2020-12-2: Ghosts, Triangles, and a Light Shroud for the FlexTube 250P

 My 10" Dobsonian (SkyWatcher FlexTube 250P) finally gets its first reasonably dark test.

I knew when I ordered the FlexTube version of the 250P that I'd want a light shroud sooner rather than later. So my mother and I bought the materials yesterday and started putting them together today. The design is almost as simple as it gets. The shroud is made of black Sport Nylon. It is simply a cylinder of nylon fabric held together with velcro, with two drawstrings at either end. Two pleats were added going up and down the length of the tube to aid in structural integrity.

Mom applying velcro to the light shroud.

The final result is a little bit too short, but it is usable. I have to occasionally check that the shroud isn't sagging into the light path, and then mess with it for a few seconds to clear the aperture.

Light shroud on the 250P.

 

It is unclear how much of a difference the light shroud makes, and I think it depends very strongly upon how much moon and street/porch-light pollution there is. I removed it while it was pointed at the Pleiades with a bit of moon light shining at it, and the difference in the background was subtle but noticeable. I tried it on a fainter object near the zenith, with no moon light shining onto the scope, and saw no difference from the unshielded view.

I will continue to use and test the light shroud. It's really easy to make, comes together in less than two hours, and cost something like $15. (The AstroZap commercial option costs $100)

I started the evening by hauling the 60 pound telescope across the yard to the one spot where I can see Jupiter & Saturn. I put the scope on a contractor bag to protect it from the very soft dirt. My damn chair sinks halfway into the ground when I sit, and when pointed low the telescope was nearly impossible to keep pointed where I wanted it. Partly this is due to the soft ground, and partly this is due to the scope being front-heavy. It's quite annoying.

This is a case where I would really have benefited from having the DT6 out as well for the sake of comparison. Jupiter was going through a transit, but the shadow was not a clear pinprick as I'm used to in the DT6. The image was fairly blurry overall, despite being very well collimated. There's a few possible culprits: seeing, cooling, or bad mirror. Seeing was average, but Jupiter & Saturn were low in the sky so they weren't well placed at all. The scope had been out for close to 20 or 30 minutes at that point, but it probably wasn't totally cooled yet. I don't think the mirror is actually a bad example, certainly not worse than the DT6, since Polaris was split better than the DT6 can do. I'm not at all experienced at star-testing telescopes.

One thing I have determined is that the 6mm Goldline Eyepiece, which had been a favorite in my DT6, performs quite badly at f/5. Even in bad seeing last night, the star test with the goldline showed spherical aberration so bad even my inexperienced eye found it obvious, while the 2.5x Barlowed 10mm eyepiece showed a much more symmetrical out of focus and inside focus figure. At the f/12.5 provided by the 2.5x Barlow, I expect the 6mm to work better, but the seeing will rarely allow for 500x magnification. It's very possible I don't know what I'm talking about! This is a level of optics which I have little experience with.

Last night I observed several of my favorite objects in bright moonlight, bad seeing, and mediocre transparency. I was unimpressed with my views of most of them--I was comparing the most memorable, excellent views of those objects as seen in my DT6, with what the 250P shows in bad conditions. I was conscious of this, so I didn't let it get to me too bad. I ought to have taken the DT6 out to make a direct comparison in the same conditions. Even M42 didn't impress me, beyond the normal amount of excellence you can see in the bright inner boxy part of the nebula.

I was however impressed with my view of Messier 15. What had been a ball of light with only suspected granularity, became much more noticeably granular, with several individual stars visible. Even in worse conditions than my initial observation with the DT6, it looked better, and that was enough to satisfy me.

