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Stargazing Is it nebulosity or an artefact?

  1. Dec 16, 2017 #21

    davenn

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    that is a really nice image :smile:

    The ONLY other suggestion I would make for you to do in processing for that image, is to alter the temperature to lower the level of that "blueish" tinting that it has
    I usually use Lightroom for most edits like that and only use Photoshop for curves/levels stretching or blending.

    I'm assuming it is a single exp. or is it stacked ?
    Exp time ?, camera ISO ?, Focal Length ?


    Dave
     
  2. Dec 17, 2017 #22

    sophiecentaur

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    Thanks. Mostly down to the sky behaving itself for once, I think. Yes, I know about the blue sky. I did a fair bit of correcting but not enough! The colour of the nebula seems to agree with most of the images on line. I was pleased with the initial impression - focus and lack of trails
    The story is that I had decided to take the advice of a local expert at the Astro Society and I jacked up the gain to about 3200 from the 800 I have been using, just to see the effect. It was more of a sighting shot than anything. The frames (only four of them) were 2min and f=500mm. The histogram showed a broad peak about half way up and the frames were very pale and washed out, in terms of an equivalent daytime picture. I lowered the levels and did some 'curves' on one frame and, mainly because the stars were circular (for once!!!) I proceeded with 'inadequate data' and stacked the four frames. Comparing the stack with a single frame, I can just see a difference but of course I would need at least ten to get significant noise improvement. Polar alignment is always hard in my garden because polaris seems always to be a bit misty and the polariscope is hard to see through. Perhaps it's a dark adaption thing and the angle at 52N kills my neck.
    After my four exposures, I did four with my OIII filter (four minutes this time) and the levels looked better so my green image was hardly processed at all.

    But this artefact thing is still an issue for me. The first sidelobe maximum of the Airy pattern is at around 2% of the maximum, which is significant. It compares with a magnitude difference of between 4 and 5 (I always wonder if there should be a square factor in that % but brightness is Power and not Amplitude - isn't it?) The radius of that halo seems about right to be due to the first sidelobe. What do you think? Could it be a quantisation problem which is accentuating the step around the edge of the halo?
     
  3. Dec 17, 2017 #23

    Drakkith

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    Could it be internal/ghost reflections between the filter and lens elements (or between the lens elements themselves even without the filter)?
     
  4. Dec 17, 2017 #24

    sophiecentaur

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    I've seen the effect with only the objective and sensor. :frown:
    Could it just be an effect of charge distribution on the sensor, which is driven into limiting over a significant area for the brightest stars? It's the sort of thing that a daytime photographer would arrange never to come across but, on the other hand, it would not surprise anyone to find artefacts in the vicinity of a burned out highlight.
     
  5. Dec 17, 2017 #25

    Drakkith

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    I don't think so, but I'm not sure. I'm still betting on some sort of internal reflection instead of a sensor artifact.
     
  6. Dec 18, 2017 #26

    Tom.G

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    Perhaps you could use an "Artificial Star", typically used for telescope collimation. It could also be adapted to get an idea of the camera sensor overload characteristics (blooming) by giving a spot source without the telescope. With a stable source and no atmospherics to get in the way, you could likely get a beam profile across the image using PS.

    Using a Google search for - telescope star simulator - turned up this site with some explanations and links: http://www.hubbleoptics.com/artificial-stars.html
     
  7. Dec 18, 2017 #27

    sophiecentaur

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    Hmm. Any reflection would involve the shape of the back end of the objective which is not flat and, moreover, it is expensively coated.
     
  8. Dec 18, 2017 #28

    sophiecentaur

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    It would need to be a pretty high powered little star, though as this effect only happens on a few stars. The other stars on my above images don't show it. The ones in the trapezium are such a jumble the the halo effect wouldn't be spotted, I think. I will soon be able to try and see if I get the effect on a Zwo bottom of the range guide camera I have ordered. Cmos may be different again.
     
  9. Dec 18, 2017 #29

    Tom.G

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    Think LASER pointer at whatever distance is appropriate, perhaps shining thru a pinhole in some aluminium foil.

    Cmos may be different again. Very noisy, low sensitivity.
     
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