199 replies to this topic

[AlMuz]

Alright ladies and gentlemen, finally I found some time to get my hands on my latest FB marketplace acquisition (serial number: 4916)

As  I've mentioned - in my case the corrector lens was loose and sliding sideways when I tilted the OTA.
Also the eyepiece and 1.25" diagonal prism were pretty dirty.
 

So I did not bother to do any optical testing before giving it a full clean and service.

 

What was done:

1. Corrector plate was taken out 
2. Secondary assembly was taken apart
3. Corrector has been washed with everything I had (starting with isopropyl alcohol and then metene lens-wipes, then tryng window wash, Clorox and WD40), but few spots feel like acid-etched and did not go off (see the knife-edge picture - there are few "splatters" on the right side).
4. Secondary was washed with isopropyl alcohol spray and metene lens-wipes, looks very clean. Reflective coating has some signs of deterioration, when shining the light from the sides I could see a few tiny holes
5. Primary was sitting pretty tight and focuser was operating pretty smooth without backlash. So I did not bother taking it out. 
6. Primary was not too dirty, with lens air blower I took off some larger dust pieces and with large and ultra-soft makeup brush I took off the rest. It probably would benefit a little more from a full wash, but over all "good enough" condition was not worth the trouble. 
7. In my case the corrector retained by large threaded ring, which was not to rusted/corroded, so it went off pretty easily. 
8. Unlike on Gil's videos  - corrector was not glued with rubber compound and was shifting sideways when you tilt the tube. I did not notice any residue from the glue either. 
9. There were cork pads around the permitter of corrector frame, they were pretty packed (hard) and brittle, so did not perform their padding/cushioning function. 
10. The was an O-ring on secondary assembly to center it in the corrector plate. It does its job not perfect, but fair enough to maintain secondary more or less centered in the correctors elliptical hole. So I did not bother to improve it in any way - just gave a clean to rubber gaskets and put it back toughater.
11. To fix two problems of corrector plate cushioning and centering in OTA I've found in my drawer 3 mm thick neoprene pads with sticky adhesive on one side (these came from my phone screen protector installation kit). I've cut them to smaller pieces (5x14 mm) and placed next to existing cork pads all around the permitter.
12. Corrector plate went in nicely centered, without any excessive any pressure around its perimeter. The only exception are two pads being moderately compressed on the opposite ends of the larger diameter of the correctors elliptical shape.
13. The diagonal and 30 mm eyepiece were fully taken apart and given a good wash - now as good as new. Clean and shiny.
14. Finder only needed a wipe of its eyepiece lens and a front objective surface, I did not take it apart - was pretty clean inside.
15. The drive is still functional (rotates the scope), however the RA adjustment knob does not. When I roate RA knobn it feels lile there is a stripped off gear on the other end. Since the drive still rotates the fork - I did not bother to take it apart yet, this will be a project for another weekend.
16. I gave the OTA preliminary collimation by looking into the OTA from the front (corrector) side with one eye at different distances (3 ft, 5ft, 10ft) and adjusting secondary screws until I could see concentric reflections between its mirrors and baffle.

Things to note:

• Both outer and inner perimeters of the corrector are oval instead of perfectly round.
• There is like 5 mm difference between smallest and larges diameter on outside edge.
• There is roughly  ~1.5 mm difference on the inside hole for secondary.
• Outer edge of the corrector is also conical on some its segments.
• I don't know - maybe its a "feature" to be able to center center corrector/secondary against the primary. But anyway - even larger outer diameter were pretty loose against the old cork pads on  OTA frame.
• ​Both edges of corrector (inner and outer) are having quite rough cut, it looks like whoever was trying to get it to circular shape was chipping the sides of the glass by a chisel hit with a brick instead of using proper rotary tools with diamond blades. It is the worst piece of optical manufacturing I've ever seen.
• Much smaller size of the corrector plate than its OTA framing made me think that corrector plate might be not original. I have hard time to believe that the Criterion manufacturing quality was this poor.
• I had a thought that it could have been a piece of flat window glass which previous owner had cut at home to replace original broken corrector. However it does not seem to be the case - putting a ruler to the side of the corrector I could see its tiny figure of the surface (slightly narrower on the outer edge than in the center). 

Unfortunately I did not take any pictures during corrector re-assembly/cleaning process, it was quite a bit late and tiring after work to take pictures in the process.

Knife-edge test - you can see two "splatters"  on the right side These looks like acid damage to the glass surface of corrector. I can feel the roughness of these by run my fingernail through the surface.
Ignore bright dots - these are specs of dust which I did not manage to remove from outer surface before taking a pic​.


