Testing focal reducers on the Sun and some familiar star clusters

 After two nights with the Canon FD 300mm f/4 L lens, it seemed worthwhile to look at some optical alternatives.  I used the Astro-Tech AT72EDII f/6 refractor with four different back-end configurations:

• Orion 1x field flattener
• Astro-Tech 0.8x flattener-reducer
• Metabones 0.71x Speed Booster (Nikon F-mount)
• AT 0.8x + Metabones 0.71x combined (0.568x reduction)

 All four configurations were tested on the sun over the past two days with a Kendrick solar filter on the front of the telescope.  Effective focal lengths were derived from each image based on the sun's current angular diameter of 1951 arcsec.   The most extreme combination (0.8x + 0.71x) was also star tested with some familiar clusters currently high in the sky.

Here is a montage of solar images from the four configurations:

 The two images on the left were obtained yesterday, the two on the right were obtained today.

The four derived focal lengths are:

1.  430 mm
2.  346 mm
3.  308 mm
4.  242 mm

 All of these values are within 1% of nominal.  

The Metabones Speedbooster was attached using a Nikon-F T-thread adapter.  The backspacing between the end of the flattener/reducer and the camera focal plane was set to a nominal value of 55 mm for all combinations.  

When the Speedbooster is combined with the 0.8x reducer the telescope is converted to a 242 mm f/3.4 lens.  This combination reduces the size of the image circle so that it barely covers the micro-four-thirds sensor on the E-M5iii camera.  The following images were variously cropped to discard the dark corners.  At the outer edges of the image circle star images were distorted by coma.  There was approximately a 2-deg field of acceptable quality.  At this focal length the sensor covers a 3-deg x 4-deg field of view.

The measured sky brightness during the star tests on Monday night was a typical 19.68 mpsas.  All of these images were 30 or 40 sec single exposures at ISO 1600.

M45, the Pleiades.

 

The Pinwheel Cluster (M36) lower left.  Starfish Cluster (M38) upper right.

 

The Double Cluster in Perseus

Caroline's Rose in Cassiopeia

M31, the Great Andromeda Galaxy

Conclusions:

I was feeling a bit disappointed by the previous Canon 300mm results, but comparison of these 242-mm images to the Canon images has changed my mind.  The Canon images seem a bit better.  Next test: the 0.8x and 0.71x reducers by themselves.  That should be interesting.

The telescope:

 

Some more pre-solstice star clusters

 The sky brightness last night was a disappointing sqml=19.6 mpsas, which is normal for urban Santa Fe.  Wide-field starscapes are a futile effort under such skies, so long-focal-length photographhy is the solution.  I used the Canon FD 300mm f/4 L lens to photograph some more star clusters that were high in the sky (with an Olympus E-M5iii camera).

NGC 7789, "Caroline's Rose" in Cassiopeia

 The open cluster NGC 7789 in the constellation Cassiopeia was discovered by Caroline Herschel in 1783.

 

NGC 457

The open cluster NGC 457 in Cassiopeia is also known as the "Owl Cluster".  It was discovered by William Herschel (Caroline's older brother) in 1787.

Mirfak

 The star at the center of this image is Alpha Persei, Mirfak, the brightest star in the constellation Perseus.  It is the brightest member of the Alpha Persei Cluster, or Melotte 20, which extends beyond the field-of-view of this image.

The Double Cluster

 The "Double Cluster" in Perseus consists of NGC884 (left) and NGC 869 (right).  These star clusters are a very impressive sight in small telescopes.

M36 in Auriga.

 

M34 in Perseus

 

Star clusters and asteroids

 Yesterday interstellar comet 3I/ATLAS made its closest approach to Earth ("close" being relative: it was 168 million miles away).  I obtained seven images over the span of about ten minutes and was surprised that its motion was very evident.  It is moving at about 140,000 miles per hour, more than twice Earth's orbital speed.  Here is an animation produced from the seven images obtained with a 135mm lens.  Some of the fainter stars twinkle in and out because the exposures were not all equal.

