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1 inch APO vs 12 inch SCT

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#101 yellobeard

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Posted 03 January 2018 - 01:54 PM

Part of the new insights is the easyness of improving the instant image quality in mostly commercial SCT's with easily aviable materials involving a few dollars..

So pretty well aviable to everybody!
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#102 Astrojensen

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Posted 03 January 2018 - 01:54 PM

 

 

The is no such thing as "never". If properly made with high quality optics, and then properly collimated, an 8 inch SCT will beat a 6 inch APO all the time in similar atmospheric conditions.



Unless of course you believe the physics of optics are incorrect.

In my experience (visual observing), this is simply not true, if we are talking about bog standard, run-of-the-mill 8" SCT's, no matter how excellent their optics are. The 6" refractor (it doesn't have to be an apochromat) will be FAR less sensitive to thermal issues than the 8" SCT and so under many observing

Thomas, Denmark
Uhh ... And that is the most important thing where you're totally wrong!!

New insights on managing the temperature problems in 'thermally difficult' telescopes gave us methods to insulate the telescopes in such way, that even a 16" SCT performes at its best only some 10 minutes after put outside from a much higher room temperature!

So thermal issues are no reason anymore to place an APO refractor above a super high quality SCT.

It is possible that the new insights did not yet reach everybody here. In my country however, lots of people already know that those insights have greatly improved the stability feeling working with reflectors, especially SCT's!

 

I actually experimented quite a lot with insulating the tube, but couldn't get the good results others rave about. The best compromise I found was to use aluminum foil (also called tin foil) wrapped around the tube. This seemed to eliminate some tube currents by stopping the tube from cooling below ambient via radiational cooling.

 

I did say IN MY EXPERIENCE, as it is simply what *I* have found, not what others have found. 

 

I also talked about the average, standard construction SCT, not optimized designs like Rik Ter Horst's 8" f/25 with open back and insulated tube, which is obviously going to react VERY differently than my off-the-shelf C8. 

 

 

Clear skies!
Thomas, Denmark



#103 Astrojensen

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Posted 03 January 2018 - 01:58 PM

Part of the new insights is the easyness of improving the instant image quality in mostly commercial SCT's with easily aviable materials involving a few dollars..

So pretty well aviable to everybody!

Links to descriptions of how-to and confirmations of the improvements and such would be highly appreciated.

 

 

Clear skies!
Thomas, Denmark


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#104 yellobeard

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Posted 03 January 2018 - 02:00 PM

Of course.. There still are the factory tolerances that cause most commercial SCT's to be of less (sometimes much less!) quality then theoretically possible

#105 Astrojensen

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Posted 03 January 2018 - 02:03 PM

Of course.. There still are the factory tolerances that cause most commercial SCT's to be of less (sometimes much less!) quality then theoretically possible

Mine did have pretty good optics. On one rare, very warm summer night, I was able to observe Delta Cygni with 800x on the binoviewer and the airy disks were sharp and clear. 

 

So they do occasionally come with very good optics. 

 

 

Clear skies!
Thomas, Denmark


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#106 Erik Bakker

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Posted 03 January 2018 - 02:03 PM

I use and own different types of scopes to accommodate my preferences of the day. Even a nice compact SCT every now and then.

 

That said, there is no super high quality SCT in existence today that only needs some thermal bugs sorted out for a few dollars.

But they can have nice marketing campaigns and supportive internet buzz.


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#107 yellobeard

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Posted 03 January 2018 - 02:20 PM

@Astrojensen,
Then indeed you ended up with a good C8..
But even when you see airy disks, there mostly can (and will) be some light that is scattered by surface roughness on mainly the schmidt corrector plate.. Almost all commercial SCT's suffer from this because of the polishing method on the schmidt corrector. This also is the main reason why most have the experience that same aperture Maksutovs perform better than SCT's, the spherical meniscus corrector is polished the same way as a lens of a refractor, involving much better surface roughness.
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#108 yellobeard

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Posted 03 January 2018 - 02:32 PM

And links.... That is a tough one. Even on the dutch forums, not very much is written about it.
Some made presentations about it that were shown at ATM gatherings.

