M33 on a dewey night - this is ugly!

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by , 09-21-2017 at 01:11 AM (669 Views)
This is not a good image, but in the interest of full disclosure I have to share the bad nights, not just the good ones. There were some positives, which I will get to as well.

It was not horribly warm last Saturday, and I have been itching to improve on my M33 from last year, so I planned to get much more data on it than I did last year. It reached 30 degrees altitude at about 9:30 PM and transited at 2:30 AM Sunday morning. So I was looking at a 5 hour imaging window. I decided to go for 4 minute frames, to collect more time and spend less time dithering between images. In setting up, I used PHD2 for drift alignment. I was able to get to an alignment error of 0.10", by far the best I have ever done. Previously 2-3' was my best. Focus was set to .35 pixel, again, a personal best. I guess I am getting more patient with the fine adjustments needed to dial these things in.

I noticed that I broke into a sweat when moving the gear outside, a sign of just how high the humidity was. By the time I got going with imaging, a little closer to 10:00 than 9:30, the table I have the laptop on was feeling a little damp. I said to myself, "That mirror is way down at the bottom of a long tube, how wet could it get?". As the subs started rolling in, the stars were nice and round, which made me happy. PHD2 was behaving well, and dithering was going better than normal.

As the subs came in, I noticed that they were getting darker and darker, which is something I have only seen in the past with clouds blowing through, but there were no clouds. I had been worried that going to four minutes might make the LP glow wash out the image, and I was also noticing fewer stars showing in the subs than I normally see. Things kept getting wetter and wetter, and I finally pointed a flashlight down the tube during one of the dithering intervals (I am not doing off axis guiding, so this doesn't affect PHD2). I saw a somewhat wet primary mirror. I didn't really understand what impact that could have on the subs so I kept going. Shortly after this PHD2 complained about losing its guide star from time to time and I had to give up. Still no clouds or fog, but it was well past time to quit.

So not much to show image-wise, but some notable successes for the evening. Without further ado, M33. 1 240 light, 23 darks, 40 each of flats and dark flats, and my trusty master bias.


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  1. astrokerr's Avatar

    that is a beautiful image, made more so by the back story - truly. Also, I now know there is something called PHD2 and that it is useful, so I'm gonna go googoo it and learn something else to add to "Bob gets the Job done" - my thanks for the pic and the post!

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