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I have finally completed some preliminary tests using various UV/blue blocking filters in combination with my TEC140/Trius combination. It appears I need to do a bit more as the data I have so far is not conclusive. Perhaps it will be of interest to others. And, certainly I'm open to suggestions for improvement of these experiments. There are so many variables to consider.
I have 3 filters to consider:
1. Shottt GG420
2. Shott GG435
3. Baader Fringe Killer
I previously tested the IDAS LP filter which gave some improvement in blue resolution and fringing (cuts around 410 nm). The Baader filter has a complex cut (see image below) and the Shotts cut at 420 and 435 nm)
I decided to image NGC1977 (Running Man) with my Trius using the Baader blue filter (which passes down to 380 nm). In front of the SX filter wheel I installed an Orion 4 position 2" wheel containing the 3 filters. I decided on a series of 10 x 2 min exposures for each filter. After collecting that data I had a quick look at the average adu in an area known to be blue. After a bit of trial and error I settled on an exposure of 1.5 min for series of RGB without any UV filter in place. My reasoning (perhaps incorrect) is that a 25% time reduction in blue without a filter would approximately equal the blue transmission with the UV blocking filters in place and lead to a hopefully close to correct colour balance. It appears that I somewhat underestimated the reduction, particularly for the Baader Fringe Killer.
First, a quick comparison of resolution shows that all three filters give a significant visual improvement to raw frames with the Baader doing the best job, though not by much.
I registered all the images at once and then stacked and mean combined for each filter. I then created 4 RGB images, one for each filter and one that used no blocking filter. The results were quite surprising as I fully expected to see a difference in resolution given the bloated appearance of the blue channel in the sub frames without the blocking filter. I cannot detect a difference in the final result in this experiment. I even went back and only registered the blue results that used no blocking filter. Again I saw no difference visually though the FHWM reported by CCDStack was about 10% worse than when I registered all the images at the same time. In any case. with this particular imaging target, it is difficult to conclude that I should be using any blocking filter at all even though with a different target I struggled with quite a lot of blue fringing on medium to bright stars. This time I simply don't see the fringing. But, I do see quite a lot of reflection halos in the sub frams as soon as I use the blocking filters. Again the halo effect goes away in the RGB combine, I guess, because there are no halos in G and R to reinforce. It makes me wonder if I saw the blue fringing in my earlier experiments only because I had not removed the blocking filters shooting R and G and consequently all the channels had refection issues.
I exported the RGB images from CCDStack after slight background adjustments to eliminate blue clipping that was showing with CCDStack background left at default. The images from CCDStack are what comes out of the default settings and no colour adjustment was made to the background. I only adjusted the background to eliminate clipping when exporting to Photoshop. I used Photoshop CS5 to align and balance black points of RGB. By far - to my eyes - the RGB image with no blocking filter looks the best in terms of blue (these were all 1.5 min subframes). As the strength of the blocking filter increases I see progressively more and more green caste which decreases somewhat when the RGB black points are aligned. However, even after alignment the blocking filter images do not look as nice in blue as the image using no filter. I have gone back and measured adu in the images and conclude that perhaps the Baader image should have been 132 seconds rather than 120 seconds. The GG filters exposure times needs perhaps only a bit more time. Here I am assuming a linear CCD response. In any case it looks with the use of these filters needs at least ca 30% increase in exposure time and the Baader perhaps 40% or more. In the next round of experiments I will increase the exposure time in blue to the calculated amount but also do a series with perhaps double the amount of time. It would be interesting to see if any amount of extra blue exposure time would result in as good an image as seen without the blocking filters.
Comments are most welcome!
Peter
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