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The secrets of the Meade 7" Maksutov


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The secrets of the Meade from 7"

The secrets of the Meade 7" Maksutov

 

By Piergiovanni Salimbeni (www.glrgroup.org)

Note: I thank Fabio Lottero and Jim Phillips for the aid in the translation from the Italian language

 

Premise

 

My goal is to document the performance of the 178mm Maksutov Cassegrain produced by the American company Meade. The occasion will be also useful for answering questions that often friends ask of me. Unfortunately, trying to match information among amateurs or in mailing-lists on Internet can be difficult as more than eighty percent of those who discuss the performances of a given telescope have never owned it or never used it, not even for ten minutes. While those who have tried it, often are influenced by other persons who have already prejudged the real performance.

 

Such is born some of the "classic metropolitan legends" that are attributed to one instrument. It will seem strange, but every instrument is a case in and of itself, and the impressions that are arrived at are influenced by the experience of the observer and also by which field of observation the observer likes most. For this reason, we repeat, it is very difficult, except in rare cases of certain quality, to find an instrument which performs identically to other instruments of the same type or to introduce the same identical aberrations in different scopes of the same type.

This kind of, we will dare to say, "psychological dependency" of certain instruments is very obvious in many amateurs that, stretch to praise or condemn a type telescope.  For this reason, but being human, we will strain to be objective in the knowledge collected in the course of the years in the comparisons of this instrument, in order to answer in a way, that we hope is most impartial and objective possible.

 

 

Optical configuration.

 

Since, its first appearance, we have spoken of the Maksutov often. I will not write  about the Schmidt Cassegrain. This instrument is a compromise between these and the most expensive apochromatic refractors sold from the same house. A lot of amateurs, did not see a pushed catadrioptic system of 2670mm focal length are supplied with a reduced field, even if Meade guaranteed it as performing comparable to an apochromatic refractor from the little inferior diameter. Beginning now to analyze the optical configuration of this instrument: as it is possible to see in the outline exposed here of continuation, aspheric primary mirror with equal focal relationship to f/2.5 and from one obtained spherical secondary mirror is formed from a spherical disc that is present on  the inner part of meniscus, the corrector. This multiplies the focal of the primary mirror by a factor of six, with the final focal length equal to F/15.




The main mirror has a diameter of 218mm. This feature produces a very bright field and without vignetting despite other optical configurations. Other maksutov cassegrain instead has a underestimated primary that it isn’t able to intercept all the light beam coming from the meniscus.

 

There are as visible in the drawing following, also a series of diaphragms into the cone deflector (D) of the main mirror that is able to stop the light beams out of the axis before they can reach the primary focus, in this way it is possible to obtain images more contrasted and a deep sky very dark, without boring reflections.

 

The obstruction of the secondary mirror is 0.28, the cone hood has a diameter we measured of 0.68mm that take the obstruction relation almost like to a common Schimdt Cassegrain reaching in-fact the value of 0.38mm (178:68).




The anti-reflection coating that are on the lenses that are part of the American optical system are different. On the correction lens is a coating with MGF2, it is a “standard” aluminum coating, while on the secondary is available as an option an “ultra-high-transmission coating” that should perform a better reflection and a better brightness of the entire system.

The meniscus is really thick and composed of Bk7 glass. Its cell has a system of push and pull screws that reduces the ease of  collimation of the secondary mirror because there is not  a separate secondary mirror but an integrated one as alumination of the internal surface of the correcting meniscus. It therefore doesn’t allow simple collimation. For this reason the manufacturer suggests that if collimation is needed, to return it to Meade.

 

I don’t suggest the first time amateur try to attempt collimation that, after all, requires a lot of sensibility and too much magnification to the phase operated with a separate secondary and with collimation screw.

 

The tube that has the same diameter of the cousin SC 203, is longer. Really, when it is mounted on the Meade fork it is impossible to set it in vertical position.

 

They would have to lengthen the fork arms. This solution is very curious and atypical.

 

Let’s go now to answer point to point to a long series of questions as said in the premise.

 

Star test and optical defects

The job done by American Opticians is optimal. The Star Test, evidenced the whole correctness of aberrations, shows only slight under correction, although at other magnifications, under 250x, the difference between the spot of the secondary in intra-focal and the one in extra-focal is imperceptible.

 

It was the best Star Test I have seen on a telescope with this configuration, also the Airy disc is easily visible also at medium magnifications, disturbed only by atmospheric turbulence. Also having an obstruction relation almost similar to the telescope taken as comparison object, it seems that the working of the spherical meniscus of this Maksutov is better than a standard Schimdt Cassegrain. Obviously we may have found a very good example but this is what we have seen through the analysis of the optic we have.

