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FOV for Maksutov

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#1 Pete-LH

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Posted 17 August 2014 - 06:44 PM

Having followed a few threads here on Maksutov, I am starting to feel the need.

Out of curiosity I did a search here and on the web on FOV for Matsukovs and the use of a reducer. I did not find much but maybe my key words were not sufficient.

Anyway I'm interested to know what maximum fields of view can be achieved with 1.25" eye pieces (45 deg Silvertop ? I saw a mention going back to 2008).

Also, has anyone used a reducer with a maksutov. I saw mention of a 0.5X  reducer from GSO that screws into the eyepiece. HAs anyone here tried this? If so how is it for visual and what fields of view did you achieve?

 

 


Edited by Pete-LH, 17 August 2014 - 07:06 PM.


#2 Pete-LH

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Posted 17 August 2014 - 07:13 PM

Actually when I spelled "maksutov" correctly I received more hits. Shame on me!

 

Still I would like to know your application and success of using a reducer with a "Maksutov".



#3 Patrick

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Posted 17 August 2014 - 10:26 PM

Having followed a few threads here on Maksutov, I am starting to feel the need.

Out of curiosity I did a search here and on the web on FOV for Matsukovs and the use of a reducer. I did not find much but maybe my key words were not sufficient.

Anyway I'm interested to know what maximum fields of view can be achieved with 1.25" eye pieces (45 deg Silvertop ? I saw a mention going back to 2008).

Also, has anyone used a reducer with a maksutov. I saw mention of a 0.5X  reducer from GSO that screws into the eyepiece. HAs anyone here tried this? If so how is it for visual and what fields of view did you achieve?

 

We would need to know what kind of Mak you're talking about.  Is it a Maksutov Cassegrain or a Maksutov Newtonian and what size aperture is it with what focal ratio?

 

Patrick



#4 Pete-LH

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Posted 17 August 2014 - 10:42 PM

Sorry, Maksutov Cassegrain, let's say 7" f/15.

 

Actually though I'm interested in any Maksutov Cassegrain experience using a reducer.



#5 Dom543

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Posted 17 August 2014 - 11:52 PM

Maksutov Cassergrains excel in allowing high magnification and hence detail in planetary or lunar observations or for viewing small planetary nebulas. They are often used with short focal length eyepieces or in combination with Barlow lenses. Focal reducers are the opposite of Barlows, they reduce magnification. It is possible to use a focal reducer with a Mak but it, in a sense, defeats the purpose.

 

A couple of years ago I bought a used Meade 7" Mak. It came with a Meade f6.3 focal reducer and the seller said that he used it almost all of the time. In other words, he used the Mak, as an f/10 scope, much like a run-off-the-mill SCT. In other words, you can have a Mak and enjoy its high magnification views of planets and moon. And then, when you would just want an f/10 SCT to look at some larger objects, you put on a focal reducer...

 

--Dom



#6 Pete-LH

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Posted 18 August 2014 - 04:49 AM

Thanks Dom, So if only have enough funds for one scope then it is better to have a Mak and a 6.3 focal reducer would be my guess? Since if you have an SCT My guess would be that it would be more difficult and not desirable to extend it to f/15? Maybe I should look closer to the C8 versus 7 inch Mak discussion.



#7 Patrick

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Posted 18 August 2014 - 11:39 AM

Thanks Dom, So if only have enough funds for one scope then it is better to have a Mak and a 6.3 focal reducer would be my guess? Since if you have an SCT My guess would be that it would be more difficult and not desirable to extend it to f/15? Maybe I should look closer to the C8 versus 7 inch Mak discussion.

 

 

What is your end game with the 7" f/15 Mak Cass? The focal length of that scope will be approx. 2700mm.  Seeing typically limits most nights to about 300x which can be reached with a 9mm eyepiece without a barlow.  With an f/6.3 focal reducer you're down around 1700mm f/l and a max true field of view around 0.9°.  But, you'll need to check to see if the Mak Cass you're looking at will take a standard SCT focal reducer.  The Orion version will only take 1.25" eyepieces.

