Step up from Toucam ?

Anyone got any experience of the SAC cameras as stocked by www.opticstar.com
?
They seem to offer reasonable value, ie 3.3MB, pieter cooled and autoguiding
port for £799 . Too good to be true ?.

Cheers

"dylan" wrote in message
...
Anyone got any experience of the SAC cameras as stocked by
www.opticstar.com ?
They seem to offer reasonable value, ie 3.3MB, pieter cooled and
autoguiding port for £799 . Too good to be true ?.

Cheers
It doesn't have an 'autoguiding port'. All it has is a USB 'pass through'
port. To autoguide, you add a second camera connected to this. You can use
another one of their cameras, or a normal webcam. Beware also, that this
requires that you can control your scope from the PC, as opposed to using
a normal 'autoguider interface' connection (which on many scopes gives
finer movements than PC control can manage). For example, if you autoguide
a SS2K based scope, using the autoguider port, it can nudge in fractions
of an arcsecond. If instead you control it from a PC, using the serial
port, the smallest movement is 'rounded' in the code, and is a couple of
arc seconds. Can make a very significant difference to smooth
autoguiding...

Best Wishes

Roger Hamlett wrote:
.....
With all these cameras, to make good use of the camera, the mount needs to
be guided. Summing multiple short exposures, while a great way of doing
better than a short exposure on it's own, and having certain advantages
(combining images this way, can give improved resolution), is no
substitute for longer images.

Simply not true as a carpet statement. Resolution rules, however.

Andrea T.

wrote in message
oups.com...

Roger Hamlett wrote:
....
With all these cameras, to make good use of the camera, the mount needs
to
be guided. Summing multiple short exposures, while a great way of doing
better than a short exposure on it's own, and having certain advantages
(combining images this way, can give improved resolution), is no
substitute for longer images.

Simply not true as a carpet statement. Resolution rules, however.
Why 'not true'?. I'd like to see a sum of any number of 0.1 second
exposures, reach Mag 20+ on a normal scope. Just won't happen. Conversely
though, start with 100 second exposures, and even though such dim stars
probably won't be visible in single exposures, a sum may well show them.
There has to be some detectable signal, for summing to work.

Best Wishes

Roger Hamlett wrote:
wrote in message
oups.com...

Roger Hamlett wrote:
....
With all these cameras, to make good use of the camera, the mount needs
to
be guided. Summing multiple short exposures, while a great way of doing
better than a short exposure on it's own, and having certain advantages
(combining images this way, can give improved resolution), is no
substitute for longer images.

Simply not true as a carpet statement. Resolution rules, however.

Why 'not true'?. I'd like to see a sum of any number of 0.1 second
exposures, reach Mag 20+ on a normal scope. Just won't happen. Conversely
though, start with 100 second exposures, and even though such dim stars
probably won't be visible in single exposures, a sum may well show them.
There has to be some detectable signal, for summing to work.


It is not true in absolute sense because the only (theoritical)
difference between an exposure of length t and the sum of n exposures
of length t/n is the readout noise. If the this is zero then there is
no difference. And yes, there are such devices. As you said, there must
be some detectable signal (with some statistical significance, say
1-sigma above the noise floor) in the total integrated signal.

Besides, there are advantages in short exposures (let's define them as
exposures short enough not to reach the intended magnitude) if you are
in noisy environment, for example (think of Hubble, for example). And
if you live in badly light polluted area the major source of noise will
be that of the background anyway, so readout noise is much less of a
concern.

On the practical point of view I've detected Miranda (at mv=16.5) with
0.1s exposures at f/22 with the ATIK1HS (600 frames summed) with a 8".
I think this is as good as it gets, with the said hardware.

Regards

Andrea T.

wrote in message
oups.com...

Roger Hamlett wrote:
wrote in message
oups.com...

Roger Hamlett wrote:
....
With all these cameras, to make good use of the camera, the mount
needs
to
be guided. Summing multiple short exposures, while a great way of
doing
better than a short exposure on it's own, and having certain
advantages
(combining images this way, can give improved resolution), is no
substitute for longer images.

Simply not true as a carpet statement. Resolution rules, however.

Why 'not true'?. I'd like to see a sum of any number of 0.1 second
exposures, reach Mag 20+ on a normal scope. Just won't happen.
Conversely
though, start with 100 second exposures, and even though such dim stars
probably won't be visible in single exposures, a sum may well show
them.
There has to be some detectable signal, for summing to work.


It is not true in absolute sense because the only (theoritical)
difference between an exposure of length t and the sum of n exposures
of length t/n is the readout noise. If the this is zero then there is
no difference. And yes, there are such devices. As you said, there must
be some detectable signal (with some statistical significance, say
1-sigma above the noise floor) in the total integrated signal.
The problem is that quantisation error exists in any practical detector,
and while this is inconsequentially small, once you have a significant
number of recorded electrons, for signals falling into the area of
probability between two small counts, it becomes the largest effect.

