ZWO ASI1600MM-Pro - Darks/Bias Necessary?

[wimvb]

Here are two master dark frames from my ASI174MM-Cool, made up of 30 subs each. Both at -30 C for 30 seconds; left at unity gain (gain setting 189), right at gain of 300. Same screen stretch applied to both images. Darks look much poorer than they really are, because they are stretched so hard. In reality the difference in pixelvalues is quite small.

[mikey2000]

Yep, mine were super-stretched too.  The pixel values are all very close for mine too.  Interesting how your 'banding' is horizontal whereas mine is vertical.  But then again, gain, exp lenght and temp are all quite different.

 

Oh, I just realised, our cameras are different too :-)

[wimvb]

2 minutes ago, mikey2000 said:

Interesting how your 'banding' is horizontal whereas mine is vertical.

The sensors are from different manufacturers. The banding can be related to how the pixels are read (column wise or row wise). Just a guess. The lines can be removed with CanonBandingReduction.

[vlaiv]

3 hours ago, mikey2000 said:

I made two master darks with my camera.  One for 120s, one for 300s.  Both at -20C, Gain 0 (The driver calls it 'High Dynamic Range')  (Stretched to view with pixinsight STF)

I would not go with Gain 0 - too much quantization noise. Use Gain 139 preferably or Gain 79 if you want more dynamic range. Scale your exposure length so you don't get too much saturation. Even if you saturate bright stars, there is a way around it - just shoot some short exposures to recover signal in brightest areas.

 

2 hours ago, wimvb said:

No, just that temperature increases during an exposure, such that amp glow is worse (more than linear) at longer exposure times. You may have noticed that images with sub frame exposures in excess of about 600 s are very rare. Not only are they not needed because of the low read noise, but they are actively avoided because of the increased amp glow. Even if amp glow can be removed by careful calibration, the random shot noise that is associated with it can not.

How long it takes for the increased temperature to reach a stable level, I don't know, but it is probably several tens of seconds.

I don't think that dark current shot noise is really a problem. Look at this graph for ASI1600

At -20C dark current is 3.72e at 10 minute exposure, so associated shot noise would be 1.92e or order of read noise. That is low dark current noise in 10 minute sub.

From the graph we can see that doubling temperature is around 7C for temperatures bigger than 0C, and even a bit bigger for lower temperatures. So even if amp glow is +7C warmer (and I would be surprised that there is such temp differential across the surface of the chip without causing mayhem), dark current in amp glow area will still be less than x2 dark current elsewhere and associated noise would be less than x1.41 - again on 10 minute sub, not something to worry too much about. For amp glow area to be x4 higher in dark current - thus having x2 more noise than elsewhere, that part of sensor would need to be almost 15C hotter than the rest!

If peak temperature in amp glow area is not dependent on surrounding temperature, but on set point cooling only, then it calibration should work each time, but yes one would have to prepare master dark for each exposure length.

Edited February 7, 2018 by vlaiv
some typos ...

[xdjdx]

Hello Mickey

I have an ASI174MM cool, I was unable to remove the Amp Glow, I discovered that covering the sensor with the supplied plastic cap, and then, covering the sensor end of the camera up to the vent fins with four layers of Aluminium kitchen foil, held in place with an elastic band. The amp glow calibrates out with the darks produced. 

[ollypenrice]

13 hours ago, xdjdx said:

Hello Mickey

I have an ASI174MM cool, I was unable to remove the Amp Glow, I discovered that covering the sensor with the supplied plastic cap, and then, covering the sensor end of the camera up to the vent fins with four layers of Aluminium kitchen foil, held in place with an elastic band. The amp glow calibrates out with the darks produced. 

This is an excellent post and deserves a wide public.

Atik provide screw on metal caps for their chip windows, as do some other camera makers. I think they are necessary.

Olly

 

[Demonperformer]

I use my old film-camera changing bag for taking darks.

[xdjdx]

Thanks Olly. 