Point is that you not only should reflect the cold radiation from the sky, but also thermally insulate your scope.

With 'closed' optical systems like SCT's and mak's, best way is to slow down the internal cooling process in such way, that the optics, even the primary mirror, cool down slow enough, that there are no internal temperature differences in the material of the optics, also not in the internal air, which creates a very stable situation inside that scope.

#109 Cotts

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Posted 03 January 2018 - 02:44 PM

And links.... That is a tough one. Even on the dutch forums, not very much is written about it.
Some made presentations about it that were shown at ATM gatherings.

Point is that you not only should reflect the cold radiation from the sky, but also thermally insulate your scope.

With 'closed' optical systems like SCT's and mak's, best way is to slow down the internal cooling process in such way, that the optics, even the primary mirror, cool down slow enough, that there are no internal temperature differences in the material of the optics, also not in the internal air, which creates a very stable situation inside that scope.

That's a really good point and it flies directly in the face of the 'received wisdom' that cooling down your Catadioptric scope as rapidly as possible with fans, icepacks and whatnot is the way to go.....

 

Dave


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#110 andycknight

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Posted 03 January 2018 - 03:00 PM

Cotts, on 03 Jan 2018 - 7:44 PM, said:

That's a really good point and it flies directly in the face of the 'received wisdom' that cooling down your Catadioptric scope as rapidly as possible with fans, icepacks and whatnot is the way to go.....

Indeed...

 

Although I also think that both methods (fast or slow) are just different techniques that achieve the same goal.

 

I personally think that both methods work well, however depending on the circumstances on a given night either one could be the best method to employ.

 

Perhaps there is also another (combined solution)... To cool the optics to ambient as fast as possible with fans / icepacks etc.. and then rely on insulation to cater for gradual changes during the night?

 

Regards

 

Andy.


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#111 treadmarks

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Posted 03 January 2018 - 03:00 PM

 

treadmarks, on 03 Jan 2018 - 4:14 PM, said:

Just as some people don't understand why you'd look at the same objects over and over, I don't understand why you'd get excited about some gray blob that's so dim it could be mistaken for dew on the eyepiece.

I can only answer this for myself and obviously other opinions will differ... and that's a good thing otherwise life would be pretty dull! smile.gif :-

 

One of the most amazing things I have looked at in the night sky, was glimpsing nothing more than a 11th magnitude dot of light under really poor skies. What was amazing to me was not the quality of the image, but rather the understanding that this object was nearly 8 billion light years away.

 

Using nothing more than a tiny 90mm Mak, I was able to glimpse a jet outburst from a black hole that was 'eating' tremendous amounts of matter. I was looking back in time long before the solar system had even formed.

 

I find that exciting... far more than a showpiece object like M42.

 

Regards

 

Andy

 

I will say that there are some "specials" like Pluto, Triton, 3C 273 etc. While I don't expect them to be visually stunning, I will be impressed by what I am seeing and I think they belong in the winner's column. However when it comes to everything else I can afford to be choosy, there's no shortage of options.

 

To bring it back to the topic, what do all the Class II amazing objects have in common? Bigger telescopes are better, if you plan on ever seeing them at all. Aka, not refractors. We talk about all this x-per-inch but sooner or later we need to start putting hard numbers on things. Triton doesn't have a magnitude per inch, it's just 13.5 and if your telescope can't hit that number then thanks for playing.


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#112 Mitrovarr

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Posted 03 January 2018 - 03:01 PM

There's always three ways to reduce tube currents in a scope. You can cool the scope to ambient, so it is thermally stable at ambient. You can insulate it, so it is stable at the initial temperature. And you can circulate the air inside the tube hard enough that the air can't set up thermal gradients (this won't stop optical surfaces from warping though).

I had not thought insulation was a good method. I'm not sure how you are going to stop enough heat loss from surfaces you can't insulate (like the lens).

#113 Astrojensen

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Posted 03 January 2018 - 03:14 PM

And links.... That is a tough one. Even on the dutch forums, not very much is written about it.
Some made presentations about it that were shown at ATM gatherings.

Point is that you not only should reflect the cold radiation from the sky, but also thermally insulate your scope.