 

Personally, I think the 8" Edge HD is a better choice for versatility.  You can still go well above 300x at the top end with a 6.6mm eyepiece, and down to 1.4-1.5° TFOV with the Celestron f/7.0 focal reducer and 2" eyepieces. The optical quality of the Edge series of scopes is also highly regarded (I love mine).

 

Patrick


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#8 Dom543

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Posted 18 August 2014 - 02:19 PM

I paid $1000 + shipping for the used Meade LX200 Classic 7" Mak. The arms & OTA assembly is rather heavy. It needs to be put outside with the fan running for at least 30 minutes to have it properly cooled down. As Patrick is suggesting it, I often use it with a 9mm ortho.

I guess that one could also get a used C8 on a CG-AGST mount for about the same amount of money. It would be much lighter and the equatorial mount has its advantages.

An 8" Edge HD would give you more for more money...

 

--Dom



#9 Pete-LH

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Posted 18 August 2014 - 05:39 PM

Thanks Patrick and Dom for you experience and information.

 

I have an old C8 which is still pretty satisfying for my capabilities. I may eventually move to an Edge.



#10 P26

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Posted 18 August 2014 - 06:23 PM

Hi Pete,

 

As Patrick and Dom have pointed out, Maks are long focal length scopes that produce higher contrast images than SCTs.  The trade off is a smaller actual field of view.  Maks are typically considered better for planetary viewing as the small FOV doesn't matter and the high contrast does.

 

Actual Field of view = (eyepiece field stop diameter in mm  X  57) / scope focal length in mm

 

Assuming you're talking a Meade LX200 Classic 7" Mak the scope's 38.1 mm optical path effectively constrains your field stop to 35mm.  So a good widest field eyepiece might be the Panoptic 35 mm with it's 38.7 mm field stop.  This would produce an actual unvignetter FOV of 48.8 arc-minutes.  Going beyond this FOV will result in vignetting. 

 

Eyepiece exit pupil size isn't a factor when viewing bright objects such as planets.  But it really counts for dim deep sky objects.  

The eyepiece exit pupil = (eyepiece focal length) / telescope f/ ratio.  In the case of the Pan 35 above your exit pupil = 35mm/15 = 2.3 mm diameter.  Ideally for dim viewing one would desire a 5mm or 6mm exit pupil to better match the size of a fully dark adapted eye's pupil.

 

You can't bend the laws of physics, and trying to use a focal reducer to increase your FOV will result in vignetting once you go beyond, in the case of the Meade 7", 49 arc minutes.  Focal reducers are used by imagers with small CCD chips because they are actually image size reducers.  So they're going to further reduce the size of the view on your eye's pupil.  The better wide field eyepieces actually have a version of basic focal reducer optics incorporated into one or two of their 5 or 6 elements.  So rather than coupling a 9mm eyepiece with an f/6.3 focal reducer you'd be better off with a 14mm eyepiece to start with.

 

There is no one telescope configuration that is best for everything.  But some have less versatility than others.  For this reason the Mak is not a good "I want to see everything" compromise.  Perhaps you might seek a book on the topic as you may wish to think carefully before making a large purchase.

 

Clear skies,

 

Pete Peterson


Edited by P26, 19 August 2014 - 10:08 AM.


#11 Pete-LH

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Posted 18 August 2014 - 07:43 PM

Thanks Pete, Great advice and information.



#12 Asbytec

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Posted 19 August 2014 - 09:43 AM

"Maks are typically considered better for planetary viewing as the small FOV doesn't matter and the high contrast does."

 

I love the Mak for lunar and planetary work despite it's limited FOV. I am always up in the sub 1mm exit pupil range, anyway, and find many deep sky objects frame nicely.

 

"This would produce an actual unvignetted FOV of 48.8 arc-seconds"

 

Pete, how did you calculate this? In my own, I do not "realize" any vignetting out to about 15 arc minutes (not arc seconds.) Jupiter is about 48 arc seconds and is small relative to the FOV at low power.

 

But, I never did the math. I only look for the pinching of the out of focus pattern caused by one of the baffles and do not notice it until about 7 arc minutes from center (radius) in low power FOV. That's assuming the center is fully illuminated, and by investigation I believe it is.

