Dark universe swamped by colured pixels

[julietdiane]

Skywatcher Startravel 150 refractor and asi120 coloured camera. Mount Skywatcher EQ-6.

Live stacked M51 with 32secs exp, gain 82%, 37 frames stacked but no dark frame subtraction.

No filters fitted and used Bahtinov mask for focusing.

I expected loads of hot pixels from my camera but was taken aback with the dark universe saturation with coloured pixels, see link below:

https://photos.google.com/photo/AF1QipN ... -46aatMcYj



I must be doing something fundamentally wrong, any ideas? - Thanks


P.S. - Sharpcap camera setting:

[ZWO ASI120MC]
Debayer Preview=On
Pan=0
Tilt=0
Output Format=PNG files (*.png)
Binning=1
Capture Area=1280x960
Colour Space=RAW8
Temperature=7.2
Discard Split Frames=Off
High Speed Mode=Off
Turbo USB=80(Auto)
Flip=None
Frame Rate Limit=Maximum
Gain=82
Exposure=32.167079
Timestamp Frames=Off
White Bal (B)=61
White Bal (R)=52
Brightness=7
Auto Exp Max Gain=50
Auto Exp Max Exp M S=30000
Auto Exp Target Brightness=100
Mono Bin=Off
Banding Threshold=35
Banding Suppression=0
Apply Flat=None
Subtract Dark=None
#Black Point
Display Black Point=0
#MidTone Point
Display MidTone Point=0.5
#White Point
Display White Point=1
Notes=
TimeStamp=2020-04-24T22:34:06.9398348Z
SharpCapVersion=3.2.6281.0

[turfpit]

32secs exp, gain 82%, 37 frames stacked but no dark frame subtraction.

Have a look at my post here viewtopic.php?f=18&t=1809 particularly the dark frame for the ZWO ASI120MC with a 60s exposure. A 30s exposure would give a similar hot pixel effect.

You will need to create a master dark and apply it during Live Stack in order to remove the hot pixels.

I suggest experimenting with a lower gain (say around 50%) for that camera. If you post back your results, then include a screenshot of the histogram during capture.

Dave

[julietdiane]

Thanks Dave,

I knew dark frame subtraction would improve the image but thought it was not essential, however

if you look at my image then you would have to say it is totally essential, is this because the uncooled asi120 camera?

Also when you start stacking would you expect the dark correction to begin straight away or get better as the frames progressed?

[turfpit]

In reality, there is no escape from dark frames as these remove noise which is inherent in the sensor. Even with cooled cameras dark frames are still required. Owners of set point cooled cameras have the benefit of being able to control the temperature the camera operates at which in turn allows the ability to build a 'darks library' for re-use.

If at the start of a session you capture 20 darks (10 minutes @ 30s) it will be time well spent in obtaining better images. As the darks are temperature dependent, a good working practice would be to generate a master dark at the start of each imaging session. Remember if the exposure (or gain) changes, then you will need to generate a new dark master. The darks get stored along with their settings in the name, so could be re-used if the temperatures are similar (you would need to measure temperatures).

The dark frame being applied to the light frame will affect that frame - which will be improved as the noise has been removed. The image quality improvement will come from the number of frames stacked. You could try saving the stack at intervals and compare the results later. The benefits of stacking demonstrated here viewtopic.php?f=18&t=1808 - no fancy maths, just pictures.

The ASI 120MC is an entry level camera capable of producing respectable images.
https://www.astrobin.com/search/?q=asi1 ... ng_cameras will give you an idea of what can be achieved.


Below are some links to my (early days) ASI120 efforts.

120MC deep sky viewtopic.php?f=16&t=353 & viewtopic.php?f=16&t=523

120MC planetary viewtopic.php?f=16&t=256

120MC lunar viewtopic.php?f=16&t=1908

Dave

[julietdiane]

All good tips Dave - Thanks.

I certainly will take dark frames before I begin stacking again - not for a few weeks though.

What I like about Sharpcap Live stacking is even though I run unguided it gives me good results

especially as I do not wish to do any post processing....

Take care and happy star gazing.

[oopfan]

Like Dave I too started with the ASI120MC. I've attached a single 120-second light frame of M33 Triangulum Galaxy so you can see how the sensor was swamped by hot pixels. This camera is not designed for Deep Sky. It's best for Lunar, Planetary, and for guiding. You can still get decent Deep Sky images, as Dave shows, but limit yourself to the brightest objects. My next camera was a monochrome Altair 290M (ZWO also makes a 290-based cam.) It was only slightly more expensive than the 120 but I was able to capture much fainter objects. Still, the major problem with the 120 and 290 is that they are not cooled. You will have lots of hot pixels especially in the summer months but if you are careful with darks you can get good results. Currently I have a 10 year old used Atik 314E CCD. It is cooled but not "set point". I get great results nonetheless. I would highly recommend purchasing a cooled camera, even if it is not "set point", if your budget allows for it.

[julietdiane]

Yes exactly what I got, yes I would love a cooled camera but I will have to carry on dreaming and do the best I can with the 120. As you say the

problems get worse over the warm summer nights but I do not like staying up late and restrict my viewing accordingly.

[admin]

Hi,

If you are interested in a little bit of do-it-yourself, then it's perfectly possible to add cooling to that model of camera – the flat back has good for thermal contact to the sensor and is perfect for fitting a cheap Peltier cooler and CPU heatsink/fan - see this thread and many other similar ones that you will find online https://stargazerslounge.com/topic/1891 ... si-120-mc/ . Peltier coolers like the one shown take 12V power (might need 3 to 5 A) and can be bought cheaply on eBay for just a few pounds. You do need to make sure that you have a heatsink attached to them or they cook themselves rather quickly though!

Cheers, Robin

[julietdiane]

Robin, diy cooling fan is not for me. However, I am puzzled: I only made one attempt at imaging before and that was about 3 years ago with the same camera, I tried to image M31. Even though I had problems with my Live stack imaging I did not see any swamping of coloured pixels (as mentioned above)? I did send a post with the M31 problems ( see Nov17 "Newbie needing help with M31" ) and you can clearly see even though I had not dark subtractons my Live stack image had no cluttered with the colour pixels?

[oopfan]

Hi,

I hate saying this but my ASI120MC died within a year. It doesn't even make a good doorstop. It just sits with my stack of stuff reminding me of the 'bad old days'. I think I pushed it beyond its comfort zone. In other words, I may have killed it taking 120s exposures. either that or ZWO has problems with quality control.

Brian