18 replies to this topic

[Tom Glenn]

This is probably best called a "draft" version of this image, because it could take me several weeks to get more comfortable processing something like this.  So, don't pixel peep!  Nevertheless, I was hoping to find some darker skies to image this comet, and so I took a field trip to Mt. Laguna, about an hour east of San Diego, and over a mile higher in elevation.  It is classified as a Bortle 3 zone, although because NEOWISE is so low on the western horizon, I wasn't able to take full advantage of that sky darkness, because I was imaging right back towards the California coast that I just left!  Still, conditions were much better than at my house.  Unfortunately, there were also some intermittent clouds in the direction of the comet.  You can actually see some evidence of this in my image of the crescent Moon that I posted yesterday, which was taken as I was setting up my equipment.  In combination with some time wasted trying to figure out the best exposure, this limited my imaging session, and so the image below only represents 17x30s light frames, at 200mm, f/2.8, ISO 200, on a Nikon D5600.  I used DSS to stack, and although it is a great program (and free), it didn't do the best job combining the comet and stars, so there are some artifacts.  Gradients were also a huge pain, which speaks to my inexperience with these images.  In any event, the image is much better than what I was getting at my house!

 

Click for larger size.

 

Edit: Please also see the image in post #4 below, as I think it shows the comet in more detail, despite the star trails (from alignment method).

 

Edited by Tom Glenn, 24 July 2020 - 06:58 PM.

[james7ca]

You've even capture some detail in the ion tail, something which I've been unable to do from my red/orange zone.

 

I'm not sure when I will have another opportunity to image this comet since I've had nothing but marine layer clouds for the last several days and we've already entered into another lunar cycle. My guess is we won't have clear skies again until we're near to the full moon (as is often said here on CN, but all too true from my location near to the coast in the north county).

[cavecollector]

Beautiful!  You lucky dog, we have been clouded out here.

[Tom Glenn]

Thanks for the comments, James and cavecollector.  Yes, the ion tail is very interesting, and very long.  It completely fills the frame (and then some) at 200mm focal length.  I have some other shots at 70mm that I still have to process.  I just wish I had collected even more data, but time was limited.  I noticed that in DSS (Deep Sky Stacker), the result produced greater detail in the comet when I aligned on the comet.  I guess that makes sense, but it's too bad that the "combined" stacking mode caused some degradation of both the comet and stars.  Shown below is a version of the image aligned on the comet.  Pretty good detail is visible in the ion tail, as well as jets emanating from the nucleus.  Stars are trailed though.  

 

 

Also, my hat goes off to the folks that process really nice comet images, because this isn't simple.  Even with my limited raw data, I think a better job could be done with more specialized software, both for manipulating the comet separately from the stars, and to deal with gradients.  DSS also did a weird thing when I tried to stack the raw Nikon NEF files directly.  The final stack lacked all color information, even when stretched, and the histograms in RGB were all identical.  The image was debayered, however.  Weird.  So to get this to work I had to export the NEF files as tiffs using Adobe Camera Raw.  On the plus side, this allowed me to enable lens profile corrections, but on the down side, ACR introduces its own tone curve, which alters color, and I think made the gradients worse.  I should go back and test what happens if I use another program (such as RawTherapee) to convert the NEF to tiffs as linear files.  But these permutations could go on for a long time!  Also, numerous satellites crossed my frames, which were effectively removed using sigma clipping in DSS, although for some reason some residuals still show up in the comet aligned image.  I could probably play around with the sigma clipping value, but I'm not too bothered by them for now.  Lot's of other things could be done differently (I have reduced exposures that didn't blow out the core, etc.), but this is a start.  

Edited by Tom Glenn, 24 July 2020 - 07:32 PM.

[Kokatha man]

Very nice Tom..!     

[Tom Glenn]

Thanks Darryl.  

