Sanity check...

[Steve Olney]

Hi All,

I'm doing some ball-park calculations for the minimum flux that system consisting of a 3 m dish (50% efficiency) could detect at 1612 MHz. I am assuming 100 K Tsys, observation time 24 minutes and a pre-detection bandwidth of 1000 Hz (nominal width of spectral lines). I get an answer of Smin = 70 Jy - which seems way too optimistic.  I think I must have done something wrong - but cannot see where.  Can someone do a check of this ?

[Marcus D. Leech]

On 04/03/2023 20:15, Steve Olney wrote:

Hi All,

I'm doing some ball-park calculations for the minimum flux that system consisting of a 3 m dish (50% efficiency) could detect at 1612 MHz. I am assuming 100 K Tsys, observation time 24 minutes and a pre-detection bandwidth of 1000 Hz (nominal width of spectral lines). I get an answer of Smin = 70 Jy - which seems way too optimistic.  I think I must have done something wrong - but cannot see where.  Can someone do a check of this ?

You never get close to theoretical with amateur-scale instruments.  My professional radio astronomer friend refers to it as
  the "graunch factor".  Assume a factor of 5.

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[Steve Olney]

Hi Marcus,

Agreed - but I am aiming for ball-park.

What number did you get ?

Steve O

[Marcus D. Leech]

On 04/03/2023 20:15, Steve Olney wrote:

Assuming you used the standard radiometer equation:

Tmin = Tsys / (Ae * sqrt(Bw * Tau))

Your 70K is about correct for a 3m dish at 50% efficiency (effective
diameter of 2.12m)

[Steve Olney]

Hi Marcus,

I got Smin = 70 Jy.  What is your Tmin = 70K in Smin Jy ?

[Steve Olney]

I'm confused. For Tmin I get 0.024 K. Where did the 70K come from ?

On Sunday, March 5, 2023 at 12:57:38 PM UTC+11 Marcus D. Leech wrote:

[duncan campbell-wilson]

Hi Steve,

Have you factored in the spill and antenna losses?

Rgds Duncan

On Sun, Mar 5, 2023 at 10:45 AM Steve Olney <hawkr...@gmail.com> wrote:

Hi All,

I'm doing some ball-park calculations for the minimum flux that system consisting of a 3 m dish (50% efficiency) could detect at 1612 MHz. I am assuming 100 K Tsys, observation time 24 minutes and a pre-detection bandwidth of 1000 Hz (nominal width of spectral lines). I get an answer of Smin = 70 Jy - which seems way too optimistic.  I think I must have done something wrong - but cannot see where.  Can someone do a check of this ?

--

[Steve Olney]

Hi Duncan,

No - it's just ball-park.  As we know the ball park can be very large :-)

I'm trying to get a handle on whether my 3 m dish in transit mode is capable of successfully detecting VY CMa OH 1612.

[Marcus D. Leech]

On 04/03/2023 20:15, Steve Olney wrote:

Also, see the parts in this tutorial here that talks about the
contribution of gain stability to the perfect radiometer equation. It's
  not pretty.  Basically gain variations contribute on a square-law
basis to increasing the effective Tsys.

https://www.cv.nrao.edu/~sransom/web/Ch3.html

[fasleitung3]

Hi Steve,

I believe your calculation is basically correct, I came up with 65 Jy. However, this is for the signal beeing equal to the rms noise. This would be insufficient SNR to actually see the line. You would probably need something like a SNR of 5 to be safe, so this would increase your minimum detectable flux to ~ 350.

Cheers,

Wolfgang

[Steve Olney]

Wolfgang,

I sent a reply some 3 hours ago via the group email pathway...

It may or may not turn up eventually...

Steve O

[fasleitung3]

Some strange things are going on with the list: I did not receive your original post, I only saw the response from Marcus.

The response you mentioned has not shown up yet for me....

Wolfgang

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[Steve Olney]

Hi Wolfgang,

I think I remember many moons ago that the group forum web interface is the most reliable.  Anyway - this was the email I sent to the group via - email...

"Hi Wolfgang,

Thanks.

Yes - the 70 Jy was for the minimum flux density detectable as per Kraus. After that it depends of what multiple of that one chooses for a positive detection."

Cheers

Steve O

[Steve]

Hi Wolfgang,

Thanks.

Yes - the 70 Jy was for the minimum flux density detectable as per
Kraus. After that it depends of what multiple of that one chooses for a
positive detection.

On 6/03/2023 8:13 pm, 'fasleitung3' via Society of Amateur Radio

Astronomers wrote:
> Hi Steve,
> I believe your calculation is basically correct, I came up with 65 Jy.
> However, this is for the signal beeing equal to the rms noise. This
> would be insufficient SNR to actually see the line. You would probably
> need something like a SNR of 5 to be safe, so this would increase your
> minimum detectable flux to ~ 350.
> Cheers,
> Wolfgang

--

Cheers

Steve Olney (HawkRAO)

(https://sites.google.com/view/hawkrao)

[Steve Olney]

And here it is - 24 hours delay... LOL...

[Marcus D. Leech]

On 07/03/2023 17:13, Steve Olney wrote:

And here it is - 24 hours delay... LOL...

e-mail has to go by ship to Oz these days.  State of the world..... :) :)

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[Steve Olney]

Yes - it seems so. I'll try and remember to only use the forum web interface.