Besides, there are advantages in short exposures (let's define them as
exposures short enough not to reach the intended magnitude) if you are
in noisy environment, for example (think of Hubble, for example). And
if you live in badly light polluted area the major source of noise will
be that of the background anyway, so readout noise is much less of a
concern.

On the practical point of view I've detected Miranda (at mv=16.5) with
0.1s exposures at f/22 with the ATIK1HS (600 frames summed) with a 8".
I think this is as good as it gets, with the said hardware.
You can see though why I make the statement. :-)

Best Wishes

"Roger Hamlett" wrote in message
...

The problem is that quantisation error exists in any practical detector,
and while this is inconsequentially small, once you have a significant
number of recorded electrons, for signals falling into the area of
probability between two small counts, it becomes the largest effect.


Hi Roger,

I agree something must be preventing really faint signals being pulled out
of the noise by stacking, but doesn't the impact of quantisation noise,
like readout noise, reduce as the square root of the number of frames in the
stack? In which case increasing the stack size should bring it out of the
noise eventually?

Currently writing a stacking FAQ for QCUIAG
http://mysite.wanadoo-members.co.uk/...stacking_noise
..htm

Robin

wrote in message
oups.com...


On the practical point of view I've detected Miranda (at mv=16.5) with
0.1s exposures at f/22 with the ATIK1HS (600 frames summed) with a 8".
I think this is as good as it gets, with the said hardware.


Good going Andrea. It beats a mag 14.5 I noticed on a rather low altitude
M42 I took the other day (SC3 0.2 sec 8 inch aperture)
I don't know how deep they got but it would be interesting to repeat this
sort of thing done a few years ago by QCUIAG using 20msec video frame
stacking (ISTR they used 10^4+ frame stacks)

http://www.cometdust.demon.co.uk/QCU...en/Juergen.htm

Robin

"Robin Leadbeater" wrote in message
...

"Roger Hamlett" wrote in message
...

The problem is that quantisation error exists in any practical
detector,
and while this is inconsequentially small, once you have a significant
number of recorded electrons, for signals falling into the area of
probability between two small counts, it becomes the largest effect.


Hi Roger,

I agree something must be preventing really faint signals being pulled
out
of the noise by stacking, but doesn't the impact of quantisation noise,
like readout noise, reduce as the square root of the number of frames in
the
stack? In which case increasing the stack size should bring it out of
the
noise eventually?

Currently writing a stacking FAQ for QCUIAG
http://mysite.wanadoo-members.co.uk/...stacking_noise
.htm
Difference is between quantisation noise, and quantisation _error_.
Quantisation 'noise', would be a random process, and as such ammenable to
multiple images finding the real data underneath. Unfortunately,
quantisation error, is a non random process (it is _influenced_, by the
randomness of the data underneath). You get ADC's, that over particular
ranges, will tend to 'stick' on a particular value. If you generate psuedo
random noise, on a small scale signal, and feed it through most ADC's, and
then try to regenerate the signal by stacking, you find this fixed pattern
being seen, rather than the small scale data. I was involved some time
ago, with a number of tests, trying to perform stacking like this on small
scale audio signals, and in practice, these effects destroyed the ability
to reconstitute the data, beyond perhaps about 5 bits extra, (using in
excess of 1024 samples).

Best Wishes

"Roger Hamlett" wrote in message
...

"Robin Leadbeater" wrote in message
...

"Roger Hamlett" wrote in message
...

The problem is that quantisation error exists in any practical
detector,
and while this is inconsequentially small, once you have a significant
number of recorded electrons, for signals falling into the area of
probability between two small counts, it becomes the largest effect.


Hi Roger,

I agree something must be preventing really faint signals being pulled
out
of the noise by stacking, but doesn't the impact of quantisation noise,
like readout noise, reduce as the square root of the number of frames in
the
stack? In which case increasing the stack size should bring it out of
the
noise eventually?

Currently writing a stacking FAQ for QCUIAG

http://mysite.wanadoo-members.co.uk/...stacking_noise
.htm
Difference is between quantisation noise, and quantisation _error_.
Quantisation 'noise', would be a random process, and as such ammenable to
multiple images finding the real data underneath. Unfortunately,
quantisation error, is a non random process (it is _influenced_, by the
randomness of the data underneath). You get ADC's, that over particular
ranges, will tend to 'stick' on a particular value. If you generate psuedo
random noise, on a small scale signal, and feed it through most ADC's, and
then try to regenerate the signal by stacking, you find this fixed pattern
being seen, rather than the small scale data. I was involved some time
ago, with a number of tests, trying to perform stacking like this on small
scale audio signals, and in practice, these effects destroyed the ability
to reconstitute the data, beyond perhaps about 5 bits extra, (using in
excess of 1024 samples).


Thanks Roger,

Thats very interesting. That would seem to put an upper bound on the
effectiveness of stacking. You don't happen to have a reference to this
effect that I could add to the stacking FAQ do you?
Would increasing the gain (at the expense of dynamic range) to spread the
weak signal+noise over a wider range of the ADC range help or do the errors
still mask the true signal?

Robin