 

Mirror shift

 

Unfortunately the same professionalism used into the design of the optical system was not carried on to the focusing system. The mirror shift during the focusing in the model I tested is almost 35 arc seconds, too much for visual observation. Luckily Meade, conscious of this defect, decided to equip the telescope with a blockage system of the primary mirror that can be accomplished by the rotation of a handle, helping it with a electrical focusing system. I have also the possibility to test the telescope with a Crayford focuser manufacturer by William Optics, it has added to the electrical system’s optimal performance. The only problem I found, rose because the screw that fixes the handle of the blockage system that with the use is unscrewing, make useless the function of blockage. For this reason it is useful to have a pair of specific keys by them will be possible also the monthly “rifixing” of the screws that block the fan to the optical tube that could unscrew because of micro-vibration.

 

Focal Turbulence

 

As anticipated there is a fan to facilitate thermal acclimatization of the tube. In any case during some nights during October and November I brought outdoor the Meade together a Celestron 8”.

 

Exiting every so often, I noticed, as, without the help of the impeller, the Maksutov taken more than three hours to reach a correct acclimatization, I can’t think what can happen in the cold winter nights.

 

The difference with the Celestron 203mm, became better till to tilt in the Maksutov 178mm, which with the cooling system started can to be useful less then after thirty minutes against the 70, 80 minutes of the C 203.

At first sight  it seems impossible as similar focal turbulence, even though the meniscus is thick, we see the intubations of very high quality. Watching the optical system with more attention we were able to discover the mystery: a metal heavy flange is present, and it has the function of balancing the optical tube in the fork systems. This metal ring is also the directly responsible for the remarkable weight proportional to the dimension of the lens.

 

With a balance, in fact,  the optical tube has a weight of almost 10 kg. After we calculate the addition of an electric focuser, a diagonal of 50.4mm and an eyepiece of the same diameter, we can reach 12 Kg: so also in this case, the weight doesn’t depend upon the meniscus dimension, as often we hear, during amateur discussions.

 

The remedy exists and we don’t know if Meade will preview a OTA version (only optical tube). On Meade’s official website, the weight of only the otpical tube is 25 pounds. This seems to confirm what we had seen.

 

How to remedy ?

 

Really a remedy for this Meade exists. You’ll find in the box, thanks to the precious suggestions, a detailed explanation on how to remove the metal flange. The operation is unsuggested to newbies, because it requires a good quantity of experience, over all and  during the remounting of the system. In any case, after removal of the flange we were able to reduce the weight of this Maksutov  by almost 5 Kg, so we can use it with the commercial mounting like Vixen Great Polaris.

 

 Performances on Solar System Objects.

 

I’ll say: Satisfactorily with the  telescope completely acclimatized.

 

This Maksutov supplied very contrasted and bright images, but the image obtained isn’t equal to the one supplied by a apochromatic refractor with a slight less diameter, although it is similar to a “theme variation” that we want to evidence.  The details that are visible in respect to a apochromatic refractor of 130mm used to compare are sometimes superior, thanks to the bigger resolution power and the images are slightly brighter. It permits also the use of planetary filters respect to a smaller apochromatic refractor of 102mm that is in the same price range.

 

It is always visible a little micro-turbulence, that is far from the  contrasted calm visible under the same atmospheric conditions in the refractors. I could say, putting things together, that in regard to observing weak planetary details, the Meade has better performance in particular situations, while an apochromatic refractors it is usable in every situation. There are obviously exceptions, as, also in the case of apochromatic refractors, the tube and the spacing method of the lens is a symptom of the prevention or the generation of a high focal turbulence, remember that first of all the Zeiss APQ of 130mm.

 

For example the Cassini Division, with Saturn low on the horizon, was always visible in the refractor telescope, while in the Meade the thermal conditions were always decisive for its visibility.

 

Concerning Lunar observation, thanks to a rare night with exceptional seeing, it exceeded the performance of the catadioptric 203mm, in which it was similar,  other than a bigger glow at lunar border, till 200 magnification. To that magnification in up until 400X the American Maksutov supplied very good images, contrasted that sometimes shown thin particulars, seen with difficulty in the catadrioptic, not for the resolution, but because for the best contrast. I remember with pleasure the sight of the internal rimes of the crater Gassendi, visible with more easiness so as, the lavish impact material in the crater Ptolemaus and tens of microcraters that I had seen only in rare nights of atmospheric calm through a Celestron 9 ¼.

 

The medium optimal magnification used during the two months with the C8 was 200X, the one with the Meade was 300X.



