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Sub Exposure tables for the ZWO ASI183MM and QHY183M (and colour versions)

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#1 Shiraz

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Posted 30 April 2018 - 04:47 PM

Hi

Please find attached tables of suggested sub-exposures (broadband and narrowband) for the ASI183MM camera from ZWO. With adjustment, the tables are also applicable to the QHY183M and also to colour cameras using the 183, as explained in later text. The tables should be helpful in navigating between different combinations of camera gain, fNo and sky brightness. Should be good enough for designing a new system, or to see what might happen if you try something different.

 

attached are
BROADBAND - 21.6, 21, 20, 19 sky brightness and
NARROWBAND - 21 and 18 sky

 

To use the tables for the ASI183MM, first determine your sky brightness. If possible, measure with an SQM, but otherwise, use the Bortle scale (http://www.skyandtel....ark-sky-scale/) or find your zone from a sky brightness atlas – maybe try this as a starting point http://www.skyandtel....-your-skyglow/

 

When you have selected the table that best describes your sky brightness/zone, choose a camera gain from the first column – use lower gain for more dynamic range, higher gain for low noise short subs or for narrowband. Suggest starting out at gain 50 for broadband. Now look directly across the row for the chosen gain setting and find the sub-length that is in the fNo column for your scope. If you cannot find the exact fNo for your scope, or want to use a gain value that is not listed, look at the surrounding data and guess – that will be good enough.

 

The value in the table is the shortest sub length that you can use (for that scope, sky and gain) and still be shot noise limited (5% effect criterion) - this is the most time-efficient way to image. The sub length suggestion is definitely not a hard and fast rule though - you can still get images if you use longer subs, but more stars will be saturated and your mount needs to be better. You can use shorter subs, but the total exposure time to get to a given image quality will be longer than need be.

 

If you wish to refine the sub length, first use the camera to take a sub and, with the cursor of the acquisition software, look at the DN/ADU values in a background sky region of the scene (no stars and no hot pixels - and away from vignetting). Dither the cursor about slightly to estimate the average DN/ADU background value in the clear region. From this background DN/ADU value, subtract the bias value for your camera (again - use the cursor to estimate the average DN/ADU signal in a bias frame taken at the same gain, offset, temp as the light sub). Then compare your sub-minus-bias value with the “expected sky DN above bias” in the final column of the table. If your measured sub-minus-bias is lower than expected, increase the sub length to compensate – if you get much a higher value, consider reducing the sub length.

 

The narrowband tables should be used in the same way as the broadband ones, although there are more unknown variables to reduce the accuracy – so use as a guide only. Because of the wide possible variability in parameters, tables have only been generated for the conditions of rural dark sky and full moonlit conditions. With narrowband, you could use the same gain setting as for broadband, but may get slightly better results using higher gain – 150 would be a good value to try. 7nm filters were assumed and, if you have narrower filters, you could use longer subs – (or vice versa). The transmission estimates should be reasonably applicable to commonly used Ha, O3 and S2 filters.

 

The tables can also be used with the QHY183M - just divide the ASI gain figure by 10 - eg gain 10 on the QHY should give results close to those from gain 100 on the ASI. The broadband tables could also be used with a 183 colour camera or with RGB filters - work out the sub exposures for the mono version of your camera and then multiply by 3x to get the sub exposures for the colour version (not exact, but should be close enough). The expected DN value will be the same for both colour and mono cameras.

 

 

Some comments:

 

The tables optimise signal-to-noise/time and do not take into account dynamic range limitations (eg due to skyglow or amp glow) - if dynamic range is an issue, reduce the gain and/or sub length.

 

Don't go by the visual appearance of a sub – short broadband subs with the 183 may look very thin, but when stacked, the final result will be fine

.

The modelling assumes an average optical efficiency. If you use a high quality refractor, slightly shorter subs may suffice and if you use a highly obstructed scope, or one with standard Al mirrors, you may need longer subs.

 

Any change of temperature, gain or offset will require new bias and dark calibration data.

 

The offset should be chosen to give at least a couple of hundred DN/ADU in the bias frame - if you measure less, then increase the offset value.

 

Thanks for looking, hope the above is useful. If you find anything that looks obviously wrong, please point it out.

 

Cheers, Ray

 

 

asi183broadband.jpg

 

 

 

 

asi183narrowband.jpg


Edited by Shiraz, 30 April 2018 - 06:37 PM.

