## RE: [Telescopes] Re: Klevzov vs SCT?

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• Jesus, I m sorry I asked! Can we knock off the Klevzov arguement now? Scott Horne http://starjourney.net Honolulu, Hawaii ... From:
Message 1 of 19 , Apr 1, 2001
Jesus, I'm sorry I asked! Can we knock off the Klevzov arguement now?

Scott Horne
http://starjourney.net
Honolulu, Hawaii

-----Original Message-----
From: sbodenheimer@... [mailto:sbodenheimer@...]
Sent: Saturday, March 31, 2001 7:07 PM
To: telescopes@yahoogroups.com
Subject: [Telescopes] Re: Klevzov vs SCT?

--- In telescopes@y..., idalv@e... wrote:
> > From Makscopes Forum:
> >
> > > Valery Deryuzhin.
> >
> > "Hi,
> >
> > I read your post and wondered whether you could identify the laws
> of
> > physics to which you refer. In particular you seem to say that
the
> > areal value of a spider obstruction is to be treated the same as
> the
> > areal value of a central obstruction (c.o.). But this simply
> > doesn't make sense from a mathematics/physics standpoint. For
> > example, the areal value of a 200 mm aperture is -- (pi) times
(100
> > squared) = (pi)(10,000). If I take away 20 mm from the diameter
at
> > the exterior of the aperture, I have an areal value of (pi) times
> (90
> > squared) = (pi)(8,100) resulting in a net loss of (pi)(1,900)=
5966
> > sq.mm. On the other hand if I remove the same 20 mm from the
> > diameter at the center of the aperture I have an areal value of
(pi)
> > (10,000) minus (pi)(10 squared), resulting in a total loss of (pi)
> > (100) = 314 sq.mm. Since none of the spider obstrution is
located
> in
> > the center of the aperture (indeed, a substantial portion of the
> > spider obstruction is located at the periphery of the aperture),
> how
> > can one properly equate the area of the spider to the area of a
> > central aperature? If your analysis were correct, then removing
> 100
> > mm from a 200 mm diameter aperture at the center of the aperture
> > would be the same as removing 27.5 mm from the diameter at its
> > exterior (i.e., a 200 mm aperture with 50% central obstruction
> would
> > be the same as a 172.5 mm aperture with no obstruction). Did I
> miss
> > something?
> >
> > Also I own a TAL200K and measured the central obstruction -- It
is
> > 35% (linear) not 40%. And the images are very bright as
previously
> > reported.
> >
> > Thanks.
> >
> > Steve Bodenheimer"
> >
>
> Have to admit that can't fallow this logic. The idea seems to be
> simply that what matters is a total of the area blocking the light
> path.
>
> So, if we have a 35% (70mm) central obstruction in a 200mm aperture
> scope, with three spider vanes 5mm wide and with some 70mm of the
> length in the light path (being curved), it comes to a total of
> 3848sq. mm area of the c. obstruction + 1050sq.mm area of three
> spider vanes = 4898sq.mm area in the light path. This would
> correspond to a 39.5% (79mm) c. obstruction without spider vanes
> (say, optical window mounted).
>
> It doesn't seem to agree with what Suiter says, since in the
example
> in his book (20% obstruction+ 2mm four-vane spider) this aritmetic
> would result in a total diffraction effect nearly equal to that of
a
> 25% (spiderless) obstruction. In effect, it is roughly 21%.
According
> to Suiter, % of drop in contrast due to spader vanes (straight or
> curved, same thing) is approx. equal to the % of aperture's area
they
> make. In this case it would be somewhat over 3%, which would
> correspond to approx. 37% spiderless obstruction. This is still the
> size of c. obstruction in f/10 Meade SCT's, and that itself
> doesn't make them third-grade instruments.
>

I'll be darned. Actually you do seem to agree with my logic.
Contrast is a function of light gathering capacity -- i.e., a
function of aperture area. Thus percent drop in contrast is simply
the same thing as losing a portion of the aperture. Bill's post here
corresponds to my measurements of 4mm thickness and your 70 mm length
is the same as I measured (using thin solder wire I bent to match the
curved spiders). Total aperture area loss is 810 sq.mm. Now that
corresponds to a reduction in aperture size from 200 mm to 191.7 mm.
Thus this system apparently is the same as a no spider system of
191.7 mm with 70 mm aperture, or 36.5% linear c.o.

Clear, calm and dark skies. Thanks.

Steve Bodenheimer

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• Don t worry about it, Scott. It s a perfectly legitimate comparison and one that the market needs. Aloha, Mike ... now?
Message 2 of 19 , Apr 1, 2001
Don't worry about it, Scott. It's a perfectly legitimate comparison
and one that the market needs.

