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Splotches!- My image has dark measles!

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#1 dcm_guitar

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Posted 24 January 2021 - 08:52 PM

Hope the collective brain can help.  Please see the link below:

 

https://astrob.in/vzg6ta/0/

 

My image is covered in dark splotches.  I'm not sure what happened or how to get rid of them.

 

Details:

  • The image is a stack of 34 lights
  • 300 second exposures using a ASI017 MC Pro (the camera is less than one month old.  I bought it brand new)
  • Stacked in Pixinsght.  I calibrated with a superbias, a master dark (120 seconds) and flats taken the next morning (used a light box tracing program on my ipad through a t-shirt as a light source, kept the focus identical to the focus used the previous evening)

These dark splotches are visible on each of my individual subs.

 

I read that using optimize in Pixinsight allows the darks to be scaled appropriately to the light exposure time.  I'm HOPING that my darks didn't work because the scaling didn't work for such a large change in exposure value.  I'm creating a new set of darks at 300s exposure right now, but it's taking 2.5 hours to get the 30 darks I want.

 

  • Is there something wrong with my camera?
  • Is there something wrong with my data collection process?
  • Is this simply a calibration issue and getting a good set of 300s darks will cure it?  

 

I'm definitely at a loss as to what to do.

 

Thoughts?  Suggestions?

 

As alwsys, THANK for the insight!!!!



#2 Dynan

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Posted 24 January 2021 - 09:00 PM

Can you upload and link a stacked image and a master flat?



#3 dhaval

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Posted 24 January 2021 - 09:03 PM

For CMOS cameras, you need the darks to match lights both in terms of duration and temperature. 

 

CS!


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#4 TxStars

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Posted 24 January 2021 - 09:09 PM

Looks like dust on an optical surface.



#5 imtl

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Posted 24 January 2021 - 09:17 PM

For CMOS cameras, you need the darks to match lights both in terms of duration and temperature. 

 

CS!

With duration, it is only for the ones with prominent amp glow.


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#6 ngc7319_20

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Posted 24 January 2021 - 09:25 PM

Looks like dust / dew / frost on sensor window


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#7 Midnight Dan

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Posted 24 January 2021 - 11:12 PM

Due to how small they are, that can't be dust on the camera window or filter.  And it doesn't look like any frost I've seen.  And the fact that you see it on individual light subs means it's not a calibration artifact.

 

My guess is that one of the camera's internal desiccant tablets crumbled and you have particles all over the sensor.  Look at it with a strong light and see if you can see anything on it.

 

 

-Dan


Edited by Midnight Dan, 24 January 2021 - 11:13 PM.

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#8 dcm_guitar

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Posted 24 January 2021 - 11:45 PM

Can you upload and link a stacked image and a master flat?

Sure:

 

Stacked image with splotches:  https://ibb.co/HBvQ7zG (it's a jpeg)

 

Master flat:  https://i.ibb.co/ZYk...3-stretched.jpg



#9 dcm_guitar

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Posted 24 January 2021 - 11:51 PM

I had the camera's dew heater on, so I don't think it's dew.

 

I took a good long look at the sensor.  It looks clean.  I don't think a desiccant tab broke on it.

 

The sensor is covered.  There are tiny dust motes on the sensor glass covering, but I assume that's what the flat would correct.

 

These splotches are really dark and ruining what would otherwise be good data.  I'm stumped, and am now concerned my camera is at fault.  

 

FWIW, I've been doing this for just over a month and all my equipment is new.



#10 Cfreerksen

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Posted 25 January 2021 - 12:10 AM

Just align and stack. If it looks fine then use a flat. If that's fine then add bias. Process of elimination.

 

Chris


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#11 dcm_guitar

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Posted 25 January 2021 - 12:18 AM

Just align and stack. If it looks fine then use a flat. If that's fine then add bias. Process of elimination.

 

Chris

I've done all of this already.  Each individual sub has these dark splotches on them.

 

I have calibrated using bias, dark and flat.  I have also calibrated with all possible combinations of bias, dark and flat.  I have stacked without using calibration frames.  The images have been stacked 7 or 8 times with different options each time.  These dark spots are there and will not go away.  I can not create a mask to get rid of them in processing.

 

I'm really confused and not sure what to do?



#12 ngc7319_20

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Posted 25 January 2021 - 12:34 AM

What is the lens and optical chain?



#13 dhferguson

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Posted 25 January 2021 - 12:44 AM

Cheers,

 

Given the sharpness of the edges on the blotches, I believe them to be contamination on the surface of the detector. Do they also show up on the flat and bias frames? If so, it is definitely your sensor, and your flats/bias were not taken under quite the same temperature/exposure conditions as your light frames, so they don't cancel. If instead the individual flat and bias frames do NOT show the blotches, let us know.

