A photometric investigation of the galaxy cluster Abell 2666
1 Introduction
Amateur astronomy is experiencing a new golden age as specialized ccd cameras are becoming available and affordable. Amateur astrophotographers are collaborating with professionals on everything from variable star research to gamma ray burst studies. They are contributing important data and helping to increase our understanding of the universe around us. This paper details my attempt to familiarize myself with scientifically useful astronomical imaging using equipment accessible to a wide range of amateur astronomers, and internet-based astronomical catalogs.
The field of investigation is centered on the Abell 2666 galaxy cluster, in the constellation Pegasus at RA 23h 51m 22s, Dec 27d 10m 21s.
2 Equipment
All observing was done at the Galaxy Quest Observatory, owned by Jacob Gerritsen in Lincolnville, Maine, USA at 44 20.5° N 069 03.8° W using equipment graciously loaned by the owner. The telescope used for all the research image acquisition was a Televue NP-101 apochromatic refractor of the Nagler-Petzval design with a 101 mm aperture and 543 mm focal length mounted on an Astrophysics AP600GTO mount, and carefully polar aligned. The mount is attached to a sand-filled 12" pipe that is in turn bolted to a large 4' x 4' concrete block that has been inset into the ground.
Figure 2.1 Spectral resopnse of the KAI-2001M CCD chip.
The imaging camera was an SBIG ST-2000XM anti-blooming camera with an internal TC-237 autoguider chip and cooled to -20 C. This camera has 7.4-micron square pixels in an 11.8 by 8.9 mm array. Mounted to the NP-101, it gives a 2.8 arcseconds/pixel image scale. Figure 1 shows the spectral response of the KAI-2001M CCD chip in this camera. It is clear from the graph that this camera is weighted heavily towards the blue end.
2.1 Software and astronomical catalogs
Maxim 1version 4.60 was used for all image acquisition and stacking and astrometry in conjunction with the Pinpoint2version 5.0 astrometric engine. The final stacked image was used as a background image in Cartes Du Ciel3 version 2.76c for galaxy identification. The internet visualization software, Aladin version 4 (Bonnarel et al 2000), was also used for object identification, with the SIMBAD dataset, from the CDS at Strasbourg, France, as well as the USNO-B (Monet et al 2003), GSC 2.3 (Spagna et al 2006), J/AJ/116/1529 (Hill & Oegerle 1998), and HYPERLEDA PGC (Paturel et al 2003) catalogs overlaid on the target area image. The Tycho2 catalog (Høg, E., et al 2000) provided magnitude reference data. Sextractor4version 2.5.0 was used to extract sources from the image.
3 Methods
3.1 Image acquisition and processing
Each sub-exposure is an auto-guided 300-second image. All images were acquired on the nights of 14 and 15 Oct. 2007. Each image was dithered via the autoguider between exposures. Autoguider exposure time was 2 seconds.Ten individual 300-second dark frames were exposed on October 15th with the camera mechanical shutter closed. These were sigma combined in Maxim DL into a master dark frame.
Ten bias frames were exposed on Oct. 15 with the mechanical shutter closed. These were combined into a master dark frame in Maxim DL.
Ten individual evening twilight flats were automatically scaled and sigma combined in Maxim DL on Oct. 15. The twilight sky conditions were such that I had some difficulty getting a good set of flats. There was a partly cloudy sky, so that I had to carefully time my flats between clouds. The flats were taken in an area of the sky just east of the zenith.
Flat field frames were bias corrected and combined into a master flat, then divided into each sub-exposure. Each sub-exposure was dark frame corrected and bias corrected in Maxim DL.
44 300-second images were plate-solved using Pinpoint and then astrometrically aligned and summed in Maxim DL for a total exposure of 13200 seconds.
3.2 Image post-processing and data reduction
The final image (Figure 2) was cropped to limit any edge of field distortion inherent in the telescope design. All magnitude estimates were done using this image. To enhance the overall visual appeal of the image, I imported the fits file into Photoshop and adjusted the curves and levels and applied a noise reduction filter (see Figure 3). For visual target identification, I modified the image by applying a mild low-pass filter, and a mild unsharp mask. The image was also inverted so that the background was white (see Figure 4).
I used Sextractor to extract objects from a filtered and sharpened version of figure 2 into a database with J2000 RA, Dec, and Best_Mag as columns. I then imported the object extraction database (OED) into Aladin and used the cross-match tool to determine which objects were within 5 arcseconds of a GSC 2.3 object. These cross-matched objects were then exported to a file with J200 RA, GSC 2.3 name, and distance from OED objects as columns. This cross-matched database (CMD) contains all the objects in the image that correspond to a GSC 2.3 object. The CMD was then compared to the GSC 2.3 catalog covering the image area, and all objects present in the CMD were flagged in the GSC 2.3 list. This entire procedure was replicated using USNO-B catalog data. Catalogs were provided by the Vizier query service (Ochsenbein et al 2000).
