NGC 3718. A bit of a horror

[DaveS]

Captured 15 red and green plus 21 blue 600 sec subs on the 16th and 17th. After fighting the data through DSS, Affinity and AstroArt this is the result.

The background is HORRIBLE, yeurch! with multi coloured blotches all over the place. Needed multiple iterations of gradient reduction with a final heavy denoise.

 

There is no way I'd buy a 16200 camera today, old tech, noisy and insensitive. Am IMX 455 would be MUCH better.

[Jkulin]

You have done a lovely job on that Dave, far better than my attempt, I remember when I captured it and it was an absolute mare: -

 

https://www.astrobin.com/ve12oc/B/

[MarkAR]

You've done a lovely job on a pair of sweet little galaxies.

[DaveS]

Thanks John and Mark.

 

The blue channel data on this is *very* thin, with banding visible in the stretched stack, even after gradient reduction. I think it was Laurin Dave on "another forum"  who suggested the 16200 with Chroma filters needed about twice as much blue as the other channels. i could do with some more blue captured in 900 sec subs to get the noise floor down.

 

However, we have a rapidly brightening moon and shrinking dark so I don't know when I'll be able to pick this up again.

 

[Jkulin]

2 hours ago, DaveS said:

Thanks John and Mark.

 

The blue channel data on this is *very* thin, with banding visible in the stretched stack, even after gradient reduction. I think it was Laurin Dave on "another forum"  who suggested the 16200 with Chroma filters needed about twice as much blue as the other channels. i could do with some more blue captured in 900 sec subs to get the noise floor down.

 

However, we have a rapidly brightening moon and shrinking dark so I don't know when I'll be able to pick this up again.

 

Hi Dave, it sounds like your set up is screaming out for G2V figures, I had them for my 10" RC and only captured them at the weekend for my 10" Newt and they make the job so much easier.

 

If you need any help then let me know and I'll be pleased to help.

[DaveS]

Thanks John.

 

I've heard of G2v calibration but thought it was pretty much tied to the way PixInsight does things, and I don't have PI (Tried the trial version, did my head in).

 

I saw in your AstroBin image that you used different sub lengths for the Red, Green, and Blue channels. I've stuck to mostly 600 sec, with occasionally 300 sec for Lum in the past.

 

So how does G2v calibration work? I'm assuming that it involves sampling a known G2v star somewhere in the image, but beyond that I'm not sure.

 

There is, of course, a nice bright G2v just up there in the sky right now 😀

[Jkulin]

Hi Dave, I just looked up the co-ordinates of a whole list of G2V stars and pointed SGP to it, then captured 1/2s, 1sec and 2sec subs from each channel and then you could use any program to identify the intensity of the non-saturated capture, in my case the 1sec ones were perfect.

 

I have a copy of Maxim which I don't use for capturing but there are many other pieces of software that can look at the ADU/Intensity of the capture.

 

Once you have the reading of the RGB channel, you take green as the base lines, so in your case 600s and divide the blue and the red into the Green figure to get the ratio, so in my instance with the 300s I took with my 10" Newt this was the ratios: -

 

R- 1.892993=1.04987 300s=314.961
G- 1.803059=1.0 300s=300s
B- 1.892993=0.9167 300s=275.01

 

So if I want to take a 600s the figures would be: -

 

R=629.992s

G=600s

B=550.02s

 

That is then matched to my G2-8200 Camera and 10" Newt

 

With my 10" RC and the G2-8300 I worked

 

R=348s

G=300s

B=396s

 

It was Peter that introduced me to G2V and I have been using with my 10" RC for a good while, but have only just grabbed it with my Newt at the weekend. No point doing it with my OSC but will do when I use a Mono CCD/CMOS on my Esprits.

 

 

If you are using SGP I can send you the .sgf file that you can open and then just edit the equipment as the star I am using at the moment is quite high so should be OK for your location.

 

If not then here are the co-ords: - G2V 15 Leonis Minoris, RA-09h48m35.18s, DEC- 46°01'16.40"

 

If you need any help then don't hesitate to ask.

 

[Jkulin]

29 minutes ago, DaveS said:

There is, of course, a nice bright G2v just up there in the sky right now 😀

I think you might have a problem with it being a bit over saturated 😂🤪

[DaveS]

Thanks again John

 

I use Maxim for capture but ASA Sequence for general sequencing. As I also have CduC on the obsy computer I could go straight to the target from there.

[Jkulin]

That's good Dave, I only used Maxim to identify the intensity, I used to be able to do it in PI but it is easier in Maxim

[DaveS]

I did a preliminary trial calibration this evening before it clouded over and found the blue channel not much more than half the intensity of the others. Will do another on the next clear night without hazy cloud.

[MarkAR]

Hadn't heard of the G2v calibration before. Will have to look into it further. 👍

[DaveS]

I've seen John mention it before in other threads.

 

After doing the arithmetic, and if I use 600 sec subs for Green, then Red needs 590 sec but Blue needs a crazy 1017 sec!

 

I'm prepared to go as far as 900 sec, but beyond that I will be driving myself into possible tracking errors and maybe noise.

 

Dunno, will have to think about it.

[Padraic M]

@DaveS according to Al Kelly's paper on the subject, you can use either an equal number of different exposure length subs, or a different number of equal exposure length subs. The latter is easier from a tracking and calibration point of view.

So according to your arithmetic above, if you use 600s for all channels and using Green as the reference, you will need 2% fewer Red subs, and 70% more Blue subs.

[DaveS]

Thanks @Padraic M.

 

Looking at his paper now.