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Is a masked SCT like an APO for viewing Mars?

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#101 wilash

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Posted 19 October 2005 - 07:07 PM

In that experiment, based on the data, the ED80 was resolving 53 line/mm at the retina and the stopped down 9.25 was resolving 75 lines/mm based on an f/34 optical train for the 80 and an f/24 optical train for the SCT.



Hmh...wonder where do these numbers come from?
Eye can only see what is resolved in the focal plane
of the objective. Conventional MTF gives linear cutoff
frequency as 1/LambdaF, which for 0.00055mm Lambda comes
to 1818/F lines per mm (F being the system F#). That
gives 242 lpmm to an f/7.5 system (80mm ED), and 61 lpmm
for an f/30 (approx. C9.25 masked down to 80mm). Sure,
angular resolution is the same.

That is for bright, contrasty lines. For low-contrast lines,
number of lines resolvable will be only about half as many.
Any other factor lowering image contrast will have it further reduced.

Vlad


Vlad that is the resolving power of the image at the retina, not at the image plane. That would also be a maximum because I was not adding the combined resolving powers of the componants of the scope/EP/eye, but it illustrates approximately the difference in resolving power between the two setups.

And you are right about target contrast, but this was just to illustrate a theoritical maximum to show that focal length was not a determining factor in the comparison as the systems should be producing fairly similar images. The fact that the SCT was not performing very well means that there was something up with the optics.

#102 sixela

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Posted 20 October 2005 - 03:33 AM

[quote name="wh46gs"] [quote]I don't know what collimation tools you use, but 1.3 arcmin over 1780mm is about 1.34mm, and that's an error that's *glaringly* obvious - with an autocollimator that magnifies that error, it simply would be impossible to miss this even if you tried very hard. [/quote]

1.3 arcmin @ 1780mm f.l. comes to 0.67mm.

Depends on whether you take it as radius or diameter - either way, it's a *large* collimation error, even with merely a good cheshire, and certainly with an autocollimator.

I'm going to have to agree to disagree on the magnitudes - I've spent just about as much time debunking the myth that an APO can't show more than an SCT of larger aperture as I've done disputing the assertion that a large newtonian *can't* be better than an 8" APO in real life, and given that there is no flaw in your reasoning (just different premises leading to different *quantitative* and not qualitative results - I'm not going to dispute the total system Strehl of a 6" APO is larger than that of a 24" Newtonian) and as I'm an equal-opportunity myth debunker, I'm going to leave it at that.

#103 wh46gs

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Posted 20 October 2005 - 10:30 PM

Vlad that is the resolving power of the image at the retina, not at the image plane. That would also be a maximum because I was not adding the combined resolving powers of the componants of the scope/EP/eye, but it illustrates approximately the difference in resolving power between the two setups.



Will, looks like we're talking past each other. There is no
resolution limit other than that imposed to the image in the
focal plane. Either eyepiece or the eye could lower
resolution inherent to objective's image only if they
contribute significant aberration to it - normally not the
case on axis. Any pair of identical apertures size-wise has
identical theoretical angular resolution limit. Where we come together is the obvious: if one of such two apertures show inferior performance, it is because it does inferior imaging.

Vlad

#104 wh46gs

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Posted 20 October 2005 - 10:54 PM

1.3 arcmin @ 1780mm f.l. comes to 0.67mm.

Depends on whether you take it as radius or diameter - either way, it's a *large* collimation error, even with merely a good cheshire, and certainly with an autocollimator.



Miscollimaton diameter? Guess it comes from the "Things You
Never Knew Existed" catalog...

Well, on the diagonal it is more like 0.5mm - little wider
than a pencil line. Guess half as much could go as "small"
but still noticeable (and correctable). This would mean that
you can collimate 16" f/4.5 Newtonian to 0.98 Strehl or
better. No word of what happens with this superb collimation
in between - remember, I was talking about miscollimation
error in the use. One can only use a telescope in between collimating.

I've spent just about as much time debunking the myth



The only myth to be debunked here is that near perfect
primary in fast Newtonians means anywhere close to that quality level actual performance - the larger aperture, the more so. It is counterproductive to negate potential significantce of additional error sources in order to extend high quality level of the primary alone into that of the entire system.

Vlad

#105 sixela

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Posted 21 October 2005 - 02:30 PM

1.3 arcmin @ 1780mm f.l. comes to 0.67mm.

Depends on whether you take it as radius or diameter - either way, it's a *large* collimation error, even with merely a good cheshire, and certainly with an autocollimator.



Miscollimaton diameter? Guess it comes from the "Things You
Never Knew Existed" catalog...

Neither did I - but if you have something that's pointing 1.3 arcmin away from an axis, the ray will insersect on a plane 1780mm away approximately like (in mm) 1.3 arcmin in radians * 1780mm - and that's *my* larger figure.

I don't know where *you* lost that factor of two, but I was prepared to give you the benefit of the doubt (and to understand that with a "1.3 arcmin error", you actually meant an error somewhere in a cone whose two extremes were 1.3 arcmin from each other, i.e. with a max error measured from the axis that was half that angle).

