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"But Maks have a narrow FOV" - WTH???

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#1 hottr6

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Posted 05 February 2013 - 01:18 PM

I am always reading the most abused and over-used byline in astronomy.... Maks have a narrow FOV. I'll suggest that such statements are made be people who came down in the last rainstorm.

Anyone ever heard slow 'fracs have a narrow field of view repeated multiple times in the same thread? No? Try saying that in the Refractor forum and be prepared to be stoned with chipped flint glass.

Anyone whoever grew up with 'scopes before the optics revolution in the late 80s grew up with long 'fracs and Kellners (if they were lucky). Magnifications greater than 70x would clip the Moon.

Today, any noob can plop a 70 degree AFOV eyepiece in their long-focus 'scope and enjoy WIIIIDE views.

I'm plenty happy with today's WIIIIDE views offered by long-focus Maks and 'fracs.
:gramps: :rant:

#2 Binojunky

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Posted 05 February 2013 - 01:26 PM

Having a bad day?DA.

#3 KerryR

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Posted 05 February 2013 - 01:42 PM

Doesn't the long, narrow central baffle in most Maks impose a limit to the TFOV attainable, a limit that's not present in other designs to the same extent?

I think most peoples' perception is not that Maks have an implicitly narrow FOV (even though the size of the visual back does make this implicit to a certain degree), but rather that it's difficult to get wide TFOV's that are easier to get with other designs, which may make them limiting for certain aplications. Same would apply, it'd seem, to long focus refractors, though to a lesser degree because 2" and low power ep's can be used more effectively-- potentially much larger full field illumination and no central obstruction in the exit pupil. Perhaps this point gets over-simplified by the "Maks have a narrow FOV".

I could have all that wrong, but I don't think so. Hopefully, someone with a better grasp of the numbers than me (who has none) will post some information that should clarify (or correct) things further.

#4 watcher

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Posted 05 February 2013 - 02:40 PM

Ya mean to tell me that my IM715 isn't a Milky way scanner, and I'm not going to fit Andromeda in the eyepiece!? Well, I can live with that. That's why they make 6" F/5 refractors! :)

#5 spencerj

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Posted 05 February 2013 - 03:51 PM

I see where the OP is coming from. Any post with the term Maksutov Cassegrain generates a barrage of posts with the term narrow field of view. The focal length of the scope never seems to come up.

For a surprising number of posters, an MCT with a focal length of 1800mm has a narrow FOV, but an SCT with a 2000mm focal length is a versatile telescope that provides great DSO views. I am not saying it doesn't, just questioning why (at times) the MCT is considered almost useless for this purpose.

#6 Eddgie

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Posted 05 February 2013 - 04:47 PM

When I say that MCTs have narrow fields, it is relative to other telescopes of the same aperture.

A 5" SCT will generally have a wider true field capability than a 5" MCT.

An 11" SCT can have about the same true field as a 7" MCT.

Relative to other designs, MCTs often do have much narrower true fields.

When made at f/10, they loose the advantage of having a smaller central obstruction than an f/10 SCT. They get the same true field capability, but loose the contrast advantage. And of course they are very expensive.

The truth may hurt, but it is the truth. Compared to similar aperture reflectors, MCTs generally do have a much narrower true field capability.

But this only matters if one needs a bigger field. The contrast advantage tha an f/15 MCT offers over an SCT of the same aperture makes them better (for inch of aperture) planetary scopes.

All scope have compromises. The compromise of the MCT is that unless you take special steps (which makes them very expensive to make) you wind up with a longer focal lenght scope than you get with SCTs or with Reflectors.

Or, you can take those steps and get it f/10, but then you limit the contrast potential to the same as the f/10 SCT.

Pick your poison. Higher contrast but at the expense of being limited

#7 Eddgie

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Posted 05 February 2013 - 05:03 PM

Regarding my last post, let me do the math on it for anyone that is interested.

The 180mm MCT has a focal lenght of 2700mm.

If you put a 2" diagonal on it to be able to use an eyepiece with a bigger field stop, the focal lenght goes up about 170mm, so 2870mm.

The baffle in the MCT is about 41mm. Assuming you want a well illuminated field, you would match this with an eyepecie with a 41mm field stop to avoid vignetting.

The C11 has a focal lenght of 2800mm, but with a 2" diagonal, the focal lenght goes up to 2970mm. The C11 though can utilize an eyepeice with a 46mm field stop and have better off axis illumination than the 180mm MCT with a 2" diagonal.

