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My astronomy project:
Implement of a stepper motor focus

Stepper motor focus

  1. Introduction to stepper motor focus
  2. Making brackets to motor focus
  3. Test of focus system out in the field
  4. Focus and temperature compensation 1
  5. Focus and temperature compensation 2
  6. Collect temperature compensation data

1: Introduction to stepper motor focus

I take no responsibility or liability for what are written here, you use the information on your own risk!

So now it was time to grab this desirable wish, a stepper motor focuser.

Have pondered long about this. Shall I build everything from scratch, or try to find something used? I'm not thinking about the price of 300 Euro and upwards. But in one of the topics I have written, I received advice from Stoffe about a new focus motor.

USB Focus

They manufacture this unit and seem to be from France and owned by Vincent. The motors have different torque. The middle model cost 165 Euro from Telescopic service was not much to argue about, better to concentrate on the other issues.

01 control unit

This is how the drive box looks, one can choose between external or USB power supply with a switch, 230 volt adapter included but none of 12 volts power, this motor requires 8 volts externally, but that is the same voltage as the camera power I have and they might share the same power supply. If you choose to feed it internally from USB via the control box, the engine must cope with 5 volts and thus little less torque. A thermometer with stainless house came in the kit and it's connected with the 3-pole jack plug.

02 stepper motor with gearbox

The motor if purchased separate cost 100 Euro but in the package it is included and rather cheap. The gearbox is marked 250/3 maybe that's the ratio, it seems so. A certain backlash arises always with gearboxes but torque will be good, and the focus remains in the rest mode. The program also has a function of backlash compensation. The only thing that gives little fears are the cable attachment to the engine, should probably be strengthened if it's to survive.

03 beltdrive

To this I bought a pair of tooth wheels and a pair of toothed belts. Focus motor must be used in two different configurations. If it works good I maybe order one more motor so I don't have to move the motor between the telescopes. Curiously, with companies in Sweden, I called some of them and got astronomical prices on these tooth wheels, just drill a hole and thread in a stop screw they wanted to have a 45 Euro each, will the Swedish industry survive? Now I bought this devices at the Conrad, it turned out to be a German company, with shipping it cost 35 Euro, in Sweden they demanded anything between 150 to 250 Euro for the same equipment. The pitch of the toothed belt is 2.5 and 6 mm width, the belts are specified down to minus 30 C degrees. The big tooth wheel has 40 teeth and the smaller has 30 teeth.

04 motor with atached beltdrivewheel

This was a bit of luck that both motor shaft and the telescopic shaft has a diameter of 6 mm and the tooth wheel fit directly. Here how it looks with the smaller tooth wheel mounted on the gearbox.

05 focus before motor control

I have chosen to put the second tooth wheel on the side of the focus axis that doesn't has the reducer of 1:10.

06 focus with beltdrivewheel

Here are the tooth wheel of 40 teeth mounted. If I counted correctly will the motor shaft after the gearbox rotate 8 revolutions at 65,000 steps, which is the max. With the tooth wheels 30/40 it is the equivalent of one end position to the other in the telescope focus range.

07 beltdrive complete

Testing the motor in it's proposed place, I choose this construction before the direct coupling to get a more compact installation, I do not like when the motor hang out on the side of the telescope. It remains one problem, how do I manufactured a mounting bracket for the motor. There are not many screws on the telescope to take advantage of and I do not want to drill holes in the telescope, or?

08 beltdrive

This view from a different angle. One problem I anticipate will occur, will focus slip in the friction clutch when the telescope is pointing upwards? Has drawn the friction clutch very tight, but are likely to be problems anyway, rack and pinion had probably been better.

09 beltdrive idea for shortfocus lens

The second telescope the motor focus shall serve is a telephoto lens of 165mm which will be the wide field setup (full frame). Here, the longer tooth belt is used and it is wrapped around the lens focus ring, and should be tightened moderately, hard to get enough friction.

10 beltdrive shortfocal lens

Here too an angle bracket will be made, quite easy here as there is substantial mounting holes in the right places to fit it with.

