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Main
Date: 08 May 2007 16:42:26
From: Klaatu
Subject: Modified RC vs SC
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Any options, particularly if you've looked/imaged through both, whether the
optics of a Meade modified Ritchey-Chrétien are work the extra money over a
Celestron Schmidt-Cassegrain. Say the Meade RX200 or RCX400 10 to 12 inch
($3699 to $6999) vs the Celestron 11 inch CPC GPS ($2799 -$200 rebate).
Will be used both visually as well as some CCD imaging. Probably will want
to try and remote the scope eventually. Present observing location needs
the scope to be moved around a bit to avoid trees. TIA
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Date: 12 May 2007 17:46:56
From:
Subject: Re: Modified RC vs SC
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On May 9, 5:23 pm, "M104gal...@gmail.com" <M104gal...@gmail.com >
wrote:
> It is fascinating to me how much misinformation passes for "expertise"
> on the net. The following exchange from APML is very instructive.
> These folks for the most part know what they are doing:
>
Indeed everytime you post
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Date: 09 May 2007 15:23:49
From: M104galaxy@gmail.com
Subject: Re: Modified RC vs SC
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On May 9, 4:20 pm, Pierre Vandevenne <pierre@data_rescue.be > wrote:
> Chris L Peterson <c...@alumni.caltech.edu> wrote innews:hbj2431gnqmc0epk27oanc5f567ea8sp75@4ax.com:
>
> > On Tue, 08 May 2007 22:19:01 -0500, "William R. Mattil"
> > <wrmat...@ix.netcom.com> wrote:
>
> >>2) faster f-ratio means less exposure, or more depth for a given
> >>exposure.
>
> > The focal ratio doesn't impact the exposure time for anything other
> > than very short exposures, where readout noise is dominant. Otherwise,
> > exposure time is set by the desired S/N, and that is set by the number
> > of photons collected, i.e. the aperture of the scope. Nothing else.
>
> A good read here
>
> http://www.stanmooreastro.com/f_ratio_myth.htm
It is fascinating to me how much misinformation passes for "expertise"
on the net. The following exchange from APML is very instructive.
These folks for the most part know what they are doing:
" I agree with most of what Richard said. Let me make a few more
comments here.
1. It is possible for an RC to produce a flat field without a
flattener -- just make
the primary and the seconday have the same curvature. (This is true
for CC and
DK, too). The drawback is that the secondary will be unusually large
(0.5x the
primary, or even larger). This decreases collecting area and image
contrast.
If it's not such a flat-field design, than RC has the worst field
curvature among
the Cassegrain families (RC, CC, DK), when they have the same primary
focal ratios and the same secondary magnifications.
2. As Richard pointed out, RC (without correcting lenses) is not
aberration free.
Classic RC corrects for coma and spherical aberrations. General RC
(with two
aspherical mirrors, no correcting lenses) can correct for any two of
coma,
spherical, and astigmatism, but not all three at the same time.
Because of this,
with a careful combination of two mirrors and correcting lenses,
general RC
can produce very large, nearly aberration free photographic fields
(such as
TAK's BRC or FRC). They are better than most of other Cassegrains
with
corrector lenses, but they are very expansive.
Go back to Alan's original question. It depends on what you want.
Photography? I assume you are not using a STLxxx level CCD camera.
(If yes, go and get an RC). TAK Mewlon with a dedicated reducer/
corrector
can produce reasonably good photographic field for an APSC-size DSLR
or an STxx level small CCD. It is good for photographing small
objects.
Another possibility (perhaps a better one) is Vixen VMC 260. It
should
be cheaper than Mewlon 250. With a dedicated focal reducer (F7.15),
it
produces a photographic field fully covering a 35mm frame. See its
spot
diagram:
http://www.aoc.nrao.edu/~whwang/misc/Spot%20Diagrams/VMC260_with_reducer.jpg
At 19.6mm from the field center (almost the edge of a 35mm field),
its spot size is about 30um in diameter. This is 3 arcsec, roughly
corresponding to 2 arcsec seeing FWHM. In other words, if your seeing
condition is not "much better" than 2 arcsec, than the image quality
of VMC250
is nearly perfect, even at the edge of a 35mm field.
If prime-focus photography is the major concern, VMC250 is the one to
go.
