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Milben Zoom-to-RFT Conversion

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#1 ftwskies

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Posted 03 December 2016 - 08:15 PM

Ladies and gentlemen, I'm proud to introduce...  my new "COROnada"!  :lol:

 

This is my recently completed conversion of the Milben (Towa) 60mm terrestrial click-zoomer time capsule I acquired a few months ago into an RFT format.  My plan is to use it primarily as a grab-and-go white light solar 'scope, hence the color scheme and goofy name.  This scope will travel with me and my family for the 2017 eclipse.

 

 

gallery_240021_6638_10986.jpg

 

The primary is a 60mm x 400mm f/6.67.  I purchased the (sadly, all-plastic) 1.25" focuser from Telescope Warehouse; I think the same unit is available from a number of outlets.

 

In addition to chopping the tube, I added a number of enhancements: blackened lens edges, fully flocked tube and shade, and flat black-painted objective retainer.  I'm hoping to get as much contrast out of this thing as I can.

 

Now I just need to fabricate a nice solar filter cell and a set of rings.  I plan to make the filter cell so that it slides snugly inside of the shade, both for aesthetic purposes and to protect the Baader film.  The rings I'll make out of plywood, with a Vixen-style dovetail base in case I ever get a new mount for it.  (For now it will mount on a camera tripod.)

 

Below I'll post more pictures of the step-by-step, in case someone else out there wants to do a similar conversion... 


Edited by ftwskies, 03 December 2016 - 09:14 PM.


#2 ftwskies

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Posted 03 December 2016 - 08:16 PM

The first step in the conversion was determining the native focal length of the objective.  I whipped up this little bench to help me get an accurate measurement.  The cell rode between the two rails, and the carpenter square helped me keep the cell square to the bench and to transfer the cell position onto the rails for accurate measurement later.  I rested the cell in the rails and against the paper and traced its edge; tilting the cell to center the focused image (I took it outside on a clear night and focused an image of the Moon) in the circle assured me that I had the objective square to the target for maximum accuracy.  The carpenter square let me transfer the cell position onto the rail, so I could later measure the focal length as the distance from the rear of the cell to the target when the image was focused.  (Actually, the rear surface of the objective is tucked about 10mm up inside the rear of the cell, so I added 10mm to the measurement.)  Using this rig, I was able to measure the objective focal length as ~400mm (f/6.67).

 

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#3 ftwskies

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Posted 03 December 2016 - 08:18 PM

D-day:  The patient is prepped for surgery!  Fully extended, the Milben zoomer was ~29" long.  Of course, that's way longer than the FL of the objective; the design relied upon those crappy little lenses in the zoom focuser to stretch the tail end of the light cone -- which is the whole raison d'etre for the conversion.  I kept the Milben focuser for grins, in case I ever want to replace the original tube and revert back to the stock zoom design.

 

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Here's the Milben zoom focuser next to the new 1.25" unit:

 

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#4 Chuck Hards

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Posted 03 December 2016 - 08:19 PM

Well-done!  Beautiful OTA!



#5 ftwskies

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Posted 03 December 2016 - 08:20 PM

After a lot of careful measurement and calculation I determined that I needed to chop the tube at exactically 200mm length.  The focuser has a total of 57mm of outfocus available; my longest eyepiece is 40mm and my shortest is 5mm.  I needed the tube to be short enough to accommodate a 1.25" prism diagonal with my shortest eyepiece (5mm) and still have a little bit (~10mm) of infocus margin left over.  The 5mm EP's focal plane is about even with the shoulder where its chrome tube meets its body -- in other words, right about at the top of the eyepiece tube on the diagonal.  This made figuring out the length through the focuser and diagonal fun and easy by using one of my favorite toys:  a Bosch DLR165 laser rangefinder.  I set the focuser with about 10mm of infocus remaining, installed the diagonal, and then installed a white plastic plug into the diagonal's eyepiece tube so that the plug was flush with the top of the eyepiece tube; this gave the laser a target to bounce off of right where I want to hit focus for a 5mm eyepiece.  Then I just squared the rangefinder up to the front of the focuser, shot the laser through the diagonal and up to the plug, and it read the light path accurate to 0.1mm. Then all I had to do was subtract the distance from the front end of the focuser drawtube to the shoulder where the focuser meets the tube when it's installed, and I had the total length from the back of the chopped tube to the EP focal plane.  Subtracting this length from the 400mm FL of the objective gave me the length I needed to chop the tube at.  (Thanks to Chuck Hards for helping me figure this out!)  I wrapped a piece of paper around the tube to mark my cut line, so I knew it was square.

