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Sad Sack SCT - Criterion Dynamax 8

catadioptric classic equipment SCT
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916 replies to this topic

#901 Bomber Bob

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Posted 30 April 2017 - 08:22 AM

I was kinda hoping BB would be posting pics of the corrector figure on his scope, sure he'll get there at some point.

 

I will.  Right now I'm observing with my scopes before our Torrid Summer starts.  2017 has been a rip-off.  We didn't get the string of clear cool nights we normally see from mid-FEB to mid-APR, and nights with lows < 75F are just about over, too.  I don't mind a little sweat, but when I have to carry a rag to keep from polluting my eyepieces...  The idea of hauling out anything heavier than a 76mm refractor is a no-go.


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#902 TOM KIEHL

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Posted 30 April 2017 - 08:31 AM

JW , do you have Air Conditioning in the Shed ? 



#903 Bomber Bob

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Posted 30 April 2017 - 09:56 AM

Nope.  I have a large floor fan, but when it's 90+ with 80% humidity, it makes no difference.


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#904 Garyth64

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Posted 30 April 2017 - 10:10 AM

You don't have a swamp cooler?


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#905 terraclarke

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Posted 30 April 2017 - 10:16 AM

^ :lol: ^

 

(Of course a real swamp cooler or water-evaporation based cooling system woulf never work efficiently in the oppressive humidities you experience there in the 'swamp'.)


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#906 starman876

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Posted 01 May 2017 - 06:38 PM

 

 

Some posts back you were setting up an optical bench but you already have everything to hand to assess the DX-8 even if there is no flat (?). Why not use the Tinsley 6'' in a single pass collimated test? Not perfect but it'll give you a very good map of the sct's wavefront.

 

Layout is simple; torch/diffuser, slit at focus of DX-8 then in reverse; cassegrain facing the Dynamax and knife edge at it's focus. Or, instead of knife edge, Ronchi grating in and around focus. The knife edge can be quantitative. The two optical axes should be parallel but can be, should be off centre.

 

David

you just went above his paygrade with those instructionslol.gif

 

Not at all! But I should have taken some more time.

Perhaps you have done a knife edge test at the ROC of a spherical mirror? This is just the same. What does a perfect sphere show? Why the even greying? What if there are zones or a turned edge or it's smooth but not spherical. Why are some parts then lighter and others darker? How can you measure what you see in terms of waves of error?

 

The slit at the focus of the first telescope acts like a point source of light.. A pinhole could be used but the slit is easier. Make it fine. Although either scope can be used as the collimator it's maybe better to use the Tinsley rather than the DX-8 here. Then alternative scopes can be tested.

Light travels in reverse through the scope and exits as a collimated beam of parallel light 6inches across. This beam's wavefront is as perfectly plane as the errors of the Tinsley allows. Now, this 'perfect' collimated 'starlight' enters the front of the telescope to be tested and is brought to a focus where it can be tested by a Ronchi grating, a knife edge or viewed with an eyepiece. As the light passes through the faulty optics of the scope under test, parts or zones of the wavefront, which now should become uniformly spherical, are advanced or retarded by whatever errors they've met along the way and will then focus longer or shorter. This is what's shown by the knife edge or Ronchi. It's good to have the two alternative tests. For example the Ronchi may show a diffraction curling back effect around the edge which might be mistaken for a turned down edge. The knife edge can be used to check on this.

 

A couple of practical tips: make the slit 1/4inch long, tape the blades onto a flat eg glass surface. On the glass side away from the scope and facing the torch tape something eg tissue paper to diffuse the light.

At the other scope's focus ensure the testing knife edge or Ronchi is aligned parallel with the slit which will be obvious in practise.

 

Try it on a Zeiss and see what perfection looks like. Compare that with the DX-8 and maybe think Wow! the errors are set out in such detail and so precisely. Swap for the Ronchi and compare. Which tells me more. Swap for the eyepiece and check/confirm astigmatism and general standard of imagery.

 

Many contributing to and following this thread will be practical, especially BB. I see long hours of careful, skillful work spent on restoring classic scopes. So much for pay grades.

 

Most in this forum will have a collection of scopes including some with excellent optics, better than 1/8 wave and maybe better than many flats.

 

You can set up these tests with a Ronchi or knife edge, with a flat or auxiliary scope. Pros and cons. Versions of the same thing. I find  the knife edge test quick, easy, informative and fun. It takes maybe half an hour to set up. Why not try it and see for yourself?

