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Information on the Takahashi Mewlon 210 and Mewlon 250

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#51 HolinOL

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Posted 19 November 2016 - 01:42 PM

Hi Mewlon friends, after more than 20 years with AP 130 EDT on AP 400 mount I fullfilled my wish to add a Mewlon 210.

I use it about one year now, viewing mainly planets and moon.

Conditions here 53°North, mainly bad to medium seeing, good not often; planets for years now deep over the roofs of the city.

Under these conditions the AP does not show a  steadier image than the  Mewlon. My personal impression is: the main difference between the two scopes is, that the Mewlon shows details more obvious than AP 130 ,  more small details only, when the seeing goes to the better end. Even with the same magnification in both scopes, the objects seem to me  to be greater, easier to observe in the Mewlon.

Viewing visually, the focusshift is not any problem. But with a planetary- cam  you can hardly keep the planet on the chip due to shifting.

I mounted a moonlite-focuser and the objects remain centered, focusing works very, very well.

As I am not skilled in collimating a scope, I was glad, the Mewlon arrived nearly spot-on collimated. After half a year, the collimation was still unchanged, I tried to perfect it; I loosened  2 srews and fixed one with a tiny, tiny squeak  under the controll of my cam and notebook. Now in my opinion it was spot- on.

Great help to do this, was a french observer, who posted a " Mewlon-Collimation - helping-method" with " false spiders" in a french astroforum, as the 3 collimation-srews are not in relation to the 4 spiders.

My AP CNC 400 mount from 1994 is still in best condition and balances the Mewlon very well. As the heavy of the scope is nearly centered on it, it never is  necessary to rebalance while changing from monoviewing to binoviewing with TV Binoview. Doing this, is not possible with the refractor.

Now the situation is, that the AP is used in daylight mainly and the Mewlon during nights.

Thanks from Germany, Holger


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#52 dr.who

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Posted 19 November 2016 - 03:30 PM

Holger - Willkommen zu bewölkte Nächte Danke für Ihren Beitrag! I found the same with my TSA-120 and even my AP 155. The 210 does a better job on planets even in average seeing.

#53 elwaine

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Posted 19 November 2016 - 04:40 PM

Great thread. Thank you Carson. And thank you, too Holger for that report.

Looking forward to the comparison between the 210 and the 250.

#54 ckwastro

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Posted 19 November 2016 - 05:22 PM

The 210 does a better job on planets even in average seeing.

Interesting. For me it has been the opposite. I've never really had superb planetary views with the Mewlon. Our seeing is usually middle of the road, but even so I've found my dobs and refractors seem to do better on Jupiter and Saturn. Seeing good enough for decent Mars observations is very infrequent for any scope, but the Mewlon does seem very sensitive to seeing.

 

I also discovered the other night that I had a boundary layer issue even after five hours of being outside and only a 5* original differential from inside the house. Not sure how that happened, but it could be one explanation for less than stellar planetary performance. That's the first time I noticed it using the scope, but also the first time I deliberately looked for it with a defocused star. Sure enough, slow crawling distortions, as opposed to the normal fast moving flickers of bad seeing. Might need those TEMP-est fans after all. 

 

OTOH, the Mewlon is one of the best lunar scopes I've ever used. Luna is a forgiving target, but nevertheless the contrast and etched details are stunning. 



#55 ckwastro

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Posted 19 November 2016 - 05:28 PM

The excellent contrast also makes for some nice deep sky observations.  I find for that application the 48mm Brandon gives me just over a 1* TFOV at 50x with nice tight stars.  That eyepiece and scope were made for each other.  :)



#56 dr.who

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Posted 19 November 2016 - 10:08 PM

Cheers Larry.

Kerry - perhaps a collimation issue? Or as you noted a boundary layer issue? If you think its a boundary layer problem I have found a small desktop size fan blowing directly onto the mirror from the front for a good 30-45 minutes prior to use will knock that down. No TEMPest fans required.

#57 ckwastro

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Posted 19 November 2016 - 10:56 PM

Thanks Carson. I've got just the fan for that, so I'll give it a shot. 

 

Collimation is good. Haven't had to touch it since I got the scope!



