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Main
Date: 29 Apr 2007 10:18:39
From: Dennis Woos
Subject: More NEAF stuff
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A few other things at NEAF that caught my eye (the left one, which is
dominant):
Protostar was showing off a new phenolic tubing for telescope making
(BlackLight@) that comes pre-flocked on the inside. They also have an
innovative tube nesting/clamping system that they claim will allow the scope
tube to collapse and then be re-expanded without requiring re-collimation.
Moonlight had crayford focusers with a motorized focus controlled by a box
from Rigel Systems (makers of the Quickfinder, etc). The pulse width and
pulse interval can be individually controlled, which they say allows
precision fine focusing without stalling. This worked much better than the
JMI I have used.
Burgess had a demo of a 30mm Paragon 2" 70degree eyepiece. If this model is
as well received as the 40mm Paragon it will be a big hit. Also, APM had a
line of "Planetary" eyepieces that appeared to be of identical manufacture
as the Burgess Planetary line. It would be fascinating to hear the inside
story on Chinese manufacturing and cloning.
An eyepiece that I would have loved to take home was the Baader 8-24mm zoom.
Many folks think that this is the best 8-24mm zoom on the market. One of
its obvous advantages is that it has an afov of 50degrees at 24mm, instead
of the usual 40degrees.
I tried moving all of the Meade Lightbridge truss dobs, and I think the
bearings stink. Surprising, given that Ebony Star and Teflon is hardly a
secret. I don't get it, but then again I am not a captain of industry, nor
a Chinese scope manufacturer.
It is very cool that AstroPhysics, an American company, continues to produce
what are arguably the finest products in their market. Maybe if Roland
Christen was the CEO of Ford there would be a waiting list at dealerships,
instead of a glut of unsold inventory.
Our club purchased a new laser collimator from Howie Glatter after the one
we used to have was stolen in a burglary of our observatory. Howie's stuff
is top-notch, he stands behind it 200%, and he supports many astro events
with donations. I encourage everyone to consider giving him your business.
On the topic of collimation, a club member who's son has gotten very
involved in astro imaging
(http://www.cloudynights.com/byauthor.php?author_id=659) told me that the
son is obsessed with regularly collimating his SCT. So much for the myth
that a downside of newts is that they require regular collimation, and SCTs
don't. Maybe it is more accurate to say that SCTs don't require frequent
collimation if you don't mind crappy views, or if you can't tell the
difference between crappy and good - which given the variability of at least
my friends' eyesight is quite likely!
Dennis
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Date: 02 May 2007 10:03:57
From: RMOLLISE
Subject: Re: More NEAF stuff
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On Apr 29, 5:10 pm, "Dennis Woos" <dpw...@gmavt.net > wrote:
> > Nope, not true at all. If an SCT is PROPERLY collimated it can easily
> > go months--even years if all you do is haul it into the backyard or
> > out to the occasonal club star party--without needing collimation, or
> > at least more than the very occasional minor touchups.
>
> I did not claim that his scope is or isn't regularly out of collimation, as
> I don't directly know the fellow or his scope. I did report that he is
> always concerned about collimation, which I took to mean that he is at least
> regularly checking and tweaking it.
>
> Well constructed newts only require minor touchups, though they do require -
> or at least I think they do - these "touchups" regularly. To be honest,
> many of the folks I observe with probably would not see the difference, and
> at least some of the time I'm not sure that I would either. However,
> collimating a newt is so easy that spending the minute or two guarantees
> that the scope is performing at its best. I wish that I could get our
> club's SCT owners to similarly check their collimation, but the design makes
> this harder to do as only a star test is very useful, and this requires good
> seeing, eyesight and some skill as well as confidence in the diagonal and
> willingness to mess around with screws near the corrector in the dark.
> Maybe you should consider a trip up here to Vermont and give a presentation
> to our club on the use and care of the SCT? Why not combine it with a
> Stellafane visit?
>
> Dennis
Hi Dennis:
I would love to. There's the possiblity that I'll be spending some
time in Bath (Maine) this fall, so, who knows?, if you inivite me I
might be able to come... ;-)
Unk Rod
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Date: 30 Apr 2007 09:37:27
From:
Subject: Re: More NEAF stuff
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Brian Tung wrote:
> > I am not talking about getting the rings concentric. I am talking about
> > getting the Poisson spot centered in the rings, which is entirely
> > different. The Airy disc is simply the Poisson spot in focus.
