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What's causing those horizontal lines?

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#1 mannys

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Posted 14 October 2021 - 01:50 AM

Hi fellas, what's causing those horizontal lines in this image I took? The lines are obvious in the lighter areas.

 

 

 

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  • Sh-2 190 v3.jpg

Edited by mannys, 14 October 2021 - 02:12 AM.


#2 james7ca

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Posted 14 October 2021 - 02:30 AM

Difficult to tell given the really small size of your enclosed image. Note that CN will resize and/or recompress your image if it is larger than 1600 x 1200 pixels or larger than 500KB. So, if that is what happened you need to resize and/or recompress your original beforehand so that it will fall under those limits.



#3 mannys

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Posted 14 October 2021 - 01:58 PM

Difficult to tell given the really small size of your enclosed image. Note that CN will resize and/or recompress your image if it is larger than 1600 x 1200 pixels or larger than 500KB. So, if that is what happened you need to resize and/or recompress your original beforehand so that it will fall under those limits.

 

Here's a cropped version of the image. What's causing those horizontal lines?

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  • SH-2 190 v3.jpg


#4 jonnybravo0311

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Posted 14 October 2021 - 02:14 PM

What light source are you using for your flats?



#5 mannys

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Posted 14 October 2021 - 02:16 PM

The light source I used to obtain flats was my Mac iPad, and covered the dew shield with a white shirt.


Edited by mannys, 14 October 2021 - 02:18 PM.


#6 jonnybravo0311

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Posted 14 October 2021 - 02:18 PM

OK... reason I asked was because some panels will exhibit similar patterns if you're shooting too short a flat. Basically, you're catching the panel's refresh rate.



#7 mdavister

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Posted 14 October 2021 - 02:23 PM

Next follow on question, what exposure length are you using for the flats? I assume you are also doing bias frames for calibration.



#8 mannys

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Posted 14 October 2021 - 02:29 PM

Next follow on question, what exposure length are you using for the flats? I assume you are also doing bias frames for calibration.

Hmmm. I used APP and if I remember correctly, I chose "automatic" for exposure length, letting APP determine the best length. I don't recall actually setting the length of time.



#9 charles.tremblay.darveau

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Posted 14 October 2021 - 02:31 PM

Can you upload your master flat and see if the artefacts are there?



#10 mannys

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Posted 14 October 2021 - 02:33 PM

OK... reason I asked was because some panels will exhibit similar patterns if you're shooting too short a flat. Basically, you're catching the panel's refresh rate.

Oh, so instead of choosing "automatic", I should choose a specific length of time then? I also understand that the histogram should show the light curve to be somewhere in the middle. While shooting the flats with a manually chosen exposure length, where do I go in APP to see the histogram? In using "automatic", I assumed that APP will use the correct exposure length to get the light curve to be in the middle of the histogram.



#11 mdavister

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Posted 14 October 2021 - 02:35 PM

I would guess that since you are using a tablet for a flat source, it would likely be short exposures. When you get into the under 20ms exposures, you start getting LED flicker and sensor read. I have noticed with the modern CMOS sensors, there is banding in the image (it can be calibrated out) under certain read conditions. 



#12 mannys

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Posted 14 October 2021 - 02:46 PM

Can you upload your master flat and see if the artefacts are there?

Sorry, not familiar yet with APP. I'm looking at the files in my working directory, but can't tell which one is the master flat. I don't see any files that has "flat" on it. What part of the file name should I be looking for in order to identify which one is the master flat?



#13 mannys

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Posted 14 October 2021 - 02:49 PM

I would guess that since you are using a tablet for a flat source, it would likely be short exposures. When you get into the under 20ms exposures, you start getting LED flicker and sensor read. I have noticed with the modern CMOS sensors, there is banding in the image (it can be calibrated out) under certain read conditions. 

Ok, I'll cook up another set of flats with exposure length of greater than 20ms. How do I view the flat histogram after I take test shots to make sure the light curve is within the center of the histogram?



#14 charles.tremblay.darveau

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Posted 14 October 2021 - 05:15 PM

Sorry, not familiar yet with APP. I'm looking at the files in my working directory, but can't tell which one is the master flat. I don't see any files that has "flat" on it. What part of the file name should I be looking for in order to identify which one is the master flat?

The master flat is the integration of all individual flats done in post-processing. If you only shot 1 flat then that would be you master flat and shooting multiple frames may help smoothing the artefact. Some screens have refresh latency that show as jailbars.

 

I have been using a cheap LED tablet from Amazon:
https://www.amazon.c...e?ie=UTF8&psc=1

 

Then you can tape an acrylic slab on top which acts as light diffuser. No issue with jailbars since.


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