Finderscope Experiment

Too Many Aberrations!

When I purchased my 130 mm Stellarvue refractor years ago it came with an 80 mm f/3.75 finderscope. I was curious to see if the finderscope could function as a 300 mm focal length telephoto lens when attached to my new Nikon Z6 2 camera. On February 13th I mounted the combination on a fixed tripod as shown in the picture below and hurried outside to take advantage of some rare clear sky.

Exposure times were limited to two seconds to avoid star trails. I compensated for short exposures by setting ISO at 51200. Focusing was extremely difficult because the finderscope has a helical focus mechanism. Since the camera was fixed to the tripod, I had to turn the entire finderscope to focus. This somewhat stiff rotation caused so much vibration on the unsteady tripod that it was hard to tell if stars were focused. Consequently, star images are a little bloated in the images that follow.

Lens aberrations are immediately apparent away from center in this full frame image of the Pleiades star cluster made by stacking seven 2-second unguided exposures.

The finderscope lens is probably a simple single spherical lens. Only the very center of such a lens is relatively free from aberration. There's a reason good telephoto lenses are so expensive! Unlike the finderscope, quality telephoto lenses contain multiple lens elements which, in combination, correct for the spherical and chromatic aberration seen here. I cropped out the majority of edge distortion and ended up with the next Pleiades image.

In the cropped picture above you can see how individual star images are slightly out of focus and bloated. You can also see a blue fringe on stars from chromatic aberration. On the positive side, the seven-image stack actually captured some silvery reflection nebula around two of the stars! The image scale also nicely includes the entire cluster!

I next tried capturing stars of Orion's belt with a stack of ten 2-second unguided exposures. The same edge aberrations were present in these full frame images, so I cropped out the edges to leave the following central region, slightly more than three degrees across.

The Flame Nebula and Horsehead Nebula are slightly visible on the left near the left belt star, Alnitak.

It's hard to point a camera at Orion without trying at least one image of the Orion Nebula, M42. The final cropped image below is just a single 2-second exposure. Notice blue fringes on stars due to chromatic aberration.

I really like the image scale of these pictures, but the finderscope/camera combination is really impractical. I don't think I'll try this again. Perhaps I'll buy a real 300 mm or 400 mm telephoto lens sometime in the future. For now, I'll look forward to using the camera with my 130 mm refractor for some guided exposures.
 

New Camera and Dobsonian Telescope

Testing a New Combination

Although my 12-inch Dobsonian telescope doesn't track stellar motion caused by Earth's rotation, I thought it would be interesting to test short exposure times with my new Nikon Z6 2 camera attached to this large aperture. On February 6th I gave it a try. A special adapter is required to attach the camera to the scope. The camera would not reach focus with first adapter I purchased. Eventually, I bought another adapter that brought the camera's focal plane a couple centimeters forward, enough to achieve focus.

The telescope effectively acted like a 1500 mm telephoto lens. I quickly discovered stars would trail unless exposures were very short. An approximate rule for avoiding star trails in unguided photos is to keep exposure times in seconds less than 500 divided by the lens focal length. For my telescope the rule said 500/1500 = 0.3 seconds would be the longest exposure I could take. Trial and error proved the rule accurate. To compensate for short exposure time I cranked the ISO up to 51200! The next image is a 0.3-second exposure of the Orion Nebula, M42, at ISO 51200. In spite of background noise associated with high ISO the image captured a lot of detail in only 0.3 seconds!

The Pleiades were high in the sky above most light pollution before the Moon rose. Another 0.3-second exposure at ISO 51200 captured only part of this cluster as seen in the next image. I should have moved the field of view around, taken a series of images, and then combined them to capture the entire cluster. Maybe I'll try this another time. The diffraction spikes on bright stars are kind of pretty!

I was particularly interested to see how Jupiter would look in a short exposure. The following 0.05-second exposure at ISO 500 overexposes the planet but allows all four major Jovian moons to be visible.

Based on the previous picture it looks like this particular telescope/camera combination might be great for capturing certain kinds of planetary conjunctions.

On this night, February 6th, comet C/2022 E3 ZTF was passing near the star, Capella, in the constellation, Auriga. I searched the neighborhood of Capella while looking at the camera's viewfinder screen, but couldn't find the comet. Instead, I took a picture of bright Capella itself with a 0.3-second exposure at ISO 51200. Yellow overexposed Capella shines nicely with accompanying diffraction spikes in the next image.

