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Is a masked SCT like an APO for viewing Mars?

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#26 Rich N

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Posted 12 October 2005 - 04:33 AM

Thanks very much for the pictures of your telescopes!

How large is the fully illuminated spot when it gets to the eyepiece?

Thanks again,
Rich

#27 sixela

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Posted 12 October 2005 - 04:41 AM

I'd be looking for subtle details in low contrast dark areas of the target planet where I'd expect larger aperture to have an advantage.


Aperture helps mainly for the really small high contrast details - those that are necessarily high contrast or you won't see them. Things like the Cassini division, or the details in the edge of a feature with very hard edges.

The modulation transfer function for those lower frequency subtle details that can still be perceived when they have lower contrast is better for a refractor (at equal aperture), and this makes those features better whether the baseline intensity is dark or light (contrast necessarily means there's a relatively "dark" and "bright" part to what you observe, of course).

And no, you're wrong that the image is just more "pleasing" - at these frequencies, the MTF of a refractor is measurably higher, which means detail is *objectively* better preserved, unless you increase the aperture of the obstructed scope to compensate.

#28 Rich N

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Posted 12 October 2005 - 04:43 AM

What I was saying about M13 in my refractor is, there are a lot of little stars in the image but the image isn't as impressive as it would be in a larger aperture telescope because the stars (in M13) in my 6" refractor are not very bright. Aperture wins almost every time. I find M13 has more of a "wow factor" in my Obsession 15 (15" f/4.5 Dob).

Rich

#29 Rich N

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Posted 12 October 2005 - 04:55 AM

A gentleman in our astronomy club has a 14.5" f/4.3 StarMaster Dob. It least I'm pretty sure it is f/4.3. It could be f/4.5. This fast, good sized Newtonian (Dob) gives highly detailed planetary images. It is really an exceptionly fine instrument.

I don't think he is using an unusually small secondary mirror. I'll take a look next time I see his telescope.

Rich

#30 JerryWise

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Posted 12 October 2005 - 07:50 AM

Thinking through this thread I have to ask a question:

Is masking down an SCT going to give genuine APO refractor like views? If I take my C-14 OTA and mask it down to say the equivilant area of a 10" APO then could one say that by all means it would be equivilant to a 10" APO. Color correction, contrast and clarity on par with each other?

I have been thinking a lot about the 6" Takahashi on sale right now but looks like I can have a larger APO instrument by putting some cardboard over the front of the 9.25. I certainly bow to others with more experience and knowledge. Help please.

#31 jrcrilly

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Posted 12 October 2005 - 08:20 AM

Thinking through this thread I have to ask a question:

Is masking down an SCT going to give genuine APO refractor like views? If I take my C-14 OTA and mask it down to say the equivilant area of a 10" APO then could one say that by all means it would be equivilant to a 10" APO. Color correction, contrast and clarity on par with each other?

I have been thinking a lot about the 6" Takahashi on sale right now but looks like I can have a larger APO instrument by putting some cardboard over the front of the 9.25. I certainly bow to others with more experience and knowledge. Help please.


The problem is that it's not just a matter of masking to make the aperture smaller. That'd make the central obstruction larger in proportion and performance would be degraded. To gain an improvement you'd need to mask it off-axis to gain an unobstructed, circular aperture. On the C14 that'd give you a maximum of perhaps 4"; on the C9.25 it'd be very small.

#32 JerryWise

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Posted 12 October 2005 - 09:18 AM

Hi John. There was mention of an oblong mask and I was thinking along that vein. It just seems there would be some trade off along the way but I haven't had a clear night to give it a try. Would a Hartman with large circular or oblong holes re-introduce a circular obstruction? Sorry to bother but if we just had one clear night I could answer these with some quick experiments.

#33 sixela

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Posted 12 October 2005 - 12:48 PM

Thinking through this thread I have to ask a question:

Is masking down an SCT going to give genuine APO refractor like views? If I take my C-14 OTA and mask it down to say the equivilant [sic] area of a 10" APO


The trouble is that you can't. If you have a 33% central obstruction, an *off-axis* mask on a 14" scope is limited to roughly 4". And your scope's thermal behaviour is still not that of a 4" APO...

#34 sixela

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Posted 12 October 2005 - 12:50 PM

Hi John. There was mention of an oblong mask and I was thinking along that vein.


That doesn't really help - you'll still end up with no more resolution along the minor axis of your oblong aperture...

#35 JerryWise

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Posted 12 October 2005 - 01:22 PM


In playing with different OTAs and looking through the Tak FS-102 or even the Orion 80ED these purpose built instruments do what they do exceptionally well. It's hard to feel comfortable with one taking a "light bucket" and masking it down to 4 inches and have it perform on a par with the Tak. It's not what it was designed or optimized for. I don't know about this but it is interesting and there are positive statements so in the CN tradition the discussions go on.