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#2 Jon Rista

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Posted 30 April 2018 - 08:00 PM


Don't go by the visual appearance of a sub – short broadband subs with the 183 may look very thin, but when stacked, the final result will be fine

waytogo.gif waytogo.gif waytogo.gif waytogo.gif waytogo.gif

 

Such a critical point! Thanks!

 

As an added caveat, I would warn people away from using gains over 178 with these cameras (I guess 18 for the QHY). Beyond that gain, the FWC gets so small and the DR drops enough that it isn't all that viable for DSO imaging. You might run into issues with glows, glow magnitude vs. offset issues that lead to clipping, etc.

 

For lucky imaging, planetary imaging, etc. high gains are probably fine, but for DSO, I would avoid going over gain 178 (which is ~0.5e-/ADU, roughly equivalent to gain 200 on an ASI1600).


Edited by Jon Rista, 30 April 2018 - 08:01 PM.

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#3 szg

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Posted 01 May 2018 - 01:48 AM

Also doing research for 183 but my knowledge is just plain technical.

My well dept table shown 183 e- bucket is huge but I'm not sure I use a proper calculation?

 

Sensor Compare.png

 

Also wonder if 183 have micro-lens artifact like 1600MM & QHY163M that use the same sensor?

 

* modified for incorrect ASI1600MM-PRO wrong QE.


Edited by szg, 01 May 2018 - 08:29 AM.


#4 Shiraz

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Posted 01 May 2018 - 02:56 AM

waytogo.gif waytogo.gif waytogo.gif waytogo.gif waytogo.gif

 

Such a critical point! Thanks!

 

As an added caveat, I would warn people away from using gains over 178 with these cameras (I guess 18 for the QHY). Beyond that gain, the FWC gets so small and the DR drops enough that it isn't all that viable for DSO imaging. You might run into issues with glows, glow magnitude vs. offset issues that lead to clipping, etc.

 

For lucky imaging, planetary imaging, etc. high gains are probably fine, but for DSO, I would avoid going over gain 178 (which is ~0.5e-/ADU, roughly equivalent to gain 200 on an ASI1600).

 

I agree and suggested gain 150 for narrowband, rather than higher gain. Considered leaving gain 300 out of the table, but left it in for anyone who might want to try it out - but there is a precipitous loss of dynamic range at such high gain.

 

Also doing research for 183 but my knowledge is just plain technical.

My well dept table shown 183 e- bucket is huge but I'm not sure I use a proper calculation?

 

attachicon.gif Sensor Compare.png

 

Also wonder if 183 have micro-lens artifact like 1600MM & QHY163M that use the same sensor?

the well depth varies with gain so I guess your overall figure is for gain 0. The 183 does seem to have relatively large well depth for the pixel size. As I understand it, the main artefact on the 1600 is due to the AR coating (or lack) on the coverglass. The Sony sensor definitely has AR coated coverglass (confirmed b Sony), so should not have that problem.

 

cheers Ray


Edited by Shiraz, 01 May 2018 - 03:01 AM.


#5 danielguo

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Posted 06 May 2018 - 02:02 PM

Is the broadband table for exposures in RGB or luminance?



#6 Jon Rista

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Posted 06 May 2018 - 02:41 PM

I agree and suggested gain 150 for narrowband, rather than higher gain. Considered leaving gain 300 out of the table, but left it in for anyone who might want to try it out - but there is a precipitous loss of dynamic range at such high gain.

Gain 300 is actually too high regardless. The maximum analog gain on the IMX183 is 270, any higher gain is just digital gain and thus useless.


Edited by Jon Rista, 06 May 2018 - 02:41 PM.


#7 Shiraz

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Posted 06 May 2018 - 04:55 PM

Is the broadband table for exposures in RGB or luminance?

the broadband tables are for full broadband luminance. For RGB filters, multiply the times by 2-3x - the exact value depends on the bandpass of the filters, but 3x should be a good enough starting point. thanks.

 

Gain 300 is actually too high regardless. The maximum analog gain on the IMX183 is 270, any higher gain is just digital gain and thus useless.

thanks for that info. Both makers offer gain above 270 (or equivalent) and both show reducing read noise, so it will be tried.


Edited by Shiraz, 06 May 2018 - 05:08 PM.


#8 Jon Rista

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Posted 06 May 2018 - 05:45 PM

 

thanks for that info. Both makers offer gain above 270 (or equivalent) and both show reducing read noise, so it will be tried.

