY route and it's hard, really hard. I'm not boasting but am trying to give you the benefit of my experience, I am a pretty good programmer, I have designed and built operating systems, designed my own computer languages, I have actually designed and built CPU hardware (though that was over 20 years ago:) ). What you are trying to do, with a target of 4 hours, puts your target right up there with the very, very best equatorial mounts in the world.
By all means ask the questions here, but be prepared for answers you don't like.
All the best,
--------------------
Thanks,
Rob
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Owen
sage
Reged: 06/21/07
Posts: 314
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Quote:
I speak from bitter experience here, I've tried the DIY route and it's hard, really hard. I'm not boasting but am trying to give you the benefit of my experience, I am a pretty good programmer
Hence the lego option - it removes lots of the interfacing headaches.
The challenges then are positioning and management of the sidereal rate. If we exclude Alt-Az (to simplify the path followed, and the control loops required), then we are down to managing the PE - and we start to get into the woods.
For the drive, I would look closely at 'zero' stretch belts (aka vehicle timing belts - use them with the teeth out) driving an idler drive system - it's easier to make a consistant 'flat' drive surface with little run-out (a car brake disk for example), than a large 'perfect' round - but that depends on the equipment to hand.
You will need gearing of some kind - simple friction rollers would seem to be the simplest and best solution although they will need to be kept clean, and speed accuracy can be further managed by motor speed control.
The route, I would personally take for this, would be to set that gate at 1 minute with low PE - tinker to reduce PE, and go for 5 minutes with low PE - tinker to reduce PE further - and go for 10 minutes with low PE - etc.
Small managable chunks is easier than one big chunk, and is likely to keep you motivated in the challenge, so much more likely to succeed.
Have fun with your back-yard moonshot!
Owen
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