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Main
Date: 20 Sep 2006 16:06:46
From: Andre
Subject: RC or just LX???
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Hai everybody
After decades of sleeping my hobby has again awakened.
Now I want to buy a good telescoop for viewing and astrophotography with
CCD.
I can not decide between a Maede RCX400 of 12 inch opening or a LX200R of 14
inch
opening...
Aperture wins?????
Any good ideas???
Thanks in advance,
Andre in Belgium
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Date: 21 Sep 2006 12:18:19
From:
Subject: Re: RC or just LX???
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Andre wrote:
> Hai everybody
>
> After decades of sleeping my hobby has again awakened.
> Now I want to buy a good telescoop for viewing and astrophotography with
> CCD.
>
> I can not decide between a Maede RCX400 of 12 inch opening or a LX200R of 14
> inch
> opening...
> Aperture wins?????
In the case where your only asperations for this telescope are CCD
derived, then the F/8 speed of the RCX400 might have some benefit over
the F/10 speed of the LX200R. Engough to outweight the (slight) loss in
aperture. And, indeed, the RCX series is better adapted to imaging,
from lack of mirror flop, focus by moving secondary, and the materials
used.
In the case where CCD imaging is balanced with visual observation, it
is much closer to a coin toss.
With the 12" RCX at $7000 and the 14" LX200R at $6400 it pretty much
remains a coin toss. I, personally, would go with the 12" RCX, but I
will accept any ones argument that the $600 difference and larger
aperture would sway the boat for them.
Interesting and difficult choice, with one good point. Either choice
will satisfy a lifetime of observations.
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Date: 21 Sep 2006 19:46:06
From: Chris L Peterson
Subject: Re: RC or just LX???
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On 21 Sep 2006 12:18:19 -0700, MitchAlsup@aol.com wrote:
>In the case where your only asperations for this telescope are CCD
>derived, then the F/8 speed of the RCX400 might have some benefit over
>the F/10 speed of the LX200R...
Focal ratio is a useless spec for imagers. First you need to match your
focal length to your pixel and sensor size (that is, determine the image
scale you want to work at). Once you've done that, you want as much
aperture as you can manage.
Except for a few special cases where you are forced to make very short
exposures, focal ratio has no bearing on exposure time. S/N is
determined only by the number of photons you collect, and that is
determined only by aperture.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 24 Sep 2006 06:37:05
From: Andre
Subject: Re: RC or just LX???
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Thanks for all the input.
Andre
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Date: 24 Sep 2006 01:02:08
From: atasselli@hotmail.com
Subject: Re: RC or just LX???
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Chris L Peterson wrote:
> On 21 Sep 2006 12:18:19 -0700, MitchAlsup@aol.com wrote:
>
> >In the case where your only asperations for this telescope are CCD
> >derived, then the F/8 speed of the RCX400 might have some benefit over
> >the F/10 speed of the LX200R...
>
> Focal ratio is a useless spec for imagers. First you need to match your
> focal length to your pixel and sensor size (that is, determine the image
> scale you want to work at). Once you've done that, you want as much
> aperture as you can manage.
>
> Except for a few special cases where you are forced to make very short
> exposures, focal ratio has no bearing on exposure time. S/N is
> determined only by the number of photons you collect, and that is
> determined only by aperture.
>
In real world, not at all. Time matters and so does focal ratio.
Andrea T.
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Date: 25 Sep 2006 05:09:58
From: Chris L Peterson
Subject: Re: RC or just LX???
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On 24 Sep 2006 01:02:08 -0700, "atasselli@hotmail.com"
<atasselli@hotmail.com > wrote:
>In real world, not at all. Time matters and so does focal ratio.
Nope. Not with electronic sensors, unless your exposure is so short that
readout noise is the dominant noise source. Otherwise, noise is just the
square root of the signal, which is determined solely by the aperture.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 25 Sep 2006 08:34:01
From: atasselli@hotmail.com
Subject: Re: RC or just LX???
