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High-flying-plane spotting with an astro camera + telescope on a simple alt-azimuth mount

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#1 Marcin_78

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Posted 16 April 2024 - 01:11 PM

Once again I used my inexpensive astrophotography equipment just for fun. This time I took some pictures of high-flying planes using just an astro camera and a telescope on a simple alt-azimuth mount. Well they were actually two different telescopes on two different alt-azimuth mounts, so you can see it's possible with different kinds of telescopes.

 

My home city lies very far from any decent airport (over 100 km to the nearest such one), so almost every plane that I can see is flying at a cruising altitude of at least 10 km (32800 ft). The crucial things for me to succeed were (1) using a focal reducer 0.5x (in order to get a bigger field of view) and (2) using the program FireCapture_v2.7 in the timelapse mode, with the minimum possible delay of 1 second.

 

Thanks to the timelapse mode I don’t need a “third hand”, nor anybody else to push the “capture button” while I am moving my telescope (aiming at a plane) and setting/correcting the focus. It’s actually good to set the focus in advance while aiming at clouds that are roughly in the direction where the plane should appear (you can predict it thanks to the site www.flightradar24.com), but don’t expect to find perfect focus this way. Well, don’t expect to find perfect focus EVER, so you won’t be disappointed. And forget about all the plane-spotting pictures (that are all over the net) taken with expensive non-astro cameras.

 

I used two different telescopes, so the results vary significantly. The difference is actually overblown because the planes pictured with my 70/700 refractor were very far (first two pictures), while the plane pictured with my 102/1300 Mak (Maksutov-Cassegrain) was very close (third picture).

 

The original pictures were 1920x1080 pixels, so they were too big to be posted here and this is why I cropped them to 1600x1080 pixels.

 

Click to enlarge!

 

2024-04-14-1635_3-U-RGB-Star_0009 - cropped.jpg

 

2024-04-14-1638_0-U-RGB-Star_0017 - cropped.jpg

 

2024-04-14-1729_7-U-RGB-Star_0358 - cropped.jpg

 

By the way, has anybody any idea what kind of plane is the last one? And what are the circles on its belly?

 

Aiming with the refractor was much easier than with the Mak, for two different reasons. First reason is that the refractor is physically longer, so there is more precision to its movements. Second reason, more important, is that the field of view is almost twice as big (with the same camera) because it has a much smaller focal length (700 vs. 1300). It's worth to point out that the field of view AREA is actually 3.45x as big ((1300/700)^2), so it's a BIG difference. But a particular plane at a particular distance will be proportionally bigger on the picture taken with the Mak, so it's a double-edge sword.

 

I attempted also several other planes, but the results were either weak or non-existent because:
1. The initial focus was very bad and I couldn’t fix it in time while keeping the plane in the field of view.
2. The plane was practically invisible because the sun was on the “wrong side”.
3. The plane didn’t leave any contrails and I couldn’t “find it” at all with a telescope.

 

I have to point out that I used my better (more expensive, but still affordable) astro camera ZWO ASI 482MC-S (sensor pixel size 5.8 µm) because it has a much bigger field of view (with a particular telescope). This camera, combined with a reducer 0.5x (official reduction factor) and with my refractor gives me more than 1.8 degrees field of view. With my Mak it's almost 1 degree.

 

Plane-spotting FOV and resolution.jpg

 

Now it's time for some math. The crucial questions (and answers) are these:

 

How can I predict the size of a plane in pixels on a captured picture? By dividing the angular size of the plane in arcseconds by the equipment resolution in arcs/pixel.
How can I predict the angular size of the plane? By using my formulas that I described here:

https://www.cloudyni...ing/?p=12775631

 

The example in that post of mine was made for:
1) a plane 35m big,
2) flying at the altitude of almost exactly 10km,
3) being 10km away (horizontally).

 

The two distances created almost exactly a square 10km x 10km, so the view angle (when looking at the plane) was almost exactly 45 degrees above the horizon.

 

At the bottom of that example there was the angular size of the plane (8.509 arcminutes), so the size in arcseconds was 510.54 arcseconds (8.509 arcminutes * 60 arcseconds/arcminute).

