VLASS Calculator

claw
Posts: 26
Joined: Mon Mar 03, 2014 7:18 pm

VLASS Calculator

Postby claw » Wed Mar 26, 2014 12:55 pm

Hi Everyone,
In the interest of coordinating planning between working groups, visualizing the VLASS proposals, and optimizing proposals, I've put together a calculator for VLASS planning. The calculator is in the format of an IPython notebook, which is essentially a Python script with visualizations built in. The key point is that it is easy to see the assumptions used and rerun the calculations.
Take a look at the current snapshot of the notebook at http://goo.gl/k8TGom. At present, only I can re-run the analysis, but it is trivial to do so. Ultimately, I'd like to make it possible for any of you to run it.
For now, I'd love to have feedback:
-- You'll note that the total survey time I calculate for the Galactic survey is half that of the latest draft circulated by Cornelia. Any thoughts on why?
-- This is a first draft, so I'd appreciate a check of my assumptions
-- Are there other visualizations you'd like to see?

thanks,
casey
Last edited by claw on Thu Mar 27, 2014 3:20 pm, edited 1 time in total.

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

Re: VLASS Calculator

Postby rosten » Thu Mar 27, 2014 2:00 pm

Casey,

I think Cornelia took the numbers from the GUTS white paper. So perhaps Betsy and Lourant could explain better the discrepancy between what you're getting and what they came up with.

claw
Posts: 26
Joined: Mon Mar 03, 2014 7:18 pm

Re: VLASS Calculator

Postby claw » Thu Mar 27, 2014 3:25 pm

Thanks, Rachel.
I am working it out with Lorant now. Seems he assumes a less efficient scanning pattern. I'll need to see how to incorporate that into my calculator.

casey

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

Re: VLASS Calculator

Postby rosten » Mon Mar 31, 2014 3:28 pm

Hi Casey,

Just following up: it sounded like Lorant was using the upper end of the frequency range for Ku band, which may explain the difference in survey speed. Did you work this out with him?

rlw@stsci.edu
Posts: 13
Joined: Mon Mar 03, 2014 6:42 pm

Re: VLASS Calculator

Postby rlw@stsci.edu » Tue Apr 01, 2014 9:22 pm

rosten wrote:Just following up: it sounded like Lorant was using the upper end of the frequency range for Ku band, which may explain the difference in survey speed. Did you work this out with him?


I'd be interested to know the answer to this too. It seems to me that you have to set the the effective pointing spacing using the upper end of the frequency range. If you use a wider spacing (e.g. using the mid-frequency spacing), the sensitivity to flat or inverted spectrum sources will vary greatly from point-to-point in the sky.

Maybe this is a question for Steve Myers, who calculated the survey speed for many different frequencies?

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

Re: VLASS Calculator

Postby lsjouwerman » Wed Apr 02, 2014 3:37 pm

Sorry for my unresponsiveness, was distracted with another urgent task before signing off today for a couple of weeks.

rlw@stsci.edu wrote:It seems to me that you have to set the the effective pointing spacing using the upper end of the frequency range. If you use a wider spacing (e.g. using the mid-frequency spacing), the sensitivity to flat or inverted spectrum sources will vary greatly from point-to-point in the sky.


This is exactly the point I conveyed to Casey; if you want to homogeneously cover the frequency you are interested in you should use that frequency for your spacing - the highest possible (18GHz) for sensitivity to thermal sources in GUTS.

lorant

gsivakoff
Posts: 17
Joined: Tue Mar 25, 2014 11:28 am

Re: VLASS Calculator

Postby gsivakoff » Wed Apr 02, 2014 11:08 pm

I think that just using the upper frequency to set your time on source is overly pessimistic. We really should fold the different expected spectrum throughout the entire band for calculating sensitivities.

On the other hand, if we're talking about the mosaic pattern, than I full agree that should probably depend on the upper frequency in each band.

rlw@stsci.edu
Posts: 13
Joined: Mon Mar 03, 2014 6:42 pm

Re: VLASS Calculator

Postby rlw@stsci.edu » Thu Apr 03, 2014 12:01 am

Oh yes, we are definitely talking about the mosaic pattern. If you don't want holes all over the sky at the high frequency end of the bandpass, you have to put the pointings sufficiently close together to overlap at those frequencies.

It would be a mess to try to understand the population of inverted-spectrum sources in a survey that loses sensitivity to them in between the pointing centers.

You don't lose that much sensitivity at lower frequencies by using closer spacing with shorter integrations. You get a lot more overlap from neighboring fields at the low frequency end of the bandpass. So it's mainly the extra overheads that come with shorter integrations that reduce the efficiency.

claw
Posts: 26
Joined: Mon Mar 03, 2014 7:18 pm

Re: VLASS Calculator

Postby claw » Thu Apr 03, 2014 3:54 pm

Sorry for the delay in responding.
I'll get Steve to chime in, but this may be a red herring. The low frequencies will always be oversampled relative to the high frequencies. Also, since all the bands have large fractional bandwidth, so a Ku-band survey is not unique in this regard. As long as we are within a beamwidth, then the whole survey will get covered. It sounds like the question is whether the beamwidth definition Steve used is correct.
For now, I'd suggest that if the Galactic group thinks they need the equivalent sensitivity of a 3000-hour survey, then it would be best to use Steve's numbers to find the depth/size at band center that gives the sensitivity you need at the high side of the band. It will help comparison of surveys to have all proposals using the same assumptions.

casey

smyers
Posts: 29
Joined: Mon Sep 09, 2013 4:43 pm

Re: VLASS Calculator

Postby smyers » Thu Apr 03, 2014 5:27 pm

Yes, this is a bit of a red herring. We can space the rows as closely as desired. As I said somewhere else, this only affects the scan speed required not the time to do the mosaic or the calculated sensitivity (those calculations are done in the limit of oversampled beam). As long as they are not so close that the speeds exceed the telescope limits it is fine.


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