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Chris L Peterson wrote:
> On 25 Sep 2006 00:17:07 -0700, "atasselli@hotmail.com"
> <atasselli@hotmail.com> wrote:
>
> >You seem a bit biased by photometry, Chris. Imaging-wise, i.e. with
> >extended sources, focal ratio is a primary concern, as it is time. Try
> >to image, with the same imager, an extended object at f/5, say, and at
> >f/10 in the same amount of time and see how far you get in the f/10
> >case wrt the f/5, both in terms of depth and in terms of S/N.
>
> This has nothing to do with photometry. There is almost no basis for
> considering focal ratio when you calculate exposure times. That's
> because you _don't_ change focal ratio. A telescope isn't a camera lens.
>
> How do you suggest I change my focal ratio? Let's consider two ways:
> first, I'll take my existing scope and reduce its aperture. Will the
> exposure time with the reduced aperture be increased? Of course, because
> fewer photons are received. If you want to say that's because the focal
> ratio is increased, go right ahead, but that's really not a very useful
> approach to analyzing what happens! Now a second way: I'll add a barlow
> and double the focal length. Does the exposure time go down? Probably
> not. What does happen, of course, is that the field of view is reduced.
> Also, assuming the original configuration was optimal, I'm now
> oversampled. But if the object of interest fits on the sensor in both
> cases, both exposures will require exactly the same exposure time to
> reach the same S/N. That's because the sensor sees exactly the same
> number of photons per unit area of the sky in either case, regardless of
> focal ratio. The number of photons per unit area of the _sensor_ is
> higher with the faster configuration, but that doesn't matter for a CCD
> unless the exposure time is so short that readout noise becomes
> significant. Interestingly, that can easily happen in some photometric
> scenarios. Focal ratio can be an important consideration to
> photometrists; where it is usually irrelevant is to aesthetic imagers.
>
> BTW, I've done your experiment (in fact, Stan Moore has images posted
> showing the results). Once you scale the images to the same size-
> perfectly fair, since one set will be at a better image scale), you
> can't tell the difference between them. At f/5 or f/10, both reach the
> same degree of S/N in the same time.
>
Chris,
In your arguments here you seem to assume that read-out noise is
neglible with respect to the signal whereas it mght be not, as well as
cosmic ray noise. This also applies to longer exposures if the dynamic
range of the object being images is very large. Besides, saying that F/
ratio isn't important in imaging you're simply ignoring that fact the
for any given camera size different F/ ratios will determine different
imaged areas. Practical issues also arise, as in focusing and tracking
which ultimately lead to a better S/N.
Andrea T.
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Date: 25 Sep 2006 15:53:38
From: Chris L Peterson
Subject: Re: RC or just LX???
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On 25 Sep 2006 08:34:01 -0700, "atasselli@hotmail.com"
<atasselli@hotmail.com > wrote:
>In your arguments here you seem to assume that read-out noise is
>neglible with respect to the signal whereas it mght be not, as well as
>cosmic ray noise.
No, I have been quite careful to point out that focal ratio is
unimportant only in the case where readout noise becomes insignificant.
In reality, however, it is usually possible (especially with aesthetic
imaging) to set the minimum exposure time so that readout noise is not a
factor. I don't see that "cosmic ray noise" is really ever important;
the effects of cosmic rays are largely invisible in any properly median
combined image set.
> This also applies to longer exposures if the dynamic
>range of the object being images is very large. Besides, saying that F/
>ratio isn't important in imaging you're simply ignoring that fact the
>for any given camera size different F/ ratios will determine different
>imaged areas. Practical issues also arise, as in focusing and tracking
>which ultimately lead to a better S/N.
As I see it, focal ratio is a secondary term. Under most circumstances,
it doesn't provide much useful information. For instance, you would
never suggest to an imager that he get an f/x scope as opposed to an f/y
scope. You would first determine the focal length, since that is almost
always the single most important spec. From there, you would seek as
much aperture as possible. Of course, the focal ratio will decrease as
that aperture increases, but the actual value of that focal ratio isn't
important.
