Abell 61

[Petergoodhew]

A rarely-imaged planetary nebula in the constellation of Cygnus. It is expanding at 30 km/s. Also known as PLN 77+14.7
It's very faint, hence the Ha and OIII were captured at bin 3x3 and 1800s.

39 hours 30 mins total integration (34x1800s OIII bin 3x3, 21x1800s Ha bin 3x3, 31x600s Luminance bin 1x1, 13x600s Red bin 1x1, 13x600s Green bin 1x1, 15 x600s Blue bin 1x1)
 
Image captured on my dual rig at EyE, Extramadura, Spain

APM TMB 152 F8 LZOS, 10 Micron GM2000HPS, QSI6120ws8

[kirkster501]

Very nice Peter, I want to have a go at this too and I hope that when I do it will be as good as yours.

Two observations (nit picking...!)  I think this could be sharpened a tiny a bit to bring out more detail in the nebula.  Also, the stars to me seems to be either the same blue, or the same orange and little in between ????

I really like it though!

[Petergoodhew]

4 minutes ago, kirkster501 said:

Very nice Peter, I want to have a go at this too and I hope that when I do it will be as good as yours.

Two observations (nit picking...!)  I think this could be sharpened a tiny a bit to bring out more detail in the nebula.  Also, the stars to me seems to be either the same blue, or the same orange and little in between ???? 

I really like it though!

Thanks. I'll take a look at increasing the sharpening (Don Goldman's version has more detail - although the Capella Observatory version has less!), and also take down the saturation.

Peter

[Petergoodhew]

24 minutes ago, kirkster501 said:

Very nice Peter, I want to have a go at this too and I hope that when I do it will be as good as yours.

Two observations (nit picking...!)  I think this could be sharpened a tiny a bit to bring out more detail in the nebula.  Also, the stars to me seems to be either the same blue, or the same orange and little in between ????

I really like it though!

Try this version....

[kirkster501]

To my eyes, that is much better Peter.  The stars were a bit distracting on the first iteration.

[vlaiv]

Not sure if it is the saturation in the first or "distribution" of hot / cold stars (image "warmth").

Most stars in the image are likely to be main sequence stars, and only very small percent of those are in fact white and bluish - less than 4%. This however does not mean that there should be 94% yellow/orange stars in the image - with decreasing temperature - total luminosity also goes down - white / blue stars can be recorded over greater distances as yellow/orange ones are much fainter.

In absence of color calibration, I would expect to see at least half of "warm" (color temperature) stars in the image (unless of course it is specific object - like cluster with young hot (absolute temperature here) stars dominating). Both images show domination of white / blue stars over yellow/orange ones.

edit:

Ok, I think I probably created confusion, so I'll correct - in terms of color temperature - warm means yellow/orange, cold means white/blue. In terms of absolute temperature it is other way around, stars with smaller temperature have warmer tone, and those with high temperatures are "cooler"

 

Edited June 8, 2019 by vlaiv
Must clarify ...

[Petergoodhew]

6 minutes ago, vlaiv said:

Not sure if it is the saturation in the first or "distribution" of hot / cold stars (image "warmth").

Most stars in the image are likely to be main sequence stars, and only very small percent of those are in fact white and bluish - less than 4%. This however does not mean that there should be 94% yellow/orange stars in the image - with decreasing temperature - total luminosity also goes down - white / blue stars can be recorded over greater distances as yellow/orange ones are much fainter.

In absence of color calibration, I would expect to see at least half of warm stars in the image (unless of course it is specific object - like cluster with young hot stars dominating). Both images show domination of white / blue stars over yellow/orange ones.

An interesting point. I always struggle with colour calibration Vlaiv. Is there a technique that you would recommend?

Peter

[kirkster501]

17 minutes ago, Petergoodhew said:

An interesting point. I always struggle with colour calibration Vlaiv. Is there a technique that you would recommend?

Peter

Me too. 

Background extraction and colour calibration are, in my opinion, the two most important matters to get right.  And they come right at the very start of the workflow.  Get the colours wrong and is is difficult to recover the situation later in the workflow.

EDIT:  I use photometric colour calibration in Pixinsight.

Edited June 8, 2019 by kirkster501

[vlaiv]

26 minutes ago, Petergoodhew said:

An interesting point. I always struggle with colour calibration Vlaiv. Is there a technique that you would recommend?

Peter

 

9 minutes ago, kirkster501 said:

Me too. 

Background extraction and colour calibration are, in my opinion, the two most important matters to get right.  And they come right at the very start of the workflow.  Get the colours wrong and is is difficult to recover the situation later in the workflow.

EDIT:  I use photometric colour calibration in Pixinsight.

I've developed pretty good (in my view) method for both color combining and color calibration. After much research, I came back to beginning (reinventing the wheel :D) - "most accurate" way of color combining is simple RGB ratio. There are however two little things that need to be done

1. Do color calibration in linear RGB space

2. Apply RGB ratio in linear space and then apply gamma - convert to sRGB space.

At this time, there is no simple way to do either of these and getting it done in current software is pretty laborious. I use AstroImageJ for photometric measurement. Next is lookup in Simbad database for selected calibration stars - finding out B-V index and converting that into temperature. Then I solve some matrix equations - in LibreOffice calc - this is just the least squares method to find transform matrix between measured and expected linear RGB components.

This is what should be exactly the same as PI photometric color calibration, but I can't be 100% certain - it depends on how PI implemented that.

After calibration - color transfer is rather simple - stretch lum, take R, G and B and treat as a small stack of three subs - do maximum stacking on that. Divide each R, G and B with max stack result and multiply with stretched lum. Apply sRGB gamma correction.

This process should be "single" command - like stretch lum - histogram manipulation that would do all of the above and show you result so that you know how much to stretch. Because of gamma transform, linear stretch that looks ok will be overstretched, so it is a bit of a pain to do it one step at the time. Currently I do a hack - stretch lum while linear, apply inverse gamma approximation (gamma 2.2 which is really close to sRGB gamma function but not quite correct). Use that lum to multiply and again apply gamma of 2.2 to result. I do all of that in ImageJ (Fiji actually, but Fiji Is Just ImageJ).

However working on a software to automate all of the above. One thing that I've found very difficult is getting proper photometric data for stars. Stellarium for example lists very wrong B-V indices for stars (nothing to do with program it self - catalog that it's using is not very accurate in this segment).

[Allinthehead]

That looks very nice indeed. You image some very interesting targets.

[Petergoodhew]

4 minutes ago, Allinthehead said:

That looks very nice indeed. You image some very interesting targets.

Thanks Richard - yes I do like the less well-known (and sometimes ridiculously faint!) targets. 

Peter

[ollypenrice]

You're going great guns on these rarer planetaries, Peter, and this is another winner. Just to be awkward I'd like the red stars from the first one and the other stars from the second! 

Great result, love it. And yes, stars and background sky are the hardest things to get right in any image. (This is why I love my 'inefficient and outmoded' Atik 11000. Those big pixels just seem to do it all for you!)

Olly

[Petergoodhew]

1 hour ago, ollypenrice said:

You're going great guns on these rarer planetaries, Peter, and this is another winner. Just to be awkward I'd like the red stars from the first one and the other stars from the second! 

Great result, love it. And yes, stars and background sky are the hardest things to get right in any image. (This is why I love my 'inefficient and outmoded' Atik 11000. Those big pixels just seem to do it all for you!)

Olly

Thanks Olly - let me see what I can do for you!

Edited June 8, 2019 by Petergoodhew
star colour