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Goto 8" Newtonian (Mod 207?) Redux

classic reflector
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#1 Stew44

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Posted 25 July 2017 - 06:00 PM

I spent a delightful couple of hours at apfever's today.  First thing encountered was the mirror box of a 36" truss dob.  Many more wonderful instruments of astronomical exploration were present, but my focus was on a Goto 8" reflector that Neil first displayed here towards the end of 2014.   He graciously parted with it today and I will be spending quite a bit of time on it going forward.  We were able to sort out quite a bit about the scope, and there will be a lot of additional research on my part.  For the most part though, after enjoying the Edmund Super Space Conqueror 6" f/8 I got last week, what better could one do who appreciates Goto telescopes but acquire a little more aperture and a Goto at that. grin.gif

 

Quick take is the mirror is 210mm, focal length 1575 or f/7.5.  Neil originally estimated f/7.  Mirror is 1.25" thick.  The back of the mirror (doesn't photograph well at all) says Goto, 1959 3, and an F number that I thought might be focal length but now think is likely a serial number - F3235.  The tube is heavy walled and heavy, period.  It reminds me of some of the early Russian Mak Newts.  Neil pointed out the interesting collimation screws in his earlier thread, and really the cell is one of the more interesting aspects of the scope.  Some serious thought will be required as to actually creating a structure that will settle given that all is enclosed.

 

I only acquired the OTA and the eyepieces.  The finder was not original and I did not take that, nor the Magnuson mount.  The goal will be to restore the tube and look at fabricating a mount similar to the one in the 'instruction' sheet that Neil posted in his earlier thread and that accompany the Mod 205 4.5" and Mod 206 6".  Since I think this is easily a 200 series scope I am tentatively going to call it a Mod 207.   A couple of pictures follow, but also a link to Neil's presentation of the scope to CN in 2014.  And I'll repost the couple of pics he posted in the Mod 204 thread which I'm sure raised some eyebrows.

 

Link to apfevers original post - https://www.cloudyni...or#entry6597203

 

The first is the scope with mirror cell removed and inverted on the top of the tube for storage.  How I found it standing tall among many other tubes standing tall.

 

1.jpg

 

These are the eyepieces.  I did not take the erector T30 eyepiece and it predates my refractors and I don't believe it was part of original acquisition.  I'm sure you will all see it again real soon.

 

2.jpg

 

Finally the focuser that terraclark nailed in the quiz Neil posted as a Goto and quickly came to the conclusion it was a Goto newt.

 

3.jpg

 

And the tube with the bogus finder (and plastic mount bawling.gif)

 

4.jpg

 

I'll get the mirror back in the cell and the cell back in the right end of the tube and see how the optics perform.  Need some rings before I can get it on the heavens.


Edited by Stew44, 25 July 2017 - 09:52 PM.

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#2 Stew44

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Posted 26 July 2017 - 08:41 AM

There is a PA Clausing label on the back of the mirror.  Spoke to Clausing (now HL) this morning.  The label with PA was in use from 81-89.  So the mirror was recoated sometime during that period with their Beral coatings.  More confident that this is original mirror.  The F number I thought was serial when checking again shows F = 3235.   That would make an f/15.4 which certainly this is not.  Any thoughts what might be represented by that number if not focal length?

 

Will mount mirror back in cell today and do some daylight observation.  We're pretty much socked in.

 

P7260520.JPG

 

Also can see the Goto and F=3235 in this picture

 

P7260521.JPG


Edited by Stew44, 26 July 2017 - 08:44 AM.

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#3 Chuck Hards

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Posted 26 July 2017 - 09:13 AM

  The F number I thought was serial when checking again shows F = 3235.   That would make an f/15.4 which certainly this is not.  Any thoughts what might be represented by that number if not focal length?

 

 

Could be radius of curvature (roughtly twice focal length) in mm.   That would yield a focal length of 63.68 inches.


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#4 apfever

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Posted 26 July 2017 - 11:04 AM

Good call Chuck, that makes sense. 

 

Stew, I need serious help with those cell bolts. This picture shows a series of bolts around the inside of the tube, but I don't see matching ones on the outside edge of the cell. I have no idea what I'm looking for, for sure, and I'm not even sure how many. Can you fit the cell with the tube and tell me roughly what the bolts would be?  I should find a matching set here if I know.

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#5 Stew44

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Posted 26 July 2017 - 11:22 AM

Six of them, flat head screw.  One shows here.  They are black.  1/4x20 about 3/4 long I believe or close to that.

 

 

screws Goto.jpg


Edited by Stew44, 26 July 2017 - 02:01 PM.


#6 Stew44

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Posted 26 July 2017 - 04:28 PM

Went to local Ace and picked up some stainless steel flat head hex screws and nuts and put the mirror cell holder back on the right end of the tube.

