Cheap astrophotography dreams

[johnridley]

With the unexpected (by me) success of the Canon Digital Rebel in taking wide field astrophotos at relatively long shutter speeds (several minutes), I'm thinking it's time to build a tracking platform for the camera. Typically this has meant a "barn door" tracker. I may just go with that, but one of the problems with a barn door tracker is that it does have some tracking error because it's not a true tangential mount.

I have been toying with the idea of using a microcontroller to drive the steppers, and then I could vary the speed as needed to eliminate any tracking error.

However, it also occurs to me that it shouldn't be that hard to do a true tangential drive with a plywood circle segment. I've seen things on the web of people who have made worm drive gears by applying J.B.Weld to the edge of a wood circle and imprinting a threaded rod onto it.

I will probably still use the microcontroller to drive the stepper motor, because it's just easier and less parts, plus I'll be able to microstep using PWM. But I think I'll just try to build the damn thing first and worry about fancy drive systems later, otherwise I'll never get it done.

I'd like to have this done for the Perseid meteor showers later this month.

Comments

[ex-erikvolso370.livejournal.com]

The cool thing is how the tracking blur in the Cygnus shot lines up just so to make Brocchi's Cluster (The Coathanger) really jump out. Well, that, and the way you could take a 80 sec exposure at 3200 and get no sky glow on the horizon.

Cool!

[johnridley.livejournal.com]

I hadn't even noticed that. Pretty cool, it makes it into one object! Oh well, I'll still take "sharp" over "interestingly blurred" any day. Off to Lowes for parts...

Canon Digital Rebel

[lindaatzarquon.livejournal.com]

Apparently Jeff's friend Terry Blake also had that camera.

Well, after Berzerker, his friend Dan (the guy who shot the propane tank with his rifle) bought one. :-)

Re: Canon Digital Rebel

[johnridley.livejournal.com]

It's a dangerous camera in that regard. I know of at least 2, possibly more instances where people played with one for a few minutes and bought one within a week. It's done everything I ask, and then some. My 'reject' rate on photos from this trip was extremely low, owing to the ability to boost the ASA up to get non-blurry shots in low light conditions when needed, and the rapid shutter button response, which meant I got the shot instead of taking a picture of what happened 3 seconds later.

Re: Canon Digital Rebel

[ex-erikvolso370.livejournal.com]

Yeah. My brother has one. I'm still holding out for the soon-to-be Minolta Digital Maxxum, so that I can use the Minolta glass and flashguns I have, rather than having to bail and buy all new gear.

Owie.

[johnridley.livejournal.com]

Yesterday I bought some J.B.Weld over at Lowe's. They didn't have the plumbing fitting, but I'll live without that for now.

Last night I ripped apart an old 3.5" bad floppy drive to steal the stepper out of it. It's tiny, I figured it would be ideal. BZZZ wrong. It's small but it's also highly inefficient. It draws a chunk of current. Also it's bipolar so it's harder to drive properly than a 4-pole model. I have a pile of 4-poles already.

However, I'm thinking I'm just going to punt and use a 12V or 24V 1RPM synchronous motor. Generating 60Hz won't be much (or any) harder than driving a 4-pole stepper, and the sync motor will draw a lot less power.

The "owie" is that while I had the testbed powered up, using the power supply to play with the floppy drive stepper, something shorted out and the ATMEL-based blinkie that I'd been working on a few weeks ago suddenly got very hot and melted part of the breadboard. I smelled something, noticed the light not throbbing anymore, and of course reached over and touched the microcontroller. I'd say it was probably at about 400*F at that point. Oh well, one less working chip in the world.

Re: Owie.

[kevinnickerson.livejournal.com]

Stepper's are pretty cheap. Try All Electronics for a moderate selection. Alltronics has some too. The AstroSyn sure has the right name.

Re: Owie.

[johnridley.livejournal.com]

I know steppers are cheap. I have a box of them. The problem is that they suck a LOT of current, and I want this battery powered. Most astro stuff has moved to regular DC motors on gearboxes with optical encoder feedback to the computer, because they draw zero current when holding position. Steppers draw max current at all times.
I will probably go ahead and use steppers anyway, now that I've had some time to think about it, because I can just use a 12V gel pack (I can use it to drive dew chasers anyway), and by using a stepper I can easily get slow drive during operation, then a very rapid reset at the end of the stroke.

But first, I'm going to just build the thing, then I'm going to drive it with a 1RPM 12V synchronous motor running either from AC power or from an inverter; it'll be good enough to get going; unfortunately it'll probably also be good enough that I'll never actually get around to building the "ideal" solution.