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!