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ZWO ASI 178MM/MC -- Do They Have Pattern and Banding Noise?

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#1 james7ca

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Posted 06 January 2016 - 02:04 AM

I suspect that they do, but is it really less than what appears on the ASI 174MM/MC?

 

ZWO says that the ASI 174 is somewhat unique in the amount of banding or fixed pattern noise it produces because (perhaps) it uses Sony's relatively new Pregius global shutter technology. However, I've seen comments that suggest that the other CMOS cameras from ZWO also have similar noise problems.

 

I downloaded the sample DF that ZWO offers on the ASI 178MM-Cool product page but that seems to be from an ASI 174MM rather than the suggested 178MM (the link just says: "Here is a 60s long exposure dark file of this Camera"). However, the FITs header says the camera resolution is 1936 x 1216 and the pixel size is 5.86um which is what the ASI 174MM has, not the 178MM.

 

So, is there anyone using the ASI 178MM that can comment on the banding and fixed pattern noise that they see in their subs? Best case would be someone who actually has one of the new ASI 178MM-Cool cameras. A simple, stretched sample from a short exposure DF would be fine.

 

In any case, here are two sample DFs from an uncooled ASI 174MM, one at a 1 second exposure time and another with a 16 second exposure (these are both stretched, in the linear mode all you will see is some hot pixel artifacts). These have also been sized reduced and compressed to meet the size restrictions on CN (size reduction from 1936 x 1216 to 1600 x 1005). Top (or first) image is 1s, next is 16s.

 

The air temperature at the time the 1s DFs were captured was 38F (I have no record of the internal camera temperature because Sequence Generator Pro doesn't report the camera temps for the ZWO ASI174). The air temperature for the 16s DFs was 37F. The gain settings for both was 179 (unity gain, or the H-gain setting in the ZWO drivers).

Attached Thumbnails

  • DF 1 x 1s Autostretched.jpg
  • DF 1 x 16s Autostretched.jpg

Edited by james7ca, 06 January 2016 - 05:48 AM.


#2 james7ca

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Posted 06 January 2016 - 02:24 AM

Here are some PixInsight Aberration Inspector samples to show the banding and pattern noise at full resolution. Again, 1s DF on top (first) followed by the 16s sample. These are highly compressed but not resized (i.e. full pixel resolution).

Attached Thumbnails

  • DF 1 x 1s Autostretched Aberration Inspector.jpg
  • DF 1 x 16s Autostretched Aberration Inspector.jpg

Edited by james7ca, 06 January 2016 - 04:08 AM.


#3 james7ca

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Posted 06 January 2016 - 02:33 AM

And finally, the master DFs. The 1s master was created from 64 subs, while the 16s master had 54. These images were resized and compressed and they begin to show the vertical blocking artifacts in addition to the horizontal pattern noise. However, since the horizontal pattern noise is somewhat random the masters actually look cleaner than the individual DFs, but in the final processed DSO images the horizontal pattern noise is often quite noticeable (although probably not as prominent as seen in the individual DFs).

 

Once again, the 1s x 64 DF master is on top (first), followed by the 16s x 54 sample.

Attached Thumbnails

  • DF 64 x 1s Autostretched.jpg
  • DF 56 x 16s Autostretched.jpg

Edited by james7ca, 06 January 2016 - 04:10 AM.


#4 james7ca

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Posted 06 January 2016 - 02:37 AM

And for good measure, the master bias created from 256 subs that were each exposed for 1 millisecond. Autostretched in PixInsight, size reduced and compressed for posting to CN.

 

 

Attached Thumbnails

  • Bias 256 x 1ms Autostretched.jpg


#5 Raginar

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Posted 06 January 2016 - 07:49 AM

What kinds of defects are you seeing on the finished image?  Does it process out or is random?



#6 james7ca

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Posted 06 January 2016 - 08:06 AM

What kinds of defects are you seeing on the finished image?  Does it process out or is random?

