Wednesday, July 20, 2011

Another improvement

Now that I have my 15-inch in operation, I turned my attention to the 10-inch Dob, which needed a few improvements. Adding the threaded inserts to the base was the first, with dramatic results. I have since then rolled out the entire telescope and been observing in just a few minutes. Indeed it takes just as much time to bring the eyepieces and observing chair outside if not more than it does to roll the telescope out into the driveway. This is very helpful when I want to observe after returning home from work, I can observe for a hour or two before rolling the telescope back into the house and going to bed.

The second major improvement was the addition of a holder for the 12-volt, 18 Amp-hour battery I have been using for several years to power the dew heaters, cooling fan and digital setting circles. I have been simply laying it in the base after carrying the base, then the tube outside in two trips. However, an encoder for the digital setting circles is on the azimuth pivot bolt, and when I lift the handles to roll the telescope, the battery will slide towards the front of the rocker box and hit the encoder.  That will destroy it eventually, so I bought some steel flat bar 1/8th of an inch, or about 3mm thick and MIG welded an upside-down cage for it. Brass threaded inserts in the base secured with epoxy glue and 3/4-inch  or 19mm long quarter inch (6.35mm) stainless steel hex bolts secure the holder and batter to the bottom of the rocker box. After welding, I smoothed down the weld beads with a grinder and painted it with flat black paint. The holder can be removed at any time with a nut driver or wrench, but I will leave the battery there and charge it in the base after each observing session.

A third improvement will be sanding down the base to remove the old, degraded varnish and replacing it with three to four coats of high gloss, exterior grade polyurethane. Also known as "spar varnish," this coating is the best for telescopes because it's durable, withstands acid dew, moisture and frequent, severe temperature changes without degrading. It's also mechanically tough and easy to apply. When it gets scuffed and marred from handing and use, simply sanding it with very fine sand paper and adding another coat will make it as good as new.

Another improvement was repairing the cooling fan. I was simply running a lead directly to the switch box I made, but the soldered connections kept breaking. Then I purchased a Dew Buster, which works much better than anything I could build. So I bought a small project box, a toggle switch, a red LED and holder, grommets, lugs and a terminal strip. I made a switch box with all connections are made with loop style lugs, and the small wire was soldered to the lugs before being connected to the terminals strip. The result is a neat installation where the connections will not break or pull loose, with a red LED lamp to show when the cooling fan is on or off.

I have also been considering the matter of eyepieces for the 15-inch, whose much narrower field of view than the 10-inch makes finding and tracking objects more difficult. I decided that acquisition of a set of ultra-wide angle eyepieces was called for, so I started looking at the various brands and came across the Explore Scientific line of 82 degree eyepieces. As the model name states, they have an 82 degree apparent field of view, so great in fact looking though one is not at all like looking through an eyepiece, it's more like peering out of a space suit's helmet or a window aboard a spacecraft. These eyepieces have an excellent fit and finish, and one feature that even the top of the line TeleVue and Pentax eyepieces do not, sealed and nitrogen purged interiors. In other words, they are water proof, dust proof and sealed against anything getting inside and onto the interior lens surfaces. Here mildew and other contaminants are a real problem, and sealed and nitrogen purged construction is a plus.

I bought and tested twice a 14mm 82 degree eyepiece last Saturday night and again Tuesday morning on the waning gibbous moon, various deep sky objects as well as the planets Uranus and Neptune through my 10-inch. The deep sky objects I tested this eyepiece on were the globular clusters M-2, M-4 M-5, M-10, M-15, M-22, M-55, M-71, and M-75, the open clusters M-11, M-18, M-26, M-39, M-6, M-7, M-103, NGC-6664, M-6231, NGC-6416, NGC-6425 NGC-6709 and NGC-457.  I also looked at the planetary nebulae M-57, M-27, NGC-7009, NGC-6818 and NGC-6826. The emission nebula M-17 and the Andromeda Galaxy were also observed.

The light transmission and contrast were excellent, with the stars' colors brilliantly visible against the sky background. The high contrast made all the difference between seeing some of these objects and missing them, despite a very bright moon being present. When turned to the moon, false color was minimal but visible towards the edge, and details across the field were razor sharp. I was able to make out subtle colors in the maria on the Moon. However, the moon also showed these eyepieces do have noticeable pincushion distortion near the edges of the field of view. It's most definitely not desirable for lunar observing, but I didn't notice it much if at all when looking at deep sky objects. The eye relief is also too short for me to use the eyepiece with my glasses on, but I get good views through it with my glasses off. It doesn't seem to give up all that much to TeleVue Type6 Naglers from what I can see, but I have yet to try a head to head comparison. For the price, these are excellent values for the money, in fact a fellow member of the local club brought the 18mm and I was impressed with that one. I'm planning to acquire more of these eyepieces, for deep sky viewing they are very good indeed. I have two sets of eyepieces already that are far better for lunar and planetary observing.