Friday, February 26, 2016

The Lust for Power, Part 2

In Part 1 I looked at the power requirement of my gear with the purpose of seeing how I might replace my aging deep-cycle batteries. The required amps for several configurations can now be given. The Ah requirement for hour hours a night for four nights is given in parentheses.
  1. C 9.25 on guided CGEM, CCD, dew prevention: 6.0A
  2. AT65 on guided CGEM, CCD, dew prevention: 5.2A
  3. AT65, guided GGEM, DSLR, dew prevention: 3.4A
  4. Camera lens on DSLR, unguided CGEM, dew prevention:  0.9A
Don't worry if the numbers don't exactly map with the empirical values given in Part 1--I'm usually rounding up here. What are our power supply options for these configurations?

Commercial Portable power packs

Most of commercially produced power packs are based on 17Ah batteries. Examples are the Celestron PowerTank 17 ($122) and Orion Dynamo Pro ($145). When brand new, these may be capable of delivering 80% of that 17Ah. That's 13.6Ah. As time goes on you'll see that decrease depending on the number of times you cycle the battery and how well you maintain it. A battery pack like this is sufficient for 14 hours of Configuration 4 and marginal for one night of Configuration 3. It doesn't meet the 16-hour requirement for either case without one or more recharges.

Generally speaking power units like these are wildly overpriced--unless you put a high premium on bells and whistles like radios, spotlights, and DC outlets at other voltages. You're get much better economy if you buy a larger battery and charger. For example, a 35Ah sealed AGM battery and charger cost around $100.

Some power supplies (Duracell, Black and Decker, etc.) are more focused on cranking power and include inverters so you can run your gear as if you had a AC outlet at hand. An inverter sounds nice, but it will eat up a small portion of whatever power you need to supply; a battery build for starting cars is quite different from your need (prolonged low current for many hours).

Recommendation: Don't buy any power supply that includes car starting in its list of features--unless the low Ah rating it provides is all you need. Even in that case, you're better off to simply invest in a battery and charger.


For lowest cost you can use flooded (also known as wet) lead-acid batteries. These have caps on top for adding water and venting gas during charging. While less expensive than other battery types they have several downsides. The acid can spill or leak out and damage equipment or even cause personal injury. For this reason wet batteries have to be kept upright at all times.

While charging hydrogen gas can accumulate and cause an explosion.

Fortunately there are sealed lead-acid batteries that are spill proof and can be used in any orientation. Those that employ Absorbed Glass Mat (AGM) technology and its variations also have better deep-cycle characteristics than flooded batteries. Other advantages of sealed batteries are that they can be shipped without worries about acid spills and the need for the user to initially add the acid, and that they're maintenance-free (aside from recharging).

(Important note: You can't use a flooded battery charger on an AGM battery unless that charger specifically has an AGM capability.)

The battery size you need will be determined by your gear and the type of battery. I think the only practical type of battery to consider is AGM; other technologies (mainly lithium ion) tend to be more expensive. If you take care of your battery (keep it charged, avoid thermal extremes and physical abuse) and use it infrequently (a dozen times a year, maybe?) it will provide years of 
reliable ability to deliver between 50 and 80 percent of its Ah rating. I'll apply the 60% rate in what follows in order to be conservative.

Configuration 1 (large scope and CCD): 6A x 6h is 96Ah. This is 70% of a 160Ah battery. A single battery with that capacity weighs over 100 pounds and costs $300 or more. This doesn't fit my definition of portable power.

Two 80AH batteries would be a somewhat better solution because although being higher in cost they're a bit more portable--each is about 50 pounds. I've imaged this way, but I don't enjoy lugging the batteries around, and consider it a marginal solution in this case.

Configuration 2 (small scope and CCD)

This needs a battery with about 140Ah capacity. This is also met by a single heavy, expensive battery. The same two-battery solution works here as in Configuration 1, so we're again stuck with the non-optimal use of very heavy batteries.

Configuration 3 (short lens or scope, guiding, dew and DSLR)

This needs a 90Ah battery. Two 50Ah batteries would provide more than enough power and cost about $180. Total weight would be around 70 pounds.

Configuration 4 (short lens and DSLR)

This is clearly a case where a battery is the best solution, requiring only a 24Ah battery. It can't be much easier.

The last two configurations clearly can use batteries to meet the requirements. But what about the first two? You can either lug big batteries around or find an alternative: A generator. That's for Part 3.

