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Has anyone come across this - ASI1600MM temp decreases and FWHM increases?

CMOS imaging SCT
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#1 Dereksc

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Posted 22 April 2019 - 04:48 PM

Hi fellow CNers,

 

I have been happily taking photos with my ASI1600MM and AR127 for the last 6 months. For galaxy season I decided to use the ASI1600MM with my C9.25. Since transferring to the C9.25 I noticed that my stars were decidedly egg shaped. I have been going through the usual suspects, guiding, loose connectors, dragging cables, collimation etc but haven't tracked down the problem. Last night I was setting up my system and noticed during preliminary focusing (before I had cooled the camera) my star shapes were much better than my previous imaging session. I then set the camera cooler target to -15C. Once everything stabilized I rechecked focus and noticed that the stars were again egg shaped. I then proceeded to warm the camera up to ambient temperature and looked at the star shapes as it warmed up.  Attached image shows the sequence of images 14C -> -15C -> 1C -> 13C. According to PI sub frame selector (and visually)  the warmer images have much better FWHM and ECC numbers. I only changed the temperature in this sequence everything else (focus etc) remained the same. The images are center crops from the original subs. Has any one seen anything like this before? It seems a very strange result. 

 

Equipment

Scope: Celestron C9.25
Mount: SKY-WATCHER EQ6-R PRO
Imaging Camera: ZWO ASI1600MM PRO
Filter Wheel: ZWO EFW
Guide Camera: ZWO ASI174MM
Other: ORION TOAG

Date captured: 4/21/2019

Data captured:
Multiple x 120s LUM (ZWO 1.25")
Guiding = 0.66" RMS

 

Any ideas would be welcome.

 

Thanks

 

Derek

Attached Thumbnails

  • Workspace01.jpg


#2 fmeschia

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Posted 22 April 2019 - 05:00 PM

Was your telescope completely acclimated by the time you started collecting data?

Maybe the camera housing cools and shrinks, so that the sensor moves off the perfect focus position? 

Francesco


Edited by fmeschia, 22 April 2019 - 05:00 PM.


#3 H-Alfa

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Posted 22 April 2019 - 05:32 PM

The fan of the camera decreased the speed during warming? I read about a user that had an issue with the vibration of the camera due to the fan...Maybe you can check it not powering the camera.

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#4 Dereksc

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Posted 22 April 2019 - 05:49 PM

Was your telescope completely acclimated by the time you started collecting data?

Maybe the camera housing cools and shrinks, so that the sensor moves off the perfect focus position? 

Francesco

Hi Francesco,

 

Thanks for your quick reply.

Good questions .... I believe the telescope was completely acclimated.

 

One further piece of information - I have been struggling with the eccentric stars for a few weeks now and had previously run a full nights imaging session 10pm -> 5am collecting RGB data - all that data looked like the -15C image shown above - that's when I knew I had a problem. I used autofocus (every hour + filter change + 1degC change in temp) so during that session I would have seen a change if it was telescope acclimation.

 

Your second suggestion is intriguing .... if the -15C image was just a little soft on focus I could re-focus and everything would be fine, but my problem is that the stars become more eccentric. I wonder if cooling and sensor shift could cause that?

 

Regards

 

Derek



#5 Dereksc

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Posted 22 April 2019 - 05:56 PM

The fan of the camera decreased the speed during warming? I read about a user that had an issue with the vibration of the camera due to the fan...Maybe you can check it not powering the camera.

Enviado desde mi ANE-LX1 mediante Tapatalk

Thanks for that information, it may be possible, the power source to the camera was not connected on the 1st (14C) which gave the best result. Do you possibly  remember where you read about that?

 

Thanks Derek 



#6 H-Alfa

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Posted 22 April 2019 - 06:27 PM

Yes. I just found it.

https://r.tapatalk.c...36&share_type=t

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#7 Dereksc

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Posted 22 April 2019 - 06:52 PM

Yes. I just found it.

https://r.tapatalk.c...36&share_type=t

Enviado desde mi ANE-LX1 mediante Tapatalk

Great, thanks for that link. I've been looking through the forums for the last few weeks and did not see that. I will go through it.

 

Regards

 

Derek



#8 fmeschia

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Posted 22 April 2019 - 06:59 PM

Hi Francesco,

 

Thanks for your quick reply.

