Sharing the Back Yard

I live in a second-tier suburb south of Minneapolis. About a mile away is the Minnesota River valley, a broad expanse that's been kept relatively undeveloped. One might expect to get a lot of wild visitors from the valley, but seldom has anything out of the ordinary been observed. Aside from deer that have been seen only two or three times in over 30 years, it's the usual suburban mix: Owls, rabbits, and raccoons (although their numbers seem to be greatly reduced in the last 20 years or so).

Increasingly present are species that are reclaiming old territories. Bald eagles have become frequent visitors. There's a giant old cottonwood tree in an adjacent lot that someday could play host to a nest, but it might be too far from the river. "Location, location, location," applies in the animal world, too.

Coyotes are also returning. I've heard them howling nightly at most star parties in the open country but now they're back in the suburbs. I've gotten a few glimpses of one trotting quickly away in the last few years, and this spring I've seen one that seems more at ease. Twice it's been seen sleeping on the back yard lawn at the sky brightens at the end of dawn:

The gleam in its eyes is from the flash, not radioactive waste! This one seems more at ease with its surroundings; It showed only mild interest in people walking a few hundred feet away.

Looking perfectly at home

Starting the morning commute

There's no shortage of food available to it. All sorts of rodents are on the menu: Rabbits, squirrels, and shrews are in abundance this year. Add to that all the pet cats and small dogs that are allowed to roam outdoors and I doubt the coyote will go hungry. (Our cat is an indoor cat.)

Bobcats have also been sighted in the river valley and I'm really not eager to see one in my back yard. I consider them much more likely than a coyote to go after larger prey like an imager.

The Minnesota river feeds into the Mississippi river a few miles downstream from here, and together they form a natural impediment to animals traveling southward. Black bears, not all that uncommon in the northern areas of the Twin Cities do occasionally make the trek southward. Two years ago a black bear made it "south of the river" and roamed for a few days through this area. Only the wayward bear came to harm; it was shot in the leg by a police officer and may have survived to return north.

Virgo Cluster Mosaic Ib: A Less Aggressive Approach

Last time I settled on using a 200mm lens to make a mosaic of the entire--fifteen degree square--Virgo Cluster. This would require something on the order of 15 to 24 subimages.

A club member pointed out that the galaxies probably wouldn't look very good at that scale, and he was right. Here's a simulation of 200mm vs. 700mm.

700mm image

200mm simulation

The galaxies are starting to lose their distinction from stars, pretty much ruining the point of a galaxy cluster mosaic.

His solution is to image only the core of the cluster, an area of about 8° by 5°. That's roughly 1/5 of the entire cluster's expanse, but it does contain the densest concentration of galaxies of interest to visual observers.

I'm going to repeat the calculations from Part I for the AT65 telescope. N = 3.5 (round up to 4) and M = 3.25 (round up to 4). So this can be done with about 16 images using much better optics.

I had some time to put together the layout of the 16 frames:

Virgo Cluster mosaic layout. Blue box, suggested area to be imaged; green markers, centers of subframes; red marker, center of mosaic; red box, area of mosaic.

The final mosaic will have a pixel counts that are about three times the frame dimensions. Since I'll be using a Canon T2i, that will give a 162 megapixel mosaic!

Virgo Cluster Mosaic I: Choosing a focal length

The galaxy imaging I did this spring was so much fun that I'm considering a big project: A mosaic of the Virgo Cluster. One of  the club's members suggested that such a project could result in a poster that would be suitable for fund raising at the 2018 Astronomical League Convention (which the club is hosting). I don't know anything about poster printing and marketing, so I'll leave that to others.

The mosaic, on the other hand, interests me.

My images were all taken using my TV-102; with the 0.8X FF/FR it has a focal length of 700mm and a field of view using an APS-C DSLR of about 1.3° by 1.1°. That's too small a field to make this practical--The Cluster has a size of about 15° by 15°, which is an area about 140 times the TV-102 field. When overlapping is considered it gets even more impractical. I would like to finish this within my lifetime!

Another option is my AT65 (422mm FL) with a field that's 3° by 2°. This means I'd need 38 images not counting the area lost to overlap needed for proper alignment. This is better, but still quite a task, and nearly impossible given the fickle weather around here.

