PoleMaster--A Quantative Estimate of Accuracy

It's one thing to say how nice a PoleMaster works based on a practice run indoors, but the real measure of its usefulness is the accuracy of its polar alignment (PA). A practical measure of PA accuracy can be found by examining images of either long exposure time or a sequence of short exposures taken a considerable time apart. The other night I was able to do the latter by taking 4 minute exposures during the course of about 1.7 hours.

To get an estimate of PA error one needs to know the declination drift rate and the declination of the target.  A good reference for how this is done is an article by Frank Barrett, "Determining Polar Alignment Accuracy."
 
I shot 15 light frames using autoguiding and then aligned the first and last in the sequence using ImagesPlus. A nice feature of IP is that it tells you the field rotation it had to apply during frame alignment; when aligning two frames, it tells you the rotation it performed to bring the second frame into alignment with the first.  In my test case the rotation was 0.02 degrees (72 arcseconds); the elapsed time between the start of the two frames was 65.3 minutes.

Given the above data (and the declination of my field of view) Barrett's Equation 1 yields a polar alignment error of 4.4 arcminutes, which is almost 9 times the maximum accuracy suggested by QHY, the maker of Polemaster. Some caveats are in order:

• This was the first time I used PoleMaster. As with most things, I expect the quality of my alignments will improve. I did use the rough and fine polar alignment methods.
• The coarse reporting of rotation angle by IP means the estimate could be between 3.3 and 5.5 arcminutes.
• The rotation reported by IP is sensitive to the choice of alignment points, and will vary substantially when close to zero like it is here.

Should I be disappointed that the accuracy was not what QHY suggested it might be? Not at all--it's actually quite good if you're autoguiding!

Consider what happens when you guide. The guide star is held motionless in or near the field of view. An error in the polar alignment will cause the FOV to rotate around the guide star. The distance the stars rotate will be proportional to the alignment error and the distance from the guide star. A guide star at the center of the FOV will cause the greatest distortion of star shapes at the four corners of the field.

Since this will get highly mathematical quickly, I'll try to make a worst-case example and see how bad it can be. I'm going to assume that the guide star is within the FOV (as it was for my test case) and located at one of the corners.That star will remain fixed thanks to guiding, but the image will rotate slowly around it because of PA error. In a long exposure this motion will result in oblong or streaked stars, with the worst effect at the greatest distance from the guide star. This will be at the corner opposite the guide star.

Let's set an arbitrary limit of a star being oblong by one pixel. Any more than that and we won't be happy.

The distance between opposite corners is calculated from sensor dimensions in pixels; for my DSLR it's 6,230 pixels. The rotation rate calculated above (0.02 degrees in 65.3m) corresponds to 0.0000053 radians per minute.  The tangential star movement is therefore R * 0.0000053 pixels per minute, which equals 0.033 pixels per minute. Take the inverse of this to find the time it would take for the star to move one pixel: 30 minutes. Therefore one can expect to be able to use 30 minute exposures and have only a one pixel of star elongation at worst when the PA error is around 4 or 5 arcminutes.

Keep in mind that this is only a very rough estimate, and it ignores how the effect varies with the parto of the sky being imaged.  To minimize field rotation effects, follow a couple of rules:

• Keep your autoguiding scope reasonably well aligned with the axis of your imaging optics; if possible choose a guide star near the center of your image
• Always align light frames with both translation and rotation

The idea that a 4 to 5 arcminute PA accuracy is good matches my impression from the images I collected. My polar alignment is usually obtained using an impatient application of the drift method assisted by PHD2 guiding. It's rarely ever the case that stack of subs doesn't need some minor cropping to get rid of field rotation effects at the edges of the FOV.

My first time imaging with PoleMaster produced no edge effects. That's a huge improvement over my usual polar alignment. Combine that with the ease of using PoleMaster and it's clear to me that it represents a great innovation for astrophotography.

Polemaster--Better than advertised!

It's been a week of disruptions and minor mayhem here, with clear nights that have been out of sync with my ability to take advantage of them. Until last night, that is.

