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Criterion Dynamax 8

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#1 Chuck Hards

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Posted 30 July 2014 - 10:07 AM

I'm looking at a well cared-for Dynamax 8 with grey tube, wedge, original gold-legged tripod, plus Meade drive corrector and a handful of older eyepieces for $400.

Is this a good price, knowing what I know now? Will be going to inspect it in a day or two if things fall into place and the price is right. I want to check the opitcs before anything else.

#2 Geo31

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Posted 30 July 2014 - 10:46 AM

Heck, the tripod, drive corrector, and eyepieces are probably worth in the $300 range collectively (and quite possibly more based upon the details).

#3 youngamateur42

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Posted 30 July 2014 - 11:27 AM

Definitly check the optics like you mentioned. I happen to think it's a pretty awesome looking scope. One came up for sale here not long ago and due to lack of $$$ (and space for that matter) I had to let it go.

#4 Andy Taylor

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Posted 30 July 2014 - 12:09 PM

I've got the "Golden Pyramid" tripod and it's solid!

Worth a good part of the price...

#5 Andy Taylor

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Posted 30 July 2014 - 12:11 PM

Just thinkin'...

If it's "well cared for" then maybe it didn't get much use because of bad optics.

If it's a bit tatty then it's been used a lot = good optics.

#6 Chuck Hards

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Posted 30 July 2014 - 12:32 PM

The owner says he doesn't have time for astronomy anymore, due to career. Sounds more like he's just lost interest.

#7 DocFinance

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Posted 30 July 2014 - 02:03 PM

Sounds like a deal, even without the scope

#8 starman876

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Posted 30 July 2014 - 02:53 PM

the couple I have had were very poor optically and I almost kissed the people that bought them from me and I told them the optics were poor. I never paid more than $250 for one. The only good part is the tripod.

#9 Chuck Hards

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Posted 30 July 2014 - 06:30 PM

I'm going to go check it out tomorrow morning. Will take my laser collimator to check alignment and give it the best optical assessment I can during daylight. It's about a 40 minute drive from my home, I have a rare day off tomorrow.

The seller is apparently the second owner, he bought it from his father-in-law.

Stay tuned.

#10 Tom McDonald

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Posted 30 July 2014 - 06:39 PM

Best of luck with it, Chuck.
I was looking to get one last year but it didn't click. Still looking.

#11 Chuck Hards

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Posted 30 July 2014 - 07:01 PM

Thanks Tom. I've emailed Gil V. asking for evaluation tips. Maybe I'll get lucky and spot a specular reflection of the sun on some distant shiny object, and be able to see a diffracttion pattern. I'll take a few of my own eyepieces in case the ones that come with it won't boost the magnification high enough to get a good evaluation.

#12 Chuck Hards

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Posted 31 July 2014 - 12:29 AM

Gil V. (Mr. Criterion) has replied to my email with a checklist of things to look for. He's also verified from the ad photos that, it being an early model, my chances of good quality are better. Many thanks, Gil!

I should have the answers in about 12 hours from the time of this post.

#13 Chuck Hards

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Posted 31 July 2014 - 04:06 PM

Well, I got it. Talked the seller down 25%. There is good news, there is not so good news. The optics didn't come anywhere near to where I felt they should be, but everything else was just about pristine. I got it from the second owner, who purchased it from the original owner. Lots of accessories included, even the table-top feet. Cosmetically it looks great. Gil V. dates it to about 79.

I'm going to try some collimation and corrector rotation to see if I can sharpen it up a bit. I'll take it to the optical shop of my night job in coming weeks for proper testing. A refigure might be in the works if I can't get it where I want it.

But, as George pointed out, it was worth what I paid, for the eyepieces, accessories, and tripod alone. If I had a tile countertop I could part it out and double my money. But of course I won't do that. Even worst case, a refigure of the mirrors and/or corrector, is doable on my end.

#14 A6Q6

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Posted 31 July 2014 - 06:48 PM

"I could part it out and double my money. But of course I won't do that. Even worst case, a refigure of the mirrors and/or corrector, is doable on my end." This is good news Chuck, I love the scopes Criterion made. I had a RV-6 years ago and was always curious about the Dynamax. I wanted the Dynamax 6 after seeing it in S&T and Astronomy years ago. Its good to know you will be preserving a part of our amateur telescope history. :waytogo:

#15 starman876

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Posted 31 July 2014 - 08:21 PM

Just hope it is not the corrector. I collimated the two I had to perfection and still the startest was horrible. I hope you can fix it Chuck.

#16 Gil V

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Posted 31 July 2014 - 08:48 PM

I have a feeling that our moderator can find or fashion whatever he needs to make that scope a solid instrument.

