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ASI120MC, star trails and hot pixels?


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At the weekend my Zwo ASI120MC camera arrived, to get me started in imaging. One of the selling points of this is a 150 degree wide angle lens that can take all sky pictures.

My first couple of attempts at this last night were rubbish, mostly due to me not using sharpcap properly. However what I did notice was a huge amount of noise. What I was hoping for was some general shots of the constellations and to use the longer exposures to get some star trails. On especially the longer exposures, the image was covered in red, green and blue dots.

The following image was taken as a snapshot, with an exposure of 10 seconds and the lens cap on:

Capture29_07_201322_36_49.png

This is the same with a 90 second exposure:

Capture29_07_201322_38_44.png

I would expect a completely black image for both. The pixels on the 10 second shot are in exactly the same place as the images I took last night.

Does this look like a faulty camera with lots of hot pixels, or am I doing something silly? Is setting a long exposure time the best way to get star trails with this camera? The maximum exposure on it is supposed to be 1000 seconds.

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Hello,

I don't think you need to worry, there probably is not a fault with the camera, My ASI120MM is very similar.

The camera has a CMOS sensor which is by nature a noisier detector than a more expensive CCD sensor.

A good quality CCD sensor cost's as much as the entire ASI camera!

This camera is also uncooled so during long exposures thermal noise will be converted into pixel values which you have demonstrated in the difference between the 10 second and 90 second images.

At this time of year the camera would have been operating at around 18 deg C. when the colder weather arrives and the camera temperature falls closer to zero then the thermal noise will be reduced.

You can prove if this is the case by sealing the camera in a plastic bag to keep it dry and placing it in the refrigerator for an hour to drop the temperature to around 5 or 6 deg C. then try the same experiment with the covered camera again, you should see a much quieter image this time. NOTE: NOT THE FREEZER OR ICEBOX YOU CAN DAMAGE THE CAMERA IF IT GETS BELOW ZERO!!!!!

CMOS sensors do not work well below zero, most manufacturers quote an operating range of around 30 dec C. to zero deg C. unlike a CCD sensor that is often cooled to -40 deg C.

When logged into SGL if you search under "Peltier Cooling" you will find a thread discussing adding active cooling to the ZWO camera, this is rather a specialist DIY area but there are a few pictures showing the difference between a CMOS camera running at air temperature and one that is cooled close to freezing.

CMOS cameras like the ASI are primarily aimed at planetary imaging where the exposure time is counted in milliseconds and not minutes, with these very short exposure times then thermal noise is not a big issue.

To get around this problem you need to take a "dark" frame, this is exactly what you did by covering the sensor, then you take your "light" frame with he camera recording the sky, both the dark and light have to be taken with the same exposure time, now you subtract the two in a piece of image processing software and most of the random sensor noise and hot pixels will disappear to leave just the star field.

When you set the exposure time used you have to find by experiment a time that gives the minimum number of saturated pixels and this will depend on air/camera temperature as well as the brightness of the object and the level of background light pollution.

Here is a link to a free image processing package called IRIS, there are good tutorials on the web site as well as the software download link:

http://www.astrosurf.../buil/index.htm

And here is a link to a widely recommended book by Steve Richards, "Making Every Photon Count" aimed at newcomers as well as the more experienced, explaining the practical and theoretical knowledge required to obtain great astro-photographs.

http://www.firstligh...e-richards.html

There are several more free image processing packages available and you will also find many experienced astro photographers using paid for packages like MAXIM DL, ASTROART, NEBULOSITY and of course PHOTOSHOP in either the full CS or "Elements" versions.

If you follow the various threads on image processing here at SGL you will find plenty of help and pointers, just take your time, it took me over a year to begin achieving fair results when I started astro photography and there wasn't anything like the amount of web based information available.

You will find after a while to take a lot of manufacturers advertising for their products for the over inflated hype that it really is, they never explain just how complicated it can be to achieve those "example" images shown in the glossy literature....

So welcome to the complex world of astrophotography, a place you will find rewarding and frustrating in equal measure!

William.

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I think the camera can be quite noisy in some circumstances. Cooling may help a fair bit (someone has posted details of a Peltier cooling system they added to theirs), particularly on warm summer nights. When I was imaging in France a few weeks ago the sensor on my ASI120MM was reporting temperatures of 52C at one point (this was during the day, so I'd guess it was about 20C above ambient).