With 'closed' optical systems like SCT's and mak's, best way is to slow down the internal cooling process in such way, that the optics, even the primary mirror, cool down slow enough, that there are no internal temperature differences in the material of the optics, also not in the internal air, which creates a very stable situation inside that scope.

Yep, I understood that back then, but still really couldn't get it to work well for me and the C8. I tried using flexible foam as insulator. 

 

On that night when I observed Delta Cygni at 800x, I could see the seeing get better and better, until it was just about perfect, but then it started to degrade just as rapidly. Inspection revealed a thin layer of dew on the tube, which immediately told me that the tube was cooling to below ambient temperature, so I quickly ran inside and grabbed some newspapers and wrapped them around the tube. This stopped the tube from radiating into the night sky and the seeing inside the scope improved immediately and the image once more became steady. 

 

But when I tried insulating the tube and just take it outside and start observing, it was just a blurry mess. No, not for the first few minutes, but once it started cooling mainly through the corrector lens, things got bad. Quite horribly bad. The solution would of course be to leave it outside in the shed, so it was always closer to ambient (but it could get quite hot in the shed, even in spring and autumn, so it wasn't always so close to ambient; the same is often true of a small observatory), but if it was close to ambient, I didn't need so much insulation in the first place. I found that leaving the scope outside in the shed and only relying on a thin layer of aluminum foil to keep it from getting too cold, but not from cooling to ambient, was the best strategy for passive thermal management, short of cutting the tube open. One of my friends cut lots of holes in the tube of his C8 and found it to help dramatically. I didn't want to do that, because technically I only had the C8 on loan.

 

 

 

Clear skies!
Thomas, Denmark



#114 yellobeard

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Posted 03 January 2018 - 04:31 PM

Well thats exactly the point Thomas!
When you properly insulate a scope, dew on the scopes tube surface does absolutely not mean that the temperature of the inside of the scope goes 'below ambient'!

Insulating means that the outer layers of insulation cool down, and sometimes collect dew, but the insulation itself prevents the rapidly cooling down of the inside of the scope.

Then there is the remark about forced cooling also being a good way, well, its not a bad way, but there still is a big volume of glass that is called 'primary mirror'.. Glass still is a reasonable insulator, so it will cool down at the surface during forced cooling, but if you do not use that forced cooling for two hours or more, warmth from the inside of that primary mirror still will surface and cause turbulence in the scope.. I'm not saying that forced cooling doesn't work, but insulating works better, just because there are no temperature differences in the glass of the optical elements.

I also saw the remark about internal fans for circulation, that is a very good working setup which workes best with insulation. In both my modified C11 scopes there are three internal small fans behind the primary mirror. Combined with insulation, circulating the internal air makes it possible for the optics to better find that equilibrium you need for that stable situation in the scope.

In my 16" SCT, i used professional insulating material, which is 19mm thick, insulating in such way, that internal fans are not nessesary..

When you insulate a SCT, you still need a dew shield to protect the schmidt corrector plate from the cold radiation from above, as the reasonable insulation capacity if the correctors glass still makes is cool down at the front, enough to collect dew, although it will happen much later than normal because of the internal air continuing to warm up the inside of the corrector plate.

Edited by yellobeard, 03 January 2018 - 04:46 PM.

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#115 Jon Isaacs

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Posted 03 January 2018 - 04:40 PM

To bring it back to the topic, what do all the Class II amazing objects have in common? Bigger telescopes are better, if you plan on ever seeing them at all. Aka, not refractors. We talk about all this x-per-inch but sooner or later we need to start putting hard numbers on things. Triton doesn't have a magnitude per inch, it's just 13.5 and if your telescope can't hit that number then thanks for playing.

 

 

The thing of it is,  the larger the aperture,  the more Class Il just barely visible amazing objects there are.. 

 

Jon


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#116 Daniel Mounsey

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Posted 03 January 2018 - 06:14 PM

What I always laugh about, because it is ridiculous, is the "my refractor has pinpoint stars while SCTs give bloated star images".  I hear it all of the time, but of course it's not true.  Here's the equation valid for all telescopes for angular resolution:

 

Angular resolution = 116/D, where the wavelength is 550 nm and the (D)iameter of the scope's primary is in mm.

