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Darned darks driving me dotty


BrendanC

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Hi all,

TL:DR - I might have a light leak and/or I might have crap dark calibration. If you know of such things, your help would be very much appreciated.

Recently I had cause to worry about my calibration files. I've moved to mono - an ASI600-Cool, with a 130PDS - and the results are OK, but it's a fight.

My flats have turned out to be part of the problem at least, so I've got a Lacerta box on order, to replace my cheapo LED panel. Hopefully that will sort that.

HOWEVER...!

I also recently wondered whether I was getting light leakage through the focuser tube. Tonight on advice from @vlaiv, I covered the front of the OTA with the dust cap, two hats and a towel, and the other end with a hat (which is always on it to try and stop leakage anywaye), and another towel. I took four 10-second subs with the Luminance filter, gain 139, offset 50, while pointing my head torch at four different sides of the tube. Then, I did a stretch and wipe on the four images in StarTools, and got this.

Honeyview_Single__L_13436_10s__6C.jpg.7188ed582577d47306c4289c5a5dbf30.jpg

Honeyview_Single__L_13437_10s__6C.jpg.62ea6374c6477a0174c8ba0432caf19a.jpg

Honeyview_Single__L_13438_10s__6C.jpg.eebf27b9e6ee021e7cb64a39b581db9d.jpg

Honeyview_Single__L_13439_10s__6C.jpg.78af38f735a3f14a7d7e70a49b86f2fc.jpg

Fairly conclusive, you'd think.

THEN, out of interest, I decided to put my actual darks, from my darks library, through the StarTools test. Here are the results - from a 60s dark, then a 180s dark, then a 1140s which I'd use for narrowband, all at -15C:

Honeyview_60s.jpg.7cd32b37b3d1eb24bdd838b5cfd7d08c.jpg

Honeyview_180s.jpg.94925afe26efb92be997c97e4959afa8.jpg

Honeyview_1140s.jpg.a66c7a12234bab0c34f2059dd6e3dda8.jpg

The first and last are slightly smaller because I cropped them, to make sure StarTools wasn't doing it's 'thing' of creating weird artefacts around images if the edges aren't clean.

SO... what can this all mean?

QUESTION 1: Are my darks crap? These were taken with the lens cover on, two snoods wrapped around that, then tin foil wrapped around that, secured with elastic bands, leaving space for the inlet and outlet grills and fan, and put in the fridge simply because it's the easiest place to achieve darkness (or so I thought). It's my third attempt to get them right and honestly, I don't know what to do next. More tin foil? Secured with tape? Or, is this just the sensor response, amplified by StarTools? I can see that the 1140s dark has the typical amp glow to the right, but it's the other stuff that bothers me. A lot.

QUESTION 2: Can the ASI1600 leak from anywhere else? As in, from behind, through the fan mechanism? 

QUESTION 3: Finally, if I do have a light leak in the focuser tube, what's the best fix for a 130PDS (Newt) with an autofocuser?

Any/all advice very welcome. This is becoming quite a long, hard slog.

Thanks, Brendan

 

 

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Thanks for responding. :)

Right - so the only reason I wiped was to get some insight into what StarTools reckoned to the 'flatness' or otherwise of the files. I'm totally prepared to accept that this isn't a valid test, and it would be a great relief if not!

Here are the frames just stretched in StarTools, no wipe, but this time a bit bigger to give more detail.

For the light leak test...

Single__L_13439_10s__6C.jpg.cf7ed9f69f2395ebd1061de1ab3b7cb0.jpg

Single__L_13436_10s__6C.jpg.22ec7ee39ea42ea7185c0f5bbadb4b15.jpg

Single__L_13437_10s__6C.jpg.c719c365f6d07ade6dfc030110d4c512.jpg

Single__L_13438_10s__6C.jpg.d2dc92a19860148b9df32d0ec60ba3e1.jpg

... and for the library darks...

60s.jpg.e2ea68fb76f0bbc6b99cb9128e4923a0.jpg

180s.jpg.d6af7843bb5f231087766b3f5c291322.jpg

1140s.jpg.c65cf45561b98e762069f63038ff4f0a.jpg

I could see even when stretching them first time around that they looked a lot healthier. It was when I did a wipe that I freaked.