How about a 200mm lens? Its field is 5.1° by 3.9° and gives about 12 images needed before overlap is figured in. That's not bad. A big galaxy like M84 manages to be about 85x74 pixels, also acceptable. It's time to pin down the cost of overlapping.

Consider a single line made of identical overlapping images. We take W as the image width and A as the overlap between one image and the next.

The width of sky covered by the first image in the line is W. Each additional image in the line adds (W - A) to the line length, making it easy to write an expression for the total line width W:

W =W + (N-1)(W-A)

We can solve this for N:

N = (W - W)/(W-A) + 1

Pretty simple, huh? We can do the same for height, letting M equal the number of images of height H in a column, B be the overlap distance, and H the total height of the column.

M = (H - H)/(H-B) + 1

We can fiddle with this a little more to express the overlap as f, the fraction it is of the width or height. This is useful because we'll almost certainly try to use the same fractional overlap in both directions.

f = A / W = B / H

N = (W - W)/[W(1-f)] + 1
M = (H - H)/[H(1-f)] + 1

One more change, let's define the constant F equal to 1/(1-f), giving
N = F(W/W - 1) + 1
M = F(H/H - 1) + 1

Notice that the right hand side of these expressions doesn't necessarily guarantee that N and M are whole numbers. It's up to us to round them up or down to an integer value depending on how well the resulting grid of subframes covers the target area.

One more alteration improves the convenience:

N = F(W/W - 1) + 1
M = F(H/H - 1) + 1

We should check this pair of equations for correct behavior in the case of no overlap. In that case f = 0 is zero and F = 1. This gives 

N = (W/W - 1 + 1 = W/W
M = (H/H - 1 + 1 = H/H

The number of subframes, N times M, is equal to (WH) / (WH), the area of the mosaic divided by the area of a subframe. This is exactly right in the limit that the subframe is much smaller than the mosaic and we can ignore the need for N and M to be whole numbers.

For the 200mm lens, we have (expressing everything in degrees) W = 5.1 and H = 3.9. For the Virgo Cluster both H and W are 15, so

N = F(1.94) + 1
M = F(2.84) + 1

Let's assume a 1/3 overlap rule, so F = 1 / (1 - 0.333) = 1.5.  This gives us

N = 1.5(1.94) + 1 =  3.91 (round up to 4)
M = 1.5(2.84) + 1 = 5.26 (round up to 6)

So--as a first guess--24 images are needed to make a rectangular mosaic of the Virgo Cluster.This could be as few as 15 (3x5), 18 (3x6), or 20 (4x5) depending on the fit and composition considerations. I'll leave that for Part II.

How about a 135mm lens that gives a 9.5° by 6.3° field?

N = 1.5(0.58) + 1 =   1.87 (round up to 2)
M = 1.5(1.38) + 1 =   4.47 (round up to 5)

Only 10 images, but the galaxies would be awfully small.

And for my AT65, with a a 3.0° by 2.0° field?

N = 1.5(4.0) + 1 =   7
M = 1.5(6.5) + 1 =   10.75 (round up to 11)

77 images, a bit much for my taste, and really difficult to get done by ALCON 2018.

Just for fun, let's do the case for the TV-102.

N = 1.5(0.58) + 1 =   16.8 (round up to 17)
M = 1.5(2.38) + 1 =   19.95 (round up to 20)

That's a mosaic with 340 subimages. That's not going to happen.

Markarian's Chain

The Minnesota Astronomical Society (MAS) has sponsored Messier Marathons for many years. Recently they've begun holding the marathon twice a year by adding a fall gathering for what's called the mini-marathon.

The spring marathon gives everyone a long, good look at the Virgo cluster of galaxies, home to 16 Messier Objects. There are more than just Messier galaxies in the cluster and deciding which particular faint fuzzy you see in your eyepiece can be difficult. Navigating the Virgo cluster is difficult for anyone who hasn't done it frequently.

Beginners wander into the cluster not just during marathons but also when working on their Messier lists. It's not unusual for them to leave the cluster with a sense of frustration.