 

Here's the graphic violence and gore:

 

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The abrasive metal cutting blade I used in my miter saw left a rough edge, but it cleaned up nicely with some careful file work.  I polished the filed edge with sandpaper to de-burr and break the sharp edges; not only does this make it safer to handle, it also gives better results when it comes time to prime and paint.



#6 ftwskies

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Posted 03 December 2016 - 08:23 PM

With the tube cut to length, it was time to strip!  (The paint, that is.)  :lol:

 

I picked up a can of spray-on airplane paint remover from the local auto parts retail.  The stuff really did a fantastic job!  The wrinkles seen in the picture below formed within five minutes of application -- the paint practically fell off once I gave it a whole ten minutes to work.  No sanding required, I had a nice smooth, clean shiny aluminum tube in no time!  A quick rinse with water neutralized any residue of stripper, and a quick lick with a scotch-brite pad gave the tube surface just enough tooth to assure a good finish.

 

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#7 ftwskies

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Posted 03 December 2016 - 08:24 PM

One last thing to do before applying prime and paint:  I needed to drill the three small holes for the focuser screws.  After double-checking to make sure that the new focuser's holes were evenly spaced (they were), I used a soft tape measure to determine the outer diameter of the tube and divided by three.  I marked the edge of the tube where each hole should go, then used a caliper to measure the gap between the holes and the shoulder of the focuser.  Again, I used a piece of paper wrapped around the tube to mark a circle intersecting the three location marks so that all three holes would be co-planar.  In order to keep the drill bit from wandering and buggering up my nice shiny tube, I first used a punch to drive a small dent where each hole would go.  Once the holes were drilled, I used a small pointy file to de-burr and polish the edges.

 

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#8 ftwskies

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Posted 03 December 2016 - 08:26 PM

Time for prime and paint! 

 

These steps took the longest, since I wanted to give each coat plenty of time to dry and cure.  I couldn't find the oft-recommended "self-etching primer", but I did find something called bonding primer at my local auto parts retail.  It did a good job of adhering to the smooth aluminum tube and provided a nice bed for the paint.  I masked the objective cell threads, and then two light coats of primer were followed by three coats of Testor's spray enamel gold.  I followed this with three coats of Testor's gloss clear spray lacquer.  Once the finish had cured, I applied some Meguiar's wax and gave the tube a nice buffing using a wheel in my hand drill.

 

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#9 ftwskies

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Posted 03 December 2016 - 08:29 PM

Here are some of the other enhancements I incorporated prior to assembly.  I used some Testor's flat black paint to carefully blacken the edges of the primary with a fine artist's brush.  (The masking tape on each lens was to remind me which sides of each lens should face out when re-assembled.)  I also flocked the tube (you can see before and after below), flocked the inside of the shade, and blackened the inside of the retaining ring.  (You can see how shiny it was before paint in the picture below.)

 

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#10 ftwskies

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Posted 03 December 2016 - 08:30 PM

After cleaning the lenses, I assembled the finished 'scope earlier this evening.  As you can see, it's a lot shorter now than the original zoomer configuration.  I can't wait to get rings and Baader cell made and try this baby out on the Sun!

 

gallery_240021_6638_10986.jpg

 

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Edited by ftwskies, 03 December 2016 - 08:37 PM.


#11 ftwskies

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Posted 03 December 2016 - 08:31 PM

Well-done!  Beautiful OTA!

Thanks, Chuck!  Couldn't a'done it without your help, though. :waytogo:



#12 Chuck Hards

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Posted 03 December 2016 - 09:16 PM

You may find that a mirror diagonal introduces less spurious color than a prism, with this short objective. 



