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Blotches on 294MC image

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#1 rishigarrod

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Posted 22 September 2019 - 04:12 AM

Hi All

 

Just done my first night of imaging with my ZWO 294MC camera.

I messed up my flats so I have simply stacked my lights without any calibration frames.

 

Andromeda_294_stretched.jpg

 

60 light frames at Unity gain, Offset =8, exposure 120 seconds, temp -10.

Standard PixInsight stretch.

 

I am wondering if the blotches are due to finger prints on one of the lenses?

I hate cleaning lenses if not necessary.

 

Any suggestions what these "blotches" are?

 

Thanks

 

Rishi

 

 


Edited by rishigarrod, 22 September 2019 - 04:18 AM.


#2 einarin

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Posted 22 September 2019 - 04:22 AM

Nope, they are there because you messed up with flats smile.gif

Most likely dust motes in camera cover glass.


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#3 nimitz69

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Posted 22 September 2019 - 07:39 AM

yep dust motes ...

 

a good learning experience for why you always need to take calibration frames ....


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#4 jdupton

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Posted 22 September 2019 - 08:26 AM

Rishi,

 

   As users einarin and nimitz69 have already indicated, the dark splotches are shadows due to dust in the optical train.

 

   They do not seem to be on the sensor. My best guess due to their size is that they are more likely to be on the camera window.

 

   I calculate that they are 5.02 mm from the sensor. That number comes from:

  • I loaded your image into PixInsight and scaled it back up to full ASI294MC-Pro size
  • Plate Solving the image gives a scale of 2.585 "/px and 369.5 mm focal length
  • I think you are using a 100 mm telescope operating at F/3.7
  • The dark rings on the image span an average of 293 pixels each
  • Plugging the numbers into a dust mote calculator shows the distance of 5.019 mm from the sensor to their location.

   I have to guess the diameter scope you are using in order to get a focal ratio but did notice from your posts that you have a SW Esprit 100. I also took a guess that the image had not been significantly cropped before being resized for posting here. If either of those guesses are wrong, my numbers will be suspect. I also had to assume that you are using some sort of focal reducer / corrector on your scope.

 

   If my guesses were close, the distance of the dust from the sensor's surface is consistent with it residing on the camera's window. It is most likely on the outside, so a gentle cleaning should take care of it.

 

   As others has also said, a good set of Flat Frames will remove the artifact in the future.

 

 

John


Edited by jdupton, 22 September 2019 - 08:27 AM.

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#5 neaptide

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Posted 22 September 2019 - 08:59 AM

A good set of flats will get rid of the dust mote blotches. I’ve had good results using flats that are created with at least 2 second exposures with the 294. 

 

The dust on the sensor can be very small and hard to see. I use a 5 x jewelers loop and a bulb blower to remove the dust without actually having to come over n contact with the sensor


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#6 rishigarrod

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Posted 22 September 2019 - 09:30 AM

Rishi,

 

   As users einarin and nimitz69 have already indicated, the dark splotches are shadows due to dust in the optical train.

 

   They do not seem to be on the sensor. My best guess due to their size is that they are more likely to be on the camera window.

 

   I calculate that they are 5.02 mm from the sensor. That number comes from:

  • I loaded your image into PixInsight and scaled it back up to full ASI294MC-Pro size
  • Plate Solving the image gives a scale of 2.585 "/px and 369.5 mm focal length
  • I think you are using a 100 mm telescope operating at F/3.7
  • The dark rings on the image span an average of 293 pixels each
  • Plugging the numbers into a dust mote calculator shows the distance of 5.019 mm from the sensor to their location.

   I have to guess the diameter scope you are using in order to get a focal ratio but did notice from your posts that you have a SW Esprit 100. I also took a guess that the image had not been significantly cropped before being resized for posting here. If either of those guesses are wrong, my numbers will be suspect. I also had to assume that you are using some sort of focal reducer / corrector on your scope.

 

   If my guesses were close, the distance of the dust from the sensor's surface is consistent with it residing on the camera's window. It is most likely on the outside, so a gentle cleaning should take care of it.

 

   As others has also said, a good set of Flat Frames will remove the artifact in the future.

 

 

John

Thanks for all the info. I have an issue getting the correct adaptor for my flattener on the Esprit 100 so this image was using my ZS61. It has a flattenar (no reduction). The image has not been cropped.

 

This was first light for me with the 294. I was used to letting the DSLR work out the correct exposure for flats. I have understood that with this camera the flats should be taken with at least a 3 second exposure. I did turn my flat field generator to a minimum setting but looks like I will need to cut down the light further as my flats were horribly over exposed (dumb that I did not check).




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