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Focuser movement measurement

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#1 jaimac

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Posted 12 August 2020 - 10:55 PM

I have a WO refractor which has measurement of the distance traveled by the focuser. This is accurate only to a millimeter. Do electronic focuser have finer focuser measurement? If they do what is the minimum distance the focuser moves? I am looking for very accurate focuser with extra fine adjustments so that I know how much the focuser moves for each object focused. Some WO scopes comes with DDG focuser but I am looking for something with even more fine adjustment.



#2 SonnyE

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Posted 12 August 2020 - 11:26 PM

I felt that most of the electronic focusers were out of my reach.

So when I ran across Tekky Dave's Arduino based focuser, I could build my own, and did.

In the end, I was able to even use a 12 volt stepper motor, and set mine up to drive the fine adjust of my telescope. (A big No-No by some. Works fine for me.)

Ratio wise, it works out to: One Step = 0.00125" of travel of my focuser tube.

Pretty fine for my needs.

 

But for those less inclined, there are good ones from Feathertouch, and MoonLite.

Though, I have no idea how fine of a focusing adjustment they are capable of.

You should be able to get your questions answered with a phone call.

 

Until I got EF capabilities, focusing was the bain of my life.


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#3 jaimac

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Posted 13 August 2020 - 12:25 AM

Looks cool. Wow, you can just dial in the distance. 



#4 Jon Isaacs

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Posted 14 August 2020 - 02:08 AM

I am looking for very accurate focuser with extra fine adjustments so that I know how much the focuser moves for each object focused.

 

 

Astronomical objects are all at infinity as far as focusing goes. Terrestrial objects are closer .

 

Jon

 

Jon


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#5 t-ara-fan

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Posted 14 August 2020 - 10:50 AM

Electronic focusers have VERY small steps. With the bonus they don't shake the scope when you are tweaking focus.

 

My two MoonLite focusers moved 4µm per step.  That is 250 steps per millimeter / 6,350 steps per inch in case you are an inches guy. 

 

The FocusLynx on my refractor moves the focuser 1.69µm per step.  

 

The steps are so small that 10 steps isn't a noticeable change of focus. Which is a good thing.

 

You can get an add-on motor for most dual speed focusers so I am sure there is one for your WO.


Edited by t-ara-fan, 14 August 2020 - 10:52 AM.

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#6 rgsalinger

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Posted 14 August 2020 - 11:30 AM

Unless the manufacturer has control over both focusing elements - the focuser itself and the motor drive, I don't see that they could specify how far a step is. I guess that they could measure it themselves for various scopes.

 

So, After you install any motorized focuser motor while still using the existing rack/pinion or crayford, rack the system all the way in. Now you are at "zero". Move the focuser using the GUI all the way out. Note the number of steps. Now use a caliper to measure the distance that the focuser has moved in millimeters or inches. Then do the arithmetic and then you know very closely what a step actually is. Do it a couple of times just to see if you're measuring accurately. 

 

The alternative is to buy a focuser that does not use the existing focus mechanics - you replace it entirely - and that has absolute measuring capabilities. Optec make the TCS range that do exactly that. I have a Hedrick focuser that moves 1 step for one micron. There are others. 

 

Rgrds-Ross


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#7 SonnyE

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Posted 14 August 2020 - 05:29 PM

True that it could be hard to guesstimate your final travel. Different scopes for different folks.

I came to mine by starting at stop (0) and running it fully to it's max travel.

It was something over 25,000 steps of my stepper motor. (It wasn't important enough to me to note it. All I wanted was a mm to inch conversion. Just a curiosity on my part)

And the only sure way would be to have a gear driven focuser, not my type. Mine is a friction drive. So I don't trust it to be true to step count. And I use it basically as a manual fine focus control I can run from my computers.

I would not trust it for repeat accuracy, such as temperature auto compensations.

 

Where that could be important is with your Automation programs that can Autofocus including backlash adjustments. Sequence Generator Pro is one I know of.

More about that HERE if you would like to blow your mind. I decided I'm more hands on, and too cheap to spring for SGP. So I've been fooling around with N.I.N.A. halfheartedly.

I have to stay up to drag my stuff in anyway.

 

A friend of mine bought an ED80T CF telescope like mine, and he immediately installed a MoonLite focuser on his. (Drool....)

But it completely replaced his OEM focuser. (I wish...)

Long story, he gave me the focuser he removed because he couldn't see needing it again. And I thought I was having issues with mine.

(I had given him my tripod and counter balance weight from my AVX.)

So I have a spare manual focuser.

 

But an Electronic Focuser is the cats meow for fine focusing, computer controlled focusing, and certainly for remote imaging like I like to do.

 

Have you inquired with WO or MoonLite or Feathertouch about your aims and goals?


