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DPAC Test - C11

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#1 Jeff B

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Posted 18 February 2020 - 03:06 PM

This sample C11 started life a good 30 years ago.  I bought it used.  It's optics were decidedly mediocre at best, rough, with pronounced "spokes" visible in the figure (specifically, the out of focus rings).  Although I could indeed see more detail on Jupiter and the image was brighter, my old AP 6" F9 Starfire consistently ate its lunch in sharpness, contrast and, especially, what I call, image fidelity.  To be honest, the reasons I bought it were that it was cheap, had an active cooling system and was not bad on deep sky stuff.  And did I say it was cheap?

 

But the "ok" optics bugged me and I parked the scope for several years.

 

So about two or so years ago, at the urging of another CN's member, I contacted Celestron about the optics.  They were responsive and after some negotiation, with a promise of new and better optics, we settled on a reasonable to me price, got a return authorization, packed it up and off it went.   

 

A couple of months later, after they missed the return date, I gave them a call.   Seems the scope was done but languishing.  It went out the next day.

 

After set up, star collimation and a swing over to the Moon, it was immediately clear in the first millisecond (and I'm slow), this was now a muuuuuuuch better scope. Sure, like all SCTs of my acquaintance, focus was a bit touchy and the image did shift a little while focusing, but, unlike most SCTs of my acquaintance, the image was actually....sharp.   Very nice really.

 

Later, I used this scope to experiment with adding a couple of layers of Reflectix to have better thermal control of the system.  It works splendidly, significantly calming the scope's innards and significantly delaying the onset of corrector dewing.  Very nice.

 

But it has been screaming at me to put it into DPAC and so I relented.

 

Jeff

Attached Thumbnails

  • C11A.jpg
  • C11 In DPAC.jpg

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#2 Jeff B

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Posted 18 February 2020 - 03:30 PM

As a little bit of background, a fellow CN'er, John (precaud) put me onto a fantastic method of taking DPAC photos with a white LED and then separating out and isolating the Red, Blue, and Green channels of the JPEG image using, in my case, an ancient version of Paint Shop Pro.  This means I don't have to fiddle with discrete color LEDs.  This considerably reduces image "capture" times.  Comparisons of this method against using discrete color LEDs, shows, for the most part, the images to be very close in quality, with red, always a difficult color with discrete LEDs, to be a bit dimmer than blue and green, but, I believe, this is more a function of my cell phone camera's white balance (which, with this phone, is quite user adjustable).  

 

But one very definite leg up this new process has is that the extracted color images are all at the same focus positions(!)  This gives some objective intelligence on how each of the colors focus relative to each other as well as their spherical content.   And there are some surprises and interesting gleanings here and else where in future DPAC postings of other scopes.

 

Ok, so here we go, first in white light.

 

Hover the cursor over the image to read the titles.

 

Jeff

Attached Thumbnails

  • C11, DPAC, White, Inside of Focus.jpg
  • C11, DPAC, White, At Focus.jpg
  • C11, DPAC, White, Outside of Focus.jpg

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#3 Jeff B

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Posted 18 February 2020 - 03:33 PM

To me, this is a good optic in an absolute sense and a really excellent one for a mass produced, large aperture SCT.  Really very smooth too.  No wonder I liked it right away.


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#4 Jeff B

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Posted 18 February 2020 - 03:48 PM

Now on to the really cool stuff, separating out of the colors from the "white image".

 

First at focus.   Pay attention to how evenly illuminated the image is over the whole surface for each color paying specific attention to the shadows.  Also note the overall shape of the surface.

 

Note the dark shadow at the left hand edge of the blue image and how it transitions over to the right hand side of the red image.  Also, the shape of the surface in the blue image is a bit different than green and red.

 

Now, I do know that the RGB LEDS in the the "white" LED are small and very closely packed together so, there is a tiny, tiny bit of physical separation between them and so this may account for some of those shadow differences, however, there does appear to be a bit of something going on that depends on color.

 

 

Attached Thumbnails

  • C11, DPAC, Blue, At Focus.jpg
  • C11, DPAC, Green, At Focus.jpg
  • C11, DPAC, Red, At Focus.jpg

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#5 EricCCD

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Posted 18 February 2020 - 03:53 PM

I was wondering when you'd get around to this scope! :)



#6 davidc135

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Posted 18 February 2020 - 03:56 PM

Those optics are really impressive. Hopefully I can do colour separation with my OMD camera as well.  David



#7 Jeff B

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Posted 18 February 2020 - 03:57 PM

And Now, the outside of focus individual colors.

 

Well, well, well.  A bit of spherochromatism (shape shift of the blue bands) and a very subtle shift in focus (note the dark band creeping in from the left hand side and the right hand most band "elongating", which is a subtle shift in focus from blue to red).

 

So this SCT is not perfectly achromatic.  But very, very close.  But we knew this based on reading, educating ourselves and, well, looking through them.

 

But now, with DPAC, we actually see it.