As regards star trails, I'm really not sure how suitable this camera would be. Are you using it with the shipped lens, or something else? It may not cover much of the sky depending on the lens you're using. I think I'd go for lots of short-ish exposures (20 to 30 seconds perhaps?), take dark frames as well and stitch them all together with the software from www.startrails.de.

If you have a DSLR that can be controlled with a programmable release however, that may be the better way to go for star trail images.

James

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Wow, many thanks for taking the times to write such detailed responses. I will take a look at those links and try the camera in the fridge. It was a very warm evening, I was out in shorts feeling very comfortable.

Yes, I was using the 150 degree lens that shipped with the camera. I was going to use it for trying to record the perseids. I will try lots of 5-10 second exposures and see how that goes.

I don't have a DSLR unfortunately. I do have a Fuji FinePix S5500, which is a fixed lens camera with many DSLR like features. It has a maximum exposure of 15 seconds but no live view, so getting the focus right is guess work. With the star trails software you suggested, I probably can generate a trailing picture. It does not have a programmable release, but does have a 3,5 or 10 second count down timer and 3 frame multi-shoot. I'll have to sit and press the button every 55 seconds, but it should work.

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At the weekend my Zwo ASI120MC camera arrived, to get me started in imaging. One of the selling points of this is a 150 degree wide angle lens that can take all sky pictures.

My first couple of attempts at this last night were rubbish, mostly due to me not using sharpcap properly. However what I did notice was a huge amount of noise. What I was hoping for was some general shots of the constellations and to use the longer exposures to get some star trails. On especially the longer exposures, the image was covered in red, green and blue dots.

The following image was taken as a snapshot, with an exposure of 10 seconds and the lens cap on:

Capture29_07_201322_36_49.png

This is the same with a 90 second exposure:

Capture29_07_201322_38_44.png

I would expect a completely black image for both. The pixels on the 10 second shot are in exactly the same place as the images I took last night.

Does this look like a faulty camera with lots of hot pixels, or am I doing something silly? Is setting a long exposure time the best way to get star trails with this camera? The maximum exposure on it is supposed to be 1000 seconds.

Hi,

These are primarily planetary cameras designed to be used at short exposures and high frame rate, even then you may need a lot of frames for stacking to get rid of the noise and some users also do a dark frame subtraction to take care of the hot pixels. I have an AsI 120 MC and I think that it is a very good camera for the money, these cameras ( QHY 5 LII and OpticStar 131 AG, included , as they share the same sensor) also have some very limited "long" exposure capabilty primarily for guiding purposes, they are usable as a guide camera with up to 10~20s exposure if need be but as they are not cooled and have noisy sensors, needless to say that the noise and hot pixels are abundant. My ASI 120 MC works very well as a guide camera with a 50 mm finder scope. To put your mind at rest there is nothing wrong with your camera, it is behaving as it should. If you wish to use it for exopures up to 30s and I would not advise you to go beyond that, you need to take at least twice as many dark frames, Bias frame, and Flat frames and these need to be stacked and a calibration frame subtraction performed to get rid of the noise( to some extent ) and hot pixels and uneven illumination. The camera with the above work flow and fast optics can be used for very basic DSO imaging on the brightest of the DSOs and it will be a cheap introduction to DSO imaging. What these cameras are not and never will be are DSO imaging cameras, for that you either need a Modded DSLR as an introduction or a Cooled CCD camera. There is a reason that Cooled CCDs cost so much in comparison to even the most expensive planetary cameras. I also think that anyone who advertises these as dual purpose is at the very least being irresoponsibe towards the customer. Enjoy your new camera.

Regards,

A.G

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To be fair, a few people are using them as DSO imaging cameras and turning out quite reasonable images, as the ones here demonstrate:

http://zwoug.org/viewforum.php?f=22

I'm sure it's far from easy though.

James

I agree with you James, but I think that the majority are seasoned imagers, some of the publicity images on the site were certainly taken with large aperture scopes,12" or so I believe.

It is certainly doable but as you say not easy.

Regards,

A.G

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Cheers for the advice, it does seem to be heat related.

I stuck the camera in the fridge for 5 hours then captured some dark frames in a pretty warm house, with the camera sitting on an icepack. The sensor was reporting it was 11 degrees. The 10 second exposure was reduced to around 8 hot pixels and the 90 second looked pretty much like the 10 second from the previous night. Once I left it for half an hour to warm up to 35 degrees, the noise was back.

