A pretty good first result Frank. Possibly longer subs would extract more of the fainter detail and of course a very much shorter set to use and blend in the core area later will fix the blown core. Guiding looks OK too
If I showed you my first go at M42, you'de think effort was fantastic.
G'Day Frank, Great start to imaging with a QHY9. I have attached a section from the nebulosity user manual off the Stark Labs Nebulosity Manual. It explains the method of setting and forgetting your gain and offset figures. Well worth the read. For interest sake I run my QHY9 at Gain of 12 and Offset of 110. Somewher to start I guess.
Good luck with your new gear.
Cheers
Doug
5.4.4 How should I set my gain and offset to set it and forget it?
The best value for your camera may not be the best value for other cameras. In particular, different makers set things up differently. For example, on a Meade DSI III that I recently tested, running the gain full-out at 100% let it just hit full well at 65,535 ADU. Running below 100%
and it hit full-well at 40,000 or 30,000, or 10,000 ADU. There's no point in running this camera at anything less than 100% gain. On a CCD Labs Q8-HR I have, even at gains of 0 and 1 (on its 0-63 scale), the camera would hit 65535 on bright objects (like the ceiling above my desk).
There's no point in running this camera at gains higher than 0 or 1. Why is there no point? The camera only holds 25k e-. If a gain of 0 or 1 gets me to 0.38 e-/ADU (so that those 25k e- become 65535), running at 0.1 e-/ADU will only serve to limit my dynamic range. Each single electron already comes out to more than 2 ADU.
So, to determine the gain and offset to use:
1) Take a bias frame and look for the minimum value in it. Is it at least, say 100 and less than a thousand or a few thousand? If so, your offset is fine. If it's too low, boost the offset. If it's high,drop it. Repeat until you have a bias frame with an offset in, roughly 100 - 1000. Don't worry about precision here as it won't matter at all in the end. You now know your offset. Set it and forget it. Never change it.
2) Aim the camera at something bright or just put it on your desk with no lens or lenscap on and take a picture. Look at the max value in the image. Is it well below 65k? If so, boost the gain. Is it at 65k? If so drop the gain. Now, if you're on a real target (daylight ones are great for this) you can look at the histogram and see the bunching up at the top end as the camera is hitting fullwell.
Having that bunch-up roughly at 65,535 plus or minus a bit is where you want to be. If you pull up just shy, you'll get the "most out of your chip" but you'll also have non-linearity up there.
You've got more of a chance of having odd color casts on saturated areas, for example, as a result. If you let that just clip off, you've lost a touch but what you've lost is very non-linear data anyway (all this assumes, BTW, an ABG chip which all of these cams in question are). Record that gain and set it and forget it. Never change it.
By doing this simple, daytime, two-step process you've set things up perfectly. You'll be sure to never hit the evil of zero and you'll be making your chip's dynamic range fit best into the 16-bits of your ADC. Again, all the cameras in question have full-well capacities below 65,535 so you are sure to have enough ADUs to fit every electron you record into its own intensity value.
Oh My Gosh! Craps all over my first attempt! You can tell that the detail is there! Well done mate.. Soft softly stretching in post processing also helps.
Last edited by CoolhandJo; 19-04-2010 at 06:33 PM.
Reason: spelling
Dave, I tried the 10"RC first up but had trouble dealing with the field of view and the long focal length when trying to find objects and be sure of focus but I am going to work towards it as it is my favoured scope. I found the ED127 easier to start with.
5.4.4 How should I set my gain and offset to set it and forget it?
Thanks for that advice Doug - it is a subject I have not been able to get a straight answer on before so I have used trial and error till now...if I may I have a couple of questions...I think I get the offset routine but for GAIN I am not sure.
With my CCD I can see a relationship between GAIN and NOISE - thus more gain means more (thermal) noise for a given exposure time. Now the signal and noise rates do not increase at the same rate with gain meaning a gain increase may double signal strength but only increase noise, say, 10% thus you have better S/N ratio which is good. Go too far and the S/N ration will worsen.
So S/N ratio may set an upper limit on gain.
If it does not (you have little or no thermal noise) then increasing GAIN too far will eventually lead you to loss of bandwidth as more and more of your image is saturated (clipping).
So what is the best gain setting to use? If I try to use your suggestion and use my camera in daylight it will saturate in no time - for example to expose my flats I used an exposure of 0.05s - this gave a max signal of 16,928 average 15,158 and min of 128, in a 4 min star shot I have a max value of 65,520 but average 15,120 and min of 6,864.
So I think my offset is ok but I am not sure about the gain. Based on visual inspection of the attached charts I think I am best off in the 200-400 range.
Any comments?
Frank sorry to divert the thread....
It is a great first LRGB I am on this learning curve with a 127mm also (not the same CCD though). Tell me did you find any significant differences in the star sizes between RG and B frames? I find my B images are soft compared to the others it may be my filters though - I use the Astronomik LRGB set.
5.4.4 How should I set my gain and offset to set it and forget it?
...
2) Aim the camera at something bright or just put it on your desk with no lens or lenscap on and take a picture. ...
This is the bit I have a problem with. Nowhere does it say how long this exposure should be. Should it be 0.0001 secs like a bias or something longer.
I find that with the standard filters supplied with the camera that the B stars are bloated compared to the RG. I don't know how to fix this but tried to sharpen the B stars in PS but it gave the whole image a strange look.
