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View Full Version : Re: Detecting the flashes from the Crab Nebula Pulsar


jerry warner
July 31st 03, 05:51 AM
well done. I found your technical notes very good.
Jerry




Robin Leadbeater wrote:

> Hi,
>
> I am not a regular contributor to this group but I posted this on
> uk.sci.astronomy yesterday and it was felt it warranted a wider audience, so
> here goes!
>
> I have been attempting to record the optical pulses from the pulsar in M1
> using a strobe technique and my home built CCD camera. It has been tough
> extracting it from the noise but I am fairly convinced I have managed it. I
> have done a bit of searching and I cannot find any record on the web at
> least of this having been done on an amateur basis before, but I expect to
> be proved wrong ;-) It is certainly an interesting and challenging project
> though!
>
> You can see images of the Pulsar 'on' and 'off ' at my site here
>
> http://www.leadbeaterhome.fsnet.co.uk/astro_image_33.htm
>
> and there is a link from there to pages which explain the technique and go
> into the results in more detail
>
> Clear Skies,
> Robin
> www.leadbeaterhome.fsnet.co.uk/astro.htm

Eric
August 1st 03, 01:43 AM
Nice job, Robin. I wonder if a BW TV camera would be able
to pick this up on a 16" telescope...

Eric.

>>You can see images of the Pulsar 'on' and 'off ' at my site here
>>
>>http://www.leadbeaterhome.fsnet.co.uk/astro_image_33.htm
>>

David M. Palmer
August 3rd 03, 12:25 AM
In article >, matt
> wrote:

> it seems like the ideal equipment would be a long exposure camera with an
> electronic or electronically controlled separate shutter . Modulate the
> shutter with a 30Hz PWM signal over a long exposure , then change PWM phase
> 180deg (maintain the 30Hz timebase coherence) and do another exposure .

That's what these people are doing, using an electronicly-controlled
motor driving a disk with a slot in it. (Something like an LCD
electronic shutter would be too slow. A micromirror array could
probably respond fast enough, but is a lot more complicated to put
together.)

One way to do this more efficiently, doing all phases in one exposure,
would be to jitter the camera at the right frequency. Maybe spinning a
thin slanted piece of glass so that the image moves on the film plane
in a small circle. Maybe an off-center weight on the shaft of a motor
attached to the camera, using the flexibility of the telescope.

--
David M. Palmer (formerly @clark.net, @ematic.com)

Robin Leadbeater
August 8th 03, 11:36 AM
"Eric" > wrote in message
...
> Nice job, Robin. I wonder if a BW TV camera would be able
> to pick this up on a 16" telescope...
>
> Eric.

Hi Eric,

You would still need some sort of strobe technique as the frame rate would
not be fast enough to catch the 30/sec double pulse. I suspect it would not
be sensitive enough either. I have a very sensitive camera and it needs a
few seconds exposure to catch the mag 16 pulsar. To catch it in a 1/60sec
frame I estimate would have needed at least 100x more light ie an 80 inch
scope. I would not mind having a try though if you know where I can borrow
one ;-)

Robin

--

----------------------------------------------------------------
Robin Leadbeater
N54.75 W3.24
www.leadbeaterhome.fsnet.co.uk/astro.htm
----------------------------------------------------------------

Robin Leadbeater
August 8th 03, 12:05 PM
"David M. Palmer" > wrote in message
...

> One way to do this more efficiently, doing all phases in one exposure,
> would be to jitter the camera at the right frequency. Maybe spinning a
> thin slanted piece of glass so that the image moves on the film plane
> in a small circle. Maybe an off-center weight on the shaft of a motor
> attached to the camera, using the flexibility of the telescope.
>

Hi David,

ISTR someone suggesting I try that, (not you was it?) but there are a lot of
other structures at similar brightness close to the pulsar in the crab so I
suspect it might disappear into the background once everything gets shaken
about. Also I think you would need to be super accurate with your speed to
avoid smearing. By taking shorter exposures and stacking to increase signal
to noise, you can tolerate small drifts in frequency. Should work for an
isolated object though. It is certainly a very elegant idea

Clear Skies
Robin

--

----------------------------------------------------------------
Robin Leadbeater
N54.75 W3.24
www.leadbeaterhome.fsnet.co.uk/astro.htm
----------------------------------------------------------------

Don
August 8th 03, 06:26 PM
The strobe wheel technique was used at Lick Observatory to produce the first
pictures showing the pulsar pulsing and thus also confirming which star it was in
the field. However the first, days earlier, identification of that star was done
at Kitt Peak National Observatory by using a small pinhole aperture to isolate
the star at the focus of the 84-inch telescope. The detection of the pulses for
that was by a photomultiplier with its output being signal averaged synchronous
with the pulsar repetition rate. This same technique was used with a 36-inch
telescope for the very first detection of the optical pulses from the Crab
pulsar by Steward Obs. of the Univ. of Arizona..

