Uranus, Jupiter, Pleiades comparison.

[John Baars]

Two planets on offer this time. Uranus doesn't come by very often.
Cold, windy, lots of clouds, also with veils in between. Rainy, snowy, typically November and December weather. Observers must be patient and wait for the sparse good moments. Is is good for let us say 1.5 hours? So out the 102 mm F5 grab-n-go!

At search magnification Uranus hardly distinguishes itself from nearby stars. At higher magnifications, one notices that he is more or less without movement. Only at 140X does one begin to notice that he is not surrounded by an diffraction ring like the point light source of a bright star. Although a star of that brightness hardly shows a diffraction ring anyway. Still, I pressed on to about 200X. Even at 300X nothing extra became noticeable, except that it became fainter and in that relatively smaller aperture it was still difficult to distinguish from a nearby star. It is quite clear that Uranus is a long way from our home.
The "sketch" was made entirely digitally, that is, without the use of pencils. The color is somewhat exaggerated and has nothing to do with the proverbial color aberration of an achromatic.

As said earlier the weather wasn't good looking, neither was the seeing or the transparency. I hadn't sketched Jupiter for a long time and knowing the weather in December this could well be the last chance. So I went for it. Pencils out! A magnification of 140X turned out te be the best compromise between seeing, transparency and details. Quite near the maximum of the telescope. The most striking detail, apart from the fidgeting in the belts, is the small dark spot in the NEB, which is really not a shadow of a moon. This detail was also noted elsewhere at the same time in a 16-inch telescope.

As with Uranus, I ignored the CA of the F5 refractor in the sketches. On bright Jupiter is was better visible than on fainter Uranus, as to be expected. Never irritating though, at least not in my eyes.

Last but not least a little comparison of two sketches of the Pleiades. One with and one without blue CCD filter at low magnification. A little trick I learned from J.Gardavski, one of the best observers I know. As the nebulae around the Pleiades shine / reflect mainly in blue, a pure blue filter can be used to block out all other colors. So the nebulae become apparent. The difference is best visible while switching between the filter and filterless. Without filter nothing is visible between the Seven Sisters, apart from smaller stars. With the filter the space around the main stars is suddenly filled with a very smooth haze. You wouldn't notice it if you hadn't the comparison possibility. It is that subtle. But quite obvious for an experienced observer. I used one old sketch of M45 to demonstrate the difference.

Enjoy!

[Lady Fraktor]

Some nice observations and sketches John.
Uranus can be a difficult but rewarding target.

[helicon]

Wow, really nice observations and sketches John!

Certainly VROD worthy, congratulations on winning the award for today (12/7/2023)!

[John Baars]

Thank you for the VROD!

[Bigzmey]

Fun session and nice sketches John! While at 3.8" diameter Uranius disk is within resolution of 100mm aperture, fast achro optics and observing conditions makes it a challenging target. I don't believe I ever tried it with 100mm scope. I should next time.

Congrats on the VROD!

[kt4hx]

Excellent reporting and sketching as always John. Uranus is an interesting object, and I while I have seen it in an 8x50 RACI finder and binoculars, it appears stellar in those instruments. Of course I have seen its greenish disk in the larger scopes as well as its brightest moon Titania, but it reveals little visually. Your sketch of Jupiter is really great, and I like the comparative sketches of M45. Overall a well deserved VROD my friend.

[messier 111]

very beautiful sketch, and a very beautiful evening. a very deserved vrod, thx.

[Unitron48]

Very nice session, John...and well done sketches! Congrats on your VROD!

Dave

[OzEclipse]

Nice report John and very deserving of a VROD. Reading your report raised a couple of questions. You say that the chromatic relation is more visible on Jupiter than Uranus. I wonder if this is because Uranus is more monochromatic than Jupiter that is quite white by comparison.

The lack of a visible disc when observing Uranus also intrigued me. It is usually between 3-5” and should be easily resolved in a 4 inch scope with a theoretical 1.1 arc sec resolution. Seeing conditions must have been truly atrocious.

I really enjoyed reading this report and I love the sketches.

Kind regards

Joe

[John Baars]

Nice report John and very deserving of a VROD. Reading your report raised a couple of questions. You say that the chromatic relation is more visible on Jupiter than Uranus. I wonder if this is because Uranus is more monochromatic than Jupiter that is quite white by comparison.

The lack of a visible disc when observing Uranus also intrigued me. It is usually between 3-5” and should be easily resolved in a 4 inch scope with a theoretical 1.1 arc sec resolution. Seeing conditions must have been truly atrocious.

I really enjoyed reading this report and I love the sketches.

Kind regards

Thank you Joe for your remarks.
It gives me the opportunity to further explain a few things about the sketches.

I briefly put the sketch under a 10X magnifying glass. Uranus is about 3.8 arcseconds in the middle of this month. That's more than three times the telescope's resolving power.
The little star in the sketch is drawn as one white pixel, with a cross of light gray pixels around it. You can think of those as Airy's disk and the diffraction ring. I often add a darker gray cross around it to visually give a better perception of a "star," purely for artistic/visual reasons, that is. No physics but artistic :-)

Funny thing is that you can see something of a ring around Uranus. Those are the pixels I made darker, because otherwise Uranus would look too big compared to the star. So the surface of Uranus was seen during the observation. Perhaps my use of language suggested otherwise. I think the surface area of the Airy disc and the planet surface are reasonably well in proportion, when you see them largely magnified? Okay, Uranus should have been a little bigger.... Seeing conditions were not ideal, indeed.

Also nice, while we are at it, is the Chromatic Aberrated image of Jupiter, to the right of the previously posted image. (click on it) The right image is as I actually see it, the left one is my original sketch. You can see that I completely ignore the CA ring in the first sketch. If I didn't, it would take me a lot of sketching time and fiddling in the computer to generate it. Hence I omit it.

You can see a CA ring around both Uranus and Jupiter. The one of Uranus was created by accident, so I certainly didn't see it that bright.
The one of Jupiter was added on purpose, to show the difference. However, I really saw this ring. I think it is indeed because of the brightness of Jupiter and its polychrome that this ring around Jupiter is so pronounced.

By the way, we are freewheeling now anyway, I think the CA for an F5 system is not too bad. At least not as bad as several reflector-adepts would have us believe. The funny thing is that I don't see less detail than, say, with an expensive filter. Perhaps the contrast seems a little lower for some eyes in the chromatic aberrated picture. What I do remark with this telescope is a slightly bluer image of the planet than in a CA-free telescope. It is not difficult to see it.