Dark halo around galaxy after calibration

[Graham]

Hi.

When I calibrate a galaxy image, I seem to end up with a dark halo around the galaxy. I tried some different calibration routines and it seems to be flat related. I generally do my flats at 20-30,000 ADU on a KAF 8300 chip which seems to work well with nebulosity and the like, it only seems to be galaxies that do it.

Are my flats too bright or something like that?

Thanks.

[RickS]

Hi Graham,

There actually *is* a dark halo around M83, so perhaps you should pick another example There's a dim but detectable star stream that's at the bottom left on your image that wraps partway around the galaxy and I suspect some even fainter stuff on the other side.  A number of people have noticed the dark halo and I'm sure it's not imaginary.  It is quite obvious in the data I processed recently and other galaxies from the same scope with the same calibration files don't exhibit the halo.

20-30K ADU should be well within the linear range of the sensor and ought to work fine.  I presume you have a mechanical shutter?  You'll get a shadow from that if your flats are too short, but that's usually pretty obvious.  You could try some different calibration strategies for your flats, e.g. calibrated with bias only or flat darks if you are using longer darks and relying on dark scaling.

Cheers,
Rick.

[msmythers]

Thought it would be interesting to compare this to a Hubble shot of M83. One issue is the Hubble shot is not as wide of a field but I think you can see some of the same dark halo in the image. First I extracted the L from the Hubble image. Then rotated it 90 degrees. I then used StarAlignment to register Grahams image to the Hubble image. I used STF on the Hubble image as the original is very dark. Here is the comparison.


Mike

[RickS]

Interesting comparison, Mike.  Here's a plot of a cross section through my data that I did when I was wondering if the halo was an optical illusion.

[msmythers]

Rick

That shows a darker trough close in and than a second slightly less dark. What did you use for plotting the cross section? I've used the 3D Plot script in the past but I like yours.


Mike

[RickS]

Hi Mike,

I used the technique described by Gerald/Oldwexi with a minor tweak to do a diagonal section instead of horizontal.  His videos are here: http://www.werbeagentur.org/oldwexi/PixInsight/PixInsight.html and the specific video is this one: https://www.dropbox.com/s/d7f0thdv9y13k3p/PixelMath_Profile_english.wmv?dl=0

Cheers,
Rick.

[Graham]

Thank you RickS and msmythers, I appreciate the feedback. 

OK, so I'm not going crazy and that is why my flats work OK on other images. 

I took a look at yours again on IIS (I am Bart on IIS, by the way ) and a few others on there and the net and it would seem as though there is quite a halo encompassing the two main background galaxies and a dark ring evident. I cannot quite pick up the tidal stream at the bottom left, I may stretch my image a bit more and see. The image on here is only a crop as the field is much larger due to using a 5 inch refractor which tends to make the dark halo much more noticeable.

I might get a bit more data now that I know it is not an issue and see what else I can pull out of it.

Cheers! 

[RickS]

Nice to meet your PI alter ego, Graham/Bart    Good luck with your M83.  I'll keep an eye out for it on IIS.

[msmythers]

Rick

Thanks. I have the video I just forgot about it. Need to go back and watch all of Geralds videos again. They are so good.


Mike