#36 snorkler

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Posted 12 October 2005 - 02:02 PM

I'd be looking for subtle details in low contrast dark areas of the target planet where I'd expect larger aperture to have an advantage.


Aperture helps mainly for the really small high contrast details - those that are necessarily high contrast or you won't see them. Things like the Cassini division, or the details in the edge of a feature with very hard edges.

The modulation transfer function for those lower frequency subtle details that can still be perceived when they have lower contrast is better for a refractor (at equal aperture), and this makes those features better whether the baseline intensity is dark or light (contrast necessarily means there's a relatively "dark" and "bright" part to what you observe, of course).

And no, you're wrong that the image is just more "pleasing" - at these frequencies, the MTF of a refractor is measurably higher, which means detail is *objectively* better preserved, unless you increase the aperture of the obstructed scope to compensate.


See this link for a nice discussion that says this isn't so, that MTF is higher for equal aperture Newtonians and other obstructed scopes. The author says that "treble boost" is probably better for separating double stars. The obstructed scopes have consistently higher resulting MTFs once you drop below 3 arc second detail, and the larger Newtonian beats the APO all the way on absolute MTF.

Unless I'm reading this wrong, it indicates that APO superiority is only perceived, not quantifiable by MTF in any case.

#37 snorkler

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Posted 12 October 2005 - 02:04 PM

From my previously linked article: "The first arrow at about 5 seconds of arc is indicating that the APO should outperform both Newtonians for lower spatial frequencies. The second arrow is indicating the the APO is outperforming the Newtonian of equal size even to relative fine details of up to 2.8 seconds of arc. But for finest detail the 6 inch Newtonian is even better than the APO. The overall performance of the 8 inch Newtonian is comparable or slightly better than the performance of the 6 inch APO. I think this is quite reasonable and very often reported from practical tests. Not commonly accepted is the result that the 8 inch Newtonain should clearly outperform the 6 inch APO in the domain of finest planetary detail."

#38 sixela

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Posted 12 October 2005 - 03:08 PM

for a nice discussion that says this isn't so, that MTF is higher for equal aperture Newtonians and other obstructed scopes.


You've just missed the point of the article - there is no such thing as "the" MTF. It's a function of frequency, and you're refusing to consider the appropriate frequency for the specific things we were talking about..

There's no doubt the MTF at high frequencies is simply dominated by aperture. Heck, even boatloads of spherical aberration don't matter at this end of the spectrum. That means that very fine high contrast features is something added aperture excels at - my very words in this thread.

No doubt you will also note that the MTF curves are going *down* with higher frequencies.

That means that for *low contrast features*, none of the scopes let you see *anything* in the right portion of the MTF curve. So you're actually concerned with the *middle* portion.

The one in which obstructed scopes don't perform as well as an APO.

There's also an artefact in the MTF function for obstructed scopes, because it's based on the response of a periodic pattern - and if you scatter energy outside of the Airy disc, at certain frequencies you end up enhancing the neighbouring pattern line (and thus the MTF). But periodic gratings aren't the patterns we look for in real life.

#39 snorkler

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Posted 12 October 2005 - 03:45 PM

No, I never said "the" MTF, and I think you've got the spacial frequencies mixed up. The APO does better at higher spatial frequencies, where you say aperture dominates.

It's actually at the lower spatial frequencies where aperture dominates - the low contrast coarse details. But the graphs clearly show the bigger aperture dominates or equals the smaller APO across the entire spatial spectrum. The gap simply widens at the lower spatial frequencies. Better simply becomes much better for the bigger aperture CAT or Newt in that portion of the frequency range.

#40 sixela

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Posted 12 October 2005 - 04:06 PM

No, I never said "the" MTF,


You wrote:

See this link for a nice discussion that says this isn't so, that MTF is higher for equal aperture Newtonians and other obstructed scopes.

I'm failing to see the qualifier with respect to frequency in front of "MTF" in that sentence. Maybe you didn't mean it, but to me that looked like reducing MTF to a single number that must necessarily be "better" for one of the scopes.

and I think you've got the spacial frequencies mixed up. The APO does better at higher spatial frequencies, where you say aperture dominates.


No, I don't think so. From your own quote of the article:

The first arrow at about 5 seconds of arc is indicating that the APO should outperform both Newtonians for lower spatial frequencies.



#41 JerryWise

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Posted 12 October 2005 - 04:29 PM

Let me slip a little something in to the running gun battle.