Dennis Woos wrote:
> I just did a little web research on Poisson's (or Arago's) spot, and I think
> it is a result of the central obstruction, whereas the Airy disk is the
> result of the edge of the aperture?
Take away the question mark, Dennis; right you are.
How can an SCT with a misalignment in a non-adjustable element be
collimated?
It can't - It can only be adjusted for least aberation, Rod.
Howie
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Date: 30 Apr 2007 23:30:01
From: Brian Tung
Subject: Re: More NEAF stuff
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Howie wrote:
> How can an SCT with a misalignment in a non-adjustable element be
> collimated?
>
> It can't - It can only be adjusted for least aberation, Rod.
In theory, you're right, but in practice, it doesn't matter a whole lot,
because the primary is spherical, and the secondary nearly so (in the
mass-produced SCTs). I seem to recall Meade mates machined secondary
assemblies (including the corrector plate) to the primaries, whereas
Celestron nulls the secondaries to the primaries, but I might have that
flipped. The differences are pretty minor.
The effect is that the tilting the secondary is not much different from
shifting it over. The more recent SCTs have pretty small lateral shift,
anyway. They're really quite good.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 01 May 2007 04:39:38
From: George Normandin
Subject: Re: More NEAF stuff
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> ....
> How can an SCT with a misalignment in a non-adjustable element be
> collimated?
> It can't - It can only be adjusted for least aberation, Rod.
>
> Howie
>
John Stiles, owner of OGS, told me the same thing. To help a few friends out
who were having problems collimating their SCTs, he put the scopes on his
test rig that he uses for his RCs. He found that the owner's problem was
that they were centering the secondary but to get the best images out of the
scope the secondary had to be adjusted slightly off to compensate for the
mis-alignment of the primary. When he did that John found that he could make
a substantial improvement in some of the scopes' performance.
George N
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Date: 30 Apr 2007 18:20:29
From: Dennis Woos
Subject: Re: More NEAF stuff
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>> I just did a little web research on Poisson's (or Arago's) spot, and I
>> think
>> it is a result of the central obstruction, whereas the Airy disk is the
>> result of the edge of the aperture?
>
> Take away the question mark, Dennis; right you are.
>
> How can an SCT with a misalignment in a non-adjustable element be
> collimated?
> It can't - It can only be adjusted for least aberation, Rod.
>
> Howie
Yes, it looks like the Poisson Spot and the Airy Disk are different
creatures. Makes me wish that I had studied some Physics when I was a
student. Hopefully my older son Doug can enlighten me on his breaks from
college! I actually have a funny story on the subject. This past year Doug
has been taking first year Physics (with Calculus) at Middlebury College, as
they make some courses available to local high school students. He was
struggling a bit with one of the first problem sets, and I started asking
him questions, simplifying the problem until he and I came up with the
solution. He was surprised, and noted that I had never had a course in
Physics. I replied that neither had Isaac Newton. I will probably
periodically dredge up this small success of mine until the day I die!
I am certainly going to continue working with our club's SCT owners to
improve their collimation, and get the best views possible - using some of
the time I save by quickly and effortlessly collimating my own scopes with
my barlowed laser ;-)
And Howie, that is a terrible picture of you on page 67 of the April 2007
issue of "Astronomy Technology Today". Doesn't look like you at all ;-)
Dennis
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Date: 29 Apr 2007 13:12:48
From: RMOLLISE
Subject: Re: More NEAF stuff
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On Apr 29, 10:59 am, b...@isi.edu (Brian Tung) wrote:
> Rod Mollise wrote:
> > If your buddy's son is having problems, I'm guessing he probably
> > doesn't follow the precept: "always tighten screws to collimate...only
> > when a screw is snug should you loosen its opposite number(s) to
> > coninute in the same 'direction'". If you do this, no, you won't find
> > you have to collimate like a Newtonian user, and you won't have
> > "crappy views" either. I guar-ron-tee. ;-)
>
> Let me offer one other possibility (somewhat less likely): The corrector
> plate is slightly loose, and each time the scope is moved, the alignment
> of the corrector, the primary, and the secondary is messed up. Even if
> the secondary isn't moving in its holder, it might have to be adjusted
> anew each time just to compensate for the movement of the corrector.
>
> Even less likely but still possible is that the visual back is jiggly,
> and the same kind of compensation needs to happen.
>
> Otherwise, if mechanical tolerances are reasonably tight, I agree with
> Rod: Once properly achieved, collimation should last for quite a few
> sessions.
