Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Sad Sack SCT - Criterion Dynamax 8

catadioptric classic equipment SCT
  • Please log in to reply
916 replies to this topic

#26 Terra Nova

Terra Nova

    ISS

  • *****
  • Posts: 20546
  • Joined: 29 May 2012
  • Loc: Middle Earth

Posted 04 May 2016 - 05:00 PM

Maybe you could put a layer of flocking in there and that would keep the debris from falling on the mirror?

#27 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • topic starter
  • Posts: 15911
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 04 May 2016 - 05:06 PM

With the corrector off, I located the culprits:  The nuts that hold on the nice chrome counterweight bars.  Criterion painted these with flat black, but over 4 decades, the paint came loose in places.  I wiped those down, along with the complete interior, then used my computer vacuum to get any strays.  Then, I cleaned the primary mirror.  Now, I can look down the tube, and it's clean as a whistle!

 

DX8 - First Cleaning S05.jpg

 

I'll say one thing for the DX8:  It's very easy to work on.  Simple design, common components, and it fits back together without a hitch.

 

DX8 - First Cleaning S06.jpg


  • Live_Steam_Mad, rcwolpert and Piggyback like this

#28 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • topic starter
  • Posts: 15911
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 04 May 2016 - 05:15 PM

Another thing about my DX8:  It ain't no Questar!  The corrector is not a perfect circle, and its edges are kinda rough; whereas, my Q's meniscus is as precisely ground & finished as my Telementor's lens.  It's like I asked The Boys to make it from an old storm door...

 

The mirrors are in excellent condition.  The coatings are every bit as flawless as the Tinsley's after Majestic worked their majik on them.

 

IF the scope performs well, I'll think about flocking the interior.  For now, I just want to see what it can do as delivered.

 

BIF:  The B&L eyepiece adapter came today.  I got it on eBay, part number 63-1013, and it threads on perfectly.  It's supposed to fit the DX6, too.  Machined aluminum, complementary DX details, and holds 1.25" accessories securely.


Edited by Bomber Bob, 04 May 2016 - 05:32 PM.

  • Terra Nova and rcwolpert like this

#29 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9086
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 04 May 2016 - 06:25 PM

  If the corrector isn't round then that  makes it a problem in finding it's true optical centered so it can be properly aligned. It's also questions if the hole in the middle of it for the secondary  is also optically centered or not ? The only way I know to check it is to place it up against an optical flat so you can find the neutral zone of the corrector. Here is a picture of what I mean. You can see were the interference fringes are the strongest and that is the neutral zone of the plate. Hopefully the neutral zone will be perfectly round and then you  can measure it's diameter and from that determine the optical center of the plate.  Here is also one of Celestron corrector showing the neutral zone as well. 

 

                     - Dave 

 

SCHMIDTPLATE.JPG

C8SCHIDMTCORRECTOR.JPG

 

 

 


  • Live_Steam_Mad and Bomber Bob like this

#30 Gil V

Gil V

    Apollo

  • -----
  • Posts: 1369
  • Joined: 09 Sep 2012

Posted 04 May 2016 - 06:41 PM

David, virtually nobody has optical flats. You do, but you are not a typical amateur astronomer - not by a long shot.

And, if you find that the plate was cored off-optical center, what are you going to do, re-core the corrector plate? Maybe you could shim the secondary holder off center, but then you have the problem of centering the corrector off it's mechanical center.

I understand your point, and I appreciate your skills. I just can't agree that the methods you discuss here are in the realm of possibility of the general members here.

If a user can refigure a corrector, all the things you discuss are doable. Just realize that it's beyond the ability of most of us.
  • Terra Nova and Bomber Bob like this

#31 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • topic starter
  • Posts: 15911
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 04 May 2016 - 07:25 PM

Actually, before cleaning, the secondary assembly was centered within .1mm; I returned all the bits to their original positions, including the cork shims, and the secondary is again dead center.  Also, with the 3 mirror collimation screws loosened, it can move about 1mm in any direction horizontally & vertically.  It made nailing collimation tougher than my Tinsley, but Criterion put in enough slack to get the pieces aligned without anything more than a screwdriver.  Again, simple design & components.  I've done my part.  It boils down to the quality of the optics.  And, since the skies cleared a day earlier than predicted, I'm going to test those optics tonight -- along with the Big Eye Cee (Carton 101mm monocular).


