Vixen VC200 (Visac)
John Lagerling
According to Vixen's Japanese homepage it is supposed to:
"Combines the advantages of refractor and reflector telescopes by using a
primary
mirror, a secondary mirror and a 3-element field corrector in its optical
system."
"Provides a sharp image over a wide field of view with no aberration.
Spherical aberration is corrected by the sixth-order aspherical primary
mirror,
and coma and curvature of field are corrected by the field corrector."
Picture on:
http://www.vixen.co.jp/english/telesco/hanseki.htm
Schematics:
http://www.vixen.co.jp/english/telesco/select.htm
It is similar in price to an SCT of the same aperture. I'm considering this
instead of an INTES MN56 so I would really appreciate any comments on this
construction which seems to be used by Vixen only. (Thus the name - VIxen
Sixth-order Aspherical Catadioptic telescope.
Thanks,
John Lagerling
Simon
problems associated with meade / Celestron SCT's.
It is a one off design, no other manufacturer makes any thing like it.
Only problem so far is that with the focal reducer in I can not reach focus
with a flip mirror.
Try http://www.egroups.com/group/vc200l
for more info.
Try http://www.ozemail.com.au/~nwilliam/bas/astphot1.htm for some pics
through a VC200
and http://www.ne.jp/asahi/stellar/scenes/english/index.htm
John Lagerling <jla...@algonet.se> wrote in message
news:8al8i8$nn1$1...@cubacola.tninet.se...
John Lagerling
Thanks alot for the excellent links. Now I really feel the VC200L is right
for me :-).
Mirror-flop is actually what is keeping me from getting a Meade or Celestron
SCT.
Cheers,
John L
"Simon" <a...@me.com> wrote in message
news:8aljhs$27n$1...@uranium.btinternet.com...
Clive Gibbons
John Lagerling <jla...@algonet.se> wrote:
>Simon !
>Thanks alot for the excellent links. Now I really feel the VC200L is right
>for me :-).
>Mirror-flop is actually what is keeping me from getting a Meade or Celestron
>SCT.
Hi John.
Don't let the threat "mirror flop" (image shift?) keep you from purchasing
an SCT. The majority of new 8" SCTs I've seen recently have virtually
none of this problem (especially the Celestrons).
I know of one fellow who owns a VC200L and is quite pleased with it.
However, it does have compromises, such as a 39% central obstruction,
thick secondary spider vanes and possible collimation issues (the primary
and secondary mirrors can be adjusted... or misadjusted!). The optics are
open to the air and, thus, can get pretty grungy over time. The VC200L
also has much less back-focus capability than the typical SCT, so using
certain accessories and getting them to focus may be more problematic.
Anyways, whatever you decide, you'll likely end up with a nice instrument!
Cheers,
--
Clive Gibbons
Technician, McMaster University,
School of Geography and Geology.
Richard Simmons
its built like a tank.
A friend bought a refractor, the 100 ED, and its fabulous.
BTW, I have some Vixen decals if you want one to paste on the side of the
tube. The scopes come with "Orion" on them.
Philip
Niels Wolterink
There is also an new Vixen scope!!
It`s the brother of the Vixen VC200, and that`s the Vixen VMC-200 L.
This one uses an maksutov design!!!
But the meniscus lens isn`t placed before the secundaire mirror, but between
the 1st and the 2nd!!!
This is totally an new design scope from Vixen..!!!!
See also this page!!
http://www.vehrenberg.de/html/homepage_neu_vmc.html
But I must tell you: "It is in German"
Maybe you can use the babblefish from altavista to translate it.
I do live in The Netherlands, so I can read German.....LOL
success...
Niels Wolterink
Brian J. Hughes
<ni...@gelrevision.nl> wrote:
| There is also an new Vixen scope!!
| It`s the brother of the Vixen VC200, and that`s the Vixen VMC-200 L.
| This one uses an maksutov design!!!
| But the meniscus lens isn`t placed before the secundaire mirror, but
| between
| the 1st and the 2nd!!!
| This is totally an new design scope from Vixen..!!!!
|
| See also this page!!
|
| http://www.vehrenberg.de/html/homepage_neu_vmc.html
|
| But I must tell you: "It is in German"
| Maybe you can use the babblefish from altavista to translate it.
| I do live in The Netherlands, so I can read German.....LOL
|
I ran the page through AltaVista <http://babelfish.altavista.com/>, it
was somewhat readable. Some words don't translate but the worst part is
the syntax. Just take a look at this last paragraph from that page:
"Once more proves Vixen in such a way that innovation a self purpose is
not purposefully used, but: With the new VMC-200L a telescope is to the
ambitious asterisk friend at the disposal, whose power reserves variety
of possible observation objects does not require restrictions in that."