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First light and artificial star test

• Friday night in Boston suburbs was a bit cloudy, but I wanted to check the image quality of the scope after rebuilding it. 
• I have an artificial star which I made out of high power LED torch light, LED diffusing plastic and 1 mm hole drilled in sheet metal. Placed at 400 ft away it represent quite solid and bright test target which I use primarily to collimate my AT8RC.
• First light at earth target (a building 450 ft away) was above my expectations. I was able to see the wooden texture of a door frame through original diagonal and eyepiece.
• Focusing tilt is horrific, but when you get it right its pretty sharp for the money. It will be a while - but I a pretty sure this old boy is capable to resolve some bands on Jupiter.
• It took a scope around 1 hour 30 min to cool down, until the diffraction rings did not look like a dancing Christmas tree. Then I went back into my apartment to pickup DSLR and a 5 mm Celestron Ultima Duo to capture diffraction rings.

​Keep in mind that I live in close Boston suburbs, and my artificial star was on the other side of the parking lot. The flow of hot air from the paved surface and colling off car engines made air a bit jiggly.
But with a series of images you can get the idea of wavefront roughness introduced by the scompe itself. 

First pictures with directly attached DSLR (no diagonal, no eyepice, just a T-ring 1.25 adapter between OTA and camera) - these are cropped out diffraction rings at a larger defocusing values. 
Resolution of all images (pixel size) is original to my Pentax K5 (no up-sampling or down-sampling, just crops).
Please click on images to get into the gallery and see a full resolution
 

Few picture at the larger defocus values

A series of the shots to get the idea of air turbulence between my artificial star and the scope 

Here is focusing to the best of what I could get with attached camera.

Click on the image to navigate to full resolution in the galley.
Its not a airy-disk you would expect in properly figured optics, its a whole gang of individual manufacturing issues.  

And here is eyepiece projection, these are basically 420x, defocused and best focus I could get:

Edited by AlMuz, 04 May 2024 - 06:46 PM.

[DAVIDG]

"Much smaller size of the corrector plate make me think that corrector plate might be not original. I have hard time to believe that the Criterion manufacturing quality was this poor."

 

  You must have missed a number of threads were a number of these have been tested and shown to have VERY poor optical quality from the fact that the method Criterion used to make the the correctors was fundamentally flawed  and could never produce an good quality corrector.

   Your optical test are confirming this on your example as well. These scope are nice to look at but awful to look through.

 

 Here is  a typical example when you test one via double pass collimation showing the very rough optics, spherical aberration and astigmatism. This why you can't get a sharp focus with yours.

 

                - Dave 

 

[AlMuz]

  You must have missed a number of threads were a number of these have been tested and shown to have VERY poor optical quality from the fact that the method Criterion used to make the the correctors was fundamentally flawed  and could never produce an good quality corrector.

I've seen these threads, and I can understand if the process is not right the figure of the surface will be terrible optically.
But I did not expect inner and outer edges of corrector to look like the were chiseled by a caveman.

I wonder if the acid damage spots were there originally or just a result of poor maintenance/storage.

[DAVIDG]

I have about 10 of these 8" correctors and couple of 4" one. I bought them from the person that worked for Bausch and Lomb when Criterion/ Bausch Lomb went out of business. They all show this chipped edges and are not round. Optically they are not very good They were made from common window glass and yes this was the quality or lack of at which they were made.

   Your scope will show the bands on Jupiter but if you do a side by side test will a telescope that has good optics you'll seethe  image is like viewing the image on an old analog TV vs a modern 4K HD one. 

 

               - Dave 

[GUS.K]

"Much smaller size of the corrector plate make me think that corrector plate might be not original. I have hard time to believe that the Criterion manufacturing quality was this poor."

 

  You must have missed a number of threads were a number of these have been tested and shown to have VERY poor optical quality from the fact that the method Criterion used to make the the correctors was fundamentally flawed  and could never produce an good quality corrector.

   Your optical test are confirming this on your example as well. These scope are nice to look at but awful to look through.

 

 Here is  a typical example when you test one via double pass collimation showing the very rough optics, spherical aberration and astigmatism. This why you can't get a sharp focus with yours.

 

                - Dave 

 

DX8DOUBLEPASSOUTFOCUS.jpg

Makes you wonder if they ever tested the optics. 

[AlMuz]

   Your scope will show the bands on Jupiter but if you do a side by side test will a telescope that has good optics you'll seethe  image is like viewing the image on an old analog TV vs a modern 4K HD one. 

Yeah, no illusions here, I was just trying to say that is not as terrible as I was expecting after reading this whole thread and taking out the roughly cut corrector.

Closer to the focus Airy disk or Point Spread Function looks funny.
Its not even about single point, its like a party of "points" trying to get closely together at their best ability.
Like a bunch of drunk guys trying to high-five each other simultaneously but each misses just a bit.  
 