  • strdst, Terra Nova, rcwolpert and 2 others like this

#32 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9086
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 04 May 2016 - 08:04 PM

David, virtually nobody has optical flats. You do, but you are not a typical amateur astronomer - not by a long shot.

And, if you find that the plate was cored off-optical center, what are you going to do, re-core the corrector plate? Maybe you could shim the secondary holder off center, but then you have the problem of centering the corrector off it's mechanical center.

I understand your point, and I appreciate your skills. I just can't agree that the methods you discuss here are in the realm of possibility of the general members here.

If a user can refigure a corrector, all the things you discuss are doable. Just realize that it's beyond the ability of most of us.

  Gil,

   If your going to do a job  you need the right tools to do it and I'm showing people how to do the job with those tools.So what is the alternative then ? Just roll the dice and hope your get it right or use a telescope that doesn't give a good image ? If you have the right equipment many times it is very easy to fix a problem .

     You can buy an optical flats at places like Ebay and Surplus Shed for not much money. Rolo got a 12" one for around $120. Here is 1/20 wave at Surplus Shed that would allow one to map out the corrector and find the neutral zone http://www.surplussh...tem/pm1017.html  for $20. 

  If you want your scope to perform especially these vintage ones that who knows what has been done to them,  you need to take the time to understand how optics work and how to test them. People here have posted wonderful mechanical restoration jobs and spent a fair amount of time and money doing them, including the tools to do it. People don't think twice on dropping $50 for certain tool to get a job done or buying custom blended paint to match a vintage finish. In most cases the tools needed to test optics are  cheaper and it is no harder to test them as it is for some the mechanical work I see people doing. 

  As for what to do if the optical center doesn't match the mechanical center, you center everything up using the optical center as the reference. If not the scope won't perform. It is the same has have Newtonian  out of the collimation or element in a refractor off center. 

 

                - Dave 


Edited by DAVIDG, 04 May 2016 - 08:12 PM.

  • tim53, gts055, Live_Steam_Mad and 4 others like this

#33 Michael Covington

Michael Covington

    Author

  • *****
  • Vendors
  • Posts: 5523
  • Joined: 13 May 2014
  • Loc: Athens, Georgia, USA

Posted 04 May 2016 - 08:33 PM

Personally, although I do not make my own optics, I am excited to hear that there are beginning to be amateurs who can refigure an SCT.  We ought to be masters of our tools, at least as a community, if not as individuals.


  • tim53 likes this

#34 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • topic starter
  • Posts: 15911
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 04 May 2016 - 09:03 PM

My indoor collimation was good, but it wasn't great.  I tweaked the DX8 on Pollux, then moved to Castor with the OR20, and got two beautiful orange stars.  Had to make it a brief work night session, so I went from there to Jupiter, and saw 6 belts at 100x.  OR12 @ 176x was very nice.  Sharp limb, splitting in the SEB, and mottling browns in both EQ belts.  I only gave the DX8 about 30 mins to adjust (while I got wowed by the Carton 101mm monocular).  I give the Tinsley at least an hour.  I don't think it'll match the Cassegrain in resolution, but I'll be glad to be proven wrong.  Right now, I'm just relieved that the planetary images aren't mush!


  • deSitter, tim53, rolo and 6 others like this

#35 Chuck Hards

Chuck Hards

    You don't know Swift from Astrola

  • *****
  • Administrators
  • Posts: 25600
  • Joined: 03 May 2010

Posted 04 May 2016 - 09:19 PM

If you got decent, detailed images at 176X, that is a very good sign and I think you got a good one.  My blue DX8 produces mush at anything over 30X, my grey ones perform as yours does.  