Here's the rest of the page if your wondering:
"VMC 200L soon available!
Vixen went on the search for as versatile and mobile a reflecting optics
as possible completely new ways. The result is an optical system, which
so beforehand still nobody implemented.
The VMC-200L is a Maksutov Cassegrain system, with which in contrast to
the classical Maksutov reflector the meniscus lens in the path of rays
is installed before the sekundaerspiegel. In the comparison to the
Schmidt Cassegrain telescope a set of characteristics was improved. The
VMC-200L offers:
* Improved sharpness over the entire image field by smaller
bildfeldwoelbung
* Improves Korrektor of remainder color defects (Sphaerochromasie)
* No and Herhuepfen of the observation object when focusing (so-called
Shifting) through stationarily installed hauptspiegel
* Open tubus for faster temperature equalizing
* Higher luminous intensity of f/9, thereby shorter exposure times with
the long-term photography"
Now if only someone could translate the other words, we might have
something.
--
Brian J. Hughes
To e-mail remove UCE
Martin Brown
"Brian J. Hughes" wrote:
> In article <8amd0g$qnd$1...@thor.wirehub.nl>, "Niels Wolterink"
> <ni...@gelrevision.nl> wrote:
>
> | There is also an new Vixen scope!!
> | It`s the brother of the Vixen VC200, and that`s the Vixen VMC-200 L.
> | This one uses an maksutov design!!!
> | But the meniscus lens isn`t placed before the secundaire mirror, but
> | between
> | the 1st and the 2nd!!!
> | This is totally an new design scope from Vixen..!!!!
> |
> | See also this page!!
> |
> | http://www.vehrenberg.de/html/homepage_neu_vmc.html
> |
> | But I must tell you: "It is in German"
> | Maybe you can use the babblefish from altavista to translate it.
> | I do live in The Netherlands, so I can read German.....LOL
> |
>
> I ran the page through AltaVista <http://babelfish.altavista.com/>, it
> was somewhat readable. Some words don't translate but the worst part is
> the syntax. Just take a look at this last paragraph from that page:
>
> "Once more proves Vixen in such a way that innovation a self purpose is
> not purposefully used, but: With the new VMC-200L a telescope is to the
> ambitious asterisk friend at the disposal, whose power reserves variety
> of possible observation objects does not require restrictions in that."
Not bad for a machine translation ;-)
> Here's the rest of the page if your wondering:
>
> "VMC 200L soon available!
>
> Vixen went on the search for as versatile and mobile a reflecting optics
> as possible completely new ways. The result is an optical system, which
> so beforehand still nobody implemented.
>
> The VMC-200L is a Maksutov Cassegrain system, with which in contrast to
> the classical Maksutov reflector the meniscus lens in the path of rays
> is installed before the sekundaerspiegel. In the comparison to the
> Schmidt Cassegrain telescope a set of characteristics was improved.
Isn't this the same optical configuration as a Klevsov ?
Regards,
Martin Brown
desnar
>snip
>Only problem so far is that with the focal reducer in I can not reach focus
>with a flip mirror.
>
Agreed - I use the Taurus Tracker II system. Great for prime focus and
eyepiece projection but insufficient back focus for use with the Vixen
dedicated f6.4 focal reducer.
>
>snip
>Try http://www.ozemail.com.au/~nwilliam/bas/astphot1.htm for some pics
>through a VC200
>
I am the person who took the above photos. They were mostly taken about 3
years ago when I had just started taking astrophoto's. Some of them show
sloppy work, such as "double" stars where I nudged the OTA when opening the
Taurus shutter, without having covered the OTA. Now I use the hat trick
method.....if only I could get to a dark sky site on a new moon with no
cloud.....So, don't be put off if some stars are not quite circular on the
photos - it's an operator error, not a VC200L problem.
The Comet SOHO and the Moon piccies were taken through a Vixen ED102S f9
apo.