#17 starman876

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Posted 31 July 2014 - 09:08 PM

I believe you are correct

#18 BigC

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Posted 31 July 2014 - 11:35 PM

Congrats on purchase and intentions.

While you correct the corrector if needed, how about a couple Criterion 4000 correctors? :grin:

Oddly several of the siblings? Criterion/Dynamax/Bausch showed up this same week on the ebay. From really low priced but pick up only to rather HIGH price with costly shipping.After seeing none for months.

#19 Chuck Hards

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Posted 31 July 2014 - 11:40 PM

Gil, is the figuring on both sides of the corrector, or all on one side? I think I remember reading that it can be done either way.

I'd have to machine out a vacuum cup before doing any corrector touch-up, hopefully that's a last resort.

BigC, let me get through one, before I decide if I want to do more. And hopefully I won't have to tackle even one!

#20 Bomber Bob

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Posted 01 August 2014 - 07:33 AM

"Sounds more like he's just lost interest."

I hate that happened, but it does. Glad that you got a great deal, and are going to preserve (and improve!) a vintage scope. I can only recall looking through one in B'ham - I do remember being impressed with its build quality.

#21 DAVIDG

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Posted 01 August 2014 - 09:31 AM

Chuck,
Having refigured a couple of Celestrons SCT over the years, here is what I would do.
First is to test the complete system via double pass autocollimation to determine exactly what is wrong.
Next, pull the corrector plate and test the primary. It should be a perfect sphere but many times they are not. What is critical is the figure is optically smooth. A little bit of aspheric correction is OK. Also be sure there is no turn edge. If it has zone refigure it to a perfect sphere.
The corrector has the Schmidt curve on only one side if it was made via the Master Plate method. Both surfaces need to be optically smooth and with no astigmatism. You can test the plate against a large optical flat to check for optical smoothness and astgimatism. If you see astgimatism, you need to make a new plate since you'll never polish it out.
The big issue with working on the plate is figuring out how to support since it is thin and you can easily polish in astigmatism if it is not supported well.
If you need to make a new plate I recommend placing 50% of the correction on each side and rotating the plate 90 degrees when you remount it to the pan to grind the back side. This way if there is any astgmatism on the front surface it will be cancelled on the back. You want to use boiled water in the vacuum pan and not air. The water will support the glass much better and if you use air you'll make a very sensitive barometer and you'll have a hard time figuring the plate. To grind the plate you need to make a tile tool that have the correct convex radius ground into it and keep trueing it up as you go. The other issue is that you don't know the exact optical design so your going to have reverse engineer it, then raytrace it to be sure you have the correct design.
Once the corrector and primary are done, you can assemble the scope and test the complete assembly via double pass again. You'll then figure the secondary mirror until the complete systems null.

- Dave

#22 MtnGoat

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Posted 01 August 2014 - 03:11 PM

Thanks Tom. I've emailed Gil V. asking for evaluation tips. Maybe I'll get lucky and spot a specular reflection of the sun on some distant shiny object, and be able to see a diffracttion pattern. I'll take a few of my own eyepieces in case the ones that come with it won't boost the magnification high enough to get a good evaluation.


I broke down and picked up one of those LED flashlights with the pattern of five different sized pinholes in it for 'star' testing, it's a fantastic tool for the money. You can pick the calmest day (or night) and do some testing, cloudy or not. I'm lucky to have a driveway long enough to get a few hundred foot shot, but I recommend it it's a great tool.

#23 Gil V

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Posted 01 August 2014 - 05:51 PM

Criterion plates had a flat side and a figured side.

If you do decide to rework the glass, I'd polish the flat side first. That could help - just making the flat side better optically. And, it is a hell of a lot easier than refiguring the curve.

I expect the primary to be good. Criterion was a hell of a good mirror making company.

#24 Chuck Hards

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Posted 01 August 2014 - 10:07 PM

I'm going to collimate it as well as possible first, maybe try some corrector rotation, do a star test. If it looks like it needs either a mirror or corrector refigure, it will have to wait a bit, I have a couple of projects ahead of it. My pro mirror-maker friend is out-of-town right now, visiting Ed Byers. When he returns we'll test it at his shop.

Thanks Gil for identifying all the accessories that came with it. I have a genuine Criterion f/5 focal reducer, as well as a piggyback camera adapter, and sliding tube weight and rod!

The seller said there's a manual buried in the storage case, that I haven't dug out yet. Maybe tomorrow.

#25 Gil V

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Posted 01 August 2014 - 10:32 PM

Manual has very little usefulness, other than a novelty.

You can do a lot more than just spin the corrector. Get your auto collimated ready, and let's talk.






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