Saturday, January 17, 2009
Sh2-240, more lights
This is a second version of Sh-240 this year.
I added more H-a light. Now image is smooth and signal / noise is
improved. I might shoot more lights for this later in this spring.
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Two versions, first in Hubble palette and secon in "natural" color narrowband composoite.
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Exposures so far are:
H-alpha, 12x600s + 24x300s(binned 2x2) + 12x600s = 6h
S-II, 5x600s (binned 2x2)
O-III, 14x300s (binned 3x3)
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Optics: Canon 200mm EF f1.8 @ f1.8 Camera: QHY9 @ -50 C Guiding: Lx200 GPS 12" + LQHY5 and PHD-Guiding - The information in O-III channel is extremely weak. I used a special technique of mine to dig it out for color information. This image serie shows how weak the O-III realy is.
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2 comments:
Hi Jukka-Pekka,
first of all, congratulations for the narrowband Sh2-240 - nice image !
As you already might be aware of, your image caused a lively discussion on the amastro newsgroup yesterday which starts from here: http://tech.groups.yahoo.com/group/amastro/message/21889
One yet unsolved question is still the discrepancy between some observation reports of people that saw some parts of the nebula with [OIII] and your image, which shows OIII emission to be very weak. In addition, I found a paper which indicates that at least some filaments exhibit a rather moderate H-alpha/[OIII] ratio of about 3:1 or 4:1. In other words, it seems that for some unknown reason, the nebula appears fainter in your [OIII] image as it should be - and we simply do not find a satisfying answer to that. One of the possibilities we were thinking of are instrumental or environmental effects; this also because at least the jpeg of the OIII image that you posted in your article shows a relatively strong background glow, which at closer scrutinity reveals a distinct gradient towards the lower right corner. Can you give a more detailed description of the circumstances under which you made the image ? I.e. humidity, possible light pollution, Moon yes/no ...
Thanks,
Matthias
Hi Matthias,
This is a same question, than Rainer Vogel, from Germany, asked
a month ago. Maybe you should contact him as well.
Please send me a mail to:
jp.metsavainio at gmail.com and
I will send you better picture about O-III channel.
The filter I use is a Baader 8,5 nm
O-III filter, it's narrow enough to allmost totally block out any light pollution effects.
The reason for uneven background is, that the signal in O-III channel is extremely weak!
I think, this is not gona be a visual target in O-III.
In attached images you can see a part of my work process.
The way I do my NB images is different than moust do.
I usually create a separate starless images for each channels, they are so much easier
to handle, since I don't have to care about bloating stars.
Then I craeate animage contain only color information, I don't care about setails and shades, only
color part of information is interesting.
Then I use a best channel as a luminance, usually H-a. I cant't use just H-a as a luminance,
since there is information in O-III and S-II channels.
I add this missing information, by using the starless cahannels as source, to the H-a luminance.
Now I have a O-III, S-II boosted H-a luminance and a separate color information.
My opinion is, this line can not be visually observed.
I tryed to estimate the intensity difference between H-a and O-III but it's more difficult than I tought.
Very rough estimation is, that O-III signal is about 1/1000 - 5 000 of H-a. This is based on the well
capacity of the CCD. and the bacground offset to the actual signal.
Best regards,
J-P Metsävainio
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