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The text below was sent to all INGRID PIs to inform them of the current state of the collimator. I would like to brief you on some news regarding the optical quality of INGRID. INGRID acceptance testing is currently proceeding well and whilst we are still on course for commissioning in mid-March, we will initially be using a temporary optical component in the collimator which will slightly reduce the optical quality of INGRID until replaced. I would like to inform you of the loss of optical quality and explain how and why this occurred. The INGRID collimator is a detachable optical assembly using two lenses which are external to the cryostat - it was designed to be detachable due to the dual foci that INGRID will operate at, namely the Cass and Nasmyth foci. The second lens in the optical train, an aspheric, has been extremely difficult to build and test, indeed beyond the capabilities of the manufacturer that RGO originally awarded the contract to. Unfortunately for INGRID, the extreme difficulty in manufacturability became apparent only in recent months after discussions with the company producing the aspheric. We have now awarded a second contract to a different manufacturer who assures us of their experience and confidence in the field of aspheric production. The aspheric is due to be delivered to us in April 2000, too late for INGRID commissioning. I should stress here that a second collimator for use with the NAOMI natural guide star AO system remains unaffected, the problem affects only observations when INGRID is at Cass. As you may be aware, we had hoped to commission INGRID in Spring 1999
but were unable to due to several problems encountered during acceptance
testing. However, as we had hoped to commission at that time, a lens
was purchased which was much easier to produce than the aspheric but which
gives a slightly inferior optical quality and increased thermal background
as it uses a pair of lenses (i.e. a doublet) to complete collimation.
I list below the effects on the FWHM and 50% encircled energy of the DEGRADATIONS
OF THE RECORDED IMAGE i.e. in median seeing the optics of INGRID with the
aspheric will degrade the recorded seeing by ~4% in all bands, the figures
below refer to the increase in this degradation which should then be added
(in quadrature) to the seeing disc. It should be noted that INGRID
images with both the aspheric and doublet give a strong central peak and
hence a narrow core with a small FWHM will dominate all images. The
50% encircled energy more usefully describes the extra width of the images.
The figures below discuss the optical quality degradation when using the
doublet as compared to the aspheric, assuming the 18.5um pixels corresponds
exactly to 0.25" and imaging of a point source using a ray tracing program.
Factor Increase in the FWHM Degradations
Factor Increase in Diameter of the 50% Encircled Energy Degradations
[Array 1/4 point is defined as the position midway between the array center and the array edge] The thermal background (i.e. at the red end of the K band filter, 2.03-2.37) will be increased, possibly by as much as a factor of two. Therefore I suggest the K_s (1.99-2.31) will most likely give much better results than the more typical K band filter, as long as your science program is not affected by the slight wavelength difference. There will also be a slight reduction in throughput and an increase in ghosting. The throughput reduction will be at the level of a few percent and ghosting is still be characterized but should be minimal. If you have any questions or want further information please feel free to contact me. Best regards, Chris Packham.
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