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I saw the Corona yesterday!

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#1 Klaus_160

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Posted 17 November 2020 - 10:10 AM

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.

 

Green Corona

 

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.

 

MLSO LC
 
MLSO HC

 

 

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.

 

 

Technical details

 

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.

 

Filter 2

 

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 😊


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#2 Spectral Joe

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Posted 17 November 2020 - 01:15 PM

I'll be watching this closely....

 

Remember, most interference filters exhibit significant width and profile changes with tilts of more than a couple of degrees.

 

Example: https://arxiv.org/abs/1108.1234


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#3 Klaus_160

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Posted 18 November 2020 - 09:32 AM

I'll be watching this closely....

 

Remember, most interference filters exhibit significant width and profile changes with tilts of more than a couple of degrees.

 

Example: https://arxiv.org/abs/1108.1234

Yes, I had seen that paper but did not look at it  carefully ... the FWHM change is bigger than I would have expected - but then my filters have a 3x higher bandwidth to begin with than the one in the paper. Also, my filter index is 2.05 instead of 1.6, so that helps, but then again, I tilt by 10 degrees which is more that they go up to. I did see the effect of astigmatism, especially in my old focal plane setup.

 

The manufacturer states that "up 15 degrees' is "OK", but that is a rather generic statement and my not be true for the very narrow band versions. 

 

However, no matter of what would be preferable, the reality is that I bought the filters for $325 each, and a new, custom made (on-band) filter would be ~$1700 (each), so that made the choice easy, especially not knowing if it was really going to work ...

 

On another note, I did figure out a way to setup the camera with the new optical train, so now all I need another clear day when I have actually time to go and try. It is still very clear right now (since Monday), but there is this pesky thing called work.

 

Knowing my luck, the weather will turn for the week end, just like it did last week. But we shall see smile.gif



#4 Klaus_160

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Posted 20 November 2020 - 03:26 PM

So – despite Murphy’s law – it remained clear the last few days including today which was the first time I could take a few hours off work. As a matter of fact, the conditions today were even better than last time, which was lucky since the solar activity went down and the Corona was quite a bit fainter.

 

However, I could see one streamer, and also take pictures and videos of it. The streamer was much fainter than the structures I saw last time, but on the plus side, the camera can actually pick up faint structures better than my eye.

 

Anyway, here are the details:

 

I drove back up MT Mitchell (and this time I took a few pictures with my cell). It was MUCH warmer this time, in the mid-50s instead of low 30s. Much more pleasant, but that also brings out the insects, as you will see in the videos.

I forgot to take pictures while I was set up, but I took pictures when I hiked to the top, and then then of the equipment packed up in the car – too lazy to take it out again just for a photo opportunity. 😊

 

20201120 112152
 
20201120 111956
 
20201120 111915
 
20201120 112554
 
20201120 112616
 
20201120 112633
 

So without further ado:

 

These are pics pics of the streamer - the arches at the bottom to the right. Notice how the sun moves a bit between the pics, and how the streamer moves with it. Also note that the dust specs do not move in respect to the occulting disc.

 

online 2 2j
 
online 2 3j
 
online 2 4j
 
online 2 5j

 

 

The next pic is the same frame, but with the filter rotated off band. The dust stays, the streamer is gone.

 

offband 2 1j

 

 

Next is a video that I took. It starts with the sun ever so slightly sticking out behind the disc but the streamer already visible. I center the center the sun better and the streamer becomes more prominent. Then (second half) I play with the filter adjustment and rotate it On- and Off-band a few times, making the streamer disapperar and re-appear. You also see of the that the multiple reflections of the filters move as the filters are being rotated. The bright streaks going through the frame are insects. If the streaks are not even, that is because the wings are flapping …

 

video0014 10 12 47

 

 

I also checked the orientation more carefully – the streamer was East and a little bit to the south, the arc pointing south. The orientation is the same for all pics, south is at 8 o’clock, East at 4 (previouly 2, now corrected) o’clock. Remember that the image is since reversed because of the prism.