 

Now we can calculate the size of plane in pixels:

 

Refractor:
Plane size in pixels = 510.54 arcseconds / 3.42 arcs/pixel = 149 pixels

 

Mak:
Plane size in pixels = 510.54 arcseconds / 1.84 arcs/pixel = 277 pixels

 

The result for the Mak is very close to the actual plane size in pixels on the picture taken with the Mak (third picture). The result for the refractor is much bigger than on the pictures taken with the refractor (first and second pictures), simply because the planes were much farther away than 10km.

 

I can't post any more pictures here, so I will continue in another post.


Edited by Marcin_78, 16 April 2024 - 01:20 PM.


#2 Marcin_78

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Posted 16 April 2024 - 01:15 PM

There's no time to make such calculations on the fly, but it's good to make some tables in a spreadsheet to roughly know what to expect from different planes being at different distances (horizontally). Here, I will save you some time:

 

Plane sizes cheat-sheet.jpg

 

One thing I can't help you with is a cheat-sheet for distances – it has to be done for a particular location like this:

 

FOV - ZG.jpg

 

From my own experience I can say that any plane within the smaller circle (20km radius from my location) is great. And any plane outside the bigger circle (40km radius) is usually invisible or very weak because it's very low above the horizon, so there is lots of air in the way.

 

It was very fun and my 9-year-old son enjoyed helping me (he was using the site www.flightradar24.com on my smartphone). I will definitely do it again. The next time I will use the refractor with the same camera, but without the reducer (the FOV will be similar to the FOV in the Mak with the reducer, but the refractor is easier to operate).

 

Clear skies!


Edited by Marcin_78, 16 April 2024 - 01:52 PM.

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#3 Echolight

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Posted 18 April 2024 - 06:00 PM

Looks a lot like a Boeing 757.

 

...looking again, maybe a 737 Next Generation.

https://en.m.wikiped...Next_Generation

 

Those circles are likely air currents. Notice the projection from the bottom of the plane about where the most forward circle is.

 


Edited by Echolight, 18 April 2024 - 06:09 PM.

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#4 BlueRidgeSky

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Posted 24 April 2024 - 09:27 AM

https://globe.adsbexchange.com/

 

International flight tracker. Fun to play with and see what is flying over. Click on a plane and it tells some stuff about it. Maybe next time you can see what is flying overhead.

 

Edit: Should probably say worldwide flight tracker instead of international.


Edited by BlueRidgeSky, 24 April 2024 - 09:35 AM.

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#5 Marcin_78

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Posted 24 April 2024 - 02:45 PM

(...) Click on a plane and it tells some stuff about it. Maybe next time you can see what is flying overhead. (...)

 

The site flightradar24.com allows that too, but I was generally focused on preparing for another plane and I wasn't checking and writing down any plane info then. The next time I will try to write down as much info as can, including the info about the distance to a plane, so I can verify how precise my calculations are.

 

As for the site globe.adsbexchange.com I must say that it is very good and the info about planes is very concise visually – on my computer I don't even have to scroll down at all in order to check the most important info, except for the destination (apparently there is no info about destination on that site at all). Thanks!

 

EDIT: I LOVE the site globe.adsbexchange.com because the planes' colors show right away (without clicking/selecting a plane) which plane is at which altitude! GREAT for my purposes! THANKS again!


Edited by Marcin_78, 24 April 2024 - 03:32 PM.

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#6 BlueRidgeSky

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Posted 25 April 2024 - 07:33 AM

A site that combines the info from both would be nice.

 

Edit: And combine the way all the info is presented. 


Edited by BlueRidgeSky, 25 April 2024 - 11:13 AM.


#7 Rutilus

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Posted 01 May 2024 - 09:20 AM

The plane looks very much like a Boeing 737 Max 8, operated by the airline Ryanair  (Dublin Ireland).

I'm in the U.K., and on the flight path from Dublin, Liverpool, Manchester and Leeds Bradford airport.

I see them all the time, their blue and yellow livery shows up well in binoculars and photos..  


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#8 Marcin_78

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Posted 14 May 2024 - 12:28 PM

(...) The next time I will use the refractor with the same camera, but without the reducer (the FOV will be similar to the FOV in the Mak with the reducer, but the refractor is easier to operate). (...)

 

I didn't do it because I didn't want to use the refractor (70/700) with the focuser racked very far out in order to compensate for the lack of a mirror diagonal. The backfocus of the reducer “cancels” the difference of the missing mirror diagonal and I wanted to stick with that.