 

Jeff

Attached Thumbnails

  • C11, DPAC, Blue, Outside of Focus.jpg
  • C11, DPAC, Green, Outside of Focus.jpg
  • C11, DPAC, Red, Outside of Focus.jpg

Edited by Jeff B, 18 February 2020 - 03:59 PM.

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#8 Mike Spooner

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Posted 18 February 2020 - 03:59 PM

Excellent optics, Jeff! Nice test, too.

 

Mike



#9 Richard O'Neill

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Posted 18 February 2020 - 04:17 PM

 Very nice, indeed! If I knew I'd get at least it's equal from Celestron I'd buy one in a heartbeat. 

 

Richard


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#10 J A VOLK

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Posted 18 February 2020 - 04:41 PM

A good number of the best SCTs I have seen over the years have had factory replacement optics. 



#11 TG

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Posted 18 February 2020 - 07:53 PM

Jeff what's the grating frequency?

#12 scooke

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Posted 18 February 2020 - 07:54 PM

Very nice optics.  I really need to get a bigger flat.  Although I could probably DPAC my C14 in sections with my 8" flat.  Has anyone tried doing that?



#13 Jeff B

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Posted 18 February 2020 - 08:11 PM

Jeff what's the grating frequency?

133 LPI



#14 stevew

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Posted 19 February 2020 - 02:56 AM

 

 

Later, I used this scope to experiment with adding a couple of layers of Reflectix to have better thermal control of the system.  It works splendidly, significantly calming the scope's innards and significantly delaying the onset of corrector dewing.  Very nice.

 

 

Jeff

Not to change the subject of the thread, but I suppose I will have to give this stuff a try.

I usually only take out my C11 when I know I'm going to be out for 4 or more hours, as it does take a while to settle down. However I have always been happy with the results I get from using my Lymax CAT Cooler. Of course it doesn't help if the temps are continually dropping...

Thanks for the tip.

 

Steve



#15 precaud

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Posted 19 February 2020 - 11:57 AM

NIce test, Jeff. But the blues are way oversaturated, which is washing out the shape of the Ronchi lines in the image.

 

If this is the same camera you used a few days ago, we saw that it hypes the blues, but most cameras use the green cells to determine the exposure (capture time, aperture, etc.). Look at a histogram of the original pic and you'll see that the blue values are spiked at max level. Or, if PSP shows RGB values of individual pixels, pan it around and you'll see that the blue is almost always maxed out (255) in the white regions. This is telling you it is overexposed, and to walk back the exposure a step or two so it can capture the detail.

 

The attached image shows the RGB readout of one of the pixels in the white region. As you can see, the blue level is saturated (255), while the red and green are in-bounds. You'll see similar values throughout all of the bright-white regions in the photo.

 

Setting the exposure control back by 1/3 step at a time would be a good first move, until all the blues fit within the dynamic range of the 8-bit data. But the truth is, you really need a better camera for the purpose... one that doesn't mess with the color balance... the good news is, they're cheap!  smile.gif

Attached Thumbnails

  • jeffsct.jpg

Edited by precaud, 19 February 2020 - 12:06 PM.

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#16 Jeff B

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Posted 19 February 2020 - 01:09 PM

Hey John, thanks for the analysis!  Much appreciated.

 

I did indeed see the blue overexposure but the "story" regarding the blue was unchanged.  The overall shape of the blue lines (and the other colors for that matter) remains the same when compare to looking through the grating using an individual blue LED.  Basically, visually, I saw the same thing, even in the C11.  The blue was different than the red and green.

 

My cell phone camera is just sooooo easy to use so I'll mess around some more with the settings...actually, I did for some subsequent shots with other scopes, but I will tweak the settings some more.

 

Jeff



#17 precaud

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Posted 19 February 2020 - 01:56 PM

I did indeed see the blue overexposure but the "story" regarding the blue was unchanged.  The overall shape of the blue lines (and the other colors for that matter) remains the same when compare to looking through the grating using an individual blue LED.

 Yes, but if the blues are being overexposed using an individual blue LED, then it too is flawed, and it's not a good comparison.

 

Optics may not be my forte, but test and measurement is... you can't draw reliable conclusions from nonlinear tools. We don't know if the LED's spectrum is linear (it probably isn't); if the camera's lenses are perfect (they probably aren't); if it's sensor is linear vs intensity in all color channels; or what it's firmware is doing to the data. So check your test setup thoroughly BEFORE getting too deep in the analysis...


Edited by precaud, 19 February 2020 - 02:27 PM.


#18 starman876

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Posted 19 February 2020 - 03:31 PM

That is one of the better DPAC test I have seen of a C11.  Very nice optics.


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#19 aa6ww

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Posted 19 February 2020 - 07:10 PM

It looks painful sitting on a Losmandy GM8. I've seen these and used these on a GM8, I definitely isn't one of my top mounts for a C11.

...Ralph

#20 vahe

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Posted 19 February 2020 - 07:47 PM

It looks painful sitting on a Losmandy GM8. I've seen these and used these on a GM8, I definitely isn't one of my top mounts for a C11.
 

 

Jeff B needs to clarify, if you are referring to the photo from Post #1, it looks like G11 head sitting on GM8 tripod.

.

Vahe




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