From: "Steve Miller" To: "Ed Olszewski" , "Jim Burge" Cc: "Roger Ceragioli" Subject: l2 Status Date: Wed, 28 Nov 2001 18:00:45 -0700 Ed, We acid etched the edge and back to relieve stress in the blank because I was concerned at the change in figure measured by Lee before Thanksgiving. We have begun re-grinding the spherical surface. We have chipped the spherical surface in the process of measuring the wedge. It is a small chip coming in from the edge, but it does extend into the clear aperture a little. I'll get Jim to look at it but I think it will be OK. We will proceed with polishing the rear. THen measure the figure and axis centration again. sm ------------------------------------------------------------------------ From edo@as.arizona.edu Fri Jan 4 14:59:46 2002 Date: Thu, 29 Nov 2001 16:59:11 -0700 (MST) From: E. Olszewski To: ggwilli@as.arizona.edu, mlesser@as.arizona.edu Subject: Jim's email Ed, I have evaluated the chip in L2 and recommend that we proceed with the part, then paint the area black before installing the lens. The chip is big and ugly, but the area it extends into the clear aperture is only about 8 mm across. The beam is 200 mm across here, so worst case, it affects 0.16%. In fact, we can do much better then this. If we clock the chip so that it is oriented along the sides (not the corners) of the filters and the CCD, then it will never be seen by the focal plane. We do need to make sure that we do NOT put it on the side of the guider. Steve is proceeding with the spherical surface, assuming that he does not need to correct it. The option for fixing the lens would be to regenerate the surface. We would lose about 1 mm of thickness and several days. Since we can simply orient the part to make the problem go away, I recommend we accept the part with the ding in it. If you have questions, please call me. Otherwise, please give Steve the okay to finish the lens as is. Jim -------------------------------------------------------------------------- From: "Steve Miller" To: "Ed Olszewski" Cc: "Jim Burge" Subject: L2 Date: Wed, 5 Dec 2001 13:39:18 -0700 Ed, This morning it was reported that we suffered additional damage on the L2 spherical surface. This time it was Dean and he pinged the surface with the drive pin while adjusting it for a polishing run. It is a small (~1mm diam) on the surface near the center. It has also a typical conical fracture that extends into the depth of the glass probably more than a mm but is difficult to quantify since the fracture edge is not easily seen. Jim was over here this morning and he and I examined it closely. Jim's opinion is that the defect itself is small enough to ignore. Typically, we would grind out the fracture as we did for L3 but I am reluctant to carve a hole in the surface since it will increase the size of the unusable area. We were concerned over the possibility that the fracture may propagate so we looked at it under crossed polarizers and can detect virtually no stress birefringence. So it is unlikely that the fracture will propagate. However, if the coating temperature is very high it may make the fracture move. Jim suggested I speak with the coating vendor to determine what the optic will be subjected to in coating so I would like the contact information for the coater from you along with some way to identify the coating to the vendor so he knows which one I'm talking about. We are pretty sure the spherical surface is finished otherwise, but Jim is checking the measured radius now. Today, we are making a measurement of the asphere. miller ------------------------------------------------------------------------- From: Jim Burge To: "'Steve Miller'" , "'E. Olszewski'" Subject: RE: L2 Date: Wed, 5 Dec 2001 13:49:08 -0700 Steve, Ed, The radius of the L2 sphere is within spec. I calculate 1128.2 +/- 0.1 mm. The spec is 1128.1 +/- 1 mm. It is not clear whether the surface is polished out. I didn't inspect this. If it is polished out then it is done. The ding in the lens will cost the system a 2 mm obscuration. The beam diameter here is 200 mm, so we lose 1/10000 of the area. The ding will need to be painted out. Unless there is a compelling reason to do anything else, I think we should leave it and not try to excise it. There is no visible stress around the fracture. The only concern is during coating. Steve should talk to the coater. Jim -------------------------------------------------------------------------- From: Jim Burge To: "'E. Olszewski'" Subject: RE: L2 and L3 dings Date: Fri, 7 Dec 2001 11:22:11 -0700 Here is a brief explanation of the basic effects of small dings... The 2 mm ding is tiny compared with the 200 mm beam from a single star. For each star, the ding does 2 bad things: 1. Scatters light out of the image making it dimmer by an amount 2(Ading/Abeam)^2 = 0.02%. This loss will be present for all of the stars, and it is so tiny you will never see this effect. 2. Half of his light is absorbed, the other half is scattered. So each star will spread 0.01% of its light all over angles roughly lambda/Dding. 1 mrad. the place. This increases your background slightly. The lens is roughly 700 mm from the focal plane, so each star will have a ghost halo, about 175 um wide with 0.01% of the light in it. 0.5 arcsec images, the images are 17 um. The ghost will be about 0.0001% as bright as its star. You will not be able to see it unless your detector has dynamic range of 1000000:1 or 20 bits. For 1 arcsec images, this ratio is of course 4 times worse 18 bits. Where these can hurt you most is obviously for very high contrast observations. I hope this helps. Jim