NGC7000 HAlphaRGB

[JohnH]

Sharpstar 15028 HNT ( 419mm f2.8 ) on iOptron CEM25P
ZWO ASI1600MM Pro
ASIAir Pro control
Ha (as Luminance) - 460 secs * 9 (Gain 0)
R, G - 60 secs * 60 (Gain 139)
B - 60 secs * 31 (Gain 139) ran out of time!
All at -15C

From Bromley - Bortle 7 or 8 (NO MOON)
Processed - PI, Photoshop, Topaz DeNoise and Sharpen.

NGC7000 is just too big for my telescope/camera sensor. If I had my observatory it would be ideal for a 2 x 2 mosaic (and no doubt a world of grief trying to process that).

Yet again my colours are overblown - they have been made worse in the transition from .XISF to .JPG (honest). A satellite trail cropped up despite using Sigma Clipping, I cloned it out in Photoshop.

I am fairly pleased with this processing but there are too many stars for my liking and the bright blue stars have a horrible fringe. Because of where I am I have to have a dew band on my secondary mirror, the cable creates a clumsy diffraction spike despite being taped along the top of a mirror support.

Happy November to all,

John

(Carole: Please let me know if there is any problem downloading the file).

[Carole]

Hi John,

That's a big improvement to previous processing.  It looks a good image on the whole but see below:

Constructive help and constructive criticism:
There are methods of reducing the number of stars and the size of stars, but I only know how to do that in Photoshop.

However, zoomed in to your image the stars are not a good shape anyway and I suspect you have one or more rogue frames in your stack.  the stars are elongated throughout.  I don't believe this is your guiding but one or more exposures that got knocked and trailed somehow.  I suggest checking al your subs and re-stacking without the rogue subs.

Halos are always a problem.  It could be partly due to differing size stars in the different filters, might be worth checking, and if that is the case, again there are ways of reducing star sizes so the filters match.

I will download the image when you have done a revision if you think you would like to do so.

Carole

[JohnH]

Hi Carole,

Thanks. Criticism and comment is always useful, I am just floundering around with no real idea of what I am doing and an indication of the direction in which to head is valuable.

I am having another go at processing but being more selective as to subframes. I think that there are techniques to reduce star size and I am going to have a look.

Can anyone who is a guiding expert advise: My Dec guiding seems more erratic than RA (I have it set at 3 sec intervals). As per manufacturer's settings Aggressiveness on Dec is 100%, on RA is 70%. Should I try reducing Dec?

Regards,

John

[Carole]

Guiding: I am no expert, but 3secs sounds too long to me, mine is on 1sec.

I can help you with Star reduction/reduce star size in Photoshop.

Carole

[Roberto]

John

I think that's an excellent outcome from Bromley.   I'm thinking of doing a PixInsight processing session with other members of the society if they are interested.  I think I am getting more and more comfortable with what works and what doesn't when imaging in LRGB or SHO from light polluted London.  Let me know if of interest.  I defer to Mark Shelley's mastery of PxI but happy to try the above if there is demand.
As for guiding, I would say that DEC is bound to be more problematic than RA.   Guiding in RA is simple, the mount only has to correct in one direction (the other is simply achieved by stopping tracking) whereas DEC necessitates movement in both directions.  Most mounts have plenty of stiction when changing direction and if your scope is not well balanced or the mount is not well built, it will overcorrect in DEC.  If you use PHD2, try using their guiding assistant to get suggestions for your Minimum Movement in DEC.  Effectively, you want to watch a guidestar for a few minutes whilst not guiding to gauge the seeing.  If the seeing is not good (or the optics of your guider are not either), your star will jump up and down and side to side.  Chasing jumps produced by seeing is a futile experiment and your stars will only increase in size in the stacked image.  It is best to undercorrect than to overcorrect.   Spend time with the guiding assistant to learn what your mount is doing and how it responds to guide commands.  Once you dial in the right parameters you can almost forget about guiding.  I have a permanent setup and have the fortune of using high end mounts but even so, the seeing determines how good any given image can be. 
Good luck!

Roberto

[JohnH]

Hi Carole and Roberto,

Thanks. As I have said, all advice and help is welcome.

Roberto, the idea of an Institute of Pixinsight Studies sounds a fantastic idea and I would certainly be interested.

I have a theory about the star shapes. Because I live next to a playing field which has a culverted river running under it humidity is a problem for me. I had put a dew band around the secondary mirror. However, the lead is bigger than the mirror supports and although it was taped over the top of one of them to create as small a profile as possible there was an overhang. The result is a very clear and ugly diffraction spike as a result. I speculate that in smaller stars this creates an eccentricity. I have now removed the band and re-collimated so I will see what results.

I have just finished a long reprocess and the end result was a curdled mess. However, I have started working on a new cunning plan ...

John

[Carole]

That's a pain with the 2ndry needing a dew heater and might explain the stars.  I must admit when I had a Newtonian I never used a dew band.  Would a dew shield work better?

Carole