Long post – but I was exited so I kept typing 😊
Some time ago, I described my home-built Coronagraph and its history (see here: https://www.cloudyni...nt-wavelengths/) and showed a few results obtained with it. I did mention that the ultimate goal was to see the Corona itself (last paragraph), and yesterday was finally the day when I was successful! It was a long journey that required some learning, so here it goes:
Observation first (technical stuff later):
Since you need to get to clear air, and I am currently in NC, Mt Mitchell seems like a logical choice as it is the highest elevation east of the Mississippi. I had been there a few times, on two occasions with my smaller instrument (20mm aperture vs. the 40mm I used today), however, I was unsuccessful on both tries. I now know that this was most likely due to equipment details, more below. Also, while Mt Mitchell may well be the best site on the east coast, it is far from perfect. The elevation is too low when combined with the latitude to the effect that the mountain top is still completely covered with trees and plants – which means pollen which in turn means insects. However, not today, as it was freezing…
Anyway, I had already missed a great opportunity weather-wise on Friday, the weekend was bad, but the predictions were good for Monday morning. Also, there seemed to be activity going on as per this site (http://sss.ynao.ac.c...=lists&catid=75). So I decided to take a few hours off work and give it a try.
I arrived at 8:45 am and started to set up. Man, it was freezing! Not only was it 32F, there were almost storm-strength winds which made it feel MUCH colder. I was dressed warm, but not THAT warm ...
So I found a spot near the parking lot as much shielded from the wind as possible and started to setup my equipment (same as previous post, including the NP mount). I rough aligned the mount and centered the sun. However, here the problems started: the mount would not track. To my dismay, found that the battery was dead, even though I had tested it the evening before … Thankfully, I was smart enough to bring a spare, even though much smaller size. Now the mount tracked – but the sun drifted out from under the occulting disc within a few seconds.
I pondered for a minute and then I realized what was wrong: I day align the mount by setting solar dec + correct polar angle, and the adjust polar az and RA until the sun is in the center. I checked and saw that I had set the mount to +20deg dec instead of -20 deg dec for the sun. Once I corrected that and re-aligned, tracking was fine.
Now as I will explain in the tech section, my setup allows the filter to be on-band for one position at limb at a time only, but since I can rotate the filter, I can scan the whole perimeter. Problem is that it is not readily apparent where the on-band section is. So all I could do is concentrate on a certain part of the limb, rotate the filter and see if any thing pops up during the rotation of the filter.
And just like that, it did! After less than a minute of scanning, a brighter region appeared in a certain direction right next to the limb when the filter was oriented right! I could not believe my eyes – it was somewhat subtle, but readily apparent and noticeable. I checked whether the spot moved with filter rotation – it did not. It just got fainter as I rotated off the best direction, and vanished very quickly as it the filter rotation continued. I tapped the telescope to confirm it moved with the sun (to make sure it is not something on the field lens or somewhere else in the optical train) – and it did. I observed over a few minutes to see if it was a cloud or similar near the sun – but the spot/region stayed exactly the same.
Then I realized I was out of focus – the occulter was in focus, but the region/sun was slightly off. I fiddled for a bit (I focus by moving the objective) and after what I believed was the best position, I could see structure in the region – and it turned out what I saw was a small helmet streamer! I could only estimate the height - about 3-4 arcmin and I would say the width of the region was roughly 5 armin. I was somewhat in disbelief since after my I corrected the previous (slight) errors in my equipment, the observation was actually pretty easy. Also, the sky was good, but had seen better on many occasions, especially in the European Alps near which I grew up at.
I checked again and again, but everything pointed to it being real. I then started to scan the full limb again more carefully – and I found another bright region, roughly on the opposite side of the disc. This one was more of a pillar, a little bit smaller in size (maybe 2-3 arcmin high and 1.5 wide) but a little brighter. In moments of good seeing/sky conditions/instrument setup I could see three small spikes next to it, maybe 0.5 high and over length of 2-3 arcmin. In the similar good moments, the streamer on the opposite side showed a smaller, second streamer next to it, about 2/3rds as high and within the same diffuse 5 arcmin region.
Wow - is all can say. I have chased this for so long, and there it was – just like that!
I observed almost until 11 am because I was fascinated – but did I mention it was freezing? It was to the point where my whole body started shiver, but (thankfully) the sky conditions deteriorated just a little to the point where I could almost not make out the streamer, and barely (but still clearly) make out the pillar. I packed up at that point, but before that I tried to determine the location of the areas. This is somewhat tricky as the relay system creates an upright image, but the prism diagonal switches left and right – and then I looked through the eyepiece/prism at some crocked angle. So the only way to determine orientation seemed to move the scope and note which direction the sun moved. I tried my best but it was confusing and I was cold.
