Sunday, January 10, 2016

Comet Catalina C/2013 US10 with DSLR

After a quick single frame of comet Catalina C/2013 US10 with my Canon T5i DSLR a couple weeks ago, I was hoping for clear skies to do a more thorough treatment. I woke up at 5:00am on January 2, 2016 and took my motor driven Orion EQ-1 mount out to the back patio. It was cold but not bone-chilling cold like some mornings, and we've had a mild winter so far this year.

I did a rushed setup and got some pretty poor alignment, meaning I could only take up to 6 second exposures without star trails. I was feeling lazy so I figured 6 seconds was good enough and I didn't want to re-align everything. I was also battling the pre-dawn twilight, which would ruin my session in a couple hours.

This YouTube video shows a timelapse of my setup, getting the Canon T5i on the Orion EQ mount. The Orion EQ-1 mount has an optional motorized attachment that will turn the camera/telescope to compensate for the rotation of the Earth. The motor does not track or lock on to anything, it just tries to turn at a constant rate. While this might be good for keeping your target in the field of view longer, it's not very smooth or precise. It's much better than a fixed tripod, however.

Notice the amount of drift in the timelapse video at the end. My alignment was not great, but that should result in a constant drift in a single direction - not the shaking up and down motion we see. It looks like small variations in the motor speed are causing the mount to lag or speed up.

The sequence at the end is 239 frames, each 6 second exposures at ISO 3200, f/5.6, 300mm with Canon T5i on January 2, 2016. If you look closely you can see the small fuzzy comet move slowly to the left relative to the star below it.

Comet photos with DSLR are tricky, especially with a very basic mount like this or even on a fixed tripod. Sometimes I get good results with stacking, and sometimes I like single frames better. I was really hoping to get some of this comet's tail, like I did with comet Lovejoy last year, but 6 seconds was just not enough.

While taking over 230 individual photos, each at 6 seconds long, the comet moved a noticeable amount. This thing is moving quick! It becomes more obvious when you look at the first and last frame I took that morning. Here is an animated GIF showing the earliest and latest frame from my stack to show the change in position.

comet catalina dslr movement animation
Animation of 2 single frames taken with Canon T5i each at 6 sec, ISO 3200, f/5.6, 300mm
comet catalina canon t5i dslr
Single frame from the GIF animation above in large .png format to show detail
This was also my first time stacking a comet photo in Deep Sky Stacker with comet mode, attempting to register the position of the comet to hold it constant and let the stars trail in the background. Because the comet moved quite a few pixels across the field of view relative to the background stars, I was hoping this painstaking process would pay off with a nice comet tail.

In Deep Sky Stacker, you literally have to go in and click on the comet in every single light frame to teach the program where the comet is. This maybe isn't so bad if you have 6x 5 minutes exposures or something, but I had over 200x 6 second exposures. Ugh! I gave it the ol' college try to see what it would do, and I was underwhelmed. After about 2 hours of pin-pointing the comet in sub frames and another 2 hours of stacking, the image below is what I got out of it.

comet catalina deep sky stacker dslr
Stack of 213 light frames, 38 dark, 25 bias, ISO 3200, f/5.6, 300mm, 6 sec (no flat frames)
You can see comet Catalina to the left and bright Arcturus to the right. All of the stars are trailing and the comet is supposed to appear stationary in this kind of stacking. I forgot flat frames and I have a massive gradient hot spot in the middle of the image. I'm going to try re-stacking with my ISO 1600 flat frames and see if they're close enough to do the job. I've been using the same flats for just about every stacked photo, I'm so bad!

deep sky stacker comet screenshot
Tagging the comet frame by frame in Deep Sky Stacker

Here is another process with flat frames - do I see a hint of a blobby tail coming off to the right or is that my imagination? Honestly, I like the look of the single frames better. The stars are around and the comet has color. I just should have spent more time on the set up and gotten longer exposures!!

comet catalina dslr
Same process as above but with flat frames to remove the hot spot in the middle

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