>snip
Cheers
Dennis
Ben Siemerink
Clive Gibbons wrote:
I've done a bit of window shopping lately and considering the following choices:
Celestron G-9.25" or Vixen VC200L.
[1] The VC200L does NOT have a 39% central obstruction. If that would be the
case, the secondary mirror would be near the 125 mm of diameter. Please note that
we are talking areas here and the area of a circle is pi*r*r, with r being the
radius = half the diameter. The technical spec says that the secondary mirror is
77 mm, hence: (pi*100*100) / (pi*38.5*38.5) = 14.8% central obstruction.
[2] The size of the secondary mirror for the VC200L is exactly the same as for
the Celestron G-8. Both have a 77 mm diameter secondary mirror.
Combining [1] and [2]: on what do you base to say that the VC200L has a large
central obstruction?
[3] The "image shift" in SCTs (Schmidt Cassegrain Telescopes) is extensively
addressed by Dennis di Cicco in the April issue of Sky & Telescope (page 136).
The image shift is typical to any instrument that focusses by changing the
distance between the primary and secondary mirrors (as do the Celestron and Meade
SCTs). Dennis di Cicco states that "[...] fortunately this shift has been reduced
to negligible levels in modern commercial instruments."
[4] However, Dennis di Cicco continues with another problem due to
mirror-distance focussing: optical aberrations, mainly spherical, but also coma
and astigmatism. He states that "[...] a Cassegrain system can be corrected only
for a specific mirror separation, and to maintain diffraction-limited
performance, the separation cannot vary by more than about 0.063*f^4 millimetres,
where f is the primary's [!] focal ratio. This is a tolerance of about 1 mm in a
typical f/10 Schmidt Cassegrain telescope with an f/2 primary." The SCTs that I
saw in the shops, let you adjust by far more than 1 mm and that is exactly what
Dennis di Cicco writes. The Vixen VC200L --which is not a Schmidt Cassegrain, but
just a Cassegrain-- does not have this problem at all.
Drawback of the VC200L is the open tube: you collect dust on both mirrors and the
correcting lens. The closed tubes in the Celestron and Meade scopes only collect
dust on the front plate. Cottet in Madrid, Spain, offers an almost free service
to let Vixen clean your tube. You only pay about US$10 on transport costs.
I want to state a major drawback of the latest Celestron G telescopes: They come
with the CG-5 mount which has been addressed various times on this news group and
is considered a Tasco quality mount, or in other words: far worse than you expect
from a top-quality amateur scope manufacturer. Even for the Celestron G-8 (8"
SCT) the tripod is really shaky. The G-9.25 which weighs quite a bit more comes
with the same shaky mount. The good thing is that it allows one to obtain a good
quality optical tube for little money. When getting tired of the tripod, one can
replace the tripod legs with wooden ones, or simply upgrade to a good mount, such
as a Vixen GP-DX. However, the G-9.25 is already a bit heavy for the GP-DX, as I
understood (please correct me if that's not true).
Well, just my 2 cents or more cents on the subject.
Saludos, Ben.
PS: Still did not decide on whether to spend my money on the bigger G-9.25 or on
the VC200L with the better eye piece, better mount and less aberration.
--
Ben Siemerink
Madrid, Spain.
Chris1011
case, the secondary mirror would be near the 125 mm of diameter. Please note
that
we are talking areas here and the area of a circle is pi*r*r, with r being the
radius = half the diameter. The technical spec says that the secondary mirror
is
77 mm, hence: (pi*100*100) / (pi*38.5*38.5) = 14.8% central obstruction>>
If the VC200l has 77mm secondary, then it has a 38.5% secondary obstruction. It
is customary to measure the obstruction by linear size, not square area. It is
done so because the linear size governs the image contrast. Brightness is not
so much of a consideration until the obstruction reaches 50% by linear size.
>>[2] The size of the secondary mirror for the VC200L is exactly the same as
for
the Celestron G-8. Both have a 77 mm diameter secondary mirror.>.
The C8 secondary size is 69mm out of 202mm clear aperture, which makes it 34%.
The Meade 8" is around 36% for the F10 model.
Roland Christen
ASTRO-PHYSICS
Simon
Chris1011 <chri...@aol.com> wrote in message
news:20000401111242...@ng-fv1.aol.com...
