Another Offset Question

Discussion of using SharpCap for Deep Sky Imaging
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turfpit
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Re: Another Offset Question

Post by turfpit » Tue Dec 25, 2018 10:12 am

Thanks for that Robin - you did far more than you should have given your current medical condition.

Steve - in File > SharpCap Settings > General Tab, what is the state of Save 10/12/14 bit images in FITS files without stretching to 16-bit ?

If this is checked, then it needs unchecking for normal imaging use.

This will keep until you emerge from the fog of turkey and mince pies ;)

Dave

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Re: Another Offset Question

Post by SteveJP » Thu Dec 27, 2018 1:59 am

Hi Robin,
I'm still working though your very comprehensive answer. I'll reply properly soon.
Steve'

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Re: Another Offset Question

Post by SteveJP » Thu Dec 27, 2018 2:41 am

turfpit wrote:
Tue Dec 25, 2018 10:12 am
Thanks for that Robin - you did far more than you should have given your current medical condition.

Steve - in File > SharpCap Settings > General Tab, what is the state of Save 10/12/14 bit images in FITS files without stretching to 16-bit ?

If this is checked, then it needs unchecking for normal imaging use.

This will keep until you emerge from the fog of turkey and mince pies ;)

Dave
Hi Dave,

Well, as it happens, I do have that option checked since I thought it would be best to get a faithful representation of what the camera sees into my post processing. The effect is surely just a multiplication by 4? The non-stretched image is surely darker, but I think this just means the stretch needs to be harder which is probably just the same as the multiplication anyway. Working in PixInsight as I do, even in the linear processing stage, a default stretch is applied. If I import the 14 bit and 16 bit versions of the same thing into Pixinsight, the 14 bit image is dimmer of course, but as soon as the default stretch is applied, I can't tell them apart. Just wondering why you think it's important?

I note too, the histogram produced by SharpCap (which my Excel graphs come from) is independent of this setting. Ie for my 14 bit camera, the lines in the histogram go from 0 to 2^14 -1 regardless. So when I plot the histogram when SharpCap is set to not stretch to 16 bits, the graph I get from the SharpCap histogram agrees with FITS Liberator, but when I uncheck the option, FITS Liberator shows a multiplication by 4 whereas the SharpCap histogram does not.

I suspect this could be regarded as a bug, since the when the option is unchecked, the SharpCap histogram doesn't match the actual output file.

So much to consider !

Rgds
Steve

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Re: Another Offset Question

Post by SteveJP » Thu Dec 27, 2018 4:22 am

Thanks Robin,

That's a great explanation - I think I really get it now. Being a bit of a geek, I really feel I need to understand the underlying mechanism.
To that end, I've been running a few experiments here.

These are all graphs made from the histogram SharpCap outputs. All are 1ms darks.

The first image shows the effect of just changing the offset (aka brightness). The Gaussian distributions are clearly centred approximately around the offset setting.

The second image shows the effect of gain when the offset is constant. As you explained, the higher gain causes the distribution to spread out. In each case, the area under the curves will be the same, corresponding to my 16,000 pixels.

The third image shows the effect of even greater gain with constant offset. In this case the offset is now clearly insufficient and demonstrates why the offset should be increased with gain.

I suppose you could just set an offset big enough for all circumstances? I'll have to explore the downside of that.

Many thanks
Steve
1ms_G90_Off40+80.PNG
1ms_G90_Off40+80.PNG (42.87 KiB) Viewed 296 times
1ms_G90+150_Off40.PNG
1ms_G90+150_Off40.PNG (39.59 KiB) Viewed 296 times
1ms_G90+250_Off40.PNG
1ms_G90+250_Off40.PNG (39.32 KiB) Viewed 296 times
Last edited by SteveJP on Thu Dec 27, 2018 4:29 am, edited 1 time in total.

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Re: Another Offset Question

Post by SteveJP » Thu Dec 27, 2018 4:32 am

This is the correct third image
1ms_G90+250_Off40X.PNG
1ms_G90+250_Off40X.PNG (39.16 KiB) Viewed 294 times

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Re: Another Offset Question

Post by turfpit » Thu Dec 27, 2018 9:32 am

Steve
Well, as it happens, I do have that option checked since
It needs to be unchecked unless you are doing astrometry. Have a look at this thread
viewtopic.php?t=1108 where I describe the use of the batch processing facility in ImageJ to resolve (thanks to @oopfan for that one - cheers Brian).