 

Nevertheless, I still wanted to get more details out of the planes, so I did something that I had been pondering about as far as the planets were concerned – I bought a new (planetary) camera (ZWO ASI 715MC) with a very small sensor pixel size (1.45 µm). I hadn't bought it earlier because I didn't want to spend my money just for imaging 4 planets from time to time. My planespotting hobby changed it completely – now I have a HUGE number of targets I can go after!

 

Here are some (cropped and slightly processed) pictures of planes taken with the new camera (ZWO ASI 715MC), combined with the refractor (70/700) and the focal reducer (0.5x):

 

2024-04-27-1521_0-U-RGB-Star_0029 - cropped.jpg

 

2024-04-27-1607_3-U-RGB-Star_0064 - cropped.jpg

 

2024-04-27-1607_3-U-RGB-Star_0121 - cropped .jpg

 

2024-04-27-1614_2-U-RGB-Star_0054 - cropped.jpg

 

My new camera allowed me to greatly improve the resolution in arcs/pixel while still using the refractor with the reducer:
 

planes - better camera + refractor.jpg

 

Please notice that I wouldn't use the Mak with the new camera because the field of view would be definitely too small for planespotting (although the resolutions in arcs/pixel would be even better):
 

planes - better camera + Mak.jpg

 

I have to also point out that all the pictures above were SLIGHTLY processed in GIMP (Color Curves and a very delicate sharpening) because this time I took all the pictures as monochromatic pictures, but after debayering they all turned out kind of weird/poor and I HAD to “smooth them out”.

 

The worst thing, however, was that a single monochromatic JPG was VERY BIG (much bigger than I expected). Later I did some experimenting and I discovered that a single monochromatic JPG is ALWAYS significantly bigger (by at least 60%, but in some cases much more) than a color JPG! It's probably because on a monochromatic picture there is hardly any compression typical of JPGs due to big differences between adjacent pixels.

 

In the next post I will present pictures of planes taken with the same setup (ZWO ASI 715MC + 70/700 refractor + focal reducer 0.5x), but with the Debayer algorithm Bilinear (that is clearly better than NearestNeighbor) used right during capturing (in the program FireCapture_v2.7.14), exactly like I had done it with the pictures in the post # 1, but this time with the new camera.

 

EDIT:

 

The colors on the first screenshot above connect things in a way that means “in spite of”, so the new camera gives:
1 – green color) big enough FOV in spite of using the reducer,
2 – yellow color) very good resolution in arcs/pixel in spite of small aperture.

 

In reality aperture influences the Dawes Limit (also present on the screenshot above), but in this case the resolution in arcs/pixel is still within acceptable limits – around 1/2nd of the Dawes Limit (0.85 / 1.66 = 0.512).

 

According to the FAQ on the planetary-imaging sub-forum it's good for seeing that is a little better than “ok-to-fair seeing” – the rules of thumbs in the FAQ are based on the sensor pixel size):
1) focal ratio for “ok-to-fair seeing” = 1.45 * 3 = 4.35
2) focal ratio for “good seeing” = 1.45 * 5 = 7.25
3) focal ratio for “excellent seeing” = 1.45 * 7 = 10.15

 

My focal ratio with the reducer 0.5x was 5 (f/5), so almost exactly half of the maximum useful focal ratio (for “excellent seeing”).


Edited by Marcin_78, 15 May 2024 - 09:39 AM.


#9 Marcin_78

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Posted 16 May 2024 - 02:55 AM

(...)

 

The colors on the first screenshot above connect things in a way that means “in spite of”, so the new camera gives:
1 – green color) big enough FOV in spite of using the reducer,

 

(...)

 

I messed it up. Obviously a big enough FOV is possible thanks to the reducer, not in spite of it.



#10 Marcin_78

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Posted 16 May 2024 - 02:57 AM

(...)

 

In the next post I will present pictures of planes taken with the same setup (ZWO ASI 715MC + 70/700 refractor + focal reducer 0.5x), but with the Debayer algorithm Bilinear (that is clearly better than NearestNeighbor) used right during capturing (in the program FireCapture_v2.7.14), exactly like I had done it with the pictures in the post # 1, but this time with the new camera.

 

(...)

 

Before I post any new pictures I have to add some additional comments on debayering, if anybody (a potential newcomer) were interested.