> >>1] The VC200L does NOT have a 39% central obstruction. If that would be
the
> case, the secondary mirror would be near the 125 mm of diameter. Please
note
> that
> we are talking areas here and the area of a circle is pi*r*r, with r being
the
> radius = half the diameter. The technical spec says that the secondary
mirror
> is
> 77 mm, hence: (pi*100*100) / (pi*38.5*38.5) = 14.8% central obstruction>>
>
> If the VC200l has 77mm secondary, then it has a 38.5% secondary
obstruction. It
> is customary to measure the obstruction by linear size, not square area.
It is
> done so because the linear size governs the image contrast. Brightness is
not
> so much of a consideration until the obstruction reaches 50% by linear
size.
>
> >>[2] The size of the secondary mirror for the VC200L is exactly the same
as
> for
Ben Siemerink
Chris1011 wrote:
> >>1] The VC200L does NOT have a 39% central obstruction. If that would be the
> case, the secondary mirror would be near the 125 mm of diameter. Please note
> that
> we are talking areas here and the area of a circle is pi*r*r, with r being the
> radius = half the diameter. The technical spec says that the secondary mirror
> is
> 77 mm, hence: (pi*100*100) / (pi*38.5*38.5) = 14.8% central obstruction>>
>
> If the VC200l has 77mm secondary, then it has a 38.5% secondary obstruction. It
> is customary to measure the obstruction by linear size, not square area. It is
> done so because the linear size governs the image contrast. Brightness is not
> so much of a consideration until the obstruction reaches 50% by linear size.
Brightness is the only most important thing for telescopes. How can you say that it
is not important?
> >>[2] The size of the secondary mirror for the VC200L is exactly the same as
> for
> the Celestron G-8. Both have a 77 mm diameter secondary mirror.>.
>
> The C8 secondary size is 69mm out of 202mm clear aperture, which makes it 34%.
> The Meade 8" is around 36% for the F10 model.
Spec says 77mm.
Saludos, Ben.
Ben Siemerink
> >Try http://www.ozemail.com.au/~nwilliam/bas/astphot1.htm for some pics
> >through a VC200
I'm really impressed with the quality of the photos and the performance of an 8"
scope!
I'm doubting between a Celestron G9 and a Vixen V(M)C200L and if the Vixen gives
already this quality why bother pay twice as much (with a good mount) for the
G9?
Clive Gibbons
Ben Siemerink <siem...@lucent.com> wrote:
>
>Hello!
>
>I've done a bit of window shopping lately and considering the following choices:
>Celestron G-9.25" or Vixen VC200L.
>
>[1] The VC200L does NOT have a 39% central obstruction. If that would be the
>case, the secondary mirror would be near the 125 mm of diameter. Please note that
>we are talking areas here and the area of a circle is pi*r*r, with r being the
>radius = half the diameter. The technical spec says that the secondary mirror is
>77 mm, hence: (pi*100*100) / (pi*38.5*38.5) = 14.8% central obstruction.
Hi Ben.
I'm talking % obstruction by diameter, here. Nobody except the folks who
market telescopes (and thus, wish to quote the smallest possible number
<g>) talk about obstruction as a % of area.
>
>[2] The size of the secondary mirror for the VC200L is exactly the same as for
>the Celestron G-8. Both have a 77 mm diameter secondary mirror.
No, the Celestron has a 2.75" diameter secondary obstruction. That's not
77mm. The VISAC definitely does have a larger obstruction, plus a
thick set of 4 support veins.
>
>Combining [1] and [2]: on what do you base to say that the VC200L has a large
>central obstruction?
See my comments above.
>
>[3] The "image shift" in SCTs (Schmidt Cassegrain Telescopes) is extensively
>addressed by Dennis di Cicco in the April issue of Sky & Telescope (page 136).
>The image shift is typical to any instrument that focusses by changing the
>distance between the primary and secondary mirrors (as do the Celestron and Meade
>SCTs). Dennis di Cicco states that "[...] fortunately this shift has been reduced
>to negligible levels in modern commercial instruments."
OK.
Image shift is usually present in modern SCTs, but it often only amounts
to a Jupiter diameter (50 arc sec.) or less. That's really not an issue.
