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Mid 80's C8 as bad as Dynamax 8 ?

catadioptric classic optics SCT
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#51 starman876

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Posted 09 August 2019 - 07:54 PM

Now where have I heard that before?

over and over and over and over . 



#52 starman876

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Posted 09 August 2019 - 07:58 PM

 I am playing fair. People keep saying that  they have  a good ones but  so far we have only seen bad ones when actually tested going back to  the 1989 Sky and Tel article,  to the present with  examples posted in this forum and other forums on Cloudy Nights.  So instead of  having this be a  bashing of SCT's thread  lets get some actual test results. That will be more informative to see what is really going on with the  quality. This thread asked the question if  it  is true that the ones made during the Halley'Comet years were poor in quality. Getting more examples will help answer that question  So if you have one and can optically test it , post the results. 

 

                     - Dave 

I meant requiring DPAC results before someone states how great or bad the optics are.  Now if everyone could only comment on scope performance with DPAC results to back it up there would be a lot of folks who would never say much anymore.  That would be kind of nice now I think of it. lol.gif


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#53 starman876

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Posted 09 August 2019 - 08:06 PM

I don't have a problem with folks posting a "stacked" image, so long as they state that, because that's the way I sketch planets.  I've stayed at the eyepiece for an hour or more (with breaks to stretch my neck!), and I sketch each detail as it appears / when the seeing allows.  I've had nights with 2 or more dozen individual sketches that I amalgamate into a final single version that I then scan & post.

 

I don't see a big difference.  I have seen a difference using Snap Shot Mode on my ASI120MC:  By the time I click, that perfect view is gone due to the short lag time, and I wind up trash-canning the image.

 

As a minimum, post the scope, camera, and stacking program.  IF you run the resulting image through PhotoShop or other enhancement software, then state that.  If the image you post is average, better, or the worst compared to what you see with the scope, state that, too.  I think I did that when I posted images from the Orion StarShoot 2 that I started out with, because there were lots of my early attempts that were way below what I actually saw at the eyepiece.  (I know, cheap gear, cheap results.)

 

Honestly, if my sketches didn't stink on ice, I wouldn't bother with imaging at all.  I like being able to share a bit of what I saw with others, and that's about the best option for me.

what people tend to forget is that the stacked images still need detail that only the scopes optics can produce.  If the detail is not there all the processing in the world will not provide a great final image.  


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#54 ccwemyss

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Posted 09 August 2019 - 08:14 PM

 One could make up an oil flat as large as needed and test away. An oil surface is flat to about 1/300 wave so no error from it.  See this thread for how it is done. https://www.cloudyni...limation-test/ 

 

 

                  - Dave 

True, but if my wife ever caught me pouring motor oil into a pan on the floor, anywhere inside the house, then I would have to go to the town dump to retrieve my scopes for testing.

 

Chip W.



#55 clamchip

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Posted 09 August 2019 - 08:20 PM

One way to see if your SCT is in the major league is set the mirror spacing with the "supplied prism and 25mm ep at infinite focus" and

once you have the mirrors set now you can leave the focus knob alone and defocus by sliding the eyepiece the same amount both sides

of focus the same amount.

The view should look the same, be careful not to defocus too much just a small amount.

 

Robert


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#56 DAVIDG

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Posted 09 August 2019 - 11:08 PM

 You want to use an eyepiece that has a focal length that is close to or equal to the F-ratio of the telescope to star test.  So for a C-8 that is F/10 use a 10mm eyepiece. That will give you enough magnification to see the difference in the diffraction pattern of  a star in both the inside and outside of focus position. Also it is critical that you only slightly defocus the star say only a 1/16" of movement of the eyepiece from one side of focus to the other. Too low of magnification and/or too much defocus will make bad optics look good.

 

                       - Dave 


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#57 DAVIDG

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Posted 09 August 2019 - 11:15 PM

True, but if my wife ever caught me pouring motor oil into a pan on the floor, anywhere inside the house, then I would have to go to the town dump to retrieve my scopes for testing.

 

Chip W.

