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From: jtomsick@ssl.berkeley.edu
Subject: Why am I seeing variation in the pn frame time?
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Date: Wed, 16 Jul 2008 21:51:27 GMT
From: jtomsick@ssl.berkeley.edu
To: xmmhelp@sciops.esa.int
Cc: jtomsick@ssl.berkeley.edu
Subject: Why am I seeing variation in the pn frame time?
Full_Name: John Tomsick
Submission from: (NULL) (128.32.98.9)


Hello,

I have been working on a timing project with the pn instrument.  Ideally, I 
would like to use the full ~6 ms time resolution of the instrument (in small 
window mode), but I have been having some problems.

Prior to making light curves and power spectra, I wanted to determine the
exact pn time resolution (i.e., the frame time).  I used a technique where
I made a data array of all of the time tags in an event list, and looked at
the time differences between events in consecutive frames.  While I expected
to find a single value close to 6 ms, I actually find a range of time
differences
between 5.6346 ms and 5.6756 ms, which appears to indicate that the
frame time is not constant (note that I have seen exactly the same spread
of time differences in 2 different data sets, and I have also seen this in
the pipeline event file as well as event files that I have made with epproc
with the time randomization off).

I suspect that the non-constant frame time (if I am correct in this conclusion)
causes significant artifacts (i.e., spikes) in the pn power spectra at high
frequencies (multiple spikes between 8 and 88 Hz in data sets from observations
of 2 different sources).

I am wondering if this is a known issue and whether there is something
that can be done to allow for timing studies above 8 Hz or if I need to
restrict my study to lower frequencies.  Thank you for your help.

John Tomsick
UC Berkeley


Reply 1

Resend
From: Nora Loiseau <xmmhelp@sciops.esa.int>
To: jtomsick@ssl.berkeley.edu
Subject: Re: Why am I seeing variation in the pn frame time? (PR#36994)
Date: Fri Jul 18 15:15:00 2008
Dear Dr Tomsick

here are the comments to your problem by SAS developers:

The exact frame time for EPIC-pn Small Window (SW) mode is
141795 clocks of the oscillator (which is *approximately*
25 MHz, and thus the frame time in time units is 5.67180 ms).
The frame time is *very* constant, the observed jitter is just
the time resolution in the ODF files. See Sect.3 in
ftp://ftp.xray.mpe.mpg.de/people/mjf/public/mpe_report_288_159.pdf

FRAMETIM contains the real frame time, while FRMTIME has the integer-
valued ODF quantity.
Randomization will reduce but not vanish binning effects as the
events are only smeared within an about 70% time window of the
readout cycle (30% dead time without photons).

Best regards,

Nora


> Submission from: (NULL) (128.32.98.9)
> 
> 
> Hello,
> 
> I have been working on a timing project with the pn instrument.  Ideally, I

> would like to use the full ~6 ms time resolution of the instrument (in
small 
> window mode), but I have been having some problems.
> 
> Prior to making light curves and power spectra, I wanted to determine the
> exact pn time resolution (i.e., the frame time).  I used a technique where
> I made a data array of all of the time tags in an event list, and looked
at
> the time differences between events in consecutive frames.  While I
expected
> to find a single value close to 6 ms, I actually find a range of time
> differences
> between 5.6346 ms and 5.6756 ms, which appears to indicate that the
> frame time is not constant (note that I have seen exactly the same spread
> of time differences in 2 different data sets, and I have also seen this in
> the pipeline event file as well as event files that I have made with
epproc
> with the time randomization off).
> 
> I suspect that the non-constant frame time (if I am correct in this
conclusion)
> causes significant artifacts (i.e., spikes) in the pn power spectra at
high
> frequencies (multiple spikes between 8 and 88 Hz in data sets from
observations
> of 2 different sources).
> 
> I am wondering if this is a known issue and whether there is something
> that can be done to allow for timing studies above 8 Hz or if I need to
> restrict my study to lower frequencies.  Thank you for your help.
> 
> John Tomsick
> UC Berkeley
> 
> ----
Dr. Nora Loiseau
XMM-Newton User Support Group


Followup 1

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From: John Tomsick <jtomsick@ssl.berkeley.edu>
Subject: Re: Why am I seeing variation in the pn frame time? (PR#36994)
Date: Mon, 21 Jul 2008 15:58:55 -0700
To: Nora Loiseau <xmmhelp@sciops.esa.int>
Dear Nora,

Thank you very much for your answer.  I had success in removing the
artifacts from my power spectra when I turn off the randomization and
use the exact frame time.  Just FYI, I did not find any keyword
FRAMETIM in the event file header, but maybe it is in a different file.
Thanks again for your help.

