The performance of the current GBT 40-52 GHz receiver, described in the document 40-52 GHz Receiver Performance, limits the science that can be performed by the receiver. In summary, the cause of the limitation is a restricted bandpass and high system temperatures at the band edges . This document tries to list and prioritize the science that the GBT can produce between 40 and 52 GHz. This should help us decide how we might rebuild the current receiver or redesign another to increase the science produced by the GBT in this band.
To help answer that question, I attempt to give below the range of science that could be possible with a 'perfect' 40-52 GHz receiver. I have made some judgments on priorities based on my view of the relative productivity and merit of various science projects. I am grateful to Paul Vanden Bout, Barry Turner, Harvey Liszt, Jeff Mangum, and Al Wootten who helped me clarify the science case I present here.
By a perfect Q-band receiver I mean a receiver that has a nominal receiver temperature of 35 K across the full receiver band under typical winter (Oct. 1 - May 1) weather conditions in Green Bank. I picked 35 K since it's the typical best receiver temperature for our current Q-band receiver. Using the same weather data and weather models as I used in 40-52 GHz Receiver Performance, the expected system temperatures, including spillover and CMB. of this 'perfect' receiver would be
where the 50% and 25% percentile represents, and as explained in 40-52 GHz Receiver Performance, the typical range of weather conditions one can expect with our current definition of dynamic observing. The high system temperatures at the upper end of the band are a result of the 60 GHz atmospheric O2 absorption lines. This absorption not only increases the system temperature, with a proportional increases in the noise in an observation, it also reduces the strength of any detected signal. The O2 absorption, thus, hits the signal-to-noise in any observation twice.
Although I'll mention the science that can be performed at the lower end of the receiver's band, the above graph implies that the upper end of the band will dominate the discussion.
About 24 proposals have been accepted for observing between 40 and 52 GHz. I believe that these proposals represent a good sub sample of the range of science that astronomers wish out of our Q-band receiver. We never advertised that the frequency limits of the as-built receiver were anything but the full 40-52 GHz. And, we never advertised that the as-built receiver has only two feeds. So, proposers should not have been greatly influenced by the reduced functionality of the as-built receiver.
An interesting question for planning any renovations to the current Q-band receiver or in planning a new receiver is: how much of the proposed science can be accommodated by the current receiver? By the 'perfect' receiver?
It's apparent that many astronomers have underestimated the affects of the atmosphere on their proposed observations. How many would have actually submitted their requests if they had known they would need many more observing hours? Would the referees have given good grades or the TAC allocated observing time if they had known the actual time needed?
In July, we reviewed all of the remaining Q-band proposals and gave them rankings as to whether or not they had any chance of producing productive science. Those that were placed into group A could be done; those in group B could be done only after the receiver renovations that we were planning at that time to perform in the summer of 2006; and group C could not be done even after the planned modifications.
Since we have opened up the possibility that we'll make modification to the receiver that are more extensive than originally planned, I have again reviewed the group B and C proposals to see how many could produce useful science with the 'perfect' Q-band receiver under typical Green Bank skies. I am left with 2 accepted proposals that I judge would not be successful even with a 'perfect' receiver because of significant (4x) underestimates of the time needed to achieve the desired noise. I have ignored these proposals in the following discussion.
We have no accepted proposals for continuum mapping at Q-band. The design of the current receiver is not well optimized for continuum observing. The 1/f noise at these frequencies and bandwidths is many times that predicted from the radiometer equation. Although a correlation receiver would be ideal for reducing 1/f noise, the atmosphere at Q-band does not lend itself to sensitive continuum observing.
In most cases, observers will find the 26-40 GHz (Ka-band) correlation receiver, and the low opacities at these frequencies, more productive than Q-band for continuum work. The positive spectral indices of thermal sources and dust grains would not offset the atmospheric disadvantage of Q-band over Ka-band. I judge that there's no obvious scientific advantage of Q-band continuum observing over Ka-band observing. Since a Q-band receiver that was good for continuum observing would go underutilized, continuum Q-band observing is not a high priority.
Unlike galactic lines, high red shifted lines have velocity widths of a few hundred km/s. Observations of these wide lines require baselines that are exceptionally flat. It's usually baseline structure, rather than system temperature or frequency coverage, that limits this kind of observing on the GBT. A receiver that is best suited to these kinds of observations cannot have 'defects' like resonances in the polarizers or frequency structures in the amplifiers with a characteristic scale of 10-60 MHz.
