Monday, September 28, 2015

Moonlight Imaging (broadband version)

Two months ago I wrote about how one could narrowband image under moonlight. Actually, you don't have to use narrowband filters to image with a full moon in the sky. But you do need to choose your targets with a thought to where the moon is. And it helps to have a haze-free sky that's otherwise fairly dark--yellow zone at least.

Here was the situation the night of the 26th: One night before the "supermoon" total lunar eclipse. The moon's magnitude was a brilliant -12.6 and at meridian crossing reached an altitude of about
42 degrees here in Minneapolis:

The Moon on the night of 9/26/15 at meridian crossing
This shows the part of the sky with an altitude of over 30 degrees. It's not going to be good imaging in Pegasus, but going further north things get better. Polaris is about 48 degrees from the moon, and if we can find something between that and the northern horizon it might be worth a shot.

So I chose objects that were on the northern side of the zenith well away from the moon and close enough to the celestial pole that they'd be available all night. And I imaged from a darker site than my backyard in the inner red zone. Here's how it turned out:
NGC 2276 (Arp 25) and NGC 2300 (Arp 114)

NGC 0040
Not too bad, really. I imaged at f/10 because these are small targets and partly in the hopes it would help contrast. The galaxies were imaged in the time before midnight and the planetary after followed by dark frames. As it ended up I was out there until after 4 A.M., but a clear night is not exactly a common event this year and I wanted to take full advantage of it.

Note that exposure times were kept short because of the moonlight. Unbinned luminance frames were only 180s and 2x2 binned RGB frames were a brief 90 seconds. As it was I got about two hour total exposure time for each image.

After three hours of sleep I went home the next day, to bed at 7 P.M. and slept right through the lunar eclipse. Imaging has a  price!

Wednesday, September 16, 2015

When To Go Off-List

It's possible to get caught up in an observing list and work on it to the exclusion of everything else. That's pretty much what I did while working on the Bright Nebula list, particularly when I got to the point of having a dozen or so items to go. Everything was dedicated to completing the list to the point that it dictated what gear would be used at star parties and how my time would be spent.

Perhaps a saner approach is to slow down and mix in non-list items for imaging along the way. That's what I did the other night, and it worked out well.

Lately I've been traveling to a friend's home north of the Twin Cities and we've been observing and imaging together. His sky (suburban-rural transition) is much darker than mine (inner red zone urban) and his gear is definitely better than mine and mounted in an observatory. I set up just as if I'm at a star party, polar align, and get to imaging. Then I'm free to help him while he works on reacquainting himself with the process of imaging--he's been inactive for several years--or hunts down deep sky objects with his 16" go-to Dob. It's the best of both worlds for me to have the benefit of his sky and telescope.

And just as at any star party, it's nice to have someone else around to talk to.

Plus at 4 A.M., I have a nice soft sofa to crash on instead of a tent!

Last Sunday was my latest trip to his place for imaging, and we succeeded in getting his imaging system working for the most part. His autoguiding wasn't working, but he's implemented a fix for next time based on what I use.

I had no specific plans for the evening other than imaging Arp galaxies for that Astronomical League list and maybe during the A.M. of switching to a planetary. My friend had his own list of favorites he was eager to observe using his big Dob and we looked at some of those. About the time I was wrapping up with my second Arp he mentioned NGC 891 and showed my his first light frame. In case you don't know 891, it's a large edge-on galaxy in Andromeda. It's cut in half by a dark dust lane dotted with bright knots and has a rather large and distinct nuclear bulge. In other words, it's pretty.

To some extent imaging is all about pretty, and after all the "bright nebula" imaging in monochrome I decided I wanted to make an image in color. My friend's suggestion of 891 is all I needed to drop my list plans for the rest of the evening and try to make something pretty. Here's the result:

NGC 891
(Details here at Astrobin.) Not a perfect image, but I'm happy with it, and glad I opted to give it a try instead of doing more list imaging!

The early imaging was mundane Luminance only. The target galaxies were so small that color was basically a waste of time. Here's what I mean:

NGC 7550 (Arp 99, Hickson 93)
NGC 7578 (Arp 170, Hickson 94; Note the plate solving error)

These two bring my Arp count up to 12 compared to my planetary at 13. I'll probably add a couple more monochrome Arps next time out to make the lists even in terms of percent completed and then go off-list again!

Sunday, September 13, 2015

Celestron Field Flattener Follow-Up

This is a follow-up of a previous post.

I was able to image the other night with the short nosepiece. Recall that my goal was to get the system to match the presumed optimal f/6.3. It was f/6.0 with a long nosepiece/adapter in place, and by switching to a simpler nosepiece it allowed me to trim about 8mm off the FR/CCD separation. While the image is not very good, it's good enough to allow to plate solve it and calculate the pixel scale.

Here's the image:

NGC 7625 (small galaxy at center)
This was based on 30 minutes total exposure for each RGB channel, under near-urban sky. It could be a lot better, but sometimes one must take what the sky and gear gives.

The pixel scale is reported to be 1.51 arcseconds per pixel. This is really close to the value of 1.50 that corresponds to a focal ratio of 6.3--the error is only about 2/3 of one percent.