where we stand

lsjouwerman
Posts: 21
Joined: Mon Jul 18, 2011 3:49 pm

Re: where we stand

Postby lsjouwerman » Mon Apr 21, 2014 8:36 pm

Thanks Rachel for clarifying that - I was reading your first post here as explicitly A-config for Ku but I'm happy that is a misunderstanding. And whether it is too dissimilar to the main scope depends on your viewing angle. I'm sure the Galactic folks may think All-sky is dissimilar to their main survey ideas too ;)

rosten
Posts: 19
Joined: Fri Feb 14, 2014 3:40 pm

Re: where we stand

Postby rosten » Fri Apr 25, 2014 12:49 pm

What are the tradeoffs between sensitivity and area coverage for a Ku band galactic plane survey? Right now the strawman design is more or less as laid out in Lorant's white paper (https://science.nrao.edu/science/surveys/vlass/Sjouwerman_WP_r0.pdf), which would cover all of the galactic plane accessible to the JVLA, and extend to +/- 5 degrees in latitude. This would take 3250 hours, according to Casey & Lorant's consensus. What do you lose by decreasing the survey area? Is that preferable to pulling back on the sensitivity?

lsjouwerman
Posts: 21
Joined: Mon Jul 18, 2011 3:49 pm

Re: where we stand

Postby lsjouwerman » Sat Apr 26, 2014 12:20 pm

rosten wrote:What are the tradeoffs between sensitivity and area coverage for a Ku band galactic plane survey? .... What do you lose by decreasing the survey area? Is that preferable to pulling back on the sensitivity?


You can't go less sensitive in a homogeneous high-freq survey without increasing the data rate which in the WP is 36 MB/s. Claire assumes it will be close to 25, so we're already a bit high (due to the line resources - without lines it will be 18, with only a limited selection of lines we can make it about 25 if we really must) and less sensitivity is thus not the solution to bring down the total time. The only way to do it is to cut on the area, but for an "full-plane" survey - like an "all-sky" survey - do you really want to do that? Every one degree cut in absolute latitude over 280 deg longitude saves you about 650 hours, but SFRs and other (thermal) Galactic sources are found out to higher latitudes (eg Ori, Oph, Tau, etc). The |b|<5 is also based on complementary data from other surveys that go out that far in their definition of extent of the Galactic plane. And the larger area will also yield new high-freq calibrators desperately needed for Galactic observations. A cut in l will impact any study comparing the outer plane versus the inner plane, and will leave a gap in l when combined with MeerGAL.

rosten
Posts: 19
Joined: Fri Feb 14, 2014 3:40 pm

Re: where we stand

Postby rosten » Mon Apr 28, 2014 12:45 pm

Thanks for the reply Lorant. I'm trying to get a sense of where the trade-offs are, if we are being asked to cut down the time request. This is going to be important for the horse-trading that is going to go on at the end of this week when the different working groups get together and hash out the total VLASS proposal request.

chomiuk
Posts: 12
Joined: Fri Feb 28, 2014 4:16 pm

Re: where we stand

Postby chomiuk » Thu May 29, 2014 1:40 pm

rosten wrote:-- there was discussion amongst the extragalactic, transient, and galactic working group leads about a compromise VLASS that could serve all parties. Right now the idea is an S-band survey of ~21,000 sq. deg extragalactic to 50 uJy between -10<dec<+75, using B-config [6350 hours including 25% overhead], and ~2800 sq. deg of the Galactic Plane and Bulge to 30 uJy using A-config [2350 hours including 25% overhead]. The total time request for the VLASS would be ~8,700 hours, i.e., 20% VLA usage over 5 years. My questions to this group are: the 30 uJy rms is a thermal noise limit, is this realizable in the galactic plane? How does this compare to what you could do at L band with the GMRT? What is the science yield for going to this sensitivity? If Shami & co. could pull some numbers out of their white paper for pulsar yields I would appreciate it, and Greg Sivakoff also mentioned compact objects in our telecon -- ditto for expected source yields.


In B config, my experience with lots of novae in the Galactic plane at L and C bands implies that we should easily be able to hit thermal noise. In the more compact configs, I often have to exclude the shortest baselines, but we should be in the clear with B confg.

I'm curious about the root of the GMRT question. Wouldn't comparison with GMRT hold for the entire VLASS, not just Galactic coverage? I don't know realistic GMRT performance numbers, but in the GMRT manual they say 30 microJy/beam is the best L-band performance *ever* achieved, which I assume means that it's not easy to get to this level in a fast survey mode.

jlazio
Posts: 8
Joined: Thu Mar 20, 2014 11:13 pm

Re: where we stand

Postby jlazio » Mon Jun 02, 2014 3:57 pm

chomiuk wrote:In B config, my experience with lots of novae in the Galactic plane at L and C bands implies that we should easily be able to hit thermal noise. In the more compact configs, I often have to exclude the shortest baselines, but we should be in the clear with B confg.


That's re-assuring news. That's not been my experience, though most of my experience has been closer to the Galactic center, which may be an extreme case. Do you have some specific data or examples? What would be quite compelling would be some kind of simple listing

longitude latitude expected thermal noise ratio of actual to thermal


for a half dozen targets?

jlazio
Posts: 8
Joined: Thu Mar 20, 2014 11:13 pm

Re: where we stand

Postby jlazio » Mon Jun 02, 2014 4:04 pm

chomiuk wrote:I'm curious about the root of the GMRT question. Wouldn't comparison with GMRT hold for the entire VLASS, not just Galactic coverage? I don't know realistic GMRT performance numbers, but in the GMRT manual they say 30 microJy/beam is the best L-band performance *ever* achieved, which I assume means that it's not easy to get to this level in a fast survey mode.


That comment may have been due to me. One of the original suggestions for a Galactic component was to search for non-thermal, low surface brightness objects, such as supernova remnants. However, I tend to agree with you that one does have to make the case that this survey has to be done with the VLA as opposed to telescope X.


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