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Gain and Sub-exposure calculation spreadsheet for the ZWO ASI183 and 294

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#26 niccoc1603

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Posted 13 September 2019 - 01:56 PM

Hi Steve and others,

Thanks for the sheet!

I am sorry for asking but I am a newbie. I own an ASI294MC pro. My understanding of the table in the Exposure Calculator tab is that we theoretically don't need the long subs (60s, 180s, 300s...) that people normally use. With an urban sky, subs exposure of 10s-15s are enough at more or less any gain (above unity gain). The inconvenient is that we then need hundreds or thousands of subs to reach the desired integration time. Is that a correct interpretation? Can we have longer exposure though? I would love to have a Gain and Sub-exposure tables such as the one proposed in https://www.cloudyni...olour-versions/ applicable for the 294MC would be great to help me understand better.

Many thanks again.

Philippe


Hi in theory yes, that’s the exposure you need to swamp the RN

To avoid saturating the HD with subs you have a few options:

- use a LP or “OSC narrowband” filter (like opto long L-enhance). You will adjust the spreadsheet bandwidth accordingly.

- lower the gain, for example switch to the low conversion gain section (0-119)

In any case keeping a minimum exp of 60s won’t do any damage
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#27 StevenBellavia

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Posted 14 September 2019 - 07:56 AM

I just uploaded the latest version (revision "J")  onto the shared drive, and removed all older versions.

 

A few little bugs were fixed, but nothing that had any noticeable effect.

 

Special thanks to John, who found these bugs and pointed them out to me

 

Steve


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#28 Jon Rista

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Posted 14 September 2019 - 09:06 AM


   The significant clipping during calibration doesn't happen (at least in my experience) when using larger default offsets such as the ZWO recommended 30. The larger Offset makes the use of a large (or any) Pedestal during calibration much less important as the larger Offset acts to "cushion" the background data.

 

   I would still like to see some actual calibrated data from other users who routinely use low Offset values with or without using a Pedestal during Image Calibration.

 

   Given this information, I think I will now try using a lower value of Offset for my next imaging session. (It may be a week before we have good weather again, though.) Knowing that background signal clipping due to low offset values can be controlled with the use of a large Pedestal during calibration, I can see the value of this approach. Until I am completely comfortable with the concept, I will probably spot check the first few frames of an imaging session to ensure only minimal clipping will occur during later processing. (I tend to be overly cautious about losing all data from an otherwise promising imaging session.)

 

 

John

The offset is part of the darks as well (and biases, if you use them). So increasing the offset in the camera settings won't change how much data clips, if you are having problems with data clipping during calibration.

 

If you use a small offset, say 10, then the lights, darks, and biases all have at a minimum a signal offset of 10. When you calibrate, you subtract 10.

 

Now, if you bump the offset to say 50. Same thing...when you calibrate, you subtract 50.

 

You would clip the same 15,000 pixels in either case. The only thing that might allow you to use a smaller pedestal is a larger SKYFOG offset, not a larger camera offset. If you can expose the baseline signal enough that it pushes the mean image signal so high that all of it, including noisy pixels, clear the camera offset, then you could calibrate without a pedestal. The thing is, this generally takes very long exposures. Cameras with higher read noise and larger FWCs will succeed in getting a very large skyfog offset because such an offset is necessary...you need a lot of signal to swamp the read noise. Cameras with low read noise and smaller FWCs, however, may not be able to expose enough per sub to fully clear the bias offset with signal.


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#29 jdupton

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Posted 14 September 2019 - 09:54 AM

Jon,

 

   I will have to think about that some more. I may be having trouble seeing the forest because there are too many trees in the way. crazy.gif

 

   I understand your analogy of the 10 vs 50 Offsets but struggle with what that means in the hardware. If a Bias Offset were added as a digital number after the A/D operation, it all makes sense and would be a linear effect all being cancelled out in calibration. However, since the Bias Offset is added to the charge voltage before A/D conversion, the effect is "non-linear" (for lack of a better term). I would think that the shot noise associated with the Bias Offset would change the analogy. The higher the added Bias Offset, the more shot noise associated with it. While it also gets added to the dark frames used for calibration, my gut tells me the clipping characteristics will change. Yes, gut feelings don't always follow the laws of statistics.

 

   I'll need to think through this some more and reconcile it with my own data observations...

 

 

John

 

PS: Steve, say the word and we can take this off to another thread. It is only peripheral to your spreadsheet and the way it works.