#13 ftwskies

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Posted 03 December 2016 - 09:38 PM

I'll consider it, but my plan for solar viewing is to use a color filter on the eyepiece along with the Baader film, so I'll essentially be monochromatic anyway.



#14 Chuck Hards

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Posted 03 December 2016 - 09:40 PM

If this is to be a dedicated white-light solar scope, consider just cutting a piece of Baader filter material to the same OD as the objective, and placing it behind the rear objective element.  The cell, objective, and retainer will hold it in position, and the solar energy concentration immediately behind the objective will be negligible.  It keeps the Baader film from ever being touched or otherwise exposed to damage.   Works well, I've done it with 60mm scopes before.  And no filter cells to make.



#15 Tenacious

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Posted 03 December 2016 - 10:05 PM

Really Nice! 

 

I have a plastic focuser assembly on one of my scopes.  It's light weight. smooth, and plenty rigid.  It functions well.

 

I like your how-to on cutting the tube to length.



#16 Bomber Bob

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Posted 03 December 2016 - 10:36 PM

Great job!  Yes, the illustrated guide will help lots of folks.  Making the abandoned, obsolete vintage into a functional & beautiful scope is too cool - and purdee, too!



#17 Terra Nova

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Posted 03 December 2016 - 11:28 PM

That is very nice work! 



#18 Vesper818

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Posted 04 December 2016 - 12:01 AM

Very nice, and clever name!😄
What a great tutorial!

#19 ftwskies

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Posted 03 January 2017 - 10:08 PM

Tonight I finished the rings I built specifically for this telescope.  I built them out of plywood.  The thumbscrews mate into square nuts that are mortised into the bottom of the wings and pookied with JB Weld to fill out the shape.  I have a brass 1/4-20 insert in the bottom of the block for mounting on a camera tripod, but looking ahead I shaped the block to fit a Vixen shoe so if I want to I can get a newer mount for it in the future (or maybe use the one that came with my new AstroMaster).  This kit is nearly complete now; I plan to get a Baader ASTF and solar continuum filter for it, and a set of Expanse EPs.  I already have a case picked out that will hold the whole kit (minus the tripod).  Ultimate RFT/solar G-n-G!

 

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#20 Vesper818

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Posted 04 January 2017 - 11:13 AM

What a beautiful transformation! These seem to be good lenses all around! My Swift 818 is assembled for testing. "Pookied" what a great word! I've done it often enough with glue and putty, usually involving stuck fingers and spots on the table. Never knew the term for it.

#21 ftwskies

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Posted 04 January 2017 - 12:10 PM

Thanks, Carol.  JB Weld is amazing stuff.  Not only does it provide a strong bond, but it can be tooled and shaped like wood or metal once cured. (Admittedly, it can be a little messy to mix and work with -- it has the consistency of gooey cake frosting until it cures.)  I've used it to embed square nuts before -- like on my crutch tripod -- but this was the first time I needed to fill out a shape before.  I wanted the nuts to be invisible on this build, but they were smaller than the thickness of the plywood, so I used masking tape applied to each side of the wings to form a sort of reservoir around the nuts.  I installed bolts through the nuts to ensure the nuts were properly located over each hole, then used a sharpened popsicle stick as a trowel to carefully nudge the epoxy all in around each nut, trying to pop any bubbles or voids and being careful not to get too much on the threads, etc.  I filled each cavity to a little proud of flush, and once it was cured (24hrs) I removed the bolts and tape and used a drum sander to carefully grind off any excess back to flush.  Worked like a charm!

 

Now that I finally have a steady mount for the scope, I can't wait to get some real use out of it!  It's light enough that it should do fine on the Slik tripod / Paragon Pro that it's mounted on in the pictures for now, but ultimately I hope to get a new/better mount for it.  (That tripod used to carry my heavy OT C90 without counterweights and was just within the limit of useability, so I don't see it having any trouble with this lightweight kit.)



