Saturday, November 14, 2015

Astrophotography: Solving Some Problems, Finding New Ones

I made something I like!





These are some of my changes:

  • Better polar alignment; not seeing field rotation. This is my longest exposure too, at 480 seconds, which I would think would show that sort of thing if it were off by much at all.
  • Neodymium filter for light pollution, it blocked quite a bit!
  • More spacers between the reducer/corrector and the camera to reduce the vignette problems to a minimum.
You can see I haven't fixed the primary tube reflection yet, still waiting on materials to be delivered.

I plate solved the image using Astrometry and came up with a 1.1 arc-seconds per pixel. The theoretical limit of my 6" scope is around 0.9 arc-seconds. Seeing conditions were much worse than that, and for where I am will probably never be better than about 2 arc-seconds.So I think we are in a good place.

I also calculated out an effective focal length of 926mm f/6.2, not far from the advertised 945mm f/6,3 the reducer/corrector is supposed to get. I didn't try plate solving previous images, but I can tell you with some certainty that the Celestron recommendation of 105mm between the corrector and focal plane, as well as the internet's prediction of 85mm are wrong. Total distance from the back of the corrector to the surface of my sensor is 155mm. If you have a Celestron C6 and the standard f/6.3 reducer this is probably about where you want to be, at least if you are using an APS-C sensor sized camera.





For pure resolution, however, our tracking is not quite perfect. Here's a single color channel from the image above, cropped to the center of the nebula where the trapezium stars are very close together:


And the same spot, but with a quick 1 second exposure:


You can clearly see all four stars as separate in the 1 second shot. This means that either my guiding isn't reacting fast enough, isn't predictable enough, or doesn't have enough resolution to keep things perfectly centered. I suspect the latter is my issue. I will probably need to get a small scope with a longer focal length to keep up with the main telescope's resolution. Seeing conditions are at play here too, but unless I was particularly lucky with that 1 second picture they should be effecting that image similarly.

For reference, the two stars closer together there are 8.7 arc-seconds apart:


Also, now that the field rotation is gone/minimized I can see we have some comatic aberration on stars at the outside edges of the frame:


Note that's coma, not chroma. This is the nature of the telescope. Stars in the center of the field focus to the same point from anywhere the light is gathered on the lens, but at the edges of the field those stars are focused at slightly different places depending on how close to the center of the lens the light was gathered. The reducer/corrector may be helping this or hurting it, depending on how much you believe I have moved the camera away from the "optimum" focal distance. There are additional correctors to help this, but this is so subtle I'm going to leave it alone for now.


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