 You don't have to use motor oil, cooking oil works just as well and you are only using a few ounces. The pan holding the oil can be easily placed in a larger one so  the risk of spilling  any is greatly reduced.  So if there is the will there is  away to test your optics in your SCT even a large one like a C-14 for very little money but with an extremely sensitive test that will quickly tell you the true quality of what you have. 

 

                  - Dave 


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#58 starman876

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Posted 10 August 2019 - 07:16 AM

Wonder if the oil pan method could be used to verify a flat?  



#59 rolo

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Posted 10 August 2019 - 07:19 AM

Wonder if the oil pan method could be used to verify a flat?  

So, do you use the oil before after an oil change? A potential environmental hazard if not disposed of properly.


Edited by rolo, 10 August 2019 - 07:20 AM.

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#60 Bomber Bob

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Posted 10 August 2019 - 08:23 AM

what people tend to forget is that the stacked images still need detail that only the scopes optics can produce.  If the detail is not there all the processing in the world will not provide a great final image.  

Exactly, if the scope didn't catch the detail, stacking and/or post-processing (with software like PhotoShop) ain't gonna make it magically appear in the final image.

 

Again, when posting an image, treat it like any other observing report:  the gear, the seeing, the techniques used, etc.  And, let readers know how accurately the image corresponds with visual performance.

 

Here's an SCT example.  Jupiter & the GRS with my C5 + ASI120MC.  I had collimated & cleaned the optics before the session.  Seeing was 7 / 10, but Jupiter was very low in the SE.  My first imaging with the C5, and I hadn't done any imaging in a long time, and I forgot to wear my reading glasses!  So, focus was a bit off.  Views of Jupiter are much sharper than this image:

 

C5 - Jupiter (GRS) 20180609V03R11.jpg

 

This was from a year ago.  When I watch the AVIs (videos) from the session, I'm happy with it given all the conditions.  My C5 got red in the GRS, orange in the NTB, and a moon shadow.  Not bad for a compact general purpose scope, and much better than the images made with my DX8 (and that was with Jupiter near the meridian).

 

Terry Wood & others have posted very fine images made with their CATs (check the "Small Bore ~ " thread if you haven't already).  I have no doubt that quality varied between Celestron SCTs over the decades, and the C8 Rolo posted looks like a really bad sample.  So, it pays to test your mass-produced scope, correct the things you can, and share that with the Forum.



#61 DAVIDG

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Posted 10 August 2019 - 09:10 AM

Wonder if the oil pan method could be used to verify a flat?  

  Yes if the oil is transparent since the flat needs  to  be immersed in the oil. Water is better since it easier to clean off the flat when your done testing. Testing a flat with water is the Raleigh Water test https://opticaleds.c...test-for-flats/

  So again if your willing  to do the work to set up the test, you have a very accurate method that cost very little. 

 

               - Dave 


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#62 tim53

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Posted 10 August 2019 - 09:52 AM

One way to see if your SCT is in the major league is set the mirror spacing with the "supplied prism and 25mm ep at infinite focus" and

once you have the mirrors set now you can leave the focus knob alone and defocus by sliding the eyepiece the same amount both sides

of focus the same amount.

The view should look the same, be careful not to defocus too much just a small amount.

 

Robert

Which is why, since I have to rework the correctors anyway, I'm thinking of taking the incomplete DX8 and making a new OTA for it that focuses with a...  ...wait for it...  ...crayford or R&P focuser so the mirror spacing doesn't change.



#63 starman876

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Posted 10 August 2019 - 09:57 AM

  Yes if the oil is transparent since the flat needs  to  be immersed in the oil. Water is better since it easier to clean off the flat when your done testing. Testing a flat with water is the Raleigh Water test https://opticaleds.c...test-for-flats/

  So again if your willing  to do the work to set up the test, you have a very accurate method that cost very little. 

 

               - Dave 

I think i will just use a smaller flat and put it over different sections of the larger flat.  Just dont want to put a flat into the oil.



