Testing old and new equipment. More fun with star spectra.

The noon air was exceptionally clear on Tuesday while kayak fishing on Cochiti Lake.  That clarity seemed to carry over into the night, which provided a good opportunity to test some recent equipment upgrades.  Unfortunately, the clear air didn't result in a darker sky.  The measured sky brightness was a very typical sqml=19.72 for urban Santa Fe.

The equipment upgrade is a pier extension for the ZWO AM3 strain-wave mount.  When this mount is operated in Alt-Az mode the camera or eyepiece end of the telescope comes uncomfortably close to hitting the tripod legs while the mount is slewing around.  The pier extension raises the mount by 16 cm (6.3 in), which is enough to provide necessary clearance.

An Astro-Tech AT72EDII telescope (fL=432 mm) was fitted with an Astro-Tech 0.8x reducer/field flattener and an Olympus E-M5iii camera.   In addition to testing the new pier extension I wanted to experiment with the star-tracking ability of the AM3 in Alt-Az mode.

The first target was Polaris, for alignment purposes.  Polaris is a challenging double star for small telescopes.  I was able to spot the companion using a Baader 2" Amici-prism diagonal and an Edmund 8-mm RKE eyepiece.  This eyepiece provides a magnification of 54x.  I also tried an 4-mm Tele Vue Delite eyepiece, for 108x.  It was actually easier to see the faint companion at the lower magnification. 

 

When the visual observing was finished the camera and focal reducer were swapped in.  The companion star showed up easily in a short 1/10 s exposure at ISO 1600.  This was a pleasant surprise because a focal length of 432 mm x 0.8 = 346 mm is far from ideal for double-star photography.

Polaris A+B, enlarged 3x.

 The next target was the star cluster M45, the Pleiades.

 

M45, the Pleiades.  ISO 1600, 20 s.

 After that was the dim cluster NGC 1647 in Taurus, which lies only a few degrees from the much brighter Hyades cluster.

 Exposures were kept short (20 s) to avoid smearing from field rotation.  The reducer seems to perform well and gives nice round stars when the tracking is accurate.

AM3 mount in Alt-Az mode, with pier extension.

 The reducer was then swapped for an extension tube with a diffraction grating mounted at the end, approximately 140 mm from the camera sensor.  This is the configuration pictured above.  Four stars were targeted for another attempt at stellar spectroscopy: Procyon (Alpha Canis Minoris), Betelgeuse (Alpha Orionis), Rigel (Beta Orionis), and Regulus (Alpha Leonis). 

Four stellar spectra.  2-sec exposures

Balmer-series absorption bands of hydrogen are easily seen in the spectra of Procyon and Regulus, which are hot F and B-type stars, respectively.  Betelgeuse is a super-giant M-type star with a cool atmosphere.  Its spectrum is dominated by molecular absorption bands, such as TiO (titanium oxide).

Regulus and Betelgeuse compared. The dashed lines mark the wavelengths of the hydrogen Balmer series.

 

The ecllpse was eclipsed

 The weather in Santa Fe was not good for the lunar eclipse of 13-14 March.  Some of the early stages were visible, but totality was completely obscured by thick clouds that brought along roaring wind and rain.

Orion 80mm ED f/7.5 telescope + Olympus E-M5iii camera.  ISO 200, varying exposures.

 

Just a hint of penumbral shading.  10:41 MDT

11:13 MDT

The last view, through thickening clouds, an hour before totality.  11:32 MDT

Stellar spectroscopy

 A large fraction of my time as a working physicist involved doing neutron and gamma-ray spectroscopy of nuclear reactions.  I have always been intrigued by the possibility of trying optical spectroscopy of stars.  Fortunately, there is an easy entry into this practice for the amateur astronomer.  All it requires is an appropriate diffraction grating that can be mounted in the optical path of the telescope ahead of an eyepiece or camera.  