 

One additional interesting (and annoying) feature was the number of SpaceX's Starlink satellites that photobombed my exposures.  Here's three of them in one of the 30s subs (and there's a fourth satellite also visible in the frame).  These satellites showing up in exposures seems to be a common problem for NEOWISE shots.  Also, from this example, you can see the major source of the gradients, because even though this occurred at "night", after astronomical twilight had ended, the comet essentially points in the direction of San Diego and the CA coast, with enormous amounts of light.  Indeed, looking West was not very impressive, while looking in almost any other direction from Mt. Laguna is excellent.  

 

Example of single 30s exposure (with satellite trails).

 

[Tom Glenn]

As a slight aside from the comet, I feel compelled to add a quick snapshot that I took before leaving Mt. Laguna, which I feel captures the essence of being in darker skies.  Despite being "only" Bortle 3, this type of scene is unheard of from my house.  Also, this image qualifies as "solar system imaging", because Jupiter and Saturn are prominent, slightly upper left of center.  In fact, this is my first image of these planets this year.

 

[OldManSky]

As a slight aside from the comet, I feel compelled to add a quick snapshot that I took before leaving Mt. Laguna, which I feel captures the essence of being in darker skies.  Despite being "only" Bortle 3, this type of scene is unheard of from my house.  Also, this image qualifies as "solar system imaging", because Jupiter and Saturn are prominent, slightly upper left of center.  In fact, this is my first image of these planets this year.

 

Jupiter_Saturn_Milky_Way_TG.jpg

Pluto is also in your shot, Tom -- thought I'm not sure it's resolved at that scale! (it's not far from Jupiter)

[Tom Glenn]

Pluto is also in your shot, Tom -- thought I'm not sure it's resolved at that scale! (it's not far from Jupiter)

Excellent point!  Although at this scale, we can't discern it.  I should also say that the above photo was taken with an 18mm lens on a fixed tripod, 30s exposure, and so there is also star trailing. 

[BillHarris]

Tom, I like the Ion and Dust tail detail!

[HowardSD]

As a slight aside from the comet, I feel compelled to add a quick snapshot that I took before leaving Mt. Laguna, which I feel captures the essence of being in darker skies.  Despite being "only" Bortle 3, this type of scene is unheard of from my house.  Also, this image qualifies as "solar system imaging", because Jupiter and Saturn are prominent, slightly upper left of center.  In fact, this is my first image of these planets this year.

 

Jupiter_Saturn_Milky_Way_TG.jpg

Nice shots of the comet! Quick question in regards your Milky Way shot... I have a Nikon D5500 and using a 17-50mm lens to shoot the Milky Way (out in the Palm Springs area), what settings did you use, if you can remember. I was out in the mountains few days ago and took a few shots very similar to yours however i've a lot more noise just on the outer edges of the core. I shot at 17mm / f2.8 / 8-10 secs / ISO 3200. Any longer exposures and i'm seeing star trails hence the high ISO setting.

[Tom Glenn]

Nice shots of the comet! Quick question in regards your Milky Way shot... I have a Nikon D5500 and using a 17-50mm lens to shoot the Milky Way (out in the Palm Springs area), what settings did you use, if you can remember. I was out in the mountains few days ago and took a few shots very similar to yours however i've a lot more noise just on the outer edges of the core. I shot at 17mm / f2.8 / 8-10 secs / ISO 3200. Any longer exposures and i'm seeing star trails hence the high ISO setting.