Edited by jdupton, 14 September 2019 - 09:55 AM.


#30 Jon Rista

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Posted 14 September 2019 - 10:08 AM

Jon,

 

   I will have to think about that some more. I may be having trouble seeing the forest because there are too many trees in the way. crazy.gif

 

   I understand your analogy of the 10 vs 50 Offsets but struggle with what that means in the hardware. If a Bias Offset were added as a digital number after the A/D operation, it all makes sense and would be a linear effect all being cancelled out in calibration. However, since the Bias Offset is added to the charge voltage before A/D conversion, the effect is "non-linear" (for lack of a better term). I would think that the shot noise associated with the Bias Offset would change the analogy. The higher the added Bias Offset, the more shot noise associated with it. While it also gets added to the dark frames used for calibration, my gut tells me the clipping characteristics will change. Yes, gut feelings don't always follow the laws of statistics.

 

   I'll need to think through this some more and reconcile it with my own data observations...

 

 

John

 

PS: Steve, say the word and we can take this off to another thread. It is only peripheral to your spreadsheet and the way it works.

The bias is added as a part of A/D conversion. The bias offset that we control is the ADC offset. So in effect, it IS a DN.

 

There is a voltage bias applied to the pixels, but that is different and not something we generally control. We control the ADC offset.

 

The bias offset also does not have shot noise. It may have a voltage noise, like 1/f noise, but it is not shot noise (shot noise is poisson noise, noise that derives from the counting of things, such as electrons...a bias offset is not the same as dark current.)

 

A bias offset can be considered a linear add to the signal. Linear add...linear subtraction. It doesn't matter how large the offset is...10, 50, 100 DN...the end result is always going to be the same when you calibrate: Subtracting the dark or bias will remove the offset. Therefor, any benefit the offset has in preserving "negative signal values" (values that go below the bias offset due to noise) will be nullified if you calibrate without using a pedestal.

 

The benefit of a bias offset is solely to prevent clipping DURING A/D conversion. The offset is not intended to prevent issues during calibration, only during conversion.


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#31 niccoc1603

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Posted 14 September 2019 - 11:01 AM

Interesting, so a pedestal is in any case recommended? How to determine the pedestal value?

#32 basskep

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Posted 16 September 2019 - 10:28 AM

Hi John,

 

It's quite interesting because a lot of work has been done here for ZWO ASI294MCPRO and ASI183 models.

So if i understand well your explanations we could use whatever Offset we want if we calibrate using pedestal ?

 

 

The bias is added as a part of A/D conversion. The bias offset that we control is the ADC offset. So in effect, it IS a DN.

 

There is a voltage bias applied to the pixels, but that is different and not something we generally control. We control the ADC offset.

 

The bias offset also does not have shot noise. It may have a voltage noise, like 1/f noise, but it is not shot noise (shot noise is poisson noise, noise that derives from the counting of things, such as electrons...a bias offset is not the same as dark current.)

 

A bias offset can be considered a linear add to the signal. Linear add...linear subtraction. It doesn't matter how large the offset is...10, 50, 100 DN...the end result is always going to be the same when you calibrate: Subtracting the dark or bias will remove the offset. Therefor, any benefit the offset has in preserving "negative signal values" (values that go below the bias offset due to noise) will be nullified if you calibrate without using a pedestal.

 

The benefit of a bias offset is solely to prevent clipping DURING A/D conversion. The offset is not intended to prevent issues during calibration, only during conversion.



#33 basskep

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Posted 16 September 2019 - 10:39 AM

Hi John D Upton,

 

Do you still need some of my past calibrated subs (after reading the last post of John Rista) ?

 

basskep,

 

 

   I don't have a mailbox large enough to receive files directly. Do you have a DropBox free account or a other file sharing account like Google Drive? If so, you could upload one or two sample calibrated subs there and send me a PM (private message; option available when you hover over my CN ID) with a link to the files that I can then download.

 

   I am interested in single calibrated subs taken using a low offset. I have lots of data of my own taken at an Offset of 30 but need real sky target data taken at an offset of 4 to 10 to examine. I only need a couple of calibrated subs at most -- not a full set of data.

 

 

John



#34 jdupton

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Posted 16 September 2019 - 11:12 AM

basskep,,

 

   Sure, it will help me by giving me a different data point if it isn't too much trouble to upload a couple of files to a share site. Ideally, I might like four files -- 2 raw subs, and then 2 more being those same subs after being calibrated.