Robin - this setting again! A suggestion/request - a lot of time spent problem resolving this one on the forum. Would it be possible to have a means to dump all the Sharpcap Settings into a text file? This can be submitted with along with the forum request and we don't have to waste time working blind trying to solve issues.

Dave

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Re: Another Offset Question

Post by admin » Thu Dec 27, 2018 11:52 am

Hi folks,

the downside to selecting a constant high offset regardless of your gain value is that you are then limiting yourself to a smaller dynamic range in each image you take. For instance if you have a 12 bit ADC then the maximum ADU value is 4095. If you set an offset that puts the black level at a true ADU of perhaps 500 then you only have 3500 or so ADU available between the black point and saturation. If you can get away with setting a lower offset so that the black point is an ADU of maybe 200 then you have gained that little bit of extra dynamic range in each frame that you take.

Dave, all the SharpCap settings are in the registry and can probably be dumped out with the appropriate command prompt Reg.exe call.

Cheers, Robin

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Re: Another Offset Question

Post by turfpit » Thu Dec 27, 2018 1:17 pm

Folks
the downside to selecting a constant high offset regardless of your gain value is that you are then limiting yourself to a smaller dynamic range in each image you take
For clarification, I haven't advocated a constant high offset just a systematic way to find the correct value to give a balanced histogram on a bias frame. See my post above showing CMOS, DSLR and CCD - two of them do it for free, one type has to have user intervention to achieve the same result.

Robin
all the SharpCap settings are in the registry and can probably be dumped out with the appropriate command prompt Reg.exe call
"probably" is not a solution. Dumping out the registry with reg.exe is ok for people like me and you, for the majority of people on the forum this is not a practical thing. Maybe program in an option in SharpCap to dump out the reg settings to a file which can then be posted?

What I am conscious of the is the large amount of time being put in to debugging issues, when, if the information was easily to hand the resolution would be faster and less time consuming. To keep recycling the same stuff again and again is not time productive for anyone.

Dave

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Re: Another Offset Question

Post by SteveJP » Thu Dec 27, 2018 10:22 pm

turfpit wrote:
Thu Dec 27, 2018 9:32 am
Steve
Well, as it happens, I do have that option checked since
It needs to be unchecked unless you are doing astrometry. Have a look at this thread
viewtopic.php?t=1108 where I describe the use of the batch processing facility in ImageJ to resolve (thanks to @oopfan for that one - cheers Brian).


Robin - this setting again! A suggestion/request - a lot of time spent problem resolving this one on the forum. Would it be possible to have a means to dump all the Sharpcap Settings into a text file? This can be submitted with along with the forum request and we don't have to waste time working blind trying to solve issues.

Dave
Hi Dave/Robin,
I did read through that thread, or at least most of it. It seems folks using DSS may have trouble stacking since DSS expects to find stars with higher ADU's. I've had that option ticked for ages and stacking with PixInsight that doesn't seem worried about the values being a 1/4 of what they might otherwise be?
In regards to setting a constant offset, I realise the effect on dynamic range, but if I have a range of 16383 (2^14-1), does it really matter if I need to set an offset of say 50 to cover all circumstances? All depends on what offset you would need and the camera dynamic range.
Steve

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Re: Another Offset Question

Post by turfpit » Thu Dec 27, 2018 10:59 pm

Steve

In that thread, which I was heavily involved in, one poster said:
I unticked the setting in Sharpcap and now DSS finds dozens to hundreds of stars
the other poster said:
not getting enough stars to register in Pixinsight
not enough stars Ransac error in PIxinsight
i do not use DSS 4.1.1
Done!!! is was checked mark
To progress this, as requested previously above, please make available somewhere a bias frame which has the histogram banging into the LHS in FITS Liberator and the corresponding SharpCap histogram CSV file (posted ~Dec 23).

Dave

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