For the darks, the amp glow to the right is what I would expect to see and is a known pattern for the ASI1600.

So, if just a stretch is a more indicative test, would you say that these don't look too bad? That is, I may not have a light leak, and my darks might be OK?

Thanks, Brendan

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Hi Brendan,

Just out of interest I ran an ASI1600 90s dark through Startools and got this

After AutoDev. It has only got the noise to work on and so has applied most stretch to the noise and some to the amp-glow. Much as expected.  View it full size to see the noise.

390096253_1MasterDarkG139-6490s-25CAutoDev.thumb.jpg.7f041a76064b2f0a68594d0561761a2d.jpg

After Wipe and AutoDev. Wipe on its own gives an almost black image so needs a develop to see the result. I was surprised that it has actually removed the noise. However it has left some odd artifacts like vignetting and slight banding. This is likely to having no noise to apply a strech to, so has applied a full stretch to a few actual ADU variations, to get black to white. I suspect the Wipe module divides the image into varying size blocks to determine the gradient effects and the edges of these blocks are visible as the bands in the image.

1004430278_2MasterDarkG139-6490s-25CAutoDevWipeAutoDev.thumb.jpg.bfb6c66ab567d4d0e84da769bbfc9cf2.jpg

In a real image most of the stretch would be given to DSOs and stars and little to the actual noise as you use a ROI. I think your wiped darks are showing more of these banding artifacts having divided the image into different sized blocks. It's likely the tiny light leakage you seem to have has given the Wipe something to work on giving more visible banding.

I'd ignore these wiped darks as they are totally unrepresentative of real images and so it seems can produce strange results.  In real images the Wipe module doesn't remove high frequency noise but it is doing so here on the darks. 🤔 Gradients are really very low frequency changes across the image which is what Wipe is normally looking for.

Alan

 

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Great, thank you. I'm now going to compare the light leak darks with the library darks in the way @vlaivsuggested, to get some of idea of what's going on with the focuser, but it's good to know my comparison will be a fair one now. Thanks again all. 

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59 minutes ago, BrendanC said:

Great, thank you. I'm now going to compare the light leak darks with the library darks in the way @vlaivsuggested, to get some of idea of what's going on with the focuser, but it's good to know my comparison will be a fair one now. Thanks again all. 

Can you post linear / unaltered versions of those files you posted stretched above?

You can best verify any light leak by subtracting relevant subs while data is still linear.

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6 minutes ago, BrendanC said:

Hey Vlaiv! Yes, I can, and here's the link: https://1drv.ms/u/s!AqovBuVZMwj3k5Jx0oOVsBtGCbWzsA?e=QSp0Tx

Would be great to get your take on these too, cos you're fab. :)

These are only 10s darks, right?

Look at this:

image.png.cc9e5ba8b094987e0b228b4a67ba36ca.png

Mean ADU value varies from 787 to 811 - that is 24ADU in just 10s.

As comparison - I took random 4 dark subs from my ASI1600 taken at 240s exposure:

image.png.af8fb4decbeb021ec248b4b2813241dc.png

Difference is in second decimal place (due to noise).

In any case, here is what light leak looks like. I took those 4 subs and subtracted one with the least mean value from them all.

image.png.57bd43564afb2dfb850e6cf139de7f73.png

That is first one in top left corner. Obviously, it will be black as it is all zeros (subtracted from itself) - but rest are not zero - nor pure noise. They contain uneven signal which is additional light leak.

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Thank you Vlaiv. :)

So, looks like I do have a light leak. :(

The 10s shots are the ones I took in the garden last night with the scope covered.

The others in that folder are masters from my darks library, and are 60s, 180s and 1140s in length. I still don't know whether they're viable or not as per your darks library, so I need to find out.

So, questions now are (to help me replicate your tests and be able to do them for myself in future, for example to analyse those masters in the darks library):

1. What software are you using to get those ADU values?

2. How do you 'subtract' one sub from another? I know this is the essence of stacking but what package would I use to do this?

