72 replies to this topic

[terrapin]

I've been on the trail of this telescope for the better part of a month and I finally got the opportunity to purchase it today.

 

I'm assuming right now that it's an RV-8, but I'm not sure what Criterion's 8-inch Dynascopes looked like prior to the launch of the RV line. I'll need to start researching. There are no model markings anywhere -- no plaque or serial number. I haven't opened up the drive yet, but perhaps there will be a date indicator on the motor.

 

This telescope has seen better days. The mount is in rough shape, both in appearance and functionality. It's stiff and rusted everywhere and the slow motion controls seem pretty messed up. The paint is gone from the focuser, the rod is bent and missing a knob, and it feels pretty non-functional. It will be a long restoration process for me, but that's okay!

 

The tube is in great shape and the mirrors seem fine. Very dirty, but no apparent damage. Although I haven't pulled the cell out yet for a close look.

 

The finder is an 8x50 beauty! And the telescope came with two Orthos: a Criterion 4mm and Edscorp 25mm. Nice!

 

All in all, I'm delighted. Here it is with my RV-6...

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Edited by terrapin, 19 November 2017 - 12:09 AM.

[terrapin]

Mirrors.

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[terrapin]

Mount.

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[terrapin]

Slow motion controls.

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[terrapin]

Finder.

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[terrapin]

Focuser. 

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[terrapin]

Orthos, 25 & 4mm.

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Edited by terrapin, 19 November 2017 - 01:36 AM.

[Michael Covington]

Excellent!   It brings back fond memories of my RV-6, which I had from 1970 to 1975.

[Geo31]

Yep, that's an RV-8 alright.

 

As you say, it looks a bit rough, but it's definitely restorable.

 

I was going to offer up a focuser knob, but you need the whole knob/shaft assembly.  The threads are gone where the knob screws on.  Or,  you could straighten the shaft and put on different knobs.  I think I'm going to do the latter with my RVC-6N.

 

I think you're looking at a lot of elbow grease, but not to many replacement parts.  You'll need aircraft remover to get the old paint off the mount castings.  Some self-etching primer and black wrinkle paint for the mount castings.  Some hammer finish silver paint for the pedestal.  And some patience.  But it should restore nicely.

 

Looking forward to the restoration thread.

[rcwolpert]

Nice find!! That's going to be a LOT of fun to restore!

 

- Bob

[terrapin]

Getting back to this project after a quick three-year break! lol

 

Right now I'm re-assembling the mirror cell, but I'm uncertain how to best arrange the nuts and washers. I'm replacing the nuts on the back of the cell with wingnuts and I assume I should have at least one washer there.

 

And then I've also got a nut and washer on the other side of springs (you can see the location on the pic below)...but not sure if that's the best location. Advice?

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[terrapin]

Cleaned up the lovely 50mm finder a bit. The crosshairs were loose and completely askew. Removed them with tweezers.

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[CHASLX200]

Mine was a killer on the planets.

[terrapin]

Mine was a killer on the planets.

That's what I'm hoping for with this one. My RV-6 is a planet killer, for sure.

[JoshUrban]

AWESOME!!!  I've got an RV-8 that a buddy converted into a dob (signed by Dobson, too!)  It's a monster on Lunar/Planetary, and so nice to use with binoviewers.  What neat scopes!

[terrapin]

AWESOME!!!  I've got an RV-8 that a buddy converted into a dob (signed by Dobson, too!)  It's a monster on Lunar/Planetary, and so nice to use with binoviewers.  What neat scopes!

Cool. I've debated converting this one into a Dob. My restoration skills and building skills are both minimal, so either way is a challenge.

[DAVIDG]

 TEST the mirror. I have tested around 12 of them and they all have been spheres. You will get an image that comes to focus and it will not be a fuzzy mess. So it will easily fool most.  Just when you crank up the power, you will never see the detail that a correctly figured 8" f/8 will produce. The owners swore up and down they had great optics and it gave super sharp images but a simple  30 second Foucault test told a different story.

 

                - Dave 

[jelloptic]

Mr. Terrapin,

 

Here's what an 8-inch Criterion Dynascope looks like.

 

 

Vintage 1960's I believe, but not sure.  This was a donation to the local club, but not well documented.  The tube and right-angle finder finished in crinkle-gray paint, with polished aluminum rings on each end.  The mount is done in crinkle-black paint.  Slow-motion flex cables on this build; no clock drive. The short tripod legs would fold together and fit into a bell-shaped pier (which is MIA).  Someone added a modern-day red-dot finder.  No one has used it in years, because it weighs a robust 100 lbs in total...  not convenient to move it out of storage, compared to the many other scopes on hand.

 

DAVIDG:  I recently Foucault-tested the primary (classic Corning molded-Pyrex blank) and found it to be spherical, with a focal length of 61.3" (not quite F/8).  Also tested the secondary (mounted in its holder) in a Ritchie-Common setup and found it to be satisfactory (not warped/strained).  Have yet to use the scope myself, but I suspect it is not going to be the "planet-killer" I was hoping for.  Others expressed surprise at my findings, and claimed it was used for outreach events in the distant past with favorable results.

 

regards,   kbl

 

[DAVIDG]

DAVIDG:  I recently Foucault-tested the primary (classic Corning molded-Pyrex blank) and found it to be spherical, with a focal length of 61.3" (not quite F/8).  Also tested the secondary (mounted in its holder) in a Ritchie-Common setup and found it to be satisfactory (not warped/strained).  Have yet to use the scope myself, but I suspect it is not going to be the "planet-killer" I was hoping for.  Others expressed surprise at my findings, and claimed it was used for outreach events in the distant past with favorable results.