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#8 jaimac

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Posted 14 August 2020 - 10:37 PM

Wow, never realized so fine focusing was possible. Although focus for astronomy should be infinity, I  think that once Jupiter is focussed in a telescope, that same focus cannot be used for a star as the star is further away. Also, there should be focus change for focusing a nearer star compared to a star further away. I have also noticed that while focussing a star, if I focus in beyond the correct focus, more hidden stars are visible around that star. It is almost like in Starwars when people go at light speed. With such fine focusers, can you please correct my thinking.Temperature and other stuff can change focus but even for astronomical objects the focus needs to change with distance.



#9 t-ara-fan

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Posted 15 August 2020 - 12:04 AM

  Although focus for astronomy should be infinity, I  think that once Jupiter is focussed in a telescope, that same focus cannot be used for a star as the star is further away. Also, there should be focus change for focusing a nearer star compared to a star further away. I have also noticed that while focussing a star, if I focus in beyond the correct focus, more hidden stars are visible around that star. It is almost like in Starwars when people go at light speed. With such fine focusers, can you please correct my thinking.Temperature and other stuff can change focus but even for astronomical objects the focus needs to change with distance.

The same focus point works for the Moon, Planets, Sun, and stars.  There is no difference. For planets and the moon, I sometimes focus on a star, then slew to the planet.

 

When you focus and "hidden stars" appear, THAT means you have hit the correct focus. You were not focused perfectly before the little stars popped up.

 

Yes temperature changes can require refocusing. Changing targets does not.
 


Edited by t-ara-fan, 15 August 2020 - 12:04 AM.


#10 rgsalinger

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Posted 15 August 2020 - 12:54 AM

Of course there's always the exception when using an SCT due to mirror flop but that's about the only time you need to refocus a scope if the temperature is constant. Sadly, for fast optics a 1C change in temperature can mean that a refocus is helpful if not absolutely necessary.

Rgrds-Ross


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#11 Jon Isaacs

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Posted 15 August 2020 - 06:06 AM

Wow, never realized so fine focusing was possible. Although focus for astronomy should be infinity, I  think that once Jupiter is focussed in a telescope, that same focus cannot be used for a star as the star is further away. Also, there should be focus change for focusing a nearer star compared to a star further away. I have also noticed that while focussing a star, if I focus in beyond the correct focus, more hidden stars are visible around that star. It is almost like in Starwars when people go at light speed. With such fine focusers, can you please correct my thinking.Temperature and other stuff can change focus but even for astronomical objects the focus needs to change with distance.

 

All astronomical objects are at infinity.  You can use the equation 

 

1/f = 1/f1 + 1/f2

 

and solve for f1-f to see the shift in focus. f is the focal length of the scope, f1 is the length at focus and f2 is the distance to the object. 

 

Solving for f:

 

f1= f/(1-f/f2)

 

 

For the moon, f2 is 250,000 miles. That's 402,336,000,000 millimeters. Then for your 72 mm F/6 refractor 

 

f1 = 432/(1 - 432/402,336,000,000) = 432.000000464 mm.  Subtract 432 mm from that to find the shift in focus.

 

The change in focus between the moon and infinity is 0.000000464mm or 0.000464 nanometers or less that 1/1000 the wave length of green light. This distance is about the diameter of an oxygen molecule. Tiny, Tiny, tiny

 

The depth of focus at F/6 is about 0.076 mm or 0.003 inches. That's 76,000 nanometers.. 

 

My point is that even the moon is at astronomical infinity. With a focal length off 432 mm, an object needs to be closer than maybe 10 kilometers to see a difference from infinity.

 

This is a fair amount of math but step back and think about it, if there were a measurable shift in focus between the moon and the stars, between two stars, you could measure the distances that way. 

 

As it is, they use the diameter of the Earth's orbit as the baseline to measure the distances to the nearest stars.

 

Jon


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#12 jaimac

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Posted 15 August 2020 - 10:39 PM

All astronomical objects are at infinity.  You can use the equation 

 

1/f = 1/f1 + 1/f2

 

and solve for f1-f to see the shift in focus. f is the focal length of the scope, f1 is the length at focus and f2 is the distance to the object. 

 

Solving for f:

 

f1= f/(1-f/f2)

 

 

For the moon, f2 is 250,000 miles. That's 402,336,000,000 millimeters. Then for your 72 mm F/6 refractor 

 

f1 = 432/(1 - 432/402,336,000,000) = 432.000000464 mm.  Subtract 432 mm from that to find the shift in focus.

 

The change in focus between the moon and infinity is 0.000000464mm or 0.000464 nanometers or less that 1/1000 the wave length of green light. This distance is about the diameter of an oxygen molecule. Tiny, Tiny, tiny

 

The depth of focus at F/6 is about 0.076 mm or 0.003 inches. That's 76,000 nanometers.. 

 

My point is that even the moon is at astronomical infinity. With a focal length off 432 mm, an object needs to be closer than maybe 10 kilometers to see a difference from infinity.

 

This is a fair amount of math but step back and think about it, if there were a measurable shift in focus between the moon and the stars, between two stars, you could measure the distances that way. 

 

As it is, they use the diameter of the Earth's orbit as the baseline to measure the distances to the nearest stars.

 

Jon

Thank you Jon. That cleared my question. 




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