Thanks, Brendan

 

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13 minutes ago, BrendanC said:

The others in that folder are masters from my darks library, and are 60s, 180s and 1140s in length. I still don't know whether they're viable or not as per your darks library, so I need to find out.

I can't really tell from the masters themselves. Maybe I would be able to tell from individual dark subs.

Here is comparison between your 60s and mine 60s sub (mine was created by stacking 64 exposures - offset is a bit different, but other than that they should be comparable):

image.png.9a64f67bfee8e5e2030020f37977c731.png

left is yours and right is mine master. Both have been binned and stretched (linearly) to show amp glow, and I would say that they are comparable. Mine was taken at -20C not sure what temp did you use so that might be reason for a bit less noise / smoother looking dark.

Unfortunately, I can't say from just comparison if there is slight light leak or not. We need numbers for that.

22 minutes ago, BrendanC said:

So, questions now are (to help me replicate your tests and be able to do them for myself in future, for example to analyse those masters in the darks library):

1. What software are you using to get those ADU values?

2. How do you 'subtract' one sub from another? I know this is the essence of stacking but what package would I use to do this?

Take look at ImageJ - it is free/open source and it is made for scientific analysis of microscopy images. Works rather well for astronomy applications (there is even AstroImageJ which is fork dedicated to astronomy applications - but you don't need that as it is geared towards astrometry / photometry rather than simple operations).

You can simply open two images and use Image calculator (there are at least 3 different kinds - each will do simple subtraction with ease, or you can even do complex math operations with multiple images in some of them - like image expression parser / image expression parser (macro) - I used simple Process / Image calculator for above as well as Analyze / measure menu option for statistics on the images).

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Thank you again. This is all really useful input.

When you say 'we need numbers for that', do you mean to compare your masters with mine, or to compare my garden darks with my master darks?

The full details for both are that the 60s and 180s are taken at gain 75, and the 1140s at gain 200.  The masters are a stack of 50 each. They're all at offset 50, at -15C. I chose this temperature because the camera wouldn't go to -20C in the house, which was at 22C, so that told me I might have difficulty getting to that temperature in summer, so figured this could be a good approach for consistency and ease of calibration. However, I can see that yours is smoother, so that could be why. I might reconsider this decision.

It's certainly extremely useful to see yours. I'm pretty much 'flying blind' here - I have no idea whatsoever of what I should be seeing, nothing to compare anything against.

If you'd like to look at the individual subs, I just put five of each into that shared library darks folder. No problem if you have other stuff to do!

However, I'm going to download that software and take a look myself, to see what I can figure out.

Thank you once more Vlaiv, you're a great contributor to this forum and I really appreciate it. 

Cheers, Brendan

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1 minute ago, BrendanC said:

When you say 'we need numbers for that', do you mean to compare your masters with mine, or to compare my garden darks with my master darks?

It is very hard to determine if there is slight light leak when taking darks.

With flashlight and scope - there is significant increase between subs and that is easy to detect, but with darks - if you did not shine a torch on purpose - light leak will be much less and will be more or less constant.

Since there is no lens - whole sensor will be illuminated so there won't be any gradient to go by. It won't be much different that regular dark current.

What we can do in order to try to detect light leak in darks is to:

1. examine if all darks have same (or nearly) mean value. If there is significant variation - it can point to light leak

2. we can examine "dark current" levels to see if they match expectations. If we get significant discrepancy between expected and measured dark current levels - then we can start suspecting light leak. Here we are actually going to measure noise rather than signal itself as there is no simple way to isolate signal with this camera - we can't remove bias signal easily as bias are not correct.

Here is what mean value looks like for 180s subs:

image.png.705c1933d875272da192f242e254902a.png

that is pretty consistent.

Measured noise in 180s sub is 2.686e

This consists out of read noise at gain 75 + dark current noise at -15C

Read noise at Gain 75 according to ZWO graph is about 2.2e

image.png.799eb9417d114168a6c61e00098903f8.png

Dark current according to ZWO at -15C is about 0.009e/s/px:

image.png.74178a9039eeac5ff4648be8612712da.png

Let's see if these two add up to what we have measured.