Tuesday, February 23, 2016

The Lust for Power, Part 1

Okay, maybe not so much lust as desire.

For the last five years or so my dark sky imaging has relied on two deep cycle batteries. One of the batteries had been allowed to discharge to nearly dead but with regular recharging seems to have recovered, although there's no doubt it lost some of its life.

These are group 27 unsealed lead acid batteries that weigh about 55 pounds each. With that weight you might expect them to have good amp hour (Ah) ratings. What are their capacities in Ah? They're labeled with two RC values: 200 for a non-standard 23A drain rate, and 175 for the standard 25A rate. The higher drain rate translates to a capacity of 73Ah and as expected the slightly lower drain of 23A gives a capacity of 76.7Ah. My expected drain rate of 6.5 amps is much lower and should suggest a larger yet AH value. Another practice is to take the Ah to be half the RC; for my batteries this would be 87.5Ah.

The upshot of all that uncertainty--and battery capacity is notoriously difficult to quantify--is that I'll assume the batteries started their lives with an AH value around 80. What it is now I can't say, other than it's less.

Even that conclusion has to be questioned, for some of those amp hours are coming when the voltage is well below 12V. Will everything keep working at 11 volts? SBIG says my CCD will work even at 10V. Kendrick controllers basically turn off when the voltage drops below 11.6V. (They're quite adamant about this and have refused pleas to disable the low voltage cut-off.) The CGEM's ability to handle low voltage is questionable, though; there are reports that it will begin to fail when the voltage goes below 12V.  So even if my batteries are able to produce 80Ah, they're not all usable.

Time for some "ground truth." How have the batteries performed in the past? Probably their biggest single star party workout came at the 2014 Nebraska Star Party where I imaged for seven and one half hours at an hourly drain of about 6A (see below). This probably says more about the lack of clear sky time than it does about the batteries.

Hauling batteries like this on long road trips is a bit of work, and does present a small risk that the batteries could leak acid. So far I've never tipped them over, but an unpleasant accident almost seems inevitable. So it may be time to replace them, and what follows is my exploration of the options.

Power Requirements

Some dark sky star parties are three nights, others are four, and all of them that I attend are during the summer or early fall. A typical summer night is completely dark for only about five hours; by the equinox this stretches to nine hours. Rather than estimate a nightly power need, an hourly power consumption is probably more sensible to use. ADDED: I was able to actually measure some of the values, and those are added in [red].

  • CGEM Mount: During fast slews it can require 1.5A [1.4A], but when tracking it's more like half of that. Let's assume a 0.75A [0.35A]demand while imaging.
  • SBIG ST-8300M CCD Camera: The spec sheet says the camera draws 3A at 100% cooling. A more typical cooling load is 60% of this, so I'll assume a steady 2A draw.
  • DSLR instead of CCD? probably more like half an amp. [With the display off, my Canon T2i, draws 0.13A while idle,  0.19A while imaging. The 12VDC-to-7.4VDC converter is 0.03A of those values.] 
  • Laptop: My old Gateway's AC power adapters says it runs at a maximum output of 3.4A @ 19V, so at 12V that's more like 5.4A. This agrees with my 12DC adapter's spec sticker that says it permits up to 5.6A. That's the load when it's running and charging the battery. A more realistic load is closer to half that, so I'll say 3A to be on the high side and include losses in the 12VDC to 19VDC adapter. [While charging it draws 5.6A, and 2.1A when fully charged. These values don't take into account computational demand of autoguiding.  Included in these values is 0.12A for the 12VDC-to-19VDC converter. Plugging in the Orion StarShoot Autoguider adds about 0.5A. Dimming the display to its minimum cuts half an amp from the draw.]
  • Dew Prevention: I use Kendrick dew prevention, and the power need varies greatly with the telescope objective diameter. At 100% power the strip I use for the guide scope draws 0.3A, for the 4" scope 0.9A, and for the 9.25" scope, 2A.  So my range is 1.2 to 2.3A. Because I almost always use a power setting half this, I'll take the dew demand as .5 to 1A [The low setting actually ranges from 0.17A (6" strap) to 0.97A (28" strap)]
How does this add up?
  1. The maximum is imaging with the C9.25 on a dewy night with autoguiding: 6.75A [6.0A]
  2. Small scope on a dewy night with autoguiding, 6.25A [5.2A]
  3. On a dewless night both drop to about 5.75A [5.0A].
  4. DSLR + lens, no guiding? 4.3A [0.7A].
  5. There are more combinations, but let's stop here.

