Optical design of ULTRACAM (by Tully Peacocke, UKATC)





Initial, approved optical design (01/11/00) Iterated optical design (08/12/00) Final optical design (12/12/00) Tender documents were sent out by VSD to the following companies (21/12/00) Revised optical design by Tully (28/02/2001), which now fits the SPECAC stock tooling, replaces the unavailable N-LAK33 glass and the expensive N-LASF31 and allows the Aristarchos collimator to fit within a much smaller space envelope. Note that this design mistakenly has the g' arm straight through instead of the r' arm, and so will have to be modified slightly. The 28/02/2001 design (above) suffered from excessively large diameter CaF2 elements in the collimator. Also, the negative concave shape of the first CaF2 collimator element meant that the thickness of the substrate would have been excessively large. This drove up the cost of the Specac quote quite considerably (approximately 2000 pounds), even though we had made substantial savings (approximately 1500 pounds) by fitting to the stock tooling and removing the N-LASF31 glass. Also, SPECAC were unhappy about collimator element 3 in the 28/02/2001 design, which they felt was too thin for its diameter (and hence too difficult to manufacture). With the above in mind, Tully came up with a new design on 06/03/2001, which reduces the diameters and thicknesses of the CaF2 elements in the collimator and increases the thickness of collimator element 3. This was all achieved by using a new glass (N-PSK3) for the first element in the collimator (which we had previously checked with SPECAC is readily available and affordable). This design also (correctly) has the r' prime arm as the straight-through arm. On 24/05/2001, Tully noticed an error in the design of the WHT collimator. The spot patterns produced by the design were not forced to coincide on the same point on the focal plane (i.e. on the same CCD pixel) in the three arms. Tully therefore redesigned the WHT collimator, in the process making elements C1 and C3 slightly thicker than is probably necessary. If Specac are willing to reduce the thicknesses, we will get increased throughput (especially in the u' band). Note that the designs of the cameras are unchanged, although there are small variations in the clear apertures in the files listed below. On 20/08/2001, another problem was highlighted by Alfred Afran at Specac. Schott were unable to supply the N-SK16, N-LAK10 and N-LAK22 glasses (which are lead-free, in accordance with EU regulations). Tully used these in his design as Schott led him to believe he should, and Alfred was told the glasses would be available and so accepted the design. Schott then found, however, that they cannot supply these glasses in the thicknesses we require and so offered us SK16, LAK10 and LAKN22 instead (which are the old versions of these glasses and contain lead). Tully looked at the optical design with these old glasses (which have very similar performances to their lead-free counterparts) and found that there is a slight improvement in throughput and no noticeable degredation of image quality when they are used. So, we breathed a huge sigh of relief and Alfred ordered these old glasses for use in ULTRACAM.

The lens figuring was finally completed in January 2002. Here is the list of lenses as manufactured by Specac (in EXCEL format), detailing their thicknesses and ROC's. Six of the surfaces are slightly out of tolerance in their thickness. I then passed this list onto Tully, who reoptimised his design. His initial attempts resulted in camera barrels approximately 7mm longer than in the original design, and a collimator barrel approximately 35mm longer! This was unacceptable to both Sheffield (where the mechanical design and much of the manufacture was complete) and to Specac (who had already made the barrels, as they make these first and then edge the lenses to them). Tully then reoptimised again holding the first and last elements in each barrel at a constant separation. This resulted in barrels of identical length to the originals in which the optics inside moved around by less than 1mm, with the exception of the WHT collimator, in which element 2 moves by approximately 2mm. Specac were happy to accept this. The downside of this is that the green camera barrel has to move away from the dichroic by approximately 5.5mm (which we can just about accommodate in the mechanical design) and the image quality is down from >90% encircled energy within 1 pixel radius to more like 70-80% (dropping to approximately 40% in the extreme corners of the field, which are not of great importance to us unless we are observing stars separated by exactly 5 arcminutes at the very highest time resolution). Specac were happy to accept this. The list of revised lens spacings from Tully is given here (in WORD format). The as-built final optical prescription is given here (in ASCII format - this should agree with the preceding WORD file) and spot diagrams and ray-traces of the as-built optical layout are given here (a tar file containing 7 images and a README file). The latter two files were sent to me by Tully in July 2003 as I prepared the optics section for the journal paper on ULTRACAM.

The above prescription does not include the Aristarchos collimator (as Tully did not have time to do this). He finally sent me the prescription (given here in ASCII format) for Aristarchos on 04/02/2002, as described in this email. Specac were unhappy about this reoptimisation, as it increased the separation between the first and last lenses in the collimator by 1.5mm. Their barrel design could not accommodate this, so Tully re-reoptimised and removed this problem. The final prescription for the Aristarchos collimator is given here in ASCII format and is described in this email.

That's the end of the story - Specac delivered the coated CCD windows on 09/02/2002 and we are expecting the barrels to be delivered by 15/02/2002.

UPDATE: ULTRACAM was commissioned on the VLT on 16 May 2005. The Zemax file containing Tully Peacocke's optical design of ULTRACAM, including the new VLT collimator, is here, with its two associated files here and here.