Bright planets. New telescope.

 There are three bright planets in the evening sky.  Venus is hard to miss at magnitude -4.5 in the western sky at dusk.  Jupiter is high in the sky at magnitude -2.6.  Late in the evening Mars climbs higher in the east at magnitude -1.4.  Saturn is still visible only about 2.6 deg from Saturn, but at magnitude 1.1 it is 174 times fainter.

Jupiter and the Hyades.  Nikon 85mm f/2 Ai-S, ISO 800, 20 s, softon filter.

Mars lined up with Castor and Pollux.  Nikon 85mm f/2 Ai-S, softon filter.

The sky brightness prior to moonrise was typical for Santa Fe, sqml = 19.68.

The new telescope is a GSO 4.5 inch (114 mm) classical Cassegrain, nominally f/12.  Star-field and moon images from the last two nights give an actual focal  length of 1427 mm, which puts it at f/12.5.

Unlike commercial Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain (MCT) telescopes, a classical Cassegrain telescope has fixed mirrors and a fixed focal plane (at infinity focus).  Achieving  focus with an eyepiece or camera therefore requires a proper combination of spacers and drawtube adjustments.  This particular scope has a light-weight carbon-fiber tube and the center of gravity is at the rear where the primary mirror and focuser are located.   An extra dovetail was added so that the scope could be moved forward far enough on the mount to obtain proper balance.

GSO 4.5-inch classical Cassegrain, set up for imaging.

I don't yet have a proper Bahtinov mask for precise focusing, so for the following images I had to wing it by looking at the camera monitor and trying to judge position of best focus.  Additionally, the CG-4 mount is not polar-aligned well enough for long exposures, so they were kept short.

M35 in Gemini.  Sony A7iii, ISO 1600, 15 s.

NGC 457, the "Owl Cluster" in Cassiopeia. Sony A7iii, ISO 1600, 15 s.

One nice thing about this scope is that it illuminates a "full-frame" sensor (36 mm x 24 mm) with no obvious vignetting.  I haven't used it enough to draw conclusions about edge-of-field aberrations. The above images were binned by 4 to hide the imprecise focusing blur.

 

The Owl Cluster again.  E-M5iii (micro four thirds).  ISO 1600, 15 s.

Mizar (top) and Alcor (bottom). E-M5iii. ISO 800, 1 s.

The Mizar and Alcor pair form a naked-eye double star in the handle of the Big Dipper.  Mizar itself is a visual double star in a telescope.  Each of the two components seen here is also a spectroscopic double, which makes Mizar a quadruple-star system.

The 18.1-d moon.  E-M5iii, ISO 400, 1/160 s.

Moon. Polaris. Sky-Watcher SkyMax 102 MCT.

 The 11.8-day moon was only three days past perigee on Friday night and therefore slightly larger (32.45 arcmin) than average.  I used this angular size to measure some effective focal lengths for a Sky-Watcher SkyMax 102 MCT (Maksutov-Cassegrain telescope).  Like the previously discussed Celestron C6, this scope focuses by changing the mirror spacing, which moves the focal plane and changes the focal length. The nominal focal length is 1300 mm, which corresponds to a focal ratio of 12.75, but that is true for only one specific location of the focal plane.

An Olympus E-M5 camera with T-adapter was used to image the moon directly inserted into a SCT-2" adapter and then inserted into a 2" diagonal.  When inserted directly into the SCT adapter, the camera image sensor was 88 mm from the base of the scope and yielded a focal length of 1290 mm.  At this focal length the moon image just fits onto the micro-four-thirds sensor.

 

A 2-inch mirror diagonal plus 5-mm parfocal ring adds 110.5 mm to the back-focus position.  The measured focal length for this configuration was 1671 mm.  The entire moon no longer fits onto the sensor.

From these two measurements the focal-length differential was determined to be 3.45-mm focal length per mm of extension.  This gives a focal length 1526 mm at the exit flange of the 2-inch diagonal, for a focal ratio of f/15.  This result is useful for estimating the magnification provided by an eyepiece when used for visual observing.

With the moon measurements completed, the scope was turned toward Polaris in the straight-through (1290 mm FL) configuration, this time with an E-M5iii.  The "North Star", Polaris, lies about 2/3-deg from the north celestial pole.  Polaris is actually a triple-star system.  The magnitude-2 primary Polaris A is a spectroscopic double star.  Polaris B, a 9.1-magnitude companion star, is separated by 18 arcsec.   This visual pair is a challenge for small telescopes because of the large magnitude difference.

Polaris A and B.  E-M5iii, ISO 1600, 1 sec.

The star colors are usually described as yellow(ish) for Polaris A and white for the fainter Polaris B.  However, in Burnham's Celestial Handbook, Vol. Three, (Dover, 1978) p.2009, Polaris B is described as "a small companion which seems of a pale bluish tint."

Straight-through configuration.

With a 2-inch mirror diagonal added.