#58 pippo

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Posted 23 November 2016 - 02:16 AM

Dear all,

in the Mewlons the entrance pupil is at the entrance of the tube, the primary mirror being oversized with respect to the tube inner-diameter. Does this solution reduce the coma with respect to a classical DK-design with an entrance pupil at the primary-mirror surface? Can some expert user of a software like Olso evaluate the eventual coma-reduction given by the advancement of the entrance pupil?

 

In some forum there are statement supporting this effect but I did not find any evaluation of resulting coma-reduction.
 

Thanks

 

Francesco


Edited by pippo, 23 November 2016 - 02:25 AM.


#59 bottlecrusher

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Posted 23 November 2016 - 08:58 AM

Dear all,

in the Mewlons the entrance pupil is at the entrance of the tube, the primary mirror being oversized with respect to the tube inner-diameter. Does this solution reduce the coma with respect to a classical DK-design with an entrance pupil at the primary-mirror surface? Can some expert user of a software like Olso evaluate the eventual coma-reduction given by the advancement of the entrance pupil?

 

In some forum there are statement supporting this effect but I did not find any evaluation of resulting coma-reduction.
 

Thanks

 

Francesco

I wouldn't think in relation to the actual true aperature and focal ratio that the system is working at this should matter.  I would think the oversized mirror is more to prevent any issues of the mirror cell affecting the image.  However, if it is for instance 250mm mirror and is masked by the tube down then the published specs wouldn't be accurate.  It would have a smaller aperature and higher F ratio.  It is an interesting question because other D-K manufacturers use a F ratio of 20 to remove coma and provide contrast.  I asked CFF about this and they recommended F15 max for D-K and F 13.5 for CC.    



#60 ckwastro

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Posted 23 November 2016 - 10:24 AM

From the specs I've found the actual diameter of the M210 primary is 220mm. Effective aperture 210mm. 

 

Here is a link to a page on Bob Royce's site. Under "continue cassegrain primer" scroll to the seventh paragraph for Dall-Kirkham design.  He gives a nice explanation on this system. 

 

http://www.rfroyce.com/cassegrains.htm

 

Tak has has done a nice job with it but I imagine the faster system is not quite ideal for this optical design, given most manufacturers seemingly like to keep the primary at f/4 rather than the f/2.9 used in the M210. My best guess is that Tak chose to use a faster system to make it marketable to a larger audience (i.e. more generalized use - some deep sky, shorter tube, etc.). Again, it's just a guess but I can't think of another good reason to modify the optimal design to that extent. 



#61 elwaine

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Posted 23 November 2016 - 10:33 AM

I thought the purpose of an oversized primary is to avoid aberrations caused by a turned down edge, and that the easiest way to avoid that is by making an oversized primary mirror. Am I misinformed?



#62 ckwastro

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Posted 23 November 2016 - 12:33 PM

TDE can be one reason yes. Although not impossible, I would think it would be rare for Takahashi to have a TDE in its optics.  I've seen some descriptions of the M210 on retailers' sites that say it's to help with any astigmatism that might arise from mirror cell stresses.  Not sure, but I would think that would be more likely than a TDE in this case.


Edited by ckwastro, 23 November 2016 - 12:35 PM.


#63 dr.who

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Posted 23 November 2016 - 03:10 PM

Dear all,
in the Mewlons the entrance pupil is at the entrance of the tube, the primary mirror being oversized with respect to the tube inner-diameter. Does this solution reduce the coma with respect to a classical DK-design with an entrance pupil at the primary-mirror surface? Can some expert user of a software like Olso evaluate the eventual coma-reduction given by the advancement of the entrance pupil?
 
In some forum there are statement supporting this effect but I did not find any evaluation of resulting coma-reduction.
 
Thanks
 
Francesco


My understanding is that this is correct. Hopefully Daniel M. can chime in since he is a Takahashi expert. The mirror is actually 260mm in size with 10mm masked.

#64 ckwastro

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Posted 23 November 2016 - 04:21 PM

 

Dear all,
in the Mewlons the entrance pupil is at the entrance of the tube, the primary mirror being oversized with respect to the tube inner-diameter. Does this solution reduce the coma with respect to a classical DK-design with an entrance pupil at the primary-mirror surface? Can some expert user of a software like Olso evaluate the eventual coma-reduction given by the advancement of the entrance pupil?
 
