Variable Stars with SharpCap 3.1

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oopfan
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Variable Stars with SharpCap 3.1

Post by oopfan » Fri Feb 09, 2018 10:52 pm

Hi Robin,

I've gotten to the stage where I am ready to test out my new knowledge. For what I need to do I have my old ways but for this application that would be labor intensive. I'd like to describe what I need to have done and perhaps SC 3.1 can expedite. Keep in mind I have never tried Live Stack so please excuse my ignorance:

I need to determine exposure. Gain and black level are predetermined to be 100 and 0, respectively, that are not subject to change. In computer terms I need to execute a 'binary search' algorithm. There will be a comparison star of known magnitude that must read 2340 ADU (out of 4096). That value of 2340 must be the maximum pixel value of the Airy disk in a stack of 10 frames.

This does not have to be super-precise. I know that there will still be noise in a 10 frame stack but if I can see that it looks like it will converge on or about 2340 I would be happy.

What do you think?

Thanks,
Brian

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Re: Variable Stars with SharpCap 3.1

Post by admin » Sat Feb 10, 2018 10:42 pm

Hi Brian,

well, I think you can make some progress - for instance

* Select a small ROI and make sure your reference star is the brightest star in it
* Live Stack for 10 frames
* Pause live stacking
* Read off the max pixel value from the screen (live stacking panel). You will be aiming for 374400 (2340 * 16 * 10 - the 16 is because values are scaled up by a factor of 16 from a range 0..4096 to the range 0..65536 and times 10 for 10 frames)

Now, no need to do a binary search, just make two measurements one too low and one too high - ie

Measurement 1: e1=4s exposure, max pixel value v1= 352000
Measurement 2: e2=5s exposure, max pixel value v2= 406400

Now you can interpolate between the two -

new exposure = e1 + (e2-e1) * (vtarget-v1) / (v2 - v1)

If everything is linear then that should get you very close to the right exposure with just the 3rd try. If it's not close enough then pick the two closest exposures each side of the target as your new measurement 1 and measurement 2 and repeat until close enough.

You can then set the correct exposure and if necessary expand/move the ROI selected to bring your target star into view. As long as it is the brightest thing in view then you can read off the max pixel value in a live stack in the same way.

cheers,

Robin

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Re: Variable Stars with SharpCap 3.1

Post by oopfan » Sun Feb 11, 2018 4:02 pm

Thanks, Robin. That's an excellent idea that takes advantage of the linearity of the sensor.

I've attached a snapshot from C2A:
  • The large red rectangle is my imaging FOV at 1920x1080 rotated -30 degrees. This orientation maximizes the capture of the most number of comparison stars.
  • The small yellow rectangle is the ROI that I will define in SharpCap at 480x270 in order to isolate the comparison star labeled '95'.
As you described I see the 'Max Pixel Value' in Live Stack's Status panel. And also as you described I can calculate the 12-bit pixel value by dividing 'Max Pixel Value' by (16 times the 'Frame Count'). That should do the trick.

This ROI technique works for this variable star but there will be others where it is just not possible. FX Selection Box would be ideal if it is something you could work into your long list of to-do items but certainly not critical.

Thanks,
Brian
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C2A Calibrate.jpg
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