 

Happy observing always,

 

Don



#14 dcm_guitar

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Posted 25 January 2021 - 01:04 AM

What is the lens and optical chain?

 

Optical chain:

William Optics GT81IV

Flat 6AIII flattener

ZWO Duoband Filter

ZWO ASI071MC Pro

 

All the equipment is new.

 

 

Cheers,

 

Given the sharpness of the edges on the blotches, I believe them to be contamination on the surface of the detector. Do they also show up on the flat and bias frames? If so, it is definitely your sensor, and your flats/bias were not taken under quite the same temperature/exposure conditions as your light frames, so they don't cancel. If instead the individual flat and bias frames do NOT show the blotches, let us know.

 

Happy observing always,

 

Don

They are not on the flats.  I've looked at all the flats I took this morning, and the did not have any of the splotches.

 

My bias is a superbias I created a few weeks ago from the Light Vortex tutorial.  I've been using this same superbias to calibrate all my lights.


Edited by dcm_guitar, 25 January 2021 - 01:16 AM.


#15 ngc7319_20

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Posted 25 January 2021 - 01:22 AM

Review on ZWO website:

 

"The Bad: Prone to frost or dew issues even with the heater on and/or issues with cooling performance with warmer ambient temperatures. I have two of these because one is having its cooling section repaired which has to be done in China. I also had to return one because it came with dust on the sensor out of the box. I suspect this is typical for such cameras."

 

There was another review that said the built-in heater was not enough:

 

"On my first night imaging, the sensor began to ice up and the front window fogged up adding insult to injury. Condensation management is critical with this camera. I bought an external dew strip offered by ASI which I view as a necessity with this camera in humid environments like mine. The built-in dew control feature must be turned on in the native driver, but it still requires the additional heating. I also dried the desiccant tablets and reassembled the camera. I haven't experienced the dew issue since, but I don't think the customer should be expected to work quite so hard to compensate for what I feel is a design weakness."

 

My feeling is that this is dew or frost, even though the heater was on.  Maybe compare the first and last "light" images to see if there is evidence of dew / frost increasing or changing.  I think if it was hard dust particles it would not change too much.


Edited by ngc7319_20, 25 January 2021 - 01:38 AM.


#16 dcm_guitar

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Posted 25 January 2021 - 02:04 AM

Review on ZWO website:

 

"The Bad: Prone to frost or dew issues even with the heater on and/or issues with cooling performance with warmer ambient temperatures. I have two of these because one is having its cooling section repaired which has to be done in China. I also had to return one because it came with dust on the sensor out of the box. I suspect this is typical for such cameras."

 

There was another review that said the built-in heater was not enough:

 

"On my first night imaging, the sensor began to ice up and the front window fogged up adding insult to injury. Condensation management is critical with this camera. I bought an external dew strip offered by ASI which I view as a necessity with this camera in humid environments like mine. The built-in dew control feature must be turned on in the native driver, but it still requires the additional heating. I also dried the desiccant tablets and reassembled the camera. I haven't experienced the dew issue since, but I don't think the customer should be expected to work quite so hard to compensate for what I feel is a design weakness."

 

My feeling is that this is dew or frost, even though the heater was on.  Maybe compare the first and last "light" images to see if there is evidence of dew / frost increasing or changing.  I think if it was hard dust particles it would not change too much.

Good thought about the dew / frost.  I took a look at the first image of the evening and the last image of the evening, and the dark spots match up exactly.  I'm trying to find spots n the last image that weren't on the first image (and vice versa), but that's not the case.

 

If these were dust, I would expect to see them on the flats, but I do not.  I DO see some expected dust "halos" on my flats, but all of the flats are clear of these dark spots.



#17 TxStars

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Posted 25 January 2021 - 03:15 AM

I would try another quick image set 10 or so exposures with new data/flats/bias at a higher temp.

Then UPS handle it on the table and shoot another set and see if they match.



#18 Midnight Dan

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Posted 25 January 2021 - 10:03 AM

If these were dust, I would expect to see them on the flats, but I do not.  I DO see some expected dust "halos" on my flats, but all of the flats are clear of these dark spots.

Well that's just wacky! :-). Just to be clear, when you say you see these on every sub, are you referring to calibrated subs or raw subs?

 

And to reiterate, it looks nothing like any frost I've seen, and I've seen quite a bit.  If this occurred on your raw subs, but didn't occur on your flats, then it's got to be a camera issue.  Hopefully a transient one.

 

I'd try shooting some more lights and see if you still get it.  Try to recreate the same conditions for exposure length, gain, and temperature.  You could also try some lights in very different conditions, like during the day at a distant target.  Those tests might give you some insight.

 

-Dan



#19 Cfreerksen

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Posted 25 January 2021 - 10:43 AM

What are the acq parameters? Gain, and offset? Maybe share a link to some un-touched lights/subframes? 