In some forum there are statement supporting this effect but I did not find any evaluation of resulting coma-reduction.
 
Thanks
 
Francesco


My understanding is that this is correct. Hopefully Daniel M. can chime in since he is a Takahashi expert. The mirror is actually 260mm in size with 10mm masked.

 

 

It appears both the 210 & 250 then are oversized by 10mm. 



#65 dr.who

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Posted 23 November 2016 - 08:12 PM

Correct.

#66 pippo

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Posted 24 November 2016 - 07:20 AM

In my Mewlon 210 the external part of the edge of the primary mirror is blackened, probably  to cover an eventual turned down edge (TDE). However, the alluminated surface of the primary has a 220mm diameter, namely it is still oversized by 10mm compared to the 210 aperture-stop at the entrance of the tube. Thus the reason of the 10mm opening reduction at the entrance of the tube can not be the prevention of TDE (the blackened part of the mirror takes care of this!). I think that the 10mm oversized is likely motivated by a the control of coma.

 

An example of such an approach is the the Lensless Schmidt camera with a spherical primary mirror and an entrance pupil at the center of curvature of the primary, this approach by symmetry kills the off-axis abberations (coma and astigmatims), see chapter 7.7 of the book Telescope optics by Harrie Rutten and Martin van Venrooij.

 

In the DK Mewlon the diaframmated entrance can not fully eliminate the off-axis coma (that remains very visible)  but probably can reduce it compared to that experienced without an oversized primary.

 

I am interested in knowing by how much....


Edited by pippo, 25 November 2016 - 05:57 AM.


#67 TheFacelessMen

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Posted 24 November 2016 - 04:47 PM

Ok....so there is all this talk and theory of Masked of Mirrors and Coma........my challenge to all you Mewlon owners (any version) is to take your scopes out and provide an observation report on this thread describing the actual perceived visible Coma that you can see along with your personal impression of the extent/degree of the effect.

 

:flowerred:



#68 dr.who

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Posted 24 November 2016 - 06:17 PM

I can do that from memory since I won't be able to observe with the Mewlon for a while as it is about to rain and I have to be up at 5 am over the next several days. ;)

Coma was minimal and in only the outer 5-10% of edge of the total field of view using Ethos eyepieces in the 17mm and below range in the 210 when the 210 was properly cooled and had (still does) very good collimation. I am sensitive to coma and can't stand it when I look in a scope. Thus why I am such a APO refractor fan. And why I have the Mewlon's. They have the least amount of coma I have seen in a reflecting scope. My old UC15 at f/4.2 had it badly to my eyes (I could see it easily but others sometimes did not as they were less sensitive to it) so I had to use a Paracorr with it at all times or else I would get very frustrated since I hate seagull stars.

The other thing that drives me to distraction is wooly fuzzy flairing stars. Something I would get in my EdgeHD scopes if they were not cooled well enough. Again why I am such a refractor guy. And something that is reduced to near zero much quicker in the Mewlon's than the closed cell SCT's.
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#69 pippo

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Posted 25 November 2016 - 01:39 AM

My impression is that in the Mewlon 210 the coma is slightly stronger than in my old f/5 12" Dobson, and much less than my actual f/4 20" Dobson (withour paracorr)...but I am comparing by memory non-simultaneous observations and I could be wrong.

 

The concept of reduction of off-axis aberration by aperture stops at the entrance of the tube is also discussed here:

http://www.telescope...#Schmidt_camera

 

Quoting:

"Finnish astronomer Y. Väisälä, described similar arrangement, so this type of camera is sometimes called Schmidt-Väisälä (usually when incorporating field-flattener). Its concept is based on the unique property of spherical mirror with the aperture stop at the center of curvature to be free from off-axis aberrations."


Edited by pippo, 25 November 2016 - 05:59 AM.


#70 Seiko4169

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Posted 25 November 2016 - 08:06 AM

Did we actually get a side by side comparison review result? 210 owner here who I think will still be happy that I chose it over the 250 due to size and weight differences. The 210 is a really nice weight and size and still pulls in quite a bit. Still, if someone can post the their impressions between the 210 and 250?