 

I can do optical testing at my friend's shop, when he's around (we tested Keith's Tinsley objective a few weeks ago using double-pass autocollimation), but I really need to make room in the basement and set up to do it there.  Right now there just isn't room.  It's a path only from one spot to another, I need to get my ex-wife to come and get the last of her stuff- it's only been six years.  Or maybe I can charge her a storage fee that will offset the alimony?  Hmmm....    :lol:


  • deSitter, Geo31, Bomber Bob and 1 other like this

#36 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • topic starter
  • Posts: 15911
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 04 May 2016 - 09:31 PM

Y'all should see that huge DX8 balanced on the D&S.  Two of its bolt holes aligned with the head's long slot, so I had it secured, but I'm sure the DX8 could've pulled it over!  But setup on the lawn is a big advantage over my pedestal mounted scopes.  With a proper wedge & tripod, this heavy SCT should be a breeze to use.

 

Don't know if this is common, but my DX8's focuser feels too slick.  I'll have to adjust it if possible, because it slipped going from horizon to zenith.  Also, as humid as The Swamp is, it'll have to have a dewshield.

 

My initial views are pretty typical for an 8" SCT.  I think it would've improved some during the next hour.  Seeing was a good 8/10, and the Carton was ready in just a few minutes.  On nights like this one, my Edmund 4 rules on any object I target, but my Questar requires patience to get its best views.  I think the DX8 will be similar.

 

Will it be a planet killer like my Tinsley?  Probably not.  But the extra 2" aperture should do well on DSOs.  And the DX8 can go to dark sites, which is not practical for the old 6" Cass.


  • Chuck Hards, rcwolpert and TSSClay like this

#37 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • topic starter
  • Posts: 15911
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 04 May 2016 - 09:44 PM

One thing I noticed immediately was that the background sky wasn't black at the zenith, as it is in both the Questar & the Tinsley (and all my long refractors, of course).  Also, at 200x the Tinsley shows the 4 Galilean moons as disks, with hints of color on Io.  The DX8 doesn't.  All four had intermittent / broken rings.  But like my other reflectors (and my Goto f/20), Jupiter's disk is a pearly white, which makes the belts jump out.  Terra suggested flocking, and I think that'll help with the contrast.  I don't think Criterion's black absorbs as well as the chalkboard black.

 

OTOH, the DX8 should do well imaging DSOs, and I have a camera now that can catch those nebulae & galaxies.


  • Live_Steam_Mad, Terra Nova and Piggyback like this

#38 Terra Nova

Terra Nova

    ISS

  • *****
  • Posts: 20546
  • Joined: 29 May 2012
  • Loc: Middle Earth

Posted 04 May 2016 - 10:20 PM

Looks like you got one of the good ones! That's great JW! Congrats. It sure is a pretty scope.
  • Bomber Bob likes this

#39 dawsonian2000

dawsonian2000

    Surveyor 1

  • *****
  • Posts: 1788
  • Joined: 26 Jul 2006
  • Loc: Riverview, FL, USA

Posted 04 May 2016 - 10:54 PM

Got ready to collimate my DX8, until I saw yet another Big Flake of black paint on the mirror:

 

attachicon.gifDX8 - First Cleaning S03.jpg

 

Good Grief!!  That won't do.  So, I took off the corrector, and gave these optics a proper cleaning.  The engraved outer ring is held on by 8 small screws:

 

attachicon.gifDX8 - First Cleaning S02.jpg

 

Criterion put a handy alignment mark on the corrector and the inner cell.  4 small cork bits shim the glass, keep it centered, and keep it from contacting the aluminum ring.  I marked the locations with a pencil before removing them.  As you can see, the corrector & secondary had paint debris, too.

 

attachicon.gifDX8 - First Cleaning S04.jpg

 

Hey Bob,

 

I see that the tradition continues in how telescope manufacturers align and center the corrector plate using cork and strategic placed engravings. My "modern day" manufactured LX200-R/ ACF SCT is assembled similarly.

