Sunday, February 13, 2011

Testing under the stars


At long last the moment I was awaiting for nearly two years has come, first light with my very own large Dobsonian telescope. Friday night I took the telescope outside after coming from from work and set it up in the driveway. I found the truss poles were indeed much too long for the telescope to focus and I had to cut off one and three quarter inches off of them to get any sort of image. I did not shorten the poles enough to allow the telescope to focus with a coma corrector lens in place, but I was able to get my Orion Stratus and Optiluxe eyepieces to just reach focus. Once that was done, I was able to take my first looks through the telescope at Saturn and the bright Messier objects M-3, M-5, M-13 and M-65. While the galaxy was underwhelming due to the interference from stray light, the globular clusters were another matter. All shattered into thousands and thousands of stars, many of which showed a reddish or yellowish tint. The sketches I had made of these star clusters cannot begin to show the magnificence of these objects through a large and powerful telescope. Indeed the views were impressive despite the bad seeing. Saturn was a squirming mess, but the rings and larger moons were easy to see. While the lack of a shroud made stray light annoying and the seeing was horrible, I was able to see that for the most part the telescope is in good order. However, I found the secondary mirror's sleeve seems to be cutting off some light and making an oddly shaped outline in out of focus star images, as though some light is is not reaching the the eyepiece.

The following night I was able to take the telescope outside at dusk, after making a ramp to roll the telescope out of the house and into the yard. I trimmed off another half an inch from the truss poles and then rolled the telescope outside. The handles I made for the telescope made moving it outside less effort than carrying my solid-tube 10-inch Dob outside. The first object I looked was the moon, on which I tried all of my eyepieces to see if they would focus, and all of them did. The most memorable view was through a 3.5mm Stratus, with which the telescope yielded 572X. The seeing as just as bad as the night before, but the crater Clavius looked downright huge in the field. I was struck by how narrow the field of view is through a 15-inch F/4.5 Dob, vindicating my choice to pay extra for a faster mirror. Even with my 21mm Stratus the moon took up most of the field of view! However, the view of Jupiter was quite nice and when the sky grew dark I had nice views of the Orion Nebula, the double stars Rigel, 52 Orionis and Sigma Orionis. I even looked at Sirius at 572X in a futile effort to see the Pup, but the seeing was just too bad. Then I located and found the Ghost of Jupiter in Hydra before taking the telescope down and rolling it back into the house. Otherwise known as NGC-3242, it is one of the finest planetary nebulae in the sky.
Overall, I am happy how the telescope turned out, except that the diagonal sleeve seems to be interfering with light from the primary. It also appears to possibly be a tad too small, but so far I have not noticed any real light loss at the edge of the field of view. Many commercially made telescopes of this aperture use diagonal mirrors with a minor axis of 2.6-inches as I have. I do not notice any serious light loss at the edges of the field, so after fixing the issue with the diagonal mirror's holder I might opt to leave it in place.

The telescope damps out vibration very quickly, no more than two seconds even with the highest power eyepiece in my possession. Moreover the telescope moves smoothly and will not"weather vane" in light wind or breezes. The poles insert easily into the pole blocks, and the blocks hold them tightly. The same is true when it comes to the upper cage fitting onto the poles, putting it on and taking it off is simple even in the dark. The cover of course will be put on the mirror box when doing this to prevent anything from falling onto the primary mirror. Following objects is smooth and effortless. As expected, no ladder is needed to reach the eyepiece no matter where the telescope is pointed, and I can still use my homemade, folding oak observing chair.

In spite of the bad seeing, I conclude that the primary mirror is free of major defects and the floatation system in it's cell is working properly. No astigmatism, not sign of the mirror being stressed or touching one of the side pins or mirror clips, which would show up in the views. The telescope also seems to hold collimation quite well between sessions, I do not have to do much tweaking with the primary mirror to get it ready for use. The handles make moving it easy on me, and it's wonderful to not have to lift and carry the mirror box, which weighs between 55 and 60 pounds with the mirror in it. Carrying it is risky both to the mirror and my back, the wheelbarrow handles and the ramp too really do make setting it up a pleasure. Now all that remains is to make the shroud which will keep out stray light, dew, falling objects and my body heat as well. I will use the remaining black Ripstop Nylon fabric to make a bag to store the truss poles in and keep my young and rambunctious cat Merlin from attacking the foam insulation on them. Once I resolve the issue with the secondary mirror and make the shroud, the next step is to get the dew heaters in place and working, and to acquire digital setting circles and a dew heater controller. For many years I dreamed of building my very own large but transportable telescope, and now it's a reality. Now that I have the telescope, I intend to take it to a regional or national star party where I can see favorite objects in a new light literally. I will be drawing favorite objects through this telescope that I have drawn previously from the same sites as I did with my 6 and 10-inch Dobsonians. That way the effect of the much larger aperture would be apparent.