For the Milky Way shot I used the 18-140mm kit lens that came with the D5600, and shot at 18mm, f/3.5, ISO 1600, 30s.  At that exposure, there is definitely star trailing, but there is also considerably less noise than doing a 10s exposure.  If you shoot RAW, which you should, then you can pull up the exposure of a 10s shot in post, but there is still considerably more noise than the 30s shot.  I did both of these variations, and decided that I liked the 30s exposures at ISO 1600 better, and could live with the trails.  Better camera bodies (like a d850, etc), would perform much better at these higher ISOs...the D5000 series cameras are very good, but not very good at high ISO.  Also, when reducing the original 24mp image down to the forum size limits, the resolution becomes limited anyway, so the trailing didn't bother me.  If you look at examples of really good Milky Way landscape photos, you will find two basic varieties.  Many will have slight star trailing, but it does not detract from the image at normal reproduction scales.  Some other images are actually composites, with a different foreground exposure superimposed on a stacked (and possibly tracked) background sky.  Some photographs even use a foreground shot taken at a different time of day, which I'm not a huge fan of, as it seems too synthetic and not in the spirit of photography, but that's just my opinion.  Also, the jpegs straight from the camera looked pretty good, but these are examples of images that can benefit from tweaking the RAW image in post. 

 

And speaking of star trails, if it wasn't obvious earlier in the post, the comet shots were taken using a tracking mount (but not guided).  I mounted my Nikon 70-200mm f/2.8 lens onto my CGEM mount using the lens foot screwed into the universal mounting plate.  Unfortunately, I didn't have the proper adapter to mount my 18-140mm lens to the CGEM (that lens has no foot, and I didn't have the adapter for my camera to connect to the mounting plate).  Hence the fixed tripod shots with that setup.  If you can mount and track, this would be a case in which you could take a tracked image of the sky, at 30s (or longer) exposures, and then blend together with a fixed shot of the same scene to get the foreground sharp.

Edited by Tom Glenn, 26 July 2020 - 03:34 AM.

[HentySky]

Fantastic image Tom, probably the best I have seen.  Tonight is the first night us southerns get to see the comet, and as usual it is clouded out here.

[Tom Glenn]

Fantastic image Tom, probably the best I have seen.  Tonight is the first night us southerns get to see the comet, and as usual it is clouded out here.

Thanks Brett, although truth be told, I've seen a lot of really fantastic images of this comet (including right here on CN, as well as other sites).  I think I could have done better with this one if I had collected many more frames, but this was just the way it played out, so I have to take what I can get.  This was probably the last serious chance I will have, with the Moon now preventing another go, and by the time the Moon exits again, I suspect it will be too dim.  

[DMach]

Nice capture Tom!

 

Yes, I saw a news article recently highlighting just how often people's photos are ruined by those pesky Starlink satellites ... and that it makes the job of asteroid tracking all the more difficult.

[Tom Glenn]

Nice capture Tom!

 

Yes, I saw a news article recently highlighting just how often people's photos are ruined by those pesky Starlink satellites ... and that it makes the job of asteroid tracking all the more difficult.

Thanks Darren, and yes, there have been several articles on websites about this.  As annoying as the satellites are, the articles are somewhat over-hyped, because unless you are taking a single long exposure, then the image won't be ruined.  In a stack of a dozen or so exposures, you can use sigma rejection to delete outlier pixels and use the average of remaining pixels. This effectively eliminates the trails.  The photos that appear in some of the web articles have been processed in reverse, to purposely highlight the satellites.  In this respect, calling the images "ruined" is somewhat misleading, as that was the goal. If I do this with my own image, I get the following.  The image acquisition here spanned only 13 minutes, yet there are 10 distinct satellites visible, although not all are Starlink. 

 

[Tom Glenn]

Here is an animated gif showing the first and last raw frames, which shows the motion of the comet relative to the stars during a 13 minute interval. 

 

[yock1960]

Really nice Tom! It's 2+ hours of driving to get to anywhere similar for me and now I think I should've went, since this doesn't happen more than once or twice in a lifetime. Missed opportunities!

 

Steve 

[Tom Glenn]

Thanks Steve! 

 

I've been playing around with the image some more.  I noticed an interesting feature, that I have now identified as the spiral galaxy NGC 3198.  Shown below is the plate solve annotation for the image (only showing part of the full image), and an additional cropped version showing NGC 3198.  Also visible are the limitations of using a zoom telephoto lens at f/2.8.  Great for terrestrial photography, but significant coma and chromatic aberration showing in the bright stars away from the very center!