I re-did the blue channel in 3-minute subs, getting 27 of them before the target sank below my hedge. This larger number of subs reduced the noise in the channel quite a lot (although it is still a bit noisy).
Also reprocessed more carefully, especially by taking the individual colour channels into Photoshop and removing the skyglow gradient from each (using Gradient Xterminator).
This is starting to look good; I’m pleased with the direction. If there are more chances this season, I’ll add more subs in all the channels.
I have gathered at least some data now for 3 colour channels and luminance, and did a first combination tonight. It needs more colour data to improve the signal-to-noise ratio, especially in the Blue channel which was taken through light cloud and is very noisy. However, it’s still quite pretty and I’m pleased with the result so far.
This would be 90 minutes of Luminance, and 60 each of Red, Green, and Blue binned 2×2. QSI583 camera on an AT8RC, autoguided with a Lodestar, all on a Losmandy G11.
I’ve started a multi-night project for M51. July 28 was the first chance to catch Luminance data, 1.5 hours so far (in 5-minute subs). Sky-glow is fairly bad in this part of the sky (removed manually in the attached shot), and will require some careful processing as I get more data.
A satellite trail wasn’t completely removed via the median-combine. Once I get more exposure time, I’ll probably just discard the frame with the satellite.
Update: Problem in Colour Darks
I’ve had a couple of evenings chance now to gather colour data, and have captured R, G, and B, all binned 2×2, on separate nights. I was quite disappointed with the results – all are showing nasty black blotches. Looking for the cause of this, I believe my 2×2 calibration dark frames are faulty (I keep a library of darks at different reference temperatures and different exposure times, and think my 3-minute, -15 degree darks are bad).
It’s supposed to be cloudy for a few days now, so this will be a good chance to take new darks, then test those against the acquired data to see if that was the problem.
After a discouraging evening the other night, trying to do too many things at the same time, last night was more organized and more successful.
I took the SCT back to pure visual configuration and did a careful collimation using an out-of-focus star. More tuning later, using software help, but for now the collimation is pretty good.
Next, I tried a more methodical set of experiments with focal reducers. I couldn’t get the OpTec 0.5x reducer to work in any configuration that occured to me – only in a couple could I bring it to focus, and quality wasn’t good. I’ll try it more later but, for last night, I switched to the Celestron 0.63x reducer. It needs about 100mm distance to CCD so I had to mount the reducer outside the Crayford focuser, which meant a couple of threaded adaptors. That worked, and I was able to take test images and achieve focus. The unusual placement of the reducer means I’m not getting 0.63x reduction or f/6 results, but I haven’t done the math yet to figure out what reduction is happening. (Update: doing a plate solve on an image, this setup is yielding an effective reduction of 0.68x, for a resolution of 0.83 arc seconds per pixel.)
I took an unguided test image of M51 to get a sense of what this resolution will do. It’s quite a pleasing image scale – once guiding is working it should be good. I was surprised at the rather steep image vignetting that was happening in this setup – substantial flat correction was needed.