Configured for visual observing

Added the following day (11 Jan):

E-M1iii. ISO 800, 1/640 s. (binned x 2)

 A 1:1 crop of the southern quadrant:

Another shot of Polaris (looks a little bit cleaner than the previous):

A couple snapshots with the Celestron C6

 Some winter weather has moved in, but last night was still clear and calm.  This provided an opportunity to do some focal-length measurements with a Celestron C6 SCT (Schmidt-Cassegrain telesceope).  This scope has a 6-inch (150 mm) mirror with a nominal system focal length of 1500 mm. This type of scope is focused by changing the spacing between the primary and secondary mirrors, which moves the focal plane.  The actual focal length therefore depends on the specific location of the focal plane 

The picture below shows the telescope with a 2-inch mirror diagonal and a camera inserted into the diagonal.  

 The measured optical path from the exit flange of the telescope to the camera sensor was 165 mm.  Star-field images obained with this arrangement showed that the effective focal length was 1815 mm.  For this particular  optical train, the effective focal ratio was therefore f/12.1.

Previous measurements showed that the effective focal length changes by 3.3 mm for each millimeter of extension.  The optical path to the exit flange of the diagonal, where an eyepiece would be inserted, is 43.5 mm shorter.  At this distance the effective focal length is 1671 mm and the focal ratio is f/11.1

After the star-field focal-length measurements were complete, the telescope was turned to a couple bright objects in Orion for some quick exposures.

Rigel is the brightest star in the constellation Orion.  It is also a visual double star in moderately-sized telescopes.  This image is a single exposure at 1/4 sec at ISO 1600.

The companion star is magnitude 6.8 and is separated from Rigel A by 9.4 arcsec.  It is challenging to separate in small telescopes because of the large brightness difference compared to mag-0.3 Rigel A.  Investigation has shown that this secondary star actually consists of three stars, so Rigel is therefore a quadruple star system.

The Orion Nebula, M42, is a favorite subject for astrophotographers.  This image is a 15-sec exposure at ISO 1600.

The constellation Orion:

credit: SkySafariAstronomy.com

Perihelion

 The earth reached perihelion (closest to the sun) at 6:28 am MST this morning (04 January).  This picture was taken with an Orion 80mm ED refractor and Lunt solar wedge at 11:19 am.  The perihelion distance is 91.4 million miles, which is  3% smaller than the aphelion (farthest) distance of 94.5 million miles.

New Year's star trails

 This image was obtained  between 8:30 - 9:30 pm Wednesday evening, 01 January.  Instead of the Rokinon 7.5mm fisheye lens, this was obtained with a Laowa 7.5mm f/2 rectilinear lens.  It has the same nominal focal length as the Rokinon, but the field-of-view (FOV) is drastically different.  The fisheye has a 180° FOV. This lens offers a 110° FOV, which is still very wide but not as extreme as the fisheye lens.

Olympus E-M1iii + Laowa 7.5mm f/2, 1-hr livecomp exposure.

There are reports of auroral activity and an SAR arc visible as far south as Taos, but none of that was apparent in Santa Fe during this exposure.

Here is a side by side comparison of the two lenses:

Star trails over Santa Fe on New Year's Eve

 The Rokinon (Samyang) 7.5mm f/3.5 fisheye lens for micro-4/3 captures a 180° field of view.  This presents some compositional challenges for night-sky photography in a confined urban setting. 

This image was captured with an Olympus E-M1iii camera in "livecomp" mode with a 1-hour exposure.  The last picture of 2024.  A nice way to wrap up the year.

The shoulders of Orion: trying out a Nikkor 85mm Ai-S f/2 lens

 The sky over urban Santa Fe is not really dark enough to work well with fast aperture lenses and long exposures.  Last night the sky brightness was a typical sqml = 19.68 mpsas in a NE direction.  The bulk of the city lies to south.  In that direction the brightness increases to about 19.5 mpsas.

I have an old Nikkor (Nikon) 85mm f/2 Ai-S lens that works extremely well for daytime photography with a Sony A7iii camera.  A clear moonless Monday night provided an opportunity to see how it performs for astrophotography.  

The following image was taken with a Sony A7iii + Nikkor 85mm f/2 + Kase AstroBlast filter, ISO 800, 20 s (on a Vixen Polarie star-tracking mount).

The Shoulders of Orion.  Red supergiant Betelgeuse is left of center.

An image taken without the diffusion filter reveals some off-axis astigmatism, but it is much less than I expected.  The serial number on this lens suggests that it was manufactured around 1990 (I bought it used).  The design is typical of that era: 5/5, five elements in five groups, all spherical and no special glass.  In contrast, the recent Rokinon AF 85mm f/1.4 FE ii has an 11/8 configuration with four High-Refractive-Index (HR) and one Extra-low Dispersion (ED) elements.  This newer and faster lens is also much larger:

The same night I also took an image with the Olympus m.Zuiko 75mm f/1.8 ED lens.  This modern lens has a 10/9 configuration with two HR and three ED elements.

Olympus E-M5iii + 75mm f/1.8 + Kase Astroblast filter.

Here are two 1:1 crops from the Nikkor and Olympus lenses, matched for scale, without the diffusion filter (the Olympus image had a cross-star filter in place).

Nikkor 85mm (left), Olympus 75mm (right)