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From rgt Fri Dec 13 13:37:39 1996 Date: Fri, 13 Dec 1996 13:21:30 -0600 From: rgt (Bob Tull) To: observers Subject: 2dcoude performance Cc: don, ebakker, esb, pjm, pwk, rgt, tgb, wdc Status: RO If you have no interest in 2dcoude you can delete this message. The 2dcoude' high-resolution focus F1 has shown resolving power as high as 270,000. However, observers have reported variable resolution and I found last February that one quadrant of the collimated beam produced an image shifted in wavelength, reducing the overall resolving power. Maximum resolution could be obtained by blocking the offending quadrant, with a resulting loss of efficiency. Further work in May resulted in improved performance by optimizing the collimator focus, but following observers continued to report lower than desired resolving power. Recently Bill Cochran found that spectra showed image structure which was resolved into doubled monochromatic images in the spatial direction. He suggested it is due to the partial loss of refractive- index-matching fluid in the interface between prisms. Ed Barker found that the two images were separated by, typically, 10 pixels at the F1 focus. He further found that the second image could be eliminated by blocking the upper half of the collimated beam, corresponding to the region of the cross-dispersing prisms where index-matching fluid has been lost. Don Barry and I inspected the prisms during Ed's coude' run in early December. This interpretation seems to be correct: the variable performance of the F1 focus appears to be related to variable area of the index-matching fluid. This could also explain the observation by Mike Marcario that the Ronchi shadow pattern seen visually at the F1 focus markedly improved when I tightened a retaining screw on the prism mount, squeezing the prisms together. White light Newton's Fringes due to interference in the "un- oiled" region showed a wedge of about 8 fringes in the air gap, when that gap was of order 1/4 the height of the prisms. We can estimate the separation of the images due to the two regions of the prism by use of the "thin prism" formula deviation = (n-1) alpha where alpha is the apex angle of the thin prism. There are two prism interfaces used in double pass, each with similar air gaps forming an air wedge of about 8 visible fringes over about 50 mm of prism height or about 0.002 degrees in each prism interface. Applying the thin prism formula to 4 prisms in series, with apex angle 0.002 degrees, actually does predict an image shift of about 10 pixels in the vertical direction. This seems to confirm the interpretation. Due to the shorter focal length at the R=60,000 F3 focus the effects are reduced by a factor 4 and will not normally be a problem at that focus. POSSIBLE SOLUTIONS. The loss of fluid in the prism interfaces has not been explained. Don and I added some fluid, with the prisms in place, but not much actually seeped into the interface, probably due to the compression of the prisms together by gravity. Clearly the fluid does not form a good permanent interface and something better is needed. Possibilities: (1) Clean the prisms and send them to Continental Optics for AR coating of the interior surfaces. Re-mount them with an air gap. The reflectance for one coated surface averages 0.5% over the spectral passband. The total loss due to reflectance at these surfaces will be of order 4%, equal to the loss at the exterior coated surfaces. This is probably the most reliable solution and the light loss is far less than that due to masking off part of the beam. The prisms are tilted at an angle that will prevent any reflected light from becoming stray light at the camera focus. (2) Cement the surfaces. These are rather large surfaces. It requires a professional optician to accomplish. I don't know the probability of success but a failure could result in loss of the prisms. (3) Put a spacer shim between the prisms and fill the gap with fluid. This is the cheapest and easiest solution; we could try this first, then later consider coating the surfaces. The only concern I have is that any distortion of prism shape, due perhaps to imperfect compression of the shim along the prism apex, might result in a non- uniform fluid thickness resulting in wavefront distortions. If the loss of fluid is due to, e.g., its absorption into the RTV cement used to seal the joints, this might solve the problem by separating the fluid from the sealant. We can try this with a couple of pieces of thick plate glass. I solicit your comments or alternate suggestions. Bob Tull
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