 

My highest magnification, 6mm TMB II - 58 degrees AFOV at about 320x, measures (star drift method) about 9 arc minutes (0.15 degrees) across, so should be fully illuminated.



#13 dotnet

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Posted 19 August 2014 - 10:09 AM

Assuming you're talking a Meade LX200 Classic 7" Mak the scope's 38.1 mm optical path effectively constrains your field stop to 35mm.  So a good widest field eyepiece might be the Panoptic 35 mm with it's 38.7 mm field stop.  This would produce an actual unvignetted FOV of 48.8 arc-seconds.  Going beyond this FOV will result in vignetting. 

 

 

Hi Pete, I'm sure you mean 48.8 arc minutes (off by 60 ;) ).

 

Cheers

Steffen.



#14 P26

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Posted 19 August 2014 - 10:16 AM

Pete incorrectly said: "This would produce an actual unvignetted FOV of 48.8 arc-seconds"

 

Norme asked "Pete, how did you calculate this? In my own, I do not "realize" any vignetting out to about 15 arc minutes (not arc seconds.) Jupiter is about 48 arc seconds and is small relative to the FOV at low power."

 

Sorry All,

 

That's arc minutes and not arc seconds.  My original posting has been corrected.  

 

The 48 or 49 arc minute unvignetted FOV is more than I can get out of my EyeOpener equipped 14" Meade GPS with a Nagler 31.  So as has been said before, it's all relative.

 

Sorry for the confusion.

 

Pete


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#15 dotnet

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Posted 19 August 2014 - 10:31 AM

Minutes or seconds aside, a Mak-Cass is certainly no wide-field scope, that's for sure. Even my 6" f/12 is hardly ever used at the wide end, that's what I've got a Dob for. Likewise, I wouldn't think of using the Dob for observing doubles.

 

Cheers

Steffen.



#16 bicparker

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Posted 19 August 2014 - 11:14 AM

As an alternative, a Vixen VMC200L is an f/9.75 with an open tube and the meniscus in front of the secondary.  Closer to an f/10 instrument at that point.  Plus it uses an external focuser instead of a mirror focuser, so no image shift.  They are a pretty nice 8" instrument for observing.


Edited by bicparker, 19 August 2014 - 11:16 AM.


#17 Eddgie

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Posted 19 August 2014 - 05:23 PM

Having followed a few threads here on Maksutov, I am starting to feel the need.

Out of curiosity I did a search here and on the web on FOV for Matsukovs and the use of a reducer. I did not find much but maybe my key words were not sufficient.

Anyway I'm interested to know what maximum fields of view can be achieved with 1.25" eye pieces (45 deg Silvertop ? I saw a mention going back to 2008).

Also, has anyone used a reducer with a maksutov. I saw mention of a 0.5X  reducer from GSO that screws into the eyepiece. HAs anyone here tried this? If so how is it for visual and what fields of view did you achieve?

 

A great deal depends on the model MCT.

 

The smaller Intes Micro MCTs have very tight baffles, and are optimized for 1.25" eyepieces and a 1.25" diagonal.

 

If you use a focal reducer with an MCT with moving mirrors, you have to move the primary much further forward to be able to reach focus.  When you do this, you run the risk of having the outside of the light cone cut off either by the secondary baffle or the primary baffle.   This is not at all a good thing.  Not only is the aperture reduced, but the size of the obstruction becomes larger by percentage. 

 

Not all MCTs will have this issue, but some will, and the Intes Micro are likely to have the most exposure to this kind of issue.  Many small MCTs sold under various brands may also experience this kind of problem.    

 

For larger MCTs with larger baffles, or those designed to work with 2" diagonals, this may not be an issue, but if it has a 2" diagonal, most likely, the use of a focal reducer would again present an issue.  Better to use it with a 2" low power eyepiece to avoid aperture loss. 



#18 astroneil

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Posted 22 August 2014 - 09:06 AM

Having followed a few threads here on Maksutov, I am starting to feel the need.

