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QHY183M & Flats = Frustration

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#26 kingjamez

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Posted 16 September 2019 - 01:14 PM

Wow! Looks great. What an improvement!! Was it just the pedestal that removed that last bit of ampglow?

-Jim

Edited by kingjamez, 16 September 2019 - 01:15 PM.


#27 terry59

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Posted 16 September 2019 - 02:35 PM

Wow! Looks great. What an improvement!! Was it just the pedestal that removed that last bit of ampglow?

-Jim

Thanks Jim...yeah that did the trick.  It took a few tries to get the right amount added. Do you use a pedestal?



#28 OldManSky

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Posted 16 September 2019 - 04:01 PM

terry, I'm glad you figured it out.  But I'm also a bit confused.

With the ZWO version of this chip/camera, I don't need to add a pedestal to the darks.  But then I don't use bias, either...maybe that's the difference?

Did you try it without the bias, and with just darks?

Just trying to figure out what's going on...:)



#29 terry59

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Posted 16 September 2019 - 04:45 PM

terry, I'm glad you figured it out.  But I'm also a bit confused.

With the ZWO version of this chip/camera, I don't need to add a pedestal to the darks.  But then I don't use bias, either...maybe that's the difference?

Did you try it without the bias, and with just darks?

Just trying to figure out what's going on...smile.gif

No, I haven't tried without bias....the effect of calibrating with them is very noticeable as is the difference in the master dark with and without the pedestal added. I suspect that it is from the bias since each is 0.00005 seconds long vs multiple seconds with flat darks. 

 

Here is the master dark with no pedestal and the one with 100

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#30 kingjamez

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Posted 16 September 2019 - 05:32 PM

terry, I'm glad you figured it out. But I'm also a bit confused.
With the ZWO version of this chip/camera, I don't need to add a pedestal to the darks. But then I don't use bias, either...maybe that's the difference?
Did you try it without the bias, and with just darks?
Just trying to figure out what's going on...:)


Good point. I wonder what the median ADU is of the bias. Wanna wager on it being around 100?

I don’t use a bias or a pedestal. While it’s great that you are able to get the +100 pedestal to get you to useable images, it seems like a bandaid covering a yet discovered underlying issue.

If it were me, I’d try not using a bias at all including during the creation of the master dark.

-Jim

#31 terry59

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Posted 16 September 2019 - 06:01 PM

Good point. I wonder what the median ADU is of the bias. Wanna wager on it being around 100?

I don’t use a bias or a pedestal. While it’s great that you are able to get the +100 pedestal to get you to useable images, it seems like a bandaid covering a yet discovered underlying issue.

If it were me, I’d try not using a bias at all including during the creation of the master dark.

-Jim

Jim, I'm willing to try...it's how I learn best. Are you suggesting I create a master dark with no calibration first?

 

Here are a bias and the two master darks if you'd like to look at them. 

 

https://1drv.ms/u/s!...C_yh52?e=uY26dk



#32 kingjamez

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Posted 16 September 2019 - 07:00 PM

Jim, I'm willing to try...it's how I learn best. Are you suggesting I create a master dark with no calibration first?

 

Here are a bias and the two master darks if you'd like to look at them. 

 

https://1drv.ms/u/s!...C_yh52?e=uY26dk

Those were very helpful, thank you.

 

One thing that stood out in looking at those files and the previous files is that the bias has a median of 1936 while your flat dark of 50 seconds has a median of 1904. This is not right. It means your bias is at a bare minimum 32 too high, probably more like 35-40 ADU high.

 

Looking at your bias-calibrated, non-pedestal added dark, it is clearly clipped which means your bias is over calibrating.

 

I'm certainly no expert, but I think this is clear evidence that you should not use a bias in any part of the calibration process. I would recreate your master darks without using a bias, and calibrate your flats with only a dark flat (again, no bias). 

 

-Jim


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#33 OldManSky

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Posted 16 September 2019 - 07:14 PM

Jim, you beat me to it.  I was going to say the same thing.

I don't use bias frames with my ZWO version of the 183...and I don't have to offset the darks with a pedastal :)


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#34 View2

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Posted 16 September 2019 - 07:35 PM

If I were having your trouble I would consider using dark Flats. You can use them instead of a bias. Here you take an equal number of darks at the same setting as your Flats. Don't get used to not using any Flats!

#35 View2

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Posted 16 September 2019 - 07:35 PM

Really nice image by the way

#36 kingjamez

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Posted 16 September 2019 - 11:42 PM

I learned some things tonight. 

 

I took a variety of bias frames. I used unity gain (111 for me) at -15c temperature.

 

Exposure Time:            Median ADU:      Offset:

0.000s (AFAP)              705.6                   10

0.20s                             706.08                 10

0.50s                             706.8                   10

15s                                706.56                 10

 

0.000s                          1985.4                  30

0.20s                            1986.0                  30

 

Now we know the source of the dramatic difference in ADU between Terry's frames and mine. It's simply offset. I didn't expect offset to have that much of a... well.... offset. I need to learn more about what exactly 10 and 30 mean... it's clearly not 10 and 30 ADU in 16 bit format.

