With all the lousy weather in Sydney lately, I have been collecting data from the iTelescope (previously GRAS) service from New Mexico. Its a great system and lets me image in daylight (from Sydney anyway).
I have been collecting LRGB data with a Tak FSQ106 - all very nice.
However, after aligning and combining with Maxim, I find stray coloured pixels. In a dark field, there will be a single red or green pixel (rarely blue for some reason).
The frames are pre-calibrated. iTelescope also supplies the calibration frames and I had a go at calibrating the lights myself - same result.
Clearly there are a few pixels that show up as white in a R or G frame and not in any others. When combined and stretched I have a nasty brightly coloured pixel.
I suspect it is a function of my combine process. I use the Sigma Clip option with 3 passes and a sigma factor of 3.00. I have Ignore Black Pixels and Ignore pixels over 64,999 checked.
I've spent some time playing with this today. I can reduce the impact of the stray coloured pixels when combining using the HST option in Combine and setting the Luminence factor at 100%.
However, there seems to be a consistent mismatch between the R and G/B frames when combining. This results in heaps of colour noise. Have a look at the screenshot attached. This is after DD in Maxim and then a manual stretch in Photoshop. The red frames don't line up with the G/B (which appear aqua) - they seem shifted down and slightly to the right. This is odd as it appears in both Astrometric and Auto-Correlation alignment modes. More oddly (to me anyway) is that the stars display no shadowing - ie there is no difference in colour in the star images themselves.
This image comprises about 1 hour of L and 50 minutes each of R, G and B.
So what am I missing? Is it a processing issue or a combine issue?
It looks like you have the answer already but I have an unrelated suggestions.
If you are interested in continuing using these type of services you may want to check out Lightbuckets. They have gone through a bit of an upheaval recently but I found them a lot easier to deal with and more responsive when there is a problem. As an added advantage they are cheaper and you do no have to be on a plan. For example they have a CDK 17 available at $55 per hour. The same scope/camera on GRAS/iTelescope setup is $145 on the lowest plan.
This is the first I have heard of GRAS changing their name. I wonder is there is a law suit on the horizon from a certain phone maker....lol
Another way around this problem is to use the "reject hot/cold pixels" command in CCDStack on each of your subs, then you have to interpolate the rejected pixels (0.2, 3 iterations). I find this cleans up most of the strays, then a little PS work later fixed the few that survive.
Another way around this problem is to use the "reject hot/cold pixels" command in CCDStack on each of your subs, then you have to interpolate the rejected pixels (0.2, 3 iterations). I find this cleans up most of the strays, then a little PS work later fixed the few that survive.
Cheers
Stuart
Cheers Stuart
I haven't used CCD Stack before - it will be worth it if I can save the data I have.
This looks like poor calibration to me. LB maybe need to sort out their calibration files. I sometimes get this every 6 months or so when calibration files need to be redone.
This looks like poor calibration to me. LB maybe need to sort out their calibration files. I sometimes get this every 6 months or so when calibration files need to be redone.
Thanks paul
I've sent a query off - will let you know what the response is.
Like Stuart says, if the problem pixels are relatively bright, you may be able to get good software to pull them out based on brightness and size criteria. (Pixinsight has a nice function for this that does a reasonable job).
At first glance it looks a bit like the images are not aligned properly.
Median combine is the best method for combine as it will get rid of many artifacts.
Sigma clip may be fine if you have enough subs for the maths to work. It assumes quite a few subs being worked over.
Greg.
Greg
the scope was an FSQ106 and CCD was a SBIG STL11000M.
The odd thing is that the stars are well aligned - I tried both Astrometric and Star Matching in Maxim - it doesn't show up well in the screenshots I posted but the stars are fine, its the imperfections in the sensor response to the R vs G/B frames that don't match. I also tried combining with median, sum and average for the same result.
Word from iTelescope is that the darks may be as old as October - they plan on redoping them and having another crack at calibration with new master darks. I'm hoping that sorts it.
I'd be interested in finding out how it goes with the new darks. I wouldn't've thought darks from October particularly old. CCDs age but not that quickly.