The "No EQ" DSO Challenge!

[The Admiral]

6 hours ago, alacant said:

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.

Not quite sure why that should be so, as it looks as though the semi-apo cuts more into the blue than the Baader Moon & Skyglow + UV/IR cut combined. Still, if that's what you found, so be it.

Ian

[The Admiral]

7 minutes ago, Nigel G said:

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. 

I guess Nige that with a Newt hanging off it a wider stance is likely to prove more stable.

Ian

[Stub Mandrel]

44 minutes ago, Filroden said:

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.

No, because the read noise stays the same regardless of exposure and its better at ISO 800/1600 than, say, at ISO 400. By reducing the exposure I'm reducing the dark noise, whatever ISO I use.

They shouldn't really call it ISO; over the last month or so I've unpicked enough to see its completely different from ISO with print films - the print film effect is closer to adding pixels together to get more sensitivity but less resolution. DSLRs don't change resolution, it's more like push processing.

Here's a test with three 30-second darks at ISOs 100, 800 and 1600, all stretched twice, first linear stretch to get the same exposure, then an identical gamma stretch (like when you process to show a DSO) to reveal the noise. 1600 is marginally better than 800, but 100 is useless.

Edited November 11, 2016 by Stub Mandrel
Inserted one pic twice!

[The Admiral]

14 minutes ago, Stub Mandrel said:

No, because the read noise stays the same regardless of exposure and its better at ISO 800/1600 than, say, at ISO 400. By reducing the exposure I'm reducing the dark noise, whatever ISO I use.

OK, I can see that the 100ISO is far noisier, but the read noise surely won't stay the same because if you reduce ISO you will increase the sub-duration and therefore you'd need fewer of them. If you changed ISO from 1600 to say 800, you'd double the sub duration for the same result, and only need half the number of subs (i.e. same total duration). Consequently the read noise would also halve. And the signal will be twice as large. So too might be the dark noise, but that can be subtracted out. Or is my logic illogical?

Ian

Edited November 11, 2016 by The Admiral

[The Admiral]

19 minutes ago, Stub Mandrel said:

1600 is marginally better than 800, but 100 is useless.

Is that because at 100ISO the sensor is operating below its native ISO?

Ian

Edited November 11, 2016 by The Admiral

[parallaxerr]

50 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

I was using less leg extension with the ZS66 but wanted the ST120 a bit higher, there's still a good bit to go, I never go full extension. Doesn't seem to have made any difference though to be honest, the most vibration prone part of the whole SE setup is the single fork arm.

[parallaxerr]

Reprocessed with flats & dark flats which have got rid of the amp glow or gradient or whatever it was and left the colours a little more "natural". I stretched this image further and applied less noise reduction but the background has still come out pretty dark just using as simple ST workflow. The final conversion to TIFF then JPEG appears to adjust contrast slightly too.

I really don't like the CA. I tried the fringe killer filter but didn't get on with it. My next attempt (hopefully tomorrow night) will be with the ZS66 AND the semi-apo filter to cut back moon & sky-glow. I'll see how the histograms look through the different setup but hope to try ISO800.

 

Edited November 11, 2016 by parallaxerr

[Nigel G]

That's a lovely image, well done.

Nice and crisp, stars are nice, round and clear. There's depth as well, something hard to capture with Alt-AZ mounts.

Cheers

Nige.

 

[Stub Mandrel]

4 minutes ago, The Admiral said:

OK, I can see that the 100ISO is far noisier, but the read noise surely won't stay the same because if you reduce ISO you will increase the sub-duration and therefore you'd need fewer of them. If you changed ISO from 1600 to say 800, you'd double the sub duration for the same result, and only need half the number of subs (i.e. same total duration). Consequently the read noise would also halve. And the signal will be twice as large. Or is my logic illogical?

Ian

Yes, I'm afraid!

Change from 1600 to 800, you double the number of subs but you would need the same number of them, so twice the duration.

 

 

 

[The Admiral]

Yes that's a good one, but if you think it's still a bit dark try stretching even more and see what happens. Nothing ventured and all that. You might reveal the complete arc a bit more, which is certainly visible even now. Pity you've only got half of the 'running man'. If you did image it again it'd be worth seeing what a vertical format will do. Then again, if you use the '66 it'll have a wider FOV.

I'm getting lost amongst all these filters! Which one's the fringe killer, and how does it differ from the semi-apo?

Ian

[Stub Mandrel]

1 hour ago, The Admiral said:

Is that because at 100ISO the sensor is operating below its native ISO?

Ian

I think its because the read noise is a much larger proportion of the small signal at 100 ISO.