With all these parts, I came a long way, it felt good to get this finished so I can concentrate me on the final goal. I have had time to test-drive motor, some problems with the program but Vincent struggling on and it is not worse than that it should be possible to use.

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2: Making brackets for motor focus

Have now come to the part where mechanics must be designed / adapted between the telescope and the motor focus.

01 motor holder

Apparently there are not many holes at the telescope that can be used. I want to get this to work without drilling holes in the telescope. I begin to plan to use the big locking screw for focus and the set screw for the holder block. It is the big front screw and the countersunk allen screw to the right. It was lucky that it's M5 thread, otherwise it is a little bit mixing between US Inch and Metric standard.

02 motor holders

Looked for aluminum profiles but found none exactly as I wanted. Took a tour to various warehouses elsewhere, at the Ants Flea Market (Myrorna) I found a scrap box of assorted profiles. Bought me 4 parts of those, above shows the three which did not get in use this time.

03 motor holder with holes and a bent

Here is the profile that I began working with and it came from the famous IKEA, it's a curtain rod to be attached to the motor focuser. It is made of stainless steel, that material is not so easy to drill, you need a drill of high quality. Have curled up on one end which can be attached to the stepper motor connector. Also drilled hole for the motor and fastening screws to the telescope.

04 belt tighter adjuster

The outer fixing screw against the stand may also be a device to tension the tooth belt. It is done by nut is tightened against the profile and lift it.

05 motor axis

The axis from gearbox has been flattened a bit so that the tooth wheel will lock properly.

06 motor mechanic

A first sample mounting bracket to ensure that no surprises emerged.

07 motor mechanic

Here is the bracket from below and the nut which was mentioned earlier and who shall serve as tighter for the tooth belt. (Update, I will replace the nut with a distance so I can tight the profile harder to the the telescope and then reduce some flexure I noticed when used it later)

08 motor mechanic

The motor is mounted and the tooth belt to be tested so that it is sufficiently tight and that tooth wheel are rotating as they should.

09 motor electric connector

The electric connector mounted on the bracket's outer edge. Feels good that it is mechanically locked to prevent the wires to the motor wears off.

10 beltdrive system from sideview

Stepper motor assembly seen from the tooth wheels side.

11 motor and controlbox

Focus motor seen from the stepper motor side and the control box attached. The DB9 connector has lock screws so that it doesn't jump out, they give a warning in the manual of the stepper motor because the inductive discharge may arise if the cable comes off and thus destroy the electronics.

The tooth wheel has a gear ratio of 3:4 which allows the digital scale to be better utilized. The focuser has a free run length of 100 mm and it goes from 0 to 56,500 step, max on the scale is 65,500 so the range is used well. An alternative solution would have been with a shaft coupling. But it gives clumsy montage, and you miss the opportunity to have a ratio between them.

Also notes that it is mechanically manually possible to focus, it draws around the gearbox and stepper motor, but is not recommended as dangerous voltage spikes can occur that destroy the electronics. It also drops sync between focus motor and the digital scale.

This really is something that I wanted for long time that now will be exciting to try in practice. Have the little problem with the software but suppose it will be solved with time. In early test the stepper motor stalled, but that was due to the tooth belt was unnecessarily hard tightened. One can imagine that there will be problems when connecting a heavier camera, but then you can connect the control electronics to an external supply of 8 volts and thus get more torque on the motor. (Update, today I run the stepper motor at 8 volt externally, huge difference)

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3: Test of focus system out in the field

Tonight there was little holes in the clouds so I could do some testing of the focus motor and the software that controls it.

Here how it looks on screen two that I have connected to my laptop. It shows the astro server that I remote control, that's the computer in the battery box connected to the telescope. It easily becomes a little messy with all the applications you need to have control over. My big screen I bought helps and it has 1980x1200 in resolution, the screens are cheap nowadays. Whatever monitor one buy it seem to never get big enough. One way you can do is too increase the resolution to a higher value than the screen has, then get pan around, I do not like it. In addition to this display, I have of course also the laptop screen.