 

Also to note: I did observe visually as well. As a matter of fact, I found the streamer through the eyepiece first by rotating the filter on-band and the centering with the x-y-stage. However, while I could see some structures in the bright regions during good moments, I never saw any of the arcs as in the pictures. Again, the region was really faint!

 

 

Now for the external sources. There was no good picture from the 5303 website – it seems to have a lot of stray light, and it is hard to make out the region period.

 

20201120201507 90831[1]

 

There is a better pic available from the MLSO Coronagraph. One has to keep in mind though that that image is of the K-Corona, and not of the emission line Corona. However, I believe it does show the feature that I saw - that brighter hook in the East pointing south. It is more apparent in the high contrast picture.

 

20201120 174333 kcor L2 cropped[1]

 

20201120 175007 kcor L2 nrgf cropped[1]

 

Anyway, I am glad that it worked out before I am gong back home before Thanksgiving. I might try again tomorrow if the weather holds out for another day.


Edited by Klaus_160, 20 November 2020 - 10:14 PM.

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#5 Spectral Joe

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Posted 20 November 2020 - 06:56 PM

"You do not have permission to view this image" is all I can see.



#6 Klaus_160

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Posted 20 November 2020 - 08:30 PM

Must be that I uploaded the tiff files ...

 

Anyway, I converted them into jpegs and re-uploaded - please let me know if you can see them now.

 

Also, does the movie work?



#7 Klaus_160

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Posted 20 November 2020 - 08:52 PM

... and just for fun (and comparison), here is what Lyot did about 90 years ago :)

https://m.youtube.co...h?v=g3sgEwJlv4w

#8 Spectral Joe

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Posted 20 November 2020 - 10:14 PM

Images working now, the movie still doesn't work for me. These are very compelling images, this is exciting stuff!



#9 Klaus_160

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Posted 21 November 2020 - 04:18 PM

Racing the clouds ...

 

Today I had another successful observation, even though I almost did not go.

 

The weather forecast predicted high clouds rolling in and I could see them already in the morning when I took a look out the window. However, I checked the MLSO site and seemed that over the course of yesterday, the region I observed brightened and the ‘hook’ started to detach itself from the limb and be ejected.

 

20201121 022236 kcor L2 nrgf

 

So off I went to try if I could see this. I really hurried since I knew that there would likely only be a short time window. I arrived at 8:40 with more than ½ the sky already covered in cirrus. The direction of the sun was still clear, so I setup in a quickly.

 

First, I observed visually and I saw the region from yesterday right away. It was quite a bit brighter than yesterday. Cirrus clouds moved close by (I think a contrail from a plane) and obscured half of the sun. The region was unobscured though and still readily observable, along with the cirrus – quite a sight! The first cloud image below shows it somewhat similarly, but the cirrus was much more defined when I observed visually.

 

Since the time window seemed short with the cirrus moving towards the sun, I switched to the camera after only 3-4 min observing. This is what I captured:

 

ol1j

 

You can see how the loop is similar to yesterday but is a little higher and has a diffuse area on the right next to it now. Please note that the direction is rotated, because the scope was rotated in its rings. True location still the same – east and a little bit south.

 

Next, I rotated the filter a tad and the loop diminished as did the diffuse area on the right – but a new, faint area on the left appears.

 

ol7j

 

I rotated the filter further to the optimal position for the new structure, and it they look like faint rays pointing upwards and away from adjacent region.

 

ol4j

 

I then rotated the filter a little further and both regions disappear.

 

off1j
 

Then, the cirrus clouds reached the sun and I was done for good. That happened at 9:20.

 

clouds1j
 
clouds2j
 

I went to the top again an snapped a few pictures for comparison form yesterday.

 

20201121 095928

 

20201121 100254j
 
20201121 100112j
 

Also, I am including this pic from yesterday

 

20201120 112416j

 

Vs. today

 

20201121 095714j

 

taken with a shed roof just covering the sun to give an impression of the sky brightness and how much it increase with high clouds.

