A combined image of M8. 24 X 5 min exposures stacked and calibrated in IP3. Not sure about this Ha stuff but it looks like I have quite a way to go with it. CS3 stuffed me up in curves with the grey scale image. All upside down quite unusual to me. The image was shot tonight under the full moon so the result is interesting.
Nice shot doug. Did you take that with the QHY8? Did you bin 2x2 or debayer the red channel only? I'm still waiting for my Ha 2" baader filter
Yes shot with the QHY8 Binned 2X2. I* have done a few leading up to this using 1 X 1 and debayering but think this is a better result.
Mike,
Lack of sleep. What is sleep, after 20 years of 12 hr shiftwork I had forgotten all about sleep. Age is now getting the better of me and I am starting to remember.
I was not going to post this as my alignment was WAY out but thought I might share it as I have just started my Ha learning too.
For comparison this was a few nights back so the moon was 3/4 ish. This was 6*4 min shots through the 127mm APO fitted with a 0.8x FR (image scale 1.73" per pixel) 1.25" Astronomic Ha (13nm passband I think) and the Opticstar 145m camera.
Processing was levels and a little sharpening in aa4, a better result would result from better alignment I am sure.
Looks good Doug... I only had time to run one sub of M42 in Ha the other night before intermittent clouds got the better of me.. Bin 1x1 it came out looking very weird, converted to greyscale it wasn't so bad, but still not great...
Nice one. Good to see you got that filter. Did you get the 13nm Astronomik? Welcome to the brave new world of combining Ha with RGB when you get round to it. No sleep by night or day!
for the guys using QHY8's i remember seeing somewhere that you should re-adjust your offset/gain when you put a Ha filter in otherwise you will have to expose forever to get any real signal.
just make sure the other settings are as you would have them when taking a light frame (i had the ccd bin2x2 and had spent a while getting it dialled in when i realised )
Is there a way to optimise the Gain/Offset settings for a particular CCD - the linked article addresses offset by taking bias shots and looking for the singnal "floor" if I am reading it correctly but the English is not crash hot. There is no mention of GAIN settings at all. In the case of the 145m I have set the gain to 400 (there are 1024 levels available). I did this based on the two attached graphs which show me S/N ratio increases with gain to at least that level.
I would love to confirm my interpretation, what is the best setting to use? I guess I will be trading some dynamic range for sensitivity if I go too far? What is a good rule of thumb 3/4 of max? Half?
Should I set it perhaps based on the sky glow level - eg set an exposure length I can track well to routinely like 240s and then adjust the gain to get the sky glow histogram showing nicely and then set the relevant offset?
Is there a way to optimise the Gain/Offset settings for a particular CCD - the linked article addresses offset by taking bias shots and looking for the singnal "floor" if I am reading it correctly but the English is not crash hot. There is no mention of GAIN settings at all. In the case of the 145m I have set the gain to 400 (there are 1024 levels available). I did this based on the two attached graphs which show me S/N ratio increases with gain to at least that level.
I would love to confirm my interpretation, what is the best setting to use? I guess I will be trading some dynamic range for sensitivity if I go too far? What is a good rule of thumb 3/4 of max? Half?
Should I set it perhaps based on the sky glow level - eg set an exposure length I can track well to routinely like 240s and then adjust the gain to get the sky glow histogram showing nicely and then set the relevant offset?
Any advice welcome.
Hi John, for the QHY8 [but I think it can be generalised to any CCD cam] I use the minimum gain 0 or 1. I set the offset to have a sky ADU [pedestal] between 500 and 1000. The higher the offset the higher the pedestal. The higher the gain the quicker you'll white clip the right end of the histogram.
It will depend on CCD chips/Camera , sky conditions, temperature, DSO you're imaging, etc... The bottom line is to maximise the amount of data between the left and right of the histogram to get the maximum range. My QHY8 will saturate real quick on bright stars at gain 0/exp 15min and over so I never had to push the gain settings. Works for me so far.
Agreed.. I run gain at 0 or 1 most of the time... the first few times I used the QHY8 I had it at 50 or 70 thinking more gain would compensate for short exposures.... Well, it clipped bright stars, and resulting images were a little noisy. Since going to 0 - 1 noise is non existent, and stars are a little easier to control...
Hmm, what range is there on gain setting of a QHY8 is it 0-100? Surely it is worth lifting it a little way above minimum to get better sensitivity and a corresponding reduction in required exposure times?
Setting gain/exposure to avoid clipping of bright stars will not give the best result for the fainter objects will it? Multiple exposures seem to be the way to go to handle high dynamic range?
Is this gain setting not the analog of ISO on a DSLR? If yes most would not shoot at ISO100 (I know I don't go below 400 for Astro shots) but shoot at 4,8 or even 1600 to keep the exposure times down.
Perhaps this is not a simple issue and depends on the CCD hardware and software to a high degree so I will have to do more experiments?
Surely there is a better way to set about this than trial and error?
gain is just your 'iso' setting, not really increasing sensitivity but amplifying the signal you are already getting (noise and all)
i dont think it would be ideal to have it set as low as 0 but i can see why people would try that. from what ive seen with a quick play around a few nights ago, if your gain is low your offset will have to be higher to get within the 400 - 1000 range.
from what i remember my ETA Carina shot was taken with a gain of 20% and an offset of 116, i still have the bias frames and a quick check showed that my max pixel value for one of those was ~900.
the point i mentioned that is because my carina shot looks fairly clean, not a lot of noise but as soon as i had finished shooting the subs for that i also took a few of triffid (90 mins worth). the triffid frame was relatively empty though compared to the carina one and the background was pretty grainy after a hard stretch so im not sure there is an ideal setting for the camera to be on 100% of the time, if the frame is going to be full of nebulosity it might be better to boost the gain a little because you can get away with it but for smaller objects you will probably just get a noisy background
so look out or you will go 
from lack of sleep 
