The "No EQ" DSO Challenge!

[GuLinux]

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Edited November 11, 2016 by GuLinux
wrong topic, please remove

[Nigel G]

It's looking good Jon, look forward to seeing it after the offsets are added

Edited November 11, 2016 by Nigel G

[alacant]

2 hours ago, parallaxerr said:

disappointed with the CA still

The semi-apo doesn't really do CA; I think it removes around 50%. A uv-ir plus a skyglow I find works better but still a great shot. HTH.

[The Admiral]

11 hours ago, parallaxerr said:

I took 30s subs on Capella (the only star that seem to be dodging the cloud) at ISO's from 100 up to 6400 and Hi1 (12800) and checked the histogram peaks. Only ISO100 showed clipping on the left side but from 200 up, there was no clipping. The lower the ISO, the darker and smoother the background sky with grain starting to show at ISO 800. As it happens, with my sensor, there is a dip in read noise level at ISO400, sooooooo ISO400 is where I'm currently at. At this level I also have 50% more dynamic range available and a higher saturation capacity, although I'm probably not reaching it with 30s subs.

I think at ISO1600, despite marginally lower read noise, the reduction in dynamic range raised the noise floor and it was all getting amplified by the ADC. Scope is now on M42 clicking away, surprising amount of detail in the subs and a very nice dark background sky. Watch this space......

 

You obviously seem to have a good handle on the way your sensor is performing, so I'm wary of teaching you to suck eggs here. I'm interpreting what you say, perhaps incorrectly, that you are concerned with clipping at the low end of the histogram, which is important not to do, so I understand. But of course what is important here is whether the stars are clipped at the top end, and I find it well night impossible to tell from my own histogram display whether that's occurring because they are such small peaks (though switching on blinkies might help). The only way I've checked this is to put the files through a RAW developer and inspected the centres of the stars. I find some are, some aren't, when I use 30s at ISO1600.

This discussion has been useful because it's made me think a bit about how I use my ISO. When I started imaging I used 100 x 15s lights on M42. M42 is very bright, and from that perspective is easy to image, but there is also a wide brightness range that needs to be captured. For my next attempt later in the year I had planned on using a mix of short exposure and long exposure subs, but now I'm minded to use the same maximum exposure length and reduce the ISO for a set. That way it should (a) increase the dynamic range of the sensor, and (b) allow more photons to be recorded per sub which should improve the SNR. If it works that way! I have to say that my sky background isn't horrendous, so 30s at 1600ISO doesn't put the peak of the histogram too far up anyway. Note also that I use the Fuji-X sensor which has its own proprietary colour filter array, and as such there is no readily available data as can be found for Canon and Nikon sensors.

Ian

[The Admiral]

3 hours ago, parallaxerr said:

OK, I wasn't going to post this just yet but I have a question - in comparison to other M42's I've seen, there seems to be quite a lot less stars in mine. Probably a result of setting ISO to 400, but I really do like the dark background. What do you think? Stars vs noise?

Note: this is far from fully processed. I threw this together at 2am so there's no flats or dark flats, just 101x30s light frames and x50 darks. The image has 4 noise hot spots towards the corners which I think is amp glow, I tamed it by over adjusting contrast and colour. Also it appears my computer monitor colour needs calibrating as this image looks far less vivid on the PC than on my other devices!

Also to note, this was taken with the Baader semi-apo filter. It made processing infinitely easier without the sky glow - subs were captured between 20°-30° Alt! After Autodev in ST, the image was washed out a lime green colour instead of red/brown but the wipe tool handled it much better, less noise probably helped. A little disappointed with the CA still.

2am!!! If only

I agree with Ken, the background is a bit dark. If you stretch the image a bit more in ST in the develop (I use manual, not auto), you can bring up a bit more of the hidden detail. At the same time, you'll probably think that the background is too noisy, but go with it because by the time you've performed all the other actions you'll find it'll quieten down considerably. At the end of the day it is easy to adjust the black level in a photo application if you're not happy with it, certainly easier than trying to bring out otherwise hidden detail. And of course, the more you stretch the image the more stars you'll likely see, though this doesn't appear to be a particularly star rich field anyway.

Still, an excellent image, but I suspect that there is more to be had from it.