Motion of comet 3I/ATLAS over ten minutes.

 

 New Moon also occurred on 19 Dec, so it was a good opportunity to do some more lens testing before the sky gets any brighter.  Last night the measured brightness was 19.68 mpsas. I set up the Canon FD 300mm f/4 L lens on a ZWO AM3 harmonic-drive mount to look at some star clusters in the eastern sky.  

Collinder 69 (Cr69) in Orion

 Collinder 69 is a loose open cluster in the head of Orion.

Cluster M35 and asteroid (10) Hygiea (circled)

 M35 is a nice open cluster in the constellation Gemini.  Asteroid (10) Hygiea was discovered in 1849 and is the 4th largest asteroid, after Ceres, Vesta, and Pallas.  Last night it was shining at magnitude 10.4.

M38 and M37 are open clusters in the constellation Auriga.  M38 is also known as the "Starfish Cluster".

M38 (above center) in Auriga

M37 in Auriga

 

NGC 2264, the "Christmas Tree Cluster"

The open cluster NGC 2264 in the constellation Monoceros is also known as the "Christmas Tree Cluster".  It is embedded in a larger nebulous region that is too faint to show up in this short 30 sec exposure (just a hint is visible near the bright star).

 

Open cluster NGC 1647 in Taurus (upper right) and asteroid (16) Psyche (circled)

The open cluster NGC 1647 is close to the much larger Hyades star cluster in Taurus.  Asteroid (16) Psyche was discovered in 1852.  It was once listed as the 16th largest asteroid, but new size measurements have moved it farther down the list.  It is the 11th most massive asteroid.  Last night it was shining at magnitude 10.

Two comets and an asteroid in the constellation Leo

Interstellar comet 3i/ATLAS made its closest approach to Earth today (168 million miles or 270 million km).  It reached  peak brightest of about magnitude 10 over a month ago.  It is currently near magnitude 11 and on its way out of the solar system.  It is designated as an "interstellar" comet because it has a hyperbolic orbit, meaning it is going too fast to be gravitationally bound to the sun. 

These images were obtained at 5:30 am MST while the comet was near its highest point in the sky.  Equipment: Olympus E-M5iii camera, Rokinon 135mm f/2 lens, ZWO AM3 mount.

Sky brightness was measured as sqml=19.8 mpsas, slightly darker than the previous evening.

Comet 3i/ATLAS, stack of 7 images. 1-deg FOV

 Comet 3i/ATLAS is the fuzzy blob at the center of the above image, which is a stack of seven images obtained over a period of about 10 minutes.  The stars are elongated because the comet was moving.  When the images are indexed to a fixed star, the comet looks elongated instead:

Comet 3i/ATLAS, just left of center.

Not too far away in the constellation Leo is periodic comet 24P/Schaumasse.  This comet is currently the third brightest in the sky (3i/ATLAS is fourth) at about magnitude 10.9.

Comet 24P/Schaumasse.  1-deg FOV.

 Also in the constellation Leo is asteroid (7) Iris.  As its name indicates, it was the seventh asteroid discovered, in 1847.  It is the fourth brightest object in the asteroid belt.  It is currently at magnitude 10.

Asteroid (7) Iris (circled)

Santa Fe skies with a Voigtlander 35mm f/1.4 lens

 The velvet-black sky so common in Creede is just not possible in urban Santa Fe.  This is a time, then, for experimentation.  Last night I tried out the Voigtlander 35mm f/1.4 Nokton Classic.  This is a compact Leica M-mount rangefinder lens.  It was mounted on a Sony A7iii using a Hawk's Factory L-M to E adapter.

Wide-angle  wide-open exposures just don't work under the bright sky of Santa Fe.  The measured sky brightness last night was sqml=19.63 mpsas, about six times brighter than a typical Creede sky on a moonless night.  My first exposures were done at f/2.8.  I later came back for a couple additional shots at f/1.4.  