These programs are currently running on the astro server:
CdC (Skychart) star chart application: CdC

EQMOD that control the EQ6 mount, it's started through the CdC: EQMOD

PHD2 auto guider program: PHD2

APT which controls the Canon EOS camera and USB-focus: APT

All four programs are also connected through ASCOM interface. It allows drift align, dithering and auto-guidance in an intelligent way. That I will do in this test is to set the focus of the telescope and the main camera (DSLR).

02 screendump focus

Before I turn off the focuser and the astro server I always check that the focus is in the the bottom position, that is position 0.

Important before starting up is to check that the focus really is mechanically in bottom so calibration is not changed. As a starting point I set the focus to position 37,410 when I start up. I do so in the program APT and connect to the "USB Focus" by ASCOM. Because there are always some backlash in the focusing system it's good practice to focus only in one direction. Run out focus to 38010 and then change to focus inward in 10 steps each.

Centers the telescope on a rather dim star. Takes ISO800 exposures of 5 second (Canon EOS 5D and telescope f5.3) between each adjustment of focus. See below, apparently, quite a lot out of focus. The temperature is plus 1 degree, which can be read on the supplied thermometer to focus the unit (at the top of the window). The camera temperature is slightly lower. Obviously wrong because the camera gets a little bit warmer inside. But not so important, if it always shows that difference, then it's stable in time.

03 screendump focus

Opens up the focus tool in APT. A small box that I center around the star I will use to focus on. Here you get information about the FWHM, Full-Width-Half-Max, measuring the star's width. The best FWHM value achieved stored and displayed on the right. Low value corresponds to a narrow star and well focused. Furthermore, you get a profile of the star, and a plotted a graph of the FWHM value for each exposure. Very handy, you can even add auto temperature compensation, so far I haven't come there yet in the field use. Although autofocus is available.

In the first photo ended up focusing on FWHM = 38.61 pixels.

03a screendump mathematical star

How you measure FWHM, Full-Width-Half-Max, sorry, only in Swedish but the illustration shows how it works.

04 screendump focus

Here the focus has stepped in 300 steps and the FWHM value improved to 17.90.

05 screendump focus

200 steps to the inward and the value 12.45 is achieved.

06 screendump focus

400 steps to the inward and value of 6:46 is achieved. Observe what happens to the profile of the star, when you get better focus, concentrated more and more light on the central pixels. In the end they go into saturation, it interferes with the FWHM measurement, then you must reduce the iso setting or exposure time. The maximum value of the peak-scale here is 255.

07 screendump focus

400 steps to the inward and the value 3.73 is achieved. Further than this, I couldn't go because the clouds come in and distracting. FWHM measured in this program, in pixels, interpreted as the star halfway up in level is 3.73 pixels wide. Do you know the pixel scale, one can easily translate this to arc. seconds. My system is pretty under-sampled, 2.5arcsec / pixel, i.e. nearly FWHM 10 arc. seconds in this case, very bad, but I rarely come below 5 arc. seconds from my balcony. Pixel scale is a little rough to get better values. It must be solved in other ways.

08 screendump focus

Used the last time to set focus to the guide camera. It should normally only needed to do this once if you do not change anything on the mechanical or optical setup. The first thing is to get the little small rectangle you see in the CdC program above to conform to the guide camera field, the rectangle is set in the CdC setup> Display. Once it's done, it's very easy to find guide stars, this one has a magnitude of 8 and gives good signal strength. When it is greatly out of focus the star profile gives the form of the tube that connects the prism with a guide camera. It would really have needed to be wider, rather than 6x9mm, perhaps 9X13mm had been good, but then you have not a thin off-axis adapter which in many cases is required to fit in place. I also moved out prism as far as possible in order that it will not cause vignetting on the large full frame sensor. We can not avoid the current mechanical use of adapters and other, even the camera body puts Limitations. If you have too small image circle of your flat field corrector the image becomes weak and out of focus as well too weak for the guide camera.