Good questions .... I believe the telescope was completely acclimated.

 

One further piece of information - I have been struggling with the eccentric stars for a few weeks now and had previously run a full nights imaging session 10pm -> 5am collecting RGB data - all that data looked like the -15C image shown above - that's when I knew I had a problem. I used autofocus (every hour + filter change + 1degC change in temp) so during that session I would have seen a change if it was telescope acclimation.

 

Your second suggestion is intriguing .... if the -15C image was just a little soft on focus I could re-focus and everything would be fine, but my problem is that the stars become more eccentric. I wonder if cooling and sensor shift could cause that?

 

Regards

 

Derek

Somebody suggested vibrations from the fan, that’d be worth investigating. Also, is by any chance dew/fog forming on the optical window as temperature goes down?

Francesco



#9 Dereksc

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Posted 22 April 2019 - 07:09 PM

Somebody suggested vibrations from the fan, that’d be worth investigating. Also, is by any chance dew/fog forming on the optical window as temperature goes down?

Francesco

I just read through the link that "H-Alfa" supplied regarding fan vibrations - the images posted there look remarkably similar to mine, so I think I will focus on fan vibration for now.  Thanks for your suggestions. 

 

Regards

 

Derek



#10 HxPI

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Posted 24 April 2019 - 02:11 PM

I would have suggested condensation on sensor window but your images don't suggest that. My guess would be fan vibration is detectable with the C9.25 focal length. I use the ASI1600MM-Cool with an ES127 CF refractor and haven't really noticed or looked into this. Usually the main issue I have is frost on the sensor. Hopefully this is not an issue but regardless it's yet another thing to look out for. Why can't this be a simple hobby, guess because then it wouldn't be interesting!!!

 

Ciao,

Mel


Edited by HxPI, 24 April 2019 - 02:14 PM.

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#11 jerahian

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Posted 24 April 2019 - 02:51 PM

Well heck, I think I have had the same issue and have been attributing the eggs to my poor little AVX, even when it performs quite well w.r.t. tracking.  I’m going to have to perform this controlled test myself now to validate.

 

So, if it is camera vibration with the 1600MM-COOL, what would be my options to dampen the vibration?  Anyone resolved this?

 

-Ara

 

UPDATE:  Nevermind, I just fully read the other talpatalk link attached in the previous post.  Solutions are presented within.


Edited by jerahian, 24 April 2019 - 03:02 PM.


#12 Tapio

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Posted 24 April 2019 - 03:03 PM

I remember someone changing fan motor.
I'm pretty sure I read it here in CN.

#13 kathyastro

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Posted 24 April 2019 - 03:16 PM

I always allow my camera to stabilize at operating temperature for an hour before using it, to ensure that condensation/frost on the camera's window has time to evaporate.  Be sure to focus after the temperature has stabilized, in case the camera's housing expands or contracts slightly when the cooler is operating.  It would not be surprising if the housing expands, since some of the heat removed from the sensor gets dumped into the metal housing as a heat sink.


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#14 Dereksc

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Posted 24 April 2019 - 04:38 PM

Thanks for the comments and suggestions.

 

I believe my problem is fan vibration. I added a small clip to the fan grill on the back of the camera in an attempt to dampen the vibration or change the vibration frequency. The star shape looks a whole lot better. I didn't delve into this any deeper since I wanted to collect some images - cloudless nights have been rare recently. I took 20xR 20xG and 25xB, the median FWHM is < 2 and the median ECC = 0.51.

 

I will probably look at replacing the fan, however I want to investigate some more.

 

I hadn't noticed this originally since I was imaging at a wider field using my AR127, it was only when I started using the C9.25 that I noticed the problem. 

 

Regards

 

Derek


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#15 jerahian

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Posted 24 April 2019 - 05:31 PM

As I mentioned earlier, I've been getting the egg-shaped stars too, but hadn't consideration camera fan vibration as the potential source of the problem.

 

So, I just measured the vibrational frequency of my 1600MM-C using the Vibro Checker app for iOS.  Although a vibration was not discernible by touch, when I did have the fan running powered from the AC adapter, the app measured a clear avg. vibration of ~19.45Hz, with a measured acceleration of 0.0049g.  Converting the acceleration and frequency to displacement, that results in 0.006mm peak-to-peak, which is 6 microns peak-to-peak.  Given that the 1600MM has a pixel size of 3.8 microns, that is a displacement of 1.6 pixels peak-to-peak.  Not insignificant.