 

I hope your efforts cleaning the optics helped to produce splendid images.

 

 

Thanks,

Mel


Edited by dawsonian2000, 04 May 2016 - 10:58 PM.

  • Bomber Bob likes this

#40 Datapanic

Datapanic

    Soyuz

  • *****
  • Posts: 3825
  • Joined: 17 Oct 2009
  • Loc: Tucson, AZ

Posted 05 May 2016 - 12:29 AM

 

David, virtually nobody has optical flats. You do, but you are not a typical amateur astronomer - not by a long shot.

And, if you find that the plate was cored off-optical center, what are you going to do, re-core the corrector plate? Maybe you could shim the secondary holder off center, but then you have the problem of centering the corrector off it's mechanical center.

I understand your point, and I appreciate your skills. I just can't agree that the methods you discuss here are in the realm of possibility of the general members here.

If a user can refigure a corrector, all the things you discuss are doable. Just realize that it's beyond the ability of most of us.

  Gil,

   If your going to do a job  you need the right tools to do it and I'm showing people how to do the job with those tools.So what is the alternative then ? Just roll the dice and hope your get it right or use a telescope that doesn't give a good image ? If you have the right equipment many times it is very easy to fix a problem .

     You can buy an optical flats at places like Ebay and Surplus Shed for not much money. Rolo got a 12" one for around $120. Here is 1/20 wave at Surplus Shed that would allow one to map out the corrector and find the neutral zone http://www.surplussh...tem/pm1017.html  for $20. 

  If you want your scope to perform especially these vintage ones that who knows what has been done to them,  you need to take the time to understand how optics work and how to test them. People here have posted wonderful mechanical restoration jobs and spent a fair amount of time and money doing them, including the tools to do it. People don't think twice on dropping $50 for certain tool to get a job done or buying custom blended paint to match a vintage finish. In most cases the tools needed to test optics are  cheaper and it is no harder to test them as it is for some the mechanical work I see people doing. 

  As for what to do if the optical center doesn't match the mechanical center, you center everything up using the optical center as the reference. If not the scope won't perform. It is the same has have Newtonian  out of the collimation or element in a refractor off center. 

 

                - Dave 

 

 

"People don't think twice on dropping $50 for certain tool to get a job done or buying custom blended paint to match a vintage finish. In most cases the tools needed to test optics are  cheaper and it is no harder to test them as it is for some the mechanical work I see people doing."

 

This is an interesting point from Dave.  Sadly, not many people go for a scientific test to determine optical quality; instead, it's the eyeball factor.  The best determination for the given quality of an optic set is to put it on the bench.


  • rcwolpert likes this

#41 Michael Covington

Michael Covington

    Author

  • *****
  • Vendors
  • Posts: 5523
  • Joined: 13 May 2014
  • Loc: Athens, Georgia, USA

Posted 05 May 2016 - 08:13 AM

Have you given any thought to the resale value of a Dynamax 8 with known good optics and in good condition?  I can't afford to add such a thing to my collection right now, but there must be others who would jump on it!



#42 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • topic starter
  • Posts: 15911
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 05 May 2016 - 08:36 AM

Have you given any thought to the resale value of a Dynamax 8 with known good optics and in good condition?

 

No, not really.  I don't buy vintage scopes as an investment, though I do want my heirs to get their money's worth once I'm gone.  That's why I wrote up a descriptive inventory with my estimates of market & auction prices -- plus the lowest amount I would take for a given scope.