In the calibrated and processed DSO images you see patterns just like those shown above. Except that the visibility of the patterns and their obviousness varies with the subject and the quality (I believe) of the calibration frames. This is one reason why you will probably need to run so-called debanding routines during your post processing (at least on the ASI174, I don't know for certain about the other CMOS cameras from ZWO). Unfortunately, in my hands those debanding routines may not remove all traces of the "banding" and they also seem to have side effects and thus they sometimes leave artifacts that are difficult to remove or hide in the final image.

 

ZWO seems to suggest that this issue is more evident on the ASI174 cameras, but I'd like to know whether anyone is seeing similar results with the ASI178 (color or monochrome, cooled or uncooled). It might be somewhat of a judgement call, since I'm sure if you look close enough you can probably see this kind of "noise" in just about any CMOS sensor.

 

Here is what a calibrated and highly stretched integration of the Orion Nebula looks like with an uncooled ASI174MM. This was created from a stack of 192 subs that were each exposed for 16 seconds using a Baader 7nm H-Alpha filter (total integration time 51 minutes). Note that you could reduce this pattern noise by collecting even more subs and by using dithering during the capture. There was a bit of dithering during this sequence, but more would have been helpful.

 

You can see what a relatively quick final processing did for this data at this link here on CN (not too bad, but I had to clip the background a little bit to get rid of all traces of the pattern noise):

 

  http://www.cloudynig...4/#entry6971374

Attached Thumbnails

  • Integration 16s x 192 Stretched Size Reduced.jpg

Edited by james7ca, 06 January 2016 - 09:03 AM.

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#7 james7ca

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Posted 06 January 2016 - 08:56 AM

And here is what a calibrated stack of 383 subs exposed for 1 second each looks like (total integration time 6 minutes and 23 seconds, again in H-alpha). This obviously didn't receive enough exposure and to get a satisfactory result you'd have to stack a lot more subs (probably several thousand). It could be used in an HR composition, which is exactly what I wanted. So, here we may have a worse case scenario, way too few subs and too much of a stretch in the processing (something you obviously don't want to do, but it does show the pattern noise).

 

So, would a similarly exposed image look about the same on an ASI 178MM? Maybe even worse because of the smaller pixel size and the lower effective sensitivity?

Attached Thumbnails

  • Integration 1s x 383 Stretched Size Reduced.jpg

Edited by james7ca, 06 January 2016 - 09:03 AM.


#8 Thirteen

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Posted 06 January 2016 - 10:07 AM

I have been using my 174mm-cool for long exposures. With short exposures I have found sometimes SNR isn't enough to devorce the signal from the banding. It is completely gone for longer exposures because it is simply a miniscule amount of the bulk signal.

I haven't had any evidence of banding whatsoever except my one attempt at stacking hundreds of 1s shots. I still can't figure out where the vertical bands were coming from in that case.

Edited by Thirteen, 06 January 2016 - 10:11 AM.


#9 Thirteen

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Posted 06 January 2016 - 10:12 AM

Are you bias subtracting? I mean enough frames so your bias frame is averaged enough?

Edited by Thirteen, 06 January 2016 - 10:17 AM.


#10 james7ca

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Posted 06 January 2016 - 02:12 PM

Are you bias subtracting? I mean enough frames so your bias frame is averaged enough?

The above images had 256 bias frames and the calibration was done using PixInsight's BatchPreprocessing script. The master bias is shown in post #4.

 

Perhaps 256 bias frames are not enough and as yet I haven't tried to create a so-called superbias in PixInsight. In any case, the horizontal patterns are somewhat variable and change over time, so I'm not sure whether a traditional calibration (even if done "better") would completely eliminate their effects. In any case, just from visual inspection a traditional bias subtraction does not seem to change the pattern or banding noise in either the DFs or the final processed images. And no, the bias is not being subtracted twice.

