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Is the great f/ratio debate dated?

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#1 bigbangbaby

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Posted 29 March 2022 - 09:27 AM

As someone probably less experienced than many here, I'd like to better understand why you'd ever choose a slow refractor over a faster one. Common wisdom suggests a slow "planet killer" scope for planetary, double stars and lunar, and a faster one for wide field views of DSOs the Milky Way and, of course, for AP. Before the advent of fast and fairly affordable apochromats, most achromats were comparatively slow (typically f/8 or higher). Consider, for example, a fast and slow (say f/5 and f/10) apochromat of the same aperture. Further assume both have similar quality optics. Using the rule-of-thumb maximum 50X/inch of aperture, both scopes should show the same level of detail at high magnification. Obviously, the f/5 scope will need an eyepiece with double the power or a 2X Barlow to achieve the same magnification as the f/10. The question is, does the fast apochromat give up anything to the slower for high magnification views? Does the fact that a slow achromat shows less false color than a faster one apply to apochromats as well? In other words, in general, is a slow apochromat better corrected than a faster one?  Does field curvature, which I understand would be greater in faster scopes, come into play when viewing planets at higher magnifications? I guess the bigger question is, can a fast apochromat serve equally well both as a wide field and planetary/double star/lunar instrument?

 

Larry


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#2 TexasToast

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Posted 29 March 2022 - 09:58 AM

Nice, slow, focal length. Capturing the distant cosmos takes time but seeing it large is worth the trade off.  


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#3 RichA

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Posted 29 March 2022 - 10:02 AM

As someone probably less experienced than many here, I'd like to better understand why you'd ever choose a slow refractor over a faster one. Common wisdom suggests a slow "planet killer" scope for planetary, double stars and lunar, and a faster one for wide field views of DSOs the Milky Way and, of course, for AP. Before the advent of fast and fairly affordable apochromats, most achromats were comparatively slow (typically f/8 or higher). Consider, for example, a fast and slow (say f/5 and f/10) apochromat of the same aperture. Further assume both have similar quality optics. Using the rule-of-thumb maximum 50X/inch of aperture, both scopes should show the same level of detail at high magnification. Obviously, the f/5 scope will need an eyepiece with double the power or a 2X Barlow to achieve the same magnification as the f/10. The question is, does the fast apochromat give up anything to the slower for high magnification views? Does the fact that a slow achromat shows less false color than a faster one apply to apochromats as well? In other words, in general, is a slow apochromat better corrected than a faster one?  Does field curvature, which I understand would be greater in faster scopes, come into play when viewing planets at higher magnifications? I guess the bigger question is, can a fast apochromat serve equally well both as a wide field and planetary/double star/lunar instrument?

 

Larry

All else being equal.  True apochromatism is as much a function of the glass as the focal length


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#4 Astrojensen

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Posted 29 March 2022 - 10:04 AM

 

The question is, does the fast apochromat give up anything to the slower for high magnification views?

In my experience, yes. It's much more finicky to focus; is much more sensitive to cooldown; puts much more stress on diagonals, binoviewers and eyepieces.  

 

 

Does the fact that a slow achromat shows less false color than a faster one apply to apochromats as well?

Yes. This means a longer focal ratio apochromat can use a considerably simpler and cheaper design, and still achieve superb color correction.

 

 

in general, is a slow apochromat better corrected than a faster one?

Yes, all else equal.

 

 

Does field curvature, which I understand would be greater in faster scopes, come into play when viewing planets at higher magnifications?

No, usually not. 

 

 

can a fast apochromat serve equally well both as a wide field and planetary/double star/lunar instrument?

The general consensus today is that they can, but most observers today have never observed with a truly long-focal ratio apochromat, so they have no basis to actually compare on and can only use extrapolation (IE "my fast scope can show this and that, which good for its aperture, so it must be a good scope"). 

 

That all said, it's generally hard to go wrong with a modern f/7 - 8 ED or apo. They are excellent scopes for general use, including very good planetary views. 

 

 

Clear skies!

Thomas, Denmark 


Edited by Astrojensen, 29 March 2022 - 10:25 AM.