>
> --
> Brian Tung <b...@isi.edu>
> The Astronomy Corner athttp://astro.isi.edu/
> Unofficial C5+ Home Page athttp://astro.isi.edu/c5plus/
> The PleiadAtlas Home Page athttp://astro.isi.edu/pleiadatlas/
> My Own Personal FAQ (SAA) athttp://astro.isi.edu/reference/faq.html
HI Brian:
Right, thanks. If there are mechanical problems with the OTA all bets
are off. ;-)
Unk Rod
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Date: 29 Apr 2007 12:12:55
From: Mark S. Holden
Subject: Re: More NEAF stuff
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Dennis Woos wrote:
>
> Burgess had a demo of a 30mm Paragon 2" 70degree eyepiece. If this model is
> as well received as the 40mm Paragon it will be a big hit. Also, APM had a
> line of "Planetary" eyepieces that appeared to be of identical manufacture
> as the Burgess Planetary line. It would be fascinating to hear the inside
> story on Chinese manufacturing and cloning.
>
The planetary eyepieces Markus was selling come out the back door of the
same factory that makes the ones for BO/TMB, and are made using the TMB
design and tooling owned by BO/TMB.
The factory they hired to make them apparently decided they could make
more profit by making and selling extras to other people.
I've heard claims some of them don't have the retaining rings with the
sharp threads.
Otherwise, the main difference is the writing on the barrels, and the
people who own the design and tooling aren't compensated for their work.
And at NEAF, the official planetary eyepieces are being discounted to
$59 so they cost less.
> It is very cool that AstroPhysics, an American company, continues to produce
> what are arguably the finest products in their market. Maybe if Roland
> Christen was the CEO of Ford there would be a waiting list at dealerships,
> instead of a glut of unsold inventory.
>
If Roland was CEO of Ford, they'd only need a couple of dealerships
because while the cars would be great, they'd only be able to make a few
hundred cars a year.
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Date: 30 Apr 2007 01:06:17
From: Knap
Subject: Re: More NEAF stuff
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"Mark S. Holden" wrote:
> Dennis Woos wrote:
> >
> > Burgess had a demo of a 30mm Paragon 2" 70degree eyepiece. If this model is
> > as well received as the 40mm Paragon it will be a big hit. Also, APM had a
> > line of "Planetary" eyepieces that appeared to be of identical manufacture
> > as the Burgess Planetary line. It would be fascinating to hear the inside
> > story on Chinese manufacturing and cloning.
> >
>
> The planetary eyepieces Markus was selling come out the back door of the
> same factory that makes the ones for BO/TMB, and are made using the TMB
> design and tooling owned by BO/TMB.
>
> The factory they hired to make them apparently decided they could make
> more profit by making and selling extras to other people.
>
> I've heard claims some of them don't have the retaining rings with the
> sharp threads.
>
> Otherwise, the main difference is the writing on the barrels, and the
> people who own the design and tooling aren't compensated for their work.
>
No, they are made by subsidised clan of monastic gnomes in Lousiana, who have
taken a vow of poverty and honesty and praise Madonna !
>
> And at NEAF, the official planetary eyepieces are being discounted to
> $59 so they cost less.
>
> > It is very cool that AstroPhysics, an American company, continues to produce
> > what are arguably the finest products in their market. Maybe if Roland
> > Christen was the CEO of Ford there would be a waiting list at dealerships,
> > instead of a glut of unsold inventory.
> >
>
> If Roland was CEO of Ford, they'd only need a couple of dealerships
> because while the cars would be great, they'd only be able to make a few
> hundred cars a year.
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Date: 29 Apr 2007 08:37:45
From: RMOLLISE
Subject: Re: More NEAF stuff
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told me that the
> son is obsessed with regularly collimating his SCT. So much for the myth
> that a downside of newts is that they require regular collimation, and SCTs
> don't. Maybe it is more accurate to say that SCTs don't require frequent
> collimation if you don't mind crappy views, or if you can't tell the
> difference between crappy and good - which given the variability of at least
> my friends' eyesight is quite likely!
>
> Dennis
Hi Dennis:
Nope, not true at all. If an SCT is PROPERLY collimated it can easily
go months--even years if all you do is haul it into the backyard or
out to the occasonal club star party--without needing collimation, or
at least more than the very occasional minor touchups.