 

P7260523.JPG

P7260524.JPG

 

Then slid the mirror cell into the holder and put in the pull bolts to hold.

 

P7260525.JPG

 

Then collimated with the pull bolts and the push threaded sleeves

 

P7260526.JPG

 

Finally took a look at the world using an 18mm Pentax SMC Or (86X) and then a Pentax 5mm SMC Or (309X).  The focuser is nice and because of the shortened focuser tube (nothing extends into the scope tube), the draw tube is needed to come to focus.  Looks a little like a microscope.

 

P7260528.JPG

 

Finally a look down the tube.  Wish I had the original finder and the tube ring that dresses the front rim.  Something to look for and create.  Now just need some rings and we'll get on something more interesting than brick mortar joints 100 yards down the street.

 

P7260529.jpg

 

Neil, still would like the original hardware, but no rush!  grin.gif


Edited by Stew44, 26 July 2017 - 08:35 PM.

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#7 Stew44

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Posted 27 July 2017 - 11:56 AM

 

  The F number I thought was serial when checking again shows F = 3235.   That would make an f/15.4 which certainly this is not.  Any thoughts what might be represented by that number if not focal length?

 

 

Could be radius of curvature (roughtly twice focal length) in mm.   That would yield a focal length of 63.68 inches.

 

Chuck, I think you are spot on with this, within quarter of an inch. So is there a discussion of the use of radius of curvature as related to newtonians and parabolic mirror focal length or is it just close enough to spherical that it's acceptable to quote it.  And does it imply spherical or is it used with parabolic as well (perhaps repeating question).

 

Thanks


Edited by Stew44, 27 July 2017 - 12:02 PM.


#8 Chuck Hards

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Posted 27 July 2017 - 12:10 PM

Optical fabricators need to know the radius of curvature when generating and refining the curve on a piece of glass.  Also in single-pass AC testing of mirrors, it is done at the radius of curvature, not at the focal point.

 

ROC (Radius Of Curvature) alone isn't precise enough to distinguish between a sphere and parabola (or other figure of revolution).   An ellipse has two foci.  

 

So I'm going to speculate that it was just Goto's preferred method of shop nomenclature.


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#9 Stew44

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Posted 27 July 2017 - 01:00 PM

Thanks, if acceptable optical practice in 1959 to quote something like that and inscribe on a mirror,  I am feeling pretty good about this being the original mirror for this telescope.  And I will start thinking of the focal length as roughly 1620.  For the 206mm window of the retaining ring of the mirror, that makes this f/7.9 and I will round up to f/8.  So 8" f/8.

 

Very nice.  Thank you


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#10 apfever

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Posted 27 July 2017 - 07:28 PM

Stew,  if I had that mirror here, in the cell or not, I could get you the focal length to within a fraction of a mm very quickly, guaranteed. I do that measurement all the time, without the bench or tester. If I had the cell, I'd get some tapered flat head slotted stainless metric screws for the cell at Ace Hardware as well, if I didn't find the right set of 6 here. Also, one of the two mystery rings that was on top of the tube ended up on the floor here, and the papers are here.  Ace is close but the original cell bolts should be closer. A set of correctly fitted metric bolts would be a perfectly acceptable purchase and a good thing to have anyway as an extra if the originals are JIS threads. It's going to be busy here through the weekend with this rain. The rain is your fault - you just made a purchase of a rather noteworthy scope, what other weather would you expect? 



#11 Stew44

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Posted 27 July 2017 - 08:35 PM

Thanks Neil,

 

I wondered about the second ring. shocked.gif   Thought I had both in hand.  Anyway I think we're close enough in focal length.  It all makes sense.  Only need some rings to get his on a mount at this point and hoping I'll stumble across some, rather than have to get some new from Parallax.


Edited by Stew44, 27 July 2017 - 08:43 PM.


#12 Stew44

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Posted 03 August 2017 - 10:12 PM

Tonight for a few brief moments I got the 8" Goto newt out on the moon.  Thanks again to Dave Trott for locating some 10" rings.  I mounted the scope on my conservatively rated for 35 lbs. EM-200 and added an additional 11 lb. CW to the three that came with it.  Even with the short legs on the mount I needed a step stool to reach the eyepiece.  Clouds were covering and then not and then again.  Took what I could get and worked up to Pentax 3.8mmXP (407X) scanning near the terminator.  Focuser was very smooth, but I 've been spoiled by two speed focusers I'm afraid.  Still it was effective and solidly snapping into focus.  Probably taxing the mount with 40 lb. OTA, but image settled quickly and was quite stable and contrasty.   Clouds covered all after only a half hour or so, but it felt good finally getting this scope back on a mount and in use.  Saturn stayed buried in the clouds the entire time.  Now we'll have a couple days of weather, but hopefully a more lengthy period of viewing over the weekend.