Out of curiosity I did a search here and on the web on FOV for Matsukovs and the use of a reducer. I did not find much but maybe my key words were not sufficient.

Anyway I'm interested to know what maximum fields of view can be achieved with 1.25" eye pieces (45 deg Silvertop ? I saw a mention going back to 2008).

Also, has anyone used a reducer with a maksutov. I saw mention of a 0.5X  reducer from GSO that screws into the eyepiece. HAs anyone here tried this? If so how is it for visual and what fields of view did you achieve?

 

Thanks for bringing this question up Pete :bow: :bow: :bow:

 

Very helpful. :waytogo:

 

Cheers,

 

Neil. ;)



#19 Pete-LH

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Posted 22 August 2014 - 10:41 AM

Thanks Neil, You really have me interested in Maksutovs again (I had an ETX105 previously).

 

I will probably go for a 7" like yours. And as far as the wide field, will not try a reducer but just go with a light 60 or 70 mm refractor f/6 to f/8 as a finder/wide field scope as I do with my C8. I'm trying out several small refractors now for this purpose.



#20 *skyguy*

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Posted 22 August 2014 - 04:56 PM

You can also try using 2" eyepieces with a proper adapter for a greater TFOV even though most Maks use only 1.25" eyepieces. I have a Meade ETX-125 Mak and can attach 2" eyepieces to it using the 27mm. rear photo port. Even with this reduced 27mm opening my 2" 32mm Erfle eyepiece shows a 1.14º (68.4 arc-minute) TFOV ... with no noticeable vignetting at night ... versus a .9º (54 arc-minute) TFOV  using a 1.25" 40mm eyepiece. That represents a real increase in the actual viewing area of 62%. Also, the stars remain sharp and pinpoint across the entire FOV without any coma visible! I'm sure with the "right" 2" eyepiece, an even greater TFOV is obtainable.

 

Check out Siebert Optics 's eyepiece adapter page for more information on using 2" eyepieces on Maksutov scopes:

 

http://www.siebertop...ceadapters.html

 

 

 

 

Attached Files



#21 Ed Holland

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Posted 24 August 2014 - 11:34 PM

One way to check for vignetting, when one doesn't notice an obvious drop in brightness toward the edge of the field is to look at the defocussed diffraction pattern. The cutoff is noticeable as a distortion in the rings.

 

Another technique is to find a star at or near the limit of vision and move the scope to change its position in the field, noting if it becomes invisible.

 

I have  a 35mm ~55○ eyepiece that does exhibit vignetting in my Orion 127mm Mak when put to the strict tests above. For casual viewing, however, it is not objectionable.

 

Cheers,

 

Ed


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#22 hardwarezone

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Posted 25 August 2014 - 12:03 AM


I have  a 35mm ~55○ eyepiece that does exhibit vignetting in my Orion 127mm Mak when put to the strict tests above. For casual viewing, however, it is not objectionable.

 

Cheers,

 

Ed

Hi Ed,

does your 127mak have microbaffles in the primary baffle ?

 

more info about my question here

http://www.cloudynig...ffle-mod/page-7

 

Thanks.



#23 Eddgie

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Posted 25 August 2014 - 07:26 AM

You can also try using 2" eyepieces with a proper adapter for a greater TFOV even though most Maks use only 1.25" eyepieces. I have a Meade ETX-125 Mak and can attach 2" eyepieces to it using the 27mm. rear photo port. Even with this reduced 27mm opening my 2" 32mm Erfle eyepiece shows a 1.14º (68.4 arc-minute) TFOV ... with no noticeable vignetting at night ... versus a .9º (54 arc-minute) TFOV  using a 1.25" 40mm eyepiece. That represents a real increase in the actual viewing area of 62%. Also, the stars remain sharp and pinpoint across the entire FOV without any coma visible! I'm sure with the "right" 2" eyepiece, an even greater TFOV is obtainable.

 

Check out Siebert Optics 's eyepiece adapter page for more information on using 2" eyepieces on Maksutov scopes:

 

http://www.siebertop...ceadapters.html

 

It isn't visable vignetting that one should be concerned about.  It is the loss of aperture that occurs when the mirror has to be moved so far forward that the light cone is cut off by the baffles.