If your buddy's son is having problems, I'm guessing he probably
doesn't follow the precept: "always tighten screws to collimate...only
when a screw is snug should you loosen its opposite number(s) to
coninute in the same 'direction'". If you do this, no, you won't find
you have to collimate like a Newtonian user, and you won't have
"crappy views" either. I guar-ron-tee. ;-)
Unk Rod
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Date: 30 Apr 2007 00:46:21
From: Knap
Subject: Re: More NEAF stuff
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RMOLLISE wrote:
> told me that the
> > son is obsessed with regularly collimating his SCT. So much for the myth
> > that a downside of newts is that they require regular collimation, and SCTs
> > don't. Maybe it is more accurate to say that SCTs don't require frequent
> > collimation if you don't mind crappy views, or if you can't tell the
> > difference between crappy and good - which given the variability of at least
> > my friends' eyesight is quite likely!
> >
> > Dennis
>
> Hi Dennis:
>
> Nope, not true at all. If an SCT is PROPERLY collimated it can easily
> go months--even years if all you do is haul it into the backyard or
> out to the occasonal club star party--without needing collimation, or
> at least more than the very occasional minor touchups.
>
Total nonsense in the real world.
>
> If your buddy's son is having problems, I'm guessing he probably
> doesn't follow the precept: "always tighten screws to collimate...only
> when a screw is snug should you loosen its opposite number(s) to
> coninute in the same 'direction'". If you do this, no, you won't find
> you have to collimate like a Newtonian user, and you won't have
> "crappy views" either. I guar-ron-tee. ;-)
>
> Unk Rod
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Date: 29 Apr 2007 18:10:40
From: Dennis Woos
Subject: Re: More NEAF stuff
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> Nope, not true at all. If an SCT is PROPERLY collimated it can easily
> go months--even years if all you do is haul it into the backyard or
> out to the occasonal club star party--without needing collimation, or
> at least more than the very occasional minor touchups.
I did not claim that his scope is or isn't regularly out of collimation, as
I don't directly know the fellow or his scope. I did report that he is
always concerned about collimation, which I took to mean that he is at least
regularly checking and tweaking it.
Well constructed newts only require minor touchups, though they do require -
or at least I think they do - these "touchups" regularly. To be honest,
many of the folks I observe with probably would not see the difference, and
at least some of the time I'm not sure that I would either. However,
collimating a newt is so easy that spending the minute or two guarantees
that the scope is performing at its best. I wish that I could get our
club's SCT owners to similarly check their collimation, but the design makes
this harder to do as only a star test is very useful, and this requires good
seeing, eyesight and some skill as well as confidence in the diagonal and
willingness to mess around with screws near the corrector in the dark.
Maybe you should consider a trip up here to Vermont and give a presentation
to our club on the use and care of the SCT? Why not combine it with a
Stellafane visit?
Dennis
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Date: 29 Apr 2007 15:15:25
From: Brian Tung
Subject: Re: More NEAF stuff
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Dennis Woos wrote:
> Well constructed newts only require minor touchups, though they do require -
> or at least I think they do - these "touchups" regularly. To be honest,
> many of the folks I observe with probably would not see the difference, and
> at least some of the time I'm not sure that I would either. However,
> collimating a newt is so easy that spending the minute or two guarantees
> that the scope is performing at its best. I wish that I could get our
> club's SCT owners to similarly check their collimation, but the design makes
> this harder to do as only a star test is very useful, and this requires good
> seeing, eyesight and some skill as well as confidence in the diagonal and
> willingness to mess around with screws near the corrector in the dark.
Having experience with collimating both Newtonians and SCTs, I don't see
how either is harder than the other. I read of many more people finding
Newts to be more difficult, but I don't believe that either. Once you
have done it a couple of times, it should become reasonably easy.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 30 Apr 2007 00:54:06
From: Knap
Subject: Re: More NEAF stuff
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Brian Tung wrote:
> Dennis Woos wrote:
> > Well constructed newts only require minor touchups, though they do require -
> > or at least I think they do - these "touchups" regularly. To be honest,
> > many of the folks I observe with probably would not see the difference, and
> > at least some of the time I'm not sure that I would either. However,
> > collimating a newt is so easy that spending the minute or two guarantees
> > that the scope is performing at its best. I wish that I could get our
> > club's SCT owners to similarly check their collimation, but the design makes
> > this harder to do as only a star test is very useful, and this requires good
> > seeing, eyesight and some skill as well as confidence in the diagonal and
> > willingness to mess around with screws near the corrector in the dark.