 

The difference between them is 1279.8 which is awfully close to 30-10==20 and 20 * 64 == 1280. So it appears that the offset is multiplied by 64 in 16bit terms or 1x in 10 bit terms. I'm sure someone smarter than me will come along.

 

It's also interesting to note that the median ADU of my 15s dark is less than the 0.50s dark. Both were 100 frames so should be fairly accurate. 

 

My camera is doing a long run of darks right now, but I also plan to add 15s and 0.5s at 30 offset to see if the same inversion happens. If it does, I'll go in small increments to map out where the median ADU shift / inversion happens. 

 

It's also nice to see that I'm getting very similar numbers between my ASI and Terry's QHY camera. I also have a 183MC and a cursory look at it's calibration frames show very similar results to my 183MM... this is obviously expected but we test because we don't always get what we expect!

 

More investigation coming in as soon as I can take the data.

-Jim 


Edited by kingjamez, 16 September 2019 - 11:55 PM.


#37 cuivienor

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Posted 17 September 2019 - 01:16 AM

Is it just me, but I thought that Bias frames were simply not recommended for any of those CMOS cameras? Everything I've read from ZWO is a solid "Don't use bias frames" recommendation...



#38 Jon Rista

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Posted 17 September 2019 - 01:22 AM

What about what I'd heard about flats needing to be at least 2 seconds in length? 

 

With my Spike-A-Flat at 50%, I use ranges from 0.01" for Lum to 1.2" for Sii. That brings the ADU to around 28000-31000 (for the ASI1600MM-C)

 

I do hope this will work, although I'm quite adaptable--but get cranky if I need to adapt too much smile.gif

 

Terry, I just ordered it, since you liked it despite your problems. Let's see how long free shipping takes smile.gif

I am not sure where you are getting 2 seconds... 

 

The ASI1600/QHY163 needed flats at least 0.2 seconds (zero point two) in length to avoid a bias instability issue, but that is JUST the Panasonic M cameras...the guidelines for that do not apply to every CMOS camera...

 

The IMX183 is just fine with biases, so you  can calibrate your flat frames with biases if they are short enough.



#39 Jon Rista

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Posted 17 September 2019 - 01:22 AM

Is it just me, but I thought that Bias frames were simply not recommended for any of those CMOS cameras? Everything I've read from ZWO is a solid "Don't use bias frames" recommendation...

The no bias frames/0.2 second min exposure should only apply to Panasonic M based CMOS cameras. It does not apply to every CMOS camera in existence... 



#40 Jon Rista

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Posted 17 September 2019 - 01:25 AM

I've been struggling with flats until today when I decided to forego using them. What a huge difference. I once again have the issue with calibrating amp glow out but I had to use the dark flats and I think I need to shoot some regular bias frames now. 

 

Here is the result...cropped to remove the amp glow area, lightly processed in PI and PS

 

Only three months in the making

 

smile.gif

 

GM-8/SV70T/QHY183M/Baader 3.5nm Ha filter

Image looks great! You are definitely getting good resolution for your setup. 

 

Regarding flats...what exactly is the issue? Bias frames should be fine with the IMX183 if the flats are short. As with any camera, if the flats are longer then dark flats should be used...but there is no 2 second exposure limimtation, nor is there even a 0.2 second exposure limitation like the Panasonic M cameras. You should be able to use very short flats and biases if you prefer. 

 

One thing you may run into is the rolling electronic shutter may interfere with the oscillation rate of a flat panel, and if it does, that might cause some problems. I have been using open (uncovered) aperture blue sky flats with my ASI183, with flat exposures of small fractions of a second along with bias calibration. So far things have been working nicely (save the occasional airplane in a flat frame. :p



#41 Stelios

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Posted 17 September 2019 - 02:29 AM

Regarding flats...what exactly is the issue? Bias frames should be fine with the IMX183 if the flats are short. As with any camera, if the flats are longer then dark flats should be used...but there is no 2 second exposure limimtation, nor is there even a 0.2 second exposure limitation like the Panasonic M cameras. You should be able to use very short flats and biases if you prefer. 

 

OK, what's "short" and what's "longer" ? I take flats depending on filter at exposures from 0.01" to 1.2". We'll stipulate that 0.01" is short, but is 1.2" "longer"?

 

Also I keep hearing this about bias, but I have an ASI1600MM-C, which is a Panasonic "M" camera, correct? I have been using a superbias (100 bias frames) taken at shortest possible exposure with SGP for *years* now, with no ill effects (following the Warren Keller approach in Inside Pixinsight). I am tempted to take some dark flats just to compare and see if there's any discernible difference, but it's not going to be major for sure.



#42 terry59

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Posted 17 September 2019 - 06:54 AM

I learned some things tonight. 

 

I took a variety of bias frames. I used unity gain (111 for me) at -15c temperature.