#64 DAVIDG

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Posted 10 August 2019 - 12:35 PM

I think i will just use a smaller flat and put it over different sections of the larger flat.  Just dont want to put a flat into the oil.

 That is fine but the sensitivity isn't very good. As I said water works just as well as oil and no clean up problems plus you  are testing the whole surface so it has the greatest sensitivity So you can measure large flats  say 12" or larger to  a very high degree without the need for Master glass flat of that size. 

   All comes down to how much effort one wants to put into finding the best  answer. 

 

                         - Dave



#65 CHASLX200

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Posted 10 August 2019 - 02:00 PM

Exactly, if the scope didn't catch the detail, stacking and/or post-processing (with software like PhotoShop) ain't gonna make it magically appear in the final image.

 

Again, when posting an image, treat it like any other observing report:  the gear, the seeing, the techniques used, etc.  And, let readers know how accurately the image corresponds with visual performance.

 

Here's an SCT example.  Jupiter & the GRS with my C5 + ASI120MC.  I had collimated & cleaned the optics before the session.  Seeing was 7 / 10, but Jupiter was very low in the SE.  My first imaging with the C5, and I hadn't done any imaging in a long time, and I forgot to wear my reading glasses!  So, focus was a bit off.  Views of Jupiter are much sharper than this image:

 

attachicon.gif C5 - Jupiter (GRS) 20180609V03R11.jpg

 

This was from a year ago.  When I watch the AVIs (videos) from the session, I'm happy with it given all the conditions.  My C5 got red in the GRS, orange in the NTB, and a moon shadow.  Not bad for a compact general purpose scope, and much better than the images made with my DX8 (and that was with Jupiter near the meridian).

 

Terry Wood & others have posted very fine images made with their CATs (check the "Small Bore ~ " thread if you haven't already).  I have no doubt that quality varied between Celestron SCTs over the decades, and the C8 Rolo posted looks like a really bad sample.  So, it pays to test your mass-produced scope, correct the things you can, and share that with the Forum.

Now that image is more like what i see with my eyes.



#66 starman876

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Posted 10 August 2019 - 02:58 PM

Exactly, if the scope didn't catch the detail, stacking and/or post-processing (with software like PhotoShop) ain't gonna make it magically appear in the final image.

 

Again, when posting an image, treat it like any other observing report:  the gear, the seeing, the techniques used, etc.  And, let readers know how accurately the image corresponds with visual performance.

 

Here's an SCT example.  Jupiter & the GRS with my C5 + ASI120MC.  I had collimated & cleaned the optics before the session.  Seeing was 7 / 10, but Jupiter was very low in the SE.  My first imaging with the C5, and I hadn't done any imaging in a long time, and I forgot to wear my reading glasses!  So, focus was a bit off.  Views of Jupiter are much sharper than this image:

 

attachicon.gif C5 - Jupiter (GRS) 20180609V03R11.jpg

 

This was from a year ago.  When I watch the AVIs (videos) from the session, I'm happy with it given all the conditions.  My C5 got red in the GRS, orange in the NTB, and a moon shadow.  Not bad for a compact general purpose scope, and much better than the images made with my DX8 (and that was with Jupiter near the meridian).

 

Terry Wood & others have posted very fine images made with their CATs (check the "Small Bore ~ " thread if you haven't already).  I have no doubt that quality varied between Celestron SCTs over the decades, and the C8 Rolo posted looks like a really bad sample.  So, it pays to test your mass-produced scope, correct the things you can, and share that with the Forum.

My portaball 12.5" shows a lot more detail than that picture visually on good nights. Have been some nights I thought I was flying over the planet.  Maybe the reason I have kept that scope for so lone.  



#67 Bomber Bob

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Posted 10 August 2019 - 03:32 PM

My portaball 12.5" shows a lot more detail than that picture visually on good nights.

 

Okay...  Here's an image made with a Unitron 142 3" F15 -- it's not an SCT, either:

 

Unitron 142 - Jupiter (GRS) 20180607V04A64R41.jpg



#68 terraclarke

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Posted 10 August 2019 - 04:09 PM

Which is why, since I have to rework the correctors anyway, I'm thinking of taking the incomplete DX8 and making a new OTA for it that focuses with a...  ...wait for it...  ...crayford or R&P focuser so the mirror spacing doesn't change.