With that frustration in mind the MAS started what it calls the Virgo Venture, a spring evening during which all attention is on the cluster and learning how to navigate it. Special star-hopping charts are provided and there's an experienced navigator around to help observers find their way.

It was at one of these Ventures that I first observed Markarian's Chain, a gentle arc of galaxies in a small patch of sky. It was the first time that I'd seen more than two galaxies in the same field of view. In fact, I could count six, and with a little travel add a few more! I was impressed, and it marked a first appreciation of wide field viewing for me.

This year's Venture was held last Saturday despite thinning cirrus clouds running east-west over the site and a thick plume of smoke from Canadian wildfires. My imaging target was the Chain but given the overhead murk I didn't have a lot of hope. I shot an hour of light frames with the haze and another hour as the haze was disappearing, so it didn't turn out too badly.

The entire group of galaxies fits tightly in the 700mm view of a TV-102 with its focal reducer/flattener in place:

Markarian's Chain, stretching from the two fuzzies in the upper left corner to M84 near the right edge.

The acquisition details are at AstroBin. Briefly this is two hours of light frames using a modified Canon T2i at ISO 800, a TV-102 at f/7, and autoguiding.

The weather forecast suggests a week of clouds and then the moon comes back. A good cycle to do some lunar/planetary imaging, I suppose. June's new moon will bring a last shot at many Arp galaxies, and then it's summer and back to nebulae!

M81 and 82 Revisited

A nice clear night and an opportunity to image from a friend's home north of the city lights produces a pretty picture:

 

This is based on 44 x 3m light frames with 15 dark frames, Canon T2i at ISO 800 on a TV-102 at f/7.

Power saving update: the software I talked about that turned off the laptop LCD display wouldn't keep it off. Apparently something about using ImagePlus kept turning the display back on. This wouldn't be a problem if a hand control was used to run the DSLR and images were stored on-camera.

Battery Update: I ran the laptop (fully charged), dew prevention, mount and camera from the 35Ah battery for four hours; loaded voltage fell from 12.7 (unloaded, 13V)  to 11.8V. Unfortunately I popped the battery onto the charger as soon as I got home and didn't check its final unloaded voltage.

Assuming it was 12.1V it would have been at 50% capacity, meaning it would have supplied about 17Ah or about 4Ah per hour. This is about right if I take into account that the old laptop battery was in place and it could run the laptop for maybe two hours, after which the 35Ah battery was supplying 6A to both run the laptop and recharge its battery.

That's a whole lot of assumptions and guesses, though, so it doesn't mean much. I may end up rewiring the battery box to include an ammeter so that I can monitor power use more rigorously.

This Year's Star Parties

It's once again time to start planning which star parties are worth the travel expense and time to attend. A big attraction of nearby star parties is their skies which can be much darker than those nearby, which are marginal at best or abysmal (like my back yard). But that's only one attaction. Let's look at the most important factors:

• Sky darkness
• Open horizons
• Light domes
• Programs
• Amenities
• Downsides

I would list ambience as a factor, but the truth is I've never been to a star party where the people have been anything less than great.

Here are some information about the ones I plan to attend this Summer and Fall:

Nebraska Star Party

 

I put off attending this because of the comments about heat and storms. What a mistake!

• Sky darkness: Dark gray (the darkest zone possible). 
• Open horizons: Excellent in all directions; the observing fields are surrounded by low hills. 
• Light Domes: Only one dome worth mentioning, and it's a feeble one from Valentine (population 2700, 26 miles to the northeast.) The dry air seems to suppress domes.
• Programs: Exellent. The Wednesday talks at the local high school are top notch with nationally known speakers.
• Amenities: Campgrounds are available, as is primitive camping on the observing fields.  The observing fields are essentially "bring whatever you need," providing only mowed land with three pit toilets. There's no power or water (drinkable or otherwise). There are catered suppers on three nights; the food is basic but very good. Door prizes that range from very good to grab-bag stuff you'll donate to your local club.
• Downsides: Prickly pear cacti will puncture unguarded feet and tent floors that aren't protected by ground covers. By day there are occasional biting flies and just enough mosquitoes to provide distraction. DEET up and it's fine. And don't forget the possibility of withering heat and violent thunderstorms packing lightning, wind, and hail. 
• Hints: If you're concerned about the heat, make the half-hour drive to Valentine where the public library has WiFi and A/C! A great place to cool off. Also be sure to visit the showers at the nearby campground (bring quarters).