Don

"William C. Keel" wrote:

> In sci.astro.amateur Robin Leadbeater > wrote:
>
> > "Eric" > wrote in message
> > ...
> >> Nice job, Robin. I wonder if a BW TV camera would be able
> >> to pick this up on a 16" telescope...
> >>
> >> Eric.
>
> > Hi Eric,
>
> > You would still need some sort of strobe technique as the frame rate would
> > not be fast enough to catch the 30/sec double pulse. I suspect it would not
> > be sensitive enough either. I have a very sensitive camera and it needs a
> > few seconds exposure to catch the mag 16 pulsar. To catch it in a 1/60sec
> > frame I estimate would have needed at least 100x more light ie an 80 inch
> > scope. I would not mind having a try though if you know where I can borrow
> > one ;-)
>
> This may be a case for the long-forgotten art of the mechanical strobe
> disk, eliminating the need for rapid readout or >100% efficiency.
> That's how the pulsar was first identified with a particularly
> optical object, using an intensified TV camera. Getting the timing
> and phasing right are left as exercises for folks who (unlike me)
> know how to do it.
>
> Bill Keel

Robin Leadbeater
August 8th 03, 09:27 PM
"William C. Keel" > wrote in message
...
>
> This may be a case for the long-forgotten art of the mechanical strobe
> disk, eliminating the need for rapid readout or >100% efficiency.
> That's how the pulsar was first identified with a particularly
> optical object, using an intensified TV camera. Getting the timing
> and phasing right are left as exercises for folks who (unlike me)
> know how to do it.
>
> Bill Keel
>

Hi Bill,

Yep, that's the way I did it. See

http://www.leadbeaterhome.fsnet.co.uk/pulsar_detection_1.htm

Robin

Robin Leadbeater
August 9th 03, 02:36 PM
"Eric" > wrote in message
...
> Robin Leadbeater wrote:
> > "Eric" > wrote in message
> > ...
> >
> >>Nice job, Robin. I wonder if a BW TV camera would be able
> >>to pick this up on a 16" telescope...
> >>
> >>Eric.
> >
> >
> > Hi Eric,
> >
> > You would still need some sort of strobe technique as the frame rate
would
> > not be fast enough to catch the 30/sec double pulse. I suspect it would
not
> > be sensitive enough either. I have a very sensitive camera and it needs
a
> > few seconds exposure to catch the mag 16 pulsar. To catch it in a
1/60sec
> > frame I estimate would have needed at least 100x more light ie an 80
inch
> > scope. I would not mind having a try though if you know where I can
borrow
> > one ;-)
>
> I'm just writing things down quickly - 29.804 Hz and 30.0 Hz gives a
> 0.196 Hz beat frequency. This is about 5.113 s. At 30 fps, that's 153ish
> frames. If I average 9 frames at a time, I get 17 frames per beat
> period. 9 frames at 30 fps is about 1/3 s exposure. I know we can get
> the central star in the ring with a 1s exposure, so if I gather frames
> for 30ish seconds, I might be able to pull it out of the noise.
>
> Have I missed anything really terrible ?
>
> Eric.

Hi Eric,

If Pat's figure (up thread) of 40 photons in a 3.9m scope is right you are
going to be down to less than 1 photon per video frame in a 16inch so it
could be tough to pull it out of the noise. But the big problem is that your
video shutter is open for 1/30 sec for each frame so there will be only a
few frames without the main pulse in them. (2 in 300 I think, even if the
pulse is very sharp) Another complication is the pulsar actually produces a
smaller intermediate pulse. (ie a pulse every 1/60 sec, alternately large
and small) which will degrade the on/off difference further. Can you see the
(mag15?) ring central star in a stack of 1/30sec video frames? If so you
will know you are at least in the right sensitivity ballpark.

Robin