Wednesday, February 9, 2011

Ready for first light!

After shimming and bolting the pole blocks on the mirror box permanently in their final positions, I added foam pipe insulation to the truss poles so handling them during cold weather would be kinder to the hands. The black foam will also suppress reflections off the shiny aluminum tubing. I also found the bores in the pole seats were too tight for the poles to fit into them, so I had to take them off the upper cage. Then I used a cordless drill and a drum sander attachment to enlarge the pole bores enough for the poles to slip into the pole seats. The result is still a tight fit, and when the knobs on the pole seats are tightened the poles are held even more tightly. It still takes a bit of effort to take the upper tube assembly off, but at least I do not have to worry about it falling off in the dark. Now before I can take it outside in my yard under the stars for the first time, all that remains is to touch up the finish on the blocks and the drum too, put the primary mirror back in it's cell and then adjust the sling and collimate it. That event will take place this weekend, if the weather permits.I expect the truss poles to be a little too long, and they will be trimmed to final length at that time. First light will be a test to see if everything is in order before I commence full scale observing with the telescope at a dark site. After that, all that remains is to make the shroud and acquire the digital setting circles, coma corrector and dew heater controller I plan to purchase for it.



Sunday, February 6, 2011

Supernova underway in NGC-2655

Last night I had the rare chance to observe the aftermath of one of the most powerful explosions the Universe ever sees, the total destruction of a star from a supernova explosion. Stars explode as supernovae all the time in galaxies other than our own, but most are very faint and at best are visible through large amatuer telescopes, if not large professional telescopes only. However, on occassion one goes off in a nearby galaxy and become easily visible in modest amateur telescope, or even binoculars. Such was the case with supernova SN2011b, a type IA supernova that occured in the spiral galaxy NGC-2655, which is in the constellation Camelopardalis the giraffe. Type 1A supernovae occur when a white dwarf gathers so much matter from another star that mutal repulsion between electrons can no longer support it against it's own gravity. These do not occur only in star forming regions within spiral and irregular galaxies, they go off in all galaxies and even globular clusters. White dwarfs that explode as supernovae are not young stars, they are usually hundreds of millions if not billions of years old before they finally reach the tipping point. The star's core collapses, runaway carbon and oxygen fusion starts, then the entire star blows apart in a blast that outshines all the other stars in the host galaxy put together. The violent shockwaves that are started often take the outer envelope of the other star with them as the wreckage is hurled into space at speeds of 10,000 kilometers a second or more. In the fury of the runaway nuclear fusion that consumes the entire star which acts like a nuclear reactor out of control, the carbon and oxygen gets turned into everything from sodium to iron and nickel. Radioactive decay is making the supernova shine at about 13th magnitude,  and thus visible in an 8 or 10-inch telescope. It peaked at magnitude 12.8, and now it's fading as the radioactive nickel-56 generated in the blast decays into stable iron-56. Currently SN2011b shines at magnitude 13.1, but soon it will fade enough to be out of reach of most amateur telescopes. It will of course remain in the grasp of CCD cameras for much longer.

Other objects examined last night included the interacting galaxy pair NGC-1531 and 1532 in the constellation Eridanus, the river. NGC-1532 is a large nearly edge-on spiral galaxy that is drawing in the much smaller elliptical galaxy NGC-1531. At a magnitude of 10.6, it's bright enough but is low in the sky even from the Alabama Gulf Coast. NGC-1531 is much dimmer and harder to see, but averted vision reveals the magnitude 12.3 galaxy at 188X.

Another galaxy I observed was the diminutive spiral galaxy NGC-691 in Aires. Nestled among a group of foreground stars, this small galaxy was only visible as a round glow with a brighter core.
Two more galaxies in Pisces the fishes also were observed, NGC-718 and NGC-741. NGC-718  was the brighter of the two, with an oval outline and a brighter core. It's a barred spiral galaxy that was quite easy to locate and is moderately bright.

NGC-741 is a dimmer elliptical galaxy that has a smaller companion near it, NGC-742. Small and round with a brighter core, NGC-741 was barely visible even though it's magnitude was 11.6. The sky conditions might be why that was the case, because fog and frost was forming everywhere at the landing strip by nine p.m. The smaller companion galaxy NGC-742 was not visible, not that at a magnitude of 14.4 would it be visible through a 10-inch telescope from a light polluted site.
Although the fogging up of my eyepieces and the sky becomming very milky cut short observing last night, it was well worth the trip to the airstrip to observe these galaxies as well as favorites such as the Double Cluster, Crab Nebula and the Messier galaxies M-31, M-32 and M-74.