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NGC 253 Sculptor Galaxy

November 2017

NGC 253 is a familiar object to me. On nights with relatively good transparency I have put the 30" Challenger on this as a part of my public programs at Fremont Peak.  With the large aperture one can easily see the rich detail in this galaxy.  However from California the object is very low in the sky. Besides the Challenger is a visual scope so using it to photograph is not realistic.

Thus Chilescope presents an opportunity to image this object at a more favorable position.  In fact, from their site it transits almost directly overhead.

Location of NGC 253

NGC 253 from Chilescope

NGC 253 from ChileScope
click for a larger, more detailed image
NGC 253 from Fremont Peak

NGC 253 from Fremont Peak

low, but still visible on a good night
NGC 253 from home

NGC 253 from Almaden Observatory

grazing the roof of my house

Annotated Image

The image is presented as North is up. Note all of the more distant galaxies captured in the picture.

NGC 253 Annotated

The bright stars are marked with their visual magnitudes and B-V indexes. The three bright stars (BV .536, .604, and .711) should plausibly display as white.  The .980 star is a K and should appear more orange.  The .261 star is an F and should be more blue.  Using a 600 sec exposure all of these were saturated in all of the color filters which compromised an accurate measurement.

Full Image 

Here is the image presented at 1/2 resolution (1.95" / pixel).  The full resolution image shows little additional detail. Note the significant star forming areas.  These are visible visually as well.

NGC 253 in LRGB+H

Processing Details

Here are counts of the raw frames that were used for the various filters.  Naturally more images were taken.  These represented the ones that made the cut. The data from the scopes was captured at -30C and was very clean.  Thus I could proceed with far fewer images than normal. I used mostly Telescope 3 for this project.

Lum 22x600
Red 14x600
Green 5x600
Blue 15x600
Halpha 14x600

All images were processed by Pixinsight. 

Color Correction

I used the bright central core of the galaxy as a white reference.  I tried using some addition sections away from the central core and the obvious high Halpha and dust areas, but this resulted in a green galaxy which obviously was not correct. 

I am disappointed the bright stars came out white as the UCAC data shows that several are F and K class (orange and bluish).  As noted above, I attribute this to the stars being saturated.  Unfortunately it is not cost effective to shoot HDR on the Chilescope system so I had to accept the result. 

I was not able to see color in the dimmer stars that were not saturated.  This part of the sky seems to lack dimmer M and A stars.

I desaturated the background during processing to remove some color gradients from the initial combination.  This worked better than DBE. Since the Lum image did not show a gradient the gradient only presented itself as color.  Creating a good background mask allowed me to desaturate the background turning it into a neutral gray color.  Applying HT late in the processing turned most of the remaining background to black.

HAlpha Addition

I added Halpha into the image using the Vicent Perez method following color correction. 
HCorrect = Halpha - (R - Med(R)) * 0.05
Red = Red +(HCorrect - Med(HCorrect)) * 20

where 0.05 and 20 were chosen for the desired result.

Since I wanted to only add to the red within the galaxy itself, HCorrect was aggressively masked to remove all stars.

Other notes

The image was otherwise processed in my typical way.

I found HDRMT applied too severe a correction with all of the algorithms.  Thus instead of totally exposing the galaxy in the mask I set the mask to only 66% using curves which reduced the contribution of HDRMT until the result was acceptable.

For this image I found that deconvolution did not improve the result.

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Except as noted,all work on this site by Robert J. Hawley is copyrighted under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License. This permits the non commercial use of the material on this site, either in whole or in part, in other works provided that I am credited for the work.