 

Also, here is a zoomed in shot of the ‘valley’, with layers of air keeping all the smoke down there. This is why one needs to go to some elevation to see the Corona.

 

20201121 100305j
 

Weather forecast is bad for tomorrow, but I might have another shot on Monday. That will be the last one before Christmas though.

 

Also, on another note, I took more movies. They are all avi files. Does anyone know what I need to do so that they will play on CN?


Edited by Klaus_160, 21 November 2020 - 04:25 PM.

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#10 Klaus_160

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Posted 21 November 2020 - 04:39 PM

https://gofile.io/d/4Ppuw2

 

Video from yesterday - no clue if this works, please let me know.



#11 dragracingdan

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Posted 21 November 2020 - 05:59 PM

Pretty darn amazing!
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#12 Klaus_160

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Posted 22 November 2020 - 08:29 AM

I uploaded the video from the second day as well, still assuming this actually works...

https://gofile.io/d/lcgTV6

#13 Klaus_160

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Posted 23 November 2020 - 08:33 PM

I went again today, but that will be it for a while now. I may post more later, but here is an image I took today

 

r54j

 

and its level adjusted version

 

r54levelj

 

I also measured the stray light today, and sky+Coronagraph together were a little under 20 millionths of disc brightness, which is pretty good considering that the conditions were not as good as on Friday.


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#14 dragracingdan

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Posted 25 November 2020 - 08:57 AM

Hi and thanks for sharing your work. Forgive if this is an ignorant question, how do you know your not looking at prominences? I'd assume by comparing SDO data. Are prominences visible with your setup or maybe they are too close to the disk?

Thanks for sharing,
Dan

#15 Klaus_160

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Posted 25 November 2020 - 03:07 PM

Hi and thanks for sharing your work. Forgive if this is an ignorant question, how do you know your not looking at prominences? I'd assume by comparing SDO data. Are prominences visible with your setup or maybe they are too close to the disk?

Thanks for sharing,
Dan

Hey, no worries - happy to answer questions.

 

One could go here an check (and go back to the dates in the archives)

 

http://halpha.nso.edu/

 

but there is a more simple and compelling reason: I am using a 530nm filter, and there are no prominences visible in that wavelength range.

 

And yes, I can see prominences with this telescope if I change out the filter for e.g. h-alpha, see here

https://www.cloudyni...nt-wavelengths/

 

 

Also, prominence look quite different, they are much more sharply defined.



#16 dragracingdan

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Posted 25 November 2020 - 05:11 PM

Thanks for taking the time to answer. I wonder if the corona in your latest image set is from the new large active region 2786

Best,
Dan

#17 dragracingdan

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Posted 25 November 2020 - 05:58 PM

Hope you don't mind, I took the SDO AIA 171 image from Nov. 23rd and roughly oriented it to your image. Looks very similar. 

 

Best,

Dan

 

 

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  • PSX_20201125_175724.jpg

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#18 Klaus_160

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Posted 25 November 2020 - 07:40 PM

Wow! This is way cool! Thank you!

 

I was not aware that these EUV images match the 5303 so closely. This is really neat - I will use this in the future for reference to see if there is activity since the other websites I use (and mention above) have a much lower data frequency - and are weather dependent.

 

Also, if you zoom out from your picture, you should see a second region like this:

 

r22j

 

I had seen and captured that region as well, but was too lazy to post, also it is not as impressive as the first one. All the settings and the orientation are the same - the image is just shifted with the x-y adjuster.

 

For reference, the bigger region image was taken 10:29 EST, this one here 10:12.

 

 

Again - thanks for posting this reference image!

 


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#19 dragracingdan

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Posted 25 November 2020 - 08:09 PM

Here is your last picture. I actually had wondered if you caught this since it was prominant in the SDO image. I use an app for my Android phone called 'Sun Viewer 2' but it only shows the current image. Have to go to https://sdo.gsfc.nasa.gov/data/ for past data. Also I had mirror flip to match your image.

 

Cheers and thanks again for sharing

 

 

 

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#20 Klaus_160

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Posted 25 November 2020 - 08:29 PM

Nice!!