>
>[4] However, Dennis di Cicco continues with another problem due to
>mirror-distance focussing: optical aberrations, mainly spherical, but also coma
>and astigmatism. He states that "[...] a Cassegrain system can be corrected only
>for a specific mirror separation, and to maintain diffraction-limited
>performance, the separation cannot vary by more than about 0.063*f^4 millimetres,
>where f is the primary's [!] focal ratio. This is a tolerance of about 1 mm in a
>typical f/10 Schmidt Cassegrain telescope with an f/2 primary." The SCTs that I
>saw in the shops, let you adjust by far more than 1 mm and that is exactly what
>Dennis di Cicco writes. The Vixen VC200L --which is not a Schmidt Cassegrain, but
>just a Cassegrain-- does not have this problem at all.
It's only a problem if the focal point of the SCT is pushed quite a bit
outside it's "optimal" position. For most visual work, the focus position
is such that spherical aberration won't be a factor, if the optics are
properly corrected to begin with.
>
>
>Drawback of the VC200L is the open tube: you collect dust on both mirrors and the
>correcting lens. The closed tubes in the Celestron and Meade scopes only collect
>dust on the front plate. Cottet in Madrid, Spain, offers an almost free service
>to let Vixen clean your tube. You only pay about US$10 on transport costs.
Fine.
However, I'd much prefer not to have to ship my scope to have it cleaned
and recollimated, no matter how inexpensive the service might be. Just my
preference, though.
>
>I want to state a major drawback of the latest Celestron G telescopes: They come
>with the CG-5 mount which has been addressed various times on this news group and
>is considered a Tasco quality mount, or in other words: far worse than you expect
>from a top-quality amateur scope manufacturer. Even for the Celestron G-8 (8"
>SCT) the tripod is really shaky. The G-9.25 which weighs quite a bit more comes
>with the same shaky mount. The good thing is that it allows one to obtain a good
>quality optical tube for little money. When getting tired of the tripod, one can
>replace the tripod legs with wooden ones, or simply upgrade to a good mount, such
>as a Vixen GP-DX. However, the G-9.25 is already a bit heavy for the GP-DX, as I
>understood (please correct me if that's not true).
The CG-5 mount (as it now appears) is quite fine for the G-8. It's
certainly less than optimal for the 9.25. However, you're getting a very
nice OTA there, for a good price. The mount is almost a freebee.
The aluminum tripod is a major culprit when it comes to shakiness. You can
fill the aluminum extrusions with epoxy, or replace the legs with wooden
ones.
>
>
>Well, just my 2 cents or more cents on the subject.
>
>Saludos, Ben.
>
>
>PS: Still did not decide on whether to spend my money on the bigger G-9.25 or on
>the VC200L with the better eye piece, better mount and less aberration.
I'd get the G-9.25.
I've observed with a VC200L and while it's a nice scope, the high
magnification images *do* suffer from the big obstruction and thick
secondary support stalks.
Good luck.
Ben Siemerink
I just realize that I was pushing it a bit. Sorry for that.
Clive Gibbons wrote:
> >[1] The VC200L does NOT have a 39% central obstruction. If that would be the
> >case, the secondary mirror would be near the 125 mm of diameter. Please note that
> >we are talking areas here and the area of a circle is pi*r*r, with r being the
> >radius = half the diameter. The technical spec says that the secondary mirror is
> >77 mm, hence: (pi*100*100) / (pi*38.5*38.5) = 14.8% central obstruction.
>
> I'm talking % obstruction by diameter, here. Nobody except the folks who market
> telescopes (and thus, wish to quote the smallest possible number <g>) talk about
> obstruction as a % of area.
Okay, I buy that. Thank you. I shall conform.
Although the obstruction by diameter sounds misleading as well: What does it obstruct?
If it is light, one should talk about area. "Obstruction" (for area) and "diameter
ratio" (for linear) would have been better terms, technically speaking.
> No, the Celestron has a 2.75" diameter secondary obstruction. That's not 77mm. The
> VISAC definitely does have a larger obstruction, plus a thick set of 4 support
> veins.
Well, for the moment I couldn't find the 77mm back. I give you folks the advantage of
the doubt (as one would say in Dutch). Sorry for persisting in wrong data, guys.
> >[3] The "image shift" in SCTs (Schmidt Cassegrain Telescopes) is extensively
> >addressed by Dennis di Cicco in the April issue of Sky & Telescope (page 136).