Ian

Edit. It could be that your PC monitor is the correct one, especially if you've calibrated it at all in the past. I think these mobile devices tend to make images look more vivid as it's perceived that's what the general public want/expect. I find quite a difference with mine compared to my calibrated PC screen.

Edited November 11, 2016 by The Admiral

[parallaxerr]

1 hour ago, The Admiral said:

you'll probably think that the background is too noisy, but go with it because by the time you've performed all the other actions you'll find it'll quieten down considerably

I think this is where I made a mistake. I set the ISO low to quieten the background but lost detail, at ISO800 I should capture more data and be able to get nice backgrounds through ST.

1 hour ago, The Admiral said:

I think these mobile devices tend to make images look more vivid as it's perceived that's what the general public want/expect

Agreed, although my monitor at home is a 32" LCD TV so I think it needs tweaking. The image looks best here on my work PC, more natural, but looks saturated on my tablet/phone.

[parallaxerr]

1 hour ago, The Admiral said:

But of course what is important here is whether the stars are clipped at the top end, and I find it well night impossible to tell from my own histogram display whether that's occurring because they are such small peaks (though switching on blinkies might help)

Same here but I don't have the blinky option. I have a feeling I was nowhere near saturating the sensor at ISO400 which is why I didn't capture some of the higher end of the range, i.e. fainter stars.

Edited November 11, 2016 by parallaxerr

[Stub Mandrel]

It's important to realise that for most images the bulk and left hand side of the histogram is the empty background, right of the peak you generally have your DSOs and as the peak fades down it moves into being the outer edges of stars with the cores of the brighter stars forming the long, low tail to the right.

A good test is to use photoshop's dust and scratches tool and watch the histogram, watch as stars disappear and see the change in the histogram.

Edited November 11, 2016 by Stub Mandrel

[Filroden]

38 minutes ago, parallaxerr said:

Same here but I don't have the blinky option. I have a feeling I was nowhere near saturating the sensor at ISO400 which is why I didn't capture some of the higher end of the range, i.e. fainter stars.

So long as your background has cleared zero then you have all the data captured regardless of increasing ISO. Increasing ISO just multiples the strength of data that already exists. So I wouldn't increase ISO and expect to see more (once processed). However, lower ISOs would result in the histogram being lower overall and require more stretching (develop in StarTools) than a higher ISO.

The only reasons I can see why you would not capture fainter stars - the exposure time is not long enough and/or they are fainter than the background signal - neither of which could be solved with increasing ISO.

Edited November 11, 2016 by Filroden

[The Admiral]

1 hour ago, parallaxerr said:

Same here but I don't have the blinky option.

The Nikon D3200 does have the blinky option. That is, flashing highlight warning in review and replay.

See for example https://www.dpreview.com/reviews/nikon-d3200/6

Ken summed up the situation re. background level quite succinctly. You need to stretch more in ST to bring up the background, but don't fret too much if it looks noisy at that early stage in the processing sequence.

Ian

Edited November 11, 2016 by The Admiral

[Stub Mandrel]

If you avoid clipping using a higher ISO increases the dynamic range in the image.

For example, an 8-bit mono image can have up to 256 levels or shades o grey, 0 to 255.

Say a mono 8-bit image at ISO 100 covers the range 0 to 70, it will contain 71 shades of grey (not fifty!)

At ISO 200 it would cover the range 0 to 140, and have 141 shades.

At ISO 400 it would require the range 0 to 280, but 280 is more than 255.

In this case the 200 ISO image would clearly capture the most detail without losing data.

Bear in mind this is true for astro images that typically start near the black point (0) and will only approach high values at the centre of the brightest stars.

 

This also shows why a 14-bit DSLR like my 450D (16,384 levels) shows much more detail than my 10-bit 10D (1,024 levels) at the same ISO setting.

 

This is why, at face value, it makes sense to use the highest possible ISO rating without clipping.

 

But working against this is noise, both read noise and thermal noise. If noise doubles from, say, ISO800 to ISO1600 then because the signal doubles as well the signal to noise ratio doesn't change and the extra detail revealed gets lost in the extra noise.

Curiously the noise/ISO relationship appears to be different for every type camera or sensor, and perhaps in detail for every individual camera.

