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How to bench collimate a Mewlon

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#1 dag55

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Posted 01 December 2012 - 07:02 PM

Thanks and credit to Fred at Texas Nautical for this procedure that he developed.
Here is the procedure,You get a 2ft.x2ft. piece of white paper board and drilled a 3/8" hole in the center. You then set up the OTA on my mount pointed directly at you at eye level with the sec. mirror holder. This is about 50 or so inches away and look straight down the OTA at the center of the back of the sec.you will see the spider vanes and then the spider vanes reflected in the primary mirror.As you look at them if you are the right distance you will see what looks like a black doughnut which is the baffle tube. Now that you see it position yourself the correct distance to super impose the sec. holder into the black ring which is the baffle tube - primary ring nut, sit where the sec. holder and the pri. baffletube and nut are the same size, you move father away the ring nut appears larger and conversely smaller when you get closer..Now this sounds crazy until you look at the OTA this way then it will make sense. Once you see what I have described you can take the white paper board and hold it close to your face as you look through the hole in it into the axis of the scope.The white will make it easy to see the spider vanes and the pri. baffle-retaining ring nut.As you look down the optical axis you will see two complete sets of spider vanes, one set is the actual vanes the other set is the vanes reflected in the primary. Ok we are at the point of checking collimation, does the vanes align directly which each other or are they offset be sure you are looking straight down the center of the optical axis by keeping the sec. holder in the center of the prim. baffle-ringnut. On mine the three vanes would appear as six until I got it coll.You have to be steady as you do this then when you determine which direction your reflected vanes must be shifted to be in line with the actual vanes you can use the col. screws to make the adjustments.On mine the screws opposite of the of the direction I wanted to move the toward I would tighten..After you adjust it and see which way it the reflected image of the vanes move it will be easier to get them to line up. It helps to have a light illuminating the white paperboard and a light illuminating the back of the sec. mirror holder. This will require patience but once you get the hang of it you will be able to coll. the Mewlon almost perfectly while sitting comfortably indoors.

I hope this helps Dane

#2 mich_al

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Posted 01 December 2012 - 10:41 PM

Printed & saved for later. Thanks for posting.

Al

#3 Fomalhaut

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Posted 02 December 2012 - 05:01 AM

A year ago, I collimated my Mewlon using the method described much more comfortably plus exactly than would be possible by using a star.

By the way, this method works better on a Cassegrain with spider vains than on one without, because coinciding the spider vains with their reflections is much more precise than just monitoring the concentricity of the "doughnut".

So, IMO using the method described, Mewlons are collimated more easily and more precisely than SCTs.

Chris

#4 Dan McConaughy

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Posted 02 December 2012 - 09:14 AM

RCOS posts a video on collimating a R-C using the Tak collimation telescope. It works quite well and can be done in a smaller area. I would suggest still doing a final collimation on a star as the telescope is likely horizontal when collimated on a bench but is tilted up in use.

#5 Mark9473

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Posted 03 December 2012 - 07:28 AM

I have difficulty understanding the text written above, to be honest. Is there a "for dummies" version somewhere?

edit: OK I just went to have a look and here's what I see:
Looking into the scope from the front I see the main mirror, then a black ring which I suppose is the baffle, and then a bit more of reflective surface between the black ring and the edge of the secondary mirror holder.

I can then sit a bit closer so that the edge of the secondary holder touches the inside of the black ring, or if I move even closer it can touch the outside of the secondary ring. At both instances the spider vanes align with their reflections.

Is this the correct procedure? What then is the white board with the 3/8" hole in it for?
If what I did is correct then I have to say it's a great and simple procedure!

#6 Ed Holland

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Posted 03 December 2012 - 07:01 PM

By the way, this method works better on a Cassegrain with spider vains than on one without, because coinciding the spider vains with their reflections is much more precise than just monitoring the concentricity of the "doughnut".

Chris


Shooting from the hip, this gives me an idea. How about we add temporary "spider vanes" to our SCT for collimating purposes? Fine thread or wire, stretched in front of the centre of the corrector & secured with tape should suffice.

I'll try this out later to see what can be seen. Maybe even capture a pic or two.

My apologies if this post intrudes on the original subject matter of the thread.

Ed

#7 dag55

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Posted 05 December 2012 - 06:35 PM

The white paper board with the 3/8" hole is to help you see the actual spider vanes and their reflections clearly. Also looking through the hole helps you look directly down the central axis of the optics, if you're not centered in the optical path the collimation will be off, I hopes this helps. Dane

#8 jrbarnett

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Posted 25 January 2013 - 11:29 PM

That method requires a center spotted secondary. The RCOS has one. The Mewlon 210 does not.

Regards,

Jim

#9 Mark9473

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Posted 26 January 2013 - 06:18 PM

I'm curious why you say that, Jim. Care to elaborate? I've tried this method and don't seem to miss a center spot.

edit: I just now realise you're referring to the use of the Tak collimation scope mentioned above, and not to the method outlined in this thread. Sorry for that.

#10 dweller25

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Posted 27 January 2013 - 05:12 AM

Thanks and credit to Fred at Texas Nautical for this procedure that he developed.
Here is the procedure,You get a 2ft.x2ft. piece of white paper board and drilled a 3/8" hole in the center. You then set up the OTA on my mount pointed directly at you at eye level with the sec. mirror holder. This is about 50 or so inches away and look straight down the OTA at the center of the back of the sec.you will see the spider vanes and then the spider vanes reflected in the primary mirror.As you look at them if you are the right distance you will see what looks like a black doughnut which is the baffle tube. Now that you see it position yourself the correct distance to super impose the sec. holder into the black ring which is the baffle tube - primary ring nut, sit where the sec. holder and the pri. baffletube and nut are the same size, you move father away the ring nut appears larger and conversely smaller when you get closer..Now this sounds crazy until you look at the OTA this way then it will make sense. Once you see what I have described you can take the white paper board and hold it close to your face as you look through the hole in it into the axis of the scope.The white will make it easy to see the spider vanes and the pri. baffle-retaining ring nut.As you look down the optical axis you will see two complete sets of spider vanes, one set is the actual vanes the other set is the vanes reflected in the primary. Ok we are at the point of checking collimation, does the vanes align directly which each other or are they offset be sure you are looking straight down the center of the optical axis by keeping the sec. holder in the center of the prim. baffle-ringnut. On mine the three vanes would appear as six until I got it coll.You have to be steady as you do this then when you determine which direction your reflected vanes must be shifted to be in line with the actual vanes you can use the col. screws to make the adjustments.On mine the screws opposite of the of the direction I wanted to move the toward I would tighten..After you adjust it and see which way it the reflected image of the vanes move it will be easier to get them to line up. It helps to have a light illuminating the white paperboard and a light illuminating the back of the sec. mirror holder. This will require patience but once you get the hang of it you will be able to coll. the Mewlon almost perfectly while sitting comfortably indoors.

I hope this helps Dane


It does help (and work) Dane - thanks :)

#11 Don Taylor

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Posted 27 January 2013 - 04:37 PM

Just used this technique to collimate my vixen vmc200l. I'll star test as soon as the weather cooperates. I first used a laser to align the focuser with the secondary spider and then to align the secondary mirror with the focuser. Then the Texas Nautical Repair method discussed in this thread to do the primary back to the secondary.

Thanks for starting this thread - it has helped a lot!






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