M31 - Comparing 18.5 hour integration to 10 hour integration

[eshy76]

Hi everyone,

You may recall my recent M31 image -- I have now combined this with data from a year ago, taking total integration from 10 hours 6 mins to 18 hours 36 mins.

Does it make a big difference? During processing, I would say the extra data made the blues in the outer galaxy easier to coax out and there is maybe more detail in the dust lanes...I'm struggling to see much difference otherwise apart from processing - maybe I'm too tired to notice at this point!

I will post the new image below and the previous one with 10 hours integration in the next post for comparison purposes.

Key processing difference - in the new image I've brought in Ha into the red channel at a 40% weighting (vs. 50% in the 10 hour image)...I have also been less aggressive on star colour in the new image.

Thanks for looking!

18.5 hour integration - M31 2020 and 2021 data. Edit: Some extra details - taken in Bortle 7-8 skies, LHaRGB filters, William Optics Z73 refractor, ZWO ASI1600MM Pro camera, Rainbow Astro RST-135 mount in 2021, iOptron CEM25P in 2020.

Edited September 14, 2021 by eshy76
Extra details

[eshy76]

And the original 10 hour integration

[Sunshine]

It is a wonderful image!

[eshy76]

1 hour ago, Sunshine said:

It is a wonderful image!

Thank you!

[Dazzyt66]

Really REALLY great image - I'm struggling to see a major impact of the extra hours though - Looks like I've only got about 9 more hours of data to go with mine then... 😂

[wornish]

Superb images.  I too struggle to see the difference though between the 10 and 18hours versions TBH.

I managed to go for around 2 hours last week and I  am embarrassed to compare it with your 10 hour one. 

[eshy76]

1 hour ago, Dazzyt66 said:

Really REALLY great image - I'm struggling to see a major impact of the extra hours though - Looks like I've only got about 9 more hours of data to go with mine then... 😂

Thank you! Yes when I really zoom in close I can see a little bit more definition in the dust lanes, and I feel I could push the colours more in the 18 hour version if I wanted to...but the difference seems incremental. Of course processing ability could be a factor!

Edited September 14, 2021 by eshy76

[eshy76]

19 minutes ago, wornish said:

Superb images.  I too struggle to see the difference though between the 10 and 18hours versions TBH.

I managed to go for around 2 hours last week and I  am embarrassed to compare it with your 10 hour one. 

Thank you! Don't be embarrassed - we all know how the weather can impact our imaging time! I got lucky over two nights and left the rig to chug away all night on both occasions. I am finding 5 hours largely sufficient for narrowband imaging in my Bortle 7 sky, whereas it looks like 5-10 hours seems to be what I should shoot for in LRGB with my f/5.9 scope.

Edited September 14, 2021 by eshy76

[alan potts]

Lovely image, I am going to start ramping up my data on this target which stands at about 4 hours now. Though I can't see them side by side I struggle with my normally good eye for this sort of thing to see a difference worth talking about. I have 16 hours on M33 which I am also going to add to, weather permitting of course.

Alan

[eshy76]

2 hours ago, alan potts said:

Lovely image, I am going to start ramping up my data on this target which stands at about 4 hours now. Though I can't see them side by side I struggle with my normally good eye for this sort of thing to see a difference worth talking about. I have 16 hours on M33 which I am also going to add to, weather permitting of course.

Alan

Thank you Alan - I think this could be a case of diminishing returns....10 hours was already a decent level of integration, so adding another 8 hours was less impactful. However, going from, say, 2 hours to 10 hours could be more visible.

[alan potts]

2 hours ago, eshy76 said:

Thank you Alan - I think this could be a case of diminishing returns....10 hours was already a decent level of integration, so adding another 8 hours was less impactful. However, going from, say, 2 hours to 10 hours could be more visible.

Totally agree, I noticed the same with the collection of M33 data. We have plenty of clear skies here from May to November and I normally do about 2 hours a night and was checking the results of M33 at 2 hour intervals, once about 4 hours it was really 12 before you could see a difference. Though one has to consider different processing on each effort, it is unlikely to be the same.

Alan

[Xilman]

13 hours ago, eshy76 said:

Does it make a big difference? During processing, I would say the extra data made the blues in the outer galaxy easier to coax out and there is maybe more detail in the dust lanes...I'm struggling to see much difference otherwise apart from processing - maybe I'm too tired to notice at this point!

Adding the extra integration time should make a difference of perhaps half a magnitude to your limiting magnitude. I suggest that you zoom in and take a critical look at the M31 stars and globular clusters, especially the fainter ones down at 20th magnitude or so.  Finder charts and catalogues are readily available but I could provide some for you if you have difficulty finding them.