 

regards,   kbl

 

  Thanks you very much for posting your results. As I have said a number of times I have had owners swear that they had great optics that produce sharp images but when placed on the test stand that doesn't hold true.  Your RV-8 makes around 13 or 14 of them that I have tested that have been spheres. 

  

                  - Dave 

[terrapin]

DAVIDG: That is certainly interesting. Have you also tested RV-6s?

 

jelloptic: Beautiful scope you have there, despite the unpromising test. Your RV-8 is earlier version than mine. At some point, the style changed to something more like the RV-6. I definitely prefer the aesthetics of yours.

[DAVIDG]

DAVIDG: That is certainly interesting. Have you also tested RV-6s?

 

jelloptic: Beautiful scope you have there, despite the unpromising test. Your RV-8 is earlier version than mine. At some point, the style changed to something more like the RV-6. I definitely prefer the aesthetics of yours.

 Yes I have tested around 50 RV-6 including my own that I bought in 1976. All the mirrors have been under corrected and close to a sphere. Mine was basically a sphere with a turned edge.  I have refigured mine  since then. An RV-6 has  a focal length of 50" not 48"  or F8.3. They did that for a reason. In Jean Texereau's  book " How to Make a Telescope" there is chart that lists when a mirror can be left spherical and  just be at a 1/4 wave. For a 6" that is  f/8.3. So if you just put a tiny bit of correction on it your at or slightly better then 1/4 wave.  The Law Optics are on your side and the odds are pretty good that without  much work  that the mirror is 1/4 wave. This is why people say the image are  so good in a RV-6. They are finally using 1/4 wave optics but not 1/8 or 1/10 like some may believe. 

  Unfortunately that doesn't work for a 8" mirror and you need to be around f/12 for sphere to be at 1/4 wave. So for  a 8" f/8 left as a sphere  at  best focus the image is  about 1/3 wave.  It would take more time to correctly figure a 8" f/8" to be at least 1/4 wave but as I said if left a sphere you will not get a image that is a fuzzy mess. It will be acceptable and even "good" to most. You will see a craters on the Moon, the rings of Saturn and bands on Jupiter, they just will never be as sharp and detailed that true 1/8 wave or better optics will shows. It is like viewing  a standard resolution analog TV picture to a modern day HD one. 

   There have been other threads were 12" Criterions Newtonian have had their mirrors tested  and they also have been grossly under corrected and close to  a sphere.

 

                 - Dave 

Edited by DAVIDG, 16 February 2021 - 03:33 PM.

[ccwemyss]

Dave - have you found the Edmunds of the same era and sizes to be similar? I seem to recall hearing that, for a while, they were both using the same mirror maker. 

 

Chip W. 

[DAVIDG]

  Chip,

     I have seen mirrors from just about every manufacture and they all vary  in quality. You just can not buy one with the belief that from a certain manufacture or time period that it will be good You have to test them. It  is not just me saying this, if  you look around for actually test data you will  see this from other as well.   There was an article back in the late 80's in Sky and Tel were they bought a number 10" mirrors and the ones from the manufactures that had the better reputation which I believe was Galaxy Optics turned out to be the worst and the one from Coulter that was cheaper wasn't bad !  The problem is most do not understand that 1/2 wave optics and even worse don't produce an image that is fuzzy mess and won't come to focus, so people are fooled that they have "good" optics. 

  I can tell  the  worst mirror I have every seen was from a red tube, fork mounted  6" f/5 Edmund. It didn't have a figure it such a lumpy mess !  When I see these gross errors it is  apparent these optics  were never tested. They were just coated, put in a telescope and shipped.  If the scope was returned you either got your money back or another scope to try and that  is how the industry has worked for years. 

   I do car restoration and before you put $10,000 into paint job, you check the motor and transmission because a car that looks great but doesn't run isn't very useful. The same with telescope restoration in my book. A scope needs to be  as pretty to look through as to look at or it will just sit and never get used. 

      It doesn't take  Phd  and a lab full of equipment to test optics. You can build a simple Foucault test for under $10 and it will show  you in a minutes if your mirrors  have issues or not. 

 

                 - Dave 

[ccwemyss]

Yes, I know they vary. I was just curious, given how many you've seen, if there was any statistically significant difference between Criterion and Edmund. Different people will swear by one or the other, often based on a sample size of one or two, or lore about the people involved in the companies. But you're one of the few people who has probably seen enough to have a sense of whether there is a difference in reality, or if it's all a wash. If I had to guess, it would be that they're about the same. 

 

Chip W. 

[jelloptic]

Chip W,    UPCO Optics (Universal Precision company of Pennsylvania) was a common mirror supplier to both Criterion and Edmund.  Don't know over what time-frame, and there may have been other suppliers as well.  I believe that UPCO is long gone; does anyone have any historical info on them?

 

      I know this because the 8-inch Dynascope (pictured in post #18 above) has an UPCO-stickered mirror inside.  And I own an 8-inch Corning Pyrex molded mirror with both an UPCO sticker and an Edmund cautionary sticker on it, so it came my way thru Edmund.  By the way, my UPCO mirror is parabolic, not spherical, with a 64 inch focal length.  Coating is pristine, as it had been used in a lab environment, then in storage for two decades. Maybe someday I'll build a Dobsonian for it.

 

DAVIDG,  so you've tested 50  RV-6 mirrors... that's a significant sample size... and they're 0 for 50??  ...not a single paraboloid?  That's shocking.

So I consider my club lucky, as we recently received a Criterion RV-6 (donation).  Vintage 1977 (dated by the motor), and the mirror tested as a respectable paraboloid... works fine optically and mechanically, so we are fortunate.

 

regards,  kbl