2009
11.28

Go ahead…  make my day.  Tell me how good or bad your weather is.  Lemme tell ya’, this PNW weather is dragging me down.

I’ll start,  It’s Nov 28.  The last chance I had  to do some astrophotography (actually just good enough to do some mount testing) was on the 13th, the time before that was the 10th of October.  Now you know why I’m workin on the web site and blog so much….  no long nights to sleep off, no images to process.  <heavy sigh>….

al

2009
11.28

I’m going to touch on this subject with a few things I’ve found to keep me warm.  Well, more accurately, things that help lower the volume of my teeth chattering.  Please chime in with any things you’ve found that help you stay comfortable through cold winter nights.

First, some guidelines:

  • Don’t wear cotton next to your skin, and minimize it elsewhere as well.  Cotton holds water, and when damp, the fabric loses it insulating value.
  • Use layers of clothing.  This helps build in insulating boundaries and allows you add or remove layers to regulate your temperature.
  • Keep your head and feet warm.  You can lose a lot of heat from your head.  You’ll feel a lot warmer if your head is warm.  Your feet are in jeopardy because the body will constrict blood flow to the extremities trying to keep your core warm.  So you need to pay extra attention to keep your feet happy.

Some specifics:

  • Head: Get some close fitting headgear that has good insulation.  I like wearing a thin poly balacava to keep any breeze at bay, and then a fur lined insulated bombardier’s hat over that.  Keeps me toasty.  I say close fitting just so it won’t flop around and get in your way while you’re doing “telescope yoga”.
  • Feet: Remember. NO COTTON.  This is really important here.  Feet sweat a lot, and shoes tend to trap any moisture.  So:  think thin poly socks against your skin, then some wool or smart-wool heavy boot socks, and good insulating boots.  Inexpensive  “after-ski” boots may be just fine, I’ve used Ugg Boots, and of course there are some hunting boots with very high insulation values…  but they’re pricey.
  • Underwear: Wear longies.  Not cotton.  Polyester blends should be okay.  Just stay away from fabrics that absorb water.
  • Outerwear:   follow the guidlines.  Layer, no cotton, wind proof an outer layer.  A nylon shell over some layers of polar fleece or the like should do well.  One of our local club members loves his ski bibs…  seems like a good choice to me.
  • Gloves: Mittens will keep your hands warmer than fingered ones.  If you poke around you can find some that have a “flap” that can be folded back and expose your fingers so you can work or type.  I’m going to buy a pair.

Accessories:

  • Hand warmers: These are little packets that get warm when exposed to air.  They last quite a while (some heat left even after 8 hours).  They also make bigger packets made to fituonder our toes in your shoes.  Note also that these hand warmers work great to warm up the LED of your hand control so you can see the display.  I use rubber bands to hold a heat pack against the back of the hand control…  works great.
  • Portable Heaters: It’s nice to have a real warm spot nearby.  I sometimes take a catalytic propane heater with me.  Catalytic heaters are very fuel efficient and feel really good.
  • Battery powered Clothes: Socks, vests, and whatnot.  I’ve never used ’em, but if the batteries last I gotta believe it would be great.

That’s all I got for now… Please comment on your favorite ways to stay warm,

al

2009
11.27

Batteries

Going remote with your Astrophotography Rig? You’ll need power….  Quite a bit of it.  Most people won’t want to have a generator ruining the peaceful solitude of the night, so battery power is the usual solution.

So how much power do you need, what kind of batteries will do, how to connect to them, how to house them, how to charge them….  all are good questions.

How much power (capacity) do you need: This part isn’t hard to figure out, just be sure to count everything that needs power.  Here’s some rough guidlines:

  • Mount: less than 0.5 A while tracking (that’s what it’s doing most of the time).
  • Laptop:  I’ve measured a couple.  I got 2A for each of them.  You could measure your’s with a wattmeter (just divide the watts by 12, then again by 0.9 to account for inverter inefficiencies)
  • USB devices:  Unless I know otherwise, I use 0.5A for each device.
  • TEC coolers in cameras:  I use 1.5A for single stage units, 3A for two stage TECs.
  • Dew Heaters:  Check your manual.  I use my home made ones, and they are sized to use 0.5A each.

So, I conservatively  calculate 6A steady:  Mount, Laptop, 2 USB Cameras, one TEC cooled, 2 dew heaters.  In actuality, I think my steady draw is about 5A.  The difference is mainly that there isn’t quite as much draw for my dew heaters, and the TEC cooler is usually drawing at 40% or less of it’s full power draw. However, the cameras and laptop are powered through and inverter so there’s some loss there, maybe 15% more current used for those items.

So, now I know about how much current I’m using.  To determine how much battery is needed, I’ll need to now a bit about how batteries are rated..