The reality is that I seldom image more than a few hours a night. If we cap the maximum number of hours at four per night, the nightly power need for Case 2 above is about 26AH [21Ah], so a four-night party would need 104AH [83Ah] if it was clear every night.

At the other extreme is using a DSLR and using the laptop only for focusing. This would require only 21AH [11Ah]!

Next time in Part 2, can I use batteries to meet my imaging needs?

Saturday, February 20, 2016

A Truss Mirror Grinding Stand

Years ago I built a mirror grinding stand from 2x4s and plywood designed to be so solid that it could be rigid when bolted together--no glue was necessary. It was very, very heavy, which was great for stability but a bother to keep around the basement when not in use.

The club has been talking about a mirror-making workshop, and although it doesn't seem like something that's going to take place any time soon I was motivated to build something better for my mirror work. After looking around the Internet for designs I settled on something like the Stellafane design. It's a simple sort of truss barrel.

What I didn't like about the Stellafane design was 24" diameter of the top--it's too big for my purposes--and the four-corner truss. I decided to try a three-corner truss instead. The four-corner truss is a standard for big Dobs and is used by Starmaster, Teeter's, Discovery, and Obsession. The thee-corner truss has proven itself on Orion and Meade Dobs, so perhaps it would work well for ATM work. It's a little lighter and less work to assemble, too.

Because I was using a three-corner truss it seemed logical to make the top and base hexagonal instead of the circles used by Stellafane. My hexagons are inscribed in a 20" diameter circle, which will be of adequate size to handle any mirror I can foresee working on up to 12" diameter.

Here's what it looks like after first assembly:

The stand is 37" from floor to top, just right for me.

What you can't see is that there are three rubber feet under the base to help it sit flat on the floor without rocking. At this point it's held together by wood screws. It seems rigid enough, but I won't know how really solid it is until it's in use. There may very well be some glue in its future.

Still to be done is the addition of adjustable cleats for standard mirror sizes: 6", 8", 10", and 12" should do it. The entire assembly will get sealed with polyurethane, with the top getting a nice sanding and a triple coat of poly to resist the water it's going to see.

Thursday, February 18, 2016

Correcting the Too-short Celestron Hand Control Cable

One of the perennial, if minor, "what were they thinking" topics in astronomy hardware is the short coiled cable used to connect Celestron hand controls to their mounts. The cable, stiffly coiled like an old phone handset line, is simply too short.

This wasn't so bad on my old CG5 ASGT where the mount head isn't all that large, but on my CGEM it couldn't be ignored. It was possible to have the handset pulled right out of its tripod leg cradle as the scope turned in RA. That's not something you want to see when you're imaging, since it means the coiled cord was torquing the mount and then letting the handset become a free-swinging weight in whatever breeze there might be.

One solution I tried for a while was a coiled extension cable. The added cable was so heavy and droopy that it was awkward to handle and tended to get snagged on the mount. Then I came across a video that shows how to replace the stock cable with one that's more user-friendly.

All you need is
  • A piece of flat 6-conductor telephone cable, preferably one with at least one end having an RJ12 connector attached--if not, you'll have to do that yourself. These can be found many places; on Amazon they're typically around $5 to $6.
  • A soldering gun and solder (A good, fast-heating gun is much preferable to the old pencil type)
  • A wire cutter
  • A craft knife for stripping very thin wires
  • Heat-shrink wire tubing (I found this on eBay)

Extremely helpful to have is a soldering jig to hold the ends of the wires together as you solder them.

The most important consideration is getting the wire connections correct. In my case the wire colors and order in the cable exactly matched that used by Celestron, so it was easy to get things right. If you get the connections wrong its quite possible you'll ruin your handset.

The filter from the old cable will be reused on your new cable. Aside from that, you can discard the old cable. Or toss it into your pile of stuff you probably should trash but are keeping "just in case" it might be useful someday.

The job takes an hour or so, when you're done you'll have the kind of cable Celestron should have provided in the first place!

Friday, February 12, 2016

A Mirror Grinding Stand

Among my many unfinished projects is a replacement mirror grinding stand for the one I'm using now. The current stand is very heavy despite being compact. The one nice thing about it is that it can be disassembled into a top, base, and legs. I'm making this new one to see if it might serve as a template for a mirror making workshop that the local club may hold sometime in the near future.

