Double Star Interference Observations

[Nick White]

Hi, last night I attempted to observe visually the double star 78 Ursa Major using a two-slit interference mask. The intent was to use separation of the slits to determine the sub-arc second separation of the double. The image through the eyepiece was as expected, the usual diffraction pattern had the broad undulations with sub-undulations in accordance with the ratio of the slit separation to slit length. The separation of the two slits was such that for a suitable rotation of the mask the interference pattern for the double should have minimised (slits perpendicular to the axis of the binary). However, when on axis, the diffraction pattern wouldn’t minimise no matter the rotation of the mask. In other words, the diffraction pattern fine structure remained visible. However, I noted that the star disappeared from view completely when the star was off axis by about 2/3 of the way out to the edge of the fov but would re-appear further out and closer in (like there was a blind spot in the telescope). Can anyone help me understand this observation of its disappearance? For instance, could it be the experiment was working but that the lack of correct collimation meant the pattern disappearance happened off axis? (if you see what I mean). Thanks.

[Callum Potter]

Hi Nick,

I am not an expert on the technique, though I have some understanding of how it should work… What sort of telescope were you using to do the observation?

Thanks, Callum

[Nick White]

Hi Callum, I was using a 10″ Dobsonian, so I had to let the image drift across the field of view. That’s how I noted the “blind spot”. Otherwise the view was as expected, showing the usual Fraunhofer interference pattern. To clarify, I didn’t see the interference pattern disappear as a result of the double, but that might have been because the slit spacing was not set quite right. More puzzling was the star (ie. its pattern) disappearing completely from view at the blind spot. That was unexpected. Unfortunately I didn’t have presence of mind to check whether it disappeared on the opposite side of the fov as well. 

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