M31 from 2017/2018/2019 data with Ha

[kirkster501]

Added the Ha to the LRGB I posed a few months ago.  I know the Ha and red is a bit strong but thought I'd share for interest's sake.

Hope everyone is keeping safe.

FSQ85 with Atik460 LRGBha Baader filters.

[Richie092]

Just looking through some M31 images as it is my next target.  Really like this one could you share capture details please?

 

[MarkAR]

That's interesting, first time I've seen M31 with Ha. Lovely image.

[kirkster501]

On 23/07/2020 at 07:11, Richie092 said:

Just looking through some M31 images as it is my next target.  Really like this one could you share capture details please?

 

It was with my FSQ85 with Atik460 LRGBHa Baader filters.  About three hours luminance, two hours in Ha and an hour each in RGB.  There are a few tweaks I need to make to this image to make it pop a bit more.

[Richie092]

Ta, can you remember sub lengths?

[kirkster501]

18 hours ago, Richie092 said:

Ta, can you remember sub lengths?

I always do luminance/Ha at ten minutes and RGB at five.  With my skies I get zero benefit in going longer than that.

[tooth_dr]

27 minutes ago, kirkster501 said:

I always do luminance/Ha at ten minutes and RGB at five.  With my skies I get zero benefit in going longer than that.

Its interesting about your exposure times RGB subs.  Do you bin 2x2 the RGB?

[kirkster501]

8 hours ago, tooth_dr said:

Its interesting about your exposure times RGB subs.  Do you bin 2x2 the RGB?

I increasingly bin everything 1x1 nowadays.

[tooth_dr]

1 minute ago, kirkster501 said:

I increasingly bin everything 1x1 nowadays.

I've been reading exposure times, as I'm just getting started on LRGB imaging, and thought RGB should be the same (or 2x longer ) than luminance, so I wondering what was the rationale for shorter RGB subs.  l'd have thought that it you can get 10 mins on luminance then you would get 10+ minutes on RGB, and wouldnt be limited to 10mins on H-alpha.

[kirkster501]

17 hours ago, tooth_dr said:

I've been reading exposure times, as I'm just getting started on LRGB imaging, and thought RGB should be the same (or 2x longer ) than luminance, so I wondering what was the rationale for shorter RGB subs.  l'd have thought that it you can get 10 mins on luminance then you would get 10+ minutes on RGB, and wouldnt be limited to 10mins on H-alpha.

I don't agree with that rule you've read about RGB x 2 wrt Luminance.   The luminance is the absolute king of astrophotography.  I have seen APOD images where the luminance was eight times the combined RGB total and was brilliantly colourful and vibrant. 20 hours of luminance and one hour each in RGB is very acceptable if the data is good quality.

Now with that said, I have been experimenting and find that if I bin the RGB 1x1 (or even one of the colours 1x1) then the need for the separate luminance is less crucial.  You still need the /total/ integration time though.

I live under a flight path into East Midlands airport and so it hurts to throw away 20 minute subs.  I have done this length before, but I find little difference between that and ten minutes in my skies.  Sure, if I lived away from the airport and had glorious and abundant dark skies, then yes, 30 min subs would be worth it. I'd have the scope going overnight most nights and have a huge data lake of data for dozens of objects.  However, reality check is that I live in Nottingham not Nevada.

P.S.  I love your comet photo.  Very nice.

[tooth_dr]

50 minutes ago, kirkster501 said:

I don't agree with that rule you've read about RGB x 2 wrt Luminance.   The luminance is the absolute king of astrophotography.  I have seen APOD images where the luminance was eight times the combined RGB total and was brilliantly colourful and vibrant. 20 hours of luminance and one hour each in RGB is very acceptable if the data is good quality.

Now with that said, I have been experimenting and find that if I bin the RGB 1x1 (or even one of the colours 1x1) then the need for the separate luminance is less crucial.  You still need the /total/ integration time though.

I live under a flight path into East Midlands airport and so it hurts to throw away 20 minute subs.  I have done this length before, but I find little difference between that and ten minutes in my skies.  Sure, if I lived away from the airport and had glorious and abundant dark skies, then yes, 30 min subs would be worth it. I'd have the scope going overnight most nights and have a huge data lake of data for dozens of objects.  However, reality check is that I live in Nottingham not Nevada.

P.S.  I love your comet photo.  Very nice.

Thanks Steve. I don’t mean you want 2x the total exposure for RGB compared with Lum. I mean that your individual exposures should be twice the length of the luminance subs, or at least the same length.  I’ve produced images with as little as one or two subs from RGB channels but a good luminance data set. 

[newbie alert]

18 hours ago, tooth_dr said:

I've been reading exposure times, as I'm just getting started on LRGB imaging, and thought RGB should be the same (or 2x longer ) than luminance, so I wondering what was the rationale for shorter RGB subs.  l'd have thought that it you can get 10 mins on luminance then you would get 10+ minutes on RGB, and wouldnt be limited to 10mins on H-alpha.

