Juno Visits Ganymede

Up close and Personal

The Juno spacecraft flew closer to Jupiter’s largest moon than any other in more than two decades, offering dramatic glimpses of the icy orb.

Credit: NASA/JPL-Caltech/SwRI/MSSS

The first two images from NASA Juno’s June 7, 2021, flyby of Jupiter’s giant moon Ganymede have been received on Earth. The photos – one from the Jupiter orbiter’s JunoCam imager and the other from its Stellar Reference Unit star camera – show the surface in remarkable detail, including craters, clearly distinct dark and bright terrain, and long structural features possibly linked to tectonic faults.

Full story/article at JPL.

M90 - Galaxy in Virgo

M90 - Spiral Galaxy in Virgo

M90 - May 13-15, 2021
EdgeHD-11 Telescope - ZWO ASI2600mm Pro Camera
LRGB

Messier 90 (also known as NGC 4569) is an intermediate spiral galaxy with a weak inner ring structure. It lies about 60 million light-years away in the constellation of Virgo. The star formation in Messier 90 appears to be tapering off as evidenced by the galaxy's spiral arms appearing smooth and rather featureless. Galaxies with active star formation have knots and trails of knotted groups in their arms. However, the central region does show some significant activity where there exists around 50,000 stars of spectral types O and B (blue to blue-white hot stars) that formed around 5 to 6 million years ago (young).

This image was created by combining 24 blue, 18 green and 17 red subs, each 120 seconds exposure, to get the master color image (RGB). To the RGB image I added 54, 120 second luminance subs (monochrome) for the detail. I had planned to use 30 of each color, but the Maryland weather this year has been giving me fits!

Remember, complete technical details on all my photos can be found on my Astrobin site.

Creating Starless Images

Astro imaging using narrow band filters (Hydrogen alpha, Ha; Oxygen-iii, O3 and Sulphur-ii, S2) is a popular part of imaging. Not only does it enable amateur astro-photographers like me to take images comparable to the Hubble Space Telescope and other land based observatories but they are a great weapon against the ever increasing light pollution we are all experiencing. They are very effective against light pollution because they only allow a very small range of light to pass (hence, narrowband). The NB images are false color as you assign the three colors of the RGB palette to each NB filter. The Hubble palette (SHO) assigns red to the S2 filter, green to the Ha filter and blue to the O3 filter. Another palette I use is the HOO - a bicolor palette as it assigns red to the Ha and both green and blue to the O3 (no S2 data). 

An obvious down-side to NB imaging is that the stars appear highly tinted in largely purple hues because the full range of color is not processed in NB treatment. So, not only is the nebula rendered in false color so are the stars, which makes them somewhat unappealing.

To correct for this I usually image the main subject using the three NB filters and then run a set of short exposure subs using the standard broadband filters to capture the true color of the stars. Then, in post-processing I create two images, one that has all the stars removed, and another that contains only the RGB stars. Then the two images are combined resulting in a NB nebula but with stars of their proper color and brightness. 

Extracting the stars is a fairly easy process that works well. Removing them to create a starless image is much more difficult. Some of the tools I use to remove the stars leave behind artifacts that must be manually corrected, and sometimes not all the stars get removed - especially bright ones that appear bigger on the image.

Recently, a fellow forum member, Steve, over at The Sky Searchers (TSS) forum posted about a multi-step process to remove stars from an image leaving no unsightly artifacts. Although I haven't fully tested it out on multiple images the results are thus far impressive.

Here are the pre- and post- star removal images of the Rosette nebula.

Unfortunately I don't have a set of RGB stars for this image so I couldn't add them back for the final photo (some folks like star-less images standalone). 

I will certainly add in the RGB session on my next NB project.

Darks vs no darks

One of the features of my new ASI2600mm camera is the low noise levels compared to other cameras. Recall that the 2600's dark frames typically have median ADU values of about 500 (Test of the 2600). And that is consistent over the typical range I use for exposures in my imaging (60-600sec).

Earlier this week I imaged some RGB and Lum subs of M98. Clouds rolled in and ruined the complete set of 60 Luminance subs so all I got was the 20 each of RGB. Not a whole lot of subs to work with, and with short 60sec exposures the amount of data was really low - in fact, only about 20-80 ADU over the noise level of 500.