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#5 weis14

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Posted 29 March 2022 - 10:04 AM

My experience is that most fast APO refractors are so well corrected for visual use that they work equally well for all magnifications. Field curvature can be an issue if the scope is sufficiently fast, but to me it doesn't matter much since when I am using high power to view the planets or double stars I won't be looking at the edge of the field much anyways.

 

Longer focal length can have some advantages.  Most importantly, I find that it makes finding the best focus easier.  With very fast scopes, I often find myself chasing the best focus.  I often think about finding a nice 100mm f/8 or so doublet, such as a classic FC-100 just to try it out.  However, as a practical matter, I doubt there would be much difference between such a scope and my 92mm Stowaway, so I don't tend to look too hard.


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#6 Ephemeral

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Posted 29 March 2022 - 10:25 AM

As someone probably less experienced than many here, I'd like to better understand why you'd ever choose a slow refractor over a faster one. Common wisdom suggests a slow "planet killer" scope for planetary, double stars and lunar, and a faster one for wide field views of DSOs the Milky Way and, of course, for AP. Before the advent of fast and fairly affordable apochromats, most achromats were comparatively slow (typically f/8 or higher). Consider, for example, a fast and slow (say f/5 and f/10) apochromat of the same aperture. Further assume both have similar quality optics. Using the rule-of-thumb maximum 50X/inch of aperture, both scopes should show the same level of detail at high magnification. Obviously, the f/5 scope will need an eyepiece with double the power or a 2X Barlow to achieve the same magnification as the f/10. The question is, does the fast apochromat give up anything to the slower for high magnification views? Does the fact that a slow achromat shows less false color than a faster one apply to apochromats as well? In other words, in general, is a slow apochromat better corrected than a faster one?  Does field curvature, which I understand would be greater in faster scopes, come into play when viewing planets at higher magnifications? I guess the bigger question is, can a fast apochromat serve equally well both as a wide field and planetary/double star/lunar instrument?

 

Larry

A slower apo will show less CA than a fast apo. Many faster apos use triplet designs to suppress CA. 

 

One of the main factors that often make slower scopes preferable for high power is the comparative ease of fabrication (gentler curves on lenses) and lesser demand on precise alignment. This means slower instruments have a better chance of performing up to design specifications as well as being cheaper usually. Of course, this doesn't mean a high quality fast apo can't perform well. 

 

While a fast apo can be pushed to high magnifications, it will require the use of shorter FL eyepieces. There are fewer options and many of the strongest performers offer little eye relief, are out of production or are quite expensive. 


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#7 Stellar1

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Posted 29 March 2022 - 11:15 AM

Is it not a fact that fast scopes of any design are hard on eyepieces? forcing one to splurge on the best eyepieces to avoid optical anomalies of all sorts. 


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#8 russell23

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Posted 29 March 2022 - 11:30 AM

For deep sky the field curvature of the faster scopes can be a limitation with certain eyepieces.  You have more potential TFOV with the faster scope but it is more challenging to get a sharp max TFOV with the faster scope.

 

After using 120mm f/7.5 and 102mm f/7 ED doublet APOs, I've settled on the 102mm f/11 ED doublet as my favorite scope.  I just like it more even thought I give up some FOV.   I love the flexibility it provides to use pretty much any eyepiece I am interested in without concern about 50% of the field having poor sharpness.  

 

The f/6-f/8 scopes are excellent and more suited to many, maybe even most observers.  But as Thomas said - if you haven't had a chance to observe with a long f/ratio APO it might be hard to understand why someone would make that choice over the faster scopes.


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#9 Astrojensen

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Posted 29 March 2022 - 11:33 AM

Is it not a fact that fast scopes of any design are hard on eyepieces? forcing one to splurge on the best eyepieces to avoid optical anomalies of all sorts. 

This is indeed the case. At the other end of the spectrum, things get extremely relaxed. In my 85mm f/19 Zeiss apo, even the bottom of a coke bottle is sharp to the edge...

 

 

Clear skies!

Thomas, Denmark


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#10 Wildetelescope

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Posted 29 March 2022 - 12:08 PM

In my experience, yes. It's much more finicky to focus; is much more sensitive to cooldown; puts much more stress on diagonals, binoviewers and eyepieces.  

 

Yes. This means a longer focal ratio apochromat can use a considerably simpler and cheaper design, and still achieve superb color correction.

 

Yes, all else equal.

