Astronomical Observatory: Cool Images

Astr384 Photography Projects, Spring 2004

M101 Pinwheel Galaxy, Peter Cook

Ml01 is obviously a beautiful, face on spiral galaxy. Probably its most obvious feature is that its core is its brightest and biggest component. So even though the core makes up a small portion of the volume of the galaxy we can see that it is also denser. Ml0l 's arms have a very nice spiral structure. It is interesting to note that these arms are bluer than average and that there are several bright blue clumps. This is because blue stars are hot, and hot stars burn up their fuel quickly and are thus short-lived. But new stars are formed in these arms, so if we are going to find a blue hot star, it is most likely here. By comparing the distances between the arms and seeing how tightly they are ound, we can figure out the speed that the material in the arms are going out relative to the angular velocity of the galaxy. Upon closer inspection we can see little dust lanes covering up the bright arms. These huge areas of dust have scattered and blocked the light coming towards us from the arms.

On the edges of the picture, we can see two other objects of interest. On the far left towards the top is a bluish hazy area which is called NGC 5477 and it even looks like it may have came off of Ml0l. On the right side, we can see an object about the same angular size as some of the larger stars in the field. However this object is decidedly elliptical instead of circular. This is MCG 9-23-25—a nice distant lenticular galaxy.

Processing Method

Source images are all of Ml0l. Three images of each color (Open, B, V, R) in four quadrants. Note, the NE blues are from a different night because the original ones did not have the galaxy in them.

1) Subtract ghosts from all images. The ghost doesn't come out of the blues nearly as well.

2) Calibrate with the relevant night's bias and darks and with flats from 4/17/04.

3) Combine with sigma clip each color/field combination (3 in each). The Open's seem a trifle fuzzy.

4) Take each quadrant and align. Manual 2 star. On NE field, don't align to blue since it is a bit off center of the other three. Blink the fields to insure that a proper job was done.

5) Find field's background minimum count and maximum in galaxy's core for visual, red, and open. Do pixel math to subtract out the background and then to scale the core's maximum to 22000. Doing this will cause the core of the galaxy to be white. I did not go higher than 22000 because there are several stars that are much brighter than the galaxy and I do not wish to amplify the noise much. Note that there is a star above the core that is brighter than the core. Also note that the SW blues have the core in some nasty fringe.

6) For the blues, subtracting out the minimum background does not work very well. So instead set the background average to 1000 and yet keeping the core at 22000.

7) Check to insure that the core in each field is 22000, and that the background minimum is zero but for blues that the background average is 1000. If it failed, you may have saved the file in a too small of a format and Maxim tries to resize everything but fails to keep the core 22000.

8) Crop each one so that there are no black leftover areas from aligning, and also to remove the ghosts from the east blues.

9) For the blues, do a background flatten set on bilinear. Don't include a part of the galaxy's anns as part of the background.

10) Mosaic each color together, paying attention to rotation. Leave on the autobackground adjust.

11) For the mosaiced blue, do a background flatten on complex and choosing many areas.

12) Align the four mosaiced images together.

13) Use the remove bloom feature on the Open, Red, and Green filters to get rid of the nasty bloom on the star on the west side and slightly south of the core. On the blue, use the remove bloom feature persistently until most of the ghosts are gone.

14) Subtract out background (cores are comparable). Open 100; Blue 1200; Red 50; Green 330

15) Combine colors. I couldn't get the Open to work. Use Red 100%, Green 100%, Blue 90%. Auto background equalizer on.

16) Final Crop