Flintstone Stargazing

January 1, 2008

No stargazing tonight, but some reprocessing

Tonight started off cloudy, but I can see the stars clearly now. The wind is blowing like crazy though. Weather.com says up to 35mph, but I’d bet we’re getting over 50 at times. The garage door has been creaking ominously all evening. I figured it would be better to play it safe with the telscope. I’m not sure I could get over my telescope being blown over in the wind.

Instead, here’s a reprocessed Orion Nebula from last night which I re-stacked with DeepSkyStacker. I think this image does a better job of displaying the nebulosity than the one from yesterday.

M42-Great Orion Nebula on 12/31/07 - reprocessed
M42-Great Orion Nebula on 12/31/07 – reprocessed

Also, here are 2 images for Pierre, a commenter who asked about what the individual frames I’m seeing look like (though they may be interesting to others as well). I’ve also included a long-winded explanation of how I do my astrophotography image gathering so if you’re interested in that, read on…

M42 for Pierre - Single
M42 single 4 second image

This first image is what I see when I take a 4 second exposure. You can clearly see the Trapezium and the other stars are nearly maxed out on brightness. The area of the nebula lit by the Trapezium stars is showing up faintly, but there’s not a huge amount to see. That’s because the stars are pushing the top end of the histogram so hard. There’s actually quite a bit more detail there and that’s why I normally adjust the gamma (in the Envisage software, it’s called “Shadow Enhance”, but it’s really just a gamma adjustment). Below is what that does for me.

M42 for Pierre - Gamma adjusted single
M42 single 4 second image – gamma adjusted

Obviously there’s a lot more detail there. As a matter of fact you can see some geostationary satellite streaking along in the upper right left quarter of the picture which really wasn’t visible in the image above at all. (Side note: probably 1/3 of all images I take of Orion have a satellite going through it because it’s in the southern part of my sky and that’s where geostationary satellites are as well, above the equator. As a matter of fact, I’ve seen as many as 4 in the same image before) Now, since I’m capturing these images in a FTS format, changing the gamma doesn’t change the underlying data, which is good because I want to reprocess these later and I’d rather work with raw data. As a matter of fact, if you have used the RAW format in your camera, it’s pretty much the same kind of thing – there’s a lot more data than a standard 8-bit image. In the case of FTS, it can store a massive amount of data – each of the frames above is 5MB in FTS format (compared to less than 15K each in JPG). It’s this raw data that allows software like DeepSkyStacker to pull out so much detail in the images later.

Another important thing to note is that these images automatically have dark frames subtracted from them. Any digital camera chip has noise in it that gets amplified the warmer it is and the longer an exposure is taken. For images of daylight or really any well lit scenes, this noise is absolutely overwhelmed by the amount of light coming into the camera so you’d never see it. With astrophotography, you are taking pictures of things that are extremely faint (though Orion is somewhat an exeption – it’s considered very bright for a deep sky object, but it’s still pretty dim) and so you use long exposures. In this case, that noise in the camera can be pretty significant. It can add new “stars” to your image, which might seem exciting at first -hey, I discovered a new star – but wears out its welcome quickly thereafter. To combat this problem, I take exposures at numerous temperatures for many different time periods and the resulting images are subtracted from the final images. For example, I’ll take a 4 second dark frame at 34°F and use that to be subtracted from the images I’m taking above since that’s the exposure time and the temperature at which it was taken (fortunatly, my camera has a built in thermometer since it’s important to know the camera temperature) This helps the end result be much more true to the light that’s coming in. There are other kinds of adjustment images that can be used – bias and flat frames – but I haven’t had a chance to really use them. They can make a difference in the quality of your work (they account for areas of the image that are dimmer than they should be as well as some other things), but dark frames are simple to do (my capturing software does it automatically) and make a massive difference right off the bat. My capturing software (Autostar Envisage – it came with my camera) automatically can create the composite image which normally looks pretty good, and so that tends to be what I’ll throw onto my blog the night I take something. If I’ve gotten good results, I then will often pull the raw data into DeepSkyStacker or RegiStax or something else to reprocess it. Such is the case with the image near the top of this post.

I hope this helps y’all understand how I do some of what I do and if anyone has any other questions, please feel free to add a comment. I really enjoy these kinds of posts, but it’s not something I tend to think of on my own.

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2 Comments »

  1. Fascinating post Ed. Your 4 second image very closely matches what I’ve been seeing in my raw images. Your clues now show me that I’m indeed starting on the right path. Thanks.

    P

    Comment by Pierre — January 2, 2008 @ 12:10 am

  2. No problem – thanks for the post idea. If you’re getting something similar to what I’m seeing, then stacking and working with the levels/curves of the image should help bring out a lot of detail.

    Comment by Ed — January 2, 2008 @ 8:35 am


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