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And Catalina makes 50!

Posted: Wed Dec 09, 2015 7:52 am
by rwilkinson
Well, there were indeed some gaps in the clouds to the South around 7am this morning, so I went out with my 15x70 binoculars to have a look for the Comet Catalina.

Unusually, the sky was clearest down on the horizon with some hazy cloud drifting around a little higher up, but the brilliant Venus was shining through it so I had hope that the patch of sky I wanted would clear soon. There wasn't much time left, as Nautical Twilight had already ended and the sky was brightening rapidly.
I'd set up my Nexus7 tablet on the mount above my binoculars (see viewtopic.php?f=10&t=687), but running the SkySafariPlus app rather than Skeye, since the latter doesn't show comet positions.

Thankfully the cloud soon moved away, and I found my way to the 4th magnitude star Iota Virginis, and once my eyes had adapted, I could just make out the 7th magnitude stars around it. Thanks to my tablet's "head-up display" I knew exactly where to look for the comet, and could just about discern a fuzzy smear, which moved around as I nudged the binocular-mount (a dynamic alternative to "averted vision"). I couldn't make out either of the tails, or anything to confirm that it was a comet, but there was certainly a "faint fuzzy" in the right position, so this was my first sighting of C/2013 US10 Catalina.
And this sighting had another significance for me: this was the 50th comet which I've detected (either in the eyepiece or by astro-imaging), a little over 30 years since I saw 1P/Halley back in 1985.
Comet Catalina is now outbound from the Sun, so it will be getting higher in the sky each morning (and will be visible in the evenings too next month), and I'm hoping to get some images of it before the end of the year, whilst it's still sporting two tails...

Before packing away, I also had a quick look at the Moon: as well as the thin sunlit crescent, the rest of its surface was clearly visible by Earthshine. And heading up the ecliptic plane, I also visited Venus, Mars and Jupiter in quick succession.

Comet imaging before breakfast!

Posted: Wed Dec 23, 2015 12:19 pm
by rwilkinson
I saw the comet much more clearly through my binoculars on the morning of 21-Dec, and seeing that it was reasonably high in the sky around 7am, wondered about the possibility of imaging it before breakfast one morning...

I devised a minimalist imaging system which would be quick to set up, and my chance to try it came two days later. When I looked out of the window at 6:30 this morning there was a fair amount of cloud around, but by 6:45 it was clearing nicely so I decided to go for it.
There wasn't much time before sunrise, and it would take too long to set up my EQ mount and laptop, so I just used a Celestron LCM (alt-az GoTo) mount on my speaker-stand tripod, carrying my little TS70ED refractor fitted with my old Pentax K110D DSLR:
My "grab & go" comet-imaging system
cometchaser.jpg (222.4 KiB) Viewed 4558 times
I used Solar System Alignment on Mars to setup the mount, and then used Venus for additional calibration. And whilst pointing at Venus, I fitted my Bahtinov mask and did my best to focus through the camera's eyepiece (I could only see one of the diffraction-spikes, so just tried to make this most prominent).

Then I used SkySafariPlus running on my Nexus7 tablet (linked to the mount via BlueTooth) to GoTo the comet's position. I couldn't see any stars in the eyepiece, but when I took a 30-sec exposure I could see that I had a fuzzy blob in the field of view. The sky background in this 30-sec exposure was already quite bright, so I went down to 20-sec for my imaging run (and then remembered to set the 2-sec post-mirror-flip delay, so I ended up with 18-sec exposures).

I had the camera powered (through a DC converter) from the 12V battery, and the shutter operated on an interval timer, so I set it to do a run of 25 and went back inside to make some coffee. Although I had only got out of bed at 6:45, my first image was timed at 7:02!

By the time this run had finished the sky was brightening quickly, so I decided to switch over to calibration images. I turned off the drive, pointed the 'scope up to the zenith and took a series of "twilight flats". Finally, I dismantled the system, fitted the camera's body-cap and took the offset and dark-frames. This was all done by 7:45 - just in time for breakfast! 8-)

Flat-tery indeed

Posted: Wed Dec 23, 2015 1:57 pm
by rwilkinson
This morning was my first attempt at "twilight flats" - but maybe I should have waited until the sky was a little brighter, as I needed 30-sec exposures to get a good signal-to-noise ratio, and I ended up with hot-pixels and star-trails in each frame:
A single 30-sec flat-field, taken with the 'scope pointed up at the zenith and the drive turned off.
singleflat.jpg (19.1 KiB) Viewed 4556 times
Now the IRIS Digital photo.. Make a flat-field soon got rid of the star-trails when combining my 16 images, but I found that this process only subtracts the Master Offset (not the Master Dark), so I was still left with the hot-pixels. But rather than going back and calibrating each of my flat-fields (as if they were lights), I simply removed the hot-pixels directly from the master-flat using the Ring_median 2 command.

However when I first performed the flat-field division on my lights, I ended up with brown images. I eventually realised that my master-flat itself was very blue (since I'd used the scattered twilight as the light-source, rather than a white card in daylight as I'd used in the past) and so dividing my lights by this yielded an over-red result. But after using the Grey_flat command, my master-flat was reduced to a neutral tone.
And I couldn't help noticing that my camera sensor was well overdue for a spring-cleaning:
I've found some of the "missing matter" in the universe!
The vignetting of the corners is due to the use of a focal-reducer from f/6 to f/4.1.
pflat.jpg (18.03 KiB) Viewed 4556 times
So I finally got a set of 26 calibrated images, but as I'd used an alt-az mount they suffered from progressive field-rotation. Fortunately there were enough stars in each frame for IRIS's Stellar registration.. Global matching to sort this out and align all the images. Then I just needed to measure the comet's rate of movement across the stack and use the Trans2 command to re-register each image on its position.

So my final stack shows a slight trailing of the stars due to the comet's movement and a twisting of the edges of the frame caused by the field-rotation:
Stack of 26 images registered on the comet.
twilight_23dec.png (369.84 KiB) Viewed 4556 times
And this is my final processed image: Image

As well as the sky-subtraction and contrast-stretching, I've also re-scaled it x0.75 from the original and cropped and rotated it to put North at the top.