 

Note that the resolution is inversely proportional to D.  Meaning that you can resolve smaller things with larger scopes.  There is no dependency in this equation for the type of telescope.  Nor is there an different equation for APOs versus SCTs.

 

I think many times the SCTs with "bloated" images are just not in equilibrium with the environment.  I have seen this happen with APO triplets that are airspaced.  It takes awhile for them to cool down as well.

 

Sandy,

Let's be practical here and not take the matter out of context. Of course stars are smaller in larger apertures, but that doesn't mean they'll "appear" that way under most circumstances. 



#117 CHASLX200

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Posted 03 January 2018 - 06:48 PM

Mass produced scopes are just so so at best. One may be a killer and 10 others so so and 4 could be mush dogs, be it Newts or SCT's .  They just don't compare to the Zambuto and other high end optics that cost much more. I have owned over 220 scopes and have some of the best seeing one could hope for. I know what is great and what is lack luster. Same for the APO's, the cheaper stuff just can't touch my Taks.  Cheap stuff just can't be made like the high end stuff when the cost is so much cheaper. But i will say there are some insane sharp SCT's out there, just gotta luck into finding one.


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#118 BillP

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Posted 03 January 2018 - 08:41 PM

Yes I’m exaggerating lol.gif   but why is this a thing in so many refractor discussions on the web?

Why is It that so many will do a comparison of a 4 inch APO and an 8 inch SCT or Newtonian and think that they see more in the APO? Limiting magnitude and resolution will surely be better in the larger scope. Am I missing something?

 

Yes.  You are missing many of the variables that affect capability of a telescope, such as:

 

- aperture of the scope,

- precision and stability of the mechanics (scope and mount),

- thermal acclimation of the scope,

- thermal acclimation of the mount,

- ability of the scope to maintain thermal stability,

- scatter differences of the different optical designs,

- scatter differences of the particular production unit,

- contrast transfer differences of the different optical designs,

- contrast transfer differences of the particular production unit,

- stray light control differences of the different engineered designs,

- stray light control differences of the particular production unit,

- thermal stability of the observing location,

- seeing of the observing location,

- transparency of the observing location,

- temperature/humidity/winds of the observing location,

- sky darkness of the observing location,

- eyepieces used,

- diagonals used,

- specific targets observed,

- dark adaptation of the observer,

- physical condition of the observer,

- psychology of the observer,

- specific environment that the observer has adapted to (i.e., location where they live as this affects things like color pallet perception, etc.),

- skill of the observer with the equipment they are using,

- skill of the observer relative to the target they are observing,

- preference of the observer (i.e., likes, dislikes, taste).

 

Each and every one of these is a variable that will critically impact the observing experience.  Depending on the particular mix of all of the variables, the observing outcome can be radically different.  One variable does not rule. 

 

I have very much been in situations where seeing was leveling resolution between disparate apertures.  I have also been in situations where I judged that a smaller aperture scope was providing a preferred view over a larger aperture scope at same magnification.  Is it always that way?  No.  Is it that way many times?  Yes.  When one has experience though using various optical designs in various situations one gains the ability to predict how one vs. another will perform on a given target under given conditions. 

 

With experience, these IMO are the primary precepts:

 

1) A smaller scope is never a best choice for all observing situations

2) A larger scope is never a best choice for all observing situations

3) A single scope is never a best choice for all observing situations

4) Skill, experience, and expectation management are always game changers in any observing situation


Edited by BillP, 04 January 2018 - 11:13 AM.

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#119 BillP

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Posted 03 January 2018 - 09:15 PM

It’s actually a psychological issue.  When someone buys something that costs a lot of money, say a 5 inch APO for $7000, they then develop a “cognitive bias” where they need to justify to themselves that it was worth it. 

