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8" refractor RFT almost done

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#1 fishhuntmike

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Posted 17 February 2018 - 02:03 PM

Last July i purchased a 204mm f5.9 lens (Istar).  In the last month i finally am getting around to putting a scope together.  I built a simple test assembly to determine scope length, baffle location, and size.  I tested the assembly during the day on a distant mountain and was pleasantly surprised at the mostly CA free view at ~35x with the 35 panoptic.  Nice and sharp to almost the extreme edge.

 

After cutting most of the pieces out of 1/2" baltic birch plywood i started the assembly yesterday (minus the rear element, which will be 3/4" plywood).  I glued the baffles and later the lens piece to a single piece of ply first, and this morning glued the rest together (rolled up into the hexagonal bundle).

I will have to sand the focuser end flat and square before measuring and cutting the rear focuser hexagon piece out of 3/4 plywood.  Then i will predrill and glue in nuts for the 3" feathertouch focuser. Followed by glueing it to the scope tube.  

 

After that lots of sanding, preassembly to determine center of balance and location of losmandy D-plate, and drilling of those holes.  Lastly, stain and finish and flat black interior.

 

I getting excited to look through this widefield beast!!!

 

Thought Id share the progress!

 

Thanks for looking

 

Mike

 

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#2 Taosmath

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Posted 17 February 2018 - 02:52 PM

Since you've glued up the tube already, I assume you're going to spray paint the inside rather than flock it?

 

Any idea yet on what the total weight will be ?

 

Colin



#3 dan_h

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Posted 17 February 2018 - 04:07 PM

Looks great. I have used similar construction techniques for several scopes but I tend towards 1/4" ply. Never had a problem with insufficient stiffness but I wouldn't see it visually anyway and I never had an 8". Looking forward to seeing your scope finished. 

 

<< I tested the assembly during the day on a distant mountain and was pleasantly surprised at the mostly CA free view at ~35x with the 35 panoptic.>>

 

Unfortunately, daylight testing does not give a good indication of CA in an RFT. Your test setup gives an exit pupil of 5.8mm. In day light conditions your eye pupil would be quite a bit smaller and stops the aperture significantly.  If your eye pupil was 2.9mm, then you are effectively using a 4" aperture, (102mm) at f11.8 so CA would seem to be much less than it would with a larger eye pupil. If your pupil was as small as 2.5mm, CA will appear to be very well controlled as you would only be using 0.43 of the aperture or 87.9mm @ f13.69, so mostly color free. 

 

dan


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#4 fishhuntmike

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Posted 17 February 2018 - 04:28 PM

Colin,

Yes i will spray paint the inside. I will try to get an estimate of the tube weight soon and post it.

Dan,

I considered 1/4" ply but since this my second woodworking project i do not consider myself good enough for precision woodworking nor do i have a shop or real table saw so i figure 1/2" is more tempermental for errors.

Thanks for the info in CA. You are correct. I was "testing" the optic only to determine focuser position at the ~infinity position and not actually judge optical performance.

Mike

#5 junomike

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Posted 17 February 2018 - 04:38 PM

Amazing!

I'd love to see through one of these (8" F6 Achro) but alas my 7" ED will have to suffice.



#6 fishhuntmike

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Posted 17 February 2018 - 07:48 PM

Colin,

Minus diagonal and eyepiece the scope OTA will (unfortunately) be around 42-44 lbs or so which is about 10# heavier than i anticipated but by all means not an issue for me or my mount. It could double as a step to get into my travel trailer or a jack stand for changing a tire. I guess its overbuilt a little.
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#7 plyscope

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Posted 17 February 2018 - 11:25 PM

Mike well done on another great wooden scope.

What is the weight of the lens in cell?



#8 fishhuntmike

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Posted 18 February 2018 - 12:15 AM

The lens cell weighs approximately 12 lbs.
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#9 opticsguy

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Posted 18 February 2018 - 01:23 PM

Cool project and you will enjoy your new scope just as much as it is cool.

 

You will also learn a lot about construction techniques and design.  

 

There are many ways to lighten up your scope, something to think about down the road if you have to move the OTA every time you go out to observe.

 

I like what you are building and will enjoy to watch your progress here and your fun outside observing.

 

ENJOY!!

 

My edit here:  Where are you located and any plans for your scope at a star party later this year?


Edited by opticsguy, 18 February 2018 - 01:26 PM.