#22 deSitter

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Posted 05 January 2017 - 07:41 AM

Ladies and gentlemen, I'm proud to introduce...  my new "COROnada"!  :lol:

 

This is my recently completed conversion of the Milben (Towa) 60mm terrestrial click-zoomer time capsule I acquired a few months ago into an RFT format.  My plan is to use it primarily as a grab-and-go white light solar 'scope, hence the color scheme and goofy name.  This scope will travel with me and my family for the 2017 eclipse.

 

 

gallery_240021_6638_10986.jpg

 

The primary is a 60mm x 400mm f/6.67.  I purchased the (sadly, all-plastic) 1.25" focuser from Telescope Warehouse; I think the same unit is available from a number of outlets.

 

In addition to chopping the tube, I added a number of enhancements: blackened lens edges, fully flocked tube and shade, and flat black-painted objective retainer.  I'm hoping to get as much contrast out of this thing as I can.

 

Now I just need to fabricate a nice solar filter cell and a set of rings.  I plan to make the filter cell so that it slides snugly inside of the shade, both for aesthetic purposes and to protect the Baader film.  The rings I'll make out of plywood, with a Vixen-style dovetail base in case I ever get a new mount for it.  (For now it will mount on a camera tripod.)

 

Below I'll post more pictures of the step-by-step, in case someone else out there wants to do a similar conversion... 

 

I would be very interested, as I have 3 of these NIB bought for the tripod legs. I was pleasantly surprised with the build quality of the tube and converted this way it should be an excellent finder. How did you accurately cut the aluminum tube? I assume with something like a plumber's pipe cutter..

 

-drl


Edited by deSitter, 05 January 2017 - 07:43 AM.


#23 ftwskies

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Posted 05 January 2017 - 10:07 AM

Look through the thread, drl, I gave a step-by-step pictorial.  See post #5 for the answer to your question.



#24 deSitter

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Posted 05 January 2017 - 10:43 AM

Thanks! I think I can fit the focuser from my Penncrest onto this as needed for the time being. Will be looking for dead 60mms to cannibalize the focusers! How does it perform color-wise? 60mm f/7 is Sidgwick criterion, should be pretty good!

 

-drl


Edited by deSitter, 05 January 2017 - 11:48 AM.


#25 ftwskies

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Posted 05 January 2017 - 12:41 PM

It's decent -- hardly an apo, but good enough for my tastes.  It's not going to beat my Molly (f/11) or my yet-to-be-affectionately-named 70mm (f/13), much less my Dob.  If I need better color performance I'd probably use one of those.  This scope is primarily going to be used for monochromatic solar viewing, and maybe a little low-power Milky Way surfing in the summer, so color's not really going to be an issue.

 

You want to be judicious in your selection of a focuser; I followed good advice that said to look for a focuser with a short(er) drawtube.  If the drawtube is too long or has too much in-travel then it can cut into the fatter light cone of the fast objective.  Frankly, the hardest part of the whole conversion for me was completing (and double-checking) the measuring and math to make sure the finished tube length can still put the focal plane at the field stop of any EP you plan to use with it.  Leave the tube too long, and you may not be able to rack the focuser in far enough to get shorter EPs into focus; cut it too short and you may not be able to rack out far enough for longer EPs (plus the drawtube may interfere with the light cone).  I discuss my approach to this in post #5, but I didn't provide any of my diagrams illustrating my work.  What helped me a lot was getting a big 11x17 sheet of graph paper and actually drawing out a 1:1 sketch of the optical tube, focuser, diagonal and light path.  A cheap digital caliper I bought at the big-box home improvement store was an invaluable tool, too, for making critical measurements accurately.  My favorite "cheat" on the whole thing, though, was the use of the laser rangefinder.  I realize not everyone has one of those handy -- and don't get me wrong, the job is still very doable without one -- but since it uses a beam of light to measure distance it was the perfect tool for eliminating any ambiguity about the actual light path through the focuser, diagonal prism, and up into the EP, which gave me a lot of confidence in my measurements and calculations.  I was still holding my breath a little before first light, but was rewarded with good, crisp views through both my shortest (5mm) and longest (40mm) EPs.




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