#71 amys

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Posted 25 November 2016 - 08:42 AM

The excellent contrast also makes for some nice deep sky observations.  I find for that application the 48mm Brandon gives me just over a 1* TFOV at 50x with nice tight stars.  That eyepiece and scope were made for each other.  :)

I had been thinking about getting that eyepiece for the Mewlon but wasn't sure it would be worth the price.  I ended up getting the 40mm SWAN instead, which is ok.  I mostly use it as a finder eyepiece because the small, straight through finder scope is almost worthless to me except as a handle.



#72 bottlecrusher

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Posted 25 November 2016 - 09:29 AM

I can do that from memory since I won't be able to observe with the Mewlon for a while as it is about to rain and I have to be up at 5 am over the next several days. ;)

Coma was minimal and in only the outer 5-10% of edge of the total field of view using Ethos eyepieces in the 17mm and below range in the 210 when the 210 was properly cooled and had (still does) very good collimation. I am sensitive to coma and can't stand it when I look in a scope. Thus why I am such a APO refractor fan. And why I have the Mewlon's. They have the least amount of coma I have seen in a reflecting scope. My old UC15 at f/4.2 had it badly to my eyes (I could see it easily but others sometimes did not as they were less sensitive to it) so I had to use a Paracorr with it at all times or else I would get very frustrated since I hate seagull stars.

The other thing that drives me to distraction is wooly fuzzy flairing stars. Something I would get in my EdgeHD scopes if they were not cooled well enough. Again why I am such a refractor guy. And something that is reduced to near zero much quicker in the Mewlon's than the closed cell SCT's.

That is very interesting.  I wonder the difference a classical cassegrain of the same F ratio mirrors would yield?    I'm assuming maybe 25-30% larger coma free field?



#73 ckwastro

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Posted 25 November 2016 - 09:37 AM

My impression is that in the Mewlon 210 the coma is slightly stronger than in my old f/5 12" Dobson, and much less than my actual f/4 20" Dobson (withour paracorr)...but I am comparing by memory non-simultaneous observations and I could be wrong.

 

The concept of reduction of off-axis aberration by aperture stops at the entrance of the tube is also discussed here:

http://www.telescope...#Schmidt_camera

 

Quoting:

"Finnish astronomer Y. Väisälä, described similar arrangement, so this type of camera is sometimes called Schmidt-Väisälä (usually when incorporating field-flattener). Its concept is based on the unique property of spherical mirror with the aperture stop at the center of curvature to be free from off-axis aberrations."

Well I can safely say that in my 210 the coma is not nearly as strong as in my 12.5 f/5 dob without a Paracorr. I'd say it's more comparable to that of an f/6+ parabola. Unfortunately I don't have a Newt in that range to make a direct, more accurate comparison, but the cometary flare is significantly less than f/5, and also limited to about the outer 5%-7% of the field. 



#74 ckwastro

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Posted 25 November 2016 - 09:47 AM

 

The excellent contrast also makes for some nice deep sky observations.  I find for that application the 48mm Brandon gives me just over a 1* TFOV at 50x with nice tight stars.  That eyepiece and scope were made for each other.  :)

I had been thinking about getting that eyepiece for the Mewlon but wasn't sure it would be worth the price.  I ended up getting the 40mm SWAN instead, which is ok.  I mostly use it as a finder eyepiece because the small, straight through finder scope is almost worthless to me except as a handle.

 

I should have said the field was just "under" 1*, not over.  The eyepiece was worth it to me and I use it in all my scopes, but if you don't often want to use the Mewlon at 50x, then the 40 SWAN is probably more than adequate for a finder eyepiece.



#75 krakatoa1883

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Posted 25 November 2016 - 11:58 AM

Coma was minimal and in only the outer 5-10% of edge of the total field of view using Ethos eyepieces in the 17mm and below range in the 210 when the 210 was properly cooled and had (still does) very good collimation. 

Coma in Mewlon is not so strong as in short newtonians - I would say it is similar to a f/6 mirror - but it depends on the accessories used. Some Barlows tend to reduce it and also some modern eyepieces with a negative group before the positive one - Delos, Ethos, DeLite's and so on - have the same effect. This is why I collimate my unit by using of a simple eyepiece or, better, by using of the CCD, this makes the procedure more sensitive. 




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