Tonight, after returning from the far reaches of my back yard to my firm concrete driveway, I went to look for Messier 15. It's easy to find if you can find Enif. Its granularity was even better than the night before, and at 240x it was dim but structured. I regret not sketching it. It reminded me very much of the time I tried to find M13 in a well-lit parking lot. Messier 2 was a little tougher to find, but not too bad. It was also bright, but its granularity was more subtle, with just a few bright stars. This is an interesting paradox: my DT6 observations suggested M2 was more granular, but my 250P and CDK24 notes both say M15 is more granular. I think what has happened is that M2 is smoother overall, with just a few bright individual stars, whereas M15's very granular, but made of dimmer individual stars. So the DT6 picked out the individual stars of M2, and struggled on M15's, while the 250P picked out the individual stars of M15 easily.

After that, I looked at Messier 31. I could see a hint of the dust lane interrupting the core light, and M32 was easy, and seemed to be larger than it typically seems in the DT6. After a bit of searching, I was able to spot M110. I was being lazy and didn't sketch... need to be better about that. I decided that it was time to find the elusive Messier 33, the Triangulum Galaxy.

I've seen M33 twice before at the Cline Observatory. Once using a C8, and another using the CDK24. 

This year, with a manual Dobsonian and no friends, I'd tried for M33 on two other occasions, once on the night I found Uranus, once with no luck at all with my Bushnell Voyager 100x4.5 "bong scope." The first time, I was using the wrong asterism to find it and could never have done so. The second, I positively identified the field of view and stars, but saw no evidence of the galaxy--to be fair, it was only a 4.5" scope.

I started to find it, got lost, and found Caldwell 28 (NCG 752). It's a bright, sparse cluster which I would have drawn had I not been interrupted by dinner. I will be returning to it.

Finding M33 is not particularly easy. I went to Triangulum and used Stellarium to hop to a squiggly asterism, then followed that asterism to a dim triangle which the galaxy resides within. I spent several minutes trying to find the correct triangle, finding that the stellarium charts didn't go quite as dim as I'd like. Finally I identified the triangle. I saw nothing with the Russel/Meade 32mm UWA, but I wasn't paying close attention yet. The 25mm Plossl revealed it. Hell yes! It is a tough object, definitely not one for the beginner. I needed all of the tricks in the book to be sure I saw it: averted vision, heavy breathing, slightly rocking the scope up and down. But I saw it. It was a broad structureless patch of slightly brighter sky. I could almost suspect a slightly brighter pip to the upper left, which may have been me imagining the H-II region I had heard might be visible.

I was going to go dawdle on over to some open clusters I spotted in stellarium, but I decided, as a lark, to try for NGC 404: Mirach's Ghost, also known as "Comet Komorowski". Now, my scope ain't a Criterion 12.5", but clearly it didn't have to be, as I saw the galaxy almost immediately with the 25mm when I turned to Mirach. It was still faint and required averted vision, but I could see it. I was surprised it would be so easy! "Surely," I said to myself, "I haven't found Mirach's Ghost." I tried to zoom in to see if the 10mm would show it in better contrast against a darker background, but I fogged the 10mm with my breath by mistake before I could get it focused. But still! It makes an almost equilateral triangle with Mirach and an 8.5 magnitude star. At a glance it could be mistaken for a star, but with some attention it becomes clearly nebulous.

It was fairly tough spotting things so close to the zenith, and when I went back later I found it had just crossed the meridian, requiring a full 180 to find it again.

My last object was the Pleiades. Gorgeous as ever, very bright. The Russel 32 eyepiece is not quite wide enough to fully fit it in the FOV, and the edge correction is pretty bad, but I expected this. I could tell a noticeable skyglow around the Pleiades, which made me remember to test the light shroud. When I removed the light shroud, the difference in the skyglow was very subtle, but it was noticeable. I determined a better example would be something dim, so I tried Mirach's Ghost again, and I don't think I noticed a difference. At that point I was cold and running out of dark skies--the Moon was getting higher and higher, but still more than an hour away from clearing the trees, so I called it a night and packed up.

Five galaxies, two of which being new to me, on one night, from a suburban backyard? That's a pretty good night if you ask me.

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