My reading of various websites is that you need to find the 'sweet spot' for your own camera using online data as a guide to find the likely range (ISO800 and 1600 are quoted as 'best' for the 450D by different websites) and then do some experiments. My own experimentation on this is inconclusive, for my camera any given exposure 800 is less noisy than 1600, but if you stretch an 800 image to match a 1600 one, the 1600 version shows marginally less noise.

What to do?

 

 

 

[Filroden]

11 minutes ago, Stub Mandrel said:

If you avoid clipping using a higher ISO increases the dynamic range in the image.

For example, an 8-bit mono image can have up to 256 levels or shades o grey, 0 to 255.

Say a mono 8-bit image at ISO 100 covers the range 0 to 70, it will contain 71 shades of grey (not fifty!)

At ISO 200 it would cover the range 0 to 140, and have 141 shades.

At ISO 400 it would require the range 0 to 280, but 280 is more than 255.

In this case the 200 ISO image would clearly capture the most detail without losing data.

Bear in mind this is true for astro images that typically start near the black point (0) and will only approach high values at the centre of the brightest stars.

 

This also shows why a 14-bit DSLR like my 450D (16,384 levels) shows much more detail than my 10-bit 10D (1,024 levels) at the same ISO setting.

 

This is why, at face value, it makes sense to use the highest possible ISO rating without clipping.

 

But working against this is noise, both read noise and thermal noise. If noise doubles from, say, ISO800 to ISO1600 then because the signal doubles as well the signal to noise ratio doesn't change and the extra detail revealed gets lost in the extra noise.

Curiously the noise/ISO relationship appears to be different for every type camera or sensor, and perhaps in detail for every individual camera.

My reading of various websites is that you need to find the 'sweet spot' for your own camera using online data as a guide to find the likely range (ISO800 and 1600 are quoted as 'best' for the 450D by different websites) and then do some experiments. My own experimentation on this is inconclusive, for my camera any given exposure 800 is less noisy than 1600, but if you stretch an 800 image to match a 1600 one, the 1600 version shows marginally less noise.

What to do?

I think it's the other way around: dynamic range reduces with increasing ISO. Here's a chart for the Canon EOS 50 from http://photonstophotos.net/Charts/PDR.htm

However, I think ISO is non-linear in its effect on signal and what I think it does at higher ISO is use more of the dynamic range for the lowest signal, therefore giving it an apparent "boost" - something that can be achieved in post-processing. However, because it has less range to play with, it therefore has to compress the highest signals, so increasing the likelihood of saturating stars.

Edited November 11, 2016 by Filroden

[The Admiral]

Hmm, hang on, I'm not sure it works like that does it? Surely bit-depth has nothing to do with dynamic range (DR)? If I understand correctly, increasing ISO will increase the (analogue) gain and will amplify the signal from the electrons in the wells. This is passed to an analogue-digital converter to digitize the output. The trouble is, ADCs have a maximum input before saturating, so as the ISO increases, this saturation limit is met by correspondingly smaller and smaller outputs from the sensor. In other words, the DR reduces as the ISO increases. A further complication is that cameras use digital gain at some ISO value rather than analogue amplification, and that point is camera specific.

I also believe that the bit-depth relates to the conversion by the ADC, so whatever range of inputs and ISO, the ADC range will be covered by that number of bits, or levels.

That's how I've understood the case, but am prepared to be educated

Ian

Edited November 11, 2016 by The Admiral

[The Admiral]

37 minutes ago, Filroden said:

However, I think ISO is non-linear in its effect on signal and what I think it does at higher ISO is use more of the dynamic range for the lowest signal, therefore giving it an apparent "boost" - something that can be achieved in post-processing. However, because it has less range to play with, it therefore has to compress the highest signals, so increasing the likelihood of saturating stars.

Could that be because RAW is linear, and ISO works on the RAW, whereas when we observe our image the data is first modified by the application of a gamma curve, I wonder? My brain won't compute!

See for example http://www.cambridgeincolour.com/tutorials/gamma-correction.htm

Ian

Edited November 11, 2016 by The Admiral

[Filroden]

7 hours ago, parallaxerr said:

OK, I wasn't going to post this just yet but I have a question - in comparison to other M42's I've seen, there seems to be quite a lot less stars in mine. Probably a result of setting ISO to 400, but I really do like the dark background. What do you think? Stars vs noise?

