Criterion Dynamax 8
Posted 30 July 2014 - 10:07 AM
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.
Posted 30 July 2014 - 10:46 AM
Posted 30 July 2014 - 11:27 AM
Posted 30 July 2014 - 12:09 PM
Worth a good part of the price...
Posted 30 July 2014 - 12:11 PM
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.
Posted 30 July 2014 - 12:32 PM
Posted 30 July 2014 - 02:53 PM
Posted 30 July 2014 - 06:30 PM
The seller is apparently the second owner, he bought it from his father-in-law.
Posted 30 July 2014 - 06:39 PM
I was looking to get one last year but it didn't click. Still looking.
Posted 30 July 2014 - 07:01 PM
Posted 31 July 2014 - 12:29 AM
I should have the answers in about 12 hours from the time of this post.
Posted 31 July 2014 - 04:06 PM
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.
Posted 31 July 2014 - 06:48 PM
Posted 31 July 2014 - 08:21 PM
Posted 31 July 2014 - 08:48 PM
Posted 31 July 2014 - 11:35 PM
While you correct the corrector if needed, how about a couple Criterion 4000 correctors?
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.
Posted 31 July 2014 - 11:40 PM
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!
Posted 01 August 2014 - 07:33 AM
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.
Posted 01 August 2014 - 09:31 AM
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.
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.
Posted 01 August 2014 - 05:51 PM
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 **** of a lot easier than refiguring the curve.
I expect the primary to be good. Criterion was a **** of a good mirror making company.
Posted 01 August 2014 - 10:07 PM
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.
Posted 01 August 2014 - 10:32 PM
You can do a lot more than just spin the corrector. Get your auto collimated ready, and let's talk.