 

Chris


Edited by Cfreerksen, 25 January 2021 - 10:43 AM.


#20 jdupton

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Posted 25 January 2021 - 11:14 AM

dcm_guitar,

 

   To me, these look like tiny dew droplets running down the sensor cover glass. Note that nearly all of them are elongated and in the same direction. It does not look like frost but that may have been the initial cause or perhaps the dew formed directly during cooling.

 

Details:

  • The image is a stack of 34 lights
  • 300 second exposures using a ASI017 MC Pro (the camera is less than one month old.  I bought it brand new)
  • Stacked in Pixinsght.  I calibrated with a superbias, a master dark (120 seconds) and flats taken the next morning (used a light box tracing program on my ipad through a t-shirt as a light source, kept the focus identical to the focus used the previous evening)

These dark splotches are visible on each of my individual subs.

   My best guess is that dew formed on the sensor during the early part of the evening, probably during the initial cooling of the sensor. The droplets ran down the sensor slightly just after formation. Since you took the Flat Frames the next morning, the dew had time to evaporate. By the time the Flats were exposed, all traces were then gone and the Flats looked perfect.

 

   If the above was the case, the only way to recover the Lights is via the use of an Synthetic Flat. It is quite a bit of work but can be made to work well. David Ault has a good tutorial on his Website covering the generation and use of Synthetic Flats.

 

 

John



#21 dcm_guitar

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Posted 25 January 2021 - 11:45 AM

Well that's just wacky! :-). Just to be clear, when you say you see these on every sub, are you referring to calibrated subs or raw subs?

 

And to reiterate, it looks nothing like any frost I've seen, and I've seen quite a bit.  If this occurred on your raw subs, but didn't occur on your flats, then it's got to be a camera issue.  Hopefully a transient one.

 

I'd try shooting some more lights and see if you still get it.  Try to recreate the same conditions for exposure length, gain, and temperature.  You could also try some lights in very different conditions, like during the day at a distant target.  Those tests might give you some insight.

 

-Dan

I've looked at both the raw subs and the calibrated subs and the dark spots are on both.  The spots are not as prominent on the subs, but they are clearly noticeable.  The dark spots don't change shape or move from the first sub to the last sub so I don't think it's frost or dew (it might be, but I don't think so).

 

I'm going to try some exposures during the day and see what happens.

 

Tonight is going to be the only clear night for the next week (at least), so I'm going to dedicate the evening to testing.  

 

I'm thinking the cause is either a defective camera (possible, but unlikely) or user error (tremendously likely).

 

What are the acq parameters? Gain, and offset? Maybe share a link to some un-touched lights/subframes? 

 

Chris

 

I don't know what acq parameters are?  I'm still very new to this. I'm happy to share.  What are acq paramters?

 

Gain and offset.  I THINK this might be the issue, because the reality is I don't truly understand the exposure concept. I've read a LOT and I can recite the definitions.  But I really don't understand the implications, nor how to apply the concepts to set the "right" exposure.

 

Currently, my gain is at 96 and my offset is at 20.  These values come from the results of a Sharpcap image analysis.  Because I don't REALLY know how to best set my exposure, I went with a "cookbook" approach and just took the analysis results.

 

According to the Sharpcap results, 96 is where my camera is at unity gain.  

 

I've read a LOT of conflicting information about the offset setting for this camera (ASI071 MC Pro).  I read this thread:  https://www.cloudyni...ro-cmos-offset/

 

It looks like offset settings have changed with a driver change.  There used to be 3 recommended offset values, then the offset option was removed, then the native driver was updated and the offset adjustment option was returned.  

 

Last night I took a look at examples of all my subs over the last month (when I started this amazing hobby).  Here's what I noticed:

 

  • Until this past Saturday, my images are at 119s and 20 offset for exposure.
  • I've been using the ZWO duoband filter to help combat really bad light pollution (I'm in a bortle 6 zone, but I have two street lights in very close proximity the cause me no end of trouble)
  • In my earlier subs (all at 119s) I THINK i can see these dark spots.  They might be there, but they might not.  In my earlier subs, if they even exist there, they seem to be part of the background. 
  • This set of subs (from this past Saturday) is my first attempt at 300s exposure (still with a 20 offset).  Obviously, these spots are VERY prominent.
  • Looking at the stacked image that started this thread, the vast majority of dark spots have no stars in the background.  A few do have a star or two in the background, but these dark spots seem to be primarily in a dark spot of the sky.

I THINK the dark spots may be the result of too low an offset.  With the offset at 20, there just isn't very much light in the well.  Using the duoband filter restricts a lot of light from hitting the sensor.  If these dark areas on my image don't get many photons (and result in producing very low signal) AND my offset is low I'm thinking this will produce a dark spot that gets exacerbated by stacking.  I DO dither after every shot which is why some of the dark spots have a star or two in the background, but the generalized "roundish" shape of the dark spots is likely due to the dither moving around a dark area that has a low offset.

