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Problems with a Tak Mewlon 210 + binoviewer

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#1 jdown

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Posted 25 August 2019 - 11:42 AM

Yesterday afternoon, after months of waiting and a few missteps, I assembled my new system - a Takahashi Mewlon 210 telescope, to which was attached a Baader Wide-field Mark V binoviewer.  It being daylight, my goal was to see if I could bring distant terrestrial objects into sharp focus with my 2 sets of eyepieces - Docter 12.5mm EPs and APM 24mm UFF EPs.

 

I screwed a Baader 1.25x glass path corrector into the binoviewer, then attached a Baader T-2 Quick Adapter.  All this was then attached to my Takahashi 1.25" prism diagonal, itself attached to the telescope.

 

I pointed this setup to a ridge that I estimated to be ~3/4 mile away, covered with pine trees.  I assumed that if it could focus on objects that far away, it could likewise bring objects many light-years away into sharp focus.  Turning the Mewlon 210 focusing knob as far as it would go in either direction, I still could not achieve sharp focus with either the 12.5mm EPs or the 24mm EPs.

 

However,when I removed the Tak diagonal and attached the binoviewer directly to the telescope, I did achieve sharp focus within the Mewlon 210's focusing range.  When I re-attached the diagonal and pointed the telescope to objects closer to me, in this case pine trees about half the distance of the far ridge, I also achieved sharp focus.

 

[As an aside, several CN reviewers and commenters have experienced fits of ecstasy when observing celestial objects with the Mewlon 210, declaring the view only slightly inferior to what can be seen with the Hubble Space Telescope.  That was not my experience.  At best, my straight-on views of the distant ridge, using the 12.5mm Docters (193x), were soft, not as crisp as I expected.]

 

So what do I need to do differently to observe Jupiter, M51, etc. in all their glory?  Was the binoviewer + accessories not assembled properly?  Why could I not achieve focus with the diagonal attached, when I could get sharp images without it?  Why could I see closer objects sharply with the setup, when it could not achieve sharp focus on more distant objects?  I suspect I need a different glass path correcter, say the Baader 1.7x, or even the Baader 2.6x glass path corrector.  Would that solve the problem?

 

Or should I go to Plan B, which is to chuck the whole setup and go back to using my APM binocular-telescope, the best instrument I ever ever owned for stargazing?   

 

 

 

 

 

 



#2 bobhen

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Posted 25 August 2019 - 03:40 PM

First: Forget judging optical quality during the day. The daytime seeing can/will play havoc with any 210mm scope.

 

Second: Try removing the glass path corrector and using a mirror diagonal.

 

Third: check collimation using a single eyepiece

 

Fourth: Use a single eyepiece to judge optical quality

 

Fifth: Before judging optical quality make sure the scope is acclimated.

 

I have a Mewlon 210. However, I don’t use it with a bino-viewer. But I do use it with a 2” mirror diagonal, a 2” reducer and an image intensifier and that comes to focus.

 

Bob


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#3 Kunama

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Posted 25 August 2019 - 03:43 PM

Yes, you need a stronger GPC or you need to shorten your optical train as you don't have quite enough back focus.


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#4 Kunama

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Posted 25 August 2019 - 03:49 PM

Your Takahashi diagonal with its eyepiece clamp is not the shortest way to go.  

I use a Baader T2 Maxbright or a Baader Zeiss T2 diagonal with the Baader HD Quick Connect to join my Zeiss binoviewer and my Mewlon 210.

Less back focus is required for objects closer to you, so you can focus on the middle distance.

The location of the field stop in the eyepiece will also have a bearing on whether they will come to focus.

 

P.S.  ONLY the HST will give Hubble-like views, binoviewers on a Mewlon will dim the view quite a lot but being able to use both eyes has the effect of regaining much of that lost brightness due to a thing called binocular summation, the sharpness of the image during daytime has nought to do with what the scope can show at night.  There are too many thermal effects during daytime, heat plumes rising from anything the Sun is shining on etc.

 

A huge benefit of binoviewing with either binoscope or binoviewer attached to a single scope is the lack of eye strain thus you can observe longer so you don't miss those times of perfect seeing.

Most people using a single eyepiece will view for less than a minute before moving their eye from the eyepiece, with binoviewers I have viewed much much longer without shifting my gaze from the target...

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Edited by Kunama, 25 August 2019 - 04:01 PM.

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#5 jdown

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Posted 25 August 2019 - 06:29 PM

Bobhen & Kunama,

   Thanks for the tips - it seems I do need more back-focus and/or perhaps the Baader diagonal.  I assume, Kunama, that that is the diagonal shown in your photo?  BTW, what weight are you using for your counterweight? 



#6 dr.who

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Posted 29 August 2019 - 02:50 PM

Late to the party but I agree with both Bob and Matt. The key is, in part, the diagonal you are using. You need to switch to the full T2 system. I use the Baader Amici prism, the T2 2" nosepiece for the Mewlon side going into the compression fitting to threaded 72mm (I think) side that comes with the Mewlon and the 1.25" fitting on the BV side. I also put the Baader 1.25x GPC in the BV side. I have zero problem bringing my 210 to focus with the Tele Vue BV's which are some of the longer focal length BV's out there.

 

Secondly, the Mewlon is nowhere near as good as the HST! However when it is properly cooled and colimated it will provide you with very good views. The key is to cool it. Viewing in the daytime is going to give you mushy views due to heat. The BV's will also do very well and you will enjoy the views. I am a big fan of both the 210 and 250 for things that require a reflector in order to see well. 



#7 Kunama

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Posted 29 August 2019 - 04:06 PM

Bobhen & Kunama,

   Thanks for the tips - it seems I do need more back-focus and/or perhaps the Baader diagonal.  I assume, Kunama, that that is the diagonal shown in your photo?  BTW, what weight are you using for your counterweight? 

I have also just added a Baader 4-in-1 visual back which now means I can reach focus without any GPC, so my components in order are:

 

Mewlon210 +

custom M71>M68 adapter +

Baader 4in1 #2458190 +

Baader 2" Nosepiece #2408150 +

Baader Zeiss T2 Prism #2456095  +

Baader HD Quick Connect #2456322 +

Zeiss Binoviewer with Baader 1.25" Clicklocks +

Tak LE eyepieces cool.gif

 

My counterweight is a 10Kg polished s/s by Robin Casady..... waytogo.gif



#8 ckwastro

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Posted 17 September 2019 - 07:31 AM

Very late joining the thread. I hope you got the system dialed in.

 

I use a Denk binoviewer in my Mewlon 210, with the 1.2x corrector that screws into the field end of my AP MaxBright 2” diagonal. Easy setup and never had any issue reaching focus with any eyepiece set. These are all night observations. I never use it during the day. 




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