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Old 28-12-2010, 10:47 AM
DJ N
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QHY8L Second Light - This time no moon

Following on from my first light the other night, I decided on a bit of a “proper” imaging run last night seeing the moon was not rising until about 12:30am. I targeted Tarantula, seeing I had imaged that a few nights ago with the QHY8L albeit under 98% illuminated moon, and wanted to compare data with no moon. I ended up with 8 eight minute subs. Upon processing the images, I just cannot bring out the detail. I find that the image really seems clipped, especially when compared to the data imaged under a full moon. Any ideas why this would be the case? The only thing that comes to mind is that I was using the Hutech IDAS LPS P2 filter (as I did under the full moon). Should I be using a light pollution suppression filter with a CCD? I know when I was using my old 400D Canon, I definitely had to use the filter.

As a comparison, attached are the 2 images with a quick process........

Image 1 – Shot under 98% illuminated moon, 3 x 8min subs including bias and dark

Image 2 – Shot with no moon, 8 x 8min subs including bias, dark and flats.

As mentioned, both images were from metropolitan Adelaide with Hutech IDAS LPS P2 filter.

It is looking to be clear tonight again, so I am hoping for another try, however this time I will not use the IDAS filter. Should make for a good comparison.


Cheers,

Daniel
Attached Thumbnails
Click for full-size image (NGC2070 22Dec10 ED120 QHY8L 3x8m Full Moon.jpg)
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Click for full-size image (NGC2070 27Dec10 ED120 QHY8L 8x8min No Moon.jpg)
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Old 28-12-2010, 11:28 AM
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multiweb (Marc)
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That's weird indeed. The second picture is lacking a lot of data given the exposure time. Did you use the exact same gain/offset on the camera between the two sessions? Same capture program? I don't think the filter is the problem. I think some settings must have been different during acquisition. Do you have any raw sub to link to on the second shot? What's the histogram like?
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Old 28-12-2010, 11:53 AM
DJ N
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Quote:
Originally Posted by multiweb View Post
That's weird indeed. The second picture is lacking a lot of data given the exposure time. Did you use the exact same gain/offset on the camera between the two sessions? Same capture program? I don't think the filter is the problem. I think some settings must have been different during acquisition. Do you have any raw sub to link to on the second shot? What's the histogram like?

Hi Marc,

Nothing changed whatsoever. Same gain and offset, same capture program. Comparing the histograms, the one from last night is a lot more "to the left". Not sure how I can copy an image of the histogram.....

Has me a little perplexed. Would not have thought it to be the filter.
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Old 28-12-2010, 11:58 AM
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multiweb (Marc)
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Hi Marc,

Nothing changed whatsoever. Same gain and offset, same capture program. Comparing the histograms, the one from last night is a lot more "to the left". Not sure how I can copy an image of the histogram.....

Has me a little perplexed. Would not have thought it to be the filter.
It's a lot more to the left because your sky count was lower (no moon). The only other thing to look for would be calibration. You did flat fielding on the second set, not on the first one though. Maybe you killed your data and dynamic range by dimming the center (inverse vignetting). Have you tried to stretch the data on uncalibrated subs see if you get he same level of details as the old data set?
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Old 28-12-2010, 12:55 PM
DJ N
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Quote:
Originally Posted by multiweb View Post
It's a lot more to the left because your sky count was lower (no moon). The only other thing to look for would be calibration. You did flat fielding on the second set, not on the first one though. Maybe you killed your data and dynamic range by dimming the center (inverse vignetting). Have you tried to stretch the data on uncalibrated subs see if you get he same level of details as the old data set?
Last night I did reprocess the data without the flats (just in case they were having an impact), and there was no difference with regards to the result. Maybe I inadvertantly changed something. It is looking to be clear again tonight so I will try and get another run in.
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Old 28-12-2010, 01:15 PM
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I just tried stacking only the light frames....... and still the same. Looking at the files from last night, I somehow saved them accidently as xxxxxxxx.fit.fit Maybe this somehow corrupted the information.

Earlier in the night, I did take a shot of M42. I might have a quick look at that one.
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Old 28-12-2010, 01:23 PM
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Compare your ADU values on both of the raw 8 minute subs and see what you get?
Because the moon was gone the skies were darker hence you did not reach your optimum sky background level.

The longest I can go without saturating my background level in adelaide is about 10-15 minutes, where as somewhere like Arkaroola I have to go 45 min or longer depending on the subject.

Your flats are also critical, you have to establish what your optimum background value is based on your imaging scale and your cameras specs. There is a calculator for that on the CCDWare website (Sub Exposure Calculator).
You need to take some test images of an empty part of the sky first with no nebulas or galaxies, then use the values from that image to populate the values.
Flats should be roughly 30% of your Well Depth capacity.

Cheers
Mark
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Old 28-12-2010, 01:33 PM
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Here is another site which actually has your camera as a selection
http://starizona.com/acb/ccd/calc_ideal.aspx
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