[parallaxerr]

3 minutes ago, The Admiral said:

I'm getting lost amongst all these filters! Which one's the fringe killer, and how does it differ from the semi-apo?

This explains the Baader series best Ian...

http://www.baader-planetarium.uk.com/en/pdf/baader_contrast_filters_US_ATT_review_0414.pdf

[Stub Mandrel]

49 minutes ago, parallaxerr said:

Reprocessed with flats & dark flats which have got rid of the amp glow or gradient or whatever it was and left the colours a little more "natural". I stretched this image further and applied less noise reduction but the background has still come out pretty dark just using as simple ST workflow. The final conversion to TIFF then JPEG appears to adjust contrast slightly too.

I really don't like the CA. I tried the fringe killer filter but didn't get on with it. My next attempt (hopefully tomorrow night) will be with the ZS66 AND the semi-apo filter to cut back moon & sky-glow. I'll see how the histograms look through the different setup but hope to try ISO800.

 

 

Try Noel Carbineris's actions his remove large/small blue violet haloes works really well on that. I've run both on it, but nothing else:

[parallaxerr]

8 minutes ago, The Admiral said:

Pity you've only got half of the 'running man'. If you did image it again it'd be worth seeing what a vertical format will do. Then again, if you use the '66 it'll have a wider FOV.

Yes, the running man fell foul to field rotation unfortunately, so had to be cropped out. The ZS66 should capture it and now I'm starting to get some idea of what I'm doing I may experiment with rotating the camera!

[parallaxerr]

10 minutes ago, Stub Mandrel said:

Try Noel Carbineris's actions his remove large/small blue violet haloes works really well on that. I've run both on it, but nothing else:

Thanks Neil, looks good.

Guys, whad'ya think. I should be using the ZS66 not the ST120 right? If I've got a doublet apo on hand why not use it. The ST120 probably grabs more photons in one go but surely not having CA in the first place is preferable.

Am I going to lose out with the smaller scope? The pixel resolution comes in at 2.04" with my camera so I'm unlikely to see any square stars but is 66mm "enough"?

[The Admiral]

4 minutes ago, Stub Mandrel said:

Yes, I'm afraid!

Change from 1600 to 800, you double the number of subs but you would need the same number of them, so twice the duration.

Sorry, I don't understand, why would you need to double the number of subs?

OK, let's start from the premise that you want to record for a given total amount of time, and collect the same number of photons, so that we are comparing like with like. You have a choice of imaging at either 1600 or 800. If you image at 800 you will need subs twice as long in duration as at 1600 to get the same result, but only half as many. On the plus side of using 800 compared to 1600, there will be:

• half the read noise
• each pixel will accrue twice as many photons and create a signal at source twice as large. Even though this is amplified with a gain half that of the 1600 case, surely this must be better statistically
• will the sensor noise be doubled? If we are following the sensor with half the gain, why would the sensor noise double? Wouldn't the effects of reduced gain and increased sub length balance out?
• the signal is comprised of wanted signal and skyglow signal. Both will double but the difference between the two components will double so would be statistically easier to separate.

On the down side, the sensor will not operating sub-optimally.

So at risk of conflating all these effects incorrectly, wouldn't we be better to use 800 rather than 1600 overall? After all, isn't that why EQ imagers like to have subs of many minutes in duration, presumably with the gain on their astro-cameras turned down (cf. ISO) to keep sky-glow in check?

Well, I'm trying to work it out in my addled brain as I write. I think I need to get back to first principles but you have my initial reaction

Ian

[Nigel G]

22 minutes ago, parallaxerr said:

Yes, the running man fell foul to field rotation unfortunately, so had to be cropped out. The ZS66 should capture it and now I'm starting to get some idea of what I'm doing I may experiment with rotating the camera!

I had the same problem with M42, the running man ran out of the frame, I  rotated the camera to reposition but unfortunately the secondary mirror supports on a reflector cause refraction spikes and they didn't line up also the stacking artefacts made me have to crop , no problem with a refractor though and I do like an experiment 

[The Admiral]

10 minutes ago, parallaxerr said:

Guys, whad'ya think. I should be using the ZS66 not the ST120 right? If I've got a doublet apo on hand why not use it. The ST120 probably grabs more photons in one go but surely not having CA in the first place is preferable.

Am I going to lose out with the smaller scope? The pixel resolution comes in at 2.04" with my camera so I'm unlikely to see any square stars but is 66mm "enough"?

My take would be that as it's a nice bright target I expect the ZS66 would get away with it, and save you blue bloat. It will have just over 3x less light gathering power, so you'll need to up the number of subs. Not advice with any practical experience behind it mind, just a gut feeling. At best we can expect seeing worse than 2", so as you say you are more than likely to be oversampling a little.