[pyrasanth]

I think I must be sad because I miss the adventure in the 70's with my old Dynamax 8. I remember when the sky was dark and clear- sometimes we had weeks of clear sky- I miss those days and sadly with the changes in the weather I don't think they are ever coming back in my lifetime.

[oldmanastro]

This discussion reminds me so much of my former B&L 8000, basically a Dynamax 8 with black paint. The irregularly cut corrector, inability to get a good focus on a star no matter how good the seeing, the centering, the collimation, an unforgettable optical disaster. I got rid of it more than 20 years ago and still remember. It was a long time before I tempted my luck with another SCT. My old Coulter 8" f/7 ran circles around the B&L 8000. I still regret selling that Coulter. 

Your star images look better than what I remember from my B&L 8000. The original Dynamax 8 is such a nice looking scope.

[deSitter]

This discussion reminds me so much of my former B&L 8000, basically a Dynamax 8 with black paint. The irregularly cut corrector, inability to get a good focus on a star no matter how good the seeing, the centering, the collimation, an unforgettable optical disaster. I got rid of it more than 20 years ago and still remember. It was a long time before I tempted my luck with another SCT. My old Coulter 8" f/7 ran circles around the B&L 8000. I still regret selling that Coulter. 

Your star images look better than what I remember from my B&L 8000. The original Dynamax 8 is such a nice looking scope.

Yes they are so beautiful on the outside that one with good optics rebuilt by someone good at it, would be a very precious telescope.

 

-drl

[davidc135]

Whilst a replacement corrector plate from a Celestron C-8 or a non ACF Meade would be over-corrected, there would be the ATM option of figuring the secondary to match and at the same time largely correcting the scope for coma.

 

In fact, it would be interesting to see if a C-8 or non ACF 8'' corrector plate, a secondary from an 8'' ACF together with a Dynamax DX-8 or B&L 8000 primary mirror are solved for spherical aberration. I think it's close. But in any case, figuring a secondary should be manageable, at least in comparison to making a decent new corrector plate,

 

There's the expense of buying a Celestron c. plate but might be worth it.

 

David

[deSitter]

Whilst a replacement corrector plate from a Celestron C-8 or a non ACF Meade would be over-corrected, there would be the ATM option of figuring the secondary to match and at the same time largely correcting the scope for coma.

 

In fact, it would be interesting to see if a C-8 or non ACF 8'' corrector plate, a secondary from an 8'' ACF together with a Dynamax DX-8 or B&L 8000 primary mirror are solved for spherical aberration. I think it's close. But in any case, figuring a secondary should be manageable, at least in comparison to making a decent new corrector plate,

 

There's the expense of buying a Celestron c. plate but might be worth it.

 

David

This sounds like a project for you or DAVIDG!

 

-drl

[tim53]

Whilst a replacement corrector plate from a Celestron C-8 or a non ACF Meade would be over-corrected, there would be the ATM option of figuring the secondary to match and at the same time largely correcting the scope for coma.

 

In fact, it would be interesting to see if a C-8 or non ACF 8'' corrector plate, a secondary from an 8'' ACF together with a Dynamax DX-8 or B&L 8000 primary mirror are solved for spherical aberration. I think it's close. But in any case, figuring a secondary should be manageable, at least in comparison to making a decent new corrector plate,

 

There's the expense of buying a Celestron c. plate but might be worth it.

 

David

The problem with using celestron or Meade optics in a Dx8 is the need to part outa celestron or Meade to do it. I doubt you could buy optics from them without buying a whole scope, assuming they’d even fit the tube. Probably the cheapest way to do this would be to buy a used ota, but then you’d still be parting out a known functional telescope to maybe improve another. And as we all know from painful experience, finding just a corrector on the used market is a challenge, since first dibs rightfully go to an owner who’d broken their own corrector. 

Edited by tim53, 07 May 2024 - 09:03 PM.

[davidc135]

The problem with using celestron or Meade optics in a Dx8 is the need to part outa celestron or Meade to do it. I doubt you could buy optics from them without buying a whole scope, assuming they’d even fit the tube. Probably the cheapest way to do this would be to buy a used ota, but then you’d still be parting out a known functional telescope to maybe improve another. And as we all know from painful experience, finding just a corrector on the used market is a challenge, since first dibs rightfully go to an owner who’d broken their own corrector. 

Yes, it could be difficult. At one time replacement 8'' Celestron correctors could be bought in the UK for around £280 IIRC. I don't know whether that's still true.

 

I think they would fit, maybe the centre hole would be a different size.

 

David

[saemark30]

I would do away with all the bipolar transistors and use N-channel power VFETS. This eliminates 7406 driver as well.

