Advice - Flaming Star Neb

[Stub Mandrel]

I'm not getting very far with 30 minutes of astro-modded DSLR data on this one. I can see the nebula, but poor detail and very faint.

I see almost all attempts at imaging it are RGB or Ha+RGB and long exposures. Am I flogging a dead horse on this target or will more subs start to find that elusive blue bit?

 

 

[daz]

Well, I can't see the nebula either

 

[Stub Mandrel]

I didn't want to post an image of a nebula that looked 'posterised'...

I've had another go using a mid-tone slope of just 1 degree in DSS + RGB background calibration and it's worked better. At least it has some blue, but still not up with the Ha guys though!

[wxsatuser]

9 hours ago, Stub Mandrel said:

I didn't want to post an image of a nebula that looked 'posterised'...

I've had another go using a mid-tone slope of just 1 degree in DSS + RGB background calibration and it's worked better. At least it has some blue, but still not up with the Ha guys though!

 

You have a fair bit of the blue but a modded camera may suppress other colours for red.

Ha has nothing to do with the blue, as the blue is Rayleigh scattering of particles smaller than light.

What were your sub lengths?
I did 2minute subs on this with an unmodded 7DMK2 and there was plenty of red and the blue was there as well.
Your modded dslr shows little red, so I assume the subs were quite short.

[Stub Mandrel]

That is as as much as I can stretch the data, I can get more red visible but it starts to look horrible.

From other photos I can see the nebula is really much bigger with extended faint nebulosity as well.

These were 1-minute subs and my camera is 'stage 1 modded'. You give me hope that there is more to find with a dslr, I will see if I can get away with 2 minutes and when it is a bit higher in the sky.

 

[wxsatuser]

This is what I got with the 2minute subs from the unmodded 7DMK2.

I never really processed this properly as the guiding was rubbish and this was into my worst direction for LP.

This was 16x120secs 7DMK2, Canon EF 70-200mm L IS II@200mm, f/4, ISO 1600, Astronomik CLS LP filter.

[Stub Mandrel]

Ah... My 329 x 60 secs at ISO1600 f4.5 shouldn't look much different, this was at a decent dark sky site..

I may be shooting myself in the foot trying to get a black background. I'll play some more.

[wxsatuser]

You may have a lot more in there waiting to come out.

Don't forget a 7DMK2 has a far superior QE to most Canon cameras.

 

[Stub Mandrel]

Well this isn't what I was expecting all in the 'mid range', still needs more subs to be decent...

[Adam J]

The problem is that the read noise on the 450d is reasonably random and so its hard to calibrate it out. Your signal is buried in the read noise due to the short exposure.

I am guessing that the NEQ3 is the limiting factor here and you cant push past 60 seconds. Though you have round stars so your not pushing it to its limits currently.

I highly recommend that in this situation you dont use ISO1600 on a 450D...actually I would say that you should never use it. There is a massive increase in read noise( way more than double) on the 450D between ISO800 and ISO1600.

Unitary gain is ISO400 with the 450D if memory serves...but i would plunk for ISO800 to get the best signal without introducing the horrible ISO1600 read noise.

What percentage of frames are you chucking out at 60seconds? Push it further if you can. When read noise limited you may find that your better off pushing the mount to longer exposures even if you have to chuck a lot of them out, what you do get will give you better signal to noise in the end....clearly there are limits to that theory though.

In an ideal world this target needs 5 min exposures with your camera.

[Stub Mandrel]

8 hours ago, Adam J said:

The problem is that the read noise on the 450d is reasonably random and so its hard to calibrate it out. Your signal is buried in the read noise due to the short exposure.

I am guessing that the NEQ3 is the limiting factor here and you cant push past 60 seconds. Though you have round stars so your not pushing it to its limits currently.

I highly recommend that in this situation you dont use ISO1600 on a 450D...actually I would say that you should never use it. There is a massive increase in read noise( way more than double) on the 450D between ISO800 and ISO1600.

Unitary gain is ISO400 with the 450D if memory serves...but i would plunk for ISO800 to get the best signal without introducing the horrible ISO1600 read noise.

