ASI1600MM-C bright star pattern

[souls33k3r]

Hi there,

I hope someone can answer this question and hopefully how to fix this.

I'm using ASI1600MM-C camera with Astronomik 6nm NB filters using SW Esprit 100ED. The image below is a single 5 min sub on the Jelly fish at Unity Gain settings. The bright star seems very tamed.

The second image is a combination of 47 x 5 min exposures and here you can see this weird pattern in the star. WHAT is going on

Please advise. 

Thanks in advance

[RolandKol]

Sadly I do not know the answer....

Just one little observation....

it is not only the bright one... in fact, - all of them....

maybe something went wrong in the stacking routine?

[souls33k3r]

2 minutes ago, RolandKol said:

Sadly I do not know the answer....

Just one little observation....

it is not only the bright one... in fact, - all of them....

maybe something went wrong in the stacking routine?

Really all of them? I just checked but can't see it ... maybe i'm not seeing it properly.

I'm not sure what stacking routine i used (Sigma Clip or SD Mask) but i can try another routine and see if that helps?

Also i'm seeing some information about microlensing playing a part in this pattern. 

Whatever it is, how can i fix this

[RolandKol]

I tried to google it out...

one lad had the similar problem with his Canon

https://www.cloudynights.com/topic/582071-parts-of-images-are-blurred-after-stacking-need-help/

Flats were the problem.

I hope you will sort it out!

[souls33k3r]

Cheers Roland, i should have mentioned, i did not use any flats with this. Only darks

[RolandKol]

:DDDDDDDDDDD

[vlaiv]

This is known issue with ASI1600, caused by micro lens on pixels and sensor cover window (not camera one, but protective glass over sensor).

Unfortunately there is no consensus on nature of exact process that creates such artifact. Two views dominate - one is that it is feature of sensor and that you can't do anything about it - it happens no matter what on bright stars. Second view, one that I'm advocating is that this phenomena occurs when you throw interference filters in the mix, and that it depends on interference filters to sensor distance.

In both views reason is the same - light reflects of micro lenses on chip and again on other surfaces to produce constructive interference on sensor and thus artifact. In my view (and I might be wrong here, I have not done any experiments my self due to lack of time under stars recently) - changing filter distance should alter pattern - either enhance it or make it go away (or at least be much less noticeable).

Thus my recommendation would be to try to reposition filter by adding a spacer between filter wheel and sensor, of course, taking care proper spacing is preserved if you are using flatteners or anything requiring exact back focus.

[souls33k3r]

8 minutes ago, vlaiv said:

This is known issue with ASI1600, caused by micro lens on pixels and sensor cover window (not camera one, but protective glass over sensor).

Unfortunately there is no consensus on nature of exact process that creates such artifact. Two views dominate - one is that it is feature of sensor and that you can't do anything about it - it happens no matter what on bright stars. Second view, one that I'm advocating is that this phenomena occurs when you throw interference filters in the mix, and that it depends on interference filters to sensor distance.

In both views reason is the same - light reflects of micro lenses on chip and again on other surfaces to produce constructive interference on sensor and thus artifact. In my view (and I might be wrong here, I have not done any experiments my self due to lack of time under stars recently) - changing filter distance should alter pattern - either enhance it or make it go away (or at least be much less noticeable).

Thus my recommendation would be to try to reposition filter by adding a spacer between filter wheel and sensor, of course, taking care proper spacing is preserved if you are using flatteners or anything requiring exact back focus.

Cheers @vlaiv, i can try and add the 5mm spacer that came with the camera. I'll just need to figure out if my back focus doesn't get mucked up because of my custom spacers used from the FF to the FW.

[RolandKol]

5mm? 
I would not go further than +-2
You will probably need to cash out again...

https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

[souls33k3r]

1 minute ago, RolandKol said:

5mm? 
I would not go further than +-2
You will probably need to cash out again...

https://www.firstlightoptics.com/adapters/baader-t2-delrin-spacer-ring-set.html

Yeah 5mm ... the ASI camera comes with a 5mm spacer which when you remove brings the camera closer to the filters.

Fortunately, i've already got these bad boys  But i need to be sure of what's the maximum i should go.

[vlaiv]

I think it would be best to do very simple experiment - do single short sub on very bright star (like brightest one you can find in the sky at the time), then change the distance by any amount (2-5mm, what ever is the easiest) - and repeat the sub and compare. If you notice any difference - better or worse, then it's worth "dialing in" spacing that works best. There is small chance that distance that works with one filter causes problems with other filter, but you will find it out in due course

 

[RolandKol]

Just an idea,

Has anyone tried to place filters on the spacers inside the EFW?

