Posted 05 October 2013 - 11:52 AM
Posted 05 October 2013 - 01:31 PM
Posted 06 October 2013 - 03:46 PM
This was shot with the CCDT67 and image scale was .9375 arc-seconds/pixel. I know I can do better than this in good seeing (actually I did do better than this with first light and my Horsehead). Still even this is better than I EVER managed to do with my C8 and the Celestron f/6.3 focal reducer. To get sub 3.0 arc-second images with it, I had to image at f/10.
To compare the middle and corners with my C8, just go to my Astrobin account, and look at any of the old images. Whether shot at f/10 or f/6.3, they were all really bad in the corners.
The image here was a simple stretch of the stacked and calibrated data with an automatic background extraction to kill gradients. No deconvolution, sharpening, or other processes were done that would influence star shape.
Posted 14 October 2013 - 08:38 PM
What has me even more stoked is the FWHM values I am getting with this combo. My best frame so far of the night is running at 2.77 arc-seconds according to CCDInspector. That is excellent compared to what I was able to achieve with the C8 at f/6.3. And I do NOT have outstanding seeing tonight. The center is better and edges are incredibly better.
Here is the Curvature map.
Posted 14 October 2013 - 08:53 PM
Posted 15 October 2013 - 02:20 PM
1) Better stars in the corners.
2) No mirror flop (this would especially bite me with focus after meridian flips).
3) Hopefully better overall FWHM.
4) Easier (and hopefully more stable) focusing
5) Faster focal ratio, somewhat better FOV, and better matched sampling with my camera once I get the spacers I need.
6) Less problems with dew.
Better stars in the corner - this one has definitely worked out. FWHM values are much much better in the corners. Visual inspection of the stars shows the same thing. How much better depends on the configuration. With the CCDT67 screwed directly into the nosepiece of the camera and working at 1190mm (f/5.85) stars nice and round, even in the corners as shown by the corner crop a few posts back. Using the Baader Hyperion 28mm spacer tube between the nosepiece and the CCDT67 is not quite as good. Stars are still far better than the Powerstar III C8. However, the stars definitely show signs of astigmatism. In that configuration that telescope is working at 1040mm and f/5.12. So there is a tradeoff between speed, FOV, and how nice the corners are. Even in that configuration star shapes, FOV, and speed are all considerably better than the C8 using the f/6.3 focal reducer.
No Mirror Flop: My mirror actually did very little of this in my SCT. There was some focuser shift, but it was minimal. There was some mirror flop, but it didn't cause a lot of movement between the main camera and the guider. What did drive me nuts was the focus did not hold very well because of it. A meridian flip was bound to affect it and even before a meridian flip I would get focus changes that seemed to be more than just temperature changes. At least my focus seems to be far more stable with the AT8RC. Perhaps I'm ascribing that to the wrong things. Either way, I love the more stable focus. And I have the steel tube, not the carbon tube.
Hopefully better FWHM - I have been documenting this above and that has clearly been the case. It was rare I would get FWHM below 3.0 with the Powerstar III C8 at f/6.3. I had to go to f/10 and the narrower field of view. Even nights like last night with less than pristine seeing, I'm getting frames below 3.0 even at 1040mm. It will be very interesting to see how this scope does at f/8. I haven't tried that yet.
Easier and more Stable Focusing: I have already addressed the stability of focusing under mirror flop. It is much improved over the C8. What I haven't addressed is the easier focusing. Two things really help here. First you don't need to worry about approaching focus only counter-clockwise. That means if you overshoot, you don't have to back off and start over. That alone is very worthwhile. Add to that the fine focus knob of the Crayford focuser AND the larger knob (even the fine focus knob is bigger) and it is distinctly easier to get accurate focus. The focus lock seems to work well without shifting the focus much if at all.
Faster focal ratio, somewhat better FOV, and better matched sampling with my camera: This has definitely panned out. Even if I back off to 1190mm to get the real nice stars in the corner, I have better FOV and f/ratio than the C8. At 1040mm, an exposure of 7.5 minutes for the C8 can be taken in 5m. And because FWHM is better, I actually gain resolution besides. What isn't to like?
Less Problems with Dew: While this is true, the new telescope is not completely free of this problem. Imagining last night, FOG eventually rolled in. When I went out to close up the shed around 4:30 in the morning, there was a fine misting of dew on part of the primary. And I had used a Dew Shield besides. Still, the C8 would have been unusable far before the AT8RC was affected. That corrector plate in the front is a dew magnet.
Overall, I'm very pleased with how the AT8RC is working out.
Posted 15 October 2013 - 02:42 PM
This is the analysis of FWHM across the frame from PixInsight.
How do you get this report in PI?
Posted 15 October 2013 - 03:10 PM
You will need to download the script. It does not come with PixInsight.
I think there is an updated version, but 0.34 is what I use.
Posted 15 October 2013 - 03:14 PM
Posted 15 October 2013 - 03:18 PM
And grats on the great setup!
Posted 15 October 2013 - 03:18 PM
Glad to hear things are working out well for you with the AT8RC. I can tell there is lots of potential here for me as well once I get a few minor issues ironed out. I think because I have my CCDT67 a little too far back at f/5.6 and 1120mm. I think that if I can move it just a little closer then my corners will clear up.
Posted 15 October 2013 - 03:37 PM
I'm sure the charts from Astro-Physics on the amount of reduction are correct, but in my case the CCDT67 doesn't completely screw into the threads on my nose piece or the spacer, so my calculations when trying to setup my spacing have been off. I'll probably move back to 1190 mm for most of my imaging except when that last bit of FOV is required.
You might have noticed from my CCDInspector charts, the amount of curvature is way more at 1040mm than 1190mm.
(BTW, the way I am calculating those is by plate solving images and then using the image-scale and pixel pitch of the camera to figure out the focal length).
Posted 15 October 2013 - 03:40 PM
Thanks! I see that it's already in my distribution. Will be trying it out as a part of my normal process.
And grats on the great setup!
Posted 15 October 2013 - 04:04 PM
Yea, I've been verifying my estimated reduction by plate-solving. Currently with my nose-piece I'm getting .99"/pixel which puts me at 1120mm. I've also done some preliminary testing with several .8x reducers, the William Optics version II, which wasn't very good, the Orion .8x reducer, which was better but still not as good as the CCDT67. I also tried my William Optics AFR-IV, but it is quite heavy and combined with my QSI is VERY heavy, so much so that the focuser sag is obvious and I can't do anything with it. I'm planning to try it with a DSLR to see how well it works. If I can get that reducer to work as well as it does with my refractor then there is the potential for having perfectly round stars corner to corner at 1200mm focal length, which I would be happy with.
Posted 15 October 2013 - 05:04 PM
The CCDT67 threads into it. Then you would need roughly a 4mm to 5mm spacer to duplicate my spacing.
Anyway, the real point is that if that 1.4" back focus is correct, you definitely ought to be able to get the spacing I have with the CCDT67 one way or another.
Nice camera by the way. If it is the one with the integrated guider port, that is a great idea.
Posted 15 October 2013 - 05:19 PM
With the filter wheel and built in OAG, the sensor is 50.2mm back from the T-thread my nosepiece screws into, so add to that what ever extra distance the nose-piece would add when threading the CCDT67 into it. (That looks like the same nose-piece I have BTW).
Posted 15 October 2013 - 05:54 PM