Tom Field, a former contributing editor to Sky & Telescope magazine, founded a company that supplies both the hardware and software required.  The starting point is Tom's website Rspec-astro.  I purchased the Star Analyzer 100 diffraction grating as well as the Rspec analysis software, which is well-written and versatile.  The software is specific to Windows, but I had no trouble installing and running this software under Crossover (Wine) for Linux.  In spite of this success, the results shown below were processed with my own software from my working days (use what you know).

Here is a an image of the diffraction grating mounted on a 2-inch extension tube that inserts into the focuser of the telescope.

 This particular grating has a line spacing of 100 lines/mm, and is housed in a standard 1.25" filter cell.  The label on the grating says "PATON HAWKSLEY, UK STAR ANALYZER 100".

The distance from the grating to the image plane was measured with a ruler as L = 140 ± 1 mm.  The exact distance doesn't matter because the spectrum calibration was obtained from the actual data. 

Light with wavelength λ is diffracted at an angle 

sin(θ) = λ/d,

where d is the line spacing (10^-2 mm) of the diffraction grating.  The wavelength of the Hydrogen-beta transition, which is a convenient reference in the blue part of the spectrum, is λ = 4861.4 angstrom. For the configuration shown here, this line will therefore appear at a displacement of  x = L*tan(θ) = 6.8 mm from the zero-order image of the star.  This displacement is well within the width (17 mm)  of the micro-four-thirds sensor of the Olympus E-M5iii.

The telescope used was an Orion 80mm f/7.5 ED refractor.  Here is an image of the star Sirius and its diffracted spectrum, ISO 1600, 1/5 sec:

 

In this image, Hydrogen absorption bands are easily visible.  The spectrum shown below was obtained from the luminosity channel of this image.  The red dotted lines correspond to the various transitions of the Balmer series in Hydrogen.  The correspondence is very good, though there may be some non-linearities that I don't yet understand (I'm just a beginner in this field). 

 The actual spectrum of the star extends well below and above the cut-off wavelengths of 3800  and 7000 angstrom seen here.  The wavelengths visible in these data are limited by the quantum response of the CMOS camera sensor and the filter stack in front of it.

 

Preparing for the eclipse

 There will be a total eclipse of the moon on the night of 13-14 March (Thursday pm into Friday am).  It is always good to check equipment ahead of time.  This image was taken with an Orion 80mm ED f/7.5 refractor and an Olympus E-M5iii camera, ISO 200, 1/640 s.

11.9-day moon

 

The Night Watch

 I am usually alone with the telescope, but last night I had company.   I heard the scratchy sounds of its arrival, and my headlamp revealed a curious observer.  It sat and watched for about five minutes, but offered no advice or commentary.  This was not the first time it has kept me company under the stars.

Procyon lotor, the star raccoon.

In the constellation Canes Venatici, which lies south of the Big Dipper, there is a deeply-red variable star with the nickname "La Superba".  Its official designation is Y Canum Venaticorum.  At magnitude 5.2, this star is one of the brightest examples of a "carbon star".  These stars have high concentrations of carbon compounds in their atmospheres.  These compounds absorb short-wavelength light and lead to the sunset-like coloration.

La Superba lies south of the handle of the Big  Dipper, between Mizar and Cor Caroli, the two double stars featured in the previous post.

credit: SkySafariAstronomy.com

Capturing this star's color in a wide-angle image is a bit of a challenge.  Diffusion filters definitely help. A darker sky (sqml=19.8 last night) would also help.

Sony A7iii + Rokinon 85mm f/1.4 AFII + Softon filter.  ISO 1600, 30 s.

Olympus E-M1iii + 45mm f/1.8 + Sparkle-6 filter.  ISO 1600, 30 s.

The Hoya Sparkle-6 filter seems to produce the most accurate colors, although the six-pointed star patterns border on looking gimmicky.  The Hoya Softon filter tends to render blue colors better than reds.