 

   I have gone back and looked at a lot of my own real data taken at Offset 30. There were only a few subs that had clipping level more than 100 pixels. Most had fewer than a dozen with an average of around 5. (It turns out that the clipping in my subs was probably due to hot pixels that escaped calibration as no amount of Pedestal would prevent them from clipping.) The lack of clipping on my own data may be more due to my swamping read noise by an average factor of 20 to 30 rather than 10 based on what Jon Rista described. More data, especially from low offset short exposure times data, will help me understand that aspect better.

 

 

John


Edited by jdupton, 16 September 2019 - 11:14 AM.


#35 Jon Rista

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Posted 16 September 2019 - 12:49 PM

Hi John,

 

It's quite interesting because a lot of work has been done here for ZWO ASI294MCPRO and ASI183 models.

So if i understand well your explanations we could use whatever Offset we want if we calibrate using pedestal ?

You still need a bias offset to avoid clipping during ADC. If you use too small an offset, then pixels could clip during ADC, and that information would be lost for good. A large offset is usually better than a small offset, and there is definitely such a thing as too small. You definitely do not want to use zero.


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#36 basskep

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Posted 16 September 2019 - 01:24 PM

basskep,,

 

   Sure, it will help me by giving me a different data point if it isn't too much trouble to upload a couple of files to a share site. Ideally, I might like four files -- 2 raw subs, and then 2 more being those same subs after being calibrated.

 

   I have gone back and looked at a lot of my own real data taken at Offset 30. There were only a few subs that had clipping level more than 100 pixels. Most had fewer than a dozen with an average of around 5. (It turns out that the clipping in my subs was probably due to hot pixels that escaped calibration as no amount of Pedestal would prevent them from clipping.) The lack of clipping on my own data may be more due to my swamping read noise by an average factor of 20 to 30 rather than 10 based on what Jon Rista described. More data, especially from low offset short exposure times data, will help me understand that aspect better.

 

 

John

Ok John I will upload some of my low offset and short exposure subs 



#37 basskep

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Posted 16 September 2019 - 01:27 PM

You still need a bias offset to avoid clipping during ADC. If you use too small an offset, then pixels could clip during ADC, and that information would be lost for good. A large offset is usually better than a small offset, and there is definitely such a thing as too small. You definitely do not want to use zero.

Thank you for your reply Jon. 

And just for information you seem to use a ZWO ASI183MM Pro. What offset do you use with it ? And maybe your other ccds ?



#38 basskep

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Posted 17 September 2019 - 03:40 AM

Ok John I will upload some of my low offset and short exposure subs 

Hi John,

 

You will find below a link to grab some calibrated lights I have done with gain 125 offset 5 exposure 60s :

 

https://drive.google...-RfuoAFRjPfeTyC

 

Do you also need darks and flats ?



#39 jdupton

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Posted 17 September 2019 - 09:11 AM

basskep,

 

   I do not need any Dark or Flat Frames.

 

   I would like to look at these same frames in their original state prior to calibration if you don't mind uploading them also. Just upload the corresponding frames with the same names but without the "_c" suffix at the end. (Example: "frame_0.fit" through "frame_5.fit")

 

   These calibrated frames show clipping due to the calibration process. It is not particularly bad but is more than I generally like to see. I am seeing a range of clipped pixel counts in the frames that are from 756 to 783 clipped pixels.

 

   If I were able to now see the same original frames as they came from the camera, I could next evaluate how many of these pixels would not be clipped had a pedestal been used during image calibration.

 

- - - 

Separate Topic:

 

   In examining the calibrated frames I note several things that I would not recommend.

  • It appears that you are using a Master Bias Frame in your calibration. (This can be problematic with the ASI294MC-Pro camera.)
     
  • It appears that the Master Dark Frame is being calibrated (with the Master Bias Frame) and optimized within the calibration operation. Neither of these options are recommended. 
     
  • Is seems the Master Flat Frame is not being calibrated with the Master Bias Frame within the calibration process. In general, that is the correct choice but is inconsistent with the treatment of the Master Dark Frame. Were the flat frames calibrated with a Master Bias and/or Flat-Dark Frames outside of the ImageCalibration process?

   These settings may be OK depending the process flow you use but there may be some inconsistency in play which can also affect the quality of your calibrated image subs. These settings should probably be separate discussion. I follow (mostly) the same process flow documented by Chris Foster in a pinned thread here on Cloudy Nights.

- - -

 

   Processing aside, if you could also upload the raw camera frames which correspond to the calibrated frames you already shared, I can look at the required size of pedestal needed to prevent the clipping seen in these calibrated frames.