#43 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9086
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 05 May 2016 - 08:38 AM

 The issue I keep experiencing is that most commercial telescopes have optics that are in the 1/2 wave or worse range. So a majority of observers are calibrated to the image produced in those telescopes as being "good". My own DX-8 which I have posted images of how the optics tested out on the bench ( very bad)  will show me the belts on Jupiter and the craters on the Moon. I watched a transit of the one of Jupiter's  moons with it. If I took that  scope to a  public out reach I'm sure I would get a bunch of OMG's when people looked thru it BUT if I put my DX-8 next to one on my scopes with true 1/8 wave optics you could easily see the difference of what your were missing. Jupiter wouldn't have that  "fuss" to it any more and all of sudden all this fine detail in the belts would be easily visible vs blaming the lack of detail in the poor quality  scope because of the seeing. or the scope hasn't cooled enough  or the eyepiece etc etc.

   Just two days ago I tested a custom made mirror that was over 20" in diameter made by a high end mirror maker for a member of my club. He just received it and paid many thousands of dollars for. It had a pretty bad turned edge that owner could easily see as well when we tested it. It didn't require a  fancy test equipment to see the defect just a simple Foucault tester that I made when I was a kid for a few dollars.  What is just  as interesting is the fact the maker of this mirror doesn't provide test data but his only claims that his mirrors will give "good images".  Well, define "good".  The maker's definition of "good" and owners definition of "good" can be two different things vs the scientific definition of "good'.  Also from what I have seen of other commercial mirrors that are much less expensive but known to be of "lesser quality",  this mirror is no better. 

   So if you don't test your optics you can be playing the same game as the old fable of "The Emperor has new Cloths". Just because someone keeps  tells you have great optics doesn't mean you do but your wallet will be much lighter.  

 

       

                         - Dave 


  • rolo, Live_Steam_Mad, Terra Nova and 3 others like this

#44 Bomber Bob

Bomber Bob

    Hubble

  • *****
  • topic starter
  • Posts: 15911
  • Joined: 09 Jul 2013
  • Loc: The Swamp, USA

Posted 05 May 2016 - 09:14 AM

I really need to make room in the basement and set up to do it there.  Right now there just isn't room.

 

I'm in the same boat; and, we're going to be moving out to get our current home fixed up & sold.  So, no BB Optical Test Lab any time soon.  Like other serious observers, I have to rely on my memories of observing through different scopes at star parties, and my own set of "reference scopes" - like my Telementor 2, Tinsley 6" Cass, and Edmund 4" refractor.

 

The T2 would blow the sox off any observer.  It's a 63mm that thinks it's a 150mm.  I've used much larger Casses than my Tinsley 6, and larger refractors than my Edmund 4.

 

I'm pretty certain that my DX8 is typical of this class.  Good optics, but not great optics.  Given the convenience of the form factor, it's an acceptable compromise versus an 8" f/10 refractor!  Another classic bargain in my collection, and it will require very little effort to improve & maintain.


  • A6Q6, Piggyback, Michael Covington and 1 other like this

#45 Michael Covington

Michael Covington

    Author

  • *****
  • Vendors
  • Posts: 5523
  • Joined: 13 May 2014
  • Loc: Athens, Georgia, USA

Posted 05 May 2016 - 09:49 AM

 The issue I keep experiencing is that most commercial telescopes have optics that are in the 1/2 wave or worse range. So a majority of observers are calibrated to the image produced in those telescopes as being "good". My own DX-8 which I have posted images of how the optics tested out on the bench ( very bad)  will show me the belts on Jupiter and the craters on the Moon. I watched a transit of the one of Jupiter's  moons with it. If I took that  scope to a  public out reach I'm sure I would get a bunch of OMG's when people looked thru it BUT if I put my DX-8 next to one on my scopes with true 1/8 wave optics you could easily see the difference of what your were missing. Jupiter wouldn't have that  "fuss" to it any more and all of sudden all this fine detail in the belts would be easily visible vs blaming the lack of detail in the poor quality  scope because of the seeing. or the scope hasn't cooled enough  or the eyepiece etc etc.

 

I know what you mean.  My new Celestron EdgeHD appears to be a "gem," better than 3 previous 8-inch SCTs, even at center of field, where the EdgeHD design should not be appreciably different from conventional SCT.  I think I got a good one.