My new Polemaster polar alignment tool worked quickly to give me a good polar alignment (as yet unconfirmed photographically). This was my first time trying it and I doubt if it took more than ten minutes to go through the basic and precise alignments. Some comments:

• You initially need Polaris in the field of view. All I did is level the mount, set the altitude for my latitude, and get it eye-aligned with north. No bending down or stooping to sight through the polar axis. This brought Polaris into the field, although near the bottom.  I adjusted altitude and azimuth further to roughly center Polaris before beginning the alignment process.
• Don't be put off by the coma you see around stars that are away from the center of the field of view. This isn't an imaging device for making pretty pictures; think of it more as a star detector. The coma shouldn't enter into the centroid calculations in a way that matters to the result, anyway.
• Under the glow of my inner red-zone sky the device had no trouble finding the needed stars; The field looked best with the gain set to maximum. 
• Several times you are asked to use the software to rotate the field of view. The rotation steps are a little coarse, making it difficult to exactly center stars in the target display circles. I don't think this matters much at all; all you need to do is get it reasonably close to the center.
• At one point you use your hand control to rotate the field of view and see that a star stays on a displayed circle. If it goes off the circle you have to start over, but with modest care when specifying stars (using double clicks) the star will stay right on the circle.
• Unless the manual has been rewritten, ignore it for the actual process of aligning. Instead follow the on-screen guidance--it's clear and perfect.
• The USB cable is kind of short, but not so short as to cause a problem. Next time I use it I'll try adding a short USB extension cable.
• When I was done the precise alignment indicator (showing a tiny box and circle whose centers will coincide when alignment is perfect) suggested that I was within atmospheric limits of perfect. Simply touching my mount would lead to a shift away from perfect.  This makes me wonder if the process is best performed when the mount is already loaded for imaging.

In summary, the Polemaster alignment procedure was smoother and simpler than I expected. Although I don't yet have tracking data or an image to confirm the quality of the polar alignment, I'm confident it was at least as good as most of my manual efforts using PHD or visual drift. And it was much faster and easier!

Micro Update

It may be clear tonight! That means I can test my new PoleMaster. I probably won't do any imaging because I'll be in my light-polluted back yard. I could travel to Cherry Grove observatory, but the recent heavy snowfall (about 15 inches at the observatory) has probably made the site unusable.

Tomorrow I hope to add a glowing review of the PoleMaster to the many that have been written.
 

A New PoleMaster Waits for Clear Sky; Step-down Rings

Another inch or so of fresh snow here as March begins, along with continuing clouds at night.

PoleMaster update: My PoleMaster (PM) arrived and all is well so far as I can tell without some stars to try it on. I'm not going to give you an "unboxing" description other than to say it arrived quickly from OPT and in perfect condition. Withing the cardboard shipping box QHY encloses the PM in a tin box. I'm not sure if this is supposed to be for storage or marketing effect.

Some commenters have mentioned that the included USB cable is a little short. I don't think this will be a problem because the PM mounted on my CGEM's polar axis port is basically stationary.

A really nice touch concerning the cable is that it attaches solidly to the PM using two small thumbscrews. I hope this helps correct one of the banes of using the Orion StarShoot Autoguider camera, which seems almost eager to drop its connection during polar alignment.

The adapter for my CGEM mount fits perfectly; The PM camera locks with ease onto the adapter. The Camera sits on the mount without any play and can be removed easily when alignment is done.

The PM manual is every bit as difficult to understand as people have said. I would guess a better translation is in the works--it's needed!

I'll say more after I actually use the PM.


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It's common practice to stop down a lens when imaging with in order to give sharper stars and less chromatic aberration. This is usually accomplished using an iris made of metal vanes that form an adjustable diaphragm. Because the inside edge of the diaphragm resembles an equal sided polygon, it causes a spike-like diffraction pattern that can be very evident around bright stars. here's one from the image I posted last time:

A Most Imperfect Star

The purple blob just left of the star is an internal reflection from somewhere in the lens. Multi-coating can only do so much, apparently. And you can see the dimmer stars suffer from shape and aberration problems. It's a $40 lens, so I'm not expecting anything close to perfection.

The spikes in the image are from stopping the lens down to f/5.6 from its wide-open f/4. Perhaps you don't mind spikes like these in your images or think that they add esthetic appeal.  If you don't like them, or simple like round stars better consider using a step-down ring (SDR).  An SDR attaches to a lens just like a filter and act as a bladeless diaphragm. Here's what one looks like:

55mm to 37mm Step Down Ring

To figure out which one is right for you there are two numbers you'll need to know: The filter size for your lens and the iris diameter for the focal ratio you plan to use. Let's take my Tamron 135mm lens as an example.

We begin by finding a little circle with a vertical line through it on the lens. It looks like a Greek letter phi and will have a number next to it.  Usually this is found on the specs ring at the front of the lens, but on my Tamron it's on the side of the lens. As it turns out, my Tamron takes a 58mm filter.

I find that the Tamron works well at f/4 rather than its wide-open f/2.5. The f/4 objective diameter of a 135mm lens is just 135mm divided by 4, or 33.75mm.