 

 

John


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#40 basskep

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Posted 17 September 2019 - 09:24 AM

Hi Jdupton,

 

I just uploaded the corresponding raw frames to the same google drive link :-)

Hope it will help.



#41 jdupton

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Posted 17 September 2019 - 10:27 AM

basskep,

 

   Your data resembles my own in a lot of respects. Your raw subs show no clipping at all. They only experience clipping during ImageCalibration.

 

   I looked at the required Pedestal that would prevent the bulk of the clipping. Pedestal values below about 400 ADU had no to little effect. All five frames responded well to Pedestals above 700 ADU. Given your camera, I would recommend that you routinely use a Pedestal of about 800 ADU. That brings the level of clipping down from 783 pixels being clipped to a maximum of 2 pixels being clipped in each of the frames you uploaded.

 

   The exact value of the Pedestal may change a little once you start using a more optimum ImageCalibration process but I think you can safely just set it to 800 and go with that as your new default. If you would like me to reevaluate the required Pedestal after changing you calibration process, I'd be happy to do so. It only take a few minutes using PixelMath as I outlined in a prior posting above.

 

   Thanks again for uploading real live data. That has helped me a lot to be able to see your examples. I will now need a couple of clear nights to play with lower Offset values with my camera to see if helps me get better data.

 

 

John



#42 Jon Rista

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Posted 17 September 2019 - 11:13 PM

Thank you for your reply Jon. 

And just for information you seem to use a ZWO ASI183MM Pro. What offset do you use with it ? And maybe your other ccds ?

I generally use an offset setting of 10. This translates into 50 ADU, but the way Sony designed this sensor (and I think some of their other sensors), they do not allow you to set the offset in ADU directly. But generally speaking, with CMOS cameras like this, 50 ADU is a good offset to use. I recommend for the Panasonic M and IMX183 cameras, use an offset of 50 ADU for all gains. With the Panasonic M, you set the offset directly in ADU, but remember that the IMX183 uses an arbitrary setting in steps of 5 ADU each, so use a setting of 10 in the driver.

 

Some drivers may set the offset automatically. The ZWO driver, for example, will set the offset automatically for their Pro series cameras. You may have the option to disable this and set the offset manually. With the ASI183, the driver will automatically set the offset to 50 (or thereabouts, it may vary slightly depending on the gain, although for a particular gain it will be consistent.)


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#43 basskep

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Posted 02 October 2019 - 07:19 AM

basskep,

 

   Your data resembles my own in a lot of respects. Your raw subs show no clipping at all. They only experience clipping during ImageCalibration.

 

   I looked at the required Pedestal that would prevent the bulk of the clipping. Pedestal values below about 400 ADU had no to little effect. All five frames responded well to Pedestals above 700 ADU. Given your camera, I would recommend that you routinely use a Pedestal of about 800 ADU. That brings the level of clipping down from 783 pixels being clipped to a maximum of 2 pixels being clipped in each of the frames you uploaded.

 

   The exact value of the Pedestal may change a little once you start using a more optimum ImageCalibration process but I think you can safely just set it to 800 and go with that as your new default. If you would like me to reevaluate the required Pedestal after changing you calibration process, I'd be happy to do so. It only take a few minutes using PixelMath as I outlined in a prior posting above.

 

   Thanks again for uploading real live data. That has helped me a lot to be able to see your examples. I will now need a couple of clear nights to play with lower Offset values with my camera to see if helps me get better data.

 

 

John

Thank you John for your investigations. I am sorry to ask but i am quite a newbie in the cmos world and could you please tell me how to use a pedestal of 800 ADU ?



#44 basskep

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Posted 02 October 2019 - 07:23 AM

I generally use an offset setting of 10. This translates into 50 ADU, but the way Sony designed this sensor (and I think some of their other sensors), they do not allow you to set the offset in ADU directly. But generally speaking, with CMOS cameras like this, 50 ADU is a good offset to use. I recommend for the Panasonic M and IMX183 cameras, use an offset of 50 ADU for all gains. With the Panasonic M, you set the offset directly in ADU, but remember that the IMX183 uses an arbitrary setting in steps of 5 ADU each, so use a setting of 10 in the driver.

 

Some drivers may set the offset automatically. The ZWO driver, for example, will set the offset automatically for their Pro series cameras. You may have the option to disable this and set the offset manually. With the ASI183, the driver will automatically set the offset to 50 (or thereabouts, it may vary slightly depending on the gain, although for a particular gain it will be consistent.)