If you're talking about a real error that simply *is* 1.3 arcmin, that is 0.021666.. degrees, which is 0.7563e-4 radians.

sin(x) is roughly tan(x) and is roughly x, so the intercept distance from the axis on a plane 1780mm away is 0.7563e-4*1780mm = 1.346mm.

#106 sixela

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Posted 21 October 2005 - 02:56 PM

The only myth to be debunked here is that near perfect
primary in fast Newtonians means anywhere close to that quality level actual performance

Given you'll find no-one asserting that myth in this thread (and I have asserted the exact opposite in this thread many times), there's no need to debunk that myth.

It doesn't even need a lot of fancy math to realise that statement is quite plainly true : an APO simply has one objective to deal with, a Newtonian has two mirrors, and requires *two* mirrors to be aligned well.

But to claim you can't get a Strehl of .98 out of a system and insinuating the Strehl ratio would be hard to get above 0.5 are two very different things.

As I said, we're in violent agreement about the actual degradations that can occur, but if you'd ever looked through a really large and really good Newtonian in perfect seeing, you would hesitate to make the same "illustrative" statement with *those* quantitive figures (and, accessorily, to also cast doubts on Vic Menard's assertion that it's possible to read .3mm with the best cheshires).

Just try to make it fly in the reflectors forum (where owners of >20" Newtonians that live in Florida happen to lurk) and they may be less civil about it than I am ;).

#107 sixela

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Posted 21 October 2005 - 03:02 PM

No word of what happens with this superb collimation
in between


If you'd spent any time in the reflectors forum, you'd know I'm a nit about this. When looking at Mars, I do collimate the scope just before looking at the planet, and given I'm a nit, with the scope pointed at *that* exact altitude. And I check it regularly, too, though I must confess that not even the autocollimator usually reveals any error just before a platform reset is needed (afte rabout 50 minutes).

Did I mention I *love* APOs because of the simplicity in extracting good performance out of them? Unfortunately, I can't afford that mythical 16" APO ;).

#108 JerryWise

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Posted 21 October 2005 - 08:29 PM

Ok Dale. We can go with it now looks like.

"A masked down SCT is not like an APO for viewing Mars". :refractor:

#109 wh46gs

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Posted 22 October 2005 - 07:51 AM

If you're talking about a real error that simply *is* 1.3 arcmin, that is 0.021666.. degrees, which is 0.7563e-4 radians.

sin(x) is roughly tan(x) and is roughly x, so the intercept distance from the axis on a plane 1780mm away is 0.7563e-4*1780mm = 1.346mm.



0.021666 degrees is 0.021666/57.3=0.3781e-4 radians and the
linear deviation 1780mm away is 0.67mm (we shouldn't be
spending this much time on a simple math)

#110 wh46gs

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Posted 22 October 2005 - 08:25 AM

But to claim you can't get a Strehl of .98 out of a system and insinuating the Strehl ratio would be hard to get above 0.5 are two very different things.



This is pretty lose interpretation of what I wrote. 0.98
Strehl was in context of collimation error alone; 0.57
Strehl was in the context of total accumulated system
error. Those are two very different things.

As I said, we're in violent agreement about the actual degradations that can occur, but if you'd ever looked through a really large and really good Newtonian in perfect seeing, you would hesitate to make the same "illustrative" statement with *those* quantitive figures (and, accessorily, to also cast doubts on Vic Menard's assertion that it's possible to read .3mm with the



Wonder what "violent agreement" should mean? And, no, I wouldn't hesitate, and I already said why. The eye can't
measure change of energy distribution within in-focus
pattern that is caused bu a number of small mostly unrelated
errors. Also, they won't cause perceptible change in the
form of inside- and out-of-focus pattern. You can chose
between the two: (1) your eye is a perfect sensor capable
of measuring changes of intensity distribution within diffraction pattern, and (2) optical theory has got it right
about it. I'll pick the latter.

BTW, intentionally or not, you are taking things out of context. You are talking pefect seeing, expert collimators
(and being able to read down to 0.3mm doesn't in any way
guarantee that the alignment itself will be at that level of
precision even immediately after the collimation), and I was
talking about an average (good) telescope, average user and average conditions.

Just try to make it fly in the reflectors forum (where owners of >20" Newtonians that live in Florida happen to lurk) and they may be less civil about it than I am



Should that be a reason for me not to speak out what I think? I already had discussions with such owners, and
it was all civilized, even if they had different views -
which I understand.

Vlad

#111 wh46gs

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Posted 22 October 2005 - 08:38 AM

I must confess that not even the autocollimator usually reveals any error just before a platform reset is needed (afte rabout 50 minutes).



Collimation of a Newtonian is more - should I say "much
more" - complex process than what most people think. It
would take too much space to go into it here and, granted,
I wouldn't be qualified enough to do it. But you can do
some reading of your own (try Nils Olof Carlin - he went
quite deep into the subject). One thing you'll find out
is that autocollimator is not that perfect collimating
device you seem to be believing.

