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16" and 11" side by side on Jupiter tonight

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#1 edsplace

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Posted 13 December 2012 - 01:45 AM

I had both scopes out tonight and imaged with them. Seeing was average for this location at best. While the planet was steady, neither image cleaned up well. I think again most of the poor seeing was high in the atmosphere and just wouldn't let fine detail through. I am not sold on these results completely and am not sure what such a large difference. I know the C11 was more difficult to focus, and I used the best image set. I did over sample the 16" is was reduced to 75% so it matched the 11" scale. The CPC1100 image used about 350 more frames in the stack, while the 406mm reflector had a lower gain and shutter speed. Seeing was similar between the two images, but probably a little better during the 406mm image since it was taken 20 minuets later and Jupiter was higher in the evening sky. I stacked both in AS!2, used wavelet sharpening in R6, and then color combined in WinJupos, some final sharpening was made in PS. I spent similar amounts of time on the images and used similar techniques to find detail. I was expecting the C11 to perform better than the 16" since it seemed to me like my "best" images have been taken with the C11. Not sure what to make of it, a "N" of 1 doesn't say much, and this was far from double blind.

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#2 tjensen

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Posted 13 December 2012 - 07:56 AM

Interesting. You said the C11 was hard to focus... how was the collimation? That could be a major factor in the comparison. You could also try the 1.5x or 3x drizzle in AS!2 then downsize again. Under poor seeing, I'd expect both scopes to perform about the same. Or maybe even an edge to the C11.

#3 edsplace

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Posted 13 December 2012 - 09:37 AM

Yes, collimation was check via star diffraction rings on the c11. I can sharpen the c11 image further, but noise becomes a problem. I will probably spend a bit more time on both today to see if this difference holds true. I have 6 sets from the c11, and about 11 sets from the 16"

#4 Space Cowboy

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Posted 13 December 2012 - 09:45 AM

Hmmm....the C11 shot looks very soft for average seeing.

#5 edsplace

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Posted 13 December 2012 - 10:48 AM

I am reprocessing the images. It was late last night and I wonder if he problem may have been sleep deprivation. Stay tuned for further results.

#6 Space Cowboy

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Posted 13 December 2012 - 11:57 AM

I find frozen toes effects processing also. ;)

#7 sfugardi

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Posted 13 December 2012 - 12:57 PM

Ed, the 16" really whipped the 11". I am wondering if the open dob enables better mirror cooling vs the C11. Thanks for posting the comparison

Regards,
Steve

#8 edsplace

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Posted 13 December 2012 - 01:57 PM

I was worked a bit with the SCT image and if I spent some time I could get it a bit sharper without noise, but the 16" did outperform the 11". Last night I imaged with the 16" then 11" then back to the 16". Both 16" sets outperformed the 11" images.

Cooling could be an issue. I did run a series without the fans running on the 16" hoping to reduce micro vibrations. This produces a reduction in image clarity, noticeable when taking the video. The 16" has 3 80mm fans, and the flow around the mirror must have been breaking up the boundry layer. I have a mirror temperature probe on the mirror and an OTA probe. The mirror was 32 an OAT read 26, so 6 degrees difference. Both scopes had been out and covered all afternoon. The mirror and OTA temp was the same at sunset when the scopes were uncovered. The 16" never caught up to OAT.

The SCT was operating in the same conditions, it was 6 feet from the Newt. So mirror temperatures and airflow may have played a significant roll.

#9 DesertRat

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Posted 13 December 2012 - 03:30 PM

I know of no seeing condition where a C11 would yield better images than a N16. If someone knows differently I would like to hear their opinion. Visually it is true a smaller scope can offfer more pleasant views in compromised seeing. In really very poor seeing it might be a tie in imaging. However in lucky imaging in any kind of tolerable seeing aperture wins, assuming there no other issues like thermal or collimation errors.

Thanks Ed, this post is a lot more useful than some other things beng discussed in this forum. :lol:

Glenn

#10 edsplace

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Posted 13 December 2012 - 04:02 PM

There were a few variables I was wondering about.

1. Was optical quality of the primary mirror on the Newtonian. Optics in newts tend to be more variable in these scopes over SCT's. SCT is a spherical grind, with a flat grind on the corrector under pressure. Parabolic grinds are a bit more difficult, and someone walked the barrel to make this mirror. They did a good job.

2. Collimation and how variances in collimation on this home built scope would effect the image. I collimate the scope every time I set it up. I know there is flexture in my system, while I am good at collimation and check it with cheshire tube and a star it is not as simple to collimate and will not hold collimation like the SCT.

3. Would the larger scopes increase in resolution be enough to overcome the seeing condition.

4. Would the great amount of light on the chip at similar image scale allow the shorter exposure time to find more moments of good seeing to clear up the image.

5. Would the cooling of the primary mirror in the 16" (It's a full thickness Pyrex primary mirror) cause problems compared to the smaller lighter closed tube SCT.

The main question "Does it pay use the larger scope in less than perfect seeing condition?"

I agree with you Glenn, there is never just a seeing condition that would allow a 11" to outperform a 16" all other thing being the same. Members did come up with good questions about the SCT image, but they were issues I was more worried about with the Dobson. In the past it seemed like I took out the 11" and I got great images and the image quality with the 16" was hit or miss. After what happened last night I have to think maybe it was the seeing that was hit or miss and not the scope.

#11 DesertRat

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Posted 13 December 2012 - 04:35 PM

I had a long discussion with a noted optician concerning newtonian mirrors. He told me working an F/4 mirror was significantly more difficult than even a F/4.5. That more than anything else explains the optical variability. Of course if your fortunate enough to have a Zambuto or Royce class mirror the variability question is answered. With an F/4 the field of aperture limited resolution is only a few Jupiter diameters across. Due to coma, collimation becomes very sensitive, making the C11 case mere childsplay in comparison. With a F/5 you are in a transition area where really good results can happen, but even there good collimation is critical. A 40cm F/6 would be an even better planetary scope, but then it gets to be something other than a backyard instrument. In addition to the mounting and thermal problems newts can suffer more from wind vibration.

A lot of us here use SCT's not because they are the best design but because they are so convenient, packing a lot of performance in a short package. Convenience is a funny thing. More convenience means more chances and more success. But if its not too difficult, I'd use that newt you have as much as possible!

Glenn

#12 edsplace

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Posted 13 December 2012 - 09:38 PM

That is the reason I couldn't pass up the f/5 newt. I had a 12" f/5 that I would mount on an old Starfinder mount. The scope was large and heavy, the mount heavy, wobbly, finicky, and a hassle to set up. Yet the images were great. I found the SCT to be a great solution to all of the problems, good track, reasonable size, quick set up, easy to store, the f/10 focal length super, easy collimation, low maintenance, I could go on and on. The only drawbacks being low contrast and since my visual observations are very limited that didn't matter much. But the 12" was a better planetary imaging scope, and I missed it. I think it's Wayne who has the 12" newt I coveted. Anyway I have now wondered off the mark here.

Yes, the f/5 was the selling point on the 16" scope. I looked at C14's closely, it was getting a bit large for single man set up. I have used one at the observatory many times, even helped take it off the pillar one afternoon for maintenance and I couldn't see myself doing that alone. The truss dob added a couple of inches, and it was one man manageable with the only price a little extra setup time.

Anyway, I am pleased that the 16" does outperform the 11" in less than perfect seeing. I still love my C11 and will use it, but the 16" will not be a basement queen.






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