We use MTF for imaging chain evaluation of medical systems. These include both electronic and optical images. MTF is most meaningful as a measure of a specific imaging chain. In this case it can be the lenses and EP of the Refractor or Reflector. Does an aberrator representation of a hypothetical MTF calculation truly represent the performance of these devices? Will an APO triplet show a different MTF compared to a doublet? Will different APOs show a different MTF? While appearing to be a scholarly article, does the article reflect an actual measurement between at least two sample OTAs to validate the hypothesis. I don't see a set of representative data from a broad set of sample measurements to validate the proposal put forth in the article. Maybe a bit more work in the validation and correlation of data?

#42 snorkler

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Posted 13 October 2005 - 10:05 AM

Sixela,

You're right. I misinterpreted the article, and should be looking to higher spatial frequency to see the superiority of bigger aperture.

You clipped my comments, which should have left no doubt that I didn't mean "the" MTF. The part you clipped said

The obstructed scopes have consistently higher resulting MTFs once you drop below 3 arc second detail, and the larger Newtonian beats the APO all the way on absolute MTF.

That doesn't sound like I'm referring to a single static value, does it. The only thing I left off was an s at the end (MTFs), but I took it from the graph title, which is "absolute MTF of various Telescopes."

The article does reinforce the consensus that it only takes an 8" reflector to beat a 6" APO in seeing detail, both in DSOs and in planetary viewing.

#43 JerryWise

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Posted 13 October 2005 - 01:07 PM

Based upon these irrefutable scientific presentations, I have ordered an SN-10 just moments ago. I fully expect it will stomp the bee-jeebers our of an AP 8 inch APO in all areas. I was not aware of the general consensus an 8 inch reflector was better than a 6 inch APO. I was just before going for a 6 inch Tak for 7k and picked up the SN-10 on EBay outlet for 449. I can't wait to cut that thing loose on the LXD650 imaging Mars. Wow.

Thanks for the advice.

#44 Rich N

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Posted 13 October 2005 - 04:49 PM

Imaging seems to put everything in a "different light" when it comes to resolving low contrast detail.

Visually, an 8" Newt. may show more low contrast detail than a 6" APO refractor but the conditions need to be just right. If they would consistantly out perform a 6" APO, night in and night out, I would buy one and not bother with my APO. My experience is my APOs give conisitantly fine images, as good as the seeing will allow.

Rich

#45 Psa19one

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Posted 13 October 2005 - 06:15 PM

tsk...tsk...tsk...

Can't we all just get along? ;)

#46 wh46gs

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Posted 13 October 2005 - 09:50 PM

While appearing to be a scholarly article, does the article reflect an actual measurement between at least two sample OTAs to validate the hypothesis.



Doesn't appear scholarly to me. The author doesn't even
know what is the cut-off frequency (Lambda/D in radians,
or 113.4/D in arcsec, for D in mm). More importantly,
what he presents as an "absolute" MTF (there is no such
thing) is simply an usual MTF constructed for brightly illuminated high-contrast details (bars). Although it gives
general indications contrast-wise, it's pretty much useless
for determining low-contrast performance. For that, one has
to take into account both lowered inherent detail contrast and increased minimum contrast level required by the eye
(as described by Rutten and Venrooij, Telescope Optics p215). Cut-off frequency for planetary details is roughly
half that for bright contrasty details.

Also, there is a number of other factors affecting telescope
performance - inherent wavefront quality, miscollimation and thermal sensitivity, baffling, optical scatter, seeing, etc. Attempting to illustrate performance comparison based
on MTF for perfect obstructed/unobstructed aperture alone
is no more than an excercise in futility - even if the MTF graphs were properly constructed.

#47 JerryWise

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Posted 13 October 2005 - 10:42 PM

While appearing to be a scholarly article, does the article reflect an actual measurement between at least two sample OTAs to validate the hypothesis.



Hello wh46gs.

There was just a bit of tongue in cheek in there. :imawake:

Welcome to CN.

#48 snorkler

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Posted 13 October 2005 - 10:58 PM

Well, there's always this thread to add fuel to the fire. No messing with scientific theory here - just flat out statements and top notch equipment and subjective evaluation. But the answer isn't going to satisfy the refractor contingent any better than the last article.

#49 snorkler

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Posted 13 October 2005 - 11:11 PM

Nor will this classic review, where a 7" Newt beats a 5" AP in image quality (mystique doesn't count).

#50 JerryWise

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Posted 13 October 2005 - 11:17 PM


Yep, read that a long time ago. Wonder how that 14" beat the socks off that 24". But anyway, I bought an SN-10 to see for myself. Got a bunch of SCTs too.

Did we ever figure out if a masked down SCT would have APO like images? I was interested more in that than this other stuff.


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