Cheers,
Wei-Hao
On 5/8/07, Richard Crisp <rdcrisp@earthlink.net > wrote:
>
> ----- Original Message -----
> From: "William R. Mattil" <wrmattil@ix.netcom.com>
> To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
> Sent: Tuesday, May 08, 2007 8:31 PM
> Subject: Re: [APML] Best compromise on long focal length OTA?
>
>
> > Richard Crisp wrote:
> >> do you mean that it has to be a ritchey chretien design Bill or are you
> >> just saying that Takahashi and RCOS make finely executed telescopes?
> >
> > Hi Richard,
> >
> > I was making the point that in comparison with an SCT the RC is a much
> > better instrument. Obviously there are many other designs that can be
> > substituted. Such as a Cassegrain <g> or Dall Kirkham or even a large
> > corrected Newt. The Mewlon I don't believe measures up to an RC either.
> > Slower and less FOV. This may, or may not be a problem. But it should be
> > noted.
>
>
> i'd like to try a properly constructed SCT sometime and see how well it
> works.
>
> You may be interested to know that on-axis you will get smaller spots with a
> newt or classical cass over an RC. You will also get the strongest field
> curvature with an RC of any of the cassegrain variants. The RC design has
> strong astigmatism as well.
>
> The claim that RCs are "aberration free" is simply false. Both field
> curvature and astigmatism are in fact aberrations and the RC design has both
> in spades.
>
> if you are willing to use a corrector then both can be eliminated and the
> shortcomings of other cassegrain variants can be corrected by a corrector as
> well. So then the issue becomes what is the most important to you.
>
> If you are doing astrometry, you definitely want an RC: the round and or
> astigmatic stars are fine even if they are fat: all you are trying to do is
> find the centroids of stars very accurately.
>
> If you want the smallest spot size and are after tiny objects that need a
> small FOV, then the CC may be the best choice.
>
> One often overlooked factor is the point of difficulty of figuring the
> optics. If they are fundamentally difficult to figure properly then they
> will
> 1) be expensive
> 2) likely to be done improperly irrespective of #1 above
>
> A fast hyperboloidal primary is notorious for having turned down edge
> problems but that is what you must contend with in an RC design. And for
> what? Big benefit if you are doing astrometry. Not so clear what advantages
> you actually get if you are after a wide FOV and use a big sensor:
>
> the RC requires a flattener, and that costs dollars and also at least opens
> the possibility of using a corrector with another cassegrain variant that
> may be easier to make well and therefore cheaper.
>
> The DK as modified by using a corrector is another interesting possibility
> and you should take a look at what Planewave is doing if you are unaware.
>
> it is also hard to beat a Classical Cassegrain if you are willing to use an
> f/10 system or slower (native) and if you want a big FOV, then use a
> corrector. The paraboloid is a no-brainer to figure well and as a result you
> are far more likely to get a better optics set for less money. Using a
> corrector will flatten the field, which is actually curved less than an RC
> of the same primary F/ratio and secondary with the same multiplier. The Coma
> isn't bad at all but again if you use a corrector that can be addressed as
> well
>
> About the only time you can get away with not using a flattener with an RC
> is to use a small sensor or a huge ritchey or both. With today's larger
> sensors and with scopes that can be used on AP1200/Paramount class mounts,
> you will need a flattener, and once you open that can of worms you really
> should consider the possibiliy of using a corrector with another cassegrain
> variant and you may get the better telescope for taking pretty pictures.
>
> RCs are way over hyped in my estimation and most people have lost sight of
> why they were developed originally and for what problem they were intended
> to solve.
>
>
> >
> >>
> >> They are different statements and it is important to be precise as to
> >> what exactly you are saying.
> >
> > Agreed .... I can make direct comparisons between a C-11 and an RCOS. The
> > RCOS wins on pretty much every category except price <g>
> >
>
> I agree. They do a fine job in their execution and charge a king's ransom
> for their work too. And that ransom has increased even faster than gasoline
> prices if you compare 2003 prices with today's but that's an entirely
> different discussion.
>
>
> >
> >> thanks in advance for the clarification,
> >
> > No problem .....
> >
> > Enjoying the heat wave on the left coast ?
