What settings for ZWO ASI224

[mikeskor]

Can anyone offer help with how to use the ASI224 camera with Sharpcap 2.5. I would like to know what settings to use to minimize noise when shooting video of planets? I would also like to know what settings to use to take frames of DSOs. I am using the camera in prime focus mode with a 2X barlow and 9.25 inch Celestron SCT. Many thanks in advance.

[knobby]

Aim for mid gain, adjust shutter speed to  give about 75% histogram, leave gamma at 50 and capture as many frames as you can in about 90 to 120 seconds.

Then have a play with settings to see what works best for you .

[vlaiv]

According to ZWO, Gain of 350 should give you the least read noise for that model. Any gain setting above 200 is good - aim closer to 200 to get better dynamic range. Set exposure time as low as possible to get at least 50% (you can go lower than that but at expense of SNR), higher values are better (like already suggested 75% of histogram, provided you don't clip histogram or have too long exposure for given seeing). I usually leave gamma at 50 (it is only digital so no real benefit, and certainly so for planetary imaging), white balance at 50 for both blue and red (again no benefit since it is digital control and can be adjusted later after stacking). One thing to do as well - there is brightness setting (at least for 185mc, but I guess it is same for 224), when you decide on exposure length, cover telescope (as if taking dark frames) and look at the histogram. If it is clipping to the left, increase brightness setting. Minimum values of your dark frames should be just above 0. Do take at least 256 dark frames and process your recording in pipp (to do dark frame subtraction - this will remove both dark and bias signal).

For DSO there is no real benefit in going over 135 with Gain, so keep under. For shorter exposures stay above 60 for gain, for longer exposures you can go below 60 with gain. There seems to be a switch in read mode around gain value of 60 that has effect on read noise.

Last tip - don't use bias frames - take as many dark frames as you can at exactly same settings - without touching any of controls - just cover telescope with a cap.

It seems that these sony cmos sensors have some internal calibration that is applied whenever you change either gain or exposure length.

[cuivenion]

On 18/02/2016 at 22:27, vlaiv said:

According to ZWO, Gain of 350 should give you the least read noise for that model. Any gain setting above 200 is good - aim closer to 200 to get better dynamic range. Set exposure time as low as possible to get at least 50% (you can go lower than that but at expense of SNR), higher values are better (like already suggested 75% of histogram, provided you don't clip histogram or have too long exposure for given seeing). I usually leave gamma at 50 (it is only digital so no real benefit, and certainly so for planetary imaging), white balance at 50 for both blue and red (again no benefit since it is digital control and can be adjusted later after stacking). One thing to do as well - there is brightness setting (at least for 185mc, but I guess it is same for 224), when you decide on exposure length, cover telescope (as if taking dark frames) and look at the histogram. If it is clipping to the left, increase brightness setting. Minimum values of your dark frames should be just above 0. Do take at least 256 dark frames and process your recording in pipp (to do dark frame subtraction - this will remove both dark and bias signal).

For DSO there is no real benefit in going over 135 with Gain, so keep under. For shorter exposures stay above 60 for gain, for longer exposures you can go below 60 with gain. There seems to be a switch in read mode around gain value of 60 that has effect on read noise.

Last tip - don't use bias frames - take as many dark frames as you can at exactly same settings - without touching any of controls - just cover telescope with a cap.

It seems that these sony cmos sensors have some internal calibration that is applied whenever you change either gain or exposure length.

Having just bought one these I've been reading up a little and I've just come across this. Does this mean you can't calibrate flat frames properly due to this 'internal calibration', as they need bias or dark flats. Is seems that calibration of any sort is going to be tricky if this is happening.

[vlaiv]

1 minute ago, cuivenion said:

Having just bought one these I've been reading up a little and I've just come across this. Does this mean you can't calibrate flat frames properly due to this 'internal calibration', as they need bias or dark flats. Is seems that calibration of any sort is going to be tricky if this is happening.

Can be done with flat darks only, no need for bias files.

To be sure - do simple experiment. Take set of flat darks (just basically darks at short exposure - same that you use to get your flats). Then power off everything and disconnect camera. Power on again, use same settings, and do another set of flat darks. Stack each group using simple average stacking to get two "masters". Subtract second master from first and examine the result. Result should have average 0 and have only random noise in it - no pattern present. If this is so, you can simply use following calibration:

master dark = avg(darks)

master flat dark = avg(flat darks)

master flat = avg(flats - master flat dark)

calibrated light = (light - master dark) / master flat

Note that lights and darks need to be taken on same temperature and settings, and flats and flat darks on their own temp, exposure and settings (which ever suit you to get good flat field).

You can on the other hand check if bias is behaving properly by following:

Take two sets of bias subs, and do the same as above for flat darks (two stack, subtract and examine). If you get avg 0 and no pattern - that is good.

Now to be sure if bias work ok, you need to do the following as well.

Take one set of darks of certain exposure (let's say 10s), and take one set of darks with double that exposure (so 20s, same temp, same settings).

Prepare first master as avg(darks from set1) - bias. Prepare second master as avg(darks from set2) - bias. Then create using pixel math following image: master2 - master1*2 and examine it. It should also have avg 0 and no patterns visible. If you get that result then bias functions properly and you can use it (although for standard calibration it will not be necessary as you can use calibration mentioned above).

[cuivenion]

Thats great thanks.

[cuivenion]

On 13/02/2018 at 17:02, vlaiv said:

Can be done with flat darks only, no need for bias files.

