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yock1960
super member
Reged: 06/22/08
Posts: 172
Loc: SW Ohio, USA
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This sounds like a dumb question to me but....
I'm using a SPC900NC SC1 modded webcam along with wxAstrocapture. Since I've been doing AP (since June), I've pretty much 'eyeballed' my exposures, which seemed to work ok doing Jupiter or the moon. Of course those are very bright. But now that I'm doing some DSO's and have started to see/understand the importance of stretching histograms, I have noticed that some of my images start out clipped on the 'left' side (that probably should be said differently, but I don't know better yet) so I can't do anything with it without losing data. So, the question is:
Should I be centering the histogram in wxAstrocapture or whatever software I'm using? There is probably more to it, it seems like if I did that, some areas would be washed out. If it would help, the subject where this has come up is M31.
I won't ask what the best book out there is, but what's a good book that will cover this? I read reviews and there doesn't seem to be a consensus 'best' and they aren't inexpensive!
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Discovery 6" Dobsonian
ETX80 Backpack Edition + Meade #884 Tripod
Astronomiser ASTSC1C
Meade DSI II
Televue Powermate 2.5x
Gallery
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clitherowclan
professor emeritus
Reged: 02/23/05
Posts: 526
Loc: Fife Scotland
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I don't know if I can help but may I ask what sort of exposure lengths are you using, what are your gain and gamma settings and how are your saving out your images; as individual frames, and if so in what file type? Or as an avi?
Cheers,
Alan C.
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Scottish Astronomer, Firstlight Instruments 10 inch F6.3 Newtonian, Helios 8inch F5 Newtonian and ED80 Apo on driven EQ5. Various cheap cameras.
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yock1960
super member
Reged: 06/22/08
Posts: 172
Loc: SW Ohio, USA
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Hi Alan,
Exposure times are pretty short, 20 seconds in this instance. Gain would be fairly high, so far for DSO's I'm usually between 60 and 80. Gamma I usually set as low as possible, I kind of started doing that shooting Jupiter and it seemed to help, it' just carried over into DSO's.
I save as avi's. I've tried FITS once it wasn't a good experience, none of the software I used at the time liked them (Registax, Pixinsight LE, Deep Sky Stacker) plus they were grayscale.
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Discovery 6" Dobsonian
ETX80 Backpack Edition + Meade #884 Tripod
Astronomiser ASTSC1C
Meade DSI II
Televue Powermate 2.5x
Gallery
|
clitherowclan
professor emeritus
Reged: 02/23/05
Posts: 526
Loc: Fife Scotland
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Don't hold me to it but I think maybe you are setting the gain too high. This will tend to exaggerate good signal but will also raise the noise level which could be swamping your faint signal detail. That may be why your images are a bit clipped at the dark end. Really I think with deep-space objects and 20 second exposures I think you want very little gain; perhaps just enough that there is something visible in each sub frame. Then use your off-camera software to start stretching your final stacked image. That would give you a much better result than letting the camera do it. You are going to need a stack of good dark frames as well to help cancel out the amp-noise (the bright patch in the top left-hand corner of most long-exposure webcam images)
I'm surprised you haven't had more success with fits files. Both registax and deep sky stacker will work with them 'though I think DSS is better at it. If you start looking through the DSS stacking parameters you will see a tab for raw and fits development processes which will allow you to specify the camera used. That way DSS will know how to de-bayer the colour information in the fits file and give you an rgb result rather than monochrome. If your camera isn't specified then try the 'generic RGGB' setting; and if that produces a weird result try running the stack again using 'generic BGGR'. Experiment in other words. Save the final result as a tiff file (make several saves using both unstretched (linear) and stretched (logarithmic) settings) and move to some better software to view, and play with, your results. I don't know picinsight but I'm pretty sure it will allow you balance the RGB parts of the image, so that a white star genuinely looks white, and to stretch the histogram to bring out faint data.
It's probably a bit cheeky of me to say it but I wouldn't be too ambitious with an 8 bit webcam. If you stick to brighter objects like the ring neb' or bright galaxies or just about any globular cluster you will get perfectly good results. If you try the faint and fuzzies you may be a bit disappointed. I started out using SC modded webcams and really enjoyed them. Hope you do too.
Cheers,
Alan C.
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Scottish Astronomer, Firstlight Instruments 10 inch F6.3 Newtonian, Helios 8inch F5 Newtonian and ED80 Apo on driven EQ5. Various cheap cameras.