Hi Jon,

 

Yes you're right the ZWO native driver set the offset automatically at 30 for the ASI294MCPRO with unitygain 120. But it's possible to set the offset manually.



#45 basskep

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Posted 02 October 2019 - 08:57 AM

I revised the Gain and Sub-exposure calculation spreadsheet for the ZWO  ASI183 and developed a new one for the ASI294.

 

They are too large to attach, so I setup a read-only Google share drive:

 

https://drive.google...8VJ?usp=sharing

 

Notes:

 

1. Much of the theory and equations came from Jon Rista, Frank, Craig Stark, and others (Thank you!)

2. Blue boxes are for user entry, everything else is calculated for you.  If you don't want to use image data to calculate the sky background and camera dark current, just enter those values directly to the best of your ability.

3. The "Transmission Efficiency" of the entire optical system is likely the largest potential source of error.  Play with it until your data matches a hand-held SQM, or known sky brightness value.  This may change with different filters. I find 0.5 to 0.7 works for me.

4. The common thinking is you want a Swamp Factor of around 10.  However, with narrow-band, especially at darker sites, this becomes impracticable.

5. The 183 tables go from Gain 0 to 270, but Jon Rista warns to not exceed Gain 178 with that camera.

6. I arbitrarily fixed the "object signal" to 22.0 magnitude/square arc-second.  This is to compare imaging an object from two different sites, with different sky brightness, so as to have an apples-to-apples comparison.  If you actually know your object's brightness, change the "22" in that equation to the desired value. This will only affect SNR calculations.

7. It is just a guide.  No precision was intended.

8. The 183 spreadsheet is not all that different form the Gain and Sub-exposure tables posted here:

 

https://www.cloudyni...olour-versions/

 

 

Special thanks to Nik Coli, who has been running these spreadsheets on his 183 and 294 and finding and fixing all my errors.

 

Steve

Hi Steve, Niccoc1603,

 

Could you please explain me how to use the Gain-Sub-Exp spreadsheet for ASI294 ? I am a little bit confused on what inputs do i need to fill in (all the cells in Input from D6 to D21) ? What is the "desired swamp factor" ? The Sub-exp value is minute ?

Thanks a lot for all the work you have done with this great tool :-)



#46 StevenBellavia

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Posted 02 October 2019 - 10:16 AM

Hi,

 

If you know your sky background (from light pollution maps or a hand held SQM), then you only need to enter values in the blue boxes for cells D6 to D21

 

Swamp factor of around 10 is considered "the norm". (There are many CN threads on this).

I am using a Transmission Efficiency of 0.7 with success.  This includes the QE of the camera, plus whatever light is lost via coatings, etc.

For the 294, leave cells D14 and D15 as 550 and 100, respectively, as this is a one-shot-color camera, so I assume no additional filters are used (except maybe a standard UV-IR cur filter, which has no effect).

If you are cooling to -10C (as most are) you can use 0.008 to 0.010 for the Dark Current in Cell D12.  You would have to do tests to get that more accurate, but not all that important.

 

The rest are self-explanatory.

 

And yes, the resulting sub-exp, to achieve a Swamp Factor of 10, is in minutes

 

Good luck!

 

Steve


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#47 basskep

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Posted 03 October 2019 - 02:15 AM

Hi,

 

If you know your sky background (from light pollution maps or a hand held SQM), then you only need to enter values in the blue boxes for cells D6 to D21

 

Swamp factor of around 10 is considered "the norm". (There are many CN threads on this).

I am using a Transmission Efficiency of 0.7 with success.  This includes the QE of the camera, plus whatever light is lost via coatings, etc.

For the 294, leave cells D14 and D15 as 550 and 100, respectively, as this is a one-shot-color camera, so I assume no additional filters are used (except maybe a standard UV-IR cur filter, which has no effect).

If you are cooling to -10C (as most are) you can use 0.008 to 0.010 for the Dark Current in Cell D12.  You would have to do tests to get that more accurate, but not all that important.

 

The rest are self-explanatory.

 

And yes, the resulting sub-exp, to achieve a Swamp Factor of 10, is in minutes

 

Good luck!

 

Steve

Thank you Steve far the information and the great tool you have developed :-) 

Do you know what to do if I use a narrowband LP filter ? 



#48 jdupton

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Posted 03 October 2019 - 02:10 PM

basskep,

 

Thank you John for your investigations. I am sorry to ask but i am quite a newbie in the cmos world and could you please tell me how to use a pedestal of 800 ADU ?