In my light polluted sky I misidentified the star Almach in Andromeda and photographed Mirach instead. I was surprised to see a faint, fuzzy object to the lower right of bright Mirach in the following image. The faint object is actually the 10th magnitude elliptical galaxy NGC 404! It's amazing how this faint distant object actually showed up in just a 0.3-second exposure at ISO 51200!

Finally, I tried a 0.2-second exposure at ISO 51200 to capture most of the star cluster, M41, in Canis Major. 

Soon after capturing M41 the nearly full Moon rose making the sky even worse for imaging. It was time to quit.

These results with very short exposure times make me wonder how nice it would be to have a 12-inch telescope on a tracking equatorial mount. Longer exposures with no star trailing would really show amazing stuff!

Comet C/2022 E3 ZTF

Partial Success

Almost all comets I've been able to observe have been difficult targets. They usually appear inconveniently close to the horizon, often before dawn when it's hard to get out of bed. Recently, prominent comet C/2022 E3 (ZTF) had an orbit bringing it close to the north celestial pole, so, for a good stretch of days, the comet was located relatively high in the sky to the north.

The comet's unfamiliar name has the following explanation according to earthsky.org: "It was discovered on March 2, 2022 and it was the third such object discovered in the fifth half month (A, B, C, D, E) of the year". (E3) It was discovered by a 48-inch telescope on Mount Palomar which is part of the Zwicky Transient Facility (ZTF). The C indicates it is a non-periodic comet.

I made several attempts to photograph the comet with my new Nikon Z6 2 camera. The first attempt on January 15th was successful as you can see in the following image which was constructed by stacking five 30-second guided exposures with a 70 mm f/2.8 lens at ISO 800.

The green coma and brighter part of the yellow dust tail are visible. Ultraviolet light from the Sun excites diatomic carbon present in gases emitted by the comet. The excited diatomic carbon then emits a green visible color. The dust tail's yellow color comes from reflected yellow sunlight. Darker skies, larger apertures, longer exposures, and telescopic magnification would be needed to reveal the long, dim, wispy ion tail which is not visible above.

I tried again on January 24th when the comet was closer to Earth. The next image is a stack of six 6-second unguided exposures at ISO 6400 with the 70 mm lens set at f/2.8.

Now the comet tail had spread out into a fan shape with a spike pointing ahead of the coma. This is a little easier to see in the following heavily enhanced, cropped image. Enhancing the image to reveal dim details also, unfortunately, brightens background noise. Therefore, the background sky doesn't look as black as it should. (All these images should be enlarged to see the comet in better detail.)

Viewing conditions were much more difficult on January 30th, two days before the comet made its closest approach to Earth. On this night the temperature was wonderfully mild, but the Moon was almost full, and the sky was covered with hazy thin clouds. In addition, it was very humid, and dew rapidly formed on exposed surfaces. Since the comet was closer to the celestial pole this night, I tried a slightly longer exposure time with the unguided camera mounted on an ordinary stationary tripod. The next image is a stack of nineteen 8-second exposures at ISO 6400 with the 70 mm lens at f/2.8.

Unfortunately, haze and moonlight worked together to diminish what should have been a brighter coma and possibly longer dim tail. 

The sky was completely overcast when the comet came closest to Earth on February 1st. After February 1st there were a few more opportunities for interesting comet pictures. For example, on February 6th the comet passed close to the bright star, Capella, in the constellation, Auriga. The Moon was nearly full that night when I tried imaging the comet with a 12-inch Dobsonian telescope. Unfortunately, I couldn't find the comet in the telescope's relatively small field of view. On February 10th the dimming comet passed close to Mars, but the sky was cloudy again. Clouds were forecast for almost every night after February 10th. The comet will rapidly dim as it recedes from the Sun and Earth and travels out into the solar system.

Venus-Saturn Conjunction

Pretty Scene at Dusk

Close after sunset on January 23rd Venus and Saturn were only about 1.25 degrees apart as they sank in the western sky below a nice crescent Moon. My new camera produced this nice image which is best viewed at full size. (Bright Venus is above dim Saturn.)

Earthshine on the Moon adds to the beauty! On the previous evening the planets were even closer together - only about 1/3 degree apart. Unfortunately, it was cloudy on January 22nd.