>
> Having experience with collimating both Newtonians and SCTs, I don't see
> how either is harder than the other. I read of many more people finding
> Newts to be more difficult, but I don't believe that either. Once you
> have done it a couple of times, it should become reasonably easy.
depends on the newt. F8 vs F5 and below. Offset vs no offset.
And since so many junk dobs today are f5 well there's the answer.
In addition some people finds wing-nuts low at the bottom of the scope difficult
vrs allen screws at the top. Dont ask me why.
>
>
> --
> Brian Tung <brian@isi.edu>
> The Astronomy Corner at http://astro.isi.edu/
> Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
> The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
> My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 30 Apr 2007 01:11:00
From: Brian Tung
Subject: Re: More NEAF stuff
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> depends on the newt. F8 vs F5 and below. Offset vs no offset.
> And since so many junk dobs today are f5 well there's the answer.
I don't see too many junk dobs these days. Mostly junk refractors.
At f/5 the need for offset does increase, but this does not really
affect the difficulty of collimation.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
|
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Date: 29 Apr 2007 18:37:59
From: Dennis Woos
Subject: Re: More NEAF stuff
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>> Well constructed newts only require minor touchups, though they do
>> require -
>> or at least I think they do - these "touchups" regularly. To be honest,
>> many of the folks I observe with probably would not see the difference,
>> and
>> at least some of the time I'm not sure that I would either. However,
>> collimating a newt is so easy that spending the minute or two guarantees
>> that the scope is performing at its best. I wish that I could get our
>> club's SCT owners to similarly check their collimation, but the design
>> makes
>> this harder to do as only a star test is very useful, and this requires
>> good
>> seeing, eyesight and some skill as well as confidence in the diagonal and
>> willingness to mess around with screws near the corrector in the dark.
>
> Having experience with collimating both Newtonians and SCTs, I don't see
> how either is harder than the other. I read of many more people finding
> Newts to be more difficult, but I don't believe that either. Once you
> have done it a couple of times, it should become reasonably easy.
I am genuinely puzzled by your claim. Would you care to address this in
more detail? How is star testing an SCT at high power, examining the Airy
disk, and adjusting the secondary screws in the dark not harder than using a
laser collimator, and not necessarily in the dark? Is the information
contained on the site http://legault.club.fr/collim.html not accurate?
Believe me, I would love to be able to collimate an SCT as easily as a newt,
as I think every SCT owned by our club members is in need of some degree of
collimation.
Dennis
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Date: 29 Apr 2007 16:08:45
From: Brian Tung
Subject: Re: More NEAF stuff
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Dennis Woos wrote:
> I am genuinely puzzled by your claim. Would you care to address this in
> more detail? How is star testing an SCT at high power, examining the Airy
> disk, and adjusting the secondary screws in the dark not harder than using a
> laser collimator, and not necessarily in the dark?
Let's compare apples to apples: There are laser collimators for SCTs,
too. You use those, your job gets a bit easier. But even manually...
Empirically, I have very little trouble collimating an SCT. In brief,
the C5+ (like many SCTs) has three screws on the front: at 9 o'clock,
1 o'clock, and 5 o'clock. I focus on a star, and then defocus just a
tad--about enough to see two or three diffraction rings clearly.
If the scope is out of collimation, the Poisson spot is out of center
in the rings. If it's too far toward the 9 o'clock position, I turn
the 9 o'clock screw clockwise; if it's too far toward the 3 o'clock
position, I turn the 9 o'clock screw counter-clockwise. Likewise for
the 1 o'clock and 5 o'clock screws. If it's in between, I have to
split the difference between the screws, but it's pretty easy for me
to critically collimate the SCT in about one or two minutes.
The turns are very small--only a few degrees (perhaps a 50th of a turn
or so, if that). Just a little squeak on the screwdriver. I do not use
Bob's Knobs, but I imagine those'll work just fine, too. Each time out,
I check it real quick, and maybe one time out of four, I need to make
one of those turns.
> Is the information
> contained on the site http://legault.club.fr/collim.html not accurate?
Uhh, yeah...where does it say that collimating an SCT is significantly
harder than collimating a Newt?
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
|
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Date: 29 Apr 2007 20:08:43
From: Dennis Woos
Subject: Re: More NEAF stuff
|
> Let's compare apples to apples: There are laser collimators for SCTs,
> too. You use those, your job gets a bit easier. But even manually...
I have never met anyone who had success using a laser to collimate an SCT,
have you? Howie Glatter makes no claims that his collimators are useful for
initial collimation, and at best are only useful for re-collimation. I am
skeptical of their usefulness for SCTs.