I regularly monitor variable stars in M31 and have imaged many globular clusters in and around the galaxy.

[Xilman]

11 minutes ago, Xilman said:

Adding the extra integration time should make a difference of perhaps half a magnitude to your limiting magnitude. I suggest that you zoom in and take a critical look at the M31 stars and globular clusters, especially the fainter ones down at 20th magnitude or so.  Finder charts and catalogues are readily available but I could provide some for you if you have difficulty finding them.

I regularly monitor variable stars in M31 and have imaged many globular clusters in and around the galaxy.

 

For instance, a quick zoom-in around the field of AE And, a blue supergiant in M31, shows the variable very clearly and it was at an average brightness of about 16.7 I would guess over the period of your observations.  Here is a 4x zoom in the field of AE And, which marked with white lines, and it shows the blue colour of the star rather nicely. Marked with red is the 16.9 magnitude globular cluster Bol 356; note its distinctly yellow colour because it is composed of much older and cooler stars than the very young and very hot AE And. Contrast with the red supergiants in the same field. The brightest stars visible are in our galaxy but the great majority of the fainter ones are in M31.

 

The image above is very much a quick-look snapshot.  I am sure you could make the M31-resident objects much more easily visible with a bit of work.

Incidentally, the full image will likely show well over a hundred globular clusters and many thousand of stars, some of which are of great historical interest. Tracking them down should keep you busy for a few cloudy hours.

[eshy76]

1 hour ago, Xilman said:

 

For instance, a quick zoom-in around the field of AE And, a blue supergiant in M31, shows the variable very clearly and it was at an average brightness of about 16.7 I would guess over the period of your observations.  Here is a 4x zoom in the field of AE And, which marked with white lines, and it shows the blue colour of the star rather nicely. Marked with red is the 16.9 magnitude globular cluster Bol 356; note its distinctly yellow colour because it is composed of much older and cooler stars than the very young and very hot AE And. Contrast with the red supergiants in the same field. The brightest stars visible are in our galaxy but the great majority of the fainter ones are in M31.

The image above is very much a quick-look snapshot.  I am sure you could make the M31-resident objects much more easily visible with a bit of work.

Incidentally, the full image will likely show well over a hundred globular clusters and many thousand of stars, some of which are of great historical interest. Tracking them down should keep you busy for a few cloudy hours.

Thank you very much for that - it reminds me to take the time to look not just at the main objects that we capture but also what is there in their vicinity! Really appreciate it!

[ollypenrice]

Did you try to stretch the outer glow any harder with the extra data? You might find you could reveal a it more of it?  What I found in going after that very faint stuff with a CCD camera (and this wouldn't apply to CMOS) was that going from 15 min subs to 30 min made far more difference than adding extra 15 min subs.

Olly

 

[eshy76]

6 hours ago, ollypenrice said:

Did you try to stretch the outer glow any harder with the extra data? You might find you could reveal a it more of it?  What I found in going after that very faint stuff with a CCD camera (and this wouldn't apply to CMOS) was that going from 15 min subs to 30 min made far more difference than adding extra 15 min subs.

Olly

 

Thanks for this Olly - on the 18 hour integration I processed it as I would normally do, which means I tried ensure I stretched it up to the point that data would clip if I went any further...which probably means I could be more aggressive...

On your CCD point, it's interesting that the data I added was 8 hours of shorter (30 second), higher gain subs, though as you say with the noise profile of CMOS cameras, that should not be detrimental.

I'll go back in due course and see how far I can push the data - I'm curious myself about it!

[ollypenrice]

4 hours ago, eshy76 said:

Thanks for this Olly - on the 18 hour integration I processed it as I would normally do, which means I tried ensure I stretched it up to the point that data would clip if I went any further...which probably means I could be more aggressive...

On your CCD point, it's interesting that the data I added was 8 hours of shorter (30 second), higher gain subs, though as you say with the noise profile of CMOS cameras, that should not be detrimental.

I'll go back in due course and see how far I can push the data - I'm curious myself about it!

Once you have your pure background sky up to a brightness you like (which for me is about 22  in Ps for galaxies) you can pin the curve at that value and stretch just above it, restoring the curve near the top.This lets you pull out faint nebulosity without white clipping at the top. In the example below the background is pinned at its original value and the added kink, or bulge, placed in the curve affects only the fainter, outer parts of the galaxies. It leaves their cores and the stars almost unaltered.

This method allows you to avoid unnecessary stretching of the background (which can lift it above the noise floor) and keeps the bright parts down while getting more out of the faint signal.