 

> Have to go to https://sdo.gsfc.nasa.gov/data/ for past data.

 

Thanks for the link! I tried to search for the data here https://sohowww.nasc...v/data/archive/ and I failed ... so I had to resort to this https://sohowww.nasc...est-eit171.html , but that only has two pictures a day.

 

> Also I had mirror flip to match your image.

 

Yes, that is correct and I saw that you had done this. The reason is that I had left the diagonal in so I could switch more quickly between imaging and visual observing.

 

Also, just a note: The orientation in previous posts is different because I rotate the scope in its rings when setting up, so images from day to day are not comparable.

 

BTW, here are two pictures from the setup.

 

20201123 110952
 
20201123 111002

 

 

It may look warm and nice, but I assure you it wasn't. This is why I am setup in the corner by the benches to have a little bit of shelter from the wind. Still, my hand almost froze off when I had to take off one glove to type in the image names after capture ...


Edited by Klaus_160, 25 November 2020 - 08:36 PM.

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#21 George9

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Posted 26 November 2020 - 08:25 PM

Excellent! Thanks so much for posting this. I had gotten a Baader coronagraph for this purpose this summer but did not get far enough. My calculations implied that a 1nm 530.3 filter might be good enough to capture the corona from sea level on a very clear day. Sounds like something much tighter will be needed; I did not buy it yet.

 

It also seems with proper image processing, even a simple green filter might work. I don't mean image processing like Photoshop, but a mathematical function that exploits the radial drop in brightness from the Sun over many aligned images.

 

George



#22 Klaus_160

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Posted 27 November 2020 - 09:00 AM


Hi George,

 

Just a few thoughts from my side (without claiming to be 100% accurate):

 

I think a 1nm filter would be pretty marginal. Since the green line only has a width of 1A, such a 1nm filter would dilute the extra intensity by a factor of 10, so this is barely better than just observing at any wavelength/bandwidth. If so, it would make more sense to go to the red area of the spectrum where the stray light is diminished.

 

As a matter of fact, Lyot himself had done this and was able to observe the inner corona on exceptional days (see here https://academic.oup...9/8/580/1085681, middle of page 586, a good read in general!). However, I doubt that this will be possible from sea level.

But a practical note: I might have seen something with a 1nm filter on my observation days, but that is questionable and would certainly have been very marginal. I certainly would have had no chance to see the white light corona with a red filter, since the stray light level was around 20 millionths.

 

 

 

The Baader coronagraph is a solid instrument (I own one myself, but have never tried to use it to observe the corona), and it might show the corona. I suspect that the Lyot stop/Iris would have to be blackend more, and then there is the fact that the optical setup is not tele-centric which makes narrow-band filter use more cumbersome.

What you will certainly need though is an uncoated single lens of sufficient polishing quality. Cemented achromats will most likely not work, and any airspace lens will not work for sure. Also note that any speck of dust on the lens will render the corona invisible. This is from practical experience during my observation run.

 

 


As for the image processing, I am not sure what you have in mind exactly. What I can tell you though, is that in my set up, there is absolutely no radial drop-off in stray light. The stray light is constant from the very edge of the field right up to the accounting disk within 10% or less. Using many images to reduce the noise and tease out the fine details would certainly be an improvement in making faint features visible. Also taking flat fields (off-band images) would greatly help. But then, I’m not an imager, so this is not in my future plans…

 

 

Also, please do not take this the wrong way: I do encourage you to try, however one should also be realistic since this is not so simple. A good way to start and try would be with prominences. Since you have the Baader instrument already, have you tried to do this? That would certainly be the way to go to gain some experience and understand the pitfalls of such a setup and try out different imaging techniques.


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#23 George9

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Posted 27 November 2020 - 05:16 PM

Thanks so much, Klaus!

 

Yes, I have gotten the prominences in H-alpha with the coronagraph. My 10A Baader filter is shot, so I used an old prominence filter, which is about 2A I think. Because it is hodge-podged together and I have not verified the throughput, I only used a camera, not naked eye. When I feel comfortable that there is no off-band transmission, I will look through it (get more safety blockers and verify on spectrograph).