> >The image shift is typical to any instrument that focusses by changing the
> >distance between the primary and secondary mirrors (as do the Celestron and Meade
> >SCTs). Dennis di Cicco states that "[...] fortunately this shift has been reduced
> >to negligible levels in modern commercial instruments."
>
> OK.
> Image shift is usually present in modern SCTs, but it often only amounts
> to a Jupiter diameter (50 arc sec.) or less. That's really not an issue.
That's exactly what the quote says.
> >[4] However, Dennis di Cicco continues with another problem due to
> >mirror-distance focussing: optical aberrations, mainly spherical, but also coma
> >and astigmatism. He states that "[...] a Cassegrain system can be corrected only
> >for a specific mirror separation, and to maintain diffraction-limited
> >performance, the separation cannot vary by more than about 0.063*f^4 millimetres,
> >where f is the primary's [!] focal ratio. This is a tolerance of about 1 mm in a
> >typical f/10 Schmidt Cassegrain telescope with an f/2 primary." The SCTs that I
> >saw in the shops, let you adjust by far more than 1 mm and that is exactly what
> >Dennis di Cicco writes. The Vixen VC200L --which is not a Schmidt Cassegrain, but
> >just a Cassegrain-- does not have this problem at all.
>
> It's only a problem if the focal point of the SCT is pushed quite a bit outside it's
> "optimal" position. For most visual work, the focus position is such that spherical
> aberration won't be a factor, if the optics are properly corrected to begin with.
Right. Being able to do it physically, does indeed not mean that you are be doing that
all the time.
In any case, I believe/got the impression that all those little things hardly make a
difference at the end, but bigger aperture does.
> >Drawback of the VC200L is the open tube: you collect dust on both mirrors and the
> >correcting lens. The closed tubes in the Celestron and Meade scopes only collect
> >dust on the front plate. Cottet in Madrid, Spain, offers an almost free service
> >to let Vixen clean your tube. You only pay about US$10 on transport costs.
>
> Fine.
> However, I'd much prefer not to have to ship my scope to have it cleaned and
> recollimated, no matter how inexpensive the service might be. Just my preference,
> though.
I completely agree with you.
In case someone is interested, some other thoughts:
* Open tubes adapt quicker to the outside air. (good)
* Open tubes can have a current (moving air) inside the tube. (bad)
* Any dob, or newtonian collects dust. (observation)
* Concerning the design: by not moving the primary mirror, it does not decollimate
so quicky. (good)
> The CG-5 mount (as it now appears) is quite fine for the G-8. It's certainly less
> than optimal for the 9.25. However, you're getting a very nice OTA there, for a good
> price. The mount is almost a freebee. The aluminum tripod is a major culprit when it
> comes to shakiness. You can fill the aluminum extrusions with epoxy, or replace the
> legs with wooden ones.
What's epoxy? Is that insulation foam?
Since I don't have experience with any scope yet I imagine that I wouldn't notice the
so-so mount at the beginning...
> >PS: Still did not decide on whether to spend my money on the bigger G-9.25 or on
> >the VC200L with the better eye piece, better mount and less aberration.
>
> I'd get the G-9.25. I've observed with a VC200L and while it's a nice scope, the
> high magnification images *do* suffer from the big obstruction and thick secondary
> support stalks.
Finally a question about the G9.25: The VC200L has a backplane with a 2" eye piece
holder. By default it comes with an adapter to convert to 1.25" and a 1.25" eye piece,
etc. Can anybody tell me how it is with the G9.25? The G9.25's are out of stock in
Spain and until April 20th there won't be any new ones. I cannot view them in any shop
I've been to.
Thank you all for your comments!
Saludos, Ben.
desnar
>In article <38E6104D...@lucent.com>,
>big snip
>No, the Celestron has a 2.75" diameter secondary obstruction. That's not
>77mm. The VISAC definitely does have a larger obstruction, plus a
>thick set of 4 support veins.
Folks
I've just measured the spider vanes on my VC200L as follows:
Vane thickness = 5mm
Vane length = 10cms
Vane length that impinges upon mirror = 6cms
Therefore one vane area = 6 x 0.5 sq cms = 3 sq cms
Four vanes = 4 x 3 sq cms = 12 sq cms obstruction.