In 180s we will have signal that is about 0.009 x 180s = 1.62e of dark current. Associated noise will be square root of that so ~1.2728e

We have 2.2e and 1.2728e of noise added - that is sqrt( 2.2^2 + 1.2728^2) = sqrt(4.84 + 1.62) = sqrt(6.46) = ~2.542e

In theory we should have around 2.542e and we measured 2.686e so there is a bit more noise, and this could mean slight light leak - but it can also mean that we did not read graphs properly (it is very hard to read graphs like that - especially temperature graph that is in log form rather than linear).

I've also found that ZWO sometimes under estimates noise in their graphs - measured values are always a bit larger than the graphs would suggest, so this might be case as well.

In the end - from measurements we can conclude that if there is any light leak in darks - it is very small by magnitude and you should revisit darks only if you eliminate telescope light leak for lights and flats and you still have calibration issues.

 

 

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Wow.

This is incredible analysis. 

So, the upshot is: if there is a slight leak in my darks, it's not significant, so they're still viable; but I do have a leak in my focuser tube.

Meanwhile I'm going to think about getting down to -20C for my subs. I have read that there is a difference between -15C and -20C but having seen your 60s I can see that it could have a real impact.

Right, well, this means things aren't great in that I have a light leak, but not as bad as I thought in that the darks I'm using are still OK. Hopefully now I've taken delivery of my light box, my new flats will also help with calibration (bearing in mind I'll have to retake all the dark flats too...)

Again, thank you Vlaiv. I know I keep saying it, but I mean it! :)

Thanks, Brendan

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My net connection is currently dire so I can't see many of the posted images. However, we have been doing some testing of darks with an ASI 2600MC. They add noise. Theory can say what it likes, but they add noise - and a lot.  Without darks the background sky is extraordinarily clean, so much so that I do not use noise reduction yet fear that someone looking at the image will say I've over-applied it.  We took the darks carefully, with the metal cover over the chip window. (I discovered in careful tests years ago that, even on a refractor, you cannot reliably exclude light so I always do darks off-telescope.)

In fact I don't think much of darks even with a high-noise CCD camera like my Atik 11000. I found that, most of the time, subtracting a master bias and running the hot pixel filter gave the same result as using a matched dark.  On rare occasions there was a difference - in favour of using the bias and never in favour of using the dark.  When I initially dumped darks I did apply a bad pixel map but stopped bothering when I found that the stacking program's hot pixel filter was as effective or better. (I use AstroArt for stacking and calibration.)

Call this a minority report, if you like, but I've come across other imagers who've come to the same conclusion. Images done this way can be found here: https://www.astrobin.com/users/ollypenrice/

Have you tried simply not using darks? I don't know if yours is an 'amp glow' camera but, if it isn't, I'd just try it.

It's unlikely to affect a Newt or refractor but the ASI 2600 has a light on the back which played havoc with our flats on the RASA 8. It's now taped over.

Olly

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Unfortunately, it does incur amp glow.

I know what you mean though. With my DSLR I eventually stopped using darks because I got exactly the same result with just bias - and the difference in time and effort in just taking a few quick bias snaps, versus building an entire darks library, was huge.

So perhaps I can consider taking shorter exposures without darks, but I'm afraid longer subs really do show the thermal signature of the ASI1600's sensor. Bit of a bummer really, and to be honest I'm starting to regret my move to mono.

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Unfortunately the ASI1600s I have seen do suffer from amp glow on extended exposures, a general glow down the RHS.

@Tomatobro has an Atik OSC version which is particularly bad in this respect.

I’ve always used darks with my IMX571c sensor cameras, but sounds like I should experiment.

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@ollypenrice

It should be fairly easy to establish how much noise master dark adds to the image.

If you still have your dark subs, do the following:

- split them into two equal groups (must be equal so if you have odd number of frames to begin with - discard one to get even which can be then divided by two)

- stack each group using simple average method (or sum - it does not matter)

- subtract resulting images - first from second or other way around, it really does not matter.