How are batteries rated: Battery capacity is measured in Amp-Hours….  That is, how many amps for how many hours.  So you might say I need 6 amps for 10 hours, so I need a 60 amp-hour battery right?  It’s not actually that simple.  Battery amp-hour ratings are typically calculated by determining the current that will discharge the battery in 20 hours.  For example, a battery that becomes discharged in 20 hours with a current of 3 amps, would be a 60 Amp-hour battery.  The bad news is that batteries do  not behave linearly.  If for example, 6 amps were drawn from our 60 amp hour battery, it wouldn’t last 10 hours, it will become discharged much faster and might last maybe only 5 hours.  Likewise if less than 3 amps is drawn it will last quite a bit longer than would be calculated.  For a bit more detailed discussion, check these links:

You can see the rating systems are kinda mixed up and complicated.  However, there are a couple decent guidelines.  They are:

  • Suitable batteries for our overnight work will likely be labeled Deep Cycle.
  • Check the “Reserve Capacity” rating.  A rating up around 2 hours is good.
  • Use the Amp-Hour rating as a guide.  Check that the Amp-Hour rating divided by 20 is a number close to or bigger than your steady state draw.

These things together should get you a battery that will last though the night like you expect, AND be able to be cycled (discharged/charged) many times meaning you’ll get a battery that will last a long time.

Connections: I hate those cigarette lighter plugs.  I’ve had them get nudged and disconnect at the worst times.  I still use them when I have to because I have too many cords and too many configurations.  I like to use molex connectors (the white plastic connectors you find in computers).  I wire them with the ground to the two center pins, and the +12V to the two outer pins.  These are quite secure and cannot be plugged in incorrectly.  So my batteries have a mix of connectors:  Cigarette lighter, Molex, and perhaps a specific type to fit a particular mount.  There are also very good connectors available at hobby stores that are modular and indexing (can’t be reversed).

Where to buy batteries: Well, there are a lot of places.  I’ve found several online stores that sell scooter/wheel chair batteries.  These are nice in that you know they are expressly built to withstand many charge/discharge cycles.  Marine deep cycle batteries are good choices.  I like buying from places where you can get complete specs.  Sears is nice for that.  I have heard several bad reports about Everstart batteries (Walmart), but I also heard of several guys using them without incident.

Boxes: Well, suit yourself here.  The only advice I have here is to use a plastic box that is resistant to battery acids.   Here’s what Mine look like:

SmallBatt

Small Battey 40 AHrs

Big Battery

Big Battery with inverter for AC power

Chargers & Charging: I’ll keep this short.

  1. Don’t discharge your battery to below 10.5 volts.  It will shorten the battery’s life, perhaps drastically.  Additionally (fortunately in a way) your mount and photo equipment problably won’t be working well if your battery drops close to this level anyway.
  2. Recharge your battery as soon as possible after discharging.
  3. Get a good float charger (battery maintainer) to keep the battery at the proper state of charge.  The details of battery charging can get complicated.  Google the subject and you’ll get more info than you want to know.

Whew…  If you have corrections or additions please comment to this post.  Also, any recommendations for batteries that serve you well would be appreciated.

al

2009
11.25

Oh geeze….   PE.   Now this is a horrid subject.  Here’s an introductory guide.

This discussion will refer to errors in the drive systems of  German Equatorial Mounts (GEMs) and particularly to those that are driven by a worm gear arrangement.  The basics and principles will of course also apply to other drive systems, but the analysis has to be done in relation to the specific drive type and configuration.

What is PE? Periodic Error…  virtually all mechanical drive systems have error, they are not perfectly smooth.  Some of the errors in motion are “periodic”, meaning the error repeats at a regular interval or period, hence the name.  Usually, the largest periodic error in a GEM mount  is due to the rotational period of the worm drive of the RA axis.  This is because the machining of the worm is not perfect.  There are surface defects on the contact face of the worm, and there will be some deviation from a geometrically perfect worm shape.  As such, there will be tracking errors that will cycle and repeat with every rotation of the worm.  It’s worth noting that one rotation of the worm advances the worm-wheel one tooth, from that you can see why we don’t talk about the periodic error of the big wheel (since its period of rotation is one sidereal day ;)).  It is possible to see periodic errors at 2 or 3 times the worm rotation rate due to miss-machining (like miss-spacing) of the teeth on the wheel.  I’ve seen this on other machinery, but never heard it mentioned for a scope mount….  doubt I ever will.

Often folks refer to PE as PEC which is actually Periodic Error Correction.  PEC is typically a software routine built into the mount control software.  When PEC is enabled, the control software tries to counteract/correct the PE of the mount by using a “map” of the mount’s natural PE which is recorded and stored in the control memory.  We’ll talk a little more about that later.