What I found was that the regions were clearly in the east/west direction. It seemed that the pillar was almost exactly east, and the streamer west and a little bit to the south.
Re-checking this morning, I must have reversed one of the directions. I had North up and East left, but that is not possible because of the prism. I think North/South is more reliable since that was easier to check due to the manual movement of that axis.
After I was home, I checked for pictures online. The only site I know which has Green Line Corona pictures is the one cited above – I found it when searching the Norikura observatory site. Norikura used to have coronagraphs, but ceased operations a decade ago and gave one their instruments away, which now seems to be the only earth-stationed emission line Coronagraph (minus Kislovodsk – but they don’t have pictures and the website recently closed …).
In any case, I have attached the Green Line picture from yesterday below. Amazingly, the visual impression is quite similar to this picture (not so for the white light K-Coronameter images from Mauna Loa which look very different). The two regions are clearly visible and the shapes match quite well. However, the sides seem switched (I think west is to the right in their image). See above for what I now think has happened.
I also never saw the third region, probably because it was quite a bit fainter than the other two. Anyway, I feel this is pretty close and I was really exited to see that my observations matched.
The MLSO pictures below show a similar trend in the sense of number of regions and their location, but look quite different. I can only see the two bright regions, no Corona all around (by far) as in the MLSO pictures. The high contrast version is a little closer than the low contrast version, but the Green Line picture above is by far the closest.
A few thoughts before moving to the tech section:
This was easier than I thought. The features I saw were fairly obvious, and I wonder how it would look like with a narrower filter (maybe 1A vs. the 2.5A effective I have now). Also, it would be nice to have an on-band filter so I do not have to tilt and could use it in focus/field position – and see the whole Corona at once.
I also wonder how it would look under really great sky conditions. I have seen skies while skiing that were more black than blue – to have a Coronagraph under such conditions must be phenomenal. As a matter of fact, Lyot reported observing the inner corona with just an orange filter on exceptional days.
Another thing I will try to do is record pictures. I have my camera here and I may go observe again before Thanksgiving (weather and work permitting). However, as you can see from my other thread, the chip is too small for a shot of the whole sun and operating the camera is ‘fiddly’: the whole mechanical setup needs to be changed – straight through, include the x-y shifter – and the finding and keeping the sun centered while only being able to see a small section of it. All of that on a mobile setup and in freezing cold … this is why I did not bring the camera yesterday.
On the plus side though, the features were so clearly visible that I am sure I can record them. That seems trivial but it is not – the eye is quite an amazing instrument, especially when it comes to contrast and brightness differences. For example, I have since bought a filter for the 706.5nm He line and have seen prominences (very faint), but have been unsuccessful recording them with my 174/249 mono chip camera – to my great surprise.
The telescope is the same as in the ‘prominence post’ with the following changes
The lens I use is a laser Grade PCX w/ d=50.8mm and f=600mm, 10-5 scratch/dig. (instead of an achromat with the exact same d/fl I use for prominences) (https://www.cvilaser...ocal_length=600)
The filters were two identical 2-cavity CWL 532nm HWB 0.35nm T=40% surplus filters which were stacked (making it a 0.25A 4-cavity filter) and tilted by 10 deg to shift the CWL on band (filter index is 2.05).
The rest of the setup was the same as in the previous post – with the exception that I had to move the filters from the focal plane to the parallel light path between the relay lenses. What I did not realize back then was that if the filters had to be tilted significantly (in my case 10 deg), then this large pre-tilt requires a much higher f-number than 20 to not widen the bandpass (the sine starts to be non-linear at these angle, check the CWL shift formula (http://hyperphysics....pt/intfilt.html) - even though my setup telecentric so the CWL is the same across the field.
So the only way to go was to put the filters in the parallel path and tilt them there. The advantage is that this is the configuration that gives the narrowest bandpass. The price to pay is that the CWL varies across the field. As a matter of fact, for a particular tilt, only one (small) sector at the disc edge has the correct CWL. I have mounted the filter in a T2 extension tube with the provision that they can be tilted to an angle, and the rotated (from the outside) to point in any direction in respect to the telescope/sky. This allows for a scanning on the solar limb by rotating the filter. For illustration, please see the pic below (upper/counter t2 missing so the filter can be seen better. Moving the outside gear with the finger rotates the filter.
The final f-number after the relay system is f/#=20, and I used a Baader 32mm Ploessl for my observations. This gives roughly a 1.5mm exit pupil (which proved to be perfect) and allows me to see the whole disc at a time.
I could go on long with a lot of technical details, but this post is long enough …
As a summary, I am really pleased that this finally worked! I hope to repeat it soon, maybe even with pictures 😊