The EQ3 DSO Challenge

[bobro]

1 hour ago, Art Gecko said:

The results of my experiment are in!

So just when I thought I was starting to understand something about imaging......

Well, to my untrained eye, the 10 sec subs have a tiny bit more detail and the 50 sec subs more noise. So that means the 10 sec subs win for me! As to why - no idea. Makes me wonder why I'm busy with guiding to obtain longer exposures. Perhaps it's a different story with exposures into minutes, though the short duration images (great composite) seem very good.

Someone must be able to explain the result.......

 

[Art Gecko]

38 minutes ago, bobro said:

So just when I thought I was starting to understand something about imaging......

Well, to my untrained eye, the 10 sec subs have a tiny bit more detail and the 50 sec subs more noise. So that means the 10 sec subs win for me! As to why - no idea. Makes me wonder why I'm busy with guiding to obtain longer exposures. Perhaps it's a different story with exposures into minutes, though the short duration images (great composite) seem very good.

Someone must be able to explain the result.......

 

Well Bob, the best summery I can come up with is that the 50s subs had a better general luminance, but because there were only 22 subs and no darks, there was simply not enough data to remove the noise... Whereas the 10s subs had 136 frames to remove the noise... but lacked in luminance... Not sure if raising the ISO would actually help with this or whether it would bring the noise level up to a point beyond it being worth it? In any case I like the way the combined exposures compliment each other in the final image.... Unfortunately I still wasn't happy with the final image so I did another one! (cropped version to show the extra detail in the core)

Probably going to be out imaging again tonight (got to make the most of the weather!) so I might try 15-20 sec subs and see if I can get the best of both worlds.

[wimvb]

3 hours ago, bobro said:

So just when I thought I was starting to understand something about imaging......

Well, to my untrained eye, the 10 sec subs have a tiny bit more detail and the 50 sec subs more noise. So that means the 10 sec subs win for me! As to why - no idea. Makes me wonder why I'm busy with guiding to obtain longer exposures. Perhaps it's a different story with exposures into minutes, though the short duration images (great composite) seem very good.

Someone must be able to explain the result.......

 

In short exposures, read noise will dominate. In long exposures, dark current (& noise) and sky glow/lp + noise will dominate. These two sources for noise behave differently. That's why you generally can't compare many short exposures with few long exposures, even if the total integration time is constant. The transition between read noise limited exposures and sky or dark current limited exposures is unique for each setup and conditions. The only way to find out what works best, is to experiment, like @Art Gecko did.

Generally longer exposures are better, as is taking more exposures. But with modern low noise, cooled cmos cameras the balance has shifted to more and shorter exposures. Again: experimentation will give best settings for a rig/conditions combination.

In this game, theory is never simple, nor flawless. And you can't take much for granted.

[Art Gecko]

30 minutes ago, wimvb said:

In short exposures, read noise will dominate. In long exposures, dark current (& noise) and sky glow/lp + noise will dominate. These two sources for noise behave differently. That's why you generally can't compare many short exposures with few long exposures, even if the total integration time is constant. The transition between read noise limited exposures and sky or dark current limited exposures is unique for each setup and conditions. The only way to find out what works best, is to experiment, like @Art Gecko did.

Generally longer exposures are better, as is taking more exposures. But with modern low noise, cooled cmos cameras the balance has shifted to more and shorter exposures. Again: experimentation will give best settings for a rig/conditions combination.

In this game, theory is never simple, nor flawless. And you can't take much for granted.

 

4 hours ago, bobro said:

Makes me wonder why I'm busy with guiding to obtain longer exposures.

 

wimvb's argument stands as far as i can see... when you look at a graph of SNR over time, longer exposures win!

On the graph above (which I can't take credit for creating, sorry if this is a no no and please delete if I'm in the wrong for posting it) you can see how longer exposures have a better SNR over time... the problem with my set up is I'm not autoguiding... even as I sit here writing this, my camera is outside taking shots and I have no idea if they're good or not... my target area is about the thickness of the curve line on the graph, which means I'm open to drastic amounts of noise whether my shutter is open for 10 or 30 or 60 seconds... the only real variables are polar alignment, wind (and other vibrations), and the quality of the camera sensor... And how picky I am over my subs when I stack them... which is where I feel I've made the most improvement recently. I'm now quite picky, I'll sacrifice a sub that isn't quite sharp enough to my eye so that it doesn't detract from the final picture, whereas I used to just let the computer asses the quality and stack them blindly... but the computer doesn't necessarily see things the way I do.... everything is a juggle for me in this game, it's fun to experiment to see what the best you can get with what you've got is... and that's what this thread is for, is it not? I'm so glad you pointed me here @bobro thank you

[Art Gecko]

M100 and co. 250x15s subs, darks and flats applied. Shame I didn't pick out the 2 subs with the satellite streaks in them, D'oh!