If you are going to take it apart and redo the optics, what about forgetting the corrector all together and making it a Gregorian?

#69 clamchip

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Posted 10 August 2019 - 05:50 PM

I'm thinking the same thing for the DX8, why not remove the corrector altogether suspend the secondary on a optical window

or 3-vane spider, and correct the SA with corrective optics at the focal plane or invent a Schmidt eyepiece specifically to counter the SA.

 

Robert  



#70 DAVIDG

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Posted 10 August 2019 - 06:10 PM

If you are going to take it apart and redo the optics, what about forgetting the corrector all together and making it a Gregorian?

 The primary is F/2 so for  a Gregorian you have to figure that F/2 sphere into  a high quality F/2 parabola. Then you also have to deal with all the coma of an F/2 parabola.  Gregorians work best with a F/5 parabolic primary. The Devil is always in the  details. In my opinion it would much more difficult to convert the optics in SCT into another design then it would be to  make a new corrector, but that is just me.

 

                - Dave 


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#71 terraclarke

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Posted 10 August 2019 - 06:24 PM

 The primary is F/2 so for  a Gregorian you have to figure that F/2 sphere into  a high quality F/2 parabola. Then you also have to deal with all the coma of an F/2 parabola.  Gregorians work best with a F/5 parabolic primary. The Devil is always in the  details. In my opinion it would much more difficult to convert the optics in SCT into another design then it would be to  make a new corrector, but that is just me.

 

                - Dave 

Thank you Dave for clarifying that for me. No that I read this I believe I asked the question once before and you told me it was impractical then as well for the same reason. Good to have an expert to answer these questions. I guess hat a classical cassegrain would be the same thing- too deep, too short?



#72 ccwemyss

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Posted 10 August 2019 - 06:41 PM

 You don't have to use motor oil, cooking oil works just as well and you are only using a few ounces. The pan holding the oil can be easily placed in a larger one so  the risk of spilling  any is greatly reduced.  So if there is the will there is  away to test your optics in your SCT even a large one like a C-14 for very little money but with an extremely sensitive test that will quickly tell you the true quality of what you have. 

 

                  - Dave 

You have no idea how clumsy I am. But my wife does. I can just hear it: "You're going to tip that over, and then the oil will go in the cracks in the oak flooring, and the house will smell like a fryolator forever." And she'd probably be right.

 

I'm going to stick with dry optics. Besides, nearly all the scopes I have will be covered by the 6" flat. If I had a separate building for my workshop, this sounds like something that would be fun to try though.

 

Chip W. 



#73 starman876

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Posted 10 August 2019 - 06:45 PM

You have no idea how clumsy I am. But my wife does. I can just hear it: "You're going to tip that over, and then the oil will go in the cracks in the oak flooring, and the house will smell like a fryolator forever." And she'd probably be right.

 

I'm going to stick with dry optics. Besides, nearly all the scopes I have will be covered by the 6" flat. If I had a separate building for my workshop, this sounds like something that would be fun to try though.

 

Chip W. 

I feel the same way with oil.  Just sounds like an accident looking for a place to happen.



#74 tim53

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Posted 10 August 2019 - 07:34 PM

you could certainly use water, but even butterfly farts 30 feet away will cause ripples in the surface.  Or it will seem that way.



#75 tim53

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Posted 10 August 2019 - 07:37 PM

I'm thinking the same thing for the DX8, why not remove the corrector altogether suspend the secondary on a optical window

or 3-vane spider, and correct the SA with corrective optics at the focal plane or invent a Schmidt eyepiece specifically to counter the SA.

 

Robert  

Probably the only thing that would work of these ideas would be the optical window with a doublet corrector in the secondary holder, aluminized on the back - like the Meade mirror lens, or a Tal Klevtsov.

 

I'm not sure that would be easier than fixing the stock corrector, though.




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