Quote:
Originally Posted by Startrek
I’m also interested in your polar alignment method too
With no computers or cameras how do you measure your polar alignment error ( drift time and laser
angle ) ??
I’m assuming within 4 minutes your under an arc minute ?
Curious !!
Thanks
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Hi Martin,
I don't measure the PA error so I can't tell you how many minutes of error I have. As you note, with no computers or cameras I can't measure polar alignment error.
Over a couple of hours of subs, I'll see dec drift but not within a sub.
The attached diagram shows the three stars I use. The little circle is 1 arc minute radius. It's not hard to get pointed somewhere around or just over 1 min using this method.
The trick is to keep parallax small by keeping the polar axis & optical axis of finder very close to the laser beam and the binocular line of vision. Working at home, I place the laser on a tripod right in front and almost in line with the PA and I stand just in front of the laser and have it shoot past my ear. I often use 7x35 German binos. The 8.9 mag star is a little hard to see but I can use the two brighter stars. This is fine when shooting with 135 f2 or 300 f4 lenses. For the prime focus, I'll use my 9x63mm Orion's and use all three stars. I have a tripod mount for them.
There is no graticule so the accuracy does depend on your visual geometry ability with the two stars and of course, how accurate the polar finder is aligned to the polar axis. It works for me, in particular because I use a DSLR driven by an intervalometer, setting up a computer and camera is all extra work, extra wiring etc. I also used to work from an unpowered site with lots of dew so I started working this way to be able to work all night off a small 7AH SLA battery or a small car battery if running dew heaters. Now, although I have AC power in my backyard I have continued working the same simple way.
If you use an ASI air, Sharpcap or similar and have to set up computer and camera anyway, then the time & trouble saving is somewhat less than for me. I can look outside at midnight, see clouds have cleared. I can be set up and PA'd in about 30 mins shooting soon after. My DSLR runs for about 5-6 hrs on one battery, intervalometer batteries last many nights and if I don't slew much, can run most of a night on one 7AH SLA.
I'm not trying to sell these to anyone. The design is so simple, patenting would be a waste of time so I put details on my website for anyone to use. You can build one without a lathe or any machining. You could make one with glued and sanded dressed wood and use Danish wax, candle wax or soap between the surfaces to lube the wooden bearing surfaces. The key is that those surfaces need to slide smooth and stop where you leave them so you can point accurately.
Feel free to PM me if you want to build one and have any specific Q's.
Joe