To be sure - do simple experiment. Take set of flat darks (just basically darks at short exposure - same that you use to get your flats). Then power off everything and disconnect camera. Power on again, use same settings, and do another set of flat darks. Stack each group using simple average stacking to get two "masters". Subtract second master from first and examine the result. Result should have average 0 and have only random noise in it - no pattern present. If this is so, you can simply use following calibration:

master dark = avg(darks)

master flat dark = avg(flat darks)

master flat = avg(flats - master flat dark)

calibrated light = (light - master dark) / master flat

Note that lights and darks need to be taken on same temperature and settings, and flats and flat darks on their own temp, exposure and settings (which ever suit you to get good flat field).

You can on the other hand check if bias is behaving properly by following:

Take two sets of bias subs, and do the same as above for flat darks (two stack, subtract and examine). If you get avg 0 and no pattern - that is good.

Now to be sure if bias work ok, you need to do the following as well.

Take one set of darks of certain exposure (let's say 10s), and take one set of darks with double that exposure (so 20s, same temp, same settings).

Prepare first master as avg(darks from set1) - bias. Prepare second master as avg(darks from set2) - bias. Then create using pixel math following image: master2 - master1*2 and examine it. It should also have avg 0 and no patterns visible. If you get that result then bias functions properly and you can use it (although for standard calibration it will not be necessary as you can use calibration mentioned above).

Hi I tested the bias. This was the workflow.

1. Took a set of ten bias.
2. Shut down the camera.
3. Restarted the camera, took another set of ten bias.
4. Integrated both sets using these settings:

5.Subtracted integration2 from integration1 and integration2 from integration2 with these settings:

6. And these are the results side by side. Looks like the bias is not useable for calibration then. Unless something in my settings was wrong:

Here are the the original two integration files before calibration:

All the files were autostretched.

One question I have now is how do I use the Basic CCD Parameters script in Pixinsight to find my electrons per data if I can't use the bias? This also leaves me unable to use the SubframeSelector process.

I've also posted about this on the Pixinsight forum.

 

[sharkmelley]

Integration2 - Integration2 = 0   

Mark

Edited March 23, 2018 by sharkmelley

[cuivenion]

7 minutes ago, sharkmelley said:

Integration2 - Integration2 = 0   

Mark

You'll have to excuse me I'm a newbie with this stuff. Wouldn't I expect the same from integration2-integration1 too? That's if bias is functioning correctly.

[cuivenion]

I guess what I'm really wanting to find out is if the bias from this camera can be used for calibrating the same as other CCD's and dslrs can. I've even posted on the zwo website and the reply I got wasn't really that helpful.

[vlaiv]

I'm having a bit trouble following your work flow - just because I'm not used to PI screens.

If I'm reading screens above correctly you are concerned about image below point 6? Or left part of it - representing difference between two bias stacks?

For the test, best thing to do is just to make stack with straight average of subs, so avoid any sort of sigma clip, or cosmetic corrections (hot/cold pixel rejection, whatever).

Then do simple arithmetic of subtracting two images. Now image you produced (in similar fashion) - shows something that can be considered as bad - vertical streak. You want your difference to be pure uniform noise, and if you do average value of all pixels in difference you want them to be very close to 0 (it will not be exactly 0 but very very close, something like 0.00000003 or something like that).

If you get any other value than very close to 0 then bias is not stable. You can check if darks are stable. Use same approach here - do two stacks of darks at same temperature and of same duration. If these turn out to be ok (as for example is case with ASI1600) then it is all ok, just don't use bias frames. You can get all the functionality that you need, like exact photon/electron count - by using light calibrated by subtracting master dark (composed out of dark frames without subtracting bias) and dividing by flat (normalized flat if you want to do photometry or something).

[cuivenion]

Hi, how do you perform the subtraction though? I did it as if I was calibrating the frame. I though that both calibrated images would look like the 2nd, blank with avgDev of zero, if the bias was performing correctly.

[vlaiv]

5 minutes ago, cuivenion said:

I guess what I'm really wanting to find out is if the bias from this camera can be used for calibrating the same as other CCD's and dslrs can. I've even posted on the zwo website and the reply I got wasn't really that helpful.

You can do that very simply with ImageJ (open source), here is workflow:

Choose details if you need to:

It will open stack of images, then do:

After that:

and select:

This will create first master. Close opened stack of images, open second stack of images and do same process until you get second master.

At the end do:

with these options:

It will create difference of two masters. Select that image and do:

And you should get something like this:

[cuivenion]

Thanks, these are my results:

I have no idea what they mean. I hope you do.

 

[vlaiv]

I would say that for 10 bias stack result of 0.011 is pretty close to 0 (if you increase number of bias subs per stack - value should get closer to 0).

I also think based on this that ASI224 has a stable bias - meaning that for same settings you will get good bias file each time (difference close to 0 just means that there is no difference in bias signal between two different sessions). So you can use bias for calibration (although it is not really needed).

You can do one more test, just select your difference frame, go into Image/Adjust/Brightness contrast, and set it like this:

So that min slider and max slider sort of bracket main part of histogram. This will make all features in difference distinct. It should be just noisy with no particular features. In the image above, there is for example visible vertical and horizontal banding - that is something that is characteristic for CMOS sensors, and it shows that read noise is not quite gaussian in distribution, but given enough subs it should not show in final image (both calibration subs and light subs).

Edited March 23, 2018 by vlaiv

[cuivenion]

Hi, here is the result of the next test. There is one faint vertical band. it appears in all my stack frames when autostretched:

[PadrePeace]

I’d say that faint line is a bad pixel column on your sensor. I have a similar one but not all the way up the screen. Calibration removes it. Nothing else you can do.