Try it and see, but I sympathise that this weather doesn't exactly encourage experimenting!

Ian

[alacant]

2 hours ago, The Admiral said:

Not quite sure why that should be so

Hi. No expert but I believe it is because of the canon sensor recording invisible radiation as blue at one end of the spectrum and red at the other; it shouldn't. It's the same effect that makes the stars fat. A #8 reduces the blue and a #12 eliminates it, but I don't think that's what you're aiming for (?).  HTH.

[Stub Mandrel]

1 hour ago, The Admiral said:

Sorry, I don't understand, why would you need to double the number of subs?

OK, let's start from the premise that you want to record for a given total amount of time, and collect the same number of photons, so that we are comparing like with like. You have a choice of imaging at either 1600 or 800. If you image at 800 you will need subs twice as long in duration as at 1600 to get the same result, but only half as many. On the plus side of using 800 compared to 1600, there will be:

• half the read noise
• each pixel will accrue twice as many photons and create a signal at source twice as large. Even though this is amplified with a gain half that of the 1600 case, surely this must be better statistically
• will the sensor noise be doubled? If we are following the sensor with half the gain, why would the sensor noise double? Wouldn't the effects of reduced gain and increased sub length balance out?
• the signal is comprised of wanted signal and skyglow signal. Both will double but the difference between the two components will double so would be statistically easier to separate.

On the down side, the sensor will not operating sub-optimally.

So at risk of conflating all these effects incorrectly, wouldn't we be better to use 800 rather than 1600 overall? After all, isn't that why EQ imagers like to have subs of many minutes in duration, presumably with the gain on their astro-cameras turned down (cf. ISO) to keep sky-glow in check?

Well, I'm trying to work it out in my addled brain as I write. I think I need to get back to first principles but you have my initial reaction

Ian

OK, I'm flying by the seat of my pants here too...

On the plus side of using 800 compared to 1600, there will be:

• half the read noise

Not necessarily - my camera has less read noise at 1600 ISO than ISO800 and read noise is independent of exposure time (that's why you use bias frames) don't confuse it with thermal noise that builds up through the exposure.

• each pixel will accrue twice as many photons and create a signal at source twice as large. Even though this is amplified with a gain half that of the 1600 case, surely this must be better statistically

ISO 1600 counts the photons with twice the resolution. Its like measuring a distance in inches or half that distance to in half inches - the number is the same.

• will the sensor noise be doubled? If we are following the sensor with half the gain, why would the sensor noise double? Wouldn't the effects of reduced gain and increased sub length balance out?

Double exposure, double thermal noise (that's why we use darks).

• the signal is comprised of wanted signal and skyglow signal. Both will double but the difference between the two components will double so would be statistically easier to separate.

That's why 1600 may be better there would be twice as many steps between signal and skyglow for any given real difference.

 

Also say I take twice as many shorter subs with ISO 1600 than with ISO 800.

Twice as many subs doesn't double read noise, it increases by root two - 1.4 times (because it is random).

Same with thermal noise, so if one 30 second sub has 'n' thermal noise, a 60 second sub has 2n thermal noise but two 30 second subs have 1.4n.

This is why long exposures (10 minutes + etc.) use cooled cameras.

 

So you are actually better getting your total exposure from lots of short subs rather than a few short subs EXCEPT that if they are too short you may lose details (except a few people like Emil Kraikamp are showing you can get results with 1s exposures and fast scopes!)

So my understanding is use the ISO that has the best level of read noise and use the longest exposures you can get without clipping and get as many subs as you can.

My experience is that the difference between 800 and 1600 ISO is hard to call with my camera but MORE subs always improves the result.

As for exposure I always aim to see a trace of my target DSO on the preview as that means I will always get something when I stretch!

 

 

[The Admiral]

13 hours ago, Stub Mandrel said:

On the plus side of using 800 compared to 1600, there will be:

• half the read noise

Not necessarily - my camera has less read noise at 1600 ISO than ISO800 and read noise is independent of exposure time (that's why you use bias frames) don't confuse it with thermal noise that builds up through the exposure.

Well, I really need to brush up on all this , but just to throw in some extra bits. The reason I said that the read noise would halve was because there would be half the number of subs, but OK, of course the read noise for each sub will now be different because of the different ISO.

I haven't been able to track down the read noise in e terms, but I have come across this site: http://www.photonstophotos.net/ where I can compare the results in terms of DN (whatever that is?) with other cameras. Comparing your EOS10d, EOS450D, and my XT-1 (http://www.photonstophotos.net/Charts/RN_ADU.htm#Canon EOS 10D_12,Canon EOS 450D_14,Fujifilm X-T1_14)

Clearly, these cameras behave in a very different way!

