This is a bit of a brain dump so...
Last night I setup the 8" F/6 newt for planetary imaging. I had a couple of different reasons for trying this setup. I had tried shooting Venus a couple nights ago (9/24-25?) with the QHY290m through the C11 using the Baader U filter. (It was the first night out with the Siebert UV barlow.)
Much to my satisfaction, I saw some larger contrast features on Venus through the C11. (The barlow and U-Venus filter work!!!) But the pessimistic side of me wondered if the corrector plate was limiting UV transmission. (I assumed there would be more detail.) My assumption was that the corrector plate was BK-7, but upon further googling (or "binging" or "duckduckgoing"), the best guesses are 'water white glass' or green glass. I couldn't find much info on water white glass except that many versions that are offered mention around 70% reduction in UV. I can't figure out if the reduction in UV is primarily a coating effect or inherent in the glass. In any case, I figured that using a newtonian (which has no glass corrector) would afford me more UV signal and thus more contrast and definition on Venus.
Regular mirror coatings (protected aluminum) tend to drop off as they approach 400 nm. But the percentage of reflection at 400 nm is still around 50-60%. So based upon the transmission curves, I've assumed that UV transmission will be pretty good with a regular newt. (But keeping in mind we're dealing with 2 mirrors, the UV transmission is probably closer to 30% with both mirrors reflecting a very optimistic 55% at 350 nm.)
When I first got the 8" f/6 scope out, I spent a long while assembling and collimating. As I began to reacquaint myself with the scope, I started thinking about why I wasn't using it much. One thing I've been plagued by is the collimation drift. I chose a 3-vane Protostar spider and and it has 3 collimation screws. For me, 3 collimation screws often have more play in them than 4 collimation screws. Over the course of the evening, I found I needed to recollimate about every 2 hours or whenever I did a meridian flip. Even with just the laser collimator, the laser would hit about 1/16 inch away from the center of the mirror after some time. (I made sure to turn the laser in the focuser to make sure it was square and the laser dot didn't move in any appreciable way.)
Another issue was that the barlow and camera were almost completely pulled out. The focus seems to be like 5 inches away from the body of the tube. As I was finagling with things, I remembered that I initially was even further out (maybe 7 inches) when I first assembled the scope. I don't think you can tell in the image below, but I think I could pull the mirror down another 3/4 inches.
The last few times I imaged with the scope (I think it was at the Arizona Sky Village), I had to deal with mysterious elongated stars. They were in maybe 90% of the images -- no matter how long or short my exposure. I assumed it was the coma corrector since taking it out seemed to fix the issue. But I noticed that when I touched the base of the focuser OR the focuser adapter plate, the image would sometimes move dramatically. Hm... I tightened down the 4 allen-head screws holding the focuser to the focuser adapter, but I couldn't tighten the adapter since the screws were inside the tube underneath the flocking. This latter procedure would require a minor teardown of the scope. BUT even if tightened down, I'm guessing I would still have problems and this aforementioned issue may be the source of elongated stars. ONE WAY of dealing with this is to add a 12" x 5" piece of rolled aluminum to the inside of the scope centered around the focuser. This would add some stability. ALSO, I could add another wooden ring close to the focuser. In the meanwhile, my workaround was to hold the corner of the focuser with one hand and focus with the other.
With the UV barlow, I was able to collect images of Mars with the QHY462C color camera. Since my focal ratio and FOV was considerably different than the C11's (5600 mm fl vs 3000 mm fl), I had to shrink the fov to 180 x 180 pixels to not have a huge area of black space around the planet.
The downside with such a small window is the obvious lack of meaningful small details. But I was able to shoot at around 230 fps which means I collected something like 700,000 frames.
I ended my 462C session by shooting some initial Venus frames with the IR850 filter.
When I swtiched to the QHY290m, I tried a number of different combinations...
With the 8" F/6, *I think* I started my Venus experiment with the Baader U-Venus filter, the Siebert UV barlow. Somewhat to my disappointment it showed about the same results as the C11 combo I had going a couple nights ago. There was a patch of darker banding protruding across the "southern" 1/3 of the planetary disc. And there was a hint of another patch that mirrored the banding on the other 1/3 of the planet.
Every so often, it seemed like I was seeing something that looked like finer lines near the darker bands and toward the equator. But I couldn't be sure. Maybe it will come out in stacking, maybe it's just wishful thinking.
I also switched out to just the QHY290m with the U-Venus filter (no barlow) and though the planet looked tiny, it seemd like the bands were much more pronounced which is to be expected.
Then I tried using the Televue 2x barlow and I *think* the 1/3 bands were present but seemingly less so compared to when I was using the UV barlow.
Out of curiosity I also tried a short run with the #47 Violet filter and UV barlow. It didn't seem to show any detail on the surface. Maybe some hints of the 1/3 bands, but only because I knew they were there.
I finished the session with the initial combination of the U-Venus filter and the Siebert UV barlow.
The seeing was definitely worse than it was on the 24th but better than it was on the 26th when I also went out.
At this point, without seeing stacked images, it seems like the best combo was through the C11 with the the Sierbert UV barlow and the QHY290m and Baader U-Venus.
There is another way to potentially tweak things in terms of the camera. The camera itself has a "built-in" adapter that allows you to mount a c-mount? type lens. But this built-in adapter has a window in it. I wonder if that could be replaced so there's one less glass element in the imaging train.
All of this has made me wonder what the ideal Venus scope would look like. And I think we're talking a 4000mm+ scope that doesn't require a barlow. From a practical perspective, I think we're talking about an all mirror cassegrain or dall-kirkham telescope. You probably want 14+ inches to get detail that will please you for a long while.
(Later: Oh, I wanted to mention that I saw astigmatism intermittently through the session. I noticed it when I was focusing -- the out of focus image would look like an oval and then when I went to the other side of focus, I would see another oval oriented at 90 degrees relative to the first one. I need to investigate further, but I'm 99% sure it was because I had to pull out the camera so far that the locking bolt on the barlow was putting pressure on the U-Venus filter.)
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