Jupiter and Saturn First Attempt

[Gerr]

Clear skies recently allowed me to try out my new ZWO ASI 462 Colour camera on Jupiter and Saturn.

I normally image DSO's so my lack of experience probably shows here.

I used a SW200P reflector and 3xBarlow lens on a HEQ5 mount. Firecapture software to take videos which were processed in autostakkert and final image refined using registax (wavelets).

I did not use high speed transfer settings in firecapture (couldn't find it) but used highest resolution available (1096x980) on 10bit 1x1 binning. 12bit and 2x2binning made my images pixellated! Histogram of exposure was 60% - I used plenty of gain to try and reduce exposure time but even so my frame rate was still slow at about 15/sec (I do use USB3 connection). 

And yes the images are mono as I used an IR cut filter as the IR pass filter hasn't arrived yet!!

Hoping someone was CC my images so I can try and improve here.

Thanks for looking.

Gerr.

 

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[vlaiv]

First - use histogram only as a guide if you are close to saturation - which you won't be in this case.

Set exposure time to ~5ms. Use gain to select low read noise rather than to "get good histogram" - which is meaningless in planetary imaging.

If you have Color camera and you used IR cut filter - you should get color images. What software are you using for stacking? If you are using AS!3 - set bayer matrix to RGGB and let it produce color image - if you are not using AS!3 - well, switch to AS!3

I like images btw. Jupiter one seems very good - a bit more care in processing and it will be very nice image.

[Gerr]

14 minutes ago, vlaiv said:

First - use histogram only as a guide if you are close to saturation - which you won't be in this case.

Set exposure time to ~5ms. Use gain to select low read noise rather than to "get good histogram" - which is meaningless in planetary imaging.

If you have Color camera and you used IR cut filter - you should get color images. What software are you using for stacking? If you are using AS!3 - set bayer matrix to RGGB and let it produce color image - if you are not using AS!3 - well, switch to AS!3

I like images btw. Jupiter one seems very good - a bit more care in processing and it will be very nice image.

Thanks for the comments Vlaiv.  Sorry I meant to say I have the IR pass filter (the IR cut filter has been ordered). So confusing!!!

I use AS!3 for stacking (will make note of bayer matrix setting). Gain was 50 for Jupiter and a lot less for Saturn. I try and keep the image grey looking / underexposed during acquisition to hopefully produce a better final image. Exposure histogram only as a guide but will certainly look at 5ms exposure time.

I use histogram stretching and wavelet adjustments (denoise and sharpening too) in Registax.

I tried Photoshop too but struggle to make any improvements here. 

Gerr.

[Gerr]

Sorry the settings stated above were actually actually Gamma.

Gain was about 70% of full value.

BTW what do you recommend Gamma should be?

50 for Jupiter?  10 for Saturn???

Gerr.

[vlaiv]

11 minutes ago, Gerr said:

BTW what do you recommend Gamma should be?

Don't touch gamma - leave it at default setting. You want your data to be linear for stacking.

[Pete Presland]

On 29/08/2021 at 21:31, vlaiv said:

First - use histogram only as a guide if you are close to saturation - which you won't be in this case.

Set exposure time to ~5ms. Use gain to select low read noise rather than to "get good histogram" - which is meaningless in planetary imaging.

@vlaiv i have been thinking about this for a couple days, i wonder if you could expand on this a little please.

I think understand the theory of having a long enough exposure to fill the pixels adequately. But doesnt the Histogram still play an important role in ensuring the image is not overexposed? Particularly in regards to the 70%-80% levels that are talked about during caprure tutorials. 

More than happy to start another thread or communicate directly if you prefer, as dont want to hijack this thread 🙂

[vlaiv]

3 minutes ago, Pete Presland said:

@vlaiv i have been thinking about this for a couple days, i wonder if you could expand on this a little please.

I think understand the theory of having a long enough exposure to fill the pixels adequately. But doesnt the Histogram still play an important role in ensuring the image is not overexposed? Particularly in regards to the 70%-80% levels that are talked about during caprure tutorials. 

More than happy to start another thread or communicate directly if you prefer, as dont want to hijack this thread 🙂

I think it is still relevant to this thread since I gave that advice in context of the thread and I'm happy to expand on it.

Histogram as guide is inherited from daytime photography where it ensures that there is proper exposure.

In context of planetary imaging, you are right about that, it can serve to ensure that there is no over exposure, however, I have couple of things to note and couple of objections to that approach.