Therefore I would use a 58mm to 34mm SDR. Easy! Or maybe not.  SDRs don't come in every possible size, so you may need to take an inner ring size that's not quite what you want or get creative by using multiple SDRs.

Because SDRs have threads on the inner circle it's possible to fit one into another. It happens that the SDR my Tamron wants is not one I could find. There is a 58mm to 55mm SDR, a 55mm to 37mm SDR and a 37mm to 34mm SDR; used together they give me the 58mm to 34mm I want!

As it turns out, my Zuiko 200mm lens needs a 55mm to 37mm SDR, so I have two reasons to buy it. And if I want, I can use the two smaller SDRs on my Zuiko to give it a focal ratio of f/5.9.


How will this all work out? I'll let you know when the SDRs arrive from Ebay and the sky clears!

The delight of BackyardEOS, A rare night in February

Last Friday night was an exceptional evening in Minnesota.

• The Moon was out of the sky from dusk to almost 1 A.M.
• It was clear
• The dark site observing field was essentially snowless
• It was an incredibly warm evening--by 1 A.M. it had fallen only to around 40F.
• The wind was for the most part very light to calm, so there was no real wind chill factor.

 Best of all my evening was free!

It's difficult to emphasize how rare that evening was. The February 17 average high and low for nearby Cannon Falls is 29 and 10, so it was a good 20 degrees warmer than average. Usually the warmer weather correlates positively with clouds, too.

I originally intended to image the Witch Head nebula because my previous image of it is rather poor; noisy and crossed by an amazing number of geosynchronous satellites. A slow start to the evening made me go with something a little easier--the far brighter M42 area. It turned out better than i expected for 2.1 hours of 5 minute exposures at f/5.6:

I was able to faintly capture some of the dimmer clouds in the outlying areas just to the left of M42. The red patch at the upper right of center edge is an extension of the Horsehead nebula area.

When M42 began to sink I went to comet 45P and got almost an hour and a half of that:

This wasn't deep enough to get even a hint of the fainter, bluer tail. Oh, well...I'll take it!

One of the best things of the night was using BackyardEOS for the first time. It's a joy to use, and despite its power it keeps things easy to use. Next time I'm going to use it to tackle the mystery dithering!

I also got a demonstration of PoleMaster, a hardware/software tandem that makes getting an excellent polar alignment easy, fast, and actually fun (well, compared to drift aligning, anyway). It's definitely on my to-buy list ($300 from OPT). I would really like to take some very long light frames this year in Nebraska, and this would help. Even though PHD2's polar alignment utility is slick and gives good results, I can see how much easier (and probably better) PoleMaster would make getting a good PA.

Workshop presentations

The topic at the recent workshop was guiding for astrophotography, and a club member sent me a link for a really good explanation of how to guide.

Here it is (It's for PHD 1.13, but although some things have changed with PHD 2, this remains a great read!

The folks working on PHD 2 have also prepared a guide for using it. It's not as comprehensive a treatment as the above source gives, but it's a fine explanation of PHD 2 usage.

Want to download PHD? Here's where you go:

PHD 1

PHD 2

I recommend PHD 2 because of its new features--I particularly like the drift polar alignment tool. If you have multiple guide scopes, guide cameras, or imaging mounts, you may like the ability to create profiles for different combinations of those. I've found version 2 to be more stable than version 1, but that may only be on the OS I use (Windows Vista) I hope to upgrade my imaging laptop this year to a Windows 10 model and will have more to say when that happens.

Brief update

Last night was our club's second imaging workshop. The topics were guiding and focusing, and I can't say I did a good job as the sole presenter. The audience was amazingly diverse in interest and skill levels, and I had been given all of 24 hours to prepare. Regardless, it seemed as if most had a good time and there was a sense that we should meet more often.

After my presentation we went to the observatory and tried to do some guiding. The club scopes use ST-80/Starshoot Autoguider combos for guiding, and the software is good old PHD. At first we couldn't get the guiding working; the scope was wildly out of focus. Once past the focus issue, the mount simply refused to respond to movement commands from PHD during calibration.  As it turned out the SSAG to mount cable had gone missing; it may have been removed when the scope pier was replaced and never returned. A hunt failed to turn up the cable, but luckily I had one with me and we were finally able to get the mount guided. By this time most of the attendees had dispersed (it was 15F) so not much more happened.

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Next weekend is a candlelight skiing event at a central Minnesota state park, and I'll be shooting some demonstration images to complement views through a friend's refractor. I really hope it won't be as cold as it was last night!