Definitely liking the shorter exposures as 62.5 minutes is the longest total I've had so far with my DSLR.

[Stub Mandrel]

5 minutes ago, Art Gecko said:

M100 and co. 250x15s subs, darks and flats applied. Shame I didn't pick out the 2 subs with the satellite streaks in them, D'oh!

Do you use DSS? If you use sigma stacking the satellite trails will disappear.

[Art Gecko]

Just now, Stub Mandrel said:

Do you use DSS? If you use sigma stacking the satellite trails will disappear.

I'm afraid I use Lynkeos for stacking as I'm on Mac... I did get DSS running through wineskin, but my poor Mac is so old it'd take a month for DSS to stack 250 subs!

[bobro]

First attempt at (part of) the North American nebula. Didn't quite get focus. Tried a UHC filter - should this have helped with the image? Perhaps longer exposures would have been better as the subs were darker than normal due to the filter. Will try again to see if I can improve on it. 24 subs @ 240sec.

[Stub Mandrel]

1 hour ago, bobro said:

First attempt at (part of) the North American nebula. Didn't quite get focus. Tried a UHC filter - should this have helped with the image? Perhaps longer exposures would have been better as the subs were darker than normal due to the filter. Will try again to see if I can improve on it. 24 subs @ 240sec.

Much of the NAN is not particularly detailed in 'broadband' but this is a surprisingly bright target - I'm surprised you haven't got a lot more faint stars at 240S exposures, it looks like something odd was going on to me as your setup is virtually identical to mine: 450D modded - same sensor as 1000D modified, 130P-DS ~= Polaris 130.

That said your guiding is working, nice round stars, next step is a Ha filter and 10-minute exposures!

I used 90-second subs with a moon & skyglow (poor man's UHC filter) night before last, finished processing this morning, more stars and nebula. I think we could both improve our framing though!

The Pelican is much fainter, and I notice that it has barely registered in my subs despite being very clear in this image I took last year:

Really hard to know what colour to aim for...

[bobro]

1 hour ago, Stub Mandrel said:

I think we could both improve our framing though!

Believe it or not Neil, I was aiming for the Pelican but time was getting on so went for what I had as I couldn't get the final adjustment right! 

Yes, my captures don't seem to show the same number of stars as others achieve. It could be down to a number of things :

1) The OTA has been shortened by 40mm, possibly losing some light as the secondary is closer to the primary. Could also be light loss due to the focus tube being 1.25" and a long path up the tube due to the OTA being shorter and the camera relatively far out compared to scopes with shorter focusers.

2) The 1.25" Coma Corrector results in noticeable light loss and additional vignetting.

3) The UHC filter requiring longer exposure. You can see the difference in image brightness with and without the filter in the attached jpeg versions of raw images. Both are ISO800 240 secs and are of the NAN.

As you say, guiding, together with the CC, are producing good round stars. What has also made a real difference is being able to very accurately polar align using SharpCap - DEC corrections are very infrequent when guiding. No issue with finding guide stars with the more sensitive camera which means the guide scope points in the same direction as the image scope - no field rotation (shouldn't be I know if PA is spot-on).

I'm glad you posted your images of the NAN - lovely and sharp. Brings me on to something that puzzles me about filters and colour balance : If I try and do the usual lining up of RGB peaks after stacking to restore the colour balance following use of a UHC filter, will that not just result in degrading the image as some of the colour (especially green) will have been filtered out?  Trying to achieve white  stars would seem to be an issue. There must be a point I've missed about this type of filter.....

Following your feedback, I think the next step will surely be longer exposures. I will try without the UHC filter first as it seems to be cutting out so much light.

Bob

[Stub Mandrel]

37 minutes ago, bobro said:

If I try and do the usual lining up of RGB peaks after stacking to restore the colour balance following use of a UHC filter, will that not just result in degrading the image as some of the colour (especially green) will have been filtered out?

IMHO aligning the histograms compensates for colour changes introduced by modification and filters. This is the Sadr region with histogram alignment, slight saturation boost and an s-shaped luminance curve (with the steep bit aligned to the histo peak) in DSS. All done in DSS and NO other colour correction or processing at all, except for reducing the bit depth to 8 bits. Taken with modded camera (too much red) and a pale-cyan moon & skyglow filter. Yes, I know it's noisy!

[mikey2000]

On 27/05/2017 at 00:28, Art Gecko said:

 

wimvb's argument stands as far as i can see... when you look at a graph of SNR over time, longer exposures win!

I must admit to being rather interested in this sort of thing.  (not that I actually know anything about this aspect of AP!)

 

I can't help noticing the scale on the vertical axis is "SNR increase over a 30s exposure" and it only goes up to about 35% higher, and then only for superdark skies and 12minute exposures.  I suspect that many of us in this thread are living in polluted areas and managing 4mins subs maximum.  Ie, we are in the bottom left corner of the graph.  Here, even a 4 min sub only increases the SNR by 5%.