Just connect the gates to pin 12 & 13 on the 7473, sources to ground and drains where the collector of the power transistors connect.

Makes more sense (to me) to use a center tapped 24V to 120V transformer instead of the 12VCT transformer in the Saxon circuit.

Edited by saemark30, 08 May 2024 - 02:33 PM.

[LU1AR]

Yeah, no illusions here, I was just trying to say that is not as terrible as I was expecting after reading this whole thread and taking out the roughly cut corrector.

Closer to the focus Airy disk or Point Spread Function looks funny.
Its not even about single point, its like a party of "points" trying to get closely together at their best ability.
Like a bunch of drunk guys trying to high-five each other simultaneously but each misses just a bit.  
 

Perhaps thermal issues (Into the tube, or outer athmosphere).
Edgardo

[orion61]

If there is a good series of DX-8 It is the one you have. The tubes are nearly indestructible.

Congrats on a scope that will give you hundreds of hours of Deep space pleasure, and possibly some 
Lunar and Planetary work. Learn how to collimate it,
Duane

[AlMuz]

Few updates on my DX8:

I think I've reached collimation peak with this scope and it can not go any better.

Fixed an R.A, knob and 3D-printed front covers for a scope and finder.
Decided to go with red to match knobs color on the sides of the fork.

 

Did some imaging session of the moon yesterday.
it actually looks a lot better in the eyepiece than what my Pentax K5 was able to pull

SvBony SV305 does a better job

https://youtu.be/hNh...TMPHlyErWT8bjUF

Edited by AlMuz, 14 June 2024 - 12:25 PM.

[rolo]

Always nice to see one in service 

[Bomber Bob]

The Dynamax 8 is a beautiful Classic:

 

 

But as an 8" SCT, it's a dud.  True Story:  One of the first-year B-1B Bombers had to be delivered by RAIL because it could not fly - left wing was about one foot shorter than the right... The Solution:  Put it on Nuclear SIOP Alert, where it would never be required to fly.  IOW:  a Display Jet...

 

But, at least Criterion couldn't be blamed for it.

[Goofyboy1]

Few updates on my DX8:

I think I've reached collimation peak with this scope and it can not go any better.

Fixed an R.A, knob and 3D-printed front covers for a scope and finder.
Decided to go with red to match knobs color on the sides of the fork.

 

 

Did some imaging session of the moon yesterday.
it actually looks a lot better in the eyepiece than what my Pentax K5 was able to pull

SvBony SV305 does a better job

https://youtu.be/hNh...TMPHlyErWT8bjUF

Do you have the STL file and do you will share it with us? I have the original telescope cover but not from the finder

[AlMuz]

Do you have the STL file and do you will share it with us? I have the original telescope cover but not from the finder

Here is the design.
https://cad.onshape....ae8687aa2f32a86
I am printing with 0.8 mm nozzle and adjusting tolerances accordingly. 
You should be able to export STL or whatever format you need (right click on the DynmaxFinder_V2 on the bottom and click export).

Did not know these scopes had covers. My came with a shower-cap from previous owner.

Edited by AlMuz, 15 June 2024 - 12:26 PM.

[Tom Stock]

  The difference between Solar rate which the telescope is gear for and Sidereal rate in only 4 minutes per day and you'll never see that error doing visual observing. 

Why did they offer this feature? 4 minutes over 24hrs shouldn't have really effected long exposure photography either, especially since everyone was manually guiding anyway.

Edited by Tom Stock, 18 June 2024 - 09:01 PM.

[Michael Covington]

When telescope electronics were just starting to become more sophisticated in the 1970s, sidereal drive had a certan amount of snob appeal.  It's rigorously correct.  Admittedly, it differs from solar rate by only 1/24 degree per hour, which is going to be lost in the much larger errors from slightly inaccurate polar alignment, etc.

[myronwasiuta]

Hi ,
I just saved this old DX8 from an uncertain future. I don’t know anything about its original owner or the history of the telescope. Haven’t even looked through it yet. I have the tripod and will set it up for some star testing. The drive seems to work( at least the red light comes on) so I’ll do some webcam star testing and post those results when I can. I’m not expecting much though. I’m curious about the tube color. I don’t know if it came this coloe( black) or if it was painted sometime after purchase. It came with a shower cap made by Paul Mitchell. The serial number on the OTA is 750187 and on the base is 1758.

[Goofyboy1]

Here is the design.
https://cad.onshape....ae8687aa2f32a86
I am printing with 0.8 mm nozzle and adjusting tolerances accordingly. 
You should be able to export STL or whatever format you need (right click on the DynmaxFinder_V2 on the bottom and click export).

Did not know these scopes had covers. My came with a shower-cap from previous owner.

Sorry for my late reaction. Thanks!