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!