 

No, usually not. 

 

The general consensus today is that they can, but most observers today have never observed with a truly long-focal ratio apochromat, so they have no basis to actually compare on and can only use extrapolation (IE "my fast scope can show this and that, which good for its aperture, so it must be a good scope"). 

 

That all said, it's generally hard to go wrong with a modern f/7 - 8 ED or apo. They are excellent scopes for general use, including very good planetary views. 

 

 

Clear skies!

Thomas, Denmark 

For me, Thomas' first point about focus is a key advantage for longer focal lengths and solar system objects.   Having a greater Depth of Field helps with dialing in details.  That said, I have had some pretty nice views of Jupiter through an NP 101:-)  I think it is probably fair to say that, assuming similar quality level, the differences between faster and slower apo scopes are relatively subtle, compared to say the difference between an apo and an achromat of similar focal length.   For my part, I do most of my visual work with f8 or higher scopes, refractors or otherwise.   I just like that range and I have invested in eyepieces that complement that range. 

 

My biggest complaint about fast scopes for solar system is that I have to use very low focal length eyepieces to get magnification.    That is OK if I use my Delos EP's but can be challenge with my Brandon's since I wear glasses:-).  Everything is a trade off.    

 

Cheers!

 

JMD


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#11 Jon Isaacs

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Posted 29 March 2022 - 12:15 PM

My two cents:

 

A well made, fast apochromat can do an excellent job at high magnifications and still provide low power, wide field views that are as perfect as perfect can be.

 

When I bought my NP-101 twelve years ago, I really didn't know what I was buying, I thought it was one of a group of higher end 4 inch apos. I didn't realize it was a longer focal length ED doublet objective combined with a Petzval type ED reducer/flattener that gave it both essentially perfect color correction as well as those perfect wide, flat fields approaching 5°.

 

There's no doubt it's demanding when it comes to eyepieces, that adds to the cost for some but for someone like myself who already owned Newtonians as fast as F/4, I was already set. 

 

Just how good the optics are at the highest magnifications when compared to the theoretical limit, I can't say. For someone only interested in using a 4 inch at high magnifications, this isn't the scope, it's very expensive and as has.been said, it's demanding when it comes to eyepieces.

 

But in the context of a 4 inch refractor as a versatile, do it all scope, it's hard to match.

 

3925365-TV NP-101 at Jewel Valley.jpg
 
Jon

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#12 Stellar1

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Posted 29 March 2022 - 12:17 PM

This is indeed the case. At the other end of the spectrum, things get extremely relaxed. In my 85mm f/19 Zeiss apo, even the bottom of a coke bottle is sharp to the edge...

 

 

Clear skies!

Thomas, Denmark

lol.gif lol.gif lol.gif



#13 db2005

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Posted 29 March 2022 - 12:48 PM

The Tak FC-76 DCU with extender operates as a quad-lens scope at f/12.6. Its color color correction is insanely good, noticeably better than the barebone FC-76 DCU fluorite which is already excellent. The combo's out-of-this-world performance is a testiment to what can be accomplished when a slow focal ratio is combined with fluorite and figured and polished to world-class standards.


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#14 Wildetelescope

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Posted 29 March 2022 - 12:49 PM

 

My two cents:

 

A well made, fast apochromat can do an excellent job at high magnifications and still provide low power, wide field views that are as perfect as perfect can be.

 

When I bought my NP-101 twelve years ago, I really didn't know what I was buying, I thought it was one of a group of higher end 4 inch apos. I didn't realize it was a longer focal length ED doublet objective combined with a Petzval type ED reducer/flattener that gave it both essentially perfect color correction as well as those perfect wide, flat fields approaching 5°.

 

There's no doubt it's demanding when it comes to eyepieces, that adds to the cost for some but for someone like myself who already owned Newtonians as fast as F/4, I was already set. 

 

Just how good the optics are at the highest magnifications when compared to the theoretical limit, I can't say. For someone only interested in using a 4 inch at high magnifications, this isn't the scope, it's very expensive and as has.been said, it's demanding when it comes to eyepieces.

 

But in the context of a 4 inch refractor as a versatile, do it all scope, it's hard to match.

