M51 - The Whirlpool Galaxy or "lessons learned setting up a sky tracking mount"

22 March 2018

The weather gave me a short opportunity to shoot a new astro image. Originally the plan was to go out with a friend to our usual place and bring out the 11-20mm lens which I haven't used for quite a while and try another selfie - but this time utilizing my star tracker.

But clear outside didn't promise clear skies before about 9 o'clock p.m. and furthermore it was forecasted to be rather cold for the end of march. So we decided to set up the mounts in the garden of a friend where I could work on some skills I really need to work on - like polar alignment and focusing. As a byproduct of my tests I thought it would be nice to point the camera to a deep sky object. As my knowledge of deep sky objects (and the steps to find them in the night sky) currently is rather limited I opted for M51 the Whirlpool Galaxy, which can be found rather easy by pointing the camera to the "last" star of the big dipper a.k.a. Alkaid. Using a zoom lens eases finding the galaxy as I can zoom out until I find my target and then step-by-step zoom further in keeping the target at the center.

My first learning task of this evening was tho get a proper (or at least usable) polar alignment. I did a rough one and wanted to check (and refine) the alignment again just before taking my individual subs. As it turned out my intentions where good but the execution was more than sloppy: As I wanted a very comfortable position of my polar scope to do the polar alignment standing rather than kneeling on the snow covered ground I decided to expand my tripod rather high. Having done so I realized that setting up the camera at roughly a 11 o'clock position I am way to short to reach my camera and the lens to get the adjustments done, therefore I put the camera on about a 7 o'clock position. Now there's a problem which I didn't notice at all - but thankfully my friend did (after some time): When my 18-250mm lens is zoomed in all the way to 250mm it blocks the polar scope - so checking the polar alignment when everything else is set up is not possible. When I looked through the polar scope to check the alignment I didn't see any stars at all. First I (actually we) thought this was because of the clouds that where passing by - but even when they were gone we didn't see stars. Well after we figured out what the problem was I zoomed the lens back out checked the alignment (or rather my friend did that for me) and zoomed back in.

Now to the second task for the evening: As you might have seen I wasn't happy with my focusing when I took the last images at 250mm so I wanted to test a Bahtinov Mask. My friend happened to have one that fits nicely onto my lens and I was surprised which large impacts even the slightest movements of less than a millimeter of the focusing ring have on the sharpness of the resulting image.

Well after polar alignment and focusing was done there was only one thing left - attach the light pollution (LPS) filter, dial in the correct settings for a nice exposure and let the interval timer do its job.

While the camera took about 60 subs each exposed for 45 seconds we went inside, which is a huge benefit compared to shooting on the field, at least this is true during the winter season.

As I had to get up early the next day I decided to stop shooting at around 11 o'clock pm, pack my gear and head back home. When I removed the LPS filter I noticed a slight hint of dew and hoped that it affected none or only some of my subs. This hope sadly didn't came true. When I looked at the images the next day I found that only 14 of more than 60 frames where usable. Nevertheless I put these frames into Deep Sky Stacker and got the result you can find above. I also did a composition where you can see M51 at the right side of the image and Alkaid to its left:

Alkaid and M51

The stripes you can see on the images are a result of the dew that was on the images I used - if I only used the images without any dew I would only have two images to stack, which would only be a 1.5 minute exposure, and that is not very much. So I used the 14 images that were at least somehow "usable".

Lessons learned from this session:

  • Keep practicing my polar alignment.
  • Position the camera in a way that it doesn't block the polar scope.
  • Buy a cheap Bahtinov Mask - it makes a huge difference.
  • Come up with a solution for the dew - perhaps a lens heater would work despite the 100mm LPS filter in front of my lens.

But the most important lesson I can take away from this session is that I can get decent results with the gear I already own and even more important: I have a good time being out and taking pictures of the night sky.

Details
f
7.1
f (frame)
30 sec
stacked frames
14
f (total)
7 min
ISO
1600
camera
Nikon D5300
lens
Sigma 18-250mm F3.5-6.3 DC Makro OS HSM