I confirmed with Sam, as a result of another discussion about offset, that gain 270 is the max analog gain. He said strait up that any higher gain is digital. Even at gain 270, you are already rather limited...about 8.8 stops of DR. Additionally, for DSO work, the glows can clip to black due to noise, which can make proper calibration near impossible.



#9 Pauls72

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Posted 06 May 2018 - 08:59 PM

Also doing research for 183 but my knowledge is just plain technical.

My well dept table shown 183 e- bucket is huge but I'm not sure I use a proper calculation?

 

attachicon.gif Sensor Compare.png

 

Also wonder if 183 have micro-lens artifact like 1600MM & QHY163M that use the same sensor?

 

* modified for incorrect ASI1600MM-PRO wrong QE.

The published full well depth of the IMX183 sensor is 19-20ke-

 

QHY183 has gain settings of 0-54 with unity gain being 11.



#10 Jon Rista

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Posted 06 May 2018 - 10:55 PM

The published full well depth of the IMX183 sensor is 19-20ke-

 

QHY183 has gain settings of 0-54 with unity gain being 11.

We need to make sure we aren't confusing specs between the Panasonic M and Sony IMX183 sensors. 

 

The published FWC for the IMX183 is 15,000e- in 2.4 micron pixels. It's gain range is 0db to 27db. It's read noise ranges from about 3.6e- at minimum gain to about 1.4e- or a bit higher at maximum gain.

 

The published FWC for the Panasonic M series is 20,000e- in 3.8 micron pixels. It's gain range is 0dB to 30dB. It's read noise ranges from about 2.5e- at minimum gain to about 1.1e- at maximum gain. 



#11 calypsob

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Posted 06 May 2018 - 11:15 PM

We need to make sure we aren't confusing specs between the Panasonic M and Sony IMX183 sensors. 

 

The published FWC for the IMX183 is 15,000e- in 2.4 micron pixels. It's gain range is 0db to 27db. It's read noise ranges from about 3.6e- at minimum gain to about 1.4e- or a bit higher at maximum gain.

 

The published FWC for the Panasonic M series is 20,000e- in 3.8 micron pixels. It's gain range is 0dB to 30dB. It's read noise ranges from about 2.5e- at minimum gain to about 1.1e- at maximum gain. 

So zwo uses a Sony in their 183 and qhy has a panasonic?



#12 Shiraz

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Posted 07 May 2018 - 12:20 AM

the tables in this tread apply only to luminance calculations with the QHY183M and the ASI183MM - both of which use the SonyIMX183 chip. The data can also be modified for use with the colour versions of the 183 cameras and for use with RGB filters.

 

The exposure tables for the ASI1600 with the Panasonic chip are available in an entirely different thread, not this one.

 

The "gain" used by ZWO appears to be 10x that of QHY. My understanding is that ZWO gain figure is in tenths of a dB whereas QHY gain is in dB. The tables are based on the published ZWO data, so are structured around the ZWO approach to gain setting, but the text explains how to use them, with reasonable confidence, with the QHY camera.

 

Both cameras have the same gain-0 full well capacity of ~15000 electrons - since they both use the same chip.

 

The software supplied by both makers allow gains of at least 30dB (maximum is 30dB  for ZWO and 54dB for QHY). The tables incorporate a row for ZWO gain 300 (30dB) on the basis that some users may wish to try gain 300, since it is available. However, the text recommends a high gain setting of only 150 and makes the point that, if dynamic range issues intrude, turn down the gain. The tables are not a recommendation that owners should use gain 300, but they provide some guidance should anyone wish to try.

 

Cheers Ray


Edited by Shiraz, 07 May 2018 - 01:37 AM.


#13 Jon Rista

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Posted 07 May 2018 - 01:34 AM

So zwo uses a Sony in their 183 and qhy has a panasonic?

No, and that is exactly the kind of confusion we should avoid. The QHY page says the FWC is 15-16ke-. Both the QSI and ASI 183 cameras use the Sony IMX183 sensor, which has a ~15,000e- FWC according to the specs. 

 

The Panasonic M sensor is used in the ASI1600 and QHY163 cameras. These were released in 2016. THEY have a 20,000e- FWC.


Edited by Jon Rista, 07 May 2018 - 01:36 AM.

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#14 Shiraz

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Posted 07 May 2018 - 01:48 AM

No, and that is exactly the kind of confusion we should avoid. The QHY page says the FWC is 15-16ke-. Both the QSI and ASI 183 cameras use the Sony IMX183 sensor, which has a ~15,000e- FWC according to the specs. 