 

 
 
Jon

 

That is why the 101 is on my list of possible future acquisitions, should I ever decide my 102 is getting lonely:-)   That is unfortunately a ways off, and I have too many scopes now to play with in any event.   As I get older I can see paring things down to a 101 and the 102 at some point.   Covers both focal ranges for me, and imaging if I so choose.   Add my C8 for higher mag views when I want them.   Nice and portable. I love my big, f8 and F9 refractors and hope to be using them for some time to come, but I can definitely see a time when they become more than I want to deal with.  That is perhaps another advantage for the faster refractors...

 

JMD



#15 EJN

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Posted 29 March 2022 - 01:02 PM

Is it not a fact that fast scopes of any design are hard on eyepieces? forcing one to splurge on the best eyepieces to avoid optical anomalies of all sorts.


Yes because you are forcing light to do tricks that it doesn’t really want to do. :yay:

I have a 3” f16 achromat, and any eyepiece, even a Huygens, works great in it.
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#16 Wildetelescope

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Posted 29 March 2022 - 01:47 PM

This is indeed the case. At the other end of the spectrum, things get extremely relaxed. In my 85mm f/19 Zeiss apo, even the bottom of a coke bottle is sharp to the edge...

 

 

Clear skies!

Thomas, Denmark

Now you have done it!   There is going to be a run on long focal length refractors and recycled glass soda bottles.

JMD


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#17 NC Startrekker

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Posted 29 March 2022 - 02:32 PM

Is it not a fact that fast scopes of any design are hard on eyepieces? forcing one to splurge on the best eyepieces to avoid optical anomalies of all sorts. 

Yes, BUT... the optical anomalies of eyepieces are inherent to the eyepiece based on its design parameters and the quality of the manufacturing process to accurately reproduce those design specifications.  So, the anomalies of an eyepiece will be present no matter what scope it is used in.  A couple of things make those anomalies more observable or apparent.  One is, as you noted, faster f/ ratios which produce steeper light cones that require more specialized and well-corrected designs to handle well.  Another is true flat field telescopes employing designs like the Petzal lens that uses a series of lenses downstream in the optical path from a doublet objective to null out the "anomalies" inherent in the doublet.


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#18 NC Startrekker

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Posted 29 March 2022 - 02:34 PM

This is indeed the case. At the other end of the spectrum, things get extremely relaxed. In my 85mm f/19 Zeiss apo, even the bottom of a coke bottle is sharp to the edge...

 

 

Clear skies!

Thomas, Denmark

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#19 barbie

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Posted 29 March 2022 - 04:16 PM

I prefer my long, slow achromats for lunar, planetary and double stars for all of the previously mentioned reasons including better depth of focus and the fact that simple, less expensive eyepieces can be used. I've been down the expensive and complex eyepiece road already and didn't care for them. My bank account and wallet now thank me profusely!!!


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#20 Sketcher

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Posted 29 March 2022 - 04:30 PM

 

A well made, fast apochromat can do an excellent job at high magnifications and still provide low power, wide field views that are as perfect as perfect can be.

 

I agree with Jon on this.  All apochromats are not created equal, so really it wouldn't be right to generalize and say that a smaller f-ratio will always provide worse high magnification views -- or that a larger f-ratio will always provide superior high magnification views -- at least as far as the discerning human eye can differentiate.

 

There exist some relatively small f-ratio apochromats that are true masters of the light at any reasonable magnification; but also, there exist some apochromats, irrespective of f-ratio, that are not so perfect.

 

Simply put, the "apochromat" classification has different meanings for different manufacturers.  Performance can differ with different specimens of the same model depending on the manufacturer, their standards, and their quality control.  All apochromats of any given aperture and f-ratio are not going to have equal performance levels.  Some small f-ratio apochromats can indeed be essentially "perfect" -- at any reasonable magnification.


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#21 Space_Race_T.J.

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Posted 29 March 2022 - 06:17 PM

Yes, BUT... the optical anomalies of eyepieces are inherent to the eyepiece based on its design parameters and the quality of the manufacturing process to accurately reproduce those design specifications.  So, the anomalies of an eyepiece will be present no matter what scope it is used in.  A couple of things make those anomalies more observable or apparent.  One is, as you noted, faster f/ ratios which produce steeper light cones that require more specialized and well-corrected designs to handle well.  Another is true flat field telescopes employing designs like the Petzal lens that uses a series of lenses downstream in the optical path from a doublet objective to null out the "anomalies" inherent in the doublet.