 

I've used standard H-alpha and CaK filters without the coronagraph.

 

On image processing, I just meant that if I take each image as a matrix and run it through a mathematical package, I would feel more comfortable trying to process for a signal, without, for example, losing the signal via rounding errors. The fact that you see no radial pattern and I do I think implies that I have way too much scatter.

 

Yes, I was afraid my old cemented doublet would not work, as clean as it is. And I suspect I would have to rebuild the Baader platform to make it work here.

 

Don't worry. I did not see this as a short-term project.

 

George



#24 Klaus_160

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Posted 27 November 2020 - 10:09 PM

Yep, these filters deteriorate over time. I replaced mine with another 10 A filter and actually use 30 A filter with higher transmission when I want to go to higher magnification (from ~0.9mm exit pupil to 0.65/0.5).

 

A 2A filter is almost too narrow for testing out whether the instrument functions properly. As a matter fact, when the conditions are good, I use a plain red filter like a #29 to see if I can see prominences. This is more like a sport, but is also a good test for sky conditions and instrument performance. On good days, I am unusually successful. And believe it or not, I have seen prominences without any kind of filtering with this instrument as well. That was using the achromat.

 

You could try something similar, that would tell you the real performance of your setup and your sky conditions. I do not believe that your objective lens is the problem, since any scatter there should give you a uniform field brightening. If I had to guess, I would believe that you are actually seeing scattering from the Earth atmosphere. If you don’t mind, why don’t you share a picture (or maybe create a gallery and link it).

 

A few other hints: it is important that the optical axis of the Baader unit lines up with telescope – otherwise your iris will be off-center and you have to stop down your aperture too much. You can check this by closing the iris slowly and watching whether the stray light goes of a uniformly around the limb. Is that is not the case, the focuser should be re-aligned.

 

Another way of checking is to slightly modify the unit itself. As I said, in its original state, the optical train is not telecentric. However, if you insert another 40 mm T2 extension tube between the iris and the projection lens (the two last barrels of the set-up) the whole unit essentially becomes telecentric. This would be beneficial for narrowband filters as well (if your f-number is still high enough, depends on your telescope) but it also allows you to see the iris and the telescope aperture at infinity at the same time by just looking for the back of the setup without any eyepiece. Now you can see if these two round apertures are aligned. The downside is that you now only have about 40 mm of back focus, which is too short to use a diagonal and in eyepiece. However it should be enough for your camera, and also give you a much smaller picture since the magnification of the relay system is now lower. Or you just take out the extra extension tune after alignment and everything goes back to normal.


Edited by Klaus_160, 27 November 2020 - 10:16 PM.

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#25 George9

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Posted 28 November 2020 - 12:42 PM

Wow, Klaus_160, thanks!

 

Yes, I see, with 2A I could just skip the coronagraph altogether. It's just what I had. But I could try my RG630+KG3 and see what I my camera detects.

 

When you say you see if you can "see" prominences with a red filter, do you literally mean visual? Doesn't that put too much faith in your mount? 45mm at f/20 is not too powerful but a peek of Sun still enough to laser the retina, no? Is there a truly safe coronagraph set up that is not subject to solar accidents? I understand that the Baader 10A was narrow and dim enough to be safe (although bright compared to a 0.7A filter). What UV and IR blockers do you use?

 

I do have some extra Baader spacers, so I can try that out. This is no longer the season for me to set up easily, so other than the indoor testing I can do, it will take me a while.

 

Sorry for the naïveté, but the reason we cannot just order a 530.3nm 1A DayStar filter and see it is that even the innermost corona is way dimmer than a prominence? During a total eclipse, the innermost corona and prominences seem of similar brightness to the eye+brain, but I guess that is just our very wide dynamic range working. Looking at photos, the ratio seems maybe 10:1? No worse than 100:1. (Plus the chromosphere dims the disk for us in H-alpha, but most real filters let through plenty of continuum.)

 

George




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