First M42 and some questions (but not THAT one!)

[Notty]

Hello, I had my first crack at M42 last week.  I had read all about overexposing the core and so on, so I took a range of exposures of 15, 30, 120, 180, 250s.  I ended up using just the 30s and 250s ones.  It's not great, but having used the layer mask/gaussian blur technique in PS I sort of get what I'm supposed to be doing.  This was just done using lights and darks as I messed up with the setup accidentally so couldn't get any flats.  However, it did throw up a couple of questions if anyone :

1. Stacking different exposure lengths... having captured these different ranges with the idea of maybe getting a graduation of detail, it my have turned out to be a waste of time due to the processing complication involved.  However I ended up taking a bunch more data of another target I'd had a go at previously, but at different exposure.  To stack different exposures do we do each stack had been done individually with it's own calibration frames i.e. individual stacked frames on top of each other?

2. M42 was in a very bad position for me with lots of light pollution and the longer exposures I tried ended up very orange.  I know the s/n ratio goes up with exposure time, but how does it increase instead with a higher number of shorter exposures (to avoid washout)?

Many thanks for any tips.  Anyway here's what I got of M42, as I said please pardon the terrible vignetting due no flats :-( .

[Oddsocks]

1. Yes, for each light-frame exposure time, camera temperature and binning mode you need matching calibration frames.

You only stack the matching image sets and then blend the separate final images together in your image processing program.

The only exceptions are that the bias-frames are independent of light-frame exposure time, as long as the binning mode and temperature they were taken at matches the temperature and binning mode of the light-frames then they can be used for calibrating both your long and short exposure time light-frames,  also the flat-frames are independent of your light-frame exposure time so again, as long as the binning mode and temperature they are taken at matches the light-frame temperature and binning mode then you can use those too for calibrating both sets of light-frames.

2. I am not so sure how to answer that question but I think the S/N ratio is unaffected by stacking, although the noise portion and the "image" portion is averaged in the stacking process I think the ratio between the two remains the same, only when the exposure length is increased does the S/N ratio increase but I may be totally wrong about that, probably time for me to re-read my copy of "The Handbook of Astronomical Image Processing" by Richard Berry and James Burnell for the definitive answer!  (ISBN-0-943396-67-0)

When you are ready try the free trial of Pixinsight, the DBE tool will remove the gradients and orange light pollution quite well.

P.S. Nice first attempt at M42, my one was dreadful but it was done with standard 35mm film and wet processing so I use that as my excuse...

[Oddsocks]

Just for interest, this is your original .jpg image put through the DBE tool in PixInsight, processing the original .tif file would have much better results as would spending some time on the DBE sample mask, I just went with the default settings and literally only spent thirty seconds on it but is does show you have captured some worthwhile data..

 

[Notty]

WOW! ! That is a completely different image I must say. Thanks for your response to my questions as well, but I'm gobsmacked you can get results like that from this pixinsight package. Must admit I'm still a learner with processing. What is DBE? I must confess I purchased Astroart which I was hoping to be able to replace photoshop, but although I like it for stacking I have gone back to PS as I've struggled with the equivalent functions in AA. Does Pixinsight do everything we use photoshop for as well?

Again, I'm thrilled to see my data is capable of what you've displayed above, thank you!

[Oddsocks]

What is DBE?

Does Pixinsight do everything we use photoshop for as well?

Hi Notty,

In Pixinsight the DBE tool means Dynamic Background Extraction and it maps the average background intensity across the image and attempts to make a subtraction mask, then the mask is subtracted from the original image to leave a "flat" luminosity level over the full image, this removes gradients caused by light pollution and vignetting. Of course, it is always better to have a fully calibrated image to start from and leave as little as possible to deal with in post processing.

Another useful tool in PI is called SCNR, Subtractive Chromatic Noise Reduction, I did not use it on your image yesterday but it is used to remove colour casts in the image and If I had used it then it would have removed the slight green colour left over in the dark background which you can see most obviously in the corners.

While some astro imagers swear by PixInsight to handle all the image processing steps necessary in post processing and others say Photoshop, with a few plug-ins, is more than capable of completely processing an image I take the view that there are good tools and processes available in both and usually begin my processing steps in PixInsight through to an almost complete image and then make the final cosmetic "tweaks" in Photoshop.

Perhaps, besides DBE and SCNR, PixInsight has a slight edge over Photoshop in that it can handle the stacking and calibration routines as well as all the other processing steps in a one-stop package and there are many image examples on the web that are only processed in PI.

A good resource for beginners with PixInsight are the tutorial videos at "Harry's Astroshed" which can be found here: http://www.harrysastroshed.com

You can obtain a free trial of PixInsight by visiting the PI site here: http://pixinsight.com

Many beginners struggle with PixInsight to begin with, the UI (user Interface) "looks" quite intimidating as there are so many tools available to choose from and variables are displayed numerically rather than graphically, or intuitively, as you have become used to with Photoshop, so I would suggest watching Harry's videos a couple of times before trying to process an image in PI or you may, like many others, become frustrated with the package and give up before discovering how powerful the PixInsight program is.

Another useful source of PixInsight tutorials in a text-with-graphics format is to be found at the "light Vortex Astronomy" site here: http://www.lightvortexastronomy.com/tutorials.html and the PixInsight site it'self has many tutorials in video and text format available.

[Notty]

Thank you very much for the information, I will probably have a play with the trial version and see how my brain copes. As an aside, I think I will try and research a photoshop plugin which might do the same thing as this DBE function as it sounds like a bit of a magic bullet!