Comparing the new XT-2 sensor with the XT-1, that is different again (http://www.photonstophotos.net/Charts/RN_ADU.htm#Fujifilm X-T1_14,Fujifilm X-T2_14)

Apparently, the XT-2 uses a dual amplifier system before the ADC, and this results in the stepped read noise response with ISO.

In other words, there isn't really a simple answer to this, as all cameras have some differences in their characteristics.

Ian

Edit. Ah, I see you have the Nikon D520. That isn't on the list on this site, unfortunately. But some cameras show some amazingly complicated variations with ISO.

 

Edited November 12, 2016 by The Admiral

[Stub Mandrel]

Good work finding that!

As you can see for my camera the noise only goes up a small bit from one ISO to the next, but the sensitivity is doubled, so high ISO seems to be good for the 450D.

I guess the open circles are where digital amplification is used which is really just stretching the data (which we do anyway so no real gain).

The 520 is a bridge camera  with no RAW setting not limited for astro although it does excellent moons and can even resolve saturn's rings with the built in lens(that's what got me in to AP properly!)

 

[Nigel G]

I'm totally lost in all this, I didn't realise there was so much to it, so many differences with every camera. I really don't understand at all.

I'm trying to follow, here's how much I know, I thought noise was noise but there's read noise thermal noise sky glow and much more, even the exposure times baffles me still, how can 1x60s be the same as 60x1s, noise or no noise there has to be more detail in the 60s ? 

I have always been good with my hands but don't research enough on the technical side, I just do it and see what happens, but understanding it would only make room for improvement.

I'm going to have a good look into these posts and links, hopefully I'll have some sort of understanding of astrophotography rather than just doing it.

Weather's rotten, so good time to sit read and research.

First I'm going to find out what read noise is

Nige

 

[The Admiral]

39 minutes ago, Nigel G said:

I'm totally lost in all this, I didn't realise there was so much to it, so many differences with every camera. I really don't understand at all.

You're not alone Nige! It's a complicated technical subject and I feel I'm only just scratching the surface and even then it's a bit of a challenge. I've just been looking at a book called "The Astrophotography Manual" and there is a short section in that which I think at least sets the scene. I haven't bought it . It covers lots of things I'm not interested in at the moment, but Google has this book on free to view, so if you are interested have a look at the section "Image Capture", starting on p118, here I think*:

https://books.google.co.uk/books?id=r5vlCAAAQBAJ&pg=PA154&lpg=PA154&dq=the+astrophotography+manual&source=bl&ots=xLb-rLghv2&sig=VOeTf6KznNpecy_LmKq3BvirofM&hl=en&sa=X&ved=0ahUKEwiXt5qq4aLQAhVKFMAKHeZeClEQ6AEIRDAH#v=twopage&q&f=true

I'd say it's probably describing the situation in the least technical way. There's a little on filters and binning as well.

The Photonstophotos site is rather deep, somewhat out of my depth currently, but I'm determined to make some headway!

Hope that helps!

Ian

*Edit. No, it takes you to a PixInsight page, but you can switch pages on the top right.

Edited November 12, 2016 by The Admiral

[Nigel G]

22 minutes ago, The Admiral said:

You're not alone Nige! It's a complicated technical subject and I feel I'm only just scratching the surface and even then it's a bit of a challenge. I've just been looking at a book called "The Astrophotography Manual" and there is a short section in that which I think at least sets the scene. I haven't bought it . It covers lots of things I'm not interested in at the moment, but Google has this book on free to view, so if you are interested have a look at the section "Image Capture", starting on p118, here I think*:

https://books.google.co.uk/books?id=r5vlCAAAQBAJ&pg=PA154&lpg=PA154&dq=the+astrophotography+manual&source=bl&ots=xLb-rLghv2&sig=VOeTf6KznNpecy_LmKq3BvirofM&hl=en&sa=X&ved=0ahUKEwiXt5qq4aLQAhVKFMAKHeZeClEQ6AEIRDAH#v=twopage&q&f=true

I'd say it's probably describing the situation in the least technical way. There's a little on filters and binning as well.

The Photonstophotos site is rather deep, somewhat out of my depth currently, but I'm determined to make some headway!

Hope that helps!

Ian

*Edit. No, it takes you to a PixInsight page, but you can switch pages on the top right.

Thanks Ian, that's a great link, after 10 minutes I'm actually beginning to understand whats going on in my camera and whats causing the different unwanted imperfections.

Its a good start.  

Cuppa tea and carry on.

Cheers

A wiser Nige.