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Scintillation of pulsar radio signals caused by the interstellar medium canin principle be used for interstellar interferometry. Changes of the dynamicspectra as a function of pulsar longitude were in the past interpreted ashaving spatially resolved the pulsar magnetosphere. Guided by this prospect weused VLBI observations of PSR B1237+25 with the Arecibo and Green Bank radiotelescopes at 324 MHz and analyzed such scintillation at separate longitudes ofthe pulse profile. We found that the fringe phase characteristics of thevisibility function changed quasi-sinusoidally as a function of longitude.Also, the dynamic spectra from each of the telescopes shifted in frequency as afunction of longitude. Similar effects were found for PSR B1133+16. However, weshow that these effects are not signatures of having resolved the pulsarmagnetosphere. Instead the changes can be related to the effect of low-leveldigitizing of the pulsar signal. After correcting for these effects thefrequency shifts largely disappeared. Residual effects may be partly due tofeed polarization impurities. Upper limits for the pulse emission altitudes ofPSR B1237+25 would likely be well below the pulsar light cylinder radius. Inview of our analysis we think that observations with the intent of spatiallyresolving the pulsar magnetosphere need to be critically evaluated in terms ofthese constraints on interstellar interferometry.

Technical constraints on interstellar interferometry and spatially resolving the pulsar magnetosphere