#10 Taosmath

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Posted 18 February 2018 - 01:40 PM

Mike,

 

thanks for the info.  I am just beginning my venture into Refractor land - my first refractor will arrive on Wednesday.  I looked at a bunch of options and also considered an ATM build  (something I will eventually do if I can find a good objective) but OTA weight was a dominating factor since I want to use it on a Nexstar 6/8 mount , so 44# is about 3 times too heavy for that (and probably 50% to much for my CG5 even!).

 

Thanks & let us know how it works out.

 

Colin 



#11 ed_turco

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Posted 18 February 2018 - 02:02 PM

I wonder about the comparitive lack of color; at this aperture and at f/5.9, it ought to be a beast.

 

Thev 35mm eyepiece you are using would produce a roughly 6.0mm exit pupil, that in the daylight, would be stopped down by your eye's pupil to about 2mm.  This converts your f/5.9 system to an f/16!

 

In dark skies, I think you'll be able to ignore secondary color with all those stars in your field.  I hope.

 

 

Ed



#12 fishhuntmike

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Posted 18 February 2018 - 03:11 PM

Opticsguy, Im located in the Tricities. One of these days i would like to go to the Sun Lakes/Dry Fall starparty that i believe i have read that you attend and see your giant refractors! I spend a lot of time in that area and have spent many nights our under the stars on flyfishing trips up there for 35 years now.

Colin, Congrats on getting your first refractor.

Today i will finish gluing up the scope and tomorrow i can assemble it to find the balance point, drill holes for the D plate, and spray paint the interior.

#13 fishhuntmike

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Posted 19 February 2018 - 04:25 PM

Today the beast is all glued up (unsanded, in the rough) spraypainted flat black on the interior, and Losmandy D plate installed.

 

Tonight I will collimate it and check out M42 and a few other things in the back yard if it is clear.

 

Definitely not hard to manage and balances nicely on the G11 with the two 21# plates not even close to the outer end of the balance rod.

 

Not sure when i will finish it.  Probably wait for good warmer weather to stain and coat it.  Its going to get down to 17F tonight.

 

 

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#14 dan_h

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Posted 19 February 2018 - 06:14 PM

Today the beast is all glued up (unsanded, in the rough) spraypainted flat black on the interior, and Losmandy D plate installed.

 

Tonight I will collimate it and check out M42 and a few other things in the back yard if it is clear.

 

Definitely not hard to manage and balances nicely on the G11 with the two 21# plates not even close to the outer end of the balance rod.

 

Not sure when i will finish it.  Probably wait for good warmer weather to stain and coat it.  Its going to get down to 17F tonight.

I'm usually not a fan of hex tubes but yours looks great.   Nice job on the joints.

 

dan



#15 fishhuntmike

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Posted 19 February 2018 - 11:27 PM

My scope turned out spot on for collimation (both focuser and lens) so i must have square things up pretty well when i glued it together plus i got lucky too.

 

Set it up in the back yard (this thing is much easier than my C14) with no mount power and the 35 panoptic (wasnt worried about running the drive at 34x).  Unfortunately the moon was out so things were not really dark.  I measured the mpsas at 19.89 with the sky quality meter.  Pointed it at M42 and was immediately impressed with the green cobwebby nebula stucture.  The trapezium wasnt perfect, nor were some of the perimeter stars so i went and put my new eyeglasses on (corrected for astigmatism) and the entire field of stars sharpened up to tiny pin pricks all the way to the extreme edge!  I spent about 10 minutes admiring the pristine view before i started freezing.  My kid was going to bed and i needed to bring the scope back inside.  

 

 I already have two premium 5" apo  refractors (AP &Tak), a 6" f15 achro, a c5 and c14.  I came inside the house with a big grin on my face tonight because this scope does what i wanted it to do (RFT) and it does it spectacularly!  Cant wait to get it out somewhere really dark when it is not so cold out.  

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#16 Jim Curry

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Posted 20 February 2018 - 06:49 AM

Great job.  Gives me an alternate tube construction to think about.



#17 Mark Harry

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Posted 20 February 2018 - 08:32 AM

Wow, nice!!!



#18 Jim Curry

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Posted 15 May 2018 - 08:35 PM

What did you use for joint glue?



#19 fishhuntmike

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Posted 16 May 2018 - 03:57 AM

Gorilla glue


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