Smoke and Solar Eclipses; FocusCube3 Prep; A Target for June

We've had our first smoke event of the year. It came on May 12, six days earlier than it did last year. Even so, I'm hopeful about this summer's imaging. (You can use the Fire and Smoke Map to get a feel for how much smoke there is around, and track your air quality.)

While the dense smoke was bad for visibility and health, it was also remindful of the recent solar eclipse. The dimming of light by the thick smoke layer above our heads reminded us of the sunlight shortly before and after totality. Was this a reasonable comparison?

I guessed the smoke-filtered light resembled sunlight about 20 minutes before (and after) totality. The fraction of illumination 20 minutes before totality is given by this table from a 2009 paper by Können and Hinz:

Column c at 20 minutes before totality gives (with a little interpolation) sun brightness of about 27.5%. We have to use column c instead of the geometric obscuration (column b) because of the sun's limb darkening.

My wife was tracking the eclipse a little differently and she estimated that the smoky sunlight corresponded to when the sun was about 2/3 geometrically obscured. The obscuration column says that 67% obscuration takes place about 22 minutes before totality with a brightness level of about 30%. Both our estimates agree very well (22 vs 20 minutes, 30% vs 27.5%). 

Some years ago -- the early 'aughts, I think -- I was at a dark sky site trying to work my Binocular Messier list and there was heavy smoke overhead. I estimated the extinction it caused at about 1.5 magnitudes. A magnitude reduction of 1.5 means brightness is reduced to of 2.512 ^ (-1.5), or 25% of normal. While it's impossible to know how much smoke there was then compared to what there was earlier this month, it's at least close.

So the next time you have a serious air quality situation or a lot of smoke aloft, maybe take a look at what it's doing to the sunlight. You may find yourself pleasantly reminded of the last total eclipse you attended.

(It's worth mentioning that there's one big difference in how the moon and smoke reduce the the sun's brightness: color. Smoke scatters blue light leaving what penetrates to the surface looking redder, while the moon's effect on color is quite small. How much this plays into one's perception of the dimming is an open question!)

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One nice feature of NINA is its ability to use filter offsets. Offsets let you change from filter to filter without requiring time-consuming manual or automated refocusing; only occasional refocusing using a reference filter is needed. This can potentially save enough time for collecting more data.

Given that I have just replaced my FocusCube2 for version 3 I though it time to update my offsets. This process basically works in a few steps:

1. Obtain good focus manually 
2. Move the focuser until you see a significant increase in star diameters, perhaps 25 to 50%. The distance focus moved will be the autofocus step size
3. Return to the best focus setting 
4. Start the offset determination process using the Darks Customs plug-in.

Before beginning I noticed that my focuser travel (a tiny 30mm) corresponds to about 8000 FC3 movement steps. This is the same as it was for my FC2, so I made a guess the autofocus step size will be the same, too. That let me skip the above steps 2 and 3. The filter offset calculator worked perfectly, and gave me the new offsets: L = 0, R = 11, G = -6, and B = -4. The old offsets were L = 0, R = 7, G = -6, and B = -1. The changes are insignificant compared to the autofocus step; Even the filter order that minimizes backlash is the same.

Incidentally, the focuser performed perfectly during the offset calculations -- every run produced a perfect 1.0 hyperbolic fit. Nice work, Pegasus!

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My first imaging camping strip is scheduled for June, and I've selected the target. If somehow it manages to be clear I'll be imaging VdB 152 ( bright blue reflection nebula), LBN 538 (a colorful emission nebula) and possibly LBN 528 (the faint, dusty "tail" of the reflection nebula). All three are in Cepheus and fit nicely into my FSQ's field of view.

Pegasus FocusCube Version 3; Back from the Eclipse

Pegasus FocusCube Version 3

I've been using the Pegasus FocusCube Version 2 for a couple of years and it's been amazing. The only problem I ever had with it was my own fault; unknowingly I tried to run it a few times past the inward limit of my focuser and that loosened the shaft coupler to the point the FC2 lost track of where it was. When I'm not trying to torture the poor thing it has always delivered a sharper focus than I ever achieved manually. 

Pegasus recently introduced FocusCube Version 3, and FC3 has new features, only one of which really interested me -- it's USB 3 and only needs USB power to operate. My first thought was "wow, one less cable!"  

Instead I'm having to choose between two options: buying a new, big, $640+tax usb/power hub or kludging together my existing hub and an old powered USB hub. Of course my frugal nature demanded I go with the latter option!

Why?  The FC3 gives me three devices that require USB 3 power or communication bandwidth: FC3, ASI 120MM Mini guide camera, and ASI 2600MM Pro imaging camera.

Alas, my current USB hub, a Pocket Powerbox Advance Gen 2 has only two USB 3 ports. My delusional mind had assumed all four of the USB ports my existing hub (a Pegasus Pocket Powerbox Advance Gen 2) were USB 3. My delusion was reinforced by the mislabeling on the Powerbox case:

My Pocket Powerbox Advanced Gen 2 Collector's Edition with incorrect USB port label

The correct label can be seen on the Pegasus product page.

The kludge I'm putting together involves using Velcro to piggyback an old hub on the Powerbox. In the end this means instead of reducing my cable count I'm going to increase it by one. Oh well ...

On the other hand, NINA fired it all up without a glitch, so it's ready for imaging!

Moving The FocusCube 2 to the AT-65EDQ

A second reason for purchasing the FC3 was so my old FC2 could be installed on my AT-65EDQ, a small astrograph that can produce lovely images. 

I plan to use the AT-65 mainly to collect stellar spectra because the camera-to-scope connection must use a 2" nosepiece. A nosepiece makes it very easy to place a diffraction grating in the optical path. And at f/6.5 it's a little better for spectra than my FSQ at f/5.

The AT-65 isn't on the list of scopes compatible with the FC, possibly because of the diameter of its fine-focus shaft. The smallest shaft coupler provided with the FC3 was 4.0mm -- much too large for the 2.5mm AT-65 shaft. Unfortunately the smallest coupler I could find for sale was 2.3mm. (Maybe that  corresponds to some common non-metric size? I tried to file down the fine focus shaft, but it's made of an alloy harder than my files. I did find a nice attachment for my rotary tool that must have had emery or diamond on it, and that took the shaft down to size.

Both FCs are now working with Unity and NINA. Let's have some clear nights now!

Back from the Eclipse

We drove from Minneapolis to South Washington, Indiana.  Ten hours in rain to get there the day before, but it was worth it--almost perfect sky for the entire eclipse -- and most importantly at totality. The naked-eye prominence was worth it alone, and I doubt any image will be able to do it justice. 

Arriving a day early and leaving the day after meant no traffic problems.  This was my fourth eclipse (1978 North Dakota, 1998 Aruba, 2017 Nebraska were the previous) and probably my last. And I'm pleased to say I took no images, I just looked upward and enjoyed.

The fact that Texas was cloudy and the Midwest clear demonstrates perfectly the difference between climate and weather.