Doing DSO work would be nice, but my scope (NexStar SLT 90) will not allow a long exposure either. For now I'll concentrate on learning the trade with the moon planets and stacking short exposure for star trails/meteors.

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It's worth looking up that Peltier cooler mod if you fancy trying longer exposures. I think it made quite a difference.

James

Hi James,

I too have looked into the cooling and bought a couple of the coolers from ebay for about £5.00 but as yet have not done any thing about it. I also wrote to the designer of the camera and was told that they were actually working on a cooled version already. The peltier would work efficiently without being oversize and heavy if the camera had a cool finger installed in the first place, as no one has opened the camera , none that I have heard about anyway, is a mystery to me as most of these cameras are new and still under warranty. In fact I am not sure what is the cooling methode already employed by the designer as all these sensors generate a lot of heat that need to be removed, perhaps convection is the mode of heat transfer but who knows unless camera was opened. If the camera did have a cool finger then the peltier could be connected to the finger and a small unit with fan and cooling fins would suffice. This would not add to the overal weight of the camera by much either which is important as most of us have focusers with less than ideal mechanical integrity due to price constraints. As for the camera in the fridge, I would not advocate this as the camera has no provision for removal of moisture build up, nor does it have a seald chamber for the electronics. The condensation could in the long run damage the camera.

Regards,

A.G

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Ah! Here it is:

http://www.astronomy...242988-1-1.html

The page is in Chinese (I assume) and the images take a while to load, but it shows the case being opened up to do a cold finger cooling mod. I should post this back in the other thread here I guess.

James

Hi James,

Many thanks for taking the trouble of posting the link, it confirmed my intial impression that the camera has no assisted passive cooling than the air trapped inside the case, convection I believe. I also liked the mod as it used a small aluminium finger to carry the heat to the small peltier and the cooling fins. I think there is potetial for improvisation.

Regards,

A.G

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I probably will construct a cooler, but wait a couple of months. To run a cooler, I will need a power tank (I currently survive on AAs!). That is on my list, but having just bought the camera I need to wait for my astro budget to fill up again.

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Many thanks for taking the trouble of posting the link, it confirmed my intial impression that the camera has no assisted passive cooling than the air trapped inside the case, convection I believe. I also liked the mod as it used a small aluminium finger to carry the heat to the small peltier and the cooling fins. I think there is potetial for improvisation.

No problem :) Should be interesting to see what can be done.

James

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NOTE: NOT THE FREEZER OR ICEBOX YOU CAN DAMAGE THE CAMERA IF IT GETS BELOW ZERO!!!!!

CMOS sensors do not work well below zero, most manufacturers quote an operating range of around 30 dec C. to zero deg C. unlike a CCD sensor that is often cooled to -40 deg C.

Lot of good information in your post, but i'm a bit confused about this though.

The sensor in this camera is designed to have an operating temp of -30 to +70 C, as well as the complete camera itself is designed to be used outdoors, when it's dark (wich means winter and very cold for many), so basically, it's designed for both cold and outdoor usage.

Moisture will build up and can potencially cause damage if taken inside into a heated room after it's been out in very cold temp, and then taken into use before drying completly, and i guess this is why you don't recommend the freezer? But otherwise i can't see how below 0 can damage the camera (the temp itself)?

Unless i've missunderstood something very fundemetal about a CMOS sensors VS CCDs in low temperatures?

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Moisture will build up and can potencially cause damage if taken inside into a heated room after it's been out in very cold temp, and then taken into use before drying completly

The same thing can happen when the camera is being used if it is also being cooled. If the camera is cooled below the dew point whilst imaging then moisture can form inside. That's something that appears to be discussed in the link to the Chinese website that I posted, but I can't make sense of the discussion, even after google has attempted to translate it for me.

Jamesj

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  • 1 year later...
  • 8 months later...

Hi Dave

my experience to date has all been with DSLR's and with some success and lots of failures. I have just bought the ZWO 120 colour version basically as it could be used as a guide camera has its own lens but read lots about it being used for imaging planets or some DSO's.

Having just has a play was really not impressed with the hot pixels and it seems that my dream of it being a cheap all rounder has just gone t*ts up. 

I know this is an old post and you may not even be imaging anymore but can I ask if you got anywhere with this camera? as one of the replies states it is possible but not easy (but then what is in this hobby!)

Regards

Bill

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Hi Dave

my experience to date has all been with DSLR's and with some success and lots of failures. I have just bought the ZWO 120 colour version basically as it could be used as a guide camera has its own lens but read lots about it being used for imaging planets or some DSO's.