To make matters worse, my attempts at taking the flat calibration frames produced odd horizontal banding on the images so I couldn't use any flats in my post-processing. Later I found out in my research that the 2600 doesn't like short exposures using a flat panel for illumination. If I had reset the camera gain to 0 (from the 100 I used for imaging) I would have had better results.

So I decided to run a little experiment. I processed each set of 20x60sec RGB subs, one with darks (but no flats) and one without darks (again, no flats). Darks are typically used in processing to calibrate the subs by removing the camera's inherent noise from the images. Since the 2600 has very little noise I wanted to see what I would get if I didn't use dark calibration in the processing.

The results are shown below. The image on the left is the 20x60 sec RGB stack processed normally, but without darks. The one on the right is with dark frames. Neither, of course, had flats, but I cropped the images to remove the vignetting and the dust bunnies weren't obvious in these short exposure images. They look pretty much the same don't they? And indeed, I was just a little surprised at the fact that they did look very similar. The low noise is really a great feature of this camera.

M98 - No darks vs darks
20x60 sec subs EdgeHD11/ASI2600

Now, if we zoom in on the images you can see that the image processed without the darks starts to show some significant noise. The image with the darks is much cleaner. (these images have been lightened to show the background noise better.)

So, I will continue to process with darks and flats. But it was an interesting experiment nonetheless. As for the final image of M98 - although I really want to retake this one with proper exposure times and processing to bring out all the lacking detail and brightness, this image will be used in the M110 contest (award on TSS for imaging all 110 Messier objects). I only have eight more to go!

M62 - Globular Star Cluster

Messier 62 is located in the southern constellation of Ophiuchus. It was discovered by Charles Messier in 1771. It lies at a distance of about 22,000 light years from earth and has a diameter is 110 light years. Its estimated mass is one million solar masses and it contains over 200 variable stars. In 2013, astronomers discovered a stellar-mass black hole in M62, one of the first to ever be found in a globular cluster.

M62 - May 21, 2021
Celestron EdgeHD11 f/10 and ASI2600mm camera
20x30sec RGB subs and 33x30sec Lum

With a Declination of -30 degrees, this was a difficult object for me to capture as it never gets more than 25 degrees above the southern horizon and I have very limited view in that direction to boot. I wanted to capture at least 40-60 luminance subs, but with the limited view and not very ideal weather conditions I settled for only 33 good ones to go with the 20 RGB subs.

This one deserves much better treatment (and longer integration time) but it is on my list for the M110 award at TSS and I don't want to wait another year to complete that contest.

NGC 6229 - Globular Cluster in Hercules

NGC 6229 is a very distant globular star cluster in the constellation Hercules. At about 100,000 light years away it is one of the furthest clusters from earth (that is, cluster in our galaxy) sitting at the very edge of our Milky Way. The Stellarium planetarium software identifies it as the "Prize Comet Globular Cluster" - possibly because of its distance, and hence small size, it would look like a faint comet head in smaller amateur telescopes.

NGC 6229 - May 21, 2021
EdgeHD-11 f/10 ZWO ASI2600mm
30x30sec RGB; 60x60sec Lum

This image was taken with a 9.8 day old moon blaring in the west and cirrus clouds covering most of the sky. Not ideal conditions, but whenever you get a mostly clear night here in Maryland you try to take advantage of the opportunity as best as you can. Globulars are ideal for image objects in these conditions as they tend to hold up well to the sky brightness and LP. 

The Lunar-X

While testing out the new adjustments I made to my mount software and camera hardware, I took a quick image of the moon. Wasn't really interested in keeping them, they were just for testing focus points, But then I noticed that I had captured the famous Lunar-X (and Y). They are optical features on the moon, visible when the moon’s terminator, the position between light and dark on the moon, is located in just the right place, you can see what appears to be the letters "X" and "Y". A serendipitous capture, I post-processed a single frame and posted here.

Lunar X and Y
EdgeHD-11; ASI2600; 0.002 seconds f/10
May 18, 2021 - 8:10 PM

Close up

For more information on the Lunar-X see the article in EarthSky.