09 screendump setup

I have a small note up on the laptop monitor when I working on this. Especially the USB com port number is usually changed and difficult to keep track of. Here are the data required. I also build up a table of how the focus changes with temperature that can be used the day I want to add temperature compensation to focus motor. When it gets colder it shrinking the tube length and the focus must be moved outwards in order to compensate for this. Though the few measurements I have done so far seems to be in the opposite direction, will probably be changed when temperature variation becomes larger. Here you can see how big the movement are to each step of the stepper motor. The 400 step I moved when I tested the above is approximately 0.75 mm, in reality focus unit is very sensitive and has high resolution.

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4: Focus and temperature compensation 1

Now, finally when the focus project has come so far that I can start looking at the temperature compensation. Yesterday, when it was a clear night, I photographed M13, got to focus pretty good, but also noticed when it became sharply how sensitive it is to temperature variations. Already one degree Celsius change focus and the focus is not good anymore. My pixel scale is very rough, 2.5" per pixel. Good for capture many photons quickly. But the resolution of the telescope are not used at good seeing nights, although Drizzle function can be used when stacking to increase the resolution. This is yet another strong argument for Sony mirror less cameras. Imagine a Sony A7R II, then you have both large field and high-resolution, well enough for this relatively small telescope. You lose FW, Full Well capacity with small pixels but if read noise is low, it is not as serious problem. Also sensitivity will be lower with small pixels.

Now there are coming software that can communicate with Sony cameras. But my experience from my Canon 6D camera is that liveview heats up the camera to extremely high temperatures, have to wait long time to cool it down. How about mirrorless cameras? Can the liveview function be shut down when connected to a computer? Else maybe the same problem as I got with my Canon 6D camera, lot of thermal noise.

Seeing puts the limit to the maximum resolution so already this telescope of 130 mm resolving power is difficult to use. I have never earlier managed to get better focus than 5" to 15" in FWHM from my balcony, now with stepper motor focus I got some stars inside one pixel, FWHM about 2 pixel or 4" or less. The image on the M13 is stacked with Drizzled function, or moving colors, which is a variant of Drizzle. So basically consists picture of 3 * 12 Mpix = 36 Mpix data before cropping. A DSLR image are usually interpolated in the deBayer process and thus get lower resolution than necessary.

m13 during making

Here are the astro server running and shooting M13. The control of focus handle by APT and the ASCOM driver that came with the USB-Focus that I bought. Unfortunately it is not directly temperature compensating in this setup. Maybe someone will update it in the future. The question is where the function shall be, in the ASCOM driver or directly in the ATP? The temperature is already available in APT so it's really not any need of an ASCOM driver update, it can be solved in APT, implemented there it can handle other motor focuser too. I will write to the developer Ivo of APT and ask (Update 2016, Ivo is close to have this implemented in ATP).

Now it is one thing that puzzles me. If the temperature falls, then it shrinks the metal tube of the telescope. Example. aluminum, 2.3 mm for each 100 degrees Kelvin per meter. As should be reasonably to set the focuser outwards when temperature falls to compensate so it have the same distance to the sensor and keep focus, in this case 683 mm. See the table below how surprising behavior it has:

01 temperature compensation

According to this I will do the opposite when it gets colder, then the tube is 0.12 mm shorter and I will focus inwards direction 1.2 mm, it is a factor of 10 over the length of the expansion (shrinkage) and with the wrong sign. Possibly I make an error of thought now when it's 4 am in the morning.

I got an email that maybe explain this strange behavior, it could be that the glass in the lens change it's refractive index with temperature changes, and that change the focus length of the telescope much more than what the aluminum tube change in length and in opposite direction. I was also provided a link about this: topic.asp?ARCHIVE=true& TOPIC_ID=122663

It's in German language so you have to translate it maybe, use Google translator or similar. Thanks a lot Bernd for this information!

With the USB-Focus it's delivered a program that can handle the focus and temperature compensation, was not able to get it started yesterday and I did not want to cancel the exposure series I started, have to take it at the next opportunity.