The new one should be much lighter. It will be built much like the Stellafane stand, but instead of a four sided truss design it will use a three sided pattern much like some truss-tube Dobs employ. I prefer the triangular design because it will save a little weight, will probably be just as solid, and makes it easier for the stand to sit on a flat floor without rocking. Also the fewer struts allow easier addition of my favorite ballast, 40# bags of water softener salt. The target height of the stand is 37" and it will double as a platform for mirror testing.

There are only 14 main pieces: Hexagonal top and base, six struts and six cleats.

Unassembled top and bottom of stand

This view shows the underside of the top and top of the base (if that makes any sense). The six cleats will be inset from the corners of the top and base so that the 2x1" struts sit flush with their sides. The top piece will be rotated 60° relative to the base so that the cleats alternate when seen in a top view.

The ends will be 3/4" plywood, which I hope will be stiff enough for the task. If not, I'll just add another sheet to the top. The hexagon is 20" across (vertex to vertex). and should be large enough to allow working on a mirror up to 14" across. I don't plan on ever working on one over 12", and probably won't go past 10" anyway.

The above needs some sanding before assembly; for that I'm going to wait for warmer weather so that I can use the garage and keep the sawdust out of my basement. In the meantime I'll begin working on my Canon DSLR portable power supply.

Saturday, February 6, 2016

Imaging Planet X: Is It Possible?

The news is that there may be a ninth planet. It's being called "Planet X" for historical reasons, with the X designating "unknown" rather than the number 10. It gets a planet designation because even though this inferred object probably resembles a trans-Neptunian object it's thought to be be bigger than Earth.  (Please don't confuse it with the utterly fictional "Planet X" known to some as Nibiru.)

The ex-planet Pluto is easy to image as are many of the other dwarf planets; what about this new denizen?

Factors that dictate a planet's total brightness (as opposed to its surface brightness) are its distance from the Sun and from us, its average albedo, its size, and its phase. In the case of these distant objects phase is essentially always full and can be ignored; likewise the 2 AU annual change in the distance from Earth is neglected. To keep this simple I'll make a comparison between X and Pluto to estimate the former's relative brightness and magnitude.

Lets's start with some simplifying assumptions and say that X and Pluto have the same albedo and that X lacks a moon that contributes significantly to its brightness. (Charon represents about 1/5 of the Plutonian system brightness--if Pluto were to become invisible Charon would be easily imaged!)

So what are the speculated physical characteristics of X?

Size: Larger than Earth, smaller than Neptune. If it's simply a scaled up version of Pluto with 4500 times the mass, it should be about 16 times the size of Pluto. this puts it about 2.8 times the size of Earth and about 73% the size of Neptune. Close enough. This gives X a reflecting cross section about 256 times that of Pluto; we'll knock that down to 200 to account for its assumed lack of a bright moon.

Orbit: There's a lot of uncertainty here; the suggestion is that as X's orbit caries it between 200 to 1200 AU from the Sun. Unfortunately no one knows it's present distance, so we'll make estimates for both the extremes. At present Pluto's distance to the Sun is about 33 AU and it's magnitude 14.2. The distance factors are (33/200) to the fourth power and (33/1200) to the fourth power for perihelion and aphelion, respectively, using the assumption that X will resemble a point source such as a star.

Perihelion: 0.148 the brightness of Pluto, giving it a magnitude of about 16.3. This is easily within the range of amateur imaging.

Aphelion:  0.000114 the brightness of Pluto, with a magnitude of about 24.1. This is probably beyond amateurs. The deepest stars I've ever imaged are near magnitude 20; X would require a total exposure time about 40 times greater than that for a 20th magnitude star to image at its most distant point.

That perihelion magnitude is encouraging, but there are complications. Not much is known about X's orbit, so even if X is near perihelion we don't know were to look for it. Given that its orbit is probably fairly eccentric it will usually be found closer to aphelion, and therefore is usually very, very dim. And don't even think about waiting for the next perihelion, the orbital period is many times a human lifetime.

My recommendation is to leave X to the professional deep sky surveys. On the other hand, if you're feeling really lucky and have a whole lot of time, you could be the next Clyde Tombaugh.


It's been a miserably cloudy winter here. I've had only one night out to image, and that was to try to catch Barnard's Loop. Overhead power lines and a neighbor's lights really degraded the image.

Barnard's Loop (in hydrogen alpha)
 As you can see, above and left of center there are diagonal artifacts thanks to the power lines and at lower right, air traffic. Too much signal was lost trying to reject the obstructions. If the weather improves I might get another chance this winter.