Actually, I've been here for several years, just don't get out much
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Intersting to see the blue bias of the DSLR vs. your CCD. Not actually sure which colour bias I prefer. The purist would go for the red, but is terms of asthetics they are both nice. In some ways I prefer the Blue, but that could just becuase I'm used to it!!
There should be no bias in my image. It's red because the nebula is predominantly Hydrogen. My images are colour calibrated using filter ratios determined by photometry on a G2V star and extinction would be minimal because M8 was near zenith when I imaged it. The colour is not entirely accurate though because I used LRGB which results in slight desaturation. The RGB is a bit more of the traditional purple/pink colour but has nowhere near the detail of the final image.
I can see that it would've benefited from more exposures
Yes it's frustrating, but I keep saying that! I told myself years ago that I was going to stop wasting time waiting for clouds, but i just keep on doing it.
This may be one of those images that takes several nights to finish at half an hour a night. Forecast for tonight is much like last night so I might get it done. One things for sure...it's not finished!
Hi all
Heres my M8 effort. It was taken a few months ago when I got xtra keen and got up at 3 am to take it. Also, it was before I got my off axis guider for the 10 inch, was instead taken with my 6 inch F3.6 Schmidt Newtonian http://www.iceinspace.com.au/forum/a...tid=2320&stc=1
I used Curves to tweak up the red to mimic the modified DSLR cameras
Its two 5 min shots ISO 200 at F3.6
Scott
There should be no bias in my image. It's red because the nebula is predominantly Hydrogen. My images are colour calibrated using filter ratios determined by photometry on a G2V star and extinction would be minimal because M8 was near zenith when I imaged it. The colour is not entirely accurate though because I used LRGB which results in slight desaturation. The RGB is a bit more of the traditional purple/pink colour but has nowhere near the detail of the final image.
There is bias in every image. No CCD can accurately measure the entire spectrum of light. What is bias anyway? Surely we need a reference to measure it against? CCD detects more red releative to the eye. The key here is relative. And CCD will capture light that is outside the visible spectrum. Which is great!!! But I often ask, What would my eye see if a particular nebula was bright enough to register colour in the eye? It probably wouldn't be as red as Eddie's image, but it probably wouldn't be as blue as my image.
Somewhere in between I suspect. However I would say "relative" to the eye Eddie's image is red biased, which shows up more details in HA. This is great because the nebula is HA dominant. It's all abour perspective really.
No CCD can accurately measure the entire spectrum of light. What is bias anyway? Surely we need a reference to measure it against?
There are references. That's why we do a G2V star calibration. These stars are white. You do photometry on them through each of the RGB filters to get the correct ratio to give the correct colour balance. This reveals the response curves of both the CCD and filters allowing the ratios to be adjusted when the final RGB image is combined. There may still be some bias caused by atmospheric extinction, but even these quantities can be calculated based on the altitude of the target and can be adjusted-for. The final result and the reason for doing it all in the first place is to acheive true-colour.
Hi Eddie,
Thanks for your reply. I am curious as to the origin of "true colour". One would presume that true colour is what the eye would see... But I know this is not the "true colour" you are referring too. I guess you could take the visible light spectrum and normalise it. Therefore creating a a constant value for each wavelength. This is not how the eye would see colour though. The eye reduces intensity at each end. Sort of a band base filter on the visible spectrum.
However I must admit the human eye is probably not a good reference as everyone's if slightly different!
Therefore normalising the colour spectrum (as you have done) is probably the best "standard" that we can adhere to for consistency. Also to capture the most detail in an astro image we need to normalise the light curve, otherwise we may miss something.
I have been somewhat reluctant to remove the HA filter from the 300D due to the fact that the filter is put there so that the image reproduced from the camera is as close to as possible to what one would expect from the eye.
It's a bit of a non-standard view of astro-imaging and definately goes against the norm, but I have never been much of one for following the rules anyway!!!
Does this sort of hold any water with you?
PS. Just had a scout throguh your website. Stunning images!! Definately need to get me one of those CCD cameras to play with one day. Maybe once I've paid off the house .
I am curious as to the origin of "true colour". One would presume that true colour is what the eye would see...But I know this is not the "true colour" you are referring too. I guess you could take the visible light spectrum and normalise it.
Emission nebula are red not because of eyeballs, cameras or filters, but because hot hydrogen glows brightest in the red part of the spectrum. That is it's true-colour in the visible frequency spectrum.
The visible light frequencies in the colour spectrum are known and there are specific frequencies being emitted by objects. Filters are designed pass a specific range of visible frequencies and the ccd chip has varying sensitivity across these frequencies. So doing the calibration I mentioned earlier matches it all up in order to accurately represent the colours of the object within the visible spectrum. Eyballs or no eyeballs.
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I have been somewhat reluctant to remove the HA filter from the 300D due to the fact that the filter is put there so that the image reproduced from the camera is as close to as possible to what one would expect from the eye.
This camera is designed for daylight use where ambient light is all over and contrast differences are great. The rules change in the absence of all-pervading sunlight. I've had a quick look at the response curves of the 300D. The IR filter is only passing 40% of the light at around 650nm (roughly where Red starts in the spectrum) and drops off even more below that. So it is attenuating Red significantly. Taking into account the higher sensitivity of CCD chips in the Red than blue would compensate for this somewhat, but at a guess I'd say the 300D could be up to 50% Red-deficient.
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It's a bit of a non-standard view of astro-imaging and definately goes against the norm, but I have never been much of one for following the rules anyway!!!Does this sort of hold any water with you?
Yes and no. If you want to accurately represent the colours of objects, then the answer is no. The 300d's filter system was not designed for low light-level imaging in the absence of sunlight. Without colour adjustment of the resulting images to compensate for the attenuated Red frequencies you just cannot get accurate colour in astrophotos with the standard camera.
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PS. Just had a scout throguh your website. Stunning images!! Definately need to get me one of those CCD cameras to play with one day. Maybe once I've paid off the house.
Thanks Robby. I still think dedicated astronomical ccd cameras are the way to go, but consumer digital cameras are getting better and better by the day. The day will come when they will replace dedicated astro ccd cameras, but that day is not today
Thanks Robby. I still think dedicated astronomical ccd cameras are the way to go, but consumer digital cameras are getting better and better by the day. The day will come when they will replace dedicated astro ccd cameras, but that day is not today
Hi Eddie,
Thanks for taking the time to explain above. It is appreciated. Certainly food for thought.
That ST-10 of yours is truely something special, but I just can't afford a 2nd mortgage at the moment The DSLR is great for honing skills and at the same time getting some very satisfactory images. I'll be all sorted when I do eventually move to dedicated CCD.
Cheers
The DSLR is great for honing skills and at the same time getting some very satisfactory images.
Absolutely! My first astro camera was a homemade Cookbook 245 and I learned a lot with it and don't regret any of it for a second. But the 300d's capabilities are eons beyond the cookbook camera!!
This is actually a reprocessed effort from 2 separate nights last June from my backyard at Thornlands (bayside suburb of Brisbane). I used a Takahashi E160 (530mm FL f3.3) + 300D with standard filter removed and replaced with baader IR/UV filter. 30 x 90sec ISO exposures (45 minutes combined) were processed in IRIS 4.37 and then final processing in Adobe Elements 2.0. Image has been cropped and reduced. All the exposures were unguided, and I picked the best onces that had the least guiding error.
Just a correction H-Alpha sensitivity is improved over 4x when removing the filter in the 300D.
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