Having just has a play was really not impressed with the hot pixels and it seems that my dream of it being a cheap all rounder has just gone t*ts up. 

I know this is an old post and you may not even be imaging anymore but can I ask if you got anywhere with this camera? as one of the replies states it is possible but not easy (but then what is in this hobby!)

Regards

Bill

Bill did you try making some adjustments to the gain?

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Hi JayStar

This all started because the weather has been so awful I have been trying to image out of the spare bedroom window, I went back to playing with the unmodded Philps SPC 900 have which I bought a few years back and getting to learn Sharpcap which then led to this latest purchase (you know how it is) I do have a Celestron 9.25 + VX advanced mount and a pulsar observatory at the back of the garden but haven't had much use out of it so far.

By experimentation I have found that doing a 30 second long exposure with minimum gain is pushing it as compared to a DSLR. Normally I would only do 30 secs to one minute at ISO 400 before LP starts to interfere. I suppose I should take with a pinch of salt what advertisers say about deep sky objects although I think this camera is capable of some pictures but just not with me! There is a good video of how to do a very cheap mod which looks very neat with a 4cm 12 volt fan and a peltier chip and two heat sinks here

https://www.youtube.com/watch?v=rba94dKfsjs its for the USB 3 version but should work with any ZWO uncooled camera, his video is a bit long winded if you are capable of wiring low voltage stuff so would skip to seeing his images and the mod at the end as its very neat. Of course there are lots of other mods out there all good but not as neat. It will however only cool the back of the camera  so has a limitation but improvement just the same.

To round up the camera is designed for planetary use with high frame rate and short exposure times then stacking the video etc, to use it for DSO's is a challenge but if you have enough patience and skill (and luck) it can be done, its a great bit of kit so all I need is a clear view of Jupiter the Moon or Saturn and I'm away and when I get to my observatory I can use it to get in to guiding.

I am a total amateur at all of this and have been for the last five years, I feel a bit like a football club which keeps spending money but doesn't get the results!

One thing does intrigue me and that that's the live view and stacking in Sharcap, I wonder if anyone has made any images that way? the file is saved as a FITS file but can be easily converted if anyone reading has made use of this I would like to hear of your experience.

Tata for now

Bill

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Hi JayStar

This all started because the weather has been so awful I have been trying to image out of the spare bedroom window, I went back to playing with the unmodded Philps SPC 900 have which I bought a few years back and getting to learn Sharpcap which then led to this latest purchase (you know how it is) I do have a Celestron 9.25 + VX advanced mount and a pulsar observatory at the back of the garden but haven't had much use out of it so far.

By experimentation I have found that doing a 30 second long exposure with minimum gain is pushing it as compared to a DSLR. Normally I would only do 30 secs to one minute at ISO 400 before LP starts to interfere. I suppose I should take with a pinch of salt what advertisers say about deep sky objects although I think this camera is capable of some pictures but just not with me! There is a good video of how to do a very cheap mod which looks very neat with a 4cm 12 volt fan and a peltier chip and two heat sinks here

https://www.youtube.com/watch?v=rba94dKfsjs its for the USB 3 version but should work with any ZWO uncooled camera, his video is a bit long winded if you are capable of wiring low voltage stuff so would skip to seeing his images and the mod at the end as its very neat. Of course there are lots of other mods out there all good but not as neat. It will however only cool the back of the camera  so has a limitation but improvement just the same.

To round up the camera is designed for planetary use with high frame rate and short exposure times then stacking the video etc, to use it for DSO's is a challenge but if you have enough patience and skill (and luck) it can be done, its a great bit of kit so all I need is a clear view of Jupiter the Moon or Saturn and I'm away and when I get to my observatory I can use it to get in to guiding.

I am a total amateur at all of this and have been for the last five years, I feel a bit like a football club which keeps spending money but doesn't get the results!

One thing does intrigue me and that that's the live view and stacking in Sharcap, I wonder if anyone has made any images that way? the file is saved as a FITS file but can be easily converted if anyone reading has made use of this I would like to hear of your experience.

Tata for now

Bill

I actually use mine for guiding most of the time and also for planetary - agree that longer exposures are a bit hit and miss.

Great comment about the football team feeling - I am in the same boat (as a lot are).  That's part of the fun  - so much to learn and so much to aim for given the truly stunning pictures I see posted here on this forum.

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