M42 HDR

Discussion in 'Astrophotography' started by georgian82, Feb 12, 2018.

  1. georgian82

    georgian82 Mu-43 Veteran

    232
    Jan 17, 2014
    Hi guys,

    This is a high dynamic range image of M42. It consists of 102 x 180 sec light frames plus another 86 x 30 sec light frames that were used for the inner core.

    It was taken with a modified Olympus EPM2 and Nikon 180mm f/2.8 at f/4 on a Fornax Lightrack ii mount and it has super bias and dark frame subtraction. It was processed using PixInsight and there is no noise reduction.

    Cheers!

    Orion HDR 2.
     
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  2. Wow, was that taken over many sessions?
     
  3. Orbmiser

    Orbmiser Mu-43 Veteran

    376
    Nov 15, 2017
    Portland,Or.
    Hard Work & Dedication for Outstanding results.
    Is that a Giant Photon Torpedo coming out of the Nebula at the 4 o'clock position? :eek:
     
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  4. georgian82

    georgian82 Mu-43 Veteran

    232
    Jan 17, 2014
    Yes, it was taken over four nights. My tracking mount only tracks for 2 hours before I have to rewind it so most nights I only image for those two hours and then spend another 30 minutes taking dark frames. It ends up being a long process, especially the pre processing and post processing which takes many hours.

    Cheers,
     
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  5. georgian82

    georgian82 Mu-43 Veteran

    232
    Jan 17, 2014
    Thanks! Yes, you can't do this without dedication. That's the only way to overcome how tire you feel the next day at work! LOL.
     
  6. How do you ensure that the images line up? Is it a case of getting it fairly accurate by eye and then the software aligns it all nicely?
     
  7. georgian82

    georgian82 Mu-43 Veteran

    232
    Jan 17, 2014
    That's correct. In this case is fairly easy because of the wide field of view. The software aligns them very well but I did have to crop the borders a bit because of that's the region where the images don't stack well if you do it over several days due to the field rotation.
     
  8. Ulfric M Douglas

    Ulfric M Douglas Mu-43 Hall of Famer

    Mar 6, 2010
    Northumberland
    Please explain this bit,
    I have no knowledge of this stuff so tell me why you need more than one dark frame,
    or why you don't need to do your dark frames during the normal exposure series...
    ??
     
  9. My understanding of taking multiple dark frames (and bias frames) is so that any other noise that happens to occur in the frame is averaged out over multiple exposures to leave the dark noise only.

    In terms on when to take, I suppose ideally you'd take one light frame and one dark frame and then repeat. However, that would be tedious!
     
  10. georgian82

    georgian82 Mu-43 Veteran

    232
    Jan 17, 2014
    I think Levster pretty much answered those two questions.

    You want to take dark frames as close as possible to the light frames that way they are at the same temperature (and same exposure length) and you wan to take at least 20-30 and then stack them together to produce a master dark frame. I find it simpler to do it right after taking my light frames, otherwise I would have to cover the lens in between light frames and that is not practical. I could do in-camera dark frame subtraction but that is also impractical because I would be wasting precious tracking time (I only have two hours available with my tracking mount before I have to rewind and start again).

    I also go a step further and also subtract the bias frames from the dark frames and light frames in order to remove any readout signal from the sensor (this is almost like a digital fingerprint that will be unique to your camera). Both bias frame and dark frame subtraction occur before I even stack the light frames as you want those light frames to be as clean as possible.

    I hope that helps...

    Cheers,
     
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  11. Ulfric M Douglas

    Ulfric M Douglas Mu-43 Hall of Famer

    Mar 6, 2010
    Northumberland
    Yeah it helps, a very thorough answer but ;
    Well that's the bit that has me puzzled : is it obvious the time you are losing by not letting the camera do dark-frame-subtraction for each frame versus the time spent doing it yourself?
    (Levster's post answered nothing but was a good post)
    Do different models of camera body achieve dark-frame-subtraction better of quicker?
     
  12. georgian82

    georgian82 Mu-43 Veteran

    232
    Jan 17, 2014
    If I let the camera do in-camera dark frame subtraction, then I would loose imaging time. In this particular case I took 30 dark frames of 180 seconds each (same exposure length as the light frames) which is 90 minutes in total. That means that by doing in-camera dark frame subtraction I would have lost 90 minutes of imaging time. And since my tracking mount only tracks for 120 minutes, it means I would have spent very little time actually imaging. This is the reason why most people do dark frame subtraction at the end of their imaging session.

    I don't know if cameras achieve dark frame subtraction better and definitely not quicker. Remember that dark frames have to be of the same length as the light frames no matter what.

    I hope I was able to explain myself in a way that makes sense...but let me know if you have more questions.

    Cheers,
     
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  13. AllanG

    AllanG Mu-43 Veteran

    451
    Aug 26, 2014
    Brisbane, Australia
    Allan
    Dark frames have to be taken at the same temperature and the same length of time as your images. The Bias frames and flat frames have to be taken at the shortest time available and for a time which gives approximately 1/3 the adu of your images, respectively. All of these can be reused the next night and for some time if the conditions are essentially the same as the conditions the lights, darks, biases and flats are initially obtained. This scheme also gets rid of sensor glow which happens with most cameras which are not called to below 40 degrees below ambient
     
  14. Ulfric M Douglas

    Ulfric M Douglas Mu-43 Hall of Famer

    Mar 6, 2010
    Northumberland
    But surely if you are taking dark frames after the real exposures only one dark frame is required,
    not half an hour of them??
     
  15. georgian82

    georgian82 Mu-43 Veteran

    232
    Jan 17, 2014
    No, you actually need 20-30 of them and stack them to produce a master dark frame which will then be used to remove the dark frame from your light frames.

    If you were to look at individual dark frames, they would look different from each other even at the same temperature since it’s random noise. You want for the random noise to be averaged out...
     
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  16. Ulfric M Douglas

    Ulfric M Douglas Mu-43 Hall of Famer

    Mar 6, 2010
    Northumberland
    Kudos to you for taking half an hour of dark frames to even out the random noise ... serious stuff.
     
  17. georgian82

    georgian82 Mu-43 Veteran

    232
    Jan 17, 2014
    Thanks. But it’s really the only way to do it in my opinion. I have realized that astrophotography is time consuming and it requires a lot of effort (not to mention expensive at times depending on the equipment). So if I am going to spend the time in doing it at all, I may as well do it right and get the best out of my equipment and effort.

    Just my two cents...
     
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  18. RenaudVL

    RenaudVL Mu-43 Veteran Subscribing Member

    378
    Mar 21, 2011
    Nicely done...

    This lens is providing very good flatness.. I want one!
     
  19. georgian82

    georgian82 Mu-43 Veteran

    232
    Jan 17, 2014
    Thank you!

    Yes, at f/4 it works great on a micro four thirds sensor, especially at the center the field is very flat and resolution is high. However, I tried this lens on my Nikon D750 full frame the other night and the vignetting is horrible at f/4 and it shows strange gradients. I was thinking about imaging the Witch Nebula with it but I changed my mind after reviewing the test frames.

    Cheers!