This should then be added to the central obstruction to obtain the overall
obstructed area.
Regards
Dennis
Ben Siemerink
First of all: what does the <snip> mean?
> Clive Gibbons wrote in message <8ca7u5$7...@mcmail.cis.McMaster.CA>...
> >No, the Celestron has a 2.75" diameter secondary obstruction. That's not
> >77mm. The VISAC definitely does have a larger obstruction, plus a
> >thick set of 4 support veins.
>
> Folks
>
> I've just measured the spider vanes on my VC200L as follows:
>
> Vane thickness = 5mm
> Vane length = 10cms
> Vane length that impinges upon mirror = 6cms
>
> Therefore one vane area = 6 x 0.5 sq cms = 3 sq cms
> Four vanes = 4 x 3 sq cms = 12 sq cms obstruction.
>
> This should then be added to the central obstruction to obtain the overall
> obstructed area.
Right. Now it would be wonderful if someone could measure the same things for
a Celestron G8 and then we can do something.
Saludos, Ben.
Ben Siemerink
Clive Gibbons wrote:
> I'm talking % obstruction by diameter, here. Nobody except the folks who market
> telescopes (and thus, wish to quote the smallest possible number <g>) talk about
> obstruction as a % of area.
I gave your last phrase some more thought and it is just the other way around:
First let me summarize the data:
VC200L:
* 200mm primary
* 77mm secondary
* 38.5% diameter length obstruction
* 14.8% area obstruction
G8:
* 203mm primary
* 70mm secondary
* 34.5% diameter length obstruction
* 11.6% area obstruction
Now: if one would want to state that the difference is really small, one would use
your diameter length obstruction values, because in that case the difference is only
11.6%. ((38.5 - 34.5)/34.5 or if you really want to twist the statistics: (38.5 -
34.5)/38.5 is even less).
If one would want to state that the difference is big, one would use the real area
obstruction, because in that case the difference has more than doubled to 24.4%!!!
((14.8 - 11.6)/11.6)
If you want to learn more about manipulating data with statistics there is a great
book written in 1950 that I bought in December: "How to lie with statistics". I can't
remember the author from heart, but the book is a absolutely great! It makes you
suspicious of values being used incorrectly though... 8-)
Saludos, Ben.
Joseph O'Neil
<siem...@lucent.com> wrote:
-snip-
Hi Ben;
first off, "-snip-" means yolu do not wish to requote the
whole message, and thus have 4, 5 or 6 screens of previously read
material just to add a couple lines of your own.
>First let me summarize the data:
>VC200L:
-snip, again :) -
This whole thread seems to be going the way of "how many
angels can dance on the head of a pin."
For what it is worth, personally I perfer the Visac to any SCT
8 from Meade or Celestron for either visual or astro-photo use, but
then, it cost more than either one of those scopes, so it should be
better, one would hope.
This is not to say that the 8" SCT is a poor scope, not at
all, I just think the Visac is a different instrument, and i am
grateful that in telescopes we have choices, unliek software where
it's MS or the highway. :)
But back to the fine details of specs and figures, central
obstructions, etc, sometimes you can worry too much about the specs
and end up not enjoying any telescope. The Visac is a wodnerful
telescope, but if you want to spend less money, get a standard C8,
they are nice too, just go out and enjoy yourself!
:)
joe
Clive Gibbons
Ben Siemerink <siem...@lucent.com> wrote:
>
>Hi Dennis!
>> >77mm. The VISAC definitely does have a larger obstruction, plus a
>> >thick set of 4 support veins.
>>
>> I've just measured the spider vanes on my VC200L as follows:
>> Vane thickness = 5mm
>> Vane length = 10cms
>> Vane length that impinges upon mirror = 6cms
>>
>> Therefore one vane area = 6 x 0.5 sq cms = 3 sq cms
>> Four vanes = 4 x 3 sq cms = 12 sq cms obstruction.
>>
>> This should then be added to the central obstruction to obtain the overall
>> obstructed area.
>
>Right. Now it would be wonderful if someone could measure the same things for
>a Celestron G8 and then we can do something.
Hi Ben.
Sounds like you've never seen a Celestron 8" SCT, in person.
They don't have secondary support veins. The corrector plate holds the
secondary assembly in place, so you don't have to consider that factor.