The Blowdryer Galaxy (M100)

No, I didn't name this galaxy. It is one of the popular names for M100. But it does look a little bit like a blowdryer.

Names aside, this is one of the Messier objects on my list to image. I'm trying to get the final Messier Award, the M110 Award, for photographing all 110 Messier objects. With eleven more to go it may be a challenge to complete the series this season if the weather doesn't cooperate soon. 

This image of M100, also known as NGC 4321, is a spiral galaxy in the constellation Coma Berenices. One of the largest and brightest galaxies in the Virgo cluster, it is 55 million light years from earth, and spans 107,000 light years across.

Taken over two nights, May 1 and 2, 2021, it was a difficult imaging run as clouds interfered throughout both nights - more than half of the exposures had to be discarded because of either low signal (clouds) or equipment failure. My new Off Axis Guider is giving me fits trying to get the image focused sufficiently for the guide system. But I'll get it sorted out someday :) It was, nonetheless, a good test of my new camera, the ZWO ASI2600mm Pro. 

M100 - May 1 and 2, 2021
EdgeHD11 2800mm f/10, ASI2600 Camera
LRGB 3.2 hrs total integration
Full details at: Astrobin

Final Test of the ASI2600

M63 - Comparison ASI2600 vs. ASI1600

As a final test of the ASI2600 I decided to compare the results I obtained imaging M63, the Sunflower galaxy using the ASI2600 with the same object using the ASI1600.  Both images had pretty much the same number of subs, all taken with 60sec exposures. Each was calibrated and processed using the same workflow. 

First, a comparison of the field of view (FOV) of each camera. Note that both images were cropped a bit during postprocessing as needed to account for slight shifting of the masters among the multiple filters and to eliminate vignetting at the corners. As reported in my previous post, I was pleased that the vignetting due to the use of 31mm filters was minimal. The ASI2600 has a wider FOV, mainly in the long axis. The images below are not exactly to scale; the ASI1600 had more cropping than the ASI2600, so the change in FOV is a little bit exaggerated. The actual sensor sizes are:

ASI1600:  4656x3520
ASI2600:  6248x4176

Next, with some cropping of the ASI2600 image to bring the scale of each closer to 1:1 we can examine each for actual image details. Please note that the sky conditions were not completely identical in each session and so there may be some differences due to quality of subs, not the actual difference in the sensors.

However, some of the 'promised' improvements in the ASI2600 over the ASI1600 can be seen. First, the artifacts due to the microlens light scattering in the ASI1600 are completely gone in the ASI2600 (see bright star just to the right and above the galaxy). Second, the overall brightness of the galaxy is greater in the ASI2600, likely due to the greater quantum efficiency of the 2600 over the 1600 (91% vs. 60%).

In addition, but not obvious in these quick comparisons, is the much lower noise in the ASI2600. Dark frames were consistently measured as 500ADU, across all typical durations (30sec - 1200sec)! And, the total absence of any amp glow makes post-processing even easier.

One thing that is noticeable in the 2600 image is the size of the stars - they are bigger than the ones in the 1600 image. This might be due to slightly different post-processing. Another contributing factor may be the lower full well capacity of the ASI2600 at gain of 100.

Overall, I am very pleased with the performance of this new camera. 

ASI2600 Testing Continues

Test of new OAG spacing

You may recall in my last post I had discussed the plan to reset the spacing of the ZWO OAG on the scope to help with the strong vignette of the guide scope camera image. I moved the 15mm spacer in front of the OAG, mounting the OAG flush with the filter wheel. A thin 0.5 mm spacer was used between the nose piece and the 15mm spacer to act as a stop when the assembly was attached to the Moonlite 2" adapter. As expected, the pick-off prism is now rotated a bit from the center of the sensor's long axis.

Although this did improve the guide image, there is still an obvious vignetting present. The ZWO prism and light guide are just very small compared to the Celestron OAG.

Gain Settings

Early Friday morning I imaged the Sunflower galaxy, M63, so that I could compare the results of the ASI2600 with the image I took with the ASI1600 back in March. I ran a similar set of LRGB subs, 60sec exposure, but I used Gain 0. It turns out that these subs are essentially useless as I needed either a longer exposure time, or higher gain setting. The images did look rather nice, but they couldn't be stretched in postprocessing since they lacked sufficient dynamic range.

