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N00b question about focusers

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#1 adamrosner

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Posted 24 May 2022 - 08:53 PM

Greetings all. I've recently moved from using a modded DSLR on a star tracker to a full Narrowband rig and am still on the learning curve.

 

My optic is a William Optics refractor (with the 2-speed focuser) and a filter wheel and mono camera. I'm doing both Narrowband and LRGB. I use Astrophotography tool for running everything.

 

It's become clear to me my rig is missing a focuser and without one, it will not be able run unattended overnight and make filter changes.

 

So to my question. There's a few focusers out there (ZWO EAF, FocusCube, etc) that all seem to work pretty much the same way - ASCOM driver, electronic controller, stepper motor that drives the focus knob on the OTA.

 

My question is in regard to understanding how the focusing works. When I focus manually, after setting up the mount, I'll slew to a bright star, put on the Bahtinov mask, set the camera to Live View, and then adjust focus until the pattern is correct.

 

How does automated focusing work? I assume it does it without the operator needing to apply the bahtinov mask each time else it'd kinda be pointless.

 

Does it focus while on-target? Or do you have to designate a bright star for it to refocus on? How long does a focus run take (I kinda assume it has to image - adjust focus - re-image, rinse and repeat until focus is nailed)

 

I'm just totally in the dark as to how this actually works and I want to understand it before I spend money in case I ignorantly buy something that turns out to be unsuitable because of something I didn't know while I was shopping...

 

For reference - I'm looking at the "Pegasus Astro DMFC Premium v2" on Cyclops (don't know if I'm allowed to post vendor links on here so I'd rather not risk being booted, but it's easy enough to find).

 

Can anyone help me destroy another little bit of ignorance??

 

Thanks :)

 

AdamR

41°S 174°E


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#2 Oyaji

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Posted 24 May 2022 - 09:12 PM

Here's a video that explains autofocus in NINA, but the process is basically the same for other AP programs.  https://www.youtube....h?v=XsjT-We9Wn0

 

Long and short, you take a picture of a starfield.  The computer analyzes the fatness of the stars.  The computer moves the focus, analyzes the stars--and repeats the process, figuring out mathematically where the optimal focus point is.  Autofocus does NOT use a bahtinov mask; nor does it depend on any particular bright star; it analyzes all the stars (or maybe the brightest 100 or 500) in a starfield.

 

Once you have tried autofocus, you will wonder how you ever got along without it!  


Edited by Oyaji, 25 May 2022 - 11:05 AM.

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#3 bobzeq25

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Posted 24 May 2022 - 10:05 PM

Greetings all. I've recently moved from using a modded DSLR on a star tracker to a full Narrowband rig and am still on the learning curve.

 

My optic is a William Optics refractor (with the 2-speed focuser) and a filter wheel and mono camera. I'm doing both Narrowband and LRGB. I use Astrophotography tool for running everything.

 

It's become clear to me my rig is missing a focuser and without one, it will not be able run unattended overnight and make filter changes.

 

So to my question. There's a few focusers out there (ZWO EAF, FocusCube, etc) that all seem to work pretty much the same way - ASCOM driver, electronic controller, stepper motor that drives the focus knob on the OTA.

 

My question is in regard to understanding how the focusing works. When I focus manually, after setting up the mount, I'll slew to a bright star, put on the Bahtinov mask, set the camera to Live View, and then adjust focus until the pattern is correct.

 

How does automated focusing work? I assume it does it without the operator needing to apply the bahtinov mask each time else it'd kinda be pointless.

 

Does it focus while on-target? Or do you have to designate a bright star for it to refocus on? How long does a focus run take (I kinda assume it has to image - adjust focus - re-image, rinse and repeat until focus is nailed)

 

I'm just totally in the dark as to how this actually works and I want to understand it before I spend money in case I ignorantly buy something that turns out to be unsuitable because of something I didn't know while I was shopping...

 

For reference - I'm looking at the "Pegasus Astro DMFC Premium v2" on Cyclops (don't know if I'm allowed to post vendor links on here so I'd rather not risk being booted, but it's easy enough to find).

 

Can anyone help me destroy another little bit of ignorance??

 

Thanks smile.gif

 

AdamR

41°S 174°E

Programs move the focuser up and back measuring star size.  The data forms a U shaped graph, x axis is focuser position, y axis is star size.  Focus is the bottom of the U.

 

Example below.  In this program you see what's happening.  There's a picture of the star, and its profile.  Here the profile is jagged and the star is large, because it's when it's just completed the curve, on the far right.  It will then move the focuser back to the bottom of the curve.  Finished.

 

How a star or stars are selected varies with the software.  It's common to choose a medium brightness star in the field.  Or look at many stars across the field simultaneously.

 

Voyager has a somewhat unique method I like a lot.  It has a database of "good stars for autofocus".  You can tell it to use that.  It slews from the target to the nearest one, using platesolving to put it in the center of the field.  Autofocuses.  Then slews back to exactly where it was on the target, again using platesolving.  So it focuses on the center of the field with an optimal focus star.

 

You can also tell the software when to autofocus.  Like every 30 minutes.  Or when you change filters.  Or, with a temperature sensor, when the temperature changes a certain amount.

 

autofocus curve.png


Edited by bobzeq25, 24 May 2022 - 10:11 PM.

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