So while a 7" MCT has a slighly shorter focal lenght than an 11" MCT, the smaller baffle means that (if you want to really compare apples to apples) you have to use an eyepeice with a smaller field stop (again, assuming you wanted to be fair and limit the field stop to the size of the baffle).

The SCTs ability to use a 46mm field stop eyepeices balances out the slightly longer focal length.

So, a C11 can get about the same size true field as a 7" MCT.

That is why people say that MCTs have narrow fields. When compared to SCTs and reflectors of the same aperture, the fields are indeed usually narrower. Again the Intes MCTs can be f/10, but now you have a secondary obstruction that is about the same size, so loose the contrast advantage of a small obstruction.

#8 Mark Costello

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Posted 05 February 2013 - 05:16 PM

When I "window shop" for a "complementary" telescope to my 5"F6.5 achromat refractor, I keep a 7"F15 MCT on the list. Maybe it would be on an dual alt-az mount with the achro riding shotgun, finding things for the MCT and dishing out the "big picture" view before the MCT gets used for the details. The big compromise for me would be the cap on aperture. Other scopes under consideration include a 8", 9.25" or 10" SCT, or a 8-10" Dob. As you say, there're compromises all around, making this a tough pick. Oh well, between my tight budget and the good performance of my refractor, I will be taking my time on this. :)


Best Regards,

#9 Lee Jay

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Posted 05 February 2013 - 05:36 PM

I have my 5" Mak, with it's narrow field of view of around 1 degree. It's such an easy thing to fix that I don't consider it a problem. When I want a wide field, I just use my little refractor with its 5 degree field of view. What's the problem?

#10 Goodchild

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Posted 05 February 2013 - 06:01 PM

Different kinds of scopes have their individual specialty. Mak's are probably best used for planetary viewing. I like mine. BTW, mine is an Ioptron with an f/12 focal ratio.

#11 Jon Isaacs

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Posted 05 February 2013 - 06:34 PM

When I say that MCTs have narrow fields, it is relative to other telescopes of the same aperture.



:waytogo:

I say something like: "a relatively narrow maximum possible true field of view." I will make a similar comment about a long focal length refractor.

This most often comes up with smaller Maks, long focal lengths, 1.25 inch focusers. It is a valid concern. The most glaring example is the ETX-125 with it's 1900mm focal length and a 1.25 inch focuser... The maximum possible field of view is right about 0.83 degrees..., the same TFOV that is possible with a 25 inch F/5 Newtonian.

Jon

#12 iluxo

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Posted 05 February 2013 - 06:36 PM

Mark, I have a dual-scope setup with a 7" f/15 Mak exactly as you describe. While I have a choice for the second scope - either a 4" f/7 APO, or a 7" f/10 Mak - these days I tend to leave both of them behind and its only the 7" f/15 that goes to the mountain with me.

The reason is that as alluded to above, my 7" f/15 has a 2" back and a useful field of view about 40+ minutes of arc, enough to comfortably fit the full moon in my LV50 eyepiece at 54X, and frankly I've never found it desirable to go for a lower magnification than this. I can find things in the sky to this accuracy, so I really don't need the refractor as a super-finder, and the other Mak doesn't offer a useful advantage.

Where the dual-scope setup really DOES come into its own is on public star parties as I can handle two queues with the eyepieces at different heights, one for smaller kids and the other to suit taller people.

#13 azure1961p

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Posted 05 February 2013 - 08:59 PM

I totally agree and add the other nonsense line: "maks aren't suitable for deepsky" as if everything up there is the size of M31, the veil and the seven Sisters.

It is peculiar how it attracts these things. I like Jon's phrasing.

Pete

#14 cheapersleeper

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Posted 06 February 2013 - 11:03 AM

Did I miss "International Protest a Fact Day?"

If someone said, "You know, your 12" f5 homemade dob works pretty well, but doesn't it break your back carrying it around," I would tell them, "yeah, it does."

Regards,
B

#15 KerryR

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Posted 06 February 2013 - 11:18 AM

I totally agree and add the other nonsense line: "maks aren't suitable for deepsky" as if everything up there is the size of M31, the veil and the seven Sisters.

It is peculiar how it attracts these things. I like Jon's phrasing.

Pete


Who ever said that? Double stars ARE deep sky... :poke:

(joke)

#16 hottr6

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Posted 06 February 2013 - 11:35 AM

I see where the OP is coming from. Any post with the term Maksutov Cassegrain generates a barrage of posts with the term narrow field of view. The focal length of the scope never seems to come up.