I have informed Sam at ZWO, as the poor man has had to put up with my questions about the 174 Amp Glow. 

I have just become the proud owner of a 1600 MM pro, and that has had the same cover (different foil)

It appears to me that just using the plastic cover does not prevent light leaks completely, and this maybe why people struggle with Amp Glow removal. 

[vlaiv]

Plastic cover does indeed remove all visible light, but IR might get thru it. Since the mono version does not have IR protection window, it is a good thing to protect it otherwise, like using aluminum foil, or something else that is opaque in IR part of the spectrum.

I usually take my darks by placing camera "face down" on wooden desk (covered with plastic cap of course), and have not had issues with dark calibration.

[xdjdx]

39 minutes ago, vlaiv said:

Plastic cover does indeed remove all visible light, but IR might get thru it. Since the mono version does not have IR protection window, it is a good thing to protect it otherwise, like using aluminum foil, or something else that is opaque in IR part of the spectrum.

I usually take my darks by placing camera "face down" on wooden desk (covered with plastic cap of course), and have not had issues with dark calibration.

Hello vlaiv

Would it be more likely to be UV as this is the more powerful wavelength. 

[vlaiv]

2 minutes ago, xdjdx said:

Hello vlaiv

Would it be more likely to be UV as this is the more powerful wavelength. 

I don't think it has to do with individual photons energy, but rather their wavelength. Longer the wavelength, less chance to interact with matter. It is actually mismatch between wavelength of particle and atomic structure of the matter. So both long wavelengths go thru (like radio waves / microwaves), but also x-rays and gamma - very short, so cross section is small.

Anyway it can be easily checked. Do google search for "Is plastic IR transparent", and "Is plastic UV transparent". Results should give you some hints which is more likely culprit.

[xdjdx]

4 minutes ago, vlaiv said:

I don't think it has to do with individual photons energy, but rather their wavelength. Longer the wavelength, less chance to interact with matter. It is actually mismatch between wavelength of particle and atomic structure of the matter. So both long wavelengths go thru (like radio waves / microwaves), but also x-rays and gamma - very short, so cross section is small.

Anyway it can be easily checked. Do google search for "Is plastic IR transparent", and "Is plastic UV transparent". Results should give you some hints which is more likely culprit.

Looks like both. 

[wimvb]

Have a look here to see the effect of ir through plastic (almost at the bottom of the page). "Clear" glass can be a better ir blocker than plastic.

I use the same method for darks as @xdjdx (nice, cryptic palindrome, btw). And just to make sure no light gets past the foil, I shoot darks at night. Enough cloudy nights for that anyway. The problem isn't so much the amp glow, but rather the noise that comes with it. For any camera with an unhealthy amount of amp glow, it also makes sense to shoot dark flats, and not use the "bias as dark master" method.

[George 47]

3 hours ago, vlaiv said:

I don't think it has to do with individual photons energy, but rather their wavelength. Longer the wavelength, less chance to interact with matter. It is actually mismatch between wavelength of particle and atomic structure of the matter. So both long wavelengths go thru (like radio waves / microwaves), but also x-rays and gamma - very short, so cross section is small.

Anyway it can be easily checked. Do google search for "Is plastic IR transparent", and "Is plastic UV transparent". Results should give you some hints which is more likely culprit.

E= h x c / lambda       E=Energy; lambda = wavelength

[vlaiv]

11 minutes ago, George 47 said:

E= h x c / lambda       E=Energy; lambda = wavelength

Quite right , so my statement "... not energy but rather wavelength" does not make much sense. I was trying to say that it is not only energy that dictates likelihood of interaction.

Here I go again writing nonsense

There is greater chance of interaction at certain energies than on other?

\

 

Edited April 13, 2018 by vlaiv
Scratch that ...