- convert into electron count by using e/ADU for selected gain

- measure standard deviation.

This will give you noise level for complete stack - you can calculate then per sub noise level.

I would be surprised that you get significantly different result than adding read noise + dark current noise (both of which are low to begin with).

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Not sure what difference there is, if any, between the 1600MM Cool and the 1600MM Pro (I have the Pro) but the Pro certainly does have some amp glow so I'm in the 'darks no bias' school. I've never suffered from light leaks through the camera though, as far as I'm aware.

I have a 150PDS and it leaks from every joint, and I'd expect the 130PDS is similar. The lens cap on the front is fine; a hat over the primary end is also fine. The focuser draw-tube is tricky, especially as it has to provide some flexibility for focusing. My current solution is a strip of tinfoil wrapped snugly around the focuser joint. It's working fine for me now. I don't cover the camera.

I've never noticed issues in darks due to light leak, although intuitively they will be impacted if there is environmental light while you're capturing them. This really means, if you haven't light-sealed your OTA, that you take darks in the dark, or off-scope with a covered camera. My biggest impact from the leak was while capturing flats as the light from the flat panel illuminated the focuser tube and caused an off-centre 'swipe' mark across the image. 

I now have reduced the issue to the level where (if it's not gone entirely) it is negligible compared to any one of my many other problems!! 🙂

I don't have any leaks (again, that I'm aware of) with my refractor Esprit 80. I'd be interested in hearing how you get on with the Lacerta.

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 I used to have light leaks. I have a couple of layers of black plastic wrapped round the focuser and all the way to the front of the camera. I'm using a V-Power focuser (TS), ZWO OAG and a filter drawer with  a short T extension to get my 55mm backspacing. It's on a TS1506UNC newtonian. That has a stainless steel lid on the back (from a small flip top bin) covering the primary.  don't trust any of it to be light proof even when taking subs (LEDs, local streetlamps) When I take darks I put a black flocked slide in the filter drawer. Works a treat.

Duncan

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@Padraic M The only difference is the 4GB onboard RAM that the Pro has, to help with shifting data quickly. I've opted to wrap a snood around the focuser, inlaid with tin foil, which I tried last night and it seemed to work, as in the focuser could still move in and out, and the images didn't seem adversely affected. As for taking darks, I always go totally neurotic about light leaks, so the last time around I had the camera in the fridge (because it's the easiest place to assure dark without it getting too warm), with two snoods on the lens cover, tin foil around that, elastic bands holding it all together tightly with just the fan grilles left for cooling. With my new light box, unfortunately I couldn't get one small enough for the 130PDS so mine's for the 150, and I'm going to improvise with an inlay around the circle on the box so that extra light isn't cast onto the focuser tube when taking flats. 

Perhaps I should just go back to observing? It was so much easier...!

Edited by BrendanC
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16 hours ago, BrendanC said:

I'm getting close to that actually, and then throwing it in the docks.

If you're going to throw it, tell me your Lat and Long and I'll give you a precise compass bearing on which to whizz it. I'll be ready with a net!

This isn't a mono-colour issue, it's an amp glow issue. The amp glow condemns you to using darks and, as I said, I have found darks to be problematic on occasion. (The theory of darks is predicated on the idea that what you actually get in your darks, and what the software actually does with them, are what the theory assumes. Can this be assumed? And, if it can, why do I get better results without them?)

Olly

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2 hours ago, ollypenrice said:

The theory of darks is predicated on the idea that what you actually get in your darks, and what the software actually does with them, are what the theory assumes. Can this be assumed? And, if it can, why do I get better results without them?

Good thing about theories (good ones at least) is that they are testable.

 

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3 hours ago, ollypenrice said:

If you're going to throw it, tell me your Lat and Long and I'll give you a precise compass bearing on which to whizz it. I'll be ready with a net!

Watch it, Olly. The camera may be in a concrete slab rather than the original packaging:

21 hours ago, Padraic M said:

Next step is to encase the camera in concrete...  🙂

 

I agree that this is about amp glow. The newer ZWO cameras (including the ASI2600?) are supposed to be "amp-glow free".

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