>
>
> I get clear skies at least :-)
>
>
> >
> > Best Regards
> >
> > Bill
> >
> >
> > --
> >
> > William R. Mattil : http://www.celestial-images.com
> >
> > _______________________________________________
> > Astro-Photo mailing list
> > Astro-Photo@seds.org
> > http://seds.org/mailman/listinfo/astro-photo
>
> _______________________________________________
> Astro-Photo mailing list
> Astro-Photo@seds.org
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Date: 09 May 2007 07:33:16
From: atasselli@hotmail.com
Subject: Re: Modified RC vs SC
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RMOLLISE wrote:
> On May 8, 6:42 pm, Klaatu <mutster8_nosp...@netscape.net> wrote:
> > Any options, particularly if you've looked/imaged through both, whether=
the
> > optics of a Meade modified Ritchey-Chr=E9tien are work the extra money =
over a
> > Celestron Schmidt-Cassegrain. Say the Meade RX200 or RCX400 10 to 12 i=
nch
> > ($3699 to $6999) vs the Celestron 11 inch CPC GPS ($2799 -$200 rebate).
> > Will be used both visually as well as some CCD imaging. Probably will =
want
> > to try and remote the scope eventually. Present observing location nee=
ds
> > the scope to be moved around a bit to avoid trees. TIA
>
> The answer is "it depends."
>
> If you're going to be imaging with film or large CCD chips, yes, you
> might like the <ahem> Meade "Advanced Ritchey Chretien Design." ;-)
>
> It's nice to start out at f/8, too.
>
> The bells and whistles of the RCX are nice too...assuming they have or
> will conquer the apparently serious QA issues that have plagued the
> scope thus far.
>
> I've had the opportunity to use one RCX, the 10-inch, extensively, and
> was impressed.
>
> As for the LX200R? That's harder. The only change has been the swapout
> of the standard optics of the LX200GPS for the new RCX style, but in f/
> 10. Most folks are not gonna image at f/10 anywhy, and will be using a
> reducer/field flattener, which somewhat arbogates the advantage of the
> new SCT optics.
I don't see why working with a FR (say the x.67 one) should abrogate
the advantage of the better corrected field of the R line. I've tried
the combo and works fine with a medium size ccd with a pretty decent
image scale too (0.78"/px).
Andrea T.
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Date: 09 May 2007 07:25:10
From: M104galaxy@gmail.com
Subject: Re: Modified RC vs SC
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On May 8, 10:19 pm, "William R. Mattil" <wrmat...@ix.netcom.com >
wrote:
> Klaatu wrote:
> > Any options, particularly if you've looked/imaged through both, whether=
the
> > optics of a Meade modified Ritchey-Chr=E9tien are work the extra money =
over a
> > Celestron Schmidt-Cassegrain. Say the Meade RX200 or RCX400 10 to 12 i=
nch
> > ($3699 to $6999) vs the Celestron 11 inch CPC GPS ($2799 -$200 rebate).
> > Will be used both visually as well as some CCD imaging. Probably will =
want
> > to try and remote the scope eventually. Present observing location nee=
ds
> > the scope to be moved around a bit to avoid trees. TIA
>
> While I am not specifically a big fan of Meade in general due to their
> shoddy marketing hype, the RCX (Improved SCT) has a number of features
> that can help imaging.
>
> 1) Fixed primary mirror. This means less image shift
>
> 2) faster f-ratio means less exposure, or more depth for a given exposure.
>
> These are big improvements. But they come at a price.....
>
> 1) Typical Meade quality Control (read as none) Numerous problems with
> the tip/tilt collimation mechanism which is motor controlled.
>
> 2) Meade Mount, some can make it work. Others have a lot of trouble with =
it.
>
> The optics appear to be good. Probably as good as Celestron. But more
> bells and whistles mean more possibility of things going wrong.
>
> The LX200R <opinion> is a step backward. Meade reverted to a moving
> primary f/10 scope. Argh. They should have done away with the motorized
> collimation crap and left the Mirror fixed and at f/8. </opinion>
>
> Bill
If you are going to comment you ought to get your facts straight. Even
Star Instruments, one of the Plaintiffs in the Meade suit, has a white
paper that points out that the Meade RCX is better than a plain SCT by
a factor of about 10 in spot size. Read the following CAREFULLY:
http://www.star-instruments.com/comparisions.html
The fact that he claims the classical RC is slightly better does NOT
make the RCX only as good as an SCT.