>
> Empirically, I have very little trouble collimating an SCT. In brief,
> the C5+ (like many SCTs) has three screws on the front: at 9 o'clock,
> 1 o'clock, and 5 o'clock. I focus on a star, and then defocus just a
> tad--about enough to see two or three diffraction rings clearly.
>
> If the scope is out of collimation, the Poisson spot is out of center
> in the rings. If it's too far toward the 9 o'clock position, I turn
> the 9 o'clock screw clockwise; if it's too far toward the 3 o'clock
> position, I turn the 9 o'clock screw counter-clockwise. Likewise for
> the 1 o'clock and 5 o'clock screws. If it's in between, I have to
> split the difference between the screws, but it's pretty easy for me
> to critically collimate the SCT in about one or two minutes.
>
> The turns are very small--only a few degrees (perhaps a 50th of a turn
> or so, if that). Just a little squeak on the screwdriver. I do not use
> Bob's Knobs, but I imagine those'll work just fine, too. Each time out,
> I check it real quick, and maybe one time out of four, I need to make
> one of those turns.
Critical collimation requires a high-power star test, which is much more
difficult than adjusting where a laser beam hits the mirror and returns to
the laser. It seems surprising to me that you would not agree with this.
Perhaps you should test your theory by attending a star party and helping
some folks collimate their SCTs? I have, and it took some little while to
get folks to even see the rings. Be prepared to lend them an eyepiece and
maybe a barlow to get the required magnification.
>
>> Is the information
>> contained on the site http://legault.club.fr/collim.html not accurate?
>
> Uhh, yeah...where does it say that collimating an SCT is significantly
> harder than collimating a Newt?
The site in question indicates, and I have read others state, that excellent
SCT images require collimation with respect to the Airy disk. It is claimed
that getting the rings concentric, as you describe above, is not sufficient.
I have on occasion attempted to collimate my newts to this level, and it is
difficult. Fortunately, I have found that for our newts this level of
collimation is only of interest when trying to push the optics to the
absolute max (e.g. splitting doubles near the Dawes limit or with very
different magnitudes - hasn't yet helped us split Antares!)
Finally, I think you agree with my main point, which is that having to
regularly collimate a newtonian is not a valid criticism when comparing them
to SCTs, as SCTs too require regular collimation (you say one time out of
four - I am sure this varies), and in any case collimation is easy (at
least, in my view, for newts).
Dennis
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Date: 30 Apr 2007 01:09:49
From: Brian Tung
Subject: Re: More NEAF stuff
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Dennis Woos wrote:
> I have never met anyone who had success using a laser to collimate an SCT,
> have you? Howie Glatter makes no claims that his collimators are useful for
> initial collimation, and at best are only useful for re-collimation. I am
> skeptical of their usefulness for SCTs.
As I understand it, that is essentially true for all laser collimators--
they have to be calibrated first. Usefulness for re-collimation is all
that I would ask from a laser collimator...if I owned one. Of course,
I find critical collimation on both Newtonians and SCTs a straightforward
enough process that I don't bother with the laser pointer.
> Critical collimation requires a high-power star test, which is much more
> difficult than adjusting where a laser beam hits the mirror and returns to
> the laser. It seems surprising to me that you would not agree with this.
To the extent that I agree, I don't see why this should be any different
for a Newt as opposed to an SCT. I agree with Nils Olof Carlin on this
matter--see his viewpoint on critical collimation requiring a star test.
> Perhaps you should test your theory by attending a star party and helping
> some folks collimate their SCTs? I have, and it took some little while to
> get folks to even see the rings. Be prepared to lend them an eyepiece and
> maybe a barlow to get the required magnification.
I have never had any trouble seeing the rings on the SCT. Maybe the
seeing at the LAAS observing sites is better, but it is difficult for me
*not* to see the rings either there or at my backyard.
> The site in question indicates, and I have read others state, that excellent
> SCT images require collimation with respect to the Airy disk. It is claimed
> that getting the rings concentric, as you describe above, is not sufficient.
I am not talking about getting the rings concentric. I am talking about
getting the Poisson spot centered in the rings, which is entirely
different. The Airy disc is simply the Poisson spot in focus. It is
somewhat easier to see the decentering of the Poisson spot just a couple
of waves out of focus. I am not talking, for instance, of the donut that
people see when many waves out of focus.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 30 Apr 2007 10:22:08
From: Dennis Woos
Subject: Re: More NEAF stuff
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> I am not talking about getting the rings concentric. I am talking about
> getting the Poisson spot centered in the rings, which is entirely
> different. The Airy disc is simply the Poisson spot in focus. It is
> somewhat easier to see the decentering of the Poisson spot just a couple
> of waves out of focus. I am not talking, for instance, of the donut that
> people see when many waves out of focus.