 

I agree this bias exists, but it does not mean it always exists, and when it does exist it cuts many ways.  So when someone buys something that does not cost a lot of money, say a 5" Newtonian for $250, they then develop a "cognitive bias" where they need to justify to themselves that it performs as well as more expensive scopes.  The cognitive bias can be defeated though by purchasing and thoroughly evaluating each unit.  As example, I purchased a small $400 Apo and $1000 Apo of same aperture.  After more than a year of side-by-side evaluation they less expensive mass production Apo showed better performance across the celestial object types I viewed most.  Sold the more expensive premium Apo.

 

 

 

SCT's, Mak's or Newt's that are made to perfection are fantastic performers. From 2"-7", per aperture, they will never beat a high-end apo.

The is no such thing as "never".  If properly made with high quality optics, and then properly collimated, an 8 inch SCT will beat a 6 inch APO all the time in similar atmospheric conditions.

 

Unless of course you believe the physics of optics are incorrect.

 

The physics of optics state that a compound mirrored design like an SCT or MAK will always have more scatter than a purely refractive design of same aperture.  The physics also states that the SCT and MAK will also have less light throughput than the unobstructed all refractive design of an Apo of same aperture.  Physics also states that the PSF of the non-obstructed design will always be better than that from an SCT or MAK of same aperture.  The engineered thermal behavior of the refractor OTA is also superior to that of a same aperture SCT or MAK.  Physics shows that light rays in a compound design like a SCT or MAK can easily intercept a given thermal in the OTA multiple times, whereas this cannot happen in the refractor.  So actually in very many respects physics shows that the refractor design has multiple advantages over same aperture SCTs or MAKs.

 

 

What I always laugh about, because it is ridiculous, is the "my refractor has pinpoint stars while SCTs give bloated star images".  I hear it all of the time, but of course it's not true.  ...  I think many times the SCTs with "bloated" images are just not in equilibrium with the environment.  I have seen this happen with APO triplets that are airspaced.  It takes awhile for them to cool down as well.

 

This is very true.  The size of the spurious disk is related to aperture.  However, when the magnification is not sufficient to show the spurious disk or the airy disk, then there are other perception and human eye phenomena that take place which will make the star point in the larger aperture mirrored optics appear less tight/controlled then that from a smaller aperture refractor.  OTA equilibrium of the compound optical design of an SCT and MAK is such that they can take inordinately long to acclimate, and they may never acclimate over an evening of observing.  This is common experience.  The refractor OTA's design is such that it sheds heat more quickly, adapts to changes more quickly, and is less prone to body heat interference.  So all of these differences contribute to why and how star points often appear bloated in an SCT or MAK whereas they are nice and tight and stable in the Apo.  On the coldest evenings where there is more than a 50 degree delta for my OTA to cool down, my 6" doublet Apo has taken up to two hours at most, and it always acclimates passively every evening used.  Comparatively, the 4" through 9.25" catadioptric scopes I have had almost never have fully acclimated.  In all my years of using these designs here in the North East there have been only two occasions where my 8" SCTs have fully acclimated and showed a non-bloated non-"wooly" star point, all other evenings they never fully acclimated passively.  Coincidentally, on those two occasions I also had my 6" Apo out observing planetary and the SCTs did every bit as well, but not any better as far as resolution was concerned, indicating that the seeing was leveling the two scopes. 


Edited by BillP, 04 January 2018 - 07:27 PM.

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#120 Mitrovarr

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Posted 03 January 2018 - 10:44 PM

In response to the people talking about observers only looking at the Messiers and nothing else: I started to get heavily into goto when I realized I was looking at the same objects every night, because they were the ones I had the positions for memorized and tracking down others took long enough that I didn't like to do it.


Edited by Mitrovarr, 03 January 2018 - 10:46 PM.