 

SampleImage

 

My next move (when it clears up in Maine) is to take a few subs with the cooling fan off vs. cooling fan on, with all other variables controlled as best I can.

 

@Derek, before you replace your fan, I would be interested to see your results as well if you are so inclined to do so.  The app is free on the App Store and extremely easy to use.

 

-Ara


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#16 Dereksc

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Posted 24 April 2019 - 05:55 PM

Hi Ara,

 

 

 

So, I just measured the vibrational frequency of my 1600MM-C using the Vibro Checker app for iOS.  

 

 

Interesting result - thanks for finding that app and posting your result. The displacement you inferred is certainly larger than my image scale so it would be noticeable. I will check out Vibro Checker and see if I can do the measurement. I'll post the result probably tomorrow. 

 

Regards

 

Derek


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#17 Dereksc

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Posted 25 April 2019 - 12:33 AM

Hi Ara,

 

I just set up my equipment ready for tonight and did the vibration measurement:

 

My result is very similar to your result - approx 4um pk-pk. I wonder how accurate the measurement is? The tone disappears if the cooler is off so it is real and it gives a quantitative result so we can re-check after any changes. Thanks again for suggesting that app.

 

Regards

 

Derek

Attached Thumbnails

  • FrequencyImage.jpg

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#18 jerahian

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Posted 25 April 2019 - 01:01 AM

My result is very similar to your result - approx 4um pk-pk. I wonder how accurate the measurement is? The tone disappears if the cooler is off so it is real and it gives a quantitative result so we can re-check after any changes. Thanks again for suggesting that app.

 

Derek, thanks for running that vibration test and reporting back!  It's great you're seeing the same vibrational frequency.  As I mentioned, my next test is to capture images with the fan ON and OFF and qualitatively assess if this has indeed been my issue, much like your case.

 

If I'm convinced this is indeed my culprit, I am going to replace my fan with the one recommended in the other thread and rerun these tests. Actually, I may just reach out to the guy who did do the fan replacement already on the other thread and see if he is willing to run this vibration test.  I'd be curious to see what it shows on his setup, even though we won't have a baseline before he changed his fan.

 

I will update this thread accordingly, if you don't mind doing so yourself as well.

 

-Ara

 

P.S.  Thanks for bringing this topic up on the forum!!  I love this community!


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#19 Dereksc

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Posted 25 April 2019 - 02:03 PM

I was optimistic and decided to image M101 last night. I will have to address the fan vibration. I took 100x180s images split over RGB filters before the fog rolled in at 3.30am. The median FWHM = 2.5 and the median ECC = 0.65. These values are not that good with the elongated star shapes clearly visible in my images. Guiding was 0.65" RMS pre-meridian and 0.53" RMS post-meridian, polar alignment error was 1'.

 

Equipment
Scope: Celestron C9.25
Mount: SKY-WATCHER EQ6-R PRO
Imaging Camera: ZWO ASI1600MM PRO
Filter Wheel: ZWO EFW

Filters: RGB ZWO 1.25"
Guide Camera: ZWO ASI174MM
Other: ORION TOAG

 

I won't have chance to make any changes in the next week since I will be traveling but as soon as I get back the fan will be my first priority. 

 

Please feel free to add to this thread. Thanks for the help.

 

Regards

 

Derek

Attached Thumbnails

  • Master_RGB_04_25_2019_resampled.jpg
  • Master_RGB_04_25_2019_crop.jpg


#20 ChrisWhite

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Posted 25 April 2019 - 02:32 PM

interesting stuff.  Curious what you guys find out with this.  Also the problem may be compounded by soft focus.  This last image you posted looks like it is slightly out of focus.



#21 Dereksc

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Posted 25 April 2019 - 06:08 PM

interesting stuff.  Curious what you guys find out with this.  Also the problem may be compounded by soft focus.  This last image you posted looks like it is slightly out of focus.

Hi Chris,

It is interesting but I would rather not have the issue :). It's also nice to get help from the CN community, I appreciate all the feedback.

