Castle Point Astronomy Club
Castle Point Astronomy Club Diary
April 2017 by Dave Stratton
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5th April 2017

Peter said that Mercury was in the evening sky now sunset is at 19.30 and at 20.00 Mercury will be above where the Sun set. Jupiter is also looking good.

Comet 41P is overhead at mag 4.5 at the end of Ursa Minor. There is another Lovejoy comet imminent. See the in-the-sky website.

Mike spoke about the Open Night last Saturday saying that it went very well and how proud he was of the club and thanked everybody for their help.

Mike introduced Andy T for his talk:

The Hertzsprung Russell Diagram Part 1

He explained what the scales were.

He said he thought it very pretty and made no apologies for showing it so many times.

He said the top right of the diagram was the home of the very large stars and the lower left was for the very small.

It relates stars relative to the Sun which is pretty much in the middle.

He referred to the Hipparchus satellite.

Gaia will look at 1000,000,000 stars.

He spoke of Ejnar Hertzsprung and Henry Norris Russell. They both had craters named after them - Ejnar´s is 600km diameter on the far side of the moon and Henry´s is 120km diameter and on Mars.

However he said they were not the first to have the idea of having a diagram that compared temperature against luminosity of stars that was Schwarzschild and Rosenberg.

Andy said that a scatter diagram was an excellent way of presenting data - he showed us a page of 'meaningless' numbers in columns and then a scatter diagram of the same info and it was ‘CPAC‚ -simple. So a picture really is worth a thousand words.

He showed us HR diagrams of the Hyades and Pleiades circa 1910.

He asked - what is luminosity? It can be measured using the whole electromagnetic spectrum or just in visual light.

Luminosity is actually difficult to measure - we really measure the brightness.

He spoke of Hipparchus who came up with a way of talking about magnitude. He had 5 magnitudes with a factor of 2 between each - such that mag 2 was twice as bright as mag 3, so that mag 1 was 100 times as bright as mag 5.

Sir Norman Pogrom developed a slightly different system called Pogrom´s ratio where the difference is 2.512. This is better and still in use today.

Andy said that luminosity is diluted by distance so we need to compare not a star's magnitude but its absolute magnitude. This is what its magnitude would be if viewed at 10 Parsecs. If viewed at this distance our Sun would be 4.83 mag. (quite unassuming).

We look forward to parts 2 and 3.

19th April 2017

A surprisingly good turn out as several of the regulars are at Kelling Heath.

Andrew said that next week we have - if it'´s cloudy - Peter with his Observing Highlights for May and Jim with a talk on Eyepieces. He also mentioned that some of us are parking badly again.

Mike announced that Emma had some raffle tickets for sale.

Mike said the subject tonight was his and a few others thoughts on:

Websites for Astronomy

The very first was our own Castle Point Astronomy Club this is very good in its own right - Mike got it on his tablet and with Andrew´s help explained the detail of how it works.

Then the British Astronomical Association site - Mike ran through its various features.

Astronomy Now was next - the current news is that the Cassini probe is shortly to end its wonderful career by plunging into Saturn - in the name of science.

Sky and Telescope

Armagh Planetarium

National Optical Astronomy Observatory

Astronomy Picture of the Day - Apparently if you are clever enough this site´s stunning images can be displayed on your device.

Hubble Space Telescope -

Lunar and Planetary Institute

H Alpha Network - This is a series of continually updated images of the Sun from various locations on our planet. maps solar images of current activity.

Calsky If it´s worth seeing this site has it.

Aurora Service Europe

There were another 16 sites demonstrated during this session details of all are - or shortly will be - on our Club´s site which is the first detailed above.

Should keep anyone out of mischief for a while.

26th April 2017

Andrew said that next week we have Andy with the second part of his talk on the HR diagram. He also said he was considering setting up a discussion group on our website - so watch this space.

Mike introduced Jim for the first part of this evening´s content for his talk:

Telescope Eyepieces and other bits

He began by explaining how an eyepiece works with the aid of a diagram. He said that the magnification was established by dividing the focal length of the scope by that of the eyepiece.

Jim explained the various design types available. They include Huygens, Ramsden, Kellner, Plossl, Orthoscopic, Erfle and Nagler.

He said that the Field of View was the amount of sky seen through an eyepiece (called the true field of view) is determined by both the magnification and the eyepiece´s apparent field of view. Apparent field of view is a design characteristic of an eyepiece design. Some eyepieces have narrow apparent fields and some have wide apparent fields. If the magnification is kept the same (i.e., the eyepieces have the same focal length), an eyepiece with a wider apparent field will have a wider true field.

Eye Relief was an important aspect of many eyepieces. Eye relief is the distance from the eyepiece to the observer's eye. The shorter this distance, the more difficult it can be to observe. Also, if the observer must wear eyeglasses, short-eye-relief eyepieces can be very difficult or impossible to use. Long-focal-length eyepieces (usually low power) inherently have long eye relief, so they do not need to be specially designed to increase eye relief. Short-focal-length eyepieces (usually high power), on the other hand, do not inherently have long eye relief and must be specially designed to make them easier to use.

There are two standard sizes of telescope eyepieces. The sizes are determined by the diameter of the eyepiece barrel that fits into the telescope. The two standard sizes are 1.25 inch and 2 inches. A third size, 0.965 inch is a smaller standard that it is usually best to avoid.

Jim said that when considering a scope be careful of too high numbers claimed for the device performance. Scopes work in the 50 - 500 range with most about 100 the limit of magnification is 50 times per inch of objective lens.

He explained how astigmatism affects the view and can be tested by considering what is seen when the object is deliberately set off focus in each direction.

Jim explained how eyepieces have changed over the last 40 years and gave us a chart that broke down what you get for your money across the eyepiece range.

He said that zoom eyepieces although expensive were a real option as they covered a range of powers, but they are a bit heavy