Comet C/2013 Catalina UQ4
8th magnitude comet in the Evening sky in Bootes
July 18th, 2014

 This faint evening comet is closest to the Earth this week, and is screaming across the sky over 10 arc minutes per minute of time. Thats the full size of M13 in Hercules. Capturing this fast mover typifies the difficult imaging challenges that serious comet imagers face, and most of the time conquer on a routine basis.

Here, I am illustrating several different combine methods for this unique asteroid - turned - comet, each method shows a different aspect of the field and the comets details. A total of 64 frames were taken in sequence of only 30 seconds exposure time with our 12.5". Any longer, the comets fast motion trailed the nucleus on the sub frames. To keep the comet relatively stationary in the final images, you must select an exposure to keep the trailing down to a tolerable level, in the 30 arc seconds or less area to be sure.

Arizona's summer monsoon has been very cloudy and rainy for the month of July so far this year, and only a few hours of clear skies opened up for us to take these frames. By morning, the clouds were back and it was raining again!

Click all frames below for the larger view:
 For this first image, the 64 sub frames were summed with the comets nucleus in register for all the frames. The result a sharp comet in a trailed star field. A close up of the stars shows that the tiny 8 second gap that the separate images take to download to the hard drive leaves a small blank gap in between shots. This gives the stars a bead like appearance at high magnification. Also, the star trails are so intense that much of the tail is lost in the long bright star streaks. The comets head has a greenish leading edge from the ionized gas in the coma. Stars are of course colored by the spectral class they exhibit. To color the stars in this image, I used an interesting technique. Normally, you only have half a dozen individual sub frames on slow moving comets to color the star trails, and you can simply replicate six layers of the RGB data over the L channel and combine with "Color" in Photoshop. But in this case, there are 64 separate images! My poor computer would lie down and die if I tried to lay over 64 RGB frames over this set of trails. So I used a new technique. I overlaid ONE frame of the RGB stars color data over the L data with the trails. Then used Photoshops "Motion Blur" tool to stretch out at the appropriate angle the colored star image to the exact length of the black and white star trail underneath. Combining this using again "Color" did the trick.
Select an image size for a larger view: All L channel frames Summed in Maxim DL 1290 x 960
 Using the new software Nebulosity 3.2 by Stark Labs, I combined the L channel data with a sigma clip stacking method, and was pleased to see that the formerly overbearing star trails had been subdued greatly, and the comets nice twin ray tail structure could now be more plainly seen. To improve the effect, I processed the odd and even files separately to leave a bigger gap between stars on each stack, then averaged the two later. Star colors are true and done the same way as described above.
Select an image size for a larger view: All L channel frames Sigma = 2 clipped in Nebulosity v3 1290 x 960
 Combining using only Median combine in Maxim yields a strong attenuation of the star trails. The blurry appearance is an effect of the median combine method, and while this image again shows good tail details, it is not something you would probably send to a magazine for publication! You can use this median combined image however to do one more thing...
Select an image size for a larger view: All L channel frames combined with Median in Maxim DL 1290 x 960

 This image shows stationary stars, and comet. To achieve this composite accurately and stay within the bounds of imagers morality, you have to take a few extra steps. First you create an LRGB of the star background, using Sigma 2 for the combine to eliminate the comets trail. Next, you select your middle frame in your sequence and adjust to be able to see the stars and comet clearly in this short sub. Finally, you will use the median combined comet image from the above image.

You create three layers in Photoshop. The base is the middle frame in your set that you will stack everything onto. Layer on top the stars data, and register to the base layer. The top layer is the median combined image of the comets head. Register it to the base layer. Now delete the base layer, it will no longer be needed.

By combining the stationary comet on the top layer using "Lighten" in Photoshop, you can allow the comet to be very accurately be placed in the proper location in the field, during the time of your middle shot. You may have to adjust the brightness of the comet to suit. Flatten and add color data. Here is what you get. Magazines will love you:

Select an image size for a larger view: All Star L channel frames combined with Sigma Clip = 1.75 All Comet L channel frames combined with Median in Maxim DL 1290 x 960
 Finally, by enhancing the already existing diffraction spikes with "Star Spikes Pro" software, you can meet your public enrichment obligations in numerous publication venues:
Select an image size for a larger view: All Star L channel frames combined with Sigma Clip = 1.75 All Comet L channel frames combined with Median in Maxim DL 1290 x 960
Lens: 12.5" f/5 Home bult Newtonian Platform: Astrophysics AP1200 Exposure: 35m LRGB with 30sec subs Location: Payson, Arizona Elevation: 5150 ft. Sky: Seeing 4/10, Transparency 6/10 Outside Temperature: 65F Processing Tools: Nebulosity, Maxim DL, Photoshop CS2 HOME GALAXIES EMISSION NEBS REFLECTION NEBS COMETS GLOBULARS OPEN CLUST PLANETARIES LINKS