Are people getting hung up on "the optical anomalies of eyepieces" which I think is being overblown here when talking about fast f-ratio scopes?

 

The real problem is the short focal lengths of these designs and the field curvature that results from them. Whether you use the most expensive Televue eyepiece or something inexpensive, neither one is ever going to cancel out the field curvature.

 

Just like when doing astrophotography a separate field flattener is required to flatten the field. This can be achieved for visual use as well, or by buying a scope with a Petzval design.

 

T.J.



#22 Jon Isaacs

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Posted 29 March 2022 - 08:11 PM

Are people getting hung up on "the optical anomalies of eyepieces" which I think is being overblown here when talking about fast f-ratio scopes?

 

The real problem is the short focal lengths of these designs and the field curvature that results from them. Whether you use the most expensive Televue eyepiece or something inexpensive, neither one is ever going to cancel out the field curvature.

 

Just like when doing astrophotography a separate field flattener is required to flatten the field. This can be achieved for visual use as well, or by buying a scope with a Petzval design.

 

T.J.

 

The curvature of the field depends on the radius of curvature of the objective, about one third the focal length, and the field stop diameter. As magnification is increased, field curvature quickly disappears. It's only at lower powers where it's visible.

 

Consider an 80mm F/6. With a 40 mm SWA, the difference between center focus and edge focus is about 1.5 mm. The curvature is very apparent.  With a 7 mm UWA, 70x, the difference between center and edge focus is about 0.08mm, the depth of focus at F/6 is 0.08 mm, the curvature is not visible.

 

With a fast scope, off-axis eyepiece astigmatism is an issue at all magnifications. Field flatteners and Petzval designs do not help with that. With a scope like the NP-101, F/5.4, it's corrected for field curvature but unless you use eyepieces like the Naglers etc, the eyepiece's off-axis aberrations will be quite visible.

 

Jon


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#23 xiando

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Posted 29 March 2022 - 08:23 PM

For someone like me, with small scopes (6", 6", and 5.something"), and only a 1/3 sensor,  longer f number allows me to achieve a slightly larger image of the target - ex: the ring nebula,  m82, bubble, dumbell, globs like M92, M13, etc (I have an F4 6" and an f7 6" and an old celestron 130 SLT). I generally don't futz with the F7 only because it requires longer exposures (the drawback) and transferring my gear means recalibration of both, the F7 to image, and the F4 once I'm done to bring my equipment back to normal operation. call me lazy. 


Edited by xiando, 29 March 2022 - 08:26 PM.


#24 Lagrange

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Posted 29 March 2022 - 08:36 PM

A slower apo will show less CA than a fast apo. Many faster apos use triplet designs to suppress CA. 

 

One of the main factors that often make slower scopes preferable for high power is the comparative ease of fabrication (gentler curves on lenses) and lesser demand on precise alignment. This means slower instruments have a better chance of performing up to design specifications as well as being cheaper usually. Of course, this doesn't mean a high quality fast apo can't perform well. 

 

While a fast apo can be pushed to high magnifications, it will require the use of shorter FL eyepieces. There are fewer options and many of the strongest performers offer little eye relief, are out of production or are quite expensive. 

I seem to recall Roland Christen saying that producing lenses with steep curves isn't that much harder than making shallow ones but a slower design still has certain advantages. Gentler curves should enable the use of thinner elements and less glass required which could mean a cheaper and lighter weight instrument, possibly with better balance, and those thin elements should cool down faster too.

 

A major issue with faster glass is that those steeply curved surfaces mean strong refractions and the introduction of large aberrations that need to be corrected elsewhere in the design. This is one of the reasons why very high speed optics have so many lens elements - splitting the refractive power across multiple weaker elements helps to minimise those aberrations, and can also reduce the total amount and cost of optical materials needed for the design.


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#25 barbie

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Posted 29 March 2022 - 08:55 PM

...Which is why I often say simpler is better, at least for my use!! My years of complex telescope & eyepiece designs are behind me since I no longer do A.P. or hunt ultra faint fuzzies anymore.


Edited by barbie, 29 March 2022 - 09:15 PM.

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