Author Topic: ngc4565 - am I dealing with moonlight background correctly?  (Read 1222 times)

Offline mmnb

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I recently acquired some images of ngc4565 while fixing a  few thing with my setup.  There was stupidity that resulted in non-aligned images, but I am trying to see if I've dealt with moonlight correctly.  I was unsure if there was something about weighting/local normalization that I may have done incorrectly but there threads make it seem like I am not:

https://www.cloudynights.com/topic/594474-what-really-is-the-best-reference-frame-for-local-normalization/
https://pixinsight.com/doc/tools/ImageIntegration/ImageIntegration.html#description_002
(didn't realize the PI documentation was so good, things are laid out very clearly)

This is the image after drizzle integration:
https://drive.google.com/open?id=1CbhL0ghV-jLAIDee-OX_qqd10i2LJo-d


An earlier sub exposure where the bg isn't so bad:
https://drive.google.com/open?id=1XBYpe3ocNrPml0H9f2_W9ArZ0MZpTqze

A later one where it is worse, but better S/N (to the best of my understanding the better S/N is not an artifact of the brighter bg):
https://drive.google.com/open?id=1E096hK-trZ3OANngpFuemr4D9cfGITxu

Vanilla application of DBE doesn't *seem* to pick up the background correctly?
https://drive.google.com/open?id=15FJQdcmrNTLNVUe9J1uSvy_JPJ5TP_Qc


Performing DBE in a standard sort of way does drop remove a lot of the background, but it still seems too high.  Does anyone have any advice on how to reduce the background intensity?
« Last Edit: 2018 May 29 09:03:32 by mmnb »

Offline mmnb

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Re: ngc4565 - am I dealing with moonlight background correctly?
« Reply #1 on: 2018 June 01 11:15:41 »
FWIW, I don't understand the criteria behind a "bad sample".  The upper left quadrant is where the problem is, and I can't seem to tell PI that it is really background there (at least, I think it is, checked successful images to make sure that I am not ignorant of some feature of that part of the sky).  Screen shot with lots of samples shows the issue (placed to show the problem).

Is it just that the signal is too high in the upper left quadrant to correct?

Online pfile

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Re: ngc4565 - am I dealing with moonlight background correctly?
« Reply #2 on: 2018 June 01 11:38:05 »
incrementally increase the sample tolerance value and click resize samples until all the samples turn from red to white. resize just forces them to redraw. note - do not click generate, it will replace all of your samples!

rob

Offline mmnb

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Re: ngc4565 - am I dealing with moonlight background correctly?
« Reply #3 on: 2018 June 01 11:57:17 »
That sounds right.  Many thanks!

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Re: ngc4565 - am I dealing with moonlight background correctly?
« Reply #4 on: 2018 June 01 12:30:46 »
cool. i forgot to mention that a bad sample is one which is brighter than the tolerance value allows - DBE thinks it's all signal and no background.

if you have a serious gradient in a nebula-type image which has signal all over the place, you can't always tell if your samples are on mostly background or mostly signal. so you can remove the gradients without regard to that, then place the samples more carefully/properly on the DBE'd image, save the process icon, and then discard the original DBE'd image and re-run on the original image with the better sample points from the trial run. you still have to crank up the tolerance but hopefully you're only sampling the gradient that way.

rob

Offline mmnb

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Re: ngc4565 - am I dealing with moonlight background correctly?
« Reply #5 on: 2018 June 01 14:05:18 »
Very helpful, that was the explanation I needed!

Many thanks again. Will post results when I am done.
« Last Edit: 2018 June 01 16:02:26 by mmnb »

Offline mmnb

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Re: ngc4565 - am I dealing with moonlight background correctly?
« Reply #6 on: 2018 June 10 21:41:55 »
Here is how I did by the end.  Some problems during acquisition:

-Temp was high (the SBIG 402ME could only get to -10C for most of it)
-Moonlight was bad giving a high background
-Focuser screws came loose, causing a change in camera angle

I just started autofocusing between subs, and my stars look a lot better than previous attempts.

This is the image using just RGB and excluding L.  There is a lot of data in L (19 exposures, 5min each) but no amount curve adjustment etc. would make the central galactic bulge 'fade out' to the background....the change would always be abrupt.  Simply taking the PCC'd rgb image and doing a bit of curve adjustment got a much better result (where the bulge looks more correct).

luminance:
https://drive.google.com/open?id=1CbhL0ghV-jLAIDee-OX_qqd10i2LJo-d

rgb:
https://drive.google.com/open?id=1PcM0M6rkGDY3ZeWrVVM6vsTZ6IVygEOq

If anyone can do much better with the data, I'd love to know how you did it.  I presume the thin profile make it difficult to do a whole lot.