John

On Jul 18, 2008, at 3:14 PM, Nora Loiseau wrote:

> Dear Dr Tomsick
>
> here are the comments to your problem by SAS developers:
>
> The exact frame time for EPIC-pn Small Window (SW) mode is
> 141795 clocks of the oscillator (which is *approximately*
> 25 MHz, and thus the frame time in time units is 5.67180 ms).
> The frame time is *very* constant, the observed jitter is just
> the time resolution in the ODF files. See Sect.3 in
> ftp://ftp.xray.mpe.mpg.de/people/mjf/public/mpe_report_288_159.pdf
>
> FRAMETIM contains the real frame time, while FRMTIME has the integer-
> valued ODF quantity.
> Randomization will reduce but not vanish binning effects as the
> events are only smeared within an about 70% time window of the
> readout cycle (30% dead time without photons).
>
> Best regards,
>
> Nora
>
>
>> Submission from: (NULL) (128.32.98.9)
>>
>>
>> Hello,
>>
>> I have been working on a timing project with the pn instrument.   
>> Ideally, I
>> would like to use the full ~6 ms time resolution of the instrument  
>> (in small
>> window mode), but I have been having some problems.
>>
>> Prior to making light curves and power spectra, I wanted to  
>> determine the
>> exact pn time resolution (i.e., the frame time).  I used a  
>> technique where
>> I made a data array of all of the time tags in an event list, and  
>> looked at
>> the time differences between events in consecutive frames.  While  
>> I expected
>> to find a single value close to 6 ms, I actually find a range of time
>> differences
>> between 5.6346 ms and 5.6756 ms, which appears to indicate that the
>> frame time is not constant (note that I have seen exactly the same  
>> spread
>> of time differences in 2 different data sets, and I have also seen  
>> this in
>> the pipeline event file as well as event files that I have made  
>> with epproc
>> with the time randomization off).
>>
>> I suspect that the non-constant frame time (if I am correct in this
> conclusion)
>> causes significant artifacts (i.e., spikes) in the pn power  
>> spectra at high
>> frequencies (multiple spikes between 8 and 88 Hz in data sets from
> observations
>> of 2 different sources).
>>
>> I am wondering if this is a known issue and whether there is  
>> something
>> that can be done to allow for timing studies above 8 Hz or if I  
>> need to
>> restrict my study to lower frequencies.  Thank you for your help.
>>
>> John Tomsick
>> UC Berkeley
>>
>> ----
> Dr. Nora Loiseau
> XMM-Newton User Support Group
>
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Reply 2

Resend
From: Nora Loiseau <xmmhelp@sciops.esa.int>
To: jtomsick@ssl.berkeley.edu
Subject: Re: Why am I seeing variation in the pn frame time? (PR#36994)
Date: Tue Jul 22 08:46:27 2008
Dear John,

I am glad that you managed to solve the problem. 
Thank you for your feedback, I will pass your comments to the calibration
scientists.

Best regards,

Nora

> Dear Nora,
> 
> Thank you very much for your answer.  I had success in removing the
> artifacts from my power spectra when I turn off the randomization and
> use the exact frame time.  Just FYI, I did not find any keyword
> FRAMETIM in the event file header, but maybe it is in a different file.
> Thanks again for your help.
> 
> John
> 
> ----
Dr. Nora Loiseau
XMM-Newton User Support Group


Reply 3

Resend
From: Nora Loiseau <xmmhelp@sciops.esa.int>
To: jtomsick@ssl.berkeley.edu
Subject: Re: Why am I seeing variation in the pn frame time? (PR#36994)
Date: Tue Jul 22 09:12:18 2008
Dear John,

By the way, you mean "turn on the randomization"?
> Thank you very much for your answer.  I had success in removing the
> artifacts from my power spectra when I turn off the randomization and
> use the exact frame time.  

> I did not find any keyword
> FRAMETIM in the event file header, but maybe it is in a different file.

The FRAMETIM keyword is in the EXPOSUXX extension (where XX is the CCD number)
of the event list file.

Nora
----
Dr. Nora Loiseau
XMM-Newton User Support Group


Followup 2

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From: John Tomsick <jtomsick@ssl.berkeley.edu>
Subject: Re: Why am I seeing variation in the pn frame time? (PR#36994)
Date: Tue, 22 Jul 2008 09:27:12 -0700
To: Nora Loiseau <xmmhelp@sciops.esa.int>
Nora,

On Jul 22, 2008, at 9:12 AM, Nora Loiseau wrote:

> By the way, you mean "turn on the randomization"?
>> Thank you very much for your answer.  I had success in removing the
>> artifacts from my power spectra when I turn off the randomization and
>> use the exact frame time.

No, I mean turn off the randomization.  I'm not sure that that is  
absolutely
essential, but it allows me to keep track of the frame in which each  
photon
arrives.

John


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