Vlad

#112 sixela

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Posted 22 October 2005 - 01:45 PM

Collimation of a Newtonian is more - should I say "much
more" - complex process than what most people think.


Given I must have spent more than 200 posts on the subject, I'm not in a position to disagree.

That doesn't mean it's impossible to master; also, perfect axial collimation is a lot more easy to get than perfect axial collimation *and* perfect diagonal offset for optimal field illumination.

It would take too much space to go into it here and, granted,
I wouldn't be qualified enough to do it. But you can do
some reading of your own (try Nils Olof Carlin - he went
quite deep into the subject). One thing you'll find out
is that autocollimator is not that perfect collimating
device you seem to be believing.

Stop putting mouths into my word, and don't assume I'm not familiar with what both Nils and Vic have written. I have never claimed it is the only tool you need to collimate - it is not.

You still need a cheshire or a barlowed laser to evaluate an autocollimator stack, yes (and ideally, also a well collimated laser so that you can *also* check focuser axis alignment) - but if you have an error on a *single* element in the path, the error will show and be magnified, so if you have *independent* tools to check the focuser axis and primary mirror axis, it *does* help you weed out even relatively small errors (and a misaligned stack even gives you an indication of which element is misaligned, too).

And you need the independent tools anyway, given you need to already have good collimation before you start to use the autocollimator, and that neither the cheshire, laser, barlowed laser or autocollimator can help you optimise diagonal placement for correct offset and field illumination (you need a sight tube for that).

But I can assure you that you'll have quite some trouble convincing either Nils or Vic that you can't collimate a newtonian to a 1.3mm precision (or was that 0.67mm?); I wouldn't have had the existential angst I had when I saw that my Jim Fly centre spot and pre-existing centre spot were misaligned by a .1mm that was blatantly apparent (and just in case you claim you can't evaluate .1mm: Vernier calipers have a nonius).

#113 wh46gs

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Posted 22 October 2005 - 04:23 PM

Stop putting mouths into my word



Wow, you can juggle words too, not only numbers. If you said
that "even autocollimator" can't reveal any errors, it
directly implies that you consider it an ultimate
collimation tool, of high level of precision.
That is, in any reasonable interpretation. If you don't
think so, don't blame me - be more careful about your
selection of words.

But I can assure you that you'll have quite some trouble convincing either Nils or Vic that you can't collimate a newtonian to a 1.3mm precision (or was that 0.67mm?);



You seem to have very poor memory when it comes to your
errors, and very selective thinking when trying to prove
"your point". I don't know and won't be assuming to what
precission Vic or Nils think is possible to collimate a
16" f/4.5 Newtonian - that is off the subject. Should I
remaind you once more: I was talking about average mid-size
fast Newtonian in use, with good mirror and average everything else - misallignment, mechanical quality, seeing, user, etc. You are falling into your own trap by
altering it into me claiming that no better collimation
is possible whatsoever, and hating me for that. Take it easy: this is just an amateur talk...

Vlad

#114 jrcrilly

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Posted 22 October 2005 - 05:49 PM

Both of you:

Take the argument somewhere else.

Now.

#115 sixela

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Posted 22 October 2005 - 06:15 PM

and hating me for that.


I feel no hatred, I can assure you - I'm not in the habit of hating *persons* anyway.

#116 sixela

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Posted 22 October 2005 - 06:17 PM

Both of you:

Take the argument somewhere else.

Now.


God forbid! It's taken long enough as it has.

#117 JerryWise

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Posted 22 October 2005 - 07:11 PM

So I ordered a 10" SN-10 from the Meade outlet as a result of this.... ah... discussion.

So I get it all rigged up on the LXD650 last night. (I would pay good money to see one of those on an original LXD55.) Had to go drill out some LXD75 counterweights to balance the thing. Also had to hit Walmart and get one of those support belts to get in all the positions needed to align and observe with it (where did that finder come from?). Is there a comfortable OTA position in the rings for all objects using one of these.

So it does real good on Andromeda. Mars, Saturn, Orion and the rest of the stuff. Well, I still like my APO for planets and Celestron 9.25 for anything else. I think I like my RCX but really can't remember.

An APO is an APO on Mars. Very, very good. So this thread has, in general, explored some interesting territory.

On the other issue: :roflmao:

#118 Guest_**DONOTDELETE**_*

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Posted 22 October 2005 - 08:53 PM

Guys,
I really didn't mean to start a "star wars" with my question that started this thread. My simple takeaway is that, for observing Mars, an APO is really nice. If one has a Schmidt-Cassegrain, it's generally best to use it "full bore". HOWEVER, I've initiated a related question in the refractor forum. There, I noted that people with refractors (or other scopes) often observe Mars with a filter to enhance contrast. So, since a filter only passes a certain band of wavelengths, I asked whether it really matters whether one uses a filtered APO or a filtered achromat (with the same aperture and focal length). Correcting wavelengths outside the filter passband wouldn't matter in this case, I thought. You can go stir the waters there if you are so inclined!
I've enjoyed the discussion in this thread, and have been checking it every night or two. But I think that my original question has been pretty thoroughly covered.
Many thanks,
Dale


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