PGC 8961 - Arp 273


Object: PGC 8961: galaxy in Andromeda 14.4m, 1.9' x 1.0' and companion PGC 8970: SB(s)a pec 15.2m, 1.2' x 0.3'

Subimages: 52 x 300s (4.3 hrs) SD Mask combined after bias, dark and flat calibration. Input FWHM 2.8" - 4", output 3.6" (I rejected 23 frames with FWHM > 4")
Scope: C11XLT with C6.3FR, EFL=1589mm (f/5.7 effective)
Mount: Losmandy G11 with Gemini L4
Camera: ST7EI (1.17"/pixel) set to -25C oriented at 3d50' (N up, E left)
Guiding: SX716 camera on a Taurus Tracker III OAG. Exposure time 3 sec.
Acquisition and Processing: MaxIm DL/CCD v4.5
Imaged on: Nov 21-25, 2006

Processing:
Background flattened manually with 30 points
UnsharpMask: Kernel LowPass, 85% mask weight and GeometricMeanMask
DDP: bgd=680, mid=1000, no filter
Remove Bloom: Two brightest stars, they also needed edit pixels and clone from a low pass filtered version afterwards.

While processing I noticed a very faint large patch (nearly as large as 8961 itself) to the NW of the galaxy, only about 6 counts above the bgd (noise is <1 count). I was originally unsure as to what this was, an image artifact or remnant from the 8961/8970 collision? Here is a linearly stretched cropped version with only the background flattened and no other processing.


It turns out the glow is an artifact from the 8m star on the left. Since I used dithered guiding an out of focus reflection appears at different position in each subimage and is smeared out when combined (even with the SD mask, since the faint ring is larger than the dither offsets). So my lesson learned is beware of artifacts from bright sources in the field, and use dithered guiding!

Here is a image from the Palomar 5m telescope, taken in 1966 with Kodak 103a-D film (640 nm band), the image size is 3.7' x 2.9':