 

   The pedestal is added during Image Calibration in PixInsight. I have forgotten which software you use for image processing. Many astrophoto processing programs allow the addition of a pedestal during image calibration. Some may call it something slightly different. Ask about how to do it in the specific program you use.

 

   For PixInsight, it is added using the ImageCalibration Process. That process window is shown below. The red oval marks the entry box for adding a pedestal. It is specified in PI to be entered in DN or ADU. In your case, you would simply put 800 in that box when calibrating your images prior to stacking.

 

PI_Pedestal.png

Adding a pedestal in PixInsight. The default value is 0 as shown here. Pedestals are limited to 1000 DN  or ADU.

 

 

John


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#49 jdupton

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Posted 03 October 2019 - 02:27 PM

Jon & Steve,

 

I generally use an offset setting of 10. This translates into 50 ADU, but the way Sony designed this sensor (and I think some of their other sensors), they do not allow you to set the offset in ADU directly. But generally speaking, with CMOS cameras like this, 50 ADU is a good offset to use. I recommend for the Panasonic M and IMX183 cameras, use an offset of 50 ADU for all gains. With the Panasonic M, you set the offset directly in ADU, but remember that the IMX183 uses an arbitrary setting in steps of 5 ADU each, so use a setting of 10 in the driver.

 

   The Offsets in the ASI183 cameras result in 5 ADU (in 14 bit measurement space) per Offset increase of 1 (an Offset Scaling Factor of 5x). Note that this is different for different cameras. 

 

   For the ASI294 camera, an Offset change of +1 results in an ADU increase of 16 (in the 14 bit measurement space). Thus, an Offset value of 10 in the ASI294 will result in an ADU value of 160 (again in 14 bit notation). I have noted that there is a quantization error that shows up if you look closely at Offset values from the ASI294 camera. It can vary +/- 1 ADU (14 bit) for most offsets but can grow to +/- 3 ADU (14 bit) for some higher Gain values. It is possible to predict this error, if needed, by adding or subtracting and adjustment from the 16x Offset Scaling result based on the current camera Gain.

 

 

Steve,

   

   This is an error in the ASI294 Spreadsheet. It is using the Offset scaling factor of 5x (from the ASI183) rather than a factor of 16x (for the ASI294). Luckily, it doesn't affect the spreadsheet results enough to even notice.

 

   For anyone using an ASI294 and looking at their Bias or Dark Frames, keep in mind that the delivered frames have been multiplied by 4 by the ASCOM driver (conversion into the 16 bit measurement space) so Offsets will appear to have a Scaling Factor of 64x ADU rather than only 16x.

 

 

John


Edited by jdupton, 03 October 2019 - 02:33 PM.


#50 Jon Rista

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Posted 03 October 2019 - 05:07 PM

Jon & Steve,

 

 

   The Offsets in the ASI183 cameras result in 5 ADU (in 14 bit measurement space) per Offset increase of 1 (an Offset Scaling Factor of 5x). Note that this is different for different cameras. 

 

   For the ASI294 camera, an Offset change of +1 results in an ADU increase of 16 (in the 14 bit measurement space). Thus, an Offset value of 10 in the ASI294 will result in an ADU value of 160 (again in 14 bit notation). I have noted that there is a quantization error that shows up if you look closely at Offset values from the ASI294 camera. It can vary +/- 1 ADU (14 bit) for most offsets but can grow to +/- 3 ADU (14 bit) for some higher Gain values. It is possible to predict this error, if needed, by adding or subtracting and adjustment from the 16x Offset Scaling result based on the current camera Gain.

 

 

Steve,

   

   This is an error in the ASI294 Spreadsheet. It is using the Offset scaling factor of 5x (from the ASI183) rather than a factor of 16x (for the ASI294). Luckily, it doesn't affect the spreadsheet results enough to even notice.

 

   For anyone using an ASI294 and looking at their Bias or Dark Frames, keep in mind that the delivered frames have been multiplied by 4 by the ASCOM driver (conversion into the 16 bit measurement space) so Offsets will appear to have a Scaling Factor of 64x ADU rather than only 16x.

 

 

John

The ASI183 is a 12-bit camera, so a single step in the Offset setting results in 5 12-bit ADU increase. That would scale differently into 16-bit measurements than if you assumed 14-bit. The ASI178 is 14-bit, and has the same pixel size, but the ASI183 is 12-bit.


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