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Mojo
Posts about photography, contract bridge, astrophotography, astronomy, Java development, internet systems.

This story is a bit flat

Last night (Saturday, May 23, 2020) was a beautiful dark clear night in Monrovia. Tonight is forecast to be even better and warmer. I can’t resist! When will I sleep?

By the end of evening twilight, I was aligned, focused, calibrated, synced, and ready to image.

I’ve been cherry picking Messier objects that were near their highest altitude for the evening. There’s a trio of galaxies near each other in Leo, M95, M96, and M105. I thought I’d start with these.

This is where I discovered the real weakness of urban astrophotography. For M95 and M96, I could not find a guide star anywhere nearby. I took a couple of images only to have the guiding software (PHD2) hunting around and guiding on what it thought were stars. The results were all squiggly lines. So I mostly gave up on those two for tonight.

I had better luck with M105, and got a nice image of it with two nearby NGC objects.

M105

M105 (center) in Leo with two nearby galaxies

 

But wait, there’s an interesting detail. You might not see it if you’re in a bright room, or with a bright background. You might click on the image for the full resolution version.

Do you see that odd circle in the background, left center?

That’s nothing mystical, it’s just caused by a mote of dust on the imaging glass of my camera. The mote is “out of focus” so it appears as a shaded circle. I correct for those by taking a series of flat images at the end of the night. I point the telescope at the zenith and put a light panel on top of the scope. I calibrate the timing so it generates a flat grey image around 50% of the camera’s dynamic range. (I’ll take a picture of that process tonight to show you.)

Here’s the flat from last night’s imaging run, stretched in contrast so the differences show a little more:

Imaging flat

Flat image taken after the observing run. notice the darker circles from dust specks, and possibly vignetting from the telescope.

I didn’t shoot flats for years because I couldn’t figure out how to get a neutral grey image target in the field, and calibrate it for a 50% image. Now that’s handled by my light panel and software that calculates the right image duration.

You can see the dark circle on the flat that matches up with the circle in the background of the M105 image. That leaves a mystery — why didn’t the flat correct for the dust mote, like it usually does?

As a test, I recalibrated the image without the flat, and the circle was still there, but it didn’t have the “3D” look to it from the calibrated image.

You can see the same problem in two other images from last night:

M95 in Leo, I only captured 10 minutes of exposure time on this for failure to find good guide stars, but I saved the image anyway. I'll revisit soon.

M95 in Leo, I only captured 10 minutes of exposure time on this for failure to find good guide stars, but I saved the image anyway. I’ll revisit soon.

Look, there it is again — that dust mote circle in the left center of the image.

M109 Ursa Major

M109 in Ursa Major. I had to shift the galaxy out of the center of the frame so I could find a star reliable enough to guide on.

And yet again, there’s the circle just to the left of M109.

Now here’s the last image from last night. Under the circumstances, I thought this was a beautiful shot of M63 the “Sunflower” galaxy.

M63

M63 the Sunflower galaxy. Click for full resolution. The dark band on the left is an artifact of registering the sub-exposures. I need to crop the image.

Wait, where did the dust circle go? It’s not there! The flat worked perfectly for my M63 image.

That mystery dust circle was a puzzle as I was processing the images in the wee hours of Sunday. When I saw this last one, I understood what happened. Can you guess?

I always shoot my flats at the end of the evening. It’s that one dreaded task I have to do before packing up the gear in the wee hours. It has to be done while it’s still dark.

The flat worked perfectly for my last image. It could only mean one thing:

The dust mote that made the circle shifted position during all the telescope movements during the evening.

During the early evening exposures, it was in a slightly different spot on the glass cover than it was during the last image. You can see the effect — the flat compensation shows extra bright on one side, and extra dark on the other. That’s what gives it the “3D” raised, or depressed, effect.

Playing with the big guy

Evening twilight with the AP180 at Sawpit Wash, 50 yards from our front door.

The previous post mentions how I started trying out photographing Messier objects from my home in Monrovia, using my Astro-Physics Traveler 105mm f/6 refractor. The results were encouraging enough that I had to bring out the giant AP180EDT and see how that […]

Astrophotography from home

In the wee hours of May 15, hunting down Comet C/2020 F8 SWAN

Normally I wouldn’t even have considered trying this project. I’m known for hauling my telescopes to a dark sky site in the desert to do deep sky astronomy — visual or photographic.

But here we all are, staying home for the duration.

I have a […]

Many firsts at GMARS

M81

Observing report, February 23, 2020

This would be my first night observing from a powered telescope pad at GMARS, Riverside Astronomical Society’s Goat Mountain Astronomical Research Station, in Landers, CA. It was a beautifully clear but cold February Sunday evening. It was also the first real observing night for a new mount, plus a new and unfamiliar […]

Where to go from here …

This is the first new blog post from me in four years, and long overdue.

Why blog now?

I enjoy writing and reporting about stuff in my life: astronomy, travel, bridge, wine, photography, network systems.

And Facebook is Dead To Me.

Since my last post, Jane and I have both retired, I opened and closed a weekly bridge club, we […]

Working with a broken astronomy camera

Last year, I had some really bad luck with astrophotography.

I replaced my old laptop (Dell, Windows XP) and installed a new suite of tools to replace the somewhat ancient and unsupported camera software. At one point during my first “shakeout” evening (October 2014), the new software started spinning my telescope around the declination axis. It made […]

Teaching bridge with a hand dealing machine

Here are five tables worth of hands ready for my next class. Each table gets boards one through four, in a distinctive board color.

Last month I took a leap and purchased a bridge hand dealing machine. The PlayBridgeDealer4 is very popular with bridge clubs and tournament managers for preparing pre-dealt hands and duplicating them across […]

Internet service has no maverick

If you’re like me, you have exactly two places from which to buy high-speed internet service: your phone company (mine is Verizon) or your cable TV provider (mine is Time Warner Cable).

That sounds like competition, doesn’t it? If you don’t like one, you can go to the other.

Why would I switch?

After nine years (and about $10,000) […]

Home theater upgrade, part 2, fixing HDMI

(Part one found here.)

First I’ll mention that the AV preamp I started out with, a Marantz 7005 purchased as B-stock from Outlaw Audio, turned out to be hopeless. It would never successfully handshake video output to either of my displays, even with no sources connected. Sometimes on a hard reboot, it would send some video, then […]

Home theater upgrade story (Part 1)

Old system:

Rotel RSX-1065 receiver
Panasonic TH-50P9UK plasma display
Old Pioneer DVD player
InFocus ScreenPlay 5700 projector
Roku
B&W CDM7NT 5.1 system, with broken tweeters on the front mains
Logitech Harmony One universal remote

Why upgrade?

The Rotel receiver has annoyed me for years with its few seconds of silence after mode switches (like Dolby Digital to PCM during a TV broadcast). It’s volume behavior […]