 

The Panasonic M sensor is used in the ASI1600 and QHY163 cameras. These were released in 2016. THEY have a 20,000e- FWC.

No confusion with the ASI183MM tables Jon - they quite definitely apply only to the IMX183 cameras.

 

Just so everyone is clear, this thread has never been about the Panasonic chipped ASI1600/QHY163 cameras - the tables for those cameras are quite different and are described in a totally different thread.


Edited by Shiraz, 07 May 2018 - 07:29 AM.


#15 Pauls72

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Posted 07 May 2018 - 07:33 AM

No, and that is exactly the kind of confusion we should avoid. The QHY page says the FWC is 15-16ke-. Both the QSI and ASI 183 cameras use the Sony IMX183 sensor, which has a ~15,000e- FWC according to the specs. 

 

The Panasonic M sensor is used in the ASI1600 and QHY163 cameras. These were released in 2016. THEY have a 20,000e- FWC.

Sorry, my mistake. Jon is correct.



#16 vdb

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Posted 09 May 2018 - 02:35 PM

So I bought one too and I'm reading up on the best settings, camera will be installed at a dark site, and will be used with a FSQ 106.

I'm hesitating on the Gain, 53 or 111 ...

would I be ok with G53 and 5min subs for LRGB and 10 min subs for NB ...

Do not see a lot on offset, I settled on 10 for the moment.

 

Any opinions?

 

Thanks,

Yves



#17 glend

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Posted 09 May 2018 - 02:50 PM

Thanks Shiraz for your fine work on those tables. It is interesting comparing exposure times for broadband and narrowband for the 183, with your previous tables on the 1600 performance.  Also thanks to Jon for his comprehensive 183 test thread, very professional and covers all the bases.  


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#18 Shiraz

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Posted 09 May 2018 - 07:28 PM

So I bought one too and I'm reading up on the best settings, camera will be installed at a dark site, and will be used with a FSQ 106.

I'm hesitating on the Gain, 53 or 111 ...

would I be ok with G53 and 5min subs for LRGB and 10 min subs for NB ...

Do not see a lot on offset, I settled on 10 for the moment.

 

Any opinions?

 

Thanks,

Yves

should be a good setup Yves.

 

from the tables, suggest that you could use gain 50 with about 3 minute luminance subs - or gain 0 with about 4 minutes (results would be the same for both). Could use 2-3 times longer for RGB subs.

 

with narrowband, maybe try gain 150 and subs as long as you possibly can - the recommendation is for more than an hour. for maximum efficiency, but shorter will still work

 

Thanks Shiraz for your fine work on those tables. It is interesting comparing exposure times for broadband and narrowband for the 183, with your previous tables on the 1600 performance.  Also thanks to Jon for his comprehensive 183 test thread, very professional and covers all the bases.  

thanks very much Glen - it is fun to do this sort of stuff and if anyone gets a benefit from it, so much the better. The 183 is very good, but the 1600 is still the read noise king of the heap. We are fortunate to have such a choice.


Edited by Shiraz, 09 May 2018 - 08:48 PM.


#19 Jon Rista

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Posted 09 May 2018 - 09:00 PM

should be a good setup Yves.

 

from the tables, suggest that you could use gain 50 with about 3 minute luminance subs - or gain 0 with about 4 minutes (results would be the same for both). Could use 2-3 times longer for RGB subs.

 

with narrowband, maybe try gain 150 and subs as long as you possibly can - the recommendation is for more than an hour. for maximum efficiency, but shorter will still work

I would recommend avoiding Gain 150 unless absolutely, absolutely necessary. The glows are brighter with this camera than with the ASI1600, and at higher gains, they can become more problematic for proper calibration. I think Gain 178 (double unity) is the highest potentially viable for DSO, but you would need to use pretty short exposures). I would try Gain 111 before Gain 150, and see how it goes.

 

I also recommend against using Gain 0 on this camera. It's fixed banding is fairly strong, and it also has random banding that requires a good number of subs to average out. You are at a disadvantage at gain 0 here. Gain 53 (exactly half unity) has much lower banding, and you'll need to use more shorter subs, which will be good for averaging out the random banding.

 

The ASI1600 (especially the Pro, which has very clean darks and minimal FPN) and QHY163 are more versatile cameras when it comes to gain options. Even Gain 0 is an option, and you can use much higher gains as well without worrying about glow clipping. The IMX183's bright glows and banding limit the options more for DSO imaging. That said, it's small pixels allow for longer exposures even at gains above 0, so it does pretty well with LRGB imaging at higher gains than the ASI1600/QHY163.