For a surprising number of posters, an MCT with a focal length of 1800mm has a narrow FOV, but an SCT with a 2000mm focal length is a versatile telescope that provides great DSO views. I am not saying it doesn't, just questioning why (at times) the MCT is considered almost useless for this purpose.

At least one person understood my post.

#17 hottr6

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Posted 06 February 2013 - 11:46 AM

When I say that MCTs have narrow fields, it is relative to other telescopes of the same aperture.

But Eddgie, even that statement is misleading. What you mean to say is MCTs have narrow fields relative to other telescopes with shorter focal lengths.

I keep coming back to slow refractors. No one has ever accused a Unitron or Zeiss Telementor of having a narrow field of view, even though it is more true than almost all MCTs of similar aperture.

Yeah, I'm grousing, but it is because maybe well-meaning people who parrot misleading or incomplete statements.

#18 fuzzystuff4ever

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Posted 06 February 2013 - 12:05 PM

I just think of my C90 as a nice F15 refractor that's been magically shrunken down to the size of a propane cylinder. I use my AstroTelescopes 152/F5.9 for wide field views if desired.

#19 Jon Isaacs

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Posted 06 February 2013 - 12:53 PM

When I say that MCTs have narrow fields, it is relative to other telescopes of the same aperture.

But Eddgie, even that statement is misleading. What you mean to say is MCTs have narrow fields relative to other telescopes with shorter focal lengths.

I keep coming back to slow refractors. No one has ever accused a Unitron or Zeiss Telementor of having a narrow field of view, even though it is more true than almost all MCTs of similar aperture.

Yeah, I'm grousing, but it is because maybe well-meaning people who parrot misleading or incomplete statements.


The maximum possible True Field of View is a function of both focal length and the focuser size, both are important. There are other scopes with similar apertures with similar narrow maximum possible TFoVs but in general, they are quite uncommon. Anyone buying Unitron should know what they are getting. Telementors have a 840mm focal length and from what I have read can be fitted with 1.25 inch and even 2 inch accessories so field of view is not an issue with them.

The fact that the ETX-125 has the same 0.83 degree maximum TFoV as a 25 inch F/5 Newtonian should be something of a wake up call. Five time the aperture, 25 times the light but the same TFoV. Making a comparison in the 5 inch size, a 130 mm F/5 Newtonian fitted with a 2 inch focuser is capable of a 3.7 degree TFoV. There was a time when I owned both an ETX-125 and a 130mm F/5, it only took one night under dark skies to determine which one stayed and which one moved on. An issue not mentioned so far but important is the maximum possible exit pupil size/image brightness.

For someone choosing a scope, the maximum TFoV is an important issue. It can be a big surprise to buy a 90mm Mak and discover that the Pleiades wont't fit or that big bright images are not going to happen.

Slow scopes, particularly slow scopes fitted with 1.25 inch focusers provide significantly narrower maximum True Fields of View than fast telescopes, particularly those fitted with a 2 inch focuser, of the same aperture. This is just the way it is, something to accept the same way one accepts chromatic aberration in a refractor or coma in a Newtonian...

My 8 inch F/5 Newtonian is capable of a 2.4 degree TFoV. I know of no Mak-Cassegrain of any aperture than can provide anything close to that... It's just the way the world works.

Jon

#20 KerryR

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Posted 06 February 2013 - 01:15 PM

Even for same aperture and same focal length:
Take an 4" f12.72 refractor and an Orion 102mm Mak. Doesn't the refractor inherently permit a wider TFOV because the light cone is (usually) less restricted than the Mak, the Mak having it's necessarily narrow baffle and visual back?

It's easy (and common) to put a 2" diagonal on a 4" refractor, long focus or otherwise, and impossible to do so on the Mak (without cutting the light cone, owing to the width of the baffle and visual back), so there's also the issue of full field illumination, too, as well as the area occluded by the secondary in the exit pupil, which inherently puts a cap on the widest (usefull) TFOV in the Mak.

I'm not implying Maks are useless for DSO's or anything. Just saying the design as it's normally executed does inherently offer a narrower FOV than a refractor of the same focal length and aperture, unless I've got it wrong (a distinct possibility).

#21 coopman

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Posted 06 February 2013 - 01:18 PM

Many observers today have no firsthand experience of what looking thru an f/15 refractor is like, so Maks DO indeed have a narrow FOV by the standards that they are used to. I've been observing since I was a teenager (45+ years ago now)and I've never looked thru a long focal length refractor.