The thing is, most discussions of focal ratio start with the
qualification "all other things being kept the same", and that can't
ever be done. You can't have two otherwise identical optical systems
that differ only in focal ratio. Focal ratio comparisons tend to be
apples vs oranges.
Focal ratio can be an important consideration in special cases. As I
already mentioned, it may be important in some photometric or
astrometric applications where the exposure time is limited. In those
cases, pixel noise may become more important than object noise (this is
never the case with aesthetic imaging), and the researcher will be
willing to trade away resolution by reducing the focal length, thereby
reducing the focal ratio and improving the pixel S/N. Focal ratio is
also an important factor in assessing optical system performance; low
focal ratios (with their consequent high cone angles) may exceed the
acceptable input angles for some eyepieces, instruments, or filters. But
realistically, in everyday usage by the vast majority of aesthetic
imagers, focal ratio is a useless concept.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 25 Sep 2006 00:17:07
From: atasselli@hotmail.com
Subject: Re: RC or just LX???
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Chris L Peterson wrote:
> On 24 Sep 2006 01:02:08 -0700, "atasselli@hotmail.com"
> <atasselli@hotmail.com> wrote:
>
> >In real world, not at all. Time matters and so does focal ratio.
>
> Nope. Not with electronic sensors, unless your exposure is so short that
> readout noise is the dominant noise source. Otherwise, noise is just the
> square root of the signal, which is determined solely by the aperture.
>
You seem a bit biased by photometry, Chris. Imaging-wise, i.e. with
extended sources, focal ratio is a primary concern, as it is time. Try
to image, with the same imager, an extended object at f/5, say, and at
f/10 in the same amount of time and see how far you get in the f/10
case wrt the f/5, both in terms of depth and in terms of S/N.
Andrea T.
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Date: 25 Sep 2006 13:38:22
From: Chris L Peterson
Subject: Re: RC or just LX???
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On 25 Sep 2006 00:17:07 -0700, "atasselli@hotmail.com"
<atasselli@hotmail.com > wrote:
>You seem a bit biased by photometry, Chris. Imaging-wise, i.e. with
>extended sources, focal ratio is a primary concern, as it is time. Try
>to image, with the same imager, an extended object at f/5, say, and at
>f/10 in the same amount of time and see how far you get in the f/10
>case wrt the f/5, both in terms of depth and in terms of S/N.
This has nothing to do with photometry. There is almost no basis for
considering focal ratio when you calculate exposure times. That's
because you _don't_ change focal ratio. A telescope isn't a camera lens.
How do you suggest I change my focal ratio? Let's consider two ways:
first, I'll take my existing scope and reduce its aperture. Will the
exposure time with the reduced aperture be increased? Of course, because
fewer photons are received. If you want to say that's because the focal
ratio is increased, go right ahead, but that's really not a very useful
approach to analyzing what happens! Now a second way: I'll add a barlow
and double the focal length. Does the exposure time go down? Probably
not. What does happen, of course, is that the field of view is reduced.
Also, assuming the original configuration was optimal, I'm now
oversampled. But if the object of interest fits on the sensor in both
cases, both exposures will require exactly the same exposure time to
reach the same S/N. That's because the sensor sees exactly the same
number of photons per unit area of the sky in either case, regardless of
focal ratio. The number of photons per unit area of the _sensor_ is
higher with the faster configuration, but that doesn't matter for a CCD
unless the exposure time is so short that readout noise becomes
significant. Interestingly, that can easily happen in some photometric
scenarios. Focal ratio can be an important consideration to
photometrists; where it is usually irrelevant is to aesthetic imagers.
BTW, I've done your experiment (in fact, Stan Moore has images posted
showing the results). Once you scale the images to the same size-
perfectly fair, since one set will be at a better image scale), you
can't tell the difference between them. At f/5 or f/10, both reach the
same degree of S/N in the same time.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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