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5: Focus and temperature compensation 2

Tonight should be done trials with the automatic temperature compensation of focus.

What I use for this are:
USB Focus

USB Optical Telescope Controller

The ASCOM driver does not have this built-in nor the APT has, I use the supplied software to control the focus motor.

A further window that must fit on the desktop.

03 temerature compensation m3

Now it's crowded, have not seen that it is possible to have dual screens to the astro server, but can do graphics resolution greater than the screen and scroll around.

04 temperatrue compensation m3

This is the window that shows how the temperature compensation work. I would have felt better if it went to go through ASCOM and perhaps it will in the future. Or if the APT can handle it, APT can read the temperature over ASCOM so it's not far away.

05 temperature compensation m3

Here are the parameters set for temperature compensation. The value you have to figure out is how many steps per 1 degree Celsius required to maintain focus.

06 temperature compensation m3

How neat it appears on the window, their values on the quality of focus. APT measured in pixels FWHM and draws a graph of how well it keeps the focus. The roughness depends mostly on the atmosphere at my place. Pixel scale in my case is, Canon 5D 2.5"/ pixel, and for my Canon 6D 2" / pixel. Both cameras are one chip color sensors so see it like the twice. Peak value must be below 255 otherwise you get incorrect values. Select a weaker star or less exposure if you get too high values.

01 temperature compensation

With the guidance of former data at different temperatures, I have made this calculation. If it has backlash in the system, it is good if we only focus from one direction. Minor glitches can be compensated for in the software, if it has major backlash the mechanics should be rebuild, I have tooth belt connections of good quality.

07 temperature compensation focus

Here is the batch that I put up for the night. 60 exposures of 30 and 60 seconds at ISO800. The object is M3 at 38 degrees elevation. I also uses mirror lockup and a 3 second delay.

08 temperature compensation focus

Auto guider program PHD2 steering EQ6 mount synced with APT, it also provides a dithering command between each exposure so the stars should not be on the same pixels every image, suppresses static error and enables the use of Drizzle function.

09 temperature compensation focus

CdC which I use to select objects for astrophotography. An error seems to be that it does not really sync properly with the EQ6 mount, there is a drift compensation in EQMOD, but I has not really got it to be perfect (Update, Patrick at CdC explain to me it's because of bad polar align). CdC can take much computing power, I usually turn off the star update, among other things, during the shooting.

10 temperature compensation

EQMOD Info window, it starts if any other ASCOM programs connect to EQ6 mount, in my case the CdC. Although APT and PHD2 and focus motor program also connect through ASCOM.

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6: Collect temperature compensation data

Now I have collect more data points on how temperature affects the focus setting. The values I have entered into an excel sheet to obtain the value that the focus should be used when compensate for the temperature changes.

temperature compensation

New updated estimate:
The orange curve shows how many millimeters relative to a start position the focus needs to be moved to different temperatures. The blue correspond to how many steps of motor. A trend calculation gives a kind of average that shows how many steps that the stepper motor should move for each degree change in temperature. This calculation gives 55 steps / degree Celsius, or set in the unit of length 95.6 my / degree C. It is usually the value corresponding to 55 you specify in your stepper motor control applications.

95.6 / 55.0 = 1.738 gives the change in micrometers per step.

The three columns in the table at right shows telescope tube's relative length. Apparently increases the tube's length at increased temperature. But the strange thing is that I have to compensate and make the tube even further to maintain focus. If the tube is increased 0.138 mm in length, I should of course have to compensate with -0.138 mm, instead it will be 10 times the wrong direction, + 1.460 mm. The image is sharp then, probably some logical error or miscalculation even if I can not find it right now. Another option is that the temperature compensation cell that holds the triplet lens doing something crazy. It shall normally only hold the lens elements in the triplet relative each other independent of temperature variations (Update, I talked to the famous mirror maker Joel von Knorring, he told me that he noticed that the mirrors change focal length with temperature). Maybe I more temperature compensate for the triple lens than the telescope tube variation with temperature, lets see what I find out in future.

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