I just did a little web research on Poisson's (or Arago's) spot, and I think
it is a result of the central obstruction, whereas the Airy disk is the
result of the edge of the aperture? Maybe this discussion goes beyond this
thread, or even this newsgroup. However, it would be great to understand
this better. I hope Suiter explains this!
Dennis
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Date: 30 Apr 2007 23:25:00
From: Brian Tung
Subject: Re: More NEAF stuff
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Dennis Woos wrote:
> I just did a little web research on Poisson's (or Arago's) spot, and I think
> it is a result of the central obstruction, whereas the Airy disk is the
> result of the edge of the aperture? Maybe this discussion goes beyond this
> thread, or even this newsgroup. However, it would be great to understand
> this better. I hope Suiter explains this!
I think he does, but here's a brief summary.
Poisson's spot is indeed created by the obstruction. It is so-called
because Poisson predicted it using the theory of light as it was then
understood, and pointed his prediction out as an absurd conclusion of
what therefore must be a flawed theory. Imagine his consternation when
the experiment was done and the spot appeared as predicted! (That's as
I've heard it, at least. The story is so good it might be apocryphal.)
The Airy disc is the in-focus diffraction peak. It should be pointed
out that it is inaccurate to call diffraction an edge effect, although
it is often described as such. It is really the effect of the wavefront
being incomplete--you only see what's coming through the aperture,
whether it's an 4-inch aperture or a 40-inch. The more of the wavefront
you capture, the less the impact of diffraction--that's why aperture
wins!
The central obstruction in Newts and SCTs hurts the image to the extent
that it blocks part of the wavefront, not because of the edge of the
obstruction. The obstruction could have a fractal curve for its
boundary, and it wouldn't have a great effect on the image, as long as
the overall area covered is the same. That's why apodizing masks made
from screen material don't suck eggs, as they would if their monstrous
edge length (probably in the many tens of meters) were the relevant
parameter.
Back to the original topic: I use the Poisson spot because I find it
easier to see misalignment there than in the relatively bright and broad
Airy disc. I make the Poisson spot as small as I like, and it's more
straightforward to see the decentering.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 30 Apr 2007 08:23:50
From: Dennis Woos
Subject: Re: More NEAF stuff
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"Brian Tung" <brian@isi.edu > wrote in message
news:f1488d$i8c$1@praesepe.isi.edu...
> Dennis Woos wrote:
>> I have never met anyone who had success using a laser to collimate an
>> SCT,
>> have you? Howie Glatter makes no claims that his collimators are useful
>> for
>> initial collimation, and at best are only useful for re-collimation. I
>> am
>> skeptical of their usefulness for SCTs.
>
> As I understand it, that is essentially true for all laser collimators--
> they have to be calibrated first. Usefulness for re-collimation is all
> that I would ask from a laser collimator...if I owned one. Of course,
> I find critical collimation on both Newtonians and SCTs a straightforward
> enough process that I don't bother with the laser pointer.
I don't understand what you mean here by "they have to be calibrated first".
However, the reason I don't see that lasers are very useful for collimating
SCTs is the same reason that I don't use my laser in newts unless it is
barlowed, i.e. the mechanical alignment of the laser is what is being
measured rather than the optical alignment of the mirrors. The barlowed
laser technique for newts is what makes the laser useful, and I don't see
how it can be applied to SCTs.
>
>> Critical collimation requires a high-power star test, which is much more
>> difficult than adjusting where a laser beam hits the mirror and returns
>> to
>> the laser. It seems surprising to me that you would not agree with this.
>
> To the extent that I agree, I don't see why this should be any different
> for a Newt as opposed to an SCT. I agree with Nils Olof Carlin on this
> matter--see his viewpoint on critical collimation requiring a star test.
The question here is whether or not SCTs require this "critical collimation"
much more than do newts (in a reasonable focal length, say f/4 - f/10). It
is my understanding that they do, but I admit that this is based on what I
have read here and elsewhere, and I think some authoritative discussion
would be useful. I do know from much experience that it is not required for
excellent newt performance - my Glatter laser collimator does a great job
and there is very little to be gained by collimating with a star test.
