How do I use the focussing assistant?

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StuartT
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How do I use the focussing assistant?

#1

Post by StuartT »

I've read the documentation, but I must admit, I find the focus assistant somewhat baffling. I can't seem to find any youtube videos on this topic either.

I am pointed at a chimney pot and have achieved rough focus with the scope's manual focuser. My Pegasus focuser is connected and working normally (using the manual focus controls in SharpCap).

I have selected edge detection, which I assume is the most appropriate here. But I am not sure what to do next. I tried the scan out, but all the green triangles seem to be all at the same value of the y-axis whether the image is in or out of focus. I would have expected a curve of some kind (assuming the y-axis is measuring contrast)?

Can anyone point me to a video explaining all this? I find the documentation not very helpful
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StuartT
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Re: How do I use the focussing assistant?

#2

Post by StuartT »

then I tried "scan in" and it just seems to overlay red triangles in the same places as the green ones (i.e. in a horizontal line) even though focus is clearly changing. :?: :?:
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StuartT
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Re: How do I use the focussing assistant?

#3

Post by StuartT »

Now it seems to be showing gradually improving focus (the red line) while moving clearly away from focus visually!

It doesn't seem to have any idea where best focus is, and even though I have checked the box "move to best focus position when scan complete" it doesn't do anything when the scan is complete. Surely it should at least move to where it thought the best focus was.

I don't get this at all, I'm afraid
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DirtyRod
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Re: How do I use the focussing assistant?

#4

Post by DirtyRod »

Not sure if this is the way it's supposed to work but turning on auto-stretch before you run the sequence works for me. I autostretch and set it for 3 second exposures with a gain of 300 before running the AF routine. Im using a 533 so other settings may work for your camera.

If you are using the sequencer, there is a commend that will have it save the current camera settings and then reload them after AF is done. You then just need to reset the display.
StuartT
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Re: How do I use the focussing assistant?

#5

Post by StuartT »

Thanks, I tried hitting the lightning button to do an autostretch, but it didn't seem to help.

I also found there is an option called "Automatic Refocus" which (presumably) just does it all itself. But that doesn't work very well either. I focused the target perfectly by using the focus knob on the scope, then tried running Automatic refocus. It should have gone away from focus then returned to the same position, but it just defocused and gave up!
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Last edited by StuartT on Sun May 15, 2022 3:44 pm, edited 1 time in total.
DirtyRod
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Re: How do I use the focussing assistant?

#6

Post by DirtyRod »

The automatic refocus just reruns the last AF routine using the parameters that were used last time.

Took me few tries but what I stated works for both my scopes. I have to be close to focus to start, set it to 3 seconds/300 gain and let it capture a few subs, autostretch, and then run the outline using -200 to 200 with a step size of 20 on my SCT. It can work with shorter exposures but it’s not always consistent for me.

Once it runs successfully you can then just do the automatic autofocus routine.
StuartT
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Re: How do I use the focussing assistant?

#7

Post by StuartT »

Well.. I can't seem to get it to work whatever I do.

Maybe it's better on stars. Thing is, I don't need it on stars as I always use NINA for that (and the autofocus there is brilliant - you literally click one button and it runs a routine, finds the point of best focus and just goes there! It's not totally manual like SharpCap seems to be)
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Re: How do I use the focussing assistant?

#8

Post by admin »

Hi,

since you are running in contrast detection mode, SharpCap is looking for a distinct peak in the graph (it must have lower values each side of the peak, and in addition there are some statistical tests to make sure that the peak is significant rather than just a random spike in the data). Your graph in post #5 looks close to working - if you had scanned out a bit further it might have drawn a curve onto the data points which signals that you have a solution and can goto best focus. In post #3 you were moving towards better focus, but didn't get far enough to see the peak.

See this part of the user manual for more info : https://docs.sharpcap.co.uk/4.0/#(Nearl ... 20Focusing

cheers,

Robin

PS. for stars, you are looking for the lowest value of FWHM, so you will be looking for a dip in the data, not a peak
StuartT
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Re: How do I use the focussing assistant?

#9

Post by StuartT »

admin wrote: Mon May 16, 2022 3:50 pm Hi,

since you are running in contrast detection mode, SharpCap is looking for a distinct peak in the graph (it must have lower values each side of the peak, and in addition there are some statistical tests to make sure that the peak is significant rather than just a random spike in the data). Your graph in post #5 looks close to working - if you had scanned out a bit further it might have drawn a curve onto the data points which signals that you have a solution and can goto best focus. In post #3 you were moving towards better focus, but didn't get far enough to see the peak.

See this part of the user manual for more info : https://docs.sharpcap.co.uk/4.0/#(Nearl ... 20Focusing

cheers,

Robin

PS. for stars, you are looking for the lowest value of FWHM, so you will be looking for a dip in the data, not a peak
Robin

My point is, the focussing tool doesn't seem to work.

I started out with perfect focus (visually, using the knob on the telescope) but the SharpCap focussing assistant just moved me away from focus and just left it there.

In NINA, the autofocus first defocuses a little, then it scans across to find a minimum (or maximum, depending on the mode being used) then moves the focus to the best point it found. SharpCap doesn't seem to do that. Unless I'm doing it wrong. Can someone please point me to a video showing how this is supposed to work? Someone must have made one, but I can't find it
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Re: How do I use the focussing assistant?

#10

Post by admin »

Hi,

the initial use of SharpCap is designed to be started from one side of focus or the other - then tell it to scan towards best focus. It will go past, get the graph to show a peak/dip and then enable you to do best focus.

Once you have done that once, SharpCap remembers the settings used and the distance the scan needed to show a useful peak. Once that is stored, you can use the 'refocus' button which behaves in the way you describe for NINA. The difficulty with doing that first time is that you have no idea how far to move to get a meaningful defocus.

cheers,

Robin
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