[david_taurus83]

On 10/04/2018 at 18:39, xdjdx said:

Hello Mickey

I have an ASI174MM cool, I was unable to remove the Amp Glow, I discovered that covering the sensor with the supplied plastic cap, and then, covering the sensor end of the camera up to the vent fins with four layers of Aluminium kitchen foil, held in place with an elastic band. The amp glow calibrates out with the darks produced. 

Interesting. Would it be worthwhile, if taking darks still attached to the telescope, to cover the scope cap with foil?

[wimvb]

2 minutes ago, david_taurus83 said:

Interesting. Would it be worthwhile, if taking darks still attached to the telescope, to cover the scope cap with foil?

There's more risk of light leaks that way. With a newton reflector, e.g., light can leak in from the bottom, near the primary. If possible, remove the camera.

[david_taurus83]

I keep getting told that! I have an older blue 200P with the steel plate covering the bottom. I have drilled small holes in the plate to allow access to the locking screws and grub screws for collimation and covered the seams with black tape.

But I take your point about light leak. Might need to look closely at the focuser as it doesn't hug the OTA, it sits flat. And I haven't covered the camera view finder either!

[elNaboet]

I have started to play around with calibration frames and then things got really confusing.

So i have a zwo 1600MM and i understand that no bias frames are required. The problem is all the tutorials i see teach me that to build a Master dark i need to calibrate with a superbias. If i don't have Bias frames how do i build a Master Dark? Is it just stacking all the Darks?

Also how do I incorporate Dark Flats into the calibration frames. 

 

I have been doing batch pre processing and adding all the flats in the flats area and all the darks and dark flats on the darks section then all the lights and thats it. But i have to use all the files as i'm not sure how to create the masters for those.

 

Another question I have which I'm not sure it belongs on this thread is about the drizzle data. I do dithering in all my subs so then i know how to integrate them and create a drizzle imagine but if i try batch pre-processing with the drizzle data it gives me a mismatch (probably because the scale of the image changes with drizzling).

 

Should i do the drizzle work flow first (which includes the initial star alignment) and then calibration. Can i do batch pre-processing and then add the drizzle workflow afterwards? 

[wimvb]

22 minutes ago, elNaboet said:

have started to play around with calibration frames and then things got really confusing.

So i have a zwo 1600MM and i understand that no bias frames are required. The problem is all the tutorials i see teach me that to build a Master dark i need to calibrate with a superbias. If i don't have Bias frames how do i build a Master Dark? Is it just stacking all the Darks?

Also how do I incorporate Dark Flats into the calibration frames. 

Just stack the darks. I've included process icons in a later post of that thread. 

Also check out the preprocessing tutorial on lightvortexastronomy.com

Edited August 13, 2019 by wimvb

[wimvb]

20 minutes ago, elNaboet said:

Should i do the drizzle work flow first (which includes the initial star alignment) and then calibration. Can i do batch pre-processing and then add the drizzle workflow afterwards? 

Do you need to drizzle? The only time you get an improvement with drizzling is when your images are undersampled. Otherwise you just get large files. 

[elNaboet]

On 13/08/2019 at 16:51, wimvb said:

Do you need to drizzle? The only time you get an improvement with drizzling is when your images are undersampled. Otherwise you just get large files. 

I just drizzle because I be heard it's very advantageous. Basically you get a bigger image with the same quality as the original.

 

I will read your post in detail later. I just wanted a master dark and flat so I avoid always putting 60 dark files and 60 flat files everything I do pre processing.

 

Can I integrate the dark flats with the darks to create the master dark? The tutorial states that when I do batch preprocessing I can just put the dark flats with the darks and the software knows what to do with it.

[Stub Mandrel]

On 06/02/2018 at 12:27, vlaiv said:

Not sure how is that related to amp glow if we assume it is due to build up of heat on adjacent component and it is being fed into sensor via metallic circuitry. Sensor is kept at constant temperature because all excess heat is taken away by Peltier. If amp glow is rising faster than dark current - that means that part of chip is at higher temperature than the rest of the chip, and it is accumulating heat (if it were only at higher temperature but stable we would not be in trouble, since there is time linearity at different temperatures). Some of that heat will indeed be taken away by Peltier via sensor, but if it is rising, it will also be in dynamic with its surroundings - some of the heat will dissipate elsewhere because not all of it is being drawn away by Peltier. Otherwise amp glow temperature would also be stable. And if it is dissipating heat by other means than Peltier - speed of dissipation will depend on temperature gradient between that component and wherever that heat goes to.