 

In any case, this thread isn't really about the ASI174, what I'd like to see is similar samples from the ASI178.


Edited by james7ca, 06 January 2016 - 02:42 PM.


#11 rkayakr

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Posted 06 January 2016 - 03:19 PM

Here are some darks from my ASI178MC. These were 60 s exposures with gain set to 250 and reported temperature of 19 C. The master is from 10 darks.

 

800 x 800 crop around center

CR_MASTERDARK.jpg

 

 

above 800 x 800 crop with 10x stretch

 

10xCR_MASTERDARK.jpg


Edited by rkayakr, 06 January 2016 - 03:20 PM.


#12 james7ca

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Posted 06 January 2016 - 03:29 PM

rkayakr, thanks for the samples and that SEEMS to be a pretty dramatic difference, but how did you stretch that DF? I'm not familiar with what a 10X stretch would be and I'd think that if the background level on that DF was brought up it would show at least SOME pattern or banding noise. Point of fact, if I reduced the stretch on the above DF and bias samples from the ASI174MM then they too would look almost completely black. 


Edited by james7ca, 06 January 2016 - 03:35 PM.


#13 rkayakr

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Posted 06 January 2016 - 04:18 PM

James

  In ImagesPlus I did a linear stretch by setting the white point level to be 10% of its initial value.

Here is the histogram of the original cropped master dark

 

histo.jpg

 

and here is the histogram of the cropped master dark after stretching

 

10xhisto.jpg



#14 james7ca

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Posted 06 January 2016 - 07:20 PM

rkayakr, okay, but I'm asking what your DF would look like if you brought the background level all the way up to something near 18% gray. That would obviously clip the brighter portions of the image but it would also allow us to see what the pattern and banding noise actually looks like (at least in the DFs, but probably not at all a meaningful view for the final images, being pixel peeping to the extreme).


Edited by james7ca, 06 January 2016 - 07:33 PM.


#15 james7ca

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Posted 06 January 2016 - 11:12 PM

Okay, I more or less surrendered to the little child in me and I went out and picked up one of the new ZWO ASI178MM-Cool cameras at OPT. Yesterday was the first time that these were in stock and this may be one of the first reports on this particular version of the cooled ZWO cameras.

 

Everything in the package seemed fine and the camera body seems substantial and well made. The heat sink is really huge and I suspect there should be no problem in keeping the camera nice and cool (assuming that the remaining components function correctly).

 

One warning to other potential purchasers, the camera does NOT come with a power cord for the 12VDC that is needed for the cooling (I already knew that, but I don't think that the sales person at OPT did).

 

So, here are the first DF samples from an uncooled ASI178MM-Cool (I think the camera temperature was about 21C as reported in the FITS header). These were taken under maximum gain (510) and they were stretched to exactly the same level as the samples I provided earlier for the ASI174MM. These look very different, more like random read and thermal noise with just a little banding (apparently, too early to know for sure). In fact, the noise is so random that I couldn't present these samples at the same reduction and compression level as I did with the ASI174MM and still get under the size limitation on CN. The amp glow seems to occur in three corners, but it doesn't seem to extend as far into the frame as on the ASI174MM. I'm going to try and do some actual image captures tonight to see what a calibrated image sample looks like (indoors, since it's raining where I live and I may not see clear skies for several more days).

 

Note, the subs taken with the ASI174MM were taken at a much lower air temperatures (approx. 37F) while the following subs for the ASI178MM were captured indoors at about 70F. So, that would be a significant advantage for the ASI174MM. In fact, this could account for much of the difference in appearance between the two cameras. Thus, I may have to redo the DFs for the ASI174MM.

 

Here are the images for the new ASI178MM, a single DF taken at 1s (top or  first image) and then a single DF exposed for 16s. These were stretched in PixInsight and then reduced in size and converted to a highly compressed JPEG. Your mileage may vary and these represent the extreme limits of pixel peeping as far as the noise floor is concerned (but still similar to what I did earlier on the ASI174MM).

