16" and 11" side by side on Jupiter tonight
Posted 13 December 2012 - 01:45 AM
Posted 13 December 2012 - 07:56 AM
Posted 13 December 2012 - 09:37 AM
Posted 13 December 2012 - 09:45 AM
Posted 13 December 2012 - 10:48 AM
Posted 13 December 2012 - 12:57 PM
Posted 13 December 2012 - 01:57 PM
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.
Posted 13 December 2012 - 03:30 PM
Thanks Ed, this post is a lot more useful than some other things beng discussed in this forum.
Posted 13 December 2012 - 04:02 PM
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.
Posted 13 December 2012 - 04:35 PM
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!
Posted 13 December 2012 - 09:38 PM
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.