All the details are in the luminance and HA.. rgb is just for colour

[tooth_dr]

6 minutes ago, newbie alert said:

All the details are in the luminance and HA.. rgb is just for colour

Indeed, thanks for that!  I’m just trying to assertain why you would choose to expose for less, with filters that will inherently let less signal through then luminance

[newbie alert]

6 minutes ago, tooth_dr said:

Indeed, thanks for that!  I’m just trying to assertain why you would choose to expose for less, with filters that will inherently let less signal through then luminance

As far as I've read luminance covers the whole of the broadband spectrum 

[tooth_dr]

18 minutes ago, newbie alert said:

As far as I've read luminance covers the whole of the broadband spectrum 

Just to reiterate I’m asking about exposure lengths not total exposure. You will obviously want more total exposure time for luminance, but I was just wondering why choose to expose individual subs for half the time when you are already only getting a third of the signal of lum / broad band spectrum (approx) 

Edited July 26, 2020 by tooth_dr

[kirkster501]

4 hours ago, tooth_dr said:

Thanks Steve. I don’t mean you want 2x the total exposure for RGB compared with Lum. I mean that your individual exposures should be twice the length of the luminance subs, or at least the same length.  I’ve produced images with as little as one or two subs from RGB channels but a good luminance data set. 

Ok, gotcha.  TBH I don't think it matters as long as you have enough of a data pool.  I have generally converged on 10 mins L/Ha and 5 mins for RGB after much experimentation.  Sometimes I'll do 5 min luminance lights to get something in the can if it looks like clouds later on that evening.

[x6gas]

I can see in theory why you'd want individual RGB subs from a mono camera to be the same length as the luminance subs but in my experience it isn't necessary as you can stretch the colour data harder, give it a lot of noise reduction if necessary and rely on the lum for detail.

[x6gas]

Steve - absolutely love your image.  That's one of the best M31s I've seen and it's very nice to see the Ha regions in deep red rather than the neon pink that is more common.

Lovely result, well done!

[kirkster501]

16 hours ago, x6gas said:

I can see in theory why you'd want individual RGB subs from a mono camera to be the same length as the luminance subs but in my experience it isn't necessary as you can stretch the colour data harder, give it a lot of noise reduction if necessary and rely on the lum for detail.

That is exactly my rationale too.  We are not festooned with clear nights in this country to allow us the luxury of dozens of hours of data on an object and have to try and make do with less.

[kirkster501]

16 hours ago, x6gas said:

Steve - absolutely love your image.  That's one of the best M31s I've seen and it's very nice to see the Ha regions in deep red rather than the neon pink that is more common.

Lovely result, well done!

Thanks.  I also try to not overly clip the background sky too and make it too black like some folks do.

The FSQ85 is a mighty fine telescope.

[tooth_dr]

15 minutes ago, kirkster501 said:

That is exactly my rationale too.  We are not festooned with clear nights in this country to allow us the luxury of dozens of hours of data on an object and have to try and make do with less.

Just my final point - longer individual subs not a change to total exposure time. 20 x 5 minutes is the same as 10 x 10 minutes, (100 = 100)

[Richie092]

1 hour ago, tooth_dr said:

Just my final point - longer individual subs not a change to total exposure time. 20 x 5 minutes is the same as 10 x 10 minutes, (100 = 100)

That's interesting, as a newbie with just a single image under my belt I understood it differently.

Using your analogy I thought we needed 10 min exposures to get detail of the fainter stuff. If we could do it all with twice as many 5 minute exposures do we really need to worry about great guiding etc. I'm not arguing as I have no experience it is a genuine question. Why would someone risk a 30 min sub if they could play safe and capture 6x5 min subs and just reject the 1 with star trails or shakes etc and keep the other 5?

I have been trying to get good 15 min exposures and probs reject 20%  for one reason or another.

[tooth_dr]

50 minutes ago, Richie092 said:

That's interesting, as a newbie with just a single image under my belt I understood it differently.

Using your analogy I thought we needed 10 min exposures to get detail of the fainter stuff. If we could do it all with twice as many 5 minute exposures do we really need to worry about great guiding etc. I'm not arguing as I have no experience it is a genuine question. Why would someone risk a 30 min sub if they could play safe and capture 6x5 min subs and just reject the 1 with star trails or shakes etc and keep the other 5?

I have been trying to get good 15 min exposures and probs reject 20%  for one reason or another.

Richie, this is quite a widerly covered topic, there are lots of topics answering this question on the forum.  It comes down to different types of noise that are involved in an exposure - both from within the camera and from the environment you are shooting in.  With your particular camera (383L+) for example, you will want to be taking longer subs compared to someone with a new CMOS camera due to the high read noise 10+ e-.  I have the same camera as you and I would advocate longer subs.

With reference to my quoted comment - you have taken what I was saying out of context of the thread.  There was some confusion that I was suggesting spending twice as long exposing for RGB data, and that it should be the same total integration time as the luminance data.  What in fact I was asking, was that why not make up the total integration time using less but longer subs.  There is no longer time spent on integration but it just has a different make up - less longer subs rather than more shorter subs).

[Richie092]

Got it. Thanks for the explanation.

I will stick to my 10 or 15 min subs.

[kirkster501]

5 hours ago, Richie092 said:

Got it. Thanks for the explanation.

I will stick to my 10 or 15 min subs.

It is all a balancing act and there is no right or wrong.  You can have the best, dark and aeroplane free skies in the world that would bare one hour exposures, but if your tracking and guiding is out, that is an awful lot of exposure and investment to throw away. So we bring the exposure down to lessen our investment any individual subs.  That sweet spot depends on your gear and also your location.  Most people in suburban skies agree that 5-10 mins for broadband and 15 mins for narrowband is about the area to be gunning for with a CCD.  If you live somewhere with very dark skies, possibly more.  Less with  CMOS.  And it also depends on your mount's tracking accuracy.