 

Tonight I plan to shoot 300s exposures with the duoband filter.  I'm going to shoot 2 or 3 images at 20 offset, then switch to 65 offset and see what those 2 or 3 images look like.  Then, I'll remove the duoband filter from the light path and see what those images look like at 20 and 65 offset.

 

Those of you who have a MC071 Pro and shoot in relatively poor light conditions, how do you set your exposure?



#22 Midnight Dan

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Posted 25 January 2021 - 12:07 PM

I'm thinking the cause is either a defective camera (possible, but unlikely) or user error (tremendously likely).

Hmmm ... I think I'm leaning more towards the camera at this point.  I've seen a lot of problem images on these forums, but I have to say I've never seen anything that looks like this, especially if its not in the flats.  Over exposure, underexposure, wrong gain or offset ... never seen them produce this.

 

One thing I'd ask about the flats is how you're looking at them.  If this effect is mostly in the dark parts of the image, it could be a relatively small deviation compared to the overall signal level.  But the average signal level in your lights is very low since the majority of a "light" image is actually the dark background.  Stretching brings up that background level and stretches these small deviations to make them visible.

 

On the other hand, a flat has a MUCH higher average level because most of the image is near the middle of the histogram.  So if you look at a flat by just doing an auto stretch, the stretching will likely be far less and not show minor deviations.  You might want to manually stretch the flat so you can pull the blacks way down, as well as pulling the lights way up.  You want to be looking at a very narrow part of the histogram so that very small deviations will show up.

 

-Dan



#23 dcm_guitar

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Posted 25 January 2021 - 12:22 PM

Hmmm ... I think I'm leaning more towards the camera at this point.  I've seen a lot of problem images on these forums, but I have to say I've never seen anything that looks like this, especially if its not in the flats.  Over exposure, underexposure, wrong gain or offset ... never seen them produce this.

 

One thing I'd ask about the flats is how you're looking at them.  If this effect is mostly in the dark parts of the image, it could be a relatively small deviation compared to the overall signal level.  But the average signal level in your lights is very low since the majority of a "light" image is actually the dark background.  Stretching brings up that background level and stretches these small deviations to make them visible.

 

On the other hand, a flat has a MUCH higher average level because most of the image is near the middle of the histogram.  So if you look at a flat by just doing an auto stretch, the stretching will likely be far less and not show minor deviations.  You might want to manually stretch the flat so you can pull the blacks way down, as well as pulling the lights way up.  You want to be looking at a very narrow part of the histogram so that very small deviations will show up.

 

-Dan

I am using autostretch.  I looked at them in raw form (.fits) as well as debayered.

 

I pulled up a flat (uncalibrated, undebayered) and did a curve adjustment.  I pulled the blacks WAY down and pulled the lights way up.  I can see the dust "halos", but no dark spots.  I moved the points around on the curve adjustment and was able to see all sorts of different things from a dust standpoint, but no dark spots.

 

So, it's either a defective camera or dew/frost that evaporated before I took my flats (the following morning).

 

Tonight will be telling.  I'll run my test and see what happens.

 

I did see on the ZWO website that the camera has a 2 year warranty.  I contacted them, but have not heard back.

 

I sent an email to Agena Astro (where my wife bought the camera for me).  They've already replied (Agena is great!), but the first response was "It's probably dust.  Clean the camera and use calibration frames".  I sent them a jpeg of the photo with spots, so we'll see how they respond.  Unfortunately It's been about 35 days since my wife bought the camera so there will be no exchanges (assuming the camera is the issue.


Edited by dcm_guitar, 25 January 2021 - 12:33 PM.


#24 ngc7319_20

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Posted 25 January 2021 - 01:20 PM

The literature for this camera talks about "improved sealing" of the sensor chamber to prevent "frost".  I wonder if something has gone awry here and the sensor itself is frosting.



#25 dcm_guitar

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Posted 25 January 2021 - 03:04 PM

Update:

 

I've been exchanging email with Agena Astro.  I sent them the splotchy image, and the response was, "I've never seen anything like that."  The folks at Agena forwarded the image to ZWO.

 

I've been emailing back and forth with Agena Support (they really are great!!).  I sent them a flat and a light in .fits format.  I'll see how they respond, but this seems to be quite mysterious to everyone involved so far.

 

Again, I'm going to take more images this evening and see if I can replicate the result.  I'm not sure if I want it to be a frost or a dew issue since I won't trust the camera in cool conditions.  However, I would like to know what in the world happened.


Edited by dcm_guitar, 25 January 2021 - 03:05 PM.



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