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yock1960
super member
Reged: 06/22/08
Posts: 172
Loc: SW Ohio, USA
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Alan,
Thanks for the reply! I will give lower gain settings a try. What use gamma? Any?
Registax would open, align, but after optimize, I wouldn't see anything...it was strange. I've just started to use DSS and need to read over the documentation; don't know any settings adjustments yet. Maybe I'll give those FITS files another shot after a little reading.
I know that I'm limited with the 8 bit camera and your probably right about me being disappointed after a while, I didn't trust myself to try and modify my original webcam for LE and this was the least expensive route that wasn't dependant on stumbling across a good deal or getting involved in a bidding war on ebay!
I'm enjoying it and maybe by the time I get through the brighter DSO's, I'll go for something a little better, economy permitting!
At some point I may even try the RAW mod for webcams out there; it's supposed to give a little better performance. I'm not ready to try it though as there does appear to be some risk to the camera.
Thanks Again!
Steve
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Discovery 6" Dobsonian
ETX80 Backpack Edition + Meade #884 Tripod
Astronomiser ASTSC1C
Meade DSI II
Televue Powermate 2.5x
Gallery
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clitherowclan
professor emeritus
Reged: 02/23/05
Posts: 526
Loc: Fife Scotland
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Steve,
Don't get me wrong I don't think you will be disappointed with your camera at-all; just don't be too ambitious with what you image. Here is an M57 I did with an SC modded logitek quickcam pro 3000 some time back. It's certainly not the greatest picture in the world but I'm absolutely not disappointed with it! At the same time I wouldn't personally use this type of camera to shoot something faint like the bubble nebula.
As to what gamma to set well I really am no expert but it does seem that good results seem to vary from camera to camera. By that I mean my camera responds well with a low gamma setting and you might like to try that but the secret to this game seems to be experiment and see what works. With the moon and planets it's easy to vary settings until you get something nice but with deep-space objects I'm not always sure I have captured anything but stars until I do post processing and try and stretch things. Longer exposures will help but with these uncooled 8 bit webcams noise can be a big issue and I would stick with no more than 30 seconds just for the moment. The other thing I would say is really don't be discouraged by any poor initial results. I found the learning curve truly mountainous but now I am generally pleased with what I manage to produce.
Best wishes,
Alan C.
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Scottish Astronomer, Firstlight Instruments 10 inch F6.3 Newtonian, Helios 8inch F5 Newtonian and ED80 Apo on driven EQ5. Various cheap cameras.
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clitherowclan
professor emeritus
Reged: 02/23/05
Posts: 526
Loc: Fife Scotland
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Steve, here is a quick PS. I found this link:
http://www.astro-imaging.de/astro/gamma.html
which goes into some depth explaining exposure and gamma settings. It seems like very little or no gamma is good for bright objects but some gentle (ie low) gamma settings are a help on dim objects. That's kind of in line with what I found by trial and error and I hope the link is useful. It's actually from a German website but is mostly in English so is perfectly understandable.
Cheers,
Alan C.
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Scottish Astronomer, Firstlight Instruments 10 inch F6.3 Newtonian, Helios 8inch F5 Newtonian and ED80 Apo on driven EQ5. Various cheap cameras.
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yock1960
super member
Reged: 06/22/08
Posts: 172
Loc: SW Ohio, USA
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That's a pretty good site! I've bookmarked it. Also a nice image of the Ring Nebula. My 1st and only attempt so far (of M57) is a blue 'donut'. Of course I was just happy that my goto's & tracking were working well enough that it was in view and not jumping all over! Well, I have to wait a few nights and hopefully the forecast will be accurate for Saturday; mainly clear. Of course the moon is big and bright right now too!
If/when I get some new data using lower gain settings, I'll post it.
Clear Skies!
Steve
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Discovery 6" Dobsonian
ETX80 Backpack Edition + Meade #884 Tripod
Astronomiser ASTSC1C
Meade DSI II
Televue Powermate 2.5x
Gallery
|
clitherowclan
professor emeritus
Reged: 02/23/05
Posts: 526
Loc: Fife Scotland
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Look forward to seeing it.
Cheers,
Alan C.
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Scottish Astronomer, Firstlight Instruments 10 inch F6.3 Newtonian, Helios 8inch F5 Newtonian and ED80 Apo on driven EQ5. Various cheap cameras.
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