The amount of PE your mount has may or may not be important to you.  If you’re a visual observer, and objects don’t wander in you eyepiece too much, then why worry?   However, if you’re doing long exposure photography, then having a low PE and controlling it well is very important.

Measuring PE

Measuring the PE of your mount is not hard and there is software freely available to help you analyze the results of the measurement.  I’ll caution here that there are a lot of variables that effect PE so don’t make decisions (like deciding to tear down and rebuild your mount) based on only one night’s measurements.  Get several runs of data and look for trends and common traits.

The easiest way to measure PE (or to record PEC curves) is to use an imager suitable as a guide camera along with guide software to create guide logs.  This is usually done with everything working just like you’re guiding, except the correction signals are not actually sent to the mount.  What this does is records how far the guide star moves around it’s original position.  As example you might let the mount track for about an hour (guider up and running, tracking a guide star, but not sending corrections) to get data for several rotations of the worm into the guide log.  Then the data in the log would be graphed and analyzed.

It doesn’t really matter what scope is used, most any should work although I’d avoid long FLs so you can keep the guide star on the chip.

Software: There are some free programs available to analyze the log data.  Two notable ones are PEAS and PecPrep.  Which program you use will depend somewhat on what guide program you have, which in turn determines how your guide log data is formated.  I usually use PEAS.  Two reasons:  1) it uses templates to read the guide logs, so it’s easy to update it for different versions of guide software and, 2) I like the simple FFT results shown on the same graph.

Here’s a sample:

PE_GM8_MockupSM

This is not an actual PE measurement, but is representative of what you might see.  This shows about 4 worm cycles on my GM8 mount where each cycle takes 480 seconds.  Note that the “Smooth data” block is checked and the “Raw data” is not.  The raw data shows the position as captured, and adds a lot of “noise” to the graph.  Here’s the same data showing the Raw data:

PE_GM8_MockupRAW

The jagged noise seen in the raw data is a combination of the Seeing and small vibrations in the drive system.  It’s good to try and lower the amount magnitude of the noise, but how to analyze and correct for this is more than I should put in this post.  It involves identifying what parts of the drive system are causing the largest problems by using the FFT information, then finding ways to correct the issues.   I’ll write a post on the subject and review my experiences chasing these issues another time.

A small issue to overcome is getting the guider log file ready to be read by the analysis software.  Typically, when you get your guider up and running, there will be several starts and stops as you calibrate; maybe to change targets, readjust the camera, or balance, or something…  as a result your log file will have many short sections that you don’t care about.  Additionally, you may have two or three sections that you do care about or want to compare.  An easy way to deal with it is to just copy the areas of interest into individual files that contain just the data (no run info or column headers) and name the files to help identify what’s what.  For example, lets say I did some PE measurement first, then trained PEC, then shot images of M33 for the rest of the night.  I might cut up the logs and save the data of interest as:  PHD_CGE_PEraw.txt  PHD_CGE_PECTrng.txt, and PHD_CGE_M33.txt.  The content of the files would look something like this:
_________________________________________________
1,3.746,-0.17,1.32,1.7,450.1,1.19,0.0,-0.19,123518.54
2,7.772,-0.20,1.19,1.7,449.3,1.19,0.0,-0.22,128314.44
3,11.787,-0.18,1.80,1.7,655.3,1.73,81.9,-0.21,127999.39
4,23.334,-0.15,1.83,1.7,686.3,1.82,70.8,-0.18,128122.90
5,27.350,-0.03,1.62,1.6,620.9,1.64,0.0,-0.05,124805.48
6,31.346,-0.29,1.64,1.7,619.4,1.64,125.1,-0.31,126909.70
7,35.812,-0.25,1.55,1.7,587.5,1.55,110.2,-0.28,125283.11
8,39.828,-0.16,1.66,1.7,622.5,1.65,73.7,-0.18,49266.85
…..  and so on for hundreds/thousands of lines…
________________________________________________

(You’ll of course already figured out if the format can be read by your software, or set up a template so it can.)

Evaluating PE:

When evaluating the PE of your mount, compare it to what others get on that model or class of mount and to previous data runs on your mount.  For example, you might be wanting to know if your mount is behaving badly compared to others, or if some work you did made it better, or if setting the balance just so had an effect, etc.

There are two main things to look for in the PE curve:

  1. The magnitude of the PE curve.  Such as  +4/-2 arcsecs (that’s 6 arcsecs peak-to-peak) in the fake example above.
  2. How smooth the curve is.  E.g., whats the magnitude of the noise (which looks to me like 10 arcsecs p-p in the example above), and are there any big jumps that might indicate a serious problem with the worm.

