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Old 05-06-2018, 11:09 PM
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Astrofriend (Lars)
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Depth of focus and motor focus gear ratio

Hi,
Earlier I have made a calculator to find the telescopes depth of focus:
http://www.astrofriend.eu/astronomy/...-of-focus.html

Now I have continued with a gear ratio calculator for the motor focuser:
http://www.astrofriend.eu/astronomy/...box-ratio.html

Sometimes it could be tricky to find the correct gear ratio and maybe this could be of some help.

It's important to have the stepsize about 5 times smaller or even less then the depth of focus.

Note:
This is all new, I may have done some mistakes, check carefully before ordering any parts!

/Lars

Last edited by Astrofriend; 06-06-2018 at 01:10 AM.
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Old 05-06-2018, 11:51 PM
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OuterObsession (Jameson)
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Hi Astrofriend,

I find this all very interesting as I have modded the crayford focuser on my ed100 to use a nema17 stepper motor.

I got a comment from IanL over at Stargazers Lounge about calculating the CFZ (critical focus zone) for a telescope - a similar calcultation you have shown on your website.

"Yes but typically each filter would start from a known position where it is near focus. Over time that position would be invalidated. Whether you can use only full steps depends on:

With regard to whether to use full steps or not:

Critical Focus Zone in microns = focal ratio x focal ratio x 2.2

So my scope is 6.38, making a CFZ of 89.41 microns (i.e. the range of focuser travel within which the image is in good focus).

Distance moved per motor step in microns = Distance moved per focus shaft turn in mm x 1,000 / steps per motor turn x motor gear teeth / focus shaft teeth

So my motor has a step angle of 1.8 degrees, making 200 steps per turn, I have an 18 tooth pulley on the motor and a 36 tooth pulley on the focuser shaft and the distance moved for one turn of the focuser shaft is 1.43mm.

Thus I get a distance per step of 3.575 microns, and 89.41 / 3.575 equates to 25 steps in the CFZ. The distance moved per step needs to be a reasonable fraction of the scope's critical focus zone (say 10-20 steps), so mine is a bit too high but not unacceptable.

If there were too few steps, then one would try half, quarter or eighth microstepping to increase the number of steps by 2, 4 or 8 respectively. If too big then there are a number of strategies - change the motor, change the gearing or just increase the number of motor steps per reported step number in the Arduino firmware."

Some thoughts I had were with your calculations you don't really need to know the total length of the draw tube as you'd never practically need to use the full length of the draw tube to achieve perfect focus.

Also as was pointed out to me, when you use less then one step on a stepper motor the motor will 'instinctively' get pulled to the nearest step after power is removed causing the software to lose track of which step the motor is on.

IanL says to have around 10-20 steps in CFZ does this sound correct? Also I'm assuming if you're using a stepper motor with a gearbox you would deduct the backlash from the total amount of steps in the CFZ?

Cheers,
Jameson
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Old 06-06-2018, 12:59 AM
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Hi Jameson, thanks for your reply

If your telescope is a f/7 I get about +/- 30 microns or 60 micron overall for the visual wavelength. I will say att least 10 micron move per step or down to 5 micron. The blue wavelength is more critical. My setup is about 2 micron and it's also a f/7 telescope, APO TS130. That's the theory, practicaly there are a lot of limtiations, the seeing, not perfect optics and other things. Earlier I had problem with slip in the friction coupling in the focuser, I hope that problem is solved now.

In the calculations I setup it up to calucalte for the full draw tube length. When I change field flatterner it change the focuser position. 25 to 35 mm in one case and 45 to 65 mm in another case. Of my 100 mm draw tube lengt I can use about 85 mm with the choosen gear ratio.

You can read more about my focuser project here:
http://www.astrofriend.eu/astronomy/...tor-focus.html

The temperature drift is heavy when everything working perfect, the telescope is sharp and it's easy to see when it's out of focus. About one degree change and the temperature compensating kicks in.

The motor focuser and later temperature compensation is by far my most important changes of the setup.

There is a back lash compensation built in the driver for focuser, I think it work relative good. But of course better with no backlash. I use USB-focus driver and ATP to control the focuser. Setup with 1/2 micro step, no more! More and the stepper motor loss a lot of torque and the precission will be bad.

I have updated my homepage now, corrected some information and added more.

/Lars

Last edited by Astrofriend; 06-06-2018 at 07:26 AM.
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Old 06-06-2018, 12:03 PM
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I don't quite understand how to determine which wavelength I should be using for the calculations. Also is depth of focus the same thing as CFZ? When I use IanLs calc I get a CFZ of 178.2 microns, but when I use your calculation I get 45.9microns (theoretical) 117.9 (seeing) which would lead me to believe that the area for "good focus" is actually a lot smaller than I think.

This is how I set up the focuser on my ed100 currently. As you can see there's no gearing atm. Based on IanLs formulas I get 2.8512 steps in the 'perfect' focus area.

Because of that I'm replacing the nema17 with a 5:1 reduction gearbox, but unfortunately the gearbox adds about <=1 of backlash. So that would be around +/- 3 steps in the CFZ.

I do not use a flattener atm but with my current setup I haven't had issues with the focuser running close to the end of the drawtube. I usually just manually bring the focus in roughly then start the APT autofocus routine so I'm not waiting there forever.

How do you focus if your imaging target doesn't have any focus stars in frame? Goto commands with platesolve?? I haven't done a lot of long imaging sessions in APT yet so I'm still trying to work all this stuff out.
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Old 06-06-2018, 08:36 PM
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Hi Jameson,
What parametrar do you use in my calculation? I used 100 mm opening and f/7 but that gives other figures.

Be aware that my answers are +/- 45.9 microns, you have to double that to compare with some of the others. 91.8 microns in focus length. I'm not aware of CFZ, but when I did my equations I compared with other calculation and it lined up good. But there are for sure a lot of different way to do this calculation.

https://en.wikipedia.org/wiki/Depth_of_focus

If you do visual or have a green/yellow filter you use 550 nm, if doing rgb photo and with good quality optic maybe better to use the blue (shortest) wavelength, about 400 nm. Mirror telescope or APO refractor.

/Lars
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Old 10-03-2019, 03:57 AM
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After I solved the problem with my focuser which is of friction construction I got a new problem. When I increased the friction force to get rid of the slip of the focuser the load on the stepper motor got to high. I need a new stepper motor with higher torque. I have not solved that yet.

But under meantime I have updated my gear box calculator how to optimize the ratio to it:
http://www.astrofriend.eu/astronomy/...box-ratio.html

I did a massive rewritten of the code, hope I got everything correct now.

/Lars
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Old 10-03-2019, 10:42 PM
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Hi Jameson,
I see that I didn't answer all your questions.

When there is no good star in the center of the field to focus on I move the telescope around until I found a good star. I use 3 to 5 seconds exposure and stay at about 80% level (20% from the saturation point) when in perfect focus.

I haven't got the auto focus to work very good yet. I think I need to focus on more than one star to get better statistic. But that isn't easy and my software can not handle it.


I have today added the important torque calculation, at least for those who have heavy equipment. See the bottom of page:
http://www.astrofriend.eu/astronomy/...box-ratio.html

Do you get yor focuser to work as you want?

/Lars

Last edited by Astrofriend; 21-03-2019 at 09:32 AM.
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