NGC 5248 Intermediate spiral galaxy

[menardre]

The constellation Bootes is close to the meridian when I started imaging last night so I decided to try and capture this interesting galaxy (NGC 5248). If I waited any longer it would be too far west to image.

NGC 5248 is a compact intermediate spiral galaxy about 59 million light-years away in the constellation Boötes. NGC 5248 is in the constellation of Boötes but is actually part of the Virgo cluster of galaxies. NGC 5248 has its own small group which contains two other faint UGC galaxies and the group itself is part of the Virgo III group. The Virgo III chain consists of at least 8 groups of galaxies stretched into a chain at least 40 million light years long by the intense gravitational pull of the main Virgo cluster.

NGC 5248 is notable for the ring structure around its nucleus. These nuclear rings are characterized by “hot spots” of starburst activity. Starburst regions are sites where stars form at a much higher rate than usual.

You can also see the edge-on spiral galaxy UGC 08575.
I was able to capture 108 images of 2 minutes (binned 1x1, gain 100 , temp 0 deg C) with my 11 inch SCT and ZWO ASI2600 OSC camera.

Processing was with PixInsight. I tried something different this time. I always use a process called 'SubframeSelector' just after calibrating and debayering the images. SubFrameSelector essentially grades each image and gives statistics such as FWHM, SNR, eccentricity, number of stars, etc. This is useful to determine if there are images that should not be used during stacking. Another aspect of SubFrameSelector is the 'weighting' function. You provide an equation to weigh each image, and then SubFrameSelector assigns a rating ( %). The equation that I use weighs FWHM, SNR, and eccentricity. When I stack the images, the weight function then takes the weight (%) and applies that to each image... so images that have lower %, do not contribute as much as images with a higher %. This is meant to give more 'weight' to the better images. One 'problem' with SubFrameSelector is that it is likely that none of the images will be weighted at 100% -- in fact sometimes the best images are only rated at 75% .... so how does the loss of the 25% impact the final image. I decided to run a test. I used SubFrameSelector twice.. once with the standard process in which even the best images may not have a rating of 100%, and the second in which the best image is rated at 100%. I then used the same processing steps to complete both image tests. I expected to see some slight improvement in the 2nd process in which the best image is weighted at 100%, since then I am not throwing away as much data. On close inspection I did find a very, very slight improvement in the 100% weighted version... but some of that could also be due to slight differences in processing. Bottom line... since forcing the best image to be weighted at 100% is easy to do, I will probably add this to my normal routine ... but I doubt it makes a noticeable difference.

Roger

Here is the image with the standard weighting function

NGC_5248_Spiral Galaxy-restricted weight.jpg (1.38 MiB) Viewed 4272 times

Here is the same image with maximum weight forced to be 100%. The galaxy may be brighter just due to slightly more curves adjustment.

NGC_5248_Spiral Galaxy-maximum weight.jpg (1.52 MiB) Viewed 4272 times

[Pete]

Very pretty Roger. My sky had clouds that kept me in last night. How do you manage?

[menardre]

Pete

Thanks..

My sky was clear until about midnight .. then spotty clouds. I shutdown around 1:30AM due to both clouds and the galaxy getting low in the western sky.

Roger

[mark.m]

I was out at UMass last night and the sky was workable for quite a while before clouds rolled in. (I'm really growing to appreciate the great horizon available there -- almost makes up for the terrible light pollution.)

There was a skunk wandering around outside the observatory; depending on the direction of the breeze, (s)he lended a distinct aroma to the observing/testing process.

- Mark