#121 treadmarks

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Posted 04 January 2018 - 01:35 AM

This is very true.  The size of the spurious disk is related to aperture.  However, when the magnification is not sufficient to show the spurious disk or the airy disk, then there are other perception and human eye phenomena that take place which will make the star point in the larger aperture mirrored optics appear less tight/controlled then that from a smaller aperture refractor.  OTA equilibrium of the compound optical design of an SCT and MAK is such that they can take inordinately long to acclimate, and they may never acclimate over an evening of observing.  This is common experience.  The refractor OTA's design is such that it sheds heat more quickly, adapts to changes more quickly, and is less prone to body heat interference.  So all of these differences contribute to why and how star points often appear bloated in an SCT or MAK whereas they are nice and tight and stable in the Apo.  On the coldest evenings where there is more than a 50 degree delta for my OTA to cool down, my 6" doublet Apo has taken up to two hours at most, and it always acclimates passively every evening used.  Comparatively, the 4" through 9.25" catadioptric scopes I have had almost never have fully acclimated.  In all my years of using these designs here in the North East there have been only two occasions where my 8" SCTs have fully acclimated and showed a non-bloated non-"wooly" star point, all other evenings they never fully acclimated passively.  Coincidentally, on those two occasions I also had my 6" Apo out observing planetary and the SCTs did every bit as well, but not any better as far as resolution was concerned, indicating that the seeing was leveling the two scopes.

I've never bought into this. They're both closed tube designs with very similar materials in use. It seems to me that the primary variables would then be weight (thermal mass), volume, surface area etc. Of those, I'd say Cassegrains get dinged the most by low surface area compared to the long, skinny tubes of refractors. With the same aperture, the surface area per volume should be the same and I would expect them to cool at the same rate. But of course, we are usually comparing a much larger CAT to a smaller refractor.

 

An explanation I give more credence to is that it's seeing-related. The idea is that larger apertures are more vulnerable to poor seeing, and Cassegrains also operate at inherently higher magnifications than refractors due to their very different focal lengths. This high magnification also magnifies the effects of poor seeing and thermal air currents. This is consistent with my experience - whenever I have noted exceptionally clean, crisp images of stars or anything else, it was at an exceptionally low magnification - usually, with my 40mm eyepiece.


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#122 Gofr

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Posted 04 January 2018 - 03:12 AM

I agree with the last paragraph in the above post. As I mentioned in my earlier post, my experience with my C9 has been sharp pin point views at lower and medium powers with the mush only coming in at higher powers (and even then, still not unusable in any way).
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#123 bobhen

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Posted 04 January 2018 - 07:57 AM

 

This is very true.  The size of the spurious disk is related to aperture.  However, when the magnification is not sufficient to show the spurious disk or the airy disk, then there are other perception and human eye phenomena that take place which will make the star point in the larger aperture mirrored optics appear less tight/controlled then that from a smaller aperture refractor.  OTA equilibrium of the compound optical design of an SCT and MAK is such that they can take inordinately long to acclimate, and they may never acclimate over an evening of observing.  This is common experience.  The refractor OTA's design is such that it sheds heat more quickly, adapts to changes more quickly, and is less prone to body heat interference.  So all of these differences contribute to why and how star points often appear bloated in an SCT or MAK whereas they are nice and tight and stable in the Apo.  On the coldest evenings where there is more than a 50 degree delta for my OTA to cool down, my 6" doublet Apo has taken up to two hours at most, and it always acclimates passively every evening used.  Comparatively, the 4" through 9.25" catadioptric scopes I have had almost never have fully acclimated.  In all my years of using these designs here in the North East there have been only two occasions where my 8" SCTs have fully acclimated and showed a non-bloated non-"wooly" star point, all other evenings they never fully acclimated passively.  Coincidentally, on those two occasions I also had my 6" Apo out observing planetary and the SCTs did every bit as well, but not any better as far as resolution was concerned, indicating that the seeing was leveling the two scopes.

I've never bought into this. They're both closed tube designs with very similar materials in use. It seems to me that the primary variables would then be weight (thermal mass), volume, surface area etc. Of those, I'd say Cassegrains get dinged the most by low surface area compared to the long, skinny tubes of refractors. With the same aperture, the surface area per volume should be the same and I would expect them to cool at the same rate. But of course, we are usually comparing a much larger CAT to a smaller refractor.

 

An explanation I give more credence to is that it's seeing-related. The idea is that larger apertures are more vulnerable to poor seeing, and Cassegrains also operate at inherently higher magnifications than refractors due to their very different focal lengths. This high magnification also magnifies the effects of poor seeing and thermal air currents. This is consistent with my experience - whenever I have noted exceptionally clean, crisp images of stars or anything else, it was at an exceptionally low magnification - usually, with my 40mm eyepiece.