What percentage of frames are you chucking out at 60seconds? Push it further if you can. When read noise limited you may find that your better off pushing the mount to longer exposures even if you have to chuck a lot of them out, what you do get will give you better signal to noise in the end....clearly there are limits to that theory though.

In an ideal world this target needs 5 min exposures with your camera.

I'm using ISO 1600 and 60 seconds at the moment simply because it seems to work well for most targets, and to date I've just been happy to capture anything!

I as losing higher percentage of frames than I used to, but then I reduced the preload on the worm and bingo I can now stack 90% of frames. I am not sure how long I can go before periodic errors cause problems, but I can get 60 seconds using the 150PL which is over twice the fl and three time the weight so I should be able to get longer subs with the 130P-DS.

I have just taken two single frames at 120 seconds at ISO800 and 60 seconds as ISO1600 then given them, converted to ITFF in Irfan view with no adjustment then given them the same stretch in Photopaint. Unlike my old version of PS, PP gives more data on the stretch and allows you to apply the same settings easily.

This is a stretch of 25.6 times, so pretty extreme.

I have saved the results as jpegs. You are clearly right that ISO1600 is noisier, but I wouldn't go as far as saying it is double, at least not if you increase the exposure for ISO800 to get a fair comparison.

ISO800 120s

ISO1600 60s

The amp glow at left is interesting too!

I need to try 120 sec/iso 800 subs and see what happens.

[Stub Mandrel]

I must admit, I am confused now. I was convinced that I had found, some time ago, a page that showed ISO1600 had less read noise than ISO800 on the 450D and here it is:

http://www.astrosurf.com/comolli/strum45.htm

I think all I can do is experiment and see what happens. the only things I am sure of with my setup are:

1. More subs bring out more detail and less noise
2. Contrary to some suggestions, using darks visibly reduces the noise levels in processed pictures

[Adam J]

9 hours ago, Stub Mandrel said:

I'm using ISO 1600 and 60 seconds at the moment simply because it seems to work well for most targets, and to date I've just been happy to capture anything!

I as losing higher percentage of frames than I used to, but then I reduced the preload on the worm and bingo I can now stack 90% of frames. I am not sure how long I can go before periodic errors cause problems, but I can get 60 seconds using the 150PL which is over twice the fl and three time the weight so I should be able to get longer subs with the 130P-DS.

I have just taken two single frames at 120 seconds at ISO800 and 60 seconds as ISO1600 then given them, converted to ITFF in Irfan view with no adjustment then given them the same stretch in Photopaint. Unlike my old version of PS, PP gives more data on the stretch and allows you to apply the same settings easily.

This is a stretch of 25.6 times, so pretty extreme.

I have saved the results as jpegs. You are clearly right that ISO1600 is noisier, but I wouldn't go as far as saying it is double, at least not if you increase the exposure for ISO800 to get a fair comparison.

ISO800 120s

ISO1600 60s

The amp glow at left is interesting too!

I need to try 120 sec/iso 800 subs and see what happens.

The first thing is that to see the read noise you will need to take very short exposures 1/4000s , the shortest possible, like when you are taking bias frames. If you look at the noise in 60s / 120s dark frames then you are looking at bias and thermal noise together, that is ultimately a better test of total noise mind you, but its not bias.

However, you are really making the wrong comparison above, there is no point in comparing ISO800 at 120s to ISO1600 at 60s, its irrelevant if you can't track for more than 60 seconds. All that matters to you is which setting gives you the best signal to noise ratio at 60s in that situation. So in a way what you have shown is that even at twice the exposure length the ISO800 frame still has less noise and i even see horizontal banding in the ISO1600 frame and it is visible in your image too. As such you could easily expect that you will get less than half the noise if you do a like for like and take one at 60s for ISO800 and ISO1600 at the same temp. If noise is more than double at ISO1600 then its a poor trade off for only double the amplification. Exposure time vs ISO trade off is only a factor when you are not limited by tracking.

The second thing is that if you can guide for 120s or even 90s then the balance may change. Problem is my experiments have shown that ISO1600 is never worth it even in the summer with high thermal noise vs read noise. You will have to stretch more at ISO800 but that does not mean that you will end up with more noise.

The issue you have across the board is that ISO is only one factor that is effecting your signal to noise ratio.

The last chart is irrelevant go with what your eyes / your own data is telling you mate.