Help an astrophotographer???

astro11

I hope I'm not too far off base asking here, but I would like some help with an 'application'.

I'm trying to 'refine' my home built remote telescope focuser. I currently use a 'gutted" servo(I know probably makes people cringe around here) basically as a motor. Speed is controlled w/ a simple PWM circuit. Since I only need fine speed control in 1 direction, to reverse direction I just reverse polarity w/ full 6 volts(I know I should probably use an H-bridge....more cringing and hence my question).

SO, I would like to improve this by having a continuous turn potentiometer or a rotary encoder (you tell me) connected to a remote servo mounted on the scope. (replacing the gutted one). movement of the pot or encoder would directly translate to movement of the servo. Needs to be continuous and bidirectional. would like to vary the ratio(course/fine focus). Connection could be with a long(30') wire OR better yet via WiFi to laptop that is already there doing other things. so box w/ potentiometer USB-->desktop--> wifi(or cat5) --->laptop-->usb controller-->servo

Is this possible? if so how?

Thank you...for letting me in your sandbox,
Kevin

W9GFO

My first thought is to use a continuous rotation servo at the focuser. On the controller end use a pot. Preferably one that returns to center or at least has a center detent.

By reading the value of the pot you would tell the servo to move one direction or the other. The further the pot is turned - the faster the servo would move.

Rich H

erco

Cringing is fine!

You have lots of options. BTW, your servo had a controller board inside which has PWM and an H-bridge already. If you're using a Stamp, you could just do a simple continuous rotation servo mod to it and control it in both directions at any speed. No external PWM controller required.

As far as control, you can use an encoder, or a stepper motor might be better for your app. If you're using a Stamp anyway, you can certainly drive a stepper with that.

Or if you're saying that you just want to turn a pot and have a remote servo slaved to that, it's even easier. No stamp required. You can just use a simple 556 "servo tester" circuit with a pot. Then you can control a standard servo's position (0 to 180 degrees) or a continuous rotation servo's speed & direction (along with stop) which I suspect might be all you need.

What are you comfortable with?

Edit: Rich beat me to it again!

Spiral_72

If you buy a servo for this mod, you might be interested in looking at the specs of the servo. I BELIEVE the higher torque models will be slower rotational speed. This should mean smoother and higher resolution.

IMHO JR makes a very nice servo, as does Hitec.... then futaba and airtronics. Some of the no-name Chinese stuff is cheap and appealing, but rather coarse.



P.S. I DO buy the no-name servos. They are great for foam planes, general hobby stuff and the like.

astro11

thank you all for helping. I guess I'm still a bit confused if it's possible to use a continuous rotation pot (or a rotary encoder) to have the servo exactly mimic control knob movement beyond <360 degree of typoical pot. so, for example 3 turns clockwise on the pot yeilds 3 turns on the servo...or 'a few' degrees counterclockwise correlates to the same on the servo. it would be nice to be able to vary the ratio but not necessary.

If there is a specific document or any other details that describes this type of application with some sample code that someone could point me toward that would be great ...I'm googled out...and admit I may be out of my league...at least for now till I play w/ this stuff a bit.

Thanks again,

Phil Pilgrim (PhiPi)

How many rotations of the focus knob are necessary to cover the full range? How much torque is required to turn it?

-Phil

astro11

haven't counted but 20-30 rotations. torque...good question it's very minimal
thanks,

Phil Pilgrim (PhiPi)

In that case, I think the simplest method would be a direct drive stepper motor. For the finest resolution, pick one with 200 steps/revolution. You can control this at the other end using a continuous-rotation encoder, like this one, a small micro, and transistors or an IC to drive the motor. You could even dispense with the micro if you're careful not to spin the knob too fast.

-Phil

astro11

Thanks Phil. That makes sense. I'll order some stuff, give it a try and check back if specific questions come up.

Looks like fun stuff, light years beyond my old radio shack days. Think I found a new hobby... My wife will love that

thanks again to all

Spiral_72

I do not intend to be argumentative, but if you use a stepper, I think you'll need to plan a way to isolate the steps / jitters from the focus control. A refractor would probably be more forgiving, but a reflector's image would turn into a blur while the stepper is adjusting which would make your focusing task a lot more complicated.

This may be a moot point if you have a several thousand dollar mount however.

Just something to consider.

astro11

Thank you spiral_72. That's a great point, if it's jerky or jittery that would make focusing difficult. I'm using a SCT, the focus knob is connected to the motor via a belt...which should help smooth out some minor jitters.

My plan is to try a cont. rotation servo first...if that doesn't work I'll try a stepper.

thanks again.

Phil Pilgrim (PhiPi)

Steppers don't have to be jerky at all. They can be PWM-ramped between steps (i.e. microstepped) to eliminate sudden lurches.

-Phil

Humanoido

Hi Keven,
From one astrophotographer to another:

I've motorized focusers for over 20 years on most of my home-built telescopes designed for astrophotography and CCD imaging. In the cases of the 40-inch and 50-inch telescopes, a remote focuser was mandatory. On the 12.5-inch Newtonian, it was one of the most useful add-ons.

Generally you want to design the system to handle the weight of the camera (and any guider, filters, etc.) The lighter the weight and the faster damping takes place. This is a great advantage. You can use either the continuous rotation server or the stepper motor. I have used both.

In my projects the servo is an advantage because it's lighter and easier for me to use (and costs less in my setups). The continuous rotation servo is easy to adapt. In fact, there's many programmable options (a lot of Boe-Bot programs will get you started) to drive it at different rates including start and stop ramping, limit blocks, ranges, slewing, ocular memory, and a host of other fine tune options which can be modified to suit your telescope.

You can program planets, Moon, DS, and a host of objects. Yes, you can drive beyond 360 degrees. The only concern is the torque required to turn the rack & pinion or friction focuser must match the servo capability. You should be OK with the SCT as their knobs normally move easily. The only difference is they often require a greater number of turns to achieve focus. This is where a set of parfocal oculars will be handy.

One advantage is I use a small hand held box and the cord isolates any hand vibrations from the telescope tube. I set variable speeds depending on the object and the setup. A pot is used to dial in the rates and two pushbuttons are for forward and reverse. A BASIC Stamp with PBASIC is ideal for this project. You can use a red LED and LCD for setting the programs.

astro11

Thanks Humanoido (just noticed your reply). I am getting great results with a continuous rotation servo controlling w/ a home built PWM from a 555. I can adjust the rate and use up/down toggle to control direction. My goal (which I think I've just accomplished) was to have the rotation of a pot (or in my case a rotary encoder) mimic the rotation of a focus knob in a continuous rotation fashion.

Using a maestro USB controller (I know not parallax) a rotary encoder, an L293 and a stepper...motor rotation is exaclty the same as rotation of the encoder. Now I have to see if there is any vibration...shouldn't be an issue cuz I'm belt driven.

Thanks again for the input.

Humanoido

Astro11, congratulations on your progressive success and keep us informed of your final results. It would be great to see this posted in the Project Forum along with a photo of the setup and telescope.