18a filter info

I've finally gone through the 18a filter.  From the logbook, we used ctdelay-110 (3 seconds) and the 3.5 km/s slit (which gives something more like 3.8 km/s at 18 microns).

Joe looked at the images and decided if the signal for each echelle angle was on, off, or weak.  He also recorded if the data frame was clean (only one echelle order) or not.  This info was saved in a csv file.

I read this into IDL, used quantities from the blog regarding echelle angle and wavelength for given voltage, and worked out the wavelength for each setting.  To do this I started with the assumption that the wavelength getting through was in echelle order = 3 and then adjusted the lower angle measurements to order = 2 so that the wavelengths that came through, the number of orders, and the filter warm curve made sense.

I saw that the array had some weird data behavior on the first row and last row (see blog post before this one), so I reset those, as well as pixels with extreme values, with an estimate of the median reset value.

I used a function I call ORDERSPACING to determine a single value for the number of pixels between echelon orders for each clean file (see figure below).  These can be used to check models for the instrument.

I then got an estimate of a "typical" value for each good image by looking at values where there was clearly light, requiring there to be >5000 such pixels, and selecting the 25%ile (more light means lower numbers) as "typical".  I subtracted the no-light values I found for each image to get the number of data numbers corresponding to the incident light.  The plot below shows a cut along row 510 of each good image (there are 25 good images) using the small dots.  These values go across orders near the peak of the blaze.  My typical values are shown in red with each frame having a single value.  For most of the frames, these values are not at the top, but do reflect larger than median signal.  For frames with low response, the "typical" values seem too high.  The green dots are my "no-light" values.  Note that I believe there is some light on the detector beyond what is landing in the orders (see comment to post on scattered light).

I finally calculated throughput using logbook/blog values for the integration time (ctdelay = 110 or 3 seconds), gain (35.9 e- per DN), slit resolution (3.8 km/s), and assuming emissivity =1 and temperature = 295 K.

I calculated the area*omega product in two ways.  First, I used the half angle that we measured during CD13 where we mapped a soldering iron tip across the beam and divided by an estimate for the distance to the focus (note that we had a KRS-5 lens not designed for EXES, so it just has a stock focal length that is close).  This half-angle = 0.0251 radians.  I calculate the corresponding solid angle as 2*!pi*(1-cos(0.0251)) = 0.001979 sr.  I next took EXES design numbers for performance on SOFIA.  Since area*omega should be invariant, if we had the proper SOFIA lens in place, this should be equivalent.  We didn't have that lens in place, so I view this more as a sanity check if we're close.  Using the SOFIA telescope = 2.375m diameter (accounts for undersized cold stop), EXES as an f/10 system, and 25 micron pixels, I get a platescale of 0.217 arcsecond per pixel or 1.053e-6 radians per pixel.  The net result ends up that area*omega from CD13 is about 4 times larger than using SOFIA design parameters.

Anyway, this entire digression of area*omega is possibly relevant because when I calculate the number of photons that should be incident per pixel assuming 100% throughput compared with the record number of electrons, I get unrealistically high numbers for the CD13 experimental area*omega and realistic numbers for SOFIA design parameters.  I'm hoping someone will find an error in my calculations.

Below is my plot for throughput.  White is with CD13 area*omega and red is the SOFIA design.  I wouldn't be surprised if the true throughput is even a factor of two below the red points for this wavelength.  I plot values as a function of wavelength and as a function of echelle angle.  It is apparent the echelle is more efficient at angles <55 degrees.  You can also see that we need a blocker for the filter to get the 16-17.5 wavelength region.