I might look for a new set of filters but the standards are OK to learn on.
I set my gain on a star field and measure the max figure at the computer to ensure it is below 65535. I use an exposure of 10 minutes as this is the most widely used exposure by me.
Thanks for that advice Doug - it is a subject I have not been able to get a straight answer on before so I have used trial and error till now...if I may I have a couple of questions...I think I get the offset routine but for GAIN I am not sure.
With my CCD I can see a relationship between GAIN and NOISE - thus more gain means more (thermal) noise for a given exposure time. Now the signal and noise rates do not increase at the same rate with gain meaning a gain increase may double signal strength but only increase noise, say, 10% thus you have better S/N ratio which is good. Go too far and the S/N ration will worsen.
So S/N ratio may set an upper limit on gain.
If it does not (you have little or no thermal noise) then increasing GAIN too far will eventually lead you to loss of bandwidth as more and more of your image is saturated (clipping).
So what is the best gain setting to use? If I try to use your suggestion and use my camera in daylight it will saturate in no time - for example to expose my flats I used an exposure of 0.05s - this gave a max signal of 16,928 average 15,158 and min of 128, in a 4 min star shot I have a max value of 65,520 but average 15,120 and min of 6,864.
So I think my offset is ok but I am not sure about the gain. Based on visual inspection of the attached charts I think I am best off in the 200-400 range.
Any comments?
Frank sorry to divert the thread....
It is a great first LRGB I am on this learning curve with a 127mm also (not the same CCD though). Tell me did you find any significant differences in the star sizes between RG and B frames? I find my B images are soft compared to the others it may be my filters though - I use the Astronomik LRGB set.
The gain settings etc should not be changed relating to noise etc.
The AD converter on you camera is 16 big meaning that you can output 0 -65000 (aprox) different levels per pixel. I don't know the full well capacity for the CCD but it is measured in electrons. The gain is simply the number of electrons that are added together to get an output by the camera of a 0-65000 level to read into your picture.
If the full well capacity is 100000 then a gain of ~1.3 will use the full depth of the CCD and you get useful data in the entire range of the output.
As stated in Dougs post you need to first set an offset in the dark so that there is a base of about 100 in the image. This is just to prevent negative pixels.
Then take exposures of anything bright trying to completely saturate the CCD with any exposure time. The gain then needs to be adjusted to so that the totally saturated exposures read about 65000. Keep reducing the gain until you start to get readings just below this value.
This tells you that you are using the full wells of the camera.
Once you know this valie don't change it.
When you image a dim object you will end up with low values but dont increase the gain. The images are easier to stretch when you have 65000 levels to play with rather than less than that.
The camera (Opticstar 145m) has a Sony EX HAD chip in it the ICX285AL to be specific, this does not have a published well depth that I can find but estimates are in the 27,000 to 30,000 range.
So I want 30,000 electrons to give me 65,000 levels - sounds like my gain should be set to 2, or is it not tahat simple?
I do not get why this advice would work :-
'Then take exposures of anything bright trying to completely saturate the CCD with any exposure time. The gain then needs to be adjusted to so that the totally saturated exposures read about 65000. Keep reducing the gain until you start to get readings just below this value."
Surely the exposure time is relevant? Say I expose for 5s and 1s and both images have saturated the target then I choose to set the gain based on the 5s exposure I may have to wind it to zero to get max values less than 65000 whereas the 1s exposure may need the gain at 10 to get max vals of 65000.
Oh well I am going to try a run of images with low gain and see if it is better or worse than my normal setting...for now I am confused.
The camera (Opticstar 145m) has a Sony EX HAD chip in it the ICX285AL to be specific, this does not have a published well depth that I can find but estimates are in the 27,000 to 30,000 range.
So I want 30,000 electrons to give me 65,000 levels - sounds like my gain should be set to 2, or is it not tahat simple?
I do not get why this advice would work :-
'Then take exposures of anything bright trying to completely saturate the CCD with any exposure time. The gain then needs to be adjusted to so that the totally saturated exposures read about 65000. Keep reducing the gain until you start to get readings just below this value."
Surely the exposure time is relevant? Say I expose for 5s and 1s and both images have saturated the target then I choose to set the gain based on the 5s exposure I may have to wind it to zero to get max values less than 65000 whereas the 1s exposure may need the gain at 10 to get max vals of 65000.
Oh well I am going to try a run of images with low gain and see if it is better or worse than my normal setting...for now I am confused.
Yes the gain should be about 2.
What I mean by the exposure time is only for when testing the camera.
When you are imaging the total value that the brightest pixel will achieve will be determined by the brighness of the object and the length of the exposure.
If in a particular image of 10 secs you brightest object only gives a max of 10000 this is OK. If you take 60 secs the image will saturate (60000). The dim parts of the image will be only a few counts above the background (of hopefully ~100).
You could turn the gain up but this will also increase the noise. It is better to leave the gain as it was and stretch the final image. By turning up the gain to make a dim object brighter reduces the dynamic range possible in the image. If you forget to turn it down and take a longer exposure you clip all the brighter parts of the image (read stars vs nebula etc).
My interest is in photometry and when taking photometry images you want to prevent the image saturating to keep the image in the linear part of the CCD. I also want a reasonable count to improve the S/N ratio. The only way around this is the increase the exposure not to turn up the gain.
