NA2 Baffle Study

last updated February 20, 2014

Apache Point Observatory
New Mexico, USA


As a follow up to an earlier discussion on Feb 6th or 13th regarding the baffles at NA2 and M3 for the new imager (ARCTIC) with regards to scattered light it was suggested that someone should look into the current baffles, their design including drawings and models. We needed to understand what had been built by Jeff Morgan around 2004 before coming to a decision on the field of view (FOV) of ARCTIC. Verification of the current models to the as-built mechanics should be done. I started such work on February 7th, 2014. I found the solidworks models and drawings for both the M3 and NA2 baffles. I then made certain that the as-built matched those drawings. To better understand what the vignetting effects were on the NA2 guider I then improved the Zemax sequential models to properly model the baffles as obstructions instead of aperatures (based on the mechanical drawings). Going further I increased the field of view to the proposed f/8 size for the ARCTIC imager. I modeled a round 10.46 arc minute FOV and 4.7 arc minute square FOV. The bottom plot reflects the vignetting effects of the current and various proposed baffle designs. Images below show the proposed optical path from the end of the NA2 baffle to the detector. Although some investigation was done regarding modification of the M3 baffles, the effects were small and have not been included below.

Current Telescope Baffles

Current NA2 baffles

Current NA2 baffles (cross sections)

Proposed NA2 baffles - 10.46 arc minute diameter FOV

Proposed NA2 baffles - 7.4 arc minute square FOV

Proposed NA2 baffles - 7.4 arc minute square FOV notched for improved NA2 guider properties

Although this design reduces the vignetting seen at the NA2 guider. We do not have a good feeling yet what the effect might be on scattered light at the focal plane.

Vignetting Diagram