 

The program Registax6 probably uses a poor Debayer algorithm NearestNeighbor, but it is not really specified. Reportedly the program AutoStakkert is better for debayering, but I don't know how to save a color picture. When I use the option “Export Frame(s) As displayed here” the saved file is still monochromatic, even though it was displayed as a color one. No idea why.

 

The program FireCapture is simply fantastic because it has lots of different Debayer algorithms. Obviously the more advanced they are, the more time and the more CPU usage they require for debayering, but the Debayer algorithm Bilinear is more than good enough for me. On the Wikipedia site Bicubic_interpolation there are great visual examples how different debayering algorithms work. To me the Bilinear algorithm seems to be the most natural one anyway.


Edited by Marcin_78, 16 May 2024 - 03:27 AM.


#11 Marcin_78

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Posted 16 May 2024 - 03:03 AM

All (cropped) pictures below were taken as color JPGs (Debayer algorithm Bilinear in the program FireCapture_v2.7.14) with the equipment combination: the camera ZWO ASI 715MC + 70/700 refractor + focal reducer 0.5x. I used a very short shutter speed of slightly less than 0.5ms because the planes are generally moving very fast across the sky, even when they are at very high altitudes. When it's getting darker (around sunset) I increase gain to make the view a little brighter, but it's very hard to “guess” a proper gain setting in advance (especially already AFTER sunset), so I ended up with some pictures being underexposed or overexposed.

 

Because of the unpredictability of the brightness of the view I (slightly) processed every picture in the program GIMP by changing (slightly) the Color Curves (either for brightening or for darkening the view). I don't use sharpening at all because it introduces more noise (or rather enhances what noise there already is). Additional notes are below the pictures.

 

2024-05-13-1718_0-U-RGB-Star_0055 - cropped.jpg

 

2024-05-13-1718_0-U-RGB-Star_0114 - cropped.jpg

 

2024-05-13-1732_8-U-RGB-Star_0087 - cropped.jpg

 

2024-05-13-1805_7-U-RGB-Star_0017 - cropped.jpg

 

2024-05-13-1805_7-U-RGB-Star_0142 - cropped.jpg

 

2024-05-13-1805_7-U-RGB-Star_0198 - cropped.jpg

 

I can't attach any more pictures here, so I will continue in the next post.


Edited by Marcin_78, 16 May 2024 - 03:15 AM.


#12 Marcin_78

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Posted 16 May 2024 - 03:07 AM

2024-05-13-1818_3-U-RGB-Star_0043 - cropped.jpg

 

2024-05-13-1818_3-U-RGB-Star_0119 - cropped.jpg

 

2024-05-13-1830_5-U-RGB-Star_0062 - cropped.jpg

 

2024-05-13-1850_1-U-RGB-Star_0029 - cropped.jpg

 

This time I was more lucky and there were more planes flying close to my home city, but there was only one flying almost directly above me (previously there were 3 such planes). This closest plane ended up being the biggest (and most detailed) so far – 680 pixels!

 

The visibility was great and the range at which I could clearly see a plane's contrails was remarkable. There were 2 cases when I was really surprised that I could clearly see the contrails of a plane that was AT LEAST 60km away. At such distance a plane is only (around) 10 degrees above the horizon (the precise angle depends also on its altitude), so there is lots of air in the way. When I was using the site globe.adsbexchange.com I had discarded the planes (previously such distant planes were completely invisible probably because of bad transparency), so I saw them without even looking for them (this is why it was so surprising).

 

I started the imaging session 2 hours before sunset, which was not too early and not too late. Once, when I tried to image planes at an earlier (relatively to sunset) hour, I was blinded by the brightness of the sky and I had trouble seeing anything on my computer screen. Not recommended at all.

 

On the other hand when it's close to sunset the planes are still very bright on the side closer to the Sun, but clearly darker on the other side. Especially AFTER sunset, very much depends on whether you are looking/aiming at a plane towards the Sun (most of the plane is very dark then) or away from the Sun (most of the plane is very bright relatively to the background). Fortunately such pictures are still very good/interesting, but waiting just for the sunset is wrong because there is too little time to image numerous planes. Like I wrote – 2 hours before sunset is a great time for starting an imaging session.

 

Clear skies!


Edited by Marcin_78, 16 May 2024 - 05:43 PM.



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