Atik 314E CCD (Read Noise 5.3e-, Full Well Depth 13400e-)
Optolong LRGB filters
Over five sessions:
L: 250x90s bin1, 6.25 hours, SNR 17.7
R: 90x85.3s bin2, 2.13 hours, SNR 10.7
G: 90x50.2s bin2, 1.26 hours, SNR 10.6
B: 90x77.8s bin2, 1.95 hours, SNR 10.5
Total Integration time: 11.59 hours
Total target SNR: 25.6
- M81_L250x90s_R90x85.3s_G90x50.2s_B90x77.8s_Rx0.83_Gx0.90_Bx1.00_S15-3-25_SA25-25_G92_SGB2_SH70_2020-02-24.jpg (378.75 KiB) Viewed 1982 times
After capturing 11.6 hours in LRGB I went to work processing the data in AstroPixelProcessor (APP). The result was underwhelming. All regions of the galaxy were predominantly yellow/red with just a hint of the outer arms wanting to go blue. It was a mystery since I've imaged plenty of targets with very good results. What was different this time? Two big differences: I never image facing north to north-north-east, and I never image so soon after astronomical twilight.
What follows is an interesting topic but after some more thought I believe it fails to explain this anomaly. Be sure to read my post at the top of the second page which proposes a better explanation IMHO.
Astronomical twilight ends at 7:15pm this time of year so I started at 7:30pm and ended by 10:30pm before M81 became "meridian-challenged". East of the meridian I always start with the red filter, then the green, and then blue, due to atmospheric extinction. So I imaged in red from 7:30pm to 8:30pm.
What is a common human activity at this time of night? That's right, commuting and/or going out-and-about in your car. My house is above the road but as cars approach their headlights briefly reflect off of a large tree just to the right of where I imaged M81. My guess is that particles in the air, most likely water vapor, reflected some of that light down my telescope tube, onto my sensor.
The next likely culprit is 2 miles away where there is an athletic field illuminated by stadium lights. That time of night I know that the lights are on but after some time the authorities turn off the lights because there are homes (i.e. taxpayers) nearby. The lights were probably shut down shortly after capturing reds.
The bottom line is that my reds contain extra signal from those pollution sources. I could see it. The sky background of a red frame was running around 1100 ADU. Compare that to a blue frame at 900 ADU. Normally they are the same.
I used a trick described by Mabula Haverkamp to discover the color content of an image: turn on saturation, set the saturation amount to maximum, and the saturation threshold to zero. There I could see blue in the arms.
I stepped back a step to the "Combine RGB" tool. My first attempt was to simply boost the blue channel. Unfortunately that result was not pleasing. The second attempt was to keep blue at unity but attenuate red. That was the secret. It was a tad bit green so I attenuated it slightly.
The attached image is the result of that small effort. Normally I don't have to do it since the "white balancing" of my filters takes care of it.
According to a brief survey I took of Top Picks and Image Of The Day (IOTD) winners, winning images have something in common: SNR > 25.
For all intents and purposes my image is competitive with regards to SNR. Where I fail are at the following:
1. The telescope is not an APO. It does not focus blue light very well which explains the blue fringes you see around my stars.
2. My small Full Well Depth easily saturates bright stars in the field. Those are the big fat discs that you see. The only way to combat saturation is to lower exposure time of an individual frame. The unfortunate consequence is that it increases total integration time.
My Bortle 5 skies and f/5.9 scope are the cause of these long integration times. I should make an effort at finding a Bortle 4 travel site.
All comments welcome!
Thanks,
Brian