The main thing is that mounts with pretty big PE may still be able to be guided well if the curve is regular and pretty smooth (by the way the mount in the example above guides just fine, even with that amount of noise).   Don’t panic just because you mount’s PE is larger than you’d like as long as there aren’t sharp changes in the curve that would disrupt guiding .  If the PE is much bigger than what other folks are getting on that mount, then it’s time to look into it.  The first place to look would be checking the mesh/alignment of the worm to worm-wheel.

Last topic… PEC.  Your mount may or may not be able to do PEC.  As I stated, PEC is a way the mount controller reduces a mount’s PE by making little corrections based on a map (a PEC curve) that’s input into the controller by the user.  The PEC curve can be generated different ways, and typically the curve will be an average of several PE curves indexed and added together.  Again, the PEC curves are usually generated using a guide camera, but they can be done with a reticule eyepiece.  Anyway, if your mount supports it, how to build the curves should be well covered in the manual.  BTW, I’ve used the batch training mode of PECTool a program made to do the training on some Celestron mounts, and it is very easy to use.  Very Sweet.

I think that’s enough for this post.  If you have questions or comments just ask here or send me and email and I’ll do my best to answer up.

al

2009
11.24

I worry a lot.

It seems there are a lot of folks that have trouble with hooking up their guider correctly, things not working right, and even damage to the mount and/or camera….  So I worry.   After I got my Losmandy mount with the Gemini controller I reviewed the manual carefully and read lots of conflicting posts on the subject trying to be absolutely sure that I wouldn’t damage anything (in my mind, downtime + repair cost = BAD).  What I read didn’t make me feel secure. Although I’m pretty sure an SBIG ST series camera guide port can be hooked up directly to a modern Gemini without a problem, I’m not absolutely, positively, completely sure.  The issue has to do with forming ground loops between the power supplies to the camera, laptop, and mount. It seems like there’s a possibility that the signal grounds of the guider could return via different paths and form a ground loop.  Not good for guiding, an potentially dangerous.  And as I think is abundantly clear,  I worry a lot.

So, I decided to optically isolate the camera guide port from the Gemini.  I found that I could buy a optically isolated cable ready made for $70 from Tom Hilton (www.arizonaskys.com). However, being cheap, I decided to build one myself.  I’m not actually advocating building one since it’s a bit of work, and probably only saved about $50, but still, I like these projects. Plus, going through the process made me really understand how the guide circuits work.

The first step was to find how complicated a optocoupler circuit would be. I found a good diagram here: SBIG_opto_isolator.jpg (I also found an almost identical circuit from Mike Dodd posted on either the Gemini or Losmandy user groups on Yahoo).  Anyway, although the circuit might look complicated at first, with a little study you see how easy it is.

Power to run the opto-isolating IC chips is provided by the camera. For each direction, the power is supplied to pin 1 of the chip and returned to the camera direction pin via a 150 ohm resistor. An optional LED is also in the circuit to show when that direction is signaled (very cool feature).  What happens when the camera “signals” a direction, the appropriate pin on the camera is shunted to ground.  So you can see what happens is that completes the circuit which would energize the chip and light the LED.  It’s that simple.  Then the output of each chip is connected such that when energized it will shunt the corresponding signal pin on the Gemini to the gemini ground. Thus there is NO direct ground or voltage connection between the SBIG and the Gemini. Easy!

So, to build it, I got a 6 conductor phone extension cable from Radio Shack and I cut a length off one end. I already had the LEDs, resistors, little circuit board, and a male DB-9 connector, but they can be had from Rat Shack as well (total cost should be less than $15). The optocoupler ICs I bought from Digikey, This cost about $3, but then there’s the $7 worth of shipping (bleh). For the box to put it in, I used a case from a wall wart power supply (might have been a phone charger or something) that I picked up at Goodwill for a dollar or two.

This is what it looks like:

OptoBox

And this is how it looks on the camera:

OptoCam

I used it for the first time a few weeks ago.  It works great, and having the LEDs indicate each time a signal is sent is really cool.  I can tell if there’s a problem with my calibration, or if my DEC is pushing back and forth, or whatever….   This actually happened during my first use.  I wasn’t familiar with how the SBIG software (CCDOps) calibrated and that gave me trouble.  What was happening is that after CCDOps reported a successful calibration and I started guiding, my guide graph looked lousy, specifically in one direction.  Just by looking at my box it was obvious there were no pulses being sent to one of the directions.  A little checking showed me that that the software had turned off guiding in that direction.  Turns out it happened because the calibration star went off-screen during the calibration process.  Okay… re-do the cal process making sure the star stays on-screen and all was well.   Bottom line, I like the indicators.

Good deal, the guider is totally isolated and works fine. One less thing to worry about.

al