The darks at Gain 0 on the ASI2600 produce frames of 500ADU average at -10C. The 60sec lights of M63 came in at 520-560ADU. This means that once the dark frame data is subtracted from the light frames the net result will be only 20-60ADU. This is extremely low signal content, especially with a camera that has a 16bit ADC (0-65535). Any amount of stretching to bring out the faint details will produce a final image that is extremely 'posterized'.

To illustrate the point, I took a few subs of the Blackeye galaxy, M64, last evening during a very short period of clear skies in that direction. Clouds interfered with the 60sec gain100 sub so I couldn't do a comparison with the 60sec subs.

Three subs, 60secGain0, 120secGain0 and 120secGain100, are shown in the image below. A quick stretch of each was made to bring out the details. As you can see, the images themselves seem pretty nice. But this is misleading as each stretch is a different relative strength, normalized so that they all appear with the same relative level of brightness and contrast. When a background model (ABE) is created for each sub the underlying problem is made apparent. Notice the model of the 60 sec gain0 sub. There are only 11 levels of dynamic range in this image. Once stretched and processed the result will be very blotchy and posterized - there simply isn't enough data in the sub to work with. Note that one of the reasons I purchased this camera was to get higher dynamic range, 16bit vs the 12bit of the ASI1600. At gain0, 60sec, the result is worse than that of the 1600.

However, when setting the gain to 100 and increasing the exposure time to 120sec the dynamic range is greatly improved. At Gain100_120sec the mean ADU of the image is 1071, twice that of the comparison dark frame.

Reviewing the specs of the ASI2600 suggests that Gain100 should be the optimum setting as there is lower read noise at gain 100 than at gain 0, and the dynamic range is almost the same. The down side is that the full well capacity is greatly reduced (16000 down from 49000). So, for long exposures, the lower gain provides some safety in protecting from blown out star images as the camera can hold far more electrons that at gain 100. Since my general process is to take more, shorter exposure subs, the higher gain is the obvious first choice. 

Since my earlier M63 subs suffer from the low dynamic range, I will need to retake that series once the weather clears and the moon is out of the sky - probably next month - and then do the comparison. Stay tuned!

More ASI2600 Testing

The ZWO OAG finally arrived this past Monday and I was able to do some initial testing of the 2600 on the EdgeHD11.

Getting the spacing just right was a challenge but I had ample spacers in my shop, and ZWO provided a good variety with their cameras. 

My final configuration is as shown in the pic below; from left to right - nosepiece to mount to the Moonlite Focuser on the EdgeHD, ZWO OAG, 15mm spacer, ZWO Filter Wheel, 2mm adapter and ZWOASI2600. The placement of the 15mm spacer behind the OAG was chosen to allow the guide camera mounting to clear the filter wheel so that it could be rotated to get the pick-off prism to be centered over the long axis of the sensor. 

Initial test exposures yielded very good results, as can be seen in the following images. Since these are single, unprocessed images (except for stretching), the dust motes and vignetting are obvious.

Starfield 300sec

M82 in Ha 300sec

I was very pleased that the f/10 focal length of the Edge produced even less vignetting at the corners than with the f/5.7 WO GT102. Since I had already verified that proper post processing with good flats eliminated the vignetting with the GT102, I should have no problem with the edge images!

However, the placement of the spacer behind the OAG caused serious vignetting in the guide camera view, so much so that finding guide stars became a big problem.

What I had forgot to account for was the longer path of the light cone entering the guide camera. I needed to add 24mm of spacer to the camera. Now the ZWO's pick-off prism is fairly small, and coupled with the guide camera's sensor distance being a bit excessive the effect is like 'looking through a straw'. This needs to be corrected. I decided to move the 15mm spacer in front of the OAG, mounting the OAG flush with the filter wheel. A thin 0.5 mm spacer was used between the nose piece and the 15mm spacer to act as a stop when the assembly is attached to the Moonlite's 2" adapter. The pick-off prism will be a bit rotated from the center of the sensor's long axis but should still be out of the light cone. Now all I have to do is wait for clear skies to test this out.