4 Results
4.1 Images
Figure 4.1: 13200 second calibrated and cropped image of the region around Abell 2666 (north is up, east is left).
Figure 4.2: Enhanced image of the Abell 2666 region
Figure 4: PGC Galaxies in Abell 2666 overlaid on deep field image
4.2 Magnitude estimates
In the field, the dimmest star detectable is listed in the USNO-B catalog as having a blue B1 magnitude of >20, and the brightest star, Tycho 2255-00345-1, is listed as having a visual magnitude of 8.08. See §5.4 for details about measuring magnitudes from the image.
4.3 Object distribution
4.3.1 Galaxies
Reaching beyond 18th magnitude, all 24 of the PGC galaxies comprising Abell 2666 listed in Table 1 were visible.
Table 4.1: PGC Objects in Figure 4
|
RA |
dec |
Name |
Alt Name |
Class |
Dim 1 |
Dim 2 |
B1 Mag |
Obs. Mag. |
Surface Brightness |
Radial Vel. |
|
23h50m47.47s |
+27°17'16.3" |
PGC0072600 |
|
Sbc |
0.7 |
0.6' |
14.98 |
|
13.85 |
7992 |
|
23h50m49.20s |
+27°13'35.5" |
PGC0072586 |
|
S? |
0.3 |
0.2' |
17.93 |
16.91 |
14.56 |
7573 |
|
23h50m49.81s |
+27°08'20.4" |
PGC1798869 |
|
|
0.6 |
0.3' |
17.34 |
|
15.01 |
|
|
23h50m52.22s |
+27°09'58.1" |
PGC0072596 |
NGC7765 |
S? |
0.7 |
0.5' |
15.68 |
|
14.33 |
7554 |
|
23h50m55.90s |
+27°07'34.7" |
PGC0072611 |
NGC7766 |
S? |
0.9 |
0.2' |
16.20 |
|
14.23 |
7938 |
|
23h50m56.51s |
+27°05'12.6" |
PGC0072601 |
NGC7767 |
S0-a |
1.1 |
0.3' |
14.52 |
|
12.86 |
8064 |
|
23h50m56.72s |
+27°06'26.6" |
PGC1797966 |
|
S? |
0.3 |
0.3' |
17.62 |
|
14.63 |
35297 |
|
23h50m58.27s |
+27°08'51.4" |
PGC0072605 |
NGC7768 |
E/cD |
1.5 |
1.0' |
13.28 |
|
13.4 |
8212 |
|
23h50m59.06s |
+27°14'27.4" |
PGC0072606 |
|
Sbc |
0.7 |
0.2' |
16.78 |
15.47 |
14.24 |
8142 |
|
23h51m00.00s |
+27°13'08.1" |
PGC0072607 |
|
Sb |
0.9 |
0.3' |
15.42 |
|
13.79 |
8313 |
|
23h51m00.79s |
+27°17'19.4" |
PGC0072609 |
|
S? |
0.4 |
0.4' |
16.38 |
|
14.05 |
9057 |
|
23h51m01.12s |
+27°15'27.5" |
PGC0072608 |
|
S? |
0.5 |
0.4' |
16.28 |
|
14.28 |
8634 |
|
23h51m10.94s |
+27°07'36.1" |
PGC1798514 |
|
S? |
0.3 |
0.2' |
17.54 |
|
14.33 |
32310 |
|
23h51m16.13s |
+27°09'49.5" |
PGC1799639 |
|
S? |
0.3 |
0.2' |
17.96 |
|
14.58 |
|
|
23h51m17.50s |
+27°12'10.4" |
PGC1800848 |
|
|
0.2 |
0.2' |
18.72 |
|
14.94 |
|
|
23h51m18.54s |
+27°17'36.8" |
PGC0072628 |
|
S? |
0.6 |
0.3' |
16.00 |
|
13.96 |
7915 |
|
23h51m18.94s |
+27°07'43.3" |
PGC1798583 |
|
|
0.3 |
0.2' |
18.68 |
|
14.97 |
|
|
23h51m22.10s |
+27°06'25.6" |
PGC1797950 |
|
S? |
0.2 |
0.2' |
18.14 |
|
14.69 |
35024 |
|
23h51m22.86s |
+27°05'58.8" |
PGC1797722 |
|
S? |
0.3 |
0.2' |
18.20 |
|
14.67 |
34878 |
|
23h51m29.99s |
+27°14'10.4" |
PGC0072640 |
|
E? |