This is not a good lens for astrophotography.  There is heavy light falloff and vignetting in the corners when wide open, and some very noticeable astigmatism.  The astigmatism is greatly reduced at f/2.8, but the corners still look dark.  The metal lens shade made for this lens might be contributing to the dark corners, but I needed to leave it on as a filter holder.  The filter used was a Hoya Sparkle-6.  

Orion rising through the branches of an apricot tree. f/2.8, slightly cropped.

 

An hour later, f/1.4, full-frame.

Betelgeuse in the corner, f/2.8.

Betelgeuse, f/1.4.

The lens:

 

Star trails and Geminids

A clear moonless night offered a chance to capture some Geminid meteor trails.  A few showed up during this two-hour exposure between 9 and 11 pm, but nothing spectacular.

Equipment: Olympus E-M1iii + Leica 9mm f/1.7 lens.  Location: Santa Fe back yard.

 

Santa Fe sun

 Sadly, I am now back in Santa Fe for the winter.  Today was relatively warm (10–20 deg F warmer than Creede) and sunny, so solar photography was an obvious choice for experimenting with some new equipment configurations.  

Equipment: TS-Optics 50mm ED f/4 refractor, Lunt Solar Wedge, Baader Solar Continuum Filter, ZWO ASI178mm monochrome camera, and Sightron Japan Alt-Az mount.

 The large sunspot group that has lately been traversing the face of the sun is now rotating out of view on the extreme lower right-hand edge.  This is a false color image.  The ASI78mm camera is monochrome, and the Baader filter is a narrow band (7.5 nm) filter centered on 540 nm.  So realistically, this image should be green, but yellowish just looks more natural.

Here is what the equipment setup looks like:

 

Based on the ephemeris value for the sun's diameter (1945 arcsec) and the measured pixel width (839 pixels) of the solar image, the derived focal length for this scope is 213 mm, which would make it f/4.3.

One more look at the Cold Moon

The full moon occurred Thursday (04 Dec), but on Friday night it was still 98% illuminated.  These images were captured with Olympus cameras and an AT80EDT refractor and a Laowa 7.5mm f/2 lens. 

Cold Moon encore over Snowshoe Mountain, 05 Dec.

There were some thin clouds visible near the northern horizon, but it didn't look like they would be a problem.  This is how it looked at the beginning of a planned 2-hr star-trail exposure:

Laowa 7.5mm, 10 sec.

 Some clouds moved in less than an hour later, so I ended the exposure at 54 min.  The moving clouds add an unusual element to the composition.

 

Cold Moon, active sun

The full moon of December is traditionally called the "Cold Moon".  An appropriate nickname - the temperature is already in the single-digits Fahrenheit two hours before midnight.  This year it is also a perigean full moon (aka "super moon"), so it is noticeably brighter than average.

Cold Moon rising over Snowshoe Mountain, 04 December.

Cold Moon illumination.  Laowa 7.5mm f/2 lens, 2-hr live composite.

A large sunspot group is traversing the sun.  

 

White light. AT80EDT refractor + Lunt solar wedge.

Hydrogen-Alpha light.  Lunt LS50THa with double-stacked etalons.

Even though it is winter, heat currents in the air make it difficult to get sharp single images of the sun during the middle of the day.

December-01 moonlight star trails

 The landscape was lit up by the 11.6-day moon.  There was a gusty wind blowing, which required the camera to be set up on the leeward side of a shed for protection.  Temperature was 11° F heading toward an eventual low of 1.4° F the next morning.

Equipment: Olympus E-M1iii + Rokinon 7.5mm f/3.5 fisheye lens.

2-hr live composite mode

 

1-hr live composite.

New sunspots

A large sunspot group has just rotated into view.  This group is so large that it was visible to the NASA Perserverance Rover on Mars, which was not designed for high-resolution solar observation.   Who knows, more northern-lights displays may be in our near future.

Equipment: Olympus E-M1iii + AT80EDT refractor + Lunt Solar Wedge + ND0.6 filter.