#20 NorthField

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Posted 09 May 2018 - 09:09 PM

Knowing what the amp glow looks like in a 10 minute sub, I’d hate to think what it would be in an hour > sub

😳

#21 vdb

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Posted 09 May 2018 - 11:35 PM

should be a good setup Yves.

 

from the tables, suggest that you could use gain 50 with about 3 minute luminance subs - or gain 0 with about 4 minutes (results would be the same for both). Could use 2-3 times longer for RGB subs.

 

with narrowband, maybe try gain 150 and subs as long as you possibly can - the recommendation is for more than an hour. for maximum efficiency, but shorter will still work

 

Thanks,

I want to limit the number of gains, so maybe I will shoot bias and darks at gain 53 and 111 ...

I really want to know at which gain this image was taken;

http://fenyeslorand.hu/m106/

 

/Yves



#22 Stamos

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Posted 11 May 2018 - 03:41 PM

Wonderful job Ray!

 

The 5% effect criterion you mentioned in order to be shot noise limited is the same as the "20*rn" one?

Does this give you a minimum exposure length or the optimal one?



#23 vdb

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Posted 11 May 2018 - 04:50 PM

So I went for G:75 Offset:8 and 5 minute subs ...

It seems to work, I'm now fighting with the Tak Cradle, but initial testing is just wow.

17x5min subs, cropped as I don't have darks yet.

 

41997189902_aff7850228_c.jpgM101 by Yves, on Flickr


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#24 Shiraz

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Posted 11 May 2018 - 06:29 PM

Wonderful job Ray!

 

The 5% effect criterion you mentioned in order to be shot noise limited is the same as the "20*rn" one?

Does this give you a minimum exposure length or the optimal one?

thank you.

 

No, nothing to do with 20*RN. The criterion used is the 5% RN effect idea developed by Stan Moore and John Smith when they worked through the theory behind this stuff about 15 years ago (I don't know who had precedence). They concluded that a 5% change in noise in an image was imperceptible and then worked back from that to determine how much sky signal would be needed in a sub to make sure that the read noise contribution to the total noise was below 5%. 

 

On working back, they found that, in photoelectron terms, to keep the effect of the RN contribution to <5%, the sky signal in a sub must be at least

 

Sky = 9.8 * RN^2

 

Note that this 5% criterion does not mean that the RN is 5% of the sky noise as is often misinterpreted - the requirement is that adding the read noise changes the total noise by less than 5% and this criterion is met when the RN is ~ 1/3 of the sky noise. 

 

The data in the tables comes from a fairly detailed end-to-end spectrograph model. It works at a photon/photoelectron level and incorporates sky brightness, target brightness, atmospheric transparency and seeing, optics aperture, fl and efficiency, filter bandwidth and efficiency, tracking error, detector pixel size, QE, RN, gain and well depth. Typical values were used for many parameters, but the detector-specific info was taken from the ZWO website graphs. The sub lengths are determined based on the Moore/Smith criterion and they represent the minimum sub length that will keep the effect of the read noise below 5%. They could be considered "optimal" in the sense that they provide very close to the best possible SNR in the shortest possible time while keeping as much dynamic range as possible. Of course "optimal" is in the eye of the beholder and other issues could influence the decision on what sub length is best.

 

ref: http://www.hiddenlof.../DarkSubExp.pdf

 

http://www.stanmoore...com/eXtreme.htm

 

 

So I went for G:75 Offset:8 and 5 minute subs ...

It seems to work, I'm now fighting with the Tak Cradle, but initial testing is just wow.

17x5min subs, cropped as I don't have darks yet.

 

41997189902_aff7850228_c.jpgM101 by Yves, on Flickr

wow, that's looking good. Guess that cropping is a good approach for many targets.


Edited by Shiraz, 12 May 2018 - 12:36 AM.

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#25 DSO_Viewer

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Posted 12 May 2018 - 12:53 PM

I confirmed with Sam, as a result of another discussion about offset, that gain 270 is the max analog gain. He said strait up that any higher gain is digital. Even at gain 270, you are already rather limited...about 8.8 stops of DR. Additionally, for DSO work, the glows can clip to black due to noise, which can make proper calibration near impossible.

Jon, If I set the gain of the ASI 183 to 178  or ASI 1600 to 200 what would be the recommended offset or brightness be set to please?

 

Steve


Edited by DSO_Viewer, 12 May 2018 - 12:53 PM.



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