I think you'd be able to make a much more informed purchase decision, if
you could get out one night and use both of the scopes in question. Do
whatever it takes to arrange this... you might be sorry otherwise!
Best o' luck.
Bo Sandström
owner of the C8 (who was much more experienced than I was at that
time) concluded that the Visac was better a the GP-DX was a much
better mount than the C8 fork. The optical quality is influenced
by a number of factors more than the central obstruction and its
effect on contrast. The Visac is much better corrected for
various optical errors than most SCT's, giving coma-free pinhead
stars over the whole FOV, something most SCT's do not.
This is not to say a Visac will suit everybody better than a C8.
Just consider other aspects than central obstruction when making
a choise. Reading some of the books like Suiters on Star testing
and Rutten and van Venrooij on Telescope optics will give at
least an elementary insight on what is important besides central
obstruction.
Bo
Clive Gibbons wrote:
>
> In article <38EAE254...@lucent.com>,
> Ben Siemerink <siem...@lucent.com> wrote:
> >
> >Hi Dennis!
> >
> >> Clive Gibbons wrote in message <8ca7u5$7...@mcmail.cis.McMaster.CA>...
> >> >No, the Celestron has a 2.75" diameter secondary obstruction. That's not
> >> >77mm. The VISAC definitely does have a larger obstruction, plus a
> >> >thick set of 4 support veins.
> >>
> >> Folks
> >> I've just measured the spider vanes on my VC200L as follows:
> >> Vane thickness = 5mm
> >> Vane length = 10cms
> >> Vane length that impinges upon mirror = 6cms
> >>
> >> Therefore one vane area = 6 x 0.5 sq cms = 3 sq cms
> >> Four vanes = 4 x 3 sq cms = 12 sq cms obstruction.
> >>
> >> This should then be added to the central obstruction to obtain the overall
> >> obstructed area.
> >
> >Right. Now it would be wonderful if someone could measure the same things for
> >a Celestron G8 and then we can do something.
>
> Hi Ben.
> Sounds like you've never seen a Celestron 8" SCT, in person.
> They don't have secondary support veins. The corrector plate holds the
> secondary assembly in place, so you don't have to consider that factor.
>
> I think you'd be able to make a much more informed purchase decision, if
> you could get out one night and use both of the scopes in question. Do
> whatever it takes to arrange this... you might be sorry otherwise!
>
> Best o' luck.
Clive Gibbons
Bo =?iso-8859-1?Q?Sandstr=F6m?= <bo.san...@mbox302.swipnet.se> wrote:
>I have made a side by side comparison with a C8 and my Visac. The
>owner of the C8 (who was much more experienced than I was at that
>time) concluded that the Visac was better a the GP-DX was a much
>better mount than the C8 fork. The optical quality is influenced
>by a number of factors more than the central obstruction and its
>effect on contrast. The Visac is much better corrected for
>various optical errors than most SCT's, giving coma-free pinhead
>stars over the whole FOV, something most SCT's do not.
Hi Bo.
There's no doubting that many factors need to be considered when comparing
these two designs.
The VISAC will have better off-axis correction, which could be important
to astrophotographers. I've also observed with a C8 in close proximity to
a VISAC. The one thing that's inescapable with the VISAC is the
diffraction problem introduced by the 5mm thick secondary support
veins. They really affected the quality of the high-power diffraction
image, when looking at anything but faint stars.
The C8 in question was *very* well collimated... as close to perfection as
I've ever seen. For low power, wide field observing, the VISAC would
certainly offer better field correction.
However, when it comes to higher mag. planetary and double star views,
while neither instrument is ideal, the 8" SCT would get my nod, if it's
collimated and a good optical example.
Best,
Bo Sandström
I respect your choice even if I don't made the same myself.
Bo
Ben Siemerink
First of all I want to thank everybody who responded to the VISAC and C8 questions
I posted. It has all been very helpful and interesting.
However, I'm following Clive's advice and am going to look _through_ some
telescopes, instead of looking _at_ scopes and reading a lot about them. I'll join
the Madrid Astronomy Association (AAM) and they already offered me to join and
help me using a scope from the association.
One of the guys has a Takahashi, out of budget for me, but would be cool to see it
anyway.
Thank you again.
Saludos, Ben.