>
> I am not talking about getting the rings concentric. I am talking about
> getting the Poisson spot centered in the rings, which is entirely
> different. The Airy disc is simply the Poisson spot in focus. It is
> somewhat easier to see the decentering of the Poisson spot just a couple
> of waves out of focus. I am not talking, for instance, of the donut that
> people see when many waves out of focus.
In this you have educated me a bit - thanks! When I star test I typically
go for 4 rings or even a few more, as the seeing is not often good enough to
get a clean image with less. Over time I am going to experiment with this a
bit more, and get back into my Suiter! However, I would like to restate
that my laser really does an excellent job on our dobs, and it is not often
when I need to collimate with a star test. The big question for me is how
much critical collimation is required for f/10 SCTs as opposed to
newtonians. I hope the answer is a lot, because I am not very impressed
with the images these scopes are providing me, and I hope the reason is that
they are not sufficiently collimated. I know the seeing is not the problem,
as similarly-sized dobs at the same site are doing better. Cooldown? They
have been out for hours, and sometimes all night. I have had good views
from some of these SCTs, but never the best views, and I hope that critical
collimation is the answer.
Dennis
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Date: 30 Apr 2007 23:15:36
From: Brian Tung
Subject: Re: More NEAF stuff
|
Dennis Woos wrote:
> I don't understand what you mean here by "they have to be calibrated first".
> However, the reason I don't see that lasers are very useful for collimating
> SCTs is the same reason that I don't use my laser in newts unless it is
> barlowed, i.e. the mechanical alignment of the laser is what is being
> measured rather than the optical alignment of the mirrors. The barlowed
> laser technique for newts is what makes the laser useful, and I don't see
> how it can be applied to SCTs.
What you are saying is the same thing: The laser collimators have to be
calibrated, so that you know that when you see the laser hit the target,
so to speak, that the optical elements are aligned. The Barlowing makes
this more accurate, but I don't think it's inherently any different from
without the Barlow.
I admit that I don't use laser collimators, so I can't speak to the ease
of use of SCT collimators, beyond the simple fact that they exist.
> The question here is whether or not SCTs require this "critical collimation"
> much more than do newts (in a reasonable focal length, say f/4 - f/10). It
> is my understanding that they do, but I admit that this is based on what I
> have read here and elsewhere, and I think some authoritative discussion
> would be useful. I do know from much experience that it is not required for
> excellent newt performance - my Glatter laser collimator does a great job
> and there is very little to be gained by collimating with a star test.
There is little difference between an f/10 Newtonian and an f/10 SCT
with respect to the effects of miscollimation. They both have coma--I
forget which is more severe, but I think they're comparable. (With the
mass-produced SCTs, anyway. It's possible, with the many degrees of
freedom in the general SCT design, to produce something that has very
little coma at all, but it tends also to be more expensive.) That gives
you some idea of how much impact miscollimation has. The main problem
with the SCT is field curvature, which a reducer/corrector can help
with.
An f/4 Newt, on the other hand, is much more sensitive. That thing
really has to be aligned very finely to get all that one can get out of
it. Plus you'll want a Paracorr, unless perhaps you're using a narrow
field eyepiece like a monocentric.
I don't have an f/4 Newt; mine is f/6. It's not too difficult to keep
in proper collimation, even without a laser. And a properly collimated
f/10 SCT, which takes relatively little time if you're used to it, is a
joy to use.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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Date: 29 Apr 2007 08:59:47
From: Brian Tung
Subject: Re: More NEAF stuff
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Rod Mollise wrote:
> If your buddy's son is having problems, I'm guessing he probably
> doesn't follow the precept: "always tighten screws to collimate...only
> when a screw is snug should you loosen its opposite number(s) to
> coninute in the same 'direction'". If you do this, no, you won't find
> you have to collimate like a Newtonian user, and you won't have
> "crappy views" either. I guar-ron-tee. ;-)
Let me offer one other possibility (somewhat less likely): The corrector
plate is slightly loose, and each time the scope is moved, the alignment
of the corrector, the primary, and the secondary is messed up. Even if
the secondary isn't moving in its holder, it might have to be adjusted
anew each time just to compensate for the movement of the corrector.
Even less likely but still possible is that the visual back is jiggly,
and the same kind of compensation needs to happen.
Otherwise, if mechanical tolerances are reasonably tight, I agree with
Rod: Once properly achieved, collimation should last for quite a few
sessions.
--
Brian Tung <brian@isi.edu >
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.html
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