It would provoke an apoplectic fit amongst the PI developers so please don't ever show it to them. 

Olly

 

[eshy76]

4 hours ago, ollypenrice said:

Once you have your pure background sky up to a brightness you like (which for me is about 22  in Ps for galaxies) you can pin the curve at that value and stretch just above it, restoring the curve near the top.This lets you pull out faint nebulosity without white clipping at the top. In the example below the background is pinned at its original value and the added kink, or bulge, placed in the curve affects only the fainter, outer parts of the galaxies. It leaves their cores and the stars almost unaltered.

This method allows you to avoid unnecessary stretching of the background (which can lift it above the noise floor) and keeps the bright parts down while getting more out of the faint signal.

 

It would provoke an apoplectic fit amongst the PI developers so please don't ever show it to them. 

Olly

 

Wow you've blown my mind with that Olly...I had a little play around this morning (before work, ha!) using a similar Curves stretch to one you posted, but in PixInsight (you can do the same thing there with the pins)...and it looked like the Andromeda core was lying in a cloud...it looks better than it sounds!

I also found that applying an even smaller stretch than your one, but several times iteratively, instead of one bigger stretch, seemed to be a more subtle and less destructive way of bringing up the "outer rim" for my particular data.

Definite potential with this approach - I will return to this on a quiet evening - it's the sort of tweak which needs work to be done for the day, the kids in bed, a neat desk and possibly classical music! Thank you very much!

[Xilman]

On 14/09/2021 at 15:27, Xilman said:

Incidentally, the full image will likely show well over a hundred globular clusters and many thousand of stars, some of which are of great historical interest. Tracking them down should keep you busy for a few cloudy hours.

I spent a pleasant hour or so tracking down a historical image and then locating its contents in yours. The large black and white image is from Edwin Hubbles's The Realm of the Nebulae. Hubble, of course, discovered and measured Cepheid variables in M31, thereby proving beyond doubt that is an external galaxy.

 

 

Your colour image is annotated in the style of Hubble's original. Note that his had North to the top and East to the left whereas yours is rotated by approximately ninety degrees. Three of the four are readily identifiable but the Cepheid is far from obvious in your unprocessed image. Accordingly, I converted to monochrome, zoomed in even further, and performed a moderately severe contrast stretching. The Cepheid is now visible --- just.  Not bad going for an object of around 19th to 20th magnitude when you consider that Hubble used a 2.5m telescope (the world's largest at the time) or 34 times the aperture of yours. To be fair, he had to use photographic plates, which are only 2% or so as sensitive as your camera, and had to take his image on a single night, but the 1200-times difference in light grasp between the two telescopes shows by just how much technology has advanced in the last 100 years.

 

[CloudMagnet]

There was a thread on there a few months ago looking at a comparison on the Iris nebula on integration time and it seemed to be that once you reached 10 hrs, the level of improvement afterwards really tailed off and you couldn't see much difference.

I think that once you get about 10hrs, you should really look at adding only the best quality data rather than just pure amount of time to get the best results. Otherwise, adding in data that is lower quality won’t improve the final image as your noise level will already be so low anyway.

[eshy76]

11 hours ago, CloudMagnet said:

There was a thread on there a few months ago looking at a comparison on the Iris nebula on integration time and it seemed to be that once you reached 10 hrs, the level of improvement afterwards really tailed off and you couldn't see much difference.

I think that once you get about 10hrs, you should really look at adding only the best quality data rather than just pure amount of time to get the best results. Otherwise, adding in data that is lower quality won’t improve the final image as your noise level will already be so low anyway.

Thank you for that, I missed that topic - it looks like my own experience bears that out too. So for me, in a Bortle 7 sky, with my f5.9 scope, 10 hours seems to be a sweet spot for an LRGB image. I agree with your comments on the quality of the data.

[City9Town0]

What a lovely thead... Lovely images and so much information... what this site is about.👍

[CloudMagnet]

1 hour ago, eshy76 said:

Thank you for that, I missed that topic - it looks like my own experience bears that out too. So for me, in a Bortle 7 sky, with my f5.9 scope, 10 hours seems to be a sweet spot for an LRGB image. I agree with your comments on the quality of the data.

Managed to find it

 

 

[Lee_P]

1 hour ago, CloudMagnet said:

Managed to find it

 

 

I'm glad that thread is still of use! I used it as the basis for an article about tips to obtain long integration times: http://urbanastrophotography.com/index.php/2021/06/14/how-to-get-long-integration-times/

[Mr Spock]

Beautiful image!