Summary: I'm going for the third time, despite never having won a door prize! If you don't mind the primitive conditions, heat, and threat of storms you should make the pilgrimage to NSP at least once.

Iowa Star Party

 

This is a personal favorite of mine, in part because I was a charter member of the host club and did my graduate study in nearby Ames.

• Sky darkness: Light blue
• Open Horizons: Excellent if you set up in the middle of the field.
• Light Domes: There's a significant dome from Des Moines (population 611000, 50 miles to ESE). During humid conditions a few others can be seen. To the south, nothing.
• Programs: Usually there's a talk by a scientist from one of the nearby universities and another by a host club member. These are given in an old barn which can sometimes be stiflingly hot and humid. 
• Amenities: There's no power or water at the observing field, but power for recharging batteries can be had at one of the nearby buildings. There are a limited number of cottages and houses for those who want to sleep in comfort, and one is kept open during the day for those who seek respite from the heat. Saturday night there's a home-cooked supper that's always terrific. Door prizes that vary in quality from year to year.
  If you like hiking, horseback riding, and wildlife there's a lot to do during the day.
• Downsides: Summertime heat and humidity have been insane the last couple of times I've attended. This year it's on the Labor Day weekend, so maybe it will be cooler. Pests are usually not a problem.

Summary: A nice sub-regional party with dark sky and pleasant surroundings

Heart of America Star Party

 

I've only been to this once, but it's a really well organized event with a lot of things going for it. Unfortunately the one time I attended it rained all three nights so some of my information is incomplete.

• Sky darkness: Light blue
• Open horizons: Excellent
• Light domes: I think there may be one to the north from Kansas City (population 2 million, 50 miles N) and probably from Butler, MO (population 4100, 12 miles E)
• Programs: Substantial--usually at least one speaker of national significance, another visiting from a university, and members' talks. These take place in a building with good A/V adjacent to the observing field.
• Amenities: Limited field power; showers set up nearby, and good food aplenty. Food and drink are available throughout the night, as are activities (movies and bingo) for cloudy nights. Very nice door prizes.
• Downsides: People like to pack together on the observing field, but there's ample room.

Summary: A superb gathering, well organized and attended. Usually in October, so it doesn't conflict with other major parties and is unlikely to be subjected to hot weather.

Back into Planetary Imaging

I've been doing so much deep sky imaging over the last few years that I've neglected the solar system for almost three years! Last night I decided to do something about that and went out and imaged Jupiter.

One of the problems I'd had when planetary imaging was getting the mechanics of it to flow smoothly.

My polar alignment was never good enough, causing targets to drift and making it tough to use region of interest (ROI) imaging. ROI imaging lets you use a small portion of your sensor and get higher frame rates. For example, my Point Grey Research Chameleon gives only 15 frames per second (fps) at its full resolution. When that's reduced to 640 by 480 pixels it provides 24fps, and further narrowing of the ROI can get it up to 30fps.

One way around polar alignment problems is to use autoguiding. To make autoguiding work in this way you need to have a rough alignment (and I do mean rough: north and level is good enough unless you want to use very long focal lengths.) You also want to use the planet as your guide star. Doing so causes the rotation due to rough polar alignment to be around the planet's center. This keeps it fairly minimal over the time span of most captures.

This is based on about 230 frames out of 2300, processed using AutoStakkert 2.6 and Registax 6.1. FireCapture 2.4 was used for acquisition, and it worked much better than the version that was available in 2013. The scope was a C 9.25 with a 2X TeleVue Barlow (f/20, FL 4700mm), and the mentioned PGR color Chameleon. It's only a so-so image and has a number of issues caused by one or more (probably all) of the following:

• Misalignment of optics
• Rotation of Jupiter and motion of the moons (the video spanned a little over 1.6 minutes)
• Poor seeing (which it was)
• Lack of experience using AS!

Despite these problems it was good to get a not-awful image. What pleases me most about this image  is the natural color it has without the need for any tweaking. Back in the old days of using my TouCam colors never seemed quite right.