 

The image does appear a little soft. I know that at -15C the images I get from my 1600 have a much worse FWHM and eccentricity than images taken at ambient temperature with the camera cooling switched off (my first post). I'm not sure if it also makes the focus a little soft at the same time. Auto focus was run every hour, or every 1 degree change in temperature, or on filter change using SGP. Personally I think if the image becomes more out of focus, then the eccentricity may be less apparent than if the image was in sharper focus, so I  don't think the soft focus is necessarily compounding the problem, but I may be wrong. I think focus is something else for me to address after I get round star shapes.

 

Regards

 

Derek



#22 H-Alfa

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Posted 26 April 2019 - 05:00 AM

As I mentioned earlier, I've been getting the egg-shaped stars too, but hadn't consideration camera fan vibration as the potential source of the problem.

So, I just measured the vibrational frequency of my 1600MM-C using the Vibro Checker app for iOS. Although a vibration was not discernible by touch, when I did have the fan running powered from the AC adapter, the app measured a clear avg. vibration of ~19.45Hz, with a measured acceleration of 0.0049g. Converting the acceleration and frequency to displacement, that results in 0.006mm peak-to-peak, which is 6 microns peak-to-peak. Given that the 1600MM has a pixel size of 3.8 microns, that is a displacement of 1.6 pixels peak-to-peak. Not insignificant.



My next move (when it clears up in Maine) is to take a few subs with the cooling fan off vs. cooling fan on, with all other variables controlled as best I can.

@Derek, before you replace your fan, I would be interested to see your results as well if you are so inclined to do so. The app is free on the App Store and extremely easy to use.

-Ara

Excellent analysis Ara!

Anyway, I'm really worried about the fact that even changing the fan, there will be some amount of vibration (at last, it's a moving component in a camera that needs to be very stable...)

I followed the referenced thread just for curiosity, but reading your experiences, maybe is an issue that all this cameras would suffer in some amount. I will check mine too. Maybe would be useful to post this results in the qhyccd forum...

Regards.

Alberto.

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#23 Dereksc

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Posted 08 June 2019 - 04:29 PM

Just a quick update:

 

I received a new fan from ZWO via High Point Scientific - great service from them both!

 

Cloudy weather hampered my testing of the replacement fan, but finally, last night I got to take some test images.

 

Below left is when using the original fan, below right is using replacement fan:

 

Workspace011.jpg

 

Unfortunately it's not the same star field but I cropped the images to the same scale. The LH image is a center crop the RH image is off to the side a little to avoid the dense star field in M13 core. As you can see the image with the replacement fan is giving a much better star shape. The images were taken at the same temperature (-15C) and same gain but the RH image is a 4 minute sub with 0.88" rms guiding compared to the 2 minute sub with 0.66" rms guiding on the LH side. 

 

It's not an exhaustive test, but so far it seems as though my poor star shapes (LH image) have improved considerably (RH image) by changing the fan in my ASI1600. 

 

Regards

 

Derek

 


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#24 jerahian

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Posted 09 June 2019 - 01:29 AM

It's not an exhaustive test, but so far it seems as though my poor star shapes (LH image) have improved considerably (RH image) by changing the fan in my ASI1600. 

 

Wow, what a significant difference!  Congratulations.

 

Derek, was everything hardware-wise in this test the same as well, like the OTA and your imaging train configuration?

 

Ara



#25 Dereksc

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Posted 09 June 2019 - 09:40 AM

Hi Ara,

 

Thanks.

 

The hardware was the same:

 

Equipment
Scope: Celestron C9.25
Mount: SKY-WATCHER EQ6-R PRO
Imaging Camera: ZWO ASI1600MM PRO
Filter Wheel: ZWO EFW
Filters: LRGB ZWO 1.25"
Guide Camera: ZWO ASI174MM
Other: ORION TOAG

 

I didn't take any L subs on Friday night so the two comparison images are using different filters. In the original test I used the L filter, in the comparison image from Friday night I was using the G filter. However the star shapes were originally elongated when using all RGB and L filters. Now at least my RGB subs are all showing rounder stars :). Only one clear night in the last 7 weeks makes it really hard to fully evaluate changes. On the plus side I didn't miss any imaging opportunities waiting for the replacement fan. Tuesday may be my next clear night. I also want to evaluate Voyager software. Currently I am using SGP but not really happy with the auto focusing routine. Hopefully the weather begins to cooperate soon so that I can get back to imaging. I am hoping that the fan change has provided a robust solution ... more imaging time will tell. 

 

Regards

 

Derek




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