ZWO ASI294MC Pro - Bad calibration frames

[vlaiv]

36 minutes ago, Dan13 said:

!. can bumping the offset up cause extra unwanted noise

It should create less noise, or rather - noise should be better looking. It should not affect levels of noise - just "shape" of it.

36 minutes ago, Dan13 said:

2.what do you mean by making sure the "gain value" is ok?

You'll have to provide a bit more context. I have no idea when I said that, or what in relation to. Can you point me to exact sentence?

[Dan13]

2 hours ago, vlaiv said:

It should create less noise, or rather - noise should be better looking. It should not affect levels of noise - just "shape" of it.

You'll have to provide a bit more context. I have no idea when I said that, or what in relation to. Can you point me to exact sentence?

Hi Vlaiv,

3. I would just recommend you in the future to use Gain 120, to turn off AWB and check if offset has good value - and continue taking nice images 

this was your point, 

mmm i really not sure whats gone wrong last night but im really not happy with the frames for the cam i have, i used to achieve better with a dslr...

[vlaiv]

3 hours ago, Dan13 said:

Hi Vlaiv,

3. I would just recommend you in the future to use Gain 120, to turn off AWB and check if offset has good value - and continue taking nice images 

this was your point, 

mmm i really not sure whats gone wrong last night but im really not happy with the frames for the cam i have, i used to achieve better with a dslr...

Yes, I see, I did not mention "gain value" being ok, but rather offset value being good.

Good offset value for a given gain is one that has no pixels clipped to the left - no pixels have minimum value that camera produces (because if it is minimum value, there is no way of telling if it is really minimum value or just clipped to minimum value).

If you read from the beginning of the thread - you'll find instructions on how to check this.

Don't use unity gain with this camera either - it has very large read noise for CMOS camera. Look at this diagram:

Unity gain, being gain 117 is still in large read noise mode. This seems to drop to nice levels at around gain 120 - that is why I recommend gain 120 for this camera.

This could have been the problem that produced noisy result if you used unity gain - 117. Other possible causes would be - low transparency / higher than usual light pollution. Lack of astronomical darkness maybe? Was there moon out?

[Dan13]

7 hours ago, vlaiv said:

Yes, I see, I did not mention "gain value" being ok, but rather offset value being good.

Good offset value for a given gain is one that has no pixels clipped to the left - no pixels have minimum value that camera produces (because if it is minimum value, there is no way of telling if it is really minimum value or just clipped to minimum value).

If you read from the beginning of the thread - you'll find instructions on how to check this.

Don't use unity gain with this camera either - it has very large read noise for CMOS camera. Look at this diagram:

Unity gain, being gain 117 is still in large read noise mode. This seems to drop to nice levels at around gain 120 - that is why I recommend gain 120 for this camera.

This could have been the problem that produced noisy result if you used unity gain - 117. Other possible causes would be - low transparency / higher than usual light pollution. Lack of astronomical darkness maybe? Was there moon out?

Thank you. I ran a sensor analysis and it suggested 118 was best for gain. Maybe I'll try 120 or some even use 129/139.

Moon wasnt out till later but I had an L enhanced filter in anyways. Im going to try 120 and adjust offset accordingly...

 

How far away should you keep the histogram from clipping to the right? I think I might of been bit to hasty and could of probably used offset of 15 

[vlaiv]

1 hour ago, Dan13 said:

Thank you. I ran a sensor analysis and it suggested 118 was best for gain. Maybe I'll try 120 or some even use 129/139.

Moon wasnt out till later but I had an L enhanced filter in anyways. Im going to try 120 and adjust offset accordingly...

 

How far away should you keep the histogram from clipping to the right? I think I might of been bit to hasty and could of probably used offset of 15 

Clipping to the right is not the problem, or at least, not the problem that can't be solved easily. If you have clipping to the right, which means that some parts of the image are saturating (like star cores - they almost always saturate, but sometimes even very bright nebula / galaxy parts) then take just a few "filler" short exposures at the end. Something like 10-15s will do - short enough so that even bright stars don't saturate.

Just a few is enough because you'll be only using very high signal parts of these short subs - which means SNR will be good as is.

Stack both sets of subs to their respective stacks and then replace saturated parts of regular stack with short one (make sure you do scaling right if you replace while still linear).

[Dan13]

15 minutes ago, vlaiv said:

Clipping to the right is not the problem, or at least, not the problem that can't be solved easily. If you have clipping to the right, which means that some parts of the image are saturating (like star cores - they almost always saturate, but sometimes even very bright nebula / galaxy parts) then take just a few "filler" short exposures at the end. Something like 10-15s will do - short enough so that even bright stars don't saturate.

Just a few is enough because you'll be only using very high signal parts of these short subs - which means SNR will be good as is.

Stack both sets of subs to their respective stacks and then replace saturated parts of regular stack with short one (make sure you do scaling right if you replace while still linear).

Thanks Vlaiv, i guess im just starting to really understand the histogram and wanted to check im doing things right. for instance...

I took a test shot and my histogram was clipped to the right so i moved up to offset 30 and it moved the hist quite far in but looked ok. ive drawn a pic (dont laugh!) should i aim for the histogram here??

[vlaiv]

3 minutes ago, Dan13 said:

Thanks Vlaiv, i guess im just starting to really understand the histogram and wanted to check im doing things right. for instance...

I took a test shot and my histogram was clipped to the right so i moved up to offset 30 and it moved the hist quite far in but looked ok. ive drawn a pic (dont laugh!) should i aim for the histogram here??

Both yes and no.

Let me explain. In principle you should aim for histogram being the furthest left without being clipped - but this is important only if you are doing single exposure. It increases dynamic range of a single sub.

Since we are using stacking and multiple exposures - that in itself increases dynamic range. We can also use above trick to deal with any saturation. For that reason - putting offset a little bit higher than optimum value - does not really matter that much.

I use offset 64 with my camera, while most people use offset 50. Not much difference really.

In fact, there is much more difference in using 14bit over 12bit and 16bit over 14bit (x4 in histogram space to the right and here you only subtract something like 1% by putting histogram a bit more to the right) - but again, people are happily using 12bit cameras to produce excellent images.

[Dan13]

2 minutes ago, vlaiv said:

Both yes and no.

Let me explain. In principle you should aim for histogram being the furthest left without being clipped - but this is important only if you are doing single exposure. It increases dynamic range of a single sub.

Since we are using stacking and multiple exposures - that in itself increases dynamic range. We can also use above trick to deal with any saturation. For that reason - putting offset a little bit higher than optimum value - does not really matter that much.

I use offset 64 with my camera, while most people use offset 50. Not much difference really.

In fact, there is much more difference in using 14bit over 12bit and 16bit over 14bit (x4 in histogram space to the right and here you only subtract something like 1% by putting histogram a bit more to the right) - but again, people are happily using 12bit cameras to produce excellent images.

ok thank you, maybe ill tone it down a little. im just really not happy with the final stack. it doesn't look clean to me and looks messy! not the sort of image im happy with.

I had some guiding issues that night so maybe effected some resolution?! could i send you the final stack to look at and see what your thoughts are?

[vlaiv]

17 minutes ago, Dan13 said:

ok thank you, maybe ill tone it down a little. im just really not happy with the final stack. it doesn't look clean to me and looks messy! not the sort of image im happy with.

I had some guiding issues that night so maybe effected some resolution?! could i send you the final stack to look at and see what your thoughts are?

Sure