Another M31

[Bassinghamobservatory]

Having a slight problem with my CCD camera at the moment so have been giving the 1100D a bit of work. This was captured with Backyard EOS on 29 September with camera and 200mm Pentax Super Takuma lens at f4. The moon was rising and the subs looked over-exposed with not much of the outer arms so pleasantly surprised how it has turned out. The camera was piggy backed on the ED80 with NEQ6 and was guided with PHD2. 20 subs of 180s at ISO800.

I really like this lens. It has M42 thread but with an adapter it has focus to infinity in the correct position, star shapes are good out to the corners even at full aperture and cost a fraction of a modern Canon 200mm lens.

Edited October 3, 2018 by Bassinghamobservatory

[+Ibbo]

Nice galaxy and a good call on the lens.

but some odd magenta coloured stars on my monitor

[Tweedledee]

That's brought out so much fine detail in the dust lanes and a very rich star field. ?

[Bassinghamobservatory]

9 hours ago, Ibbo said:

Nice galaxy and a good call on the lens.

but some odd magenta coloured stars on my monitor

Thanks Steve. I had not noticed but agree about some of the star colours. I think there is a green tinge to the galaxy so perhaps a touch of SCNR will fix it. Have you any suggestions?

Graham

[+Ibbo]

All i can suggest is you try with colour balance or look on you tube as i think you are using PI and i have only just downloaded a trial yesterday (again)

[Bassinghamobservatory]

Thanks to all for the positive comments.

 

I have tried the SCNR process in Pixinsight which has corrected the slight green tinge to the galaxy but has not helped the magenta stars. From looking at other M31 images the magenta stars should be blue. I have used PI's colour calibration and the histograms look well aligned, so not sure what I am doing wrong.

[+Graham]

Graham 

Load your image back into PI.

Go Process  --  All processes -- curves transformation.

open preview o button.

Click on the Red curve button and gently pull down the center curve after locking off the upper and lower parts of the curve. See photo. sorry for the rubbish pic but my phone camera is shot.

Your magenta stars will turn back to white ,blue and yellow.

Whilst there you can then play with the Green and Blue channels individually to get the desired result you want.

 

 

 

 

Edited October 6, 2018 by Graham

[Bassinghamobservatory]

Thank you for your suggestion Graham. Unfortunately it does not seem to be working for me. Still have some magenta stars and no blue ones. I am puzzled why this should happen if colour calibration is having the desired effect, so perhaps I need to have a closer look at that process.

[+Graham]

Very strange. 

[8472]

I've no experience of PI, but my 135mm Takumar also exhibits colour fringes around stars.

 

Cut a long story short, it doesn't focus red, green and blue at the same point. There is an IR focus marking on my lens, which stops just short of infinity and I find this point to be the best compromise to infinity focus. The stars tend to then bloat a little, but the other alternative is annoying red/blue fringing. It's more apparent with modded cameras. There are a number of threads of this on CN, so it is not unheard of.

 

I don't know if it applies to your issue, but it might be worth investigation in the future.

 

YMMV

[Bassinghamobservatory]

Thanks Kev. I also have the 135mm and find it similar in performance to the 200. I will try your suggestion and experiment with the focus position. I have already done this with the Takumar 55mm f1.8. Initially I was disappointed with the coma, but stopping down to f2.8 and backing off to focus stars near the corners a bit better still leaves acceptable stars at the centre of the field.

Graham