From ZWO's website:

Quote

 

When it comes to CMOS cameras, “amp glow” is usually not from an amplifier. CMOS sensors are usually “fully integrated” which means that, unlike a CCD, readout electronics are included on the sensor die along with all the pixels themselves. Each sensor has at least one, often many, ADC (analog to digital conversion) and CDS (noise reduction) units on it. There are also other support circuits on the sensor die itself these days…clock generators and power supply regulators and such. These support circuits can generate heat or may even emit NIR light, both of which can cause glows. Additionally, many modern CMOS sensors include high performance image processing as part of the sensor package, either in the form of on-die processing or a secondary processor that is directly integrated into the sensor by attaching it (often to the reverse side of the sensor.) This processing circuitry can often generate heat that may produce glows.

Heat can increase the dark current accumulated in pixels. Pixels that exhibit roundish glows, usually soft without any obvious structure, are caused by heat sources. NIR, or Near Infra-Red, may also be emitted by some sensor circuitry. NIR glows can exhibit anywhere on a sensor, may often exhibit distinct structure, and are often radiated from specific locations around the periphery of the sensor. Not all pixels in a sensor will be affected, nor will those affected be affected uniformly. Glows may not “grow” with time the same way dark current itself does, and may accelerate over time becoming brighter faster than dark current itself as exposures become longer.

 

https://astronomy-imaging-camera.com/tutorials/what-is-amp-glow.html

[wimvb]

3 hours ago, elNaboet said:

just drizzle because I be heard it's very advantageous. Basically you get a bigger image with the same quality as the original.

"There ain't no such thing as a free lunch." The information that is used to "create" detail is not available anymore for noise reduction. If the drizzled image were of the same quality as the undrizzled one, you could resample it and increase the signal to noise ratio (the "free lunch").

3 hours ago, elNaboet said:

will read your post in detail later. I just wanted a master dark and flat so I avoid always putting 60 dark files and 60 flat files everything I do pre processing.

Pixinsight creates the dark and flat masters and stores them in a directory called "masters".

3 hours ago, elNaboet said:

Can I integrate the dark flats with the darks to create the master dark? The tutorial states that when I do batch preprocessing I can just put the dark flats with the darks and the software knows what to do with it.

If the exposure time of darks and dark flats differs, pixinsight will put them in different groups (called "bins"). The preprocessor script has additional features for handling grouped data

[elNaboet]

On 16/08/2019 at 20:52, wimvb said:

"There ain't no such thing as a free lunch." The information that is used to "create" detail is not available anymore for noise reduction. If the drizzled image were of the same quality as the undrizzled one, you could resample it and increase the signal to noise ratio (the "free lunch").

Pixinsight creates the dark and flat masters and stores them in a directory called "masters".

If the exposure time of darks and dark flats differs, pixinsight will put them in different groups (called "bins"). The preprocessor script has additional features for handling grouped data

Thank you very much for all your replies. I have been away for a while. I can see i had some miss conceptions about some of the stuff. really thought drizzle was like increasing the size of the image keeping all the same proprieties. Always sounded impossible but people talk about that process like its black magic so i thought there was something else special about it. (i intended to use it in targets that are small for my FL just to do an higher quality crop).

 

When it comes to masters i had already created my master dark, my only confusion was that all of the tutorials i see about creating master darks (with flats or not) include the dark subs to be integrated with the master bias (that i dont have). I havent read your links yet and i intend to do it this weekned but i read load of tutorials and they only stated the same. Darks integrated with Master bias.

 

Thank you :)