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A question about the apparent size of double star disks.

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#1 MistrBadgr


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Posted 22 October 2013 - 05:39 PM

I have been working on a TerraStar 60 that I purchased pretty much as a drowned basket case on eBay. I had to put new lenses in it and decided to play with a number of things on the scope to optimize it as fully as I can. I will talk about those changes in another post when I get finished with it. I think I am getting very close to its optimum, but have been having a bit of trouble getting splits on close double stars like I think they should. Part of the problem could be with the scope, but I am thinking that most of it is either with my eyes, or light pollution. I am really thinking the scope is "spot on" but I want to really prove it in some way before really saying so.

I have been able to split Epsilon 2 Lyrae, which has two visible stars of nearly equal magnitude with a separation of around 2.3 arc-seconds, but the gap between them has seemed a bit tight and the disks themselves have seemed a bit big. With Epsilon 1 Lyrae, there is a difference in magnitude of about 1.5 but the separation is supposed to be 2.5 or 2.6. I have not been able to split them, at least well enough to truly call it that. It could be that I simply cannot see the dimmer star well enough with the amount of light pollution I have with this 60 mm scope. Under ideal circumstances, I am thinking this scope should be able to split magnitude six double stars with a separation of 2.0 arc seconds.

I have also worked with Epsilon Sagitarius, which I encourage everyone to try out. It is relatively low from my back yard at 36 degrees North, even at zenith. There is a sizable magnitude difference, the two stars are inside a dust cloud the primary star has thrown off, and the separation is about 2.4 arc-seconds. All together, I can understand why I can only get the two stars spread apart enough to just "kiss" with my very best observation on a good night and have decided I am satisfied with that for this scope. The reason I recommend it to you is that the primary is a pale blue white, while the secondary is orange. Even without a clean split, the colors are absolutely beautiful in the little TerraStar. (Objective lens from a NG 60. Very good lenses, I think, in those scopes)

In my ponderings about the problem, I had a thought about airy disks and plan on checking it out sometime. However, I am not sure if my visual memory is good enough from one time to the next, so I thought I would ask and see if any of you have noticed this when observing close double stars.

I know that dark adaptation being greater at a dark site may blow holes in what I am about to talk about, but I am thinking more about what people have actually noticed.

With light pollution, I am thinking that the intensity of the over-all light level of what one sees goes up at least a little. Therefore, the visible part of the airy disk might grow a little and the dark portion between the disk and the first defraction ring may shrink as compared to what one sees at a dark site. With the general level of contrast being less with light pollution, the situation might or might not be made worse.

I am wondering if anyone has noticed the visible disks of a known close pair being larger with light pollution and the space between them being less as compared to the same object, same scope, same magnification at a dark site.

Even information to the contrary is useful in my pondering.


Bill Steen

Bill Steen, Sky Hunters' Haven Observatory, Broken Arrow, Oklahoma

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