 

I've just compared your star field with one of my first images taken last year. This, I think, was with the 9.25" SCT at f6.7. I think it was from a stack of 15s subs. The field of view is different but I see 100% overlap of stars.

[parallaxerr]

6 hours ago, Nigel G said:

I hope you don't mind.

Not at all Nige, you've highlighted the gradient I was masking with contrast which obviously took away some fine detail. Just reprocessing with flats & dark flats, gradients gone...

[Stub Mandrel]

Sorry I was sloppy in my explanation. Bit depth and dynamic range aren't the same.

My point is that a higher ISO does results in increased dynamic range IN THE FINAL IMAGE - if you compare the captured image and neither image is clipped. I.E. I am talking OUTPUT dynamic range not INPUT dynamic range.

So in my example the ISO200 image would have a dynamic range of 140 compared to 70 for the ISO100 image.

Obviously higher ISOs clip before lower ISOs so they can deal with less dynamic range in the subject

My argument ONLY applies when there is no clipping.

 

Remember old tape recorders? They had a VU meter so you could turn up the gain (= increase the ISO) to maximise the signal to noise ratio and dynamic range of the recording, but at a high gain you were more likely to get clipping on a loud passage.

[Filroden]

I see what you mean. So you're balancing the input range wanting lower ISO to output range wanting higher. 

So the test would be to start at the highest ISO and work down until no clipping occurs?

[The Admiral]

8 minutes ago, Stub Mandrel said:

Sorry I was sloppy in my explanation. Bit depth and dynamic range aren't the same.

My point is that a higher ISO does results in increased dynamic range IN THE FINAL IMAGE - if you compare the captured image and neither image is clipped. I.E. I am talking OUTPUT dynamic range not INPUT dynamic range.

So in my example the ISO200 image would have a dynamic range of 140 compared to 70 for the ISO100 image.

Obviously higher ISOs clip before lower ISOs so they can deal with less dynamic range in the subject

My argument ONLY applies when there is no clipping.

 

Remember old tape recorders? They had a VU meter so you could turn up the gain (= increase the ISO) to maximise the signal to noise ratio and dynamic range of the recording, but at a high gain you were more likely to get clipping on a loud passage.

Ah, OK, so really this is the "expose to the right" principle you'd use in conventional photography, in order to get more out of the darks in the image and to minimize posterisation, yet not clip the highlights.

For an object with a very large range, like M42, which might exceed the DR of the imaging system,one can additionally adopt the process of blending two or more images using different exposures. Or HDR as it is conventionally known.

Ian

Edited November 11, 2016 by The Admiral

[Stub Mandrel]

Not quite, I experiment to get the 'best' ISO (still not sure if its 1600 or 800!) but whichever I'm going with I change the EXPOSURE to get an image with detail but hopefully no clipping.

[Filroden]

Isn't it better to reduce ISO before exposure? You'd only want to reduce exposure if you couldn't reduce ISO far enough to avoid clipping.

[The Admiral]

11 minutes ago, Stub Mandrel said:

Not quite, I experiment to get the 'best' ISO (still not sure if its 1600 or 800!) but whichever I'm going with I change the EXPOSURE to get an image with detail but hopefully no clipping.

But then this comes back to the question: "Do you gain more by keeping the exposure duration long but at a lower ISO, and so capturing more photons, than you lose by reducing ISO below optimum?"

Ian

[The Admiral]

On 07/11/2016 at 20:04, parallaxerr said:

I use my Nexstar with only about a foot of leg extension. I can't prove any benefit but it strikes me that it ought to be a little more rigid.

Ian

[Nigel G]

3 minutes ago, The Admiral said:

I use my Nexstar with only about a foot of leg extension. I can't prove any benefit but it strikes me that it ought to be a little more rigid.

Ian

There's different opinions to this ☺I keep mine fully extended for wider footing and more stability , also it gives me a little more visual range over the roofs ☺

Nige. 

[Filroden]

Likewise. I keep my tripod as low as possible. It kills my back during alignment but I think it's more stable. I also have vibration pads below the tips which speeds up how quickly it settles.

Edited November 11, 2016 by Filroden