 

The baffles are so far away that you don't see this as vignetting, but the result is that the rays comeing from the edge of the mirror can no longer even reach the very center of the mirror.

 

Again, this does not look like classic vignetting, but the apeture can easily be reduced by an inch or more.

 

How do you know if you are in this state?  The easiest way is to defocus a star at the center of the field.  Now, start to move the star away from the center of the field and watch the outside edge.

 

If you immediatly see that the outside of the Fresnel pattern starts to experience a smootly curved "Slice" that gets worse as you move away from the center, then your scope is very likely in a reduced apeture mode.  To know how much, you would need to measure it with a laser, but again, depending on the scope, the reduction could be small, or it could be as much as an inch or even more, depending on the scopes exact configuration.

 

Most small MCTs simply do not tolerate a lot of back focus.

 

And by the way, some of the imported MCTs are already working at reduced apeture (118mm vs the 127mm that they are sold as) so for these scopes, putting a 2" diagonal may not show additional damage because the systems were already improperly designed even for a 1.25" eyepecie use.

 

My point though is that seeing vignetting is not the main issue.   Having the apeture reduce is the main danger.    

 

If you are going to try to turn a scope that was not designed into a wide field scope at the expense of reducing its apeture, why not just buy a smaller wide field scope?  In the end, it will not only give wider fields, but will almost always have a better illuminated field.

 

Another test once can do is to start with a very dim cluster at the center of the field.  Find the faintest stars in the cluster.

 

Now, drift the cluster to the edge of the field and see how many of those stars disappear before they get to the field stop.

Some might say that they only care about what is at the center of the field, but if this is the case, they why bother with the wider field to begin with?   My personal belief is that the best views are when the field has all of the stars around the target shown as well as the target item.  This gives "Contest" to the target.  The target appears in "Space" rather than as just something in the middle of nowhwere.

 

That is of course a highly personal preference, but the more I started understanding howbadly poor off axis illumination was affecting my enjoyment, I stopped using configurations that damaged this characterist. 

The most serious performance hit though when pushing a scope with baffles beyond the design limit is the aperture and contrast loss that starts to occur.


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#24 Ed Holland

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Posted 25 August 2014 - 12:34 PM


Hi Ed,

does your 127mak have microbaffles in the primary baffle ?

 

more info about my question here

http://www.cloudynig...ffle-mod/page-7

 

Thanks.

 

 

No, the factory setup is just the rather blunt "threading" coated with black paint. This is very poor as a stray light control. This is why, in that same thread, I mention the experiments made with blackened sandpaper.


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#25 Pete-LH

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Posted 31 August 2014 - 10:09 AM

Probably should let sleeping dogs lie. But it looks like clouds and rain now for several days at least now.

 

After a week of great viewing evenings, even from my Patio in the Philadelphia area, mainly watching Jacques pass through Cepheus with small refractors and my C9.25, I find myself obsessing over other telescope possibilities; Even though the views through the SCT were magnificent; Even though I used either a Baader Vario 60mm finder or a 60 mm Borg 60ED as finder wide field combos to have the best of both worlds (and thinking of upgrading the 60ED to a 71FL).

 

So looking through the sites offering the Skywatcher 180 Maksutov-Cassegrain, it now comes with a 2" visual back and diagonal (one review said the 2" diagonal was a bit "floppy" so they use a 1.25" diagonal).

Now I am looking for actual experience with this newer version. In addition to the quality I would like to confirm that the threads on the back are the same as those for SCT's which I believe are 2"/24tpi.

I think from comments here and other discussions the attraction is to have refractor like performance in a more compact package.

 

I guess we go around in circles on these here discussions here but ...

 

Another comparison in performance might be might be such a Mak vs a 6" f15 such as that offered by D&G. Or because of the limitation to narrower fields (this MAk would be close to 1 degree with a TV 55mm plossl ot 41mm Panoptic if one so desired) Maybe a comparison at higher magnifications to a high quality 6" Apochromatic refractor (which would be about 4X the cost).








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