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Discussion in 'Astrophotography' started by wjiang, Jan 7, 2015.

  1. The weather has been pretty good for the last month so in between roof painting, I've had a chance to take some shots of the area of the sky around Crux and Carina.

    First is a wide one, with the LMC just showing up at the top of the frame and Orion on the left:
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    This was shot at 7.5mm FE, f/3.5, 10s, ISO1600, with a 13 stack for the top half, 12 stack for the bottom half, aligned and reprojected in Hugin, then manually blended and processed in Photoshop. I decided to try and do manual dark frames this time around, whereas previously I used to stack light frames with built-in dark frame subtraction enabled. The initial stacks from this sequence were a complete disaster! I discovered that for super wide field shots with the fisheye at least, DeepSkyStacker just craps out if fed with the RAW light frames and dark frames for some reason. The result was noisy as hell and had trails of hot pixels everywhere. In the end I did some hefty initial processing, noise reduction, and manual hot pixel removal with ACR on just the light frames and stacked the exported DNGs for this salvaged result.

    Next is a close-up with the Carina Nebula and Coalsack prominently visible:
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    This one was shot at 25mm, f/1.4, 4s, ISO1600, with an 18 stack of light frames and dark frames. For this narrow FoV, DeepSkyStacker did well when fed with all the RAWs. I really should have stopped down a bit more, the coma at the corners is a bit too noticeable...

    Finally, a trial run on a different night with my new PL15:
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    This one was shot at 15mm, f/1.7, 8s, ISO400, with a 6 stack of light frames (automatic dark frame subtraction enabled). It's not quite wide enough to be this close to the foreground (a pretty poor foreground at that), but I think it will be workable in other scenarios.

    After this experience, I don't think I'll bother with shooting manual dark frames - I feel that the camera actually does a better job at this as each dark frame is taken with the same conditions as the corresponding light frame and perfectly aligned. Moreover, if anything screws up, I'll have more salvageable light frames with dark frame subtraction enabled than without.
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  2. OzRay

    OzRay Mu-43 Hall of Famer

    Jan 29, 2010
    South Gippsland, Australia
    Ray, not Oz
    That's really good, especially the last shot. From what I've read, coma is a lens characteristic and something that you'll more or less have to live with. I think I'm just going to have to wait until winter for what I'm doing, as I can't catch a clear might for love or money at the moment.
  3. MadMarco

    MadMarco Mu-43 Veteran

    Oct 30, 2014
    Guildford, England
    Those are great photos, I'm loving it! The last one is definitely my favourite, I think that the background of the house gives a really nice perspective.

    I think that Ray is right; the coma is going to be very difficult to eliminate, I've never come close to doing anything useful with it. I suspect that it is made worse if you aren't using an equatorial tracker, you will start to see field rotation in the stacking which makes everything much more difficult for the software to align. I've certainly seen this with wide stacks that I've done without a tracker.
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  4. krixoff

    krixoff Mu-43 Top Veteran

    Jul 11, 2013
    As i said in 15mm topic, greats pictures and i will trying to use yours tips this weekend. Thanks again for your topic.
  5. maritan

    maritan Mu-43 Veteran

    Oct 30, 2014
    Awesome captures. Especially, like everybody else, the last one. I assume light pollution is little to none in your area? ETA: Never mind. I see there's a light post right opposite your house as seen in the first picture. Wow. Even with that kind of light pollution, it's possible to acquire such pictures? I have much to learn.

    I read through your explanation in the other thread:

    Is the star alignment a software thing or something you do with a sky tracker? Sorry, I'm a newbie at this and don't mean to sound like a total idiot.
  6. It's not actually my house... just my neighbourhood :p 

    Star alignment is built into DeepSkyStacker (free software). As long as you make sure you have plenty of stars visible and that importantly, there are no trails, it can align them reasonably well. With wide field images it starts to struggle a bit as I noted, but still copes with just light frames okay. With a sky tracker, you can make even longer exposures to stack, allowing you to stop down for maximum sharpness while avoiding trailing or a super high ISO. You can of course just use a single really long exposure with a tracker, but that requires a very accurate and well aligned tracker.
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  7. maritan

    maritan Mu-43 Veteran

    Oct 30, 2014
    Thank you! I think I have an understanding of the general work flow to get these type of pictures. I have plans this year to go camping to certain spots and get some good pictures with a foreground and the milky way. We'll see if it materializes.
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