 

They are both closed-tube designs but the SCT primary is enclosed in a tin can and not exposed to the outside air whereas the refractor objective is exposed to the outside air and that facilitates more rapid initial cooling AND thermal stability at night during falling temperatures.

 

Heat plumes can easily be seen in a defocused star image in a SCT hours after the outside tube has acclimated. People that use Lymax coolers have to reinsert them to help eliminate heat plumes if temperatures keep falling because the “enclosed” primary cannot keep up.

 

Bob



#124 Jason B

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Posted 04 January 2018 - 08:34 AM

I pick my scope based on what objects I am focusing on.  I am lucky that I have a large open air observatory at my disposal so the scopes are at or very near optimal temp at a moments notice and between my equipment "obsession", my friend's similar "obsession" or the observatory's equipment, I pretty much have everything but a big mak or big apo at my disposal.

 

We have 4 scopes that always see some use.  A 8" F8 Newt (Dob mount), a 111mm APO (actually, a lot of 4" refractors), a 16" SCT and a 17.5" F4 Newt (dob).  The 8" has some of the best optics I have ever had the pleasure to use, a true one of a kind scope.  Planets are fantastic through it.  Actually, pretty much everything is.  But, when I want to look at galaxy clusters or similar, the 17.5" is the scope.  When I want to study the details in planetary nebula or small single galaxies, I like the 16" SCT.  On exceptional nights, the 16 is great on the planets too but those nights are far and few between here in the Great Lakes State (only 1 really comes to mind in the last 10 years...).  The 111 and it's refractor friends get used every night on larger extended objects and/or showpieces and frankly, I just like that scope.  Plus, while the 8" is awesome, it's still on a manual mount so for public observing, the ability to track with small 111 at high power on Saturn or Jupiter is a huge plus.

 

As they say, your mileage may, actually will, vary.  Just get outside and enjoy yourself with ever you have or want to use.


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#125 Jon Isaacs

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Posted 04 January 2018 - 10:08 AM

The physics of optics state that a compound mirrored design like an SCT or MAK will always have more scatter than a purely refractive design of same aperture.  The physics also states that the SCT and MAK will also have less light throughput than the unobstructed all refractive design of an Apo of same aperture.  Physics also states that the PSF of the non-obstructed design will always be better than that from an SCT or MAK of same aperture.  The engineered thermal behavior of the refractor OTA is also superior to that of a same aperture SCT or MAK.  Physics shows that light rays in a compound design like a SCT or MAK can easily intercept a given thermal in the OTA multiple times, whereas this cannot happen in the refractor.  So actually in very many respects physics shows that the refractor design has multiple advantages over same aperture SCTs or MAKs.

 

 

The topic was about comparing 4 inch refractors to 8 inch SCTs.. All those carefully placed, "same  aperture," that's not what this is about. 

 

Physics tells us that an 8 inch scope has multiple advantages over a 4 inch scope.. 

 

But that has been adequately discussed. 

 

Regarding the bias that may result because someone may feel the need to justify and expensive item..  There is also the bias that results from not buying that expensive item.. 

 

Myself,  I spent most of my adult life in a research laboratory. In a laboratory one spends a lot of time tinkering and optimizing every aspect of a measurement.  It really doesn't matter if one is dealing with simple , basic equipment or equipment that is truly state of the art, the goal is to make the best measurement possible with what one has. A good experimentalist understands the equipment,  it's limitations and capabilities and works to maximize the potential. 

 

In amateur astronomy,  this is my same attitude,  same approach.  Getting the best possible views out of my seemingly lowly 10 inch Dob is much more interesting and much more challenging than starting out with some optically perfect 4 or 5 inch refractor that's inherently less capable.  

 

I am not suggesting that my way is the only way,  rather I'm suggesting this is my own personal bias.  Getting the most out of what I've got.  Part of that is that it's nice to tinker and mess with my own equipment rather than deal with some tricky experiment that might take a week just to make a single measurement,  a measurement that only takes a few microseconds. 

 

Jon


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