 

I did recently try comparing 12x2mins against 6x4mins and the 6x4mins was easier to process.   But really, 24mins data isn't much to go on :-)   I should really try again with longer overall sessions, say 1 hour each of 2 and 4 mins subs.

 

I also need to see how far my baader neodymium filter is helping to lift me towards the dark sky curve.  Amusingly, since I bought it and got really into this imaging stuff, I haven't bothered to remove it to see if it is doing anything useful!  Maybe it's just eating my photons!

[Stub Mandrel]

I think that NAN looks ghastly in purple... this one looks a more natural colour to me:

 

[mikey2000]

I agree, I think purple is not quite right.  Orange looks a bit odd to.   The shape and textures look good, but maybe use a bit of gimp, photoshop whatever to adjust the hue.

 

I just wondered for the first time, what colour are these nebulae really?  We tend to see them as reddish in long exposure photos but I've never seen a real one with my naked eye...

[bobro]

22 minutes ago, Stub Mandrel said:

I think that NAN looks ghastly in purple... this one looks a more natural colour to me:

So how did you achieve the colour change?

[Stub Mandrel]

If you were in the nebula, you wouldn't be able to see it...

The natural colour of Ha emission is deep red, but there are other colours of light from these nebulas as well so they may not be pure deep red in colour.

1 hour ago, bobro said:

So how did you achieve the colour change?

More care in aligning histogram gave a redder colour, and then in PS I added a layer with saturation at the max so it looked psychadelic, then a colour balance layer between the original and the saturation layer. Zoom in on neutral black background and tweak colour balance so each of R-C,G-M and B-Y is at the point between where it all turns one colour or the other, then delete the saturation layer.

Alternatively just use colour balance and judge it by eye.

You can also selectively tweak the saturation of colours with a Hue & Saturation layer.

The soft light & luminance trick also makes it look redder if it starts out a 'brick red'.

I have an odd effect in the star bottom left that has a blue halo around a yellow star...

 

Another good trick is to split you image into two images, process the first image (LUM) for best contrast and clarity, the other (RGB) for best colour, perhaps blurring it a bit to help colour up small stars, then put the LUM layer on top of RGB and set it to 'luminosity'.

yet another trick is to use a Hue and Saturation layer on the luminosity layer 9assuming it has some colour in it - you might want to up it's saturation a bit first). You can then use the 'lighten' slider on different colour channels - the R and M channels will generally help brighten the densest Ha regions, reducing G and Y helps darken the background outside the Ha and helps control stars. Best to just play and see what happens.

[Nigel G]

I have added an hour to the original NGC7000 image with a good result. 2 hours of 240s. Modified 1200d, Equinox 80ED. I don't know what the natural colour is but very little colour processing and this is the result. First is the 1 hour, 2nd is 2 hours.

Nige.

 

[Stub Mandrel]

That's the colour I keep trying for, but it just won't come :-(

[bobro]

Good to see the difference with longer exposure Nige - gives me motivation to have another go at the nebula with longer individual and total exposure.

[Nigel G]

3 hours ago, Stub Mandrel said:

That's the colour I keep trying for, but it just won't come :-(

I had trouble with the unmodified 1200d camera, orange and pink being dominant but with the self modified 1200d and CLS filter the red responce is far better.

I tried the modified without the CLS filter once but theres too much light polution washing out the images.

Nige.

[Stub Mandrel]

Hmm. tried everything without luck. Then went back to the original stack, ran HLVG, a few shots of LUM/Softlight to increase the colour, two doses of make stars smaller and one of space noise reduction... Perhaps I was trying to hard?

[bobro]

Had another go at the NAN - without the UHC filter. A little better than my previous attempt, but I expected more nebulosity to show in the RAW image. 21 subs @ 360 secs.

[Art Gecko]

2 hours ago, bobro said:

Had another go at the NAN - without the UHC filter. A little better than my previous attempt, but I expected more nebulosity to show in the RAW image. 21 subs @ 360 secs.

Nice shot Bob, out of interest what ISO are you imaging at? I might join the fun on this target tonight if it stays clear as I've never even tried finding it before!

[Nigel G]

2 hours ago, bobro said:

Had another go at the NAN - without the UHC filter. A little better than my previous attempt, but I expected more nebulosity to show in the RAW image. 21 subs @ 360 secs.

Nice Bob, colours good and good detail, plenty of nebulosity.

I think you might pick up more detail if you centre the Southern half rather than centre the complete nebula. I loose detail around the edge with my reflector so try to get the most detailed parts in the middle of the frame. Maybe mosaic 2 images, but still well done.

 An example, here's the same last year with my 150p 45s exposures.

Nige.

 

[bobro]

8 minutes ago, Art Gecko said:

out of interest what ISO are you imaging at?

The camera is set to ISO800 as I understand 400/800 helps to keep noise lower.  It looks like longer exposure with my setup is still required.