Since red shifted lines can occur at any frequency in Q-band, reducing the bandwidth of the receiver reduces the likelihood that an observer's galaxy will fall into the receiver's band. Although there's some statistical advantage of a receiver that extends to 52 GHz, the atmosphere at the upper end of the band makes ~48-52 GHz observations tough. Also, the wider the band, the more likely the receiver design will be compromised and have baseline structures. Note that we have only one request to observe a source above 49 GHz and this was in a proposal that had multiple sources. Thus, a baseline-optimized bandwidth, restricted to between 40-49 or 40-50 GHz, would be an acceptable compromise.
I judge that the high priority of these observations suggest that, if necessary, we can compromise system temperatures or bandwidth if we'll get better baselines. This compromise is counter to what I will recommend for galactic lines.
One immediate conclusion from the above graph is that there will be a frequency above which the observation of weak galactic spectral-line sources will become impractical. Although the 40-52 GHz band has not been fully explored by telescopes with similar gain as the GBT, we can be somewhat confident that our catalogs already contain all of the strong galactic lines that can be observed between 40 and 52 GHz. One can then make a judgment as to the relative importance of the various strong spectral lines.
In reviewing all of the strong Q-band lines that the current receiver cannot observe, the panel of astronomers I talked with all agree that it's critical that we have good performance to 49 GHz for observing galactic Carbon monosulfide (CS). There's also a strong Cyclopropenylidene (C3H2) line near 52 GHz but we deemed this line to have a low priority. The 18 GHz transition of this same molecule would probably be a more productive observation. For completeness, have used the NIST Recommended Rest Frequencies website (https://physics.nist.gov/cgi-bin/micro/table5/start.pl) to create a list of all known astronomical lines between 40-52 GHz (Lovas -- 40-52 GHz Known Astronomical Lines).
Weak or unknown lines will be scattered pretty much randomly throughout Q-band. Restricting the band of the receiver is a matter of reducing the likelihood that the receiver would cover a desired weak line. If, for example, the receiver was restricted to 40-49 GHz, and not extend to 52 GHz, we will have reduced by 25% the likelihood of being able to observe a random line. However, the atmosphere above 49 GHz reduces the likelihood of a successful observation of a random line.
Although the number of Q-band proposals is small, the proportion of lines above and below 49 GHz that observers wanted are close to what one would expect. Only two lines -- methylacetylene (CH3C2H) and NVII -- have been proposed for above 49 GHz.
In conclusion, I judge that the science warrants setting a high priority to great, low system temperature performance between 40-49 GHz with a greatly reduced priority for good performance above 49 GHz. Since galactic lines are very narrow, top priority will go to improving system temperature and bandwidth over improving baseline shapes. This latter compromise is counter to what I have recommend for high red shifted lines.
Even though we never advertised that the Q-band receiver did not have the intended 4 feeds, only one project so far has asked for Q-band mapping. But, even if the current receiver had 4 feeds, it's not obvious to me whether or not the extra feeds would have helped. Their proposed source has an extent that is slightly smaller that the beam separation.
Multiple feeds would help with continuum mapping but we have already assigned a low priority to continuum observing at Q-band.
That leaves us, then, with judging how often we expect someone will submit a proposal for mapping an extended source. Of all the galactic lines in Q-band, probably only some CS or recombination line sources will be extended. High red shift sources, SiO maser sources are all typically point sources and will not require mapping.
Until we see a substantial increase in the number of mapping proposals, my judgment is that there's no strong, obvious science drivers for a multi-feed Q-band receiver. It appears that such a receiver would go underutilized most of the time. It then comes down to costs, not science. For our infrequent mapping projects, what is cheaper? a multi-feed receiver with a reduction in telescope time needed to map an area?, or save the cost of building a multi-feed receiver but having to spend the extra telescope time to map a large area with only the current two feeds?
The following will fulfill the top scientific priorities for Q-band observing:
Note that the compromises mentioned in 2 and 3 are in opposition. In my judgment both galactic line work and high red shift observing have equal scientific priority. The number or Q-band proposals we have in each category are about equal. Rather, it may become an engineering or management decision as to what type of Q-band science we will emphasize.