 

Exposure Time:            Median ADU:      Offset:

0.000s (AFAP)              705.6                   10

0.20s                             706.08                 10

0.50s                             706.8                   10

15s                                706.56                 10

 

0.000s                          1985.4                  30

0.20s                            1986.0                  30

 

Now we know the source of the dramatic difference in ADU between Terry's frames and mine. It's simply offset. I didn't expect offset to have that much of a... well.... offset. I need to learn more about what exactly 10 and 30 mean... it's clearly not 10 and 30 ADU in 16 bit format.

 

The difference between them is 1279.8 which is awfully close to 30-10==20 and 20 * 64 == 1280. So it appears that the offset is multiplied by 64 in 16bit terms or 1x in 10 bit terms. I'm sure someone smarter than me will come along.

 

It's also interesting to note that the median ADU of my 15s dark is less than the 0.50s dark. Both were 100 frames so should be fairly accurate. 

 

My camera is doing a long run of darks right now, but I also plan to add 15s and 0.5s at 30 offset to see if the same inversion happens. If it does, I'll go in small increments to map out where the median ADU shift / inversion happens. 

 

It's also nice to see that I'm getting very similar numbers between my ASI and Terry's QHY camera. I also have a 183MC and a cursory look at it's calibration frames show very similar results to my 183MM... this is obviously expected but we test because we don't always get what we expect!

 

More investigation coming in as soon as I can take the data.

-Jim 

This is interesting Jim. Looking forward to more results



#43 terry59

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Posted 17 September 2019 - 06:57 AM

Is it just me, but I thought that Bias frames were simply not recommended for any of those CMOS cameras? Everything I've read from ZWO is a solid "Don't use bias frames" recommendation...

 

That is what makes this forum invaluable. Real data, real tests, real results



#44 terry59

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Posted 17 September 2019 - 07:08 AM

Image looks great! You are definitely getting good resolution for your setup. 

 

Regarding flats...what exactly is the issue? Bias frames should be fine with the IMX183 if the flats are short. As with any camera, if the flats are longer then dark flats should be used...but there is no 2 second exposure limimtation, nor is there even a 0.2 second exposure limitation like the Panasonic M cameras. You should be able to use very short flats and biases if you prefer. 

 

One thing you may run into is the rolling electronic shutter may interfere with the oscillation rate of a flat panel, and if it does, that might cause some problems. I have been using open (uncovered) aperture blue sky flats with my ASI183, with flat exposures of small fractions of a second along with bias calibration. So far things have been working nicely (save the occasional airplane in a flat frame. tongue2.gif

 

Thanks Jon....I really like this camera. It is beating the tar out of the KAF8300. As to flats, it is taking many, many seconds to reach the 30k ADU level when using the Flatman and that pattern persists regardless of what I've tried so I think you are correct as to the cause because I don't see anything like that with the KAF8300 and its mechanical shutter.



#45 terry59

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Posted 17 September 2019 - 08:35 AM

I have rerun everything using an uncalibrated master dark. Here are the two results (no bias and bias with a pedestal of 100) with only a STF applied. I'm curious to know what people think

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Edited by terry59, 17 September 2019 - 08:37 AM.


#46 kingjamez

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Posted 17 September 2019 - 08:53 AM

Really hard to tell a difference between them given the forum image size restrictions. They both appear to have completely removed the amp glow. I'd be interested to hear what you see on your machine.

 

-Jim


Edited by kingjamez, 17 September 2019 - 08:54 AM.


#47 kingjamez

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Posted 17 September 2019 - 08:57 AM

The IMX183 is just fine with biases, so you  can calibrate your flat frames with biases if they are short enough.

What do you suggest as the cause of Terry's bias having a 32 ADU higher median than his 300 second dark?

 

-Jim



#48 terry59

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Posted 17 September 2019 - 09:09 AM

Really hard to tell a difference between them given the forum image size restrictions. They both appear to have completely removed the amp glow. I'd be interested to hear what you see on your machine.

 

-Jim

I don't see any difference in the amp glow....both remove it completely as far as I can tell. I do find a slight visual difference in the result. It seems to me that adding a pedestal is an improvement 


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#49 kingjamez

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Posted 17 September 2019 - 09:25 AM

I don't see any difference in the amp glow....both remove it completely as far as I can tell. I do find a slight visual difference in the result. It seems to me that adding a pedestal is an improvement.

Good deal. Since both are auto-SFT'd, I'd bet that you can make them both look the same by modifying the histogram. I think that the pedestal only needs to be roughly 40 to "work" so the extra is slightly making the whole image brighter. 

 

My camera is running all day today taking darks and bias'. Hopefully I'll be able to reasonably replicate your results on mine soon.

 

-Jim



#50 terry59

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Posted 17 September 2019 - 09:46 AM

Good deal. Since both are auto-SFT'd, I'd bet that you can make them both look the same by modifying the histogram. I think that the pedestal only needs to be roughly 40 to "work" so the extra is slightly making the whole image brighter. 

 

My camera is running all day today taking darks and bias'. Hopefully I'll be able to reasonably replicate your results on mine soon.

 

-Jim

Thanks Jim....looking forward to seeing what you get




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