#22 hottr6

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Posted 06 February 2013 - 01:35 PM

Even for same aperture and same focal length:
Take an 4" f12.72 refractor and an Orion 102mm Mak. Doesn't the refractor inherently permit a wider TFOV because the light cone is (usually) less restricted than the Mak, the Mak having it's necessarily narrow baffle and visual back?

Jon has alluded to this, and I do agree that vignetting will occur in the Mak, but if a Mak and a 'frac have the same focal length, they will still have the same FOV when using the same eyepiece, regardless of the size of the eyepiece's field stop (but the Mak will be vignetted). Is this a correct statement?

I don't have much experience with many Maks, but I have been working under the assumption that the Synta Maks have narrow baffle tubes that will contribute to vignetting. Do Questars, Russian Rumak-Maks and the 7" Meade have "undersized" baffle tubes that contribute to vignetting?

#23 orion61

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Posted 06 February 2013 - 01:49 PM

When I say that MCTs have narrow fields, it is relative to other telescopes of the same aperture.

A 5" SCT will generally have a wider true field capability than a 5" MCT.

An 11" SCT can have about the same true field as a 7" MCT.

Relative to other designs, MCTs often do have much narrower true fields.

When made at f/10, they loose the advantage of having a smaller central obstruction than an f/10 SCT. They get the same true field capability, but loose the contrast advantage. And of course they are very expensive.

The truth may hurt, but it is the truth. Compared to similar aperture reflectors, MCTs generally do have a much narrower true field capability.

But this only matters if one needs a bigger field. The contrast advantage tha an f/15 MCT offers over an SCT of the same aperture makes them better (for inch of aperture) planetary scopes.

All scope have compromises. The compromise of the MCT is that unless you take special steps (which makes them very expensive to make) you wind up with a longer focal lenght scope than you get with SCTs or with Reflectors.

Or, you can take those steps and get it f/10, but then you limit the contrast potential to the same as the f/10 SCT.

Pick your poison. Higher contrast but at the expense of being limited

Wouldn't that then call for the comment that a Mak should be far superior than the same size SCT for Planetary viewing useing the same eyepieces?
but we never hear the Positive aspects of Maks..
WE NEED A MAK FORUM..
Heck, we can have one for audio equipment and pets??
this way us true Maksutov fans can have a safe haven for
a Big Mak chat..LOL

#24 Eddgie

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Posted 06 February 2013 - 02:00 PM

The baffle tube is not "Undersized." It is the right size for the secondary mirror in the scope.

The contrast of f/15 MCTs can approach that of a refractor because the central obstruction is very small.

The Baffle doesn't need to be any wider (and in fact can be smaller) than the diameter of the secondary baffle, though usually they are made a bit narrower than this.

If you make it fatter, you have to make it shorter so that light leaving the inside edge of the primary can make it to the center of the secondary.

In other words, the size of the baffle is a function of the diameter of the secondary and the spacing between the primary and secondary.

It would not make sense to use a bigger baffle because light could squeek past the secondary mirror and fall directly into the baffle.

This means that the price you pay for having contrast that approaches that of a refractor with the same aperture is that you have to have a small baffle, and this is what limits your true field.

Again, this is why MCTs generally have narrower fields than similar or even much larger SCTs (as my earlier example of the 180MCT vs a C11 clearly shows). To keep the contrast high, you have to use a small secondary, and then you get stuck with a soda straw baffle, which limits the field.

This is what made the MCT popular when refractors were achromats. You could get better contrast from a 7" f/15 MCT than you could get from a 7" f/18 achromat!

But you can get even better contrast peformance out of a C11, and get about the same true field as the MCT and a bigger field than a 7" f/18 achromat.

Again, you can get f/10 out of the MCT, but only with a SCT sized secondary. The only advantage of an f/10 MCT over a similar aperture SCT is that the MCT is better corrected for coma.

The 10" MCT though will be expensive because in the MCT design, you have to take special steps to eliminate higher order spherical abberation, or the scope will not perform well. You can aspherise, or you can use different curves on the front and rear of the meniscus, and on the secondary, but this makes for an expensive scope.

Otherwise, your MCT will usually be limited to about f/12.5 (and even then will have some HSA), and that means that it will usually be limited to a narrower true field than similar aperture refractors, reflectors, or SCTs.

And hence, for a given aperture, the MCTs usually have the narrowest true field capabilites.

#25 moynihan

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Posted 06 February 2013 - 02:02 PM

WE NEED A MAK FORUM..


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