1. If we are utilizing lucky type imaging properly, it is very unlikely that planetary imaging will produce saturated pixels. Solar and lunar can, but Jupiter and Saturn are almost impossible to saturate pixels if approached properly.

2. Over exposure is bad even if we can't see it in histogram. Due to nature of light and Poisson distribution - we always have varying level of signal in the image. Even if most pixels are not saturated - some can be (say signal is 95% - some pixels will shoot above 100% intensity and will be clipped).

3. In light of point number two - there are other ways to warn about saturation. If I'm not mistaken, SharpCap has this - you can turn it on and it will mark saturated pixels in preview. It can also warn if any of the pixels in image are saturated. This is better indicator than histogram alone.

When we have saturation out of the way - then we can deal with "70% histogram recommendation". This is really not a good recommendation when we utilize stacking. We can image planet in 8bit mode or in 16bit mode. In these two modes - 70% of maximum is different number of photons, so which one is right?

How long do we need to expose for to get it? Exposure length should be set with regards to seeing as to avoid motion blur of atmosphere. Each sub will be distorted by seeing and each distortion will be different. We want to get to a point where two consecutive subs are almost the same - to expose so that we don't give seeing a chance to "combine" two distortions, as such combination of distortions is much harder to deal with than single distortion. In turn this gives us much more frames to choose from and select ones that have "good" kind of distortion.

Now we get to stacking part. Here read noise is important factor. It is only thing that causes distinction between 1000 x 1s stack vs 1 x 1000s in terms of SNR. If we had zero read noise camera - these two would be the same in terms of SNR.

Since in lucky imaging we go for very short exposures - we want read noise to be low in order to get best possible result - one that is closest to single exposure in terms of SNR.

Gain can alter amount of read noise and therefore we should select gain as to have the least amount of read noise - and not to make our image "brighter" - which does not really mean anything in context of stacking.

In the end, that is rationale behind - "forget histogram rule" recommendation. It is used to avoid saturation and to get "correct exposure". However, there are other tools to avoid saturation and saturation is often not even an issue, exposure length is dictated by seeing and aperture size (coherence length and time) and gain should be utilized to select lowest read noise (which does not need to be highest gain settings - some cameras have lowest read noise at different gain setting - there is graph for each model and it's worth experimenting to find best gain setting for particular camera). Aiming for certain histogram value will just cause us to select either exposure length or gain that is not optimal for lucky imaging.

[Pete Presland]

Thanks @vlaiv for taking the time to type such an in depth reply. I have read it many times and been pondering it since you posted.

Would i be correct then in thinking that as long as my exposure for Jupiter was 5-7 ms and gain was 250-350 i need not worry about a lower histgram ?

I dont think i am far away from these settings when i capture anyway.

Looking at a recent capture files from Jupiter. The capture details with my C9.25/Asi224mc were exp 6.947ms and Gain 375 (62%), this gave Histgram of 69%

 

[vlaiv]

1 hour ago, Pete Presland said:

Would i be correct then in thinking that as long as my exposure for Jupiter was 5-7 ms and gain was 250-350 i need not worry about a lower histgram ?

Yes.

As far as gain is concerned - maybe best thing to do is measure read noise at particular gain and select the best one - the lowest read noise.

Read noise measurement is straight forward. Take number of bias subs, multiply them with e/ADU value (should be reported by ASCOM driver in fits header for particular gain setting) to convert to electron count, split them into two halves, stack each with sum stack and then subtract the two. Measure resulting image standard deviation and divide with square root of subs.

I personally think that best gain values are of form: Unity gain + N * 61 for ASI cameras (because their gain is such that it uses 0.1db units and every 6.1db actual e/ADU doubles). For ASI224 unity gain is 135 and lowest read noise are 300+ so that would be 318 or 379 - but again, I'd measure several values and check to make sure.

Exposure length is something that is quite individual and depends on scope size and location / conditions. 5-7ms is general figure that is good in most conditions for 8" ish of aperture. In good seeing / on good location - it will be a bit longer. It also depends on aperture size as it is related to time that takes for turbulent cell to move over the aperture of the scope.

See https://en.wikipedia.org/wiki/Greenwood_frequency

It is best if you experiment with these and judge yourself for your location and equipment. You want your subs to look a bit like this:

(I could not find planet representation - but Moon will do). They can be distorted and individual patches / parts even a bit blurred - but you want "clean split" between frames - you don't want to see motion blur morphing between them as that would indicate that you did not freeze the seeing but rather it is causing motion blur on top of distortion.