Attached Thumbnails

  • ASI178MM DF 1s Stretched.jpg
  • ASI178MM DF 16s Stretched.jpg

Edited by james7ca, 07 January 2016 - 03:38 AM.


#16 Raginar

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Posted 07 January 2016 - 03:47 AM

Wow. That's really cool. Thanks for being so systematic about posting this. It's a good read.

#17 james7ca

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Posted 07 January 2016 - 05:36 AM

Raginar, thanks for the notice. However, I think thus far the results have been somewhat inconclusive at best and perhaps even misleading or simply incorrect at the worst. But, I plan on doing better with a closer match of conditions between the cameras.


Edited by james7ca, 07 January 2016 - 05:36 AM.


#18 Thirteen

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Posted 07 January 2016 - 07:03 AM

If you'd like anything from the cooled camera to add to your comparison let me know.

#19 james7ca

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Posted 07 January 2016 - 07:45 AM

If you'd like anything from the cooled camera to add to your comparison let me know.

Thanks, but I have one of the ASI178MM-Cool cameras now.



#20 Thirteen

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Posted 07 January 2016 - 08:21 AM

Nice! ... though i should have been clear, I meant the 174 for comparison.

If I wasn't working at a slow long focal length, that 178 would have been my buy. I still may look for one if I pick up a new scope after galaxy season.

Edited by Thirteen, 07 January 2016 - 08:23 AM.


#21 james7ca

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Posted 07 January 2016 - 09:18 AM

I'm going to use the ASI178 with my short focus refractors and for lunar and planetary work with my SCT. In both cases the sampling is just about perfect and I was getting tired of the square and diamond-shaped stars I was getting with the larger pixel ASI174. However, the ASI174 is still nice for wide field lunar work and it's probably a better match for DSOs with the SCT.



#22 N1ghtSc0p3

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Posted 08 January 2016 - 03:19 PM

James,

Do you think the ASI178 would be a good fit for either a WO Star71 or a small Borg (55FL)?  I also have a 6" RC that I'm still trying to match to a reasonably priced camera.  Apologies for my lack of comprehension of pixel size v. focal length...that's one of the more technical aspects that still confuses (confounds?) me. 



#23 james7ca

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Posted 08 January 2016 - 04:45 PM

Having smaller pixels is a two edged sword. It can help with the sampling on a short focus telescope but it also provides less sensitivity because each pixel (being smaller in area) will collect fewer photons per unit of time (exposure). Remember also that smaller pixels are likely to show more problems in the quality of your optics, focus, seeing, and guiding. Note that this sensor apparently doesn't support hardware binning, so binning the smaller pixels won't gain you anything over what a simple resampling would do during the post processing.

 

Whether the ASI178MM will be a good sensor for DSO work will depend upon its noise characteristics and here I'm not just talking about read noise (which is fairly low on this sensor), but instead things like amp glow and pattern noise. If you have some sharp and well-corrected optics, focus well,  guide well, and have good seeing then the ASI174MM could be a good sensor for DSOs when paired with a small scope. However, since its noise characteristics are somewhat unknown at this time it could also represent a bit of a risk.

 

In any case, it should be a good sensor for high resolution work on SCTs since it will come very close to providing critical sampling at the prime focus without any barlows. Similarly, with a 2X barlow or Powermate it is also a good match for f/5 to f/6 refractors.

 

The color versus mono issue is another thing to consider. Getting the one-shot-color camera certainly simplifies the process of acquiring images, but of course it limits what you can do in the narrow band and it also means that you sacrifice luminance sensitivity while gaining in speed for capturing simple RGB images (since you can capture all three RGB channels with one sub when using the "color"sensor). However, going the color camera route means you also give up some resolution, since you must use four pixels from the Bayer pattern on the color camera to make one pixel in the final RGB result. Then there is the additional cost of the filters if you are using a mono camera (and the filters themselves can cost more than the camera itself).