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Date: 09 May 2007 04:34:54
From: RMOLLISE
Subject: Re: Modified RC vs SC
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On May 8, 6:42 pm, Klaatu <mutster8_nosp...@netscape.net > wrote:
> Any options, particularly if you've looked/imaged through both, whether t=
he
> optics of a Meade modified Ritchey-Chr=E9tien are work the extra money ov=
er a
> Celestron Schmidt-Cassegrain. Say the Meade RX200 or RCX400 10 to 12 inch
> ($3699 to $6999) vs the Celestron 11 inch CPC GPS ($2799 -$200 rebate).
> Will be used both visually as well as some CCD imaging. Probably will wa=
nt
> to try and remote the scope eventually. Present observing location needs
> the scope to be moved around a bit to avoid trees. TIA
The answer is "it depends."
If you're going to be imaging with film or large CCD chips, yes, you
might like the <ahem > Meade "Advanced Ritchey Chretien Design." ;-)
It's nice to start out at f/8, too.
The bells and whistles of the RCX are nice too...assuming they have or
will conquer the apparently serious QA issues that have plagued the
scope thus far.
I've had the opportunity to use one RCX, the 10-inch, extensively, and
was impressed.
As for the LX200R? That's harder. The only change has been the swapout
of the standard optics of the LX200GPS for the new RCX style, but in f/
10. Most folks are not gonna image at f/10 anywhy, and will be using a
reducer/field flattener, which somewhat arbogates the advantage of the
new SCT optics.
Unk Rod
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Date: 08 May 2007 22:19:01
From: William R. Mattil
Subject: Re: Modified RC vs SC
|
Klaatu wrote:
> Any options, particularly if you've looked/imaged through both, whether the
> optics of a Meade modified Ritchey-Chrétien are work the extra money over a
> Celestron Schmidt-Cassegrain. Say the Meade RX200 or RCX400 10 to 12 inch
> ($3699 to $6999) vs the Celestron 11 inch CPC GPS ($2799 -$200 rebate).
> Will be used both visually as well as some CCD imaging. Probably will want
> to try and remote the scope eventually. Present observing location needs
> the scope to be moved around a bit to avoid trees. TIA
While I am not specifically a big fan of Meade in general due to their
shoddy marketing hype, the RCX (Improved SCT) has a number of features
that can help imaging.
1) Fixed primary mirror. This means less image shift
2) faster f-ratio means less exposure, or more depth for a given exposure.
These are big improvements. But they come at a price.....
1) Typical Meade quality Control (read as none) Numerous problems with
the tip/tilt collimation mechanism which is motor controlled.
2) Meade Mount, some can make it work. Others have a lot of trouble with it.
The optics appear to be good. Probably as good as Celestron. But more
bells and whistles mean more possibility of things going wrong.
The LX200R <opinion > is a step backward. Meade reverted to a moving
primary f/10 scope. Argh. They should have done away with the motorized
collimation crap and left the Mirror fixed and at f/8. </opinion >
Bill
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Date: 09 May 2007 04:28:48
From: Chris L Peterson
Subject: Re: Modified RC vs SC
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On Tue, 08 May 2007 22:19:01 -0500, "William R. Mattil"
<wrmattil@ix.netcom.com > wrote:
>2) faster f-ratio means less exposure, or more depth for a given exposure.
The focal ratio doesn't impact the exposure time for anything other than
very short exposures, where readout noise is dominant. Otherwise,
exposure time is set by the desired S/N, and that is set by the number
of photons collected, i.e. the aperture of the scope. Nothing else.
_________________________________________________
Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com
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Date: 09 May 2007 16:20:31
From: Pierre Vandevenne
Subject: Re: Modified RC vs SC
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Chris L Peterson <clp@alumni.caltech.edu > wrote in
news:hbj2431gnqmc0epk27oanc5f567ea8sp75@4ax.com:
> On Tue, 08 May 2007 22:19:01 -0500, "William R. Mattil"
> <wrmattil@ix.netcom.com> wrote:
>
>>2) faster f-ratio means less exposure, or more depth for a given
>>exposure.
>
> The focal ratio doesn't impact the exposure time for anything other
> than very short exposures, where readout noise is dominant. Otherwise,
> exposure time is set by the desired S/N, and that is set by the number
> of photons collected, i.e. the aperture of the scope. Nothing else.
A good read here
http://www.stanmooreastro.com/f_ratio_myth.htm
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