Category Archives: astronomy

astronomy, computing, eclipses

Eclipse Calculations using Python

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On 20th March there is a deep partial solar eclipse over the  UK which is total over the Faroe Islands and Svalbard. I set about trying to determine the time of maximum eclipse, and percentage eclipsed at a given location.

I used the pyephem module which is a Python implementation of the numerical routines that are behind the excellent Xephem. The module will allow a user to calculate the positions of astronomical objects (eg Sun, Moon, etc) from a given location at a given time. Using the positions of these objects it’s possible to determine the separation of the centres of these objects.

To work out the percentage eclipse at that time, I used the formula on the Wolfram Mathworld page on “Lunes” (author: Weisstein, Eric W). A “Lune” is the “plane figure bounded by two circular arcs of unequal radii, i.e., a crescent.” which exactly describes the visible surface of the sun during an eclipse.

With the area of the lune calculated, it’s trivial to work out the percent eclipse at that time – and by looping over time, it’s possible to get a list of lists to search for a maximum.

The code can be downloaded using this link: eclipse_percent.py

The result I get (using the location of the Science Oxford setup on 20th March – the SBS in Oxford, UK) is as follows:

Max Eclipse occurs at: 2015-03-20  09:30:11 GMT
Max percentage eclipse: 85.86
First contact: 08:24:21 GMT
Last contact: 10:40:04 GMT

All we need now is some decent weather…

The featured image here is an H-Alpha filtered frame taken by me on 3rd Oct 2005 from Denia, Spain, during the early phases of an annular eclipse.

astroimaging, astronomy, open clusters

M35 & NGC 2158

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Field centred at: RA: 6h 08m 25.2s, Dec: +24° 15′ 06.2″
(plate solve from astrometry.net)

M35 and NGC2158 are a pair of open clusters which, similarly to the more famous h and X Persei, are viewed nearly on the same line of sight from Earth. M35 is somewhat younger (as evidenced by the predominantly blue stars as compared to the older yellow stars in NGC2158), and a lot closer to us – while NGC2158 is more compact with more stars in the same volume.

M35 & NGC2158 in Gemini
M35 & NGC2158

This exposure was taken on the 8th February 2015 but was somewhat shortened by fog rapidly rolling in, and hence the rather short total exposure:

ST2000XM, FLT110, Losmandy Titan
LRGB: 20:9:9:9 each in 1min subs.

astroimaging, astronomy, comets

C/2014 Q2 (Lovejoy) – 8th Feb 2015

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Field centred at: RA 02:00:07, Dec +45:10:35.
Field size: 51.4 x 38.9arcmin; Field rotation: up is 179.9° E of N
(plate solve by local astrometry.net instance)

Comet Lovejoy was discovered in summer 2014 by Terry Lovejoy and is a long period comet and had a period of about 11000 years (now reduced to about 8000 years after this approach to the sun.

C2014-Q2-20150208.jpg
Comet Lovejoy (C/2014 Q2)

 

C2014-Q2-20150208-neg-tail.jpg
C/2014 Q2 (Lovejoy) – in negative, showing tail detail

 

Images were taken on the evening of the 8th Feb 2015 using an ST2000XM  through a William Optics FLT110 on Losmandy Titan. 26 each of L, R,G,B filtered frames, each taken in that sequence (so each frame of each filter starting about 5 minutes apart).

Processing of the colour image with the comet “frozen” against the stars followed Bernhard Hubl’s method of processing the comet. The negative image which enhances the tail was produced by a median combine of the data while aligned on the comet itself rather than the background stars (the comet was moving at about 2arcsec/min at the time).

 

astronomy, equipment

Orion Optics SPX – 6 Point Mirror Cell Mod

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I’ve recently completed the first stage of modifying the mirror cell to convert it to use 6 point supports instead of 3 points.

By modelling deformations of the 350mm mirror in PLOP, I get the following figures for the current 3 points:

Support radius = 123mm
P-V: 1.65 x 10-4mm (~1/5λ at 500nm)
RMS: 3.81 x 10-5mm (~1/13λ)

For a mirror with a zygo report of 1/12λ P-V, this is less than impressive!

By using equally spaced 6 point supports, these figures are:

Support radius = 110mm
P-V: 1.07 x 10-5mm (~1/46λ)
RMS: 2.37 x 10-6mm (~1/210λ!)

So it can be shown that the deformations are potentially over an order of magnitude better using 6 points. Interestingly, allowing PLOP to run for a 9-point cell for the same mirror gives a slightly worse result than the 6 point, and though the 9 point is much better than a 3 point it’s harder to adapt this cell into.

Orion Optics Mirror Cell with 6 point mod
350mm Orion SPX Mirror Cell with mod…

My modification adds three “U” shaped supports that attach to the existing cell parts (being attached loosely via an M8 bolt to allow them to rock a little). There are two nylon mushroom head bolts that screw into these parts which the mirror will sit upon (these are the black cross heads in the image).

I think I’ll most likely need some mechanism to stop the bars rotating (but this might just be a bit of tape), and I’ll need new lateral nylon grub screws as 2 of the three current ones are well and truly stuck!

astronomy, equipment

Refurbishing the Newtonian

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"Newt" Raytrace for SPX350
“Newt” Raytrace for SPX350

I’ve been working recently towards finally completing the observatory dome in my back garden (that I bought almost 4 years ago now…) – with flooring done and power connected, I need just some steps, finish security and I can start to setup.

Attention has turned towards the Orion Optics SPX350 that’s been languishing in my garage awaiting use. I’ve known that the current secondary mirror doesn’t offer anywhere near enough completely illuminated field – and so I’ve finally measured the location of the focal plane  and using the excellent “Newt” software (now available as a web based app) and determined the size of a new secondary mirror for it. Yesterday I ordered a new 89mm secondary from Galvoptics that I plan to mount in a new secondary holder from Protostar (and hoping I won’t need a new spider :-/ ). This should give me a 100% illuminated diameter approaching 20mm – more than enough to cover the KAI2020 CCD chip I use, and good enough (with flats) should I decide to try upgrading to a KAF-8300 based camera in the future.

SPX350 - Primary Mirror
Primary Mirror – warts and all.

More importantly, the primary is going to need some work to bring it back to its full potential (the original zygo report says it is ~1/12 wave P-V) as even after washing off the dead moth remains (!) and flies, the coatings look to be past it, and there’s a load of spider poo stuck to the surface as well. Here’s hoping that they will clean off well and it’ll recoat fine – and so once I have the new secondary installed, a trip to Newcastle-under-Lyme will be needed in due course to look to get the whole thing serviced.

More to follow.