Aloha,

Mike

--- In telescopes@y..., "Scott Horne" <gsm2horn@c...> wrote:
> Jesus, I'm sorry I asked! Can we knock off the Klevzov arguement
now?
>
> Scott Horne
> http://starjourney.net
> Honolulu, Hawaii
• ... aperture ... the ... here ... length ... the ... Hi Vlad and Steve, In the case with straight thin spiders only a small portion of energy is pulled out
Message 3 of 19 , Apr 1, 2001
--- In telescopes@y..., sbodenheimer@c... wrote:

> >
> > Have to admit that can't fallow this logic. The idea seems to be
> > simply that what matters is a total of the area blocking the light
> > path.
> >
> > So, if we have a 35% (70mm) central obstruction in a 200mm
aperture
> > scope, with three spider vanes 5mm wide and with some 70mm of the
> > length in the light path (being curved), it comes to a total of
> > 3848sq. mm area of the c. obstruction + 1050sq.mm area of three
> > spider vanes = 4898sq.mm area in the light path. This would
> > correspond to a 39.5% (79mm) c. obstruction without spider vanes
> > (say, optical window mounted).
> >
> > It doesn't seem to agree with what Suiter says, since in the
> example
> > in his book (20% obstruction+ 2mm four-vane spider) this aritmetic
> > would result in a total diffraction effect nearly equal to that of
> a
> > 25% (spiderless) obstruction. In effect, it is roughly 21%.
> According
> > to Suiter, % of drop in contrast due to spader vanes (straight or
> > curved, same thing) is approx. equal to the % of aperture's area
> they
> > make. In this case it would be somewhat over 3%, which would
> > correspond to approx. 37% spiderless obstruction. This is still
the
> > size of c. obstruction in f/10 Meade SCT's, and that itself
> > doesn't make them third-grade instruments.
> >
>
>
> I'll be darned. Actually you do seem to agree with my logic.
> Contrast is a function of light gathering capacity -- i.e., a
> function of aperture area. Thus percent drop in contrast is simply
> the same thing as losing a portion of the aperture. Bill's post
here
> corresponds to my measurements of 4mm thickness and your 70 mm
length
> is the same as I measured (using thin solder wire I bent to match
the
> curved spiders). Total aperture area loss is 810 sq.mm. Now that
> corresponds to a reduction in aperture size from 200 mm to 191.7 mm.

> Thus this system apparently is the same as a no spider system of
> 191.7 mm with 70 mm aperture, or 36.5% linear c.o.
>
> Clear, calm and dark skies. Thanks.
>
> Steve Bodenheimer

In the case with straight thin spiders only a small portion of
energy is pulled out from Airy disk and it spreads only in four
narrow direction. All rest space around Airy disk is undamaged
and equal for simple obstructed aperture. In the case with curved
siders - an energy spreads very uniformly around Airy disk and
approximation you mentioned is almost 100% valid.

Steve, if only the matter will be in % brightenss lost.
You can place a neutral density filter (which absorbs 15% of
energy) over a true APO and you will not see any contrast lost,
except highest magnifications.
But if you will place a c.o. with total 15% energy shadowed
(15% area lost), you will see a significant contrast lost,
because not only 15% energy lost, but also significant portion
of Airy disk energy will be spreaded in first, second and third
diffraction rings - THIS will decrease a contrast.

Valery Deryuzhin.
• ... now? ... Dear Scott, I for one have enjoyed the very informative , if somewhat overheated , discussion of the TAL-200. I doubted that Bill s assertion that
Message 4 of 19 , Apr 1, 2001
--- In telescopes@y..., "Scott Horne" <gsm2horn@c...> wrote:
> Jesus, I'm sorry I asked! Can we knock off the Klevzov arguement
now?
>
> Scott Horne
> http://starjourney.net
> Honolulu, Hawaii
>
>
Dear Scott,
I for one have enjoyed the very informative , if somewhat
overheated , discussion of the TAL-200. I doubted that Bill's
assertion that his Klevzov is "just as bright and twice as sharp" as
a C 9.5 would go unchallenged . That is quite a bold statment for a
spherically designed scope of 8" with 4.5 mm spider vanes . My 130mm
Newt has similarly sized straight vanes and I see large diffraction
spikes on bright objects . Irregardless of whether the vanes are
straight or curved they still obstruct the aperature and cause a loss
of contrast at best , large spikes like I see at worst .
Through discussions like these I (and other 'newbies' like me) am
learning the various experiences of these gentlemen . I think that's
a good thing . That there are differences of opinion being strongly
debated is also a good thing . But let us try to keep to the facts
without getting personal as that can get ugly indeed .

Thanks
AndyV in Cloudyville
• ... I try to limit my reports to what I see. I really don t know if the TAL200k is a superlative performer or not, I let other people deterimine that if they
Message 5 of 19 , Apr 1, 2001
andy_vollmer@... Wrote:

>I doubted that Bill's
>assertion that his Klevzov is "just as bright and twice as sharp" as
>a C 9.5 would go unchallenged . That is quite a bold statment for a
>spherically designed scope of 8" with 4.5 mm spider vanes

I try to limit my reports to what I see. I really don't know if the
TAL200k is a superlative performer or not, I let other people deterimine
that if they care too.

I noted in my initial report that the scope was different, that I was
seeing different patterns and images, but I didn't know it was also
different in how it would invoke controversy. As Andrea Tasselli said:

> Indeed Yuri, it was quite a good wine. But not as strong as your brandy. In
> a couple of weeks I can predict the TAL CO will reach the 100% mark while
> TEC/Aries/Intes will plunge to the 0% line, only because you can't have
> negative CO :-))
>
> Clear Skies
>
> Andrea
>

In any case, TAL200k scopes are now arriving at their new owners sites
and soon someone else can take up the reporting.

