Rob Hawley's Pages Almaden Observatory Main Page

Messier 35 (and NGC 2158)

Jan-Feb 2013

These two open clusters sit in the west end of the constellation Gemini.  Visually I never thought too much of these.  It was just another collection of stars - boring.  But photographically the yellow stars and the population of red stars in the NGC make this more interesting.

This project was done using Narrowband for Stars (nb4stars).  The filter set used was sV, sYel, and NIR.  I did this as an HDR project since the stars in the cluster were saturating at the 600s exposure. Follow the link for more information on this technique.

M35 using
                  nb4stars sV,sYel,NIR
click for a full size (9"/pixel) image

Annotated Map

This is shows the B-V values of selected stars in the image.  You can compare these to the "correct" star colors.

Annotated map
click for full size image

Processing Details

Since many of the stars in field of view were saturating with the longest exposure I decided to shoot this in HDR

Strömgren sV
600s x 22

240s x 8

060s x 20

Strömgren sYel
600s x 27

240s x 8

060x x 20

600s x 34

240s x 8

060x x 18

These were combined to form 3 HDR images for each channel.

That is only part of the story.  As described in the NB4Stars page, the raw images have to be combined to form an RGB image.  I used the following expressions in PixInsight PixelMath.

Red = "NIR_HDR*.4+sYel_HDR*.2";
Green = "sYel_HDR";
Blue = "sV_HDR+sYel_HDR*.2";

I found that the mixture I used for the Double Cluster left the stars too blue.  Thus I tweaked the red a bit higher and the green component of the blue a little lower. This result better matched the LRGB pictures available.

From that point it was just normal processing.  Since these were just stars I chose to not aggressively stretch the final image.

Area Map of M35

M35 is marked by the white square.

Area Map

HDR Images for the Channels

The differences between the 3 channels were rather striking.  This shows the images obtained by the 3 filters with similar stretching. Look particularly at the NGC.  The NIR image was multiplied by .4 before stretching.

NIR image
sYel image

Copyrights for Photos

Creative Commons License
Except as noted, all work on this site by Robert J. Hawley is licensed 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.