Starmaster 20″ – Observing Jupiter
by Daniel Mounsey
This is an observation of Jupiter which took place in early May over two nights which I emailed to some friends but didn’t give as accurate of details. During this month, I sometimes get some of the most stable seeing conditions to take advantage of large aperture. The excessive light pollution also helps wipe out diffraction spikes and excessive background darkness for more aesthetic views of the planets. I decided to set up the 20″ Starmaster, which has a premium secondary and primary with GoTo tracking. For my observing location on the second night, I had an excellent on-shore laminar flow about a mile from the ocean and could feel the cool breeze heading northeast, a perfect scenario. Around this time, Jupiter was moving past meridian, so even though I could rotate the upper cage to the opposite side, I left it in the stock, right-sided position because I would be sitting downwind in a special highchair on the north side of the telescope using a premium binoviewer.
The seeing on the first night was about 6 out of 10 while the second night provided 8-9 out of 10. On the second night I saw moisture on the roofs and hoods of the cars nearby before I moved them. This is a very good sign. Once I saw that, combined with no jet streams, I expected the seeing would take shape and drop below the sub second barrier. At the time of setup, I placed a large electric fan behind the primary and just left it on for a solid couple of hours and just forgot about it so I could do some other things. This would be paramount to the observations. Once I returned and the structure was more settled down, I did a really careful collimation. The temperature drops are extremely subtle near sea level for me, so once the primary optic was acclimated, I would rely on passive cooling unless something unusual in temperature occurred.
I compared three different telescope scenarios. I had my premium 6″ apochromatic refractor set up, as well as a custom 8″ clear aperture mask I made for the 20″. This experience would provide a clear view from the foundation up on how they all performed and compared visually on Jupiter. As many may already know, I have used a great number of high-end telescopes of numerous types and apertures . I love to compare telescopes and push their limits, but the learning curve and platform to achieve it is no easy task and so many factors have to be accounted for in order to achieve stunning results.
After the Sunset on the first night, I started out with single eyepieces using various magnifications before applying any binoviewer to the telescopes. I don’t use a binoviewer unless I have ideal conditions. It’s like having the ultimate supercar and the worst road to drive it. I would compare the 6” apo and the 8” clear aperture on the 20” using magnifications between 200-300x. Some observers have good experiences with masks when compared to apos and some don’t. The reason is because the apo is nearly dummy proof, while the masked reflector still faces many challenges, and mistakes can easily be made if the observer isn’t careful enough or aware of them.
Usually with world-class refractors, the planetary images appear chiseled out with hard outlines and the perceived contrast is really striking. Sometimes it doesn’t even matter if as much detail can be seen to the unbiased observer. The perception is that the planet may appear smaller and dimmer, but more aesthetic to look at. Reflectors on the other hand often tend to produce a sort of processed look about them unless really careful measures are taken, but some of you already know what I’ve been through on that subject. It’s like there’s always something making the image buzz and scintillate. The image just never seems to settle for whatever the countless reasons may be at the time.
FIRST NIGHT OUT / SEEING 6
I started out with the 6” apo on the first night and the image of Jupiter was pretty good. Some surface details were visible, but the image still wasn’t anything like many great nights I’ve seen. There was still some interference from seeing every few seconds or so. As a result, I decided to use the 8” mask on the 20” and see if the contrast would hold up. Incredibly, the 8” mask produced equally as good a contrast as the 6” apo and was very impressive considering the seeing wasn’t world-class. It didn’t appear to reveal much if any more scintillating around the edges of Jupiter or its moons compared to the 6” apo, which was surprising. I then determined that even though the seeing was a 6 at this time, the 8” mask was looking even more impressive than the 6” apo in every regard. The reason is because the color saturation on Jupiter’s surface was noticeably more colorful and the image could be magnified more without the subject fading.
Hands down, the 8” mask was in first place with the 6” apo in second place, so I decided to pull the mask off and go the full aperture on the reflector. Remember that the seeing was still about a 6. I immediately could tell that the seeing wasn’t ready to support the full 20” of aperture. The excessive brightness and scintillating practically washed Jupiter’s surface contrast to oblivion as the seeing tricked in and out. At certain moments it was okay, but not special, in fact the equatorial belts were even less impressive as a result at times. In other words, I could actually see the same surface detail more clearly using the mask over the full aperture. At this particular time of observation, I had the 8” mask in first place, with the 6” apo in second place and the full 20” in 3rd place.
SECOND NIGHT OUT / SEEING 8-9
This was a really interesting evening. I would compare the three optical configurations again. Even with my modest 6” apo on Jupiter, a stunning wealth of detail could be witnessed at the eyepiece with magnifications between 200x and 300x and a truly world-class image. There’s a sense of relief when you look at a planet like Jupiter on the most-steady nights. Truly a beautiful sight and I was in awe. The image was rock steady and there was absolutely no scattered light around the planet. Even the moons of Jupiter stood still. Once again, I applied all the techniques I could to get the 20” down to equilibrium. I then conducted my observations with the 8” aperture mask. After playing with various magnifications between 200x and 300x, I was startled at the sheer beauty of the image I was getting.
Jupiter was rich with color and contrast by comparison. Even the moons were solid as a rock and appeared as if they were perfect airy discs, just like the 6” refractor. Without a shred of a doubt, the 8” mask had clearly overwhelmed and surpassed the image seen in my 6” apo, even as good as it was. It was every bit as crisp and I was downwind during the observation. In my mind I was praying the full aperture could sustain this much beauty if the mask were removed. There’s always that doubt that goes through my mind about whether all this is going to come together at a time of world-class seeing because so many things can go wrong. So, here was the moment of truth. I carefully removed the mask, expecting to see at least some degree of scatter around Jupiter’s limbs and some scintillating, the kind you normally see when using really large apertures like this on planets.
The moment I looked in the eyepiece and focused carefully, the image stood completely still and my jaw just dropped to the floor! There was no scatter or scintillating! The image was rock solid and absolutely smashing! I immediately tried to get my Baader Mark V configured as quickly as possible! My hands were literally shaking from the anticipation of trying to capture this incredible moment on Jupiter using magnifications between 300x-400x in stereo. When I saw Jupiter in the Mark V, the amount of color and detail was so staggering that it would be impossible to draw. It looked exactly like an image Damian Peach had completed, only better because it was live in its full glory at the eyepieces. The color saturation of Jupiter was stunning and just shot out with incredible authority! The violet streams from the ammonia ice crystals below the troposphere were visible in full splendor across part of the globe. During this time, I pulled up my tall observing chair and just sat there in silence, concentrating on one of the greatest images of Jupiter I had ever seen. Definitely ranked in my top 5 greatest Jupiter views of all time.
My 6” apo and the 8” mask were nowhere near as incredible as this image using the maximum aperture. What was so incredible was how aesthetic, crisp and sharp the image was. After I managed to gather my thoughts again, I knew I had seen a truly world-class visual image of Jupiter, the kind of view that brings back the greatest of memories of observation. You can’t buy it and you can’t teach it. You just have to be there at the right place, at the right time and just hope the hell everything just comes together.
Steady skies!