 

Performance on deep sky

 

In this case, my opinion will be on contrary to the most of comments I have heard on this optical configuration. I think that this Meade Maksutov isn’t so bad in the observation of the deep sky objects, photography is obviously a matter apart.

 

The problem is the limited field, really, other than caused by the long focal relation it is due also by the eyepieces used. It is obvious that amateur astronomers that have a good set of eyepieces can do good observations of deep sky in respect to owners of telescopes with lower focal ratios but used with eyepiece with a little field of sight.

 

By using a very good Nagler Televue eyepiece 31mm diameter with 82 degrees of filed, I had the ability to see a field of over 1 degree, the entire structure of the Perseus double cluster, M81 and M82 and the external parts of the Orion nebula, moreover, the image brought was much more correct and point-shaped respect to Celestron 8 inch of mine, and to a chinese made newton that show more field but also an excessive coma. Also the contrast was able to help me when was necessary to observe faint details such  as faint galaxies or the structure of Orion nebula.

For those who want to improve these performances, there is available a focal reducer able to reach 2000m.

 

It is obvious that “ with a good set of accessories”  the performance on star clusters and globular clusters will be very good.

 

The telescope, proportional to its diameter and its focal length for us seems very bright, over all by using a good diagonal mirror of 50.4mm. By using the eyepieces cited above, I have never  mourned the loss of instruments of slightly larger aperture.

 

 

Box1: The real sight field

 

It is the real angular field that can be see through the system eyepiece-telescope.

 

The real field (CR) is related to the apparent field (CA) thanks to the telescope magnification (I) and the simple CR=CA/I.

 

Below we see some comparison did during out tests

 

Meade

Eyepiece

Apparent field

Magnification

obtained =
 focal of telescope                                      __________________   focal

magnification of eyepiece



Real field=                                     Apparent field of sight
__________________ 

magnification obtained

178 mm
2670 focal lenght

Televue Nagler 31mm

82°

2670/35mm=76.28X

82/76.28=1.07°

 

Televue Nagler 12mm

82°

2670/12=222.5X

82/222.5=0.36

 

 

 

 

 

 

 

Celestron

Eyepiece

Apparent field

Magnification

obtained =
 focal of telescope                                      __________________   focal

magnification of eyepiece



Real field=                                     Apparent field of sight
__________________ 

magnification obtained

203mm
2000 focal lenght

Eyepeiece

Ultima 35mm

49°

2000/35:57.14

49:57.14=0.85

 

Orthoscopic

eyepiece Japan Optic 12mm

45°

2000:12:166,6X

45:166.6=0.27

 

 

Box 2 How to remove the inner flange


For who want to remove the inner flange, these are the suggested steps:

 

First of all, its needs to be repeated that this operation needs very high technical ability, both in the phase of taking apart and  reassembling the optical and mechanical systems.

 

The tube, posed in horizontal position, will be dismounted with very high attention, You will remove the meniscus, unscrewing the threaded aluminum ring that contains it in the cell.

 

After rested with care in a secure place, best on a clean cloth to prevent scratches, you'll have to unscrew the handle of the focuser. This operation will be executed by loosing the two grains sited laterally, that work as fixing elements.
 

At this point the main mirror won't be supported anymore by the long focusing screw. To remove it you'll have to leave with a hand the security seeger ring sited at the end of the hood cone, where the mirror slide: after which you will  be able to remove the mirror.

 

After removing the  meniscus and mirror, you'll have to mount the impeller and the grid on the other aeration hole, on the opposite side, so to have two holes from which to proceed with removing the disc using hammer and chisel:

 

The metallic flange is blocked on the breech by a glue with very high resistance.

 

For removing it you'll have to hit alternatively with the hammer, first one  side and then the  other.





After the removing the Meade will weight almost 5 Kg less, and it is become more easily managed and also aiding the thermal acclimatizing.

 

The phase of remounting will be a very delicate operation, and very important to be able to take advantage of  the power of this instrument.  

 

<b><span style="font-family: Arial;" lang="EN-GB"><br>
Hots:</span></b>
<i><span style="font-family: Arial;" lang="EN-GB"> A real Killer optics.</span></i>
<i><span style="font-family: Arial;" lang="EN-GB"> Good performance on moon and planet.<br>
The e</span></i><i><span
style="font-family: Arial;">lectrical focusing system.</span></i><span
style="font-family: Arial;" lang="EN-GB"><br>
<br>
</span>
<b><span style="font-family: Arial;" lang="EN-GB">Nots:</span></b>
<i><span style="font-family: Arial;" lang="EN-GB">Long cool-down time without fan.</span></i>

The weight of the optical tube.
 


 


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