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.

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