#46 DAVIDG

DAVIDG

    Cosmos

  • *****
  • Posts: 9086
  • Joined: 02 Dec 2004
  • Loc: Hockessin, De

Posted 05 May 2016 - 09:55 AM

 I do majority of my testing on either my kitchen table or kitchen counter tops when it come to finished optics and everything gets put neatly away when I'm done.  I also setup two chairs temporarily in my living room, one with the mirror setup on it and the other with the tester on it. So you don't need a permanent  setup to do optical testing.   Yes I do have a setup in my basement but I used that when I'm figuring optics because once I get everything set up, I can quickly go from the pitch lap, to the test stand and see what is going on with the figure and right back to the pitch lap to continue work. 

   I made my first mirror as  kid in the  parent house using the  cloth hamper as  a stand with a  plywood top I made for it. The story goes that Bernard Schmidt was making optics on the top of  a chest of drawers in the room he was renting. 

 

                - Dave 


Edited by DAVIDG, 05 May 2016 - 09:57 AM.

  • Live_Steam_Mad and rcwolpert like this

#47 Chuck Hards

Chuck Hards

    You don't know Swift from Astrola

  • *****
  • Administrators
  • Posts: 25600
  • Joined: 03 May 2010

Posted 05 May 2016 - 10:01 AM

My kitchen table is about 4 feet in diamter, and my wife would kill me if I set up even a temporary optical bench there.  Ain't gonna happen until I get the basement cleaned out!  Of course, I have a friend with a Zygo- not everyone does.


  • tim53, AndreyYa, A6Q6 and 1 other like this

#48 bremms

bremms

    Fly Me to the Moon

  • *****
  • Posts: 5234
  • Joined: 31 Aug 2012
  • Loc: SC

Posted 05 May 2016 - 12:57 PM

My kitchen table is about 4 feet in diamter, and my wife would kill me if I set up even a temporary optical bench there.  Ain't gonna happen until I get the basement cleaned out!  Of course, I have a friend with a Zygo- not everyone does.

Four feet is too small for me, I need 10-12' for mirror testing. 4-5' works with most refractors using a flat.



#49 orion61

orion61

    Vendor - Clear Edge Optical

  • *****
  • Vendors
  • Posts: 7058
  • Joined: 20 Oct 2007
  • Loc: Birthplace James T Kirk

Posted 05 May 2016 - 01:01 PM

 

Based on the corrector number (740205) my OTA is the 205th made in 1974.  I'm hoping that was a good year at Criterion.  And, the 1941 base number means it's the 941st DX8 made.

they really made that many?

 

Unless you bought your Dynamax new, you really don't get a grasp on how good they can be. IF they were all as horrible as some say how could they keep selling them for 10 years. I know Criterion too complaints seriously and took care of those customers.

In reality "MOST" of the Sad Sack stories come from someone buying a used one maz has passed through the hands of Lord knows how many people. All of those folks then repeat ALL are junk, there is even proof of my theory in the Articles section on this site where the Author ends his piece with the statement they are ALL Junk. This is after the stunning detective work of owning or looking through exactly ONE scope. Many a DX8 are still in the hands of their original owners and still in use. And I have looked through good to excellent examples. They did send out scopes they shouldn't have but many of the worst offenders were built after the Celestron Lawsuit, with much of the Companies cash assets going to the only people that really WON in that action, The Lawyers.



#50 tim53

tim53

    Voyager 1

  • *****
  • Posts: 14456
  • Joined: 17 Dec 2004
  • Loc: Highland Park, CA

Posted 05 May 2016 - 01:02 PM

My kitchen table is about 4 feet in diamter, and my wife would kill me if I set up even a temporary optical bench there.  Ain't gonna happen until I get the basement cleaned out!  Of course, I have a friend with a Zygo- not everyone does.

This would fit on your kitchen table, and could involve vegetable oil (the flat).  

 

You wouldn't even have to be a Mac person, but it would certainly help if you are... :grin:

 

post-6788-0-94936300-1450056947.jpg


  • bremms and Piggyback like this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics