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I'm still new to this modern era of digital astrophotography, though I shot a little bit of film back in my teens (in the early 1980s). I'm using Nebulosity 4 and GIMP on a Mac, and a Canon EOS 7D (unmodded) camera. I've also downloaded but not yet really tinkered with SiriL and RawTherapee. The camera is set up to record both JPEG and raw images. The JPEGs look remarkably good, but the raws are just that, raw pixel data. I don't know if all 7Ds are like this, but red accumulates WAY faster than blue or green, and of course it's just linear brightness, so everything starts out down in the murk. It takes me a LOT of fiddling with Nebulosity to get to an image that even looks as good as the JPEG straight out of the camera. Often, I feel like I'm not even really getting there. So, is there some quick & easy way to match the JPEG, as a starting point? i.e., is the in-camera processing info available somehow, for white balance and whatever curve stretching it's doing? Of course, I don't want anything that can't be undone, but something that guides me to a better first look would make my life a lot better. |
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The astronomy version of a RAW file is the FITS format as far as image quality. FITS has a bunch of additional data stored/imbedded in the file that enables lots of functionality and commonality between various astronomy apps. Since you don't have a dedicated astro camera that transfers FITS files, RAW is the best you can do. STAY AWAY from JPEG (jpg) format for anything AP-related using a DSLR. Once you save/convert your image to JPG the image quality is permanently degraded (you cannot recover it once the file is ran through the JPEG algorithm), it will get worse each time you reopen and resave the file. If you are shooting DSLR and dropping RAW files you can convert them to TIF/TIFF format for processing in DPP/PI/etc. The only issue with TIF is you don't have the additional header info (not a big deal when you are starting out) and the image quality (and file sizes) is less than RAW or FITS. When you are done manipulating your files and you want to post an image online (like on a blog, webpage, etc.) then use JPEG. IMHO the better web (raster) option would be PNG since the compression method is less destructive and the format maintains a semblance of very good image quality. In the old days JPG was preferred for online viewing because image quality was balanced with keeping image file sizes down so they loaded faster in browsers. Consumer cameras catered to users' online activity and since most consumers don't want to fuss with files and software JPEG became the preferred consumer option. Once cable modems, ADSL, and the like came online image file size was not a concern, image quality was more important. IMHO again, in AP, image quality is THE primary goal. The longer you can keep images in their original source format (aka original resolution, color, etc.) the better. If you process an original file make a copy and edit that so you have a fall-back if you pork the image during processing. CS & GB! |
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Hello Rod, Though the algorithm has improved a lot, after all, JPEG is a lossy compression. With RAW you will gather every photon your sensor catches in one piece. With your post processing software you can compensate for unwanted color hues and squeeze the most from your images. Best luck Robert |
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Thanks for the quick replies. I'm very aware of the limitations of JPEG. I'm not proposing to process based on the JPEGs themselves. What I want is to use them as a *guide* for processing the raw images. The camera internally is doing a number of processing steps to get from raw to JPEG internally. I want to know what those steps are, as a shortcut. Is there any way to acquire that info? |
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| btw, when you open a raw picture in PhotoShop, it starts out with a pretty good view. Nebulosity is a much rawer (so to speak) view of the raw data. I don't actually have PS, though, I'm using Nebulosity and GIMP. |
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Hi Rod, I believe I understand your question. Some software automatically "stretches" images for screen display. Nebulosity is a good example ... irrespective of whether captur3es as JPG or raw. As you may already be aware, PixInsight (PI) is used by many folks on AstroBin for image processing But most of us came from previously doing this in the like of Nebulosity, Maxim, Photoshop, DeepSkyStacker, AstroArt, etc. PI includes a magic button called the "ScreenTransferFunction" to auto-stretch an image for screen display ... getting things in the ball park globally across the image. With a few extra mouse clicks, you can transfer this 'stretch' to the raw image data ... as a starting point for further processing. The first few minutes of the following video provides an overview: www.harrysastroshed.com/pixinsight/pixinsight video files/2013 pix vids/stffinal/stffinal.mp4 Hope this helps. |
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Rod Van Meter: Hi Rod, Rawtherapee can do a very good job at matching the JPEG if you have a DCP camera profile (which you can obtain by installing Adobe's DNG Converter as described here). The relevant settings are in the color management tab, simply chose your DCP and then enable Tone Curve, Baseline Exposure and Lookup Table. Here is a raw from my Z50 with the default settings:  And here it is with the Z50 Standard .dcp applied (I have highlighted the relevant controls):  BUT (this is important) this is for a single shot only, which you would export as JPEG and be more or less done with it (note that there are many, many other processing steps you can do in Rawtherapee, including curves transformation, denoise, gradient removal, vignetting correction, haze removal, dark and flat frame correction, retinex, wavelets, literally hundreds of things). If an advanced raw processor is what you were asking, you don't need to read further.  If on the other hand you have multiple shots which you are looking to integrate, you must: 1) uncheck "Tone Curve" as having it enabled destroys the linearity, resulting in serious information loss when integrating. But leave the other boxes checked. 2) export as 16 bit TIFF. The exported TIFF files will be your new "raw" images which you will register and stack. It does not matter that the exported linear TIFFS will look dull, they will still have correct color information and you will be able to stretch the integrated image in post processing. Doing 90% of processing after integration is the norm in deep sky and planetary imaging. Regards, Dimitris |
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Hi Rod IMHO, just try to google the words 'auto DDP stretch images' or some other relevant 'words' related to image processing of astronomical images and then decide on which SW tool you want to use/maybe buy and use. The learning curve of these SW tools can be high, but don't give up :-) Personally I use most SW tools like, Adobe Photoshop, CCDStack2 and PI. CS Niels |
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BTW, forgot to mention this one :-) A SW camera control & Image processing tool. Download for free ImagesPlus 6.5 Image Processing & 6.5 Camera Control Download, http://www.mlunsold.com/ILOrdering.html |