 

P8030548 (2).jpg

P8030547.JPG

P8030546.JPG


Edited by Stew44, 04 August 2017 - 06:41 AM.

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#13 davidmcgo

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Posted 04 August 2017 - 09:14 AM

Nice!  Parallax can roll and weld some pretty thick bands, so if you send him the ID, thickness, and width of what you need for the front as well as where you want countersunk screw holes, he can probably get close in appearance to the rear end cell.

 

I had him make a rear cell attachment band for my 10.25" Dall Kirkham.

 

Dave



#14 Stew44

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Posted 05 August 2017 - 11:19 AM

Thanks Dave,

 

I'm working with a 10" PVC cap that I've painted satin black.  Needs a little felt to tighten to the tube, but it's 1-15/16" deep and does a nice job of balancing the back cell ring when capped.  Works for now and when viewing doesn't matter that ring isn't there.  Another couple of coats and I'll get a picture up during daytime to see the effect.



#15 Stew44

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Posted 16 September 2017 - 10:14 PM

Finished a couple rounded riser blocks for mounting the 10" rings to the Cave mount.  Still have some adjustments to make to the weights so that I can switch off between the Cave 10" f/6 and this heavier Goto 8" f/8 but at least the scope looks a little more balanced and better proportioned with the Cave mount.  Of course when I got it set up clouds quickly moved in.  Another step along the path complete (and I can roll it out of the garage quietly and sneak up on the clouds pretty easily now).  My EM-200 says thank you.  lol.gif

 

P9160659.JPG

 


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#16 Stew44

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Posted 25 October 2017 - 08:28 PM

When I received this telescope it was missing some parts.  A couple were functional and one in particular was more trim and appearance, but had a minor functional part to play.  In the original thread by apfever, Neil posted an image provided by Galakuma of the 8" reflector on the Southern Skies Crawford mount.  Mine was built in 1959, years after the 8" was offered on that mount and the only difference I believe in OTA was the removal of the mirror access door that shows on that image. The front trim ring, providing balance in appearance against the rear trim ring and mirror cell was missing from mine.  Somewhere in the fifty eight years prior the owner of this scope removed it and trimmed an inch or so of tube to remove the part with the six holes that held it in place.  In addition the finder and finder shoe were missing.  That is truly sad because the finder in the picture is the same finder that came with my Model 203 and 204 Best Optics telescopes.  To make the OTA as complete as I can at this point and to restore the wonderful balance of those front and rear rings I have built a front ring and located a finder shoe (thanks to Dave Trott), and wanted to share with you how nice this telescope looks when it is as pictured in the Galakuma provided picture.

 

Here is the original pic.

8 inch reflector.jpg

 

Here is the way the scope looks now with front ring, finder shoe, and finder from Model 203.

 

PA250747.JPG

 

PA250748.JPG

 

The ring is made of pieces of a 10" PVC pipe joint cut to the required length and glued together with PVC cement.  The front trim ring was the fourth layer.  It started out looking like this.

 

PA130717.JPG

 

A little sanding and black paint and it finished up like this

 

PA140724.JPG

 

Finally I added some felt strips to allow for a friction fit and mounted it to the tube.  The finder shoe required an adapter plate to make use (and cover) the poor plastic finder ring holes and I used a piece of steel garden edging to fashion an adapter plate.  When mounted on the OTA it is very similar to the plate used to mount the focuser to the tube.

 

PA240737.JPG

 

Continued next post


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#17 Stew44

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Posted 25 October 2017 - 08:37 PM

Finally by adding one of my other telescope's finders I get the finished product.  I will likely add/paint some screw heads on the ring as they certainly stand out in the picture, but it's mostly complete.  The finder is simply a work of art as much as functional.  Don't think I will find one missing its telescope, but since I will have the 80mm for many many years to come (one of last three) this scope will have a finder to use as long as I own it.

 

PA250749.JPG

 

I also acquired a 10" PVC end cap which I've displayed before.  Again with felt strips it is a friction fit.  To use it I simply slide the ring down a bit and place the cap on the end of the tube.  Still has a very nice balance with the rear ring and cell.

 

PA250745.JPG

 

PA250746.JPG

 

I enjoy all of my classics, but this one in particular needed to have these parts to make it feel 'right'.  And now I can use a finder instead of lining up parts or the rings to find things. grin.gif

 

I do plan to recreate that Southern Skies mount if I can ever find a tripod worthy.  Also I need to recoat the mirror and strip and paint the tube.  Still, a lot closer than I was and nicer now to look at as well as use.

 

 


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#18 Dave Trott

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Posted 25 October 2017 - 10:15 PM

Looking great, Stew!




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