 

Basically, I don't think that there is an "easy" decision on either the camera (ASI178) or whether you want the color or mono version. The reason I finally pulled the trigger on the ASI178MM is that I want to try and push to the extremes of lucky imaging, both for high resolution work and on DSOs. Whether it will work well for the latter is, I think, still unknown.


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#24 mfarrell

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Posted 09 January 2016 - 02:32 PM


So, here are the first DF samples from an uncooled ASI178MM-Cool (I think the camera temperature was about 21C as reported in the FITS header). These were taken under maximum gain (510) and they were stretched to exactly the same level as the samples I provided earlier for the ASI174MM. These look very different, more like random read and thermal noise with just a little banding (apparently, too early to know for sure). In fact, the noise is so random that I couldn't present these samples at the same reduction and compression level as I did with the ASI174MM and still get under the size limitation on CN. The amp glow seems to occur in three corners, but it doesn't seem to extend as far into the frame as on the ASI174MM. I'm going to try and do some actual image captures tonight to see what a calibrated image sample looks like (indoors, since it's raining where I live and I may not see clear skies for several more days).

 

Note, the subs taken with the ASI174MM were taken at a much lower air temperatures (approx. 37F) while the following subs for the ASI178MM were captured indoors at about 70F. So, that would be a significant advantage for the ASI174MM. In fact, this could account for much of the difference in appearance between the two cameras. Thus, I may have to redo the DFs for the ASI174MM.

 

Here are the images for the new ASI178MM, a single DF taken at 1s (top or  first image) and then a single DF exposed for 16s. These were stretched in PixInsight and then reduced in size and converted to a highly compressed JPEG. Your mileage may vary and these represent the extreme limits of pixel peeping as far as the noise floor is concerned (but still similar to what I did earlier on the ASI174MM).

 

James,

 

Thanks for the early report on the new ASI178-MM COOL.  I have my eye on the 178MC but I'm waffling between that and the 224MC.  Anyway, these dark frames at indoor temperature look very encouraging for the 178.  I would say they appear better than the 174 dark frames you posted earlier in the thread, with respect to banding and amp glow.  I'm assuming that the 30F temperature difference accounts for the increase in random/thermal noise with the ASI178, but it also looks like the gain was different between the two cameras.  In fact between the sample darks on the ZWO website and your examples here, it looks like the ASI178 may have the most uniform dark frame of all the current ASI cameras with minimal banding and amp glow. 

 

Were the ASI178MM dark frames both processed identically?  If I look at the histogram stats for a center portion of the frame (were there is no amp glow) the 16s image has a lower mean and a lower standard deviation than the 1s frame.

 

Have you had an opportunity to do more dark frame tests at colder temperatures?  If I may be so bold, I would like to propose some tests as a start which I think would help determine the benefit of cooled vs uncooled.

 

Room temperature (~70F): 1s, 16s, 60s

"outdoor temp" (~40F): 1s, 16s, 60s

Cooled (~0F or similar): 1s, 16s, 60s

 

For my own use, I would primarily use a 178 for planetary but I would like to dabble in lucky-style imaging or just large stacks of short exposures on brighter DSO, whether it be planetary nebulae, M42/Trapezium, etc.  I'm wondering just how much benefit the cooled version would provide and if there's a "break point" in exposure time where the cooled version begins to show a big advantage.  I use a cooled Canon 6D (from CentralDS) for my normal DSO imaging and the cooling definitely helps there with 5-15 min exposures.

 

Regards,

Matthew



#25 james7ca

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Posted 09 January 2016 - 06:57 PM

The images from the 178MM were done at a different gain level than with the 174MM (obviously, being different cameras) and at different temperatures. The post processing stretch on the 1s samples are the same between the two cameras but the 16s samples received a different stretch than the 1s DFs. So, the 1s samples are matched and the 16s samples are matched but the 1s and 16s samples were not processed the same. 




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