[souls33k3r]

Got to risk it for a biscuit i suppose. 
Do you reckon that stacking it differently would help solve the issue?
Please don't take this the wrong way but i am slightly annoyed with this pattern and would like to know how other users with similar camera have been able to avoid it (besides cropping the bright bad boy out of the image)

[vlaiv]

4 minutes ago, souls33k3r said:

Got to risk it for a biscuit i suppose. 
Do you reckon that stacking it differently would help solve the issue?
Please don't take this the wrong way but i am slightly annoyed with this pattern and would like to know how other users with similar camera have been able to avoid it (besides cropping the bright bad boy out of the image)

I've seen it in my images as well, here is mine OIII on relatively bright star:

But, as I mentioned, I have not had a chance to try to sort it out.

Changing of stacking method will not help, I'm afraid. It is real signal, and it just "reinforces" with stacking as any other real signal. You might have noticed that is invisible or barely visible in single sub, but stack shows it clearly - same as with other signals in the image - faint background galaxy might not be visible in single sub - but stack enough of them and it will pop out of the background.

You can think of it as being artifact of equipment used - same as diffraction spikes or halos around bright stars. Yes, I know that it's troubling because it "ruins" aesthetics of the image somewhat, and I guess some people choose not to be bothered by it in their images. I personally would not be bothered by small amount of it, but if it's too pronounced and distracts viewer from looking at the image - then it's certainly worth trying to deal with it in some way - first thing that comes to my mind is changing filter spacing.

You could also try to process it out in some way - a bit of clone stamp tool or something like that, at least for images already acquired. Another way for making it less visible with certain filters is to process in such way that luminance is from filter that does not contain artifact. I did something similar for above star, this is result:

I used Ha luminance (notice tighter star in Ha - another benefit of using it as luminance), and OIII as color (SHO pallete) - OIII artifact is now "shifted" to hue part of signal and eye is less sensitive to changes of hue, especially if brightness is low - so effect is not so intrusive.

[souls33k3r]

1 hour ago, vlaiv said:

I've seen it in my images as well, here is mine OIII on relatively bright star:

But, as I mentioned, I have not had a chance to try to sort it out.

Changing of stacking method will not help, I'm afraid. It is real signal, and it just "reinforces" with stacking as any other real signal. You might have noticed that is invisible or barely visible in single sub, but stack shows it clearly - same as with other signals in the image - faint background galaxy might not be visible in single sub - but stack enough of them and it will pop out of the background.

You can think of it as being artifact of equipment used - same as diffraction spikes or halos around bright stars. Yes, I know that it's troubling because it "ruins" aesthetics of the image somewhat, and I guess some people choose not to be bothered by it in their images. I personally would not be bothered by small amount of it, but if it's too pronounced and distracts viewer from looking at the image - then it's certainly worth trying to deal with it in some way - first thing that comes to my mind is changing filter spacing.

You could also try to process it out in some way - a bit of clone stamp tool or something like that, at least for images already acquired. Another way for making it less visible with certain filters is to process in such way that luminance is from filter that does not contain artifact. I did something similar for above star, this is result:

I used Ha luminance (notice tighter star in Ha - another benefit of using it as luminance), and OIII as color (SHO pallete) - OIII artifact is now "shifted" to hue part of signal and eye is less sensitive to changes of hue, especially if brightness is low - so effect is not so intrusive.

Very good advice. 

I'm also thinking (not sure how practical this approach would be) to take some shorter exposures of the same target using and/or different gain settings and then combine with the original? Just thinking it out loud. Just like what people do with taming down the orion nebula core.

[vlaiv]

7 minutes ago, souls33k3r said:

Very good advice. 

I'm also thinking (not sure how practical this approach would be) to take some shorter exposures of the same target using and/or different gain settings and then combine with the original? Just thinking it out loud. Just like what people do with taming down the orion nebula core.

You could try, but you don't have to do it like that - no need to get shorter exposures, since you don't have any data clipped. What you could do instead is use two different levels of stretch - if you don't stretch your data too much it will not show artifact as much - so you do one stretch to show central target and do another stretch of star and blend the two together. Just make sure you set background to equal value to get seamless transition between the two.

[souls33k3r]

9 minutes ago, vlaiv said:

You could try, but you don't have to do it like that - no need to get shorter exposures, since you don't have any data clipped. What you could do instead is use two different levels of stretch - if you don't stretch your data too much it will not show artifact as much - so you do one stretch to show central target and do another stretch of star and blend the two together. Just make sure you set background to equal value to get seamless transition between the two.

Good shout ... can try this.