 

Yes, as Greyday says, pictures would explain things best. I must work on that.

 

David

 

You need to simplify things a bit more for Bomber Boblol.gif



#907 Bomber Bob

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Posted 04 May 2017 - 10:01 PM

Using my Mystery Mak 6 has been an eye-opener.  An older "kit CAT" beats a larger commercial SCT on lunar & planetary.  The only advantage my DX8 has is aperture for low & medium power DSO viewing.  That's still something, but it's not worth what Criterion charged for these back when they were brand new.  And not when a good quality 8" or 10" Newtonian would've been cheaper and better 40 odd years ago.


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#908 terraclarke

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Posted 05 May 2017 - 03:24 PM

And often as cheap today!



#909 Bomber Bob

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Posted 05 May 2017 - 05:03 PM

How much are the RV-8's going for these days?  Gotta be less than the original DX8 price (scope + wedge + tripod).



#910 terraclarke

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Posted 05 May 2017 - 05:12 PM

I could have purchased a deluxe version of the 8" F7 Dynascope last summer for $400! It was just too big for me to deal with.


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#911 Bomber Bob

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Posted 05 May 2017 - 05:21 PM

That's too bad.  Cheaper than my DX-8, and most likely a far better scope in terms of views.


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#912 terraclarke

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Posted 05 May 2017 - 06:19 PM

Yeah, the deluxe Dynascopes are beautiful. That chrome nameplate with the scroll script is wonderful. Like a big old Chrysler of a telescope. The OTA was in a wooden crate like a coffin. It was longer than the truckbed of my pickup! It had the heavy GEM with the bell shaped pedistal and the short tripod that fit inside. Just way too big for me. I would never use it. Just too big and heavy. I have learned my lesson. A gal's gotta know her capability and I know mine, and that scope ain't it.


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#913 Gil V

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Posted 05 May 2017 - 07:59 PM

I use this Clint Eastwood movie line often.

"A man's gotta know his limitations."

When it comes to owning telescopes and aging, no truer words were ever spoken.
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#914 Bomber Bob

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Posted 05 May 2017 - 08:08 PM

Good point.  Despite its very limited repertoire, I can carry my complete DX8 rig around the yard for tree-dodging.



#915 clamchip

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Posted 05 May 2017 - 08:54 PM

Yeah, the deluxe Dynascopes are beautiful. That chrome nameplate with the scroll script is wonderful. Like a big old Chrysler of a telescope. The OTA was in a wooden crate like a coffin. It was longer than the truckbed of my pickup! It had the heavy GEM with the bell shaped pedistal and the short tripod that fit inside. Just way too big for me. I would never use it. Just too big and heavy. I have learned my lesson. A gal's gotta know her capability and I know mine, and that scope ain't it.

I'd love one!

Probably one of the coolest most fabulous telescopes to ever grace these pages is

that metallic green Dynascope that was here. It was so long ago now I wouldn't even

know where to begin looking for it. I think someone here made longer maple tripod legs

for it, thats all I can remember.

 

Robert 


Edited by actionhac, 05 May 2017 - 08:59 PM.


#916 astro140

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Posted 05 May 2017 - 09:09 PM

Criterion 8 inch Dynascope.jpg

 

Yeah, the deluxe Dynascopes are beautiful. That chrome nameplate with the scroll script is wonderful. Like a big old Chrysler of a telescope. The OTA was in a wooden crate like a coffin. It was longer than the truckbed of my pickup! It had the heavy GEM with the bell shaped pedistal and the short tripod that fit inside. Just way too big for me. I would never use it. Just too big and heavy. I have learned my lesson. A gal's gotta know her capability and I know mine, and that scope ain't it.

I'd love one!

Probably one of the coolest most fabulous telescopes to ever grace these pages is

that metallic green Dynascope that was here. It was so long ago now I wouldn't even

know where to begin looking for it. I think someone here made longer maple tripod legs

for it, thats all I can remember.

 

Robert 

 

Happiness is owning a 8" Dynascope smile.gif

 

Steve

New Mexico


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#917 GreyDay

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Posted 06 May 2017 - 02:56 AM

attachicon.gifCriterion 8 inch Dynascope.jpg

 

Happiness is owning a 8" Dynascope smile.gif


 

 

Steve

New Mexico

and a step ladder grin.gif

 

Love the idea of these long f/l reflectors but at 5'8" tall I know where the impracticality starts and the fun ends.... gotta love the design and finish though.


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