Wm. "Bill" Brady, Harwood MD 38°51'30"N 76°41'00"W - Clouds - Rain -
Clear then WIND!
• ... here ... length ... the ... mm. ... Hi Steve, The reason I said can t follow your logic is that what Valery said (spider area should be treated same as
Message 6 of 19 , Apr 1, 2001
--- In telescopes@y..., sbodenheimer@c... wrote:

>
> I'll be darned. Actually you do seem to agree with my logic.
> Contrast is a function of light gathering capacity -- i.e., a
> function of aperture area. Thus percent drop in contrast is simply
> the same thing as losing a portion of the aperture. Bill's post
here
> corresponds to my measurements of 4mm thickness and your 70 mm
length
> is the same as I measured (using thin solder wire I bent to match
the
> curved spiders). Total aperture area loss is 810 sq.mm. Now that
> corresponds to a reduction in aperture size from 200 mm to 191.7
mm.
> Thus this system apparently is the same as a no spider system of
> 191.7 mm with 70 mm aperture, or 36.5% linear c.o.

Hi Steve,

The reason I said can't follow your logic is that what Valery
said (spider area should be treated same as that of c. obstruction -
the way you put it) doesn't imply that image contrast is a function
of the remaining clear aperture area in the way that it is
appropriate to an unobstructed aperture of that area. Those are
different assumptions.

As for the vanes thickness, that was an estimate on my side.
Actually, it was closer to 4mm, but that Markus's photo in my memory
tricked me into thinking that it has to be thicker. Doesn't make much
difference, anyway.

I arrived to my figure using my shortcut for contrast% and Suiter's
mention of contrast drop caused by spider wanes being aprrox. same
as % of their area in that of the aperture's. I don't follow your
figures (again!), but there's little difference in the end result.

When it comes to figuring out contrast level in an obstructed vs.
aprropriate contrast-wise unobstructed aperture, there is no simple
answer. It is different for high-contrast/high-brightness (Moon,
doubles), low-contrast/high-brightness(planets) and low-contrast/
low-brightness (nebulae) objects, with all in-betweens. For the
first group, the obstructed is worse in the lower resolution range,
and slightly better in the higher one (a 35% obstructed aperture here
would have greatest contrast loss at about 1/3 of its resolution
range, being not much better than a 40% smaller unobstructed
aperture - note that the obstruction takes out only 12% of the area).
For the other two, always worse, but not necessarily in the same
degree. What about the in-betweens? It is complex enough without
factoring in optical quality and optics' alignment, possible effects
on contrast of either corrector, focusing mechanism, baffling,
changing factors (personal, weather-wise, atmospheric, as Mike
mentioned), should I go on?

As for the spider effect, it is there and it's minor, even for
one with 4mm thick vanes. Combined c.o./spider effect is not.
Valery's objections have merit, although my impression is that
your impressions? We all would be interested to hear.

• ... ... Hi Vladimir, Thank you for your detailed explanation and analysis. As you must have guessed, I am no expert on telescope design or
Message 7 of 19 , Apr 2, 2001
--- In telescopes@y..., idalv@e... wrote:
> --- In telescopes@y..., sbodenheimer@c... wrote:

<chop> <chop>

> Hi Steve,
> <snip>
> When it comes to figuring out contrast level in an obstructed vs.
> aprropriate contrast-wise unobstructed aperture, there is no simple
> It is complex enough without factoring in optical quality and
> optics' alignment, possible effects on contrast of either
> corrector, focusing mechanism, baffling, changing factors
> (personal, >weather-wise, atmospheric, as Mike mentioned), should I
> go on?
>
> As for the spider effect, it is there and it's minor, even for
> one with 4mm thick vanes. Combined c.o./spider effect is not.
> <snip>
>

Thank you for your detailed explanation and analysis. As you must
have guessed, I am no expert on telescope design or optics; I do have
a sufficient physics background and interest to be dangerous. :-)

I am sure that I dropped the ball on my quantitative estimate to
which you kindly replied. I was basing my analysis on the general
optical principle that the total effect of multiple obstructions is
simply additive, unless the shape and regularity of arrangement of
the obstructions is sufficient to result in interference. A single
central obstruction, on the other hand, inherently generates
interference. However, you are undoubtedly correct that with so many
different real world factors involved, the theoretical is a mere
guesstimate -- the best test is the real world.

I am sad that my comments may have fueled a fire that hurt
Bill Brady. He has such wonderful enthusiasm -- a friend once told
me he believed enthusiam to be the most important predictor of
success -- that the word itself had origins in the Greek
word 'theus' -- and just as 'theus' is the heart of 'enthusiasm', a
person with enthusiam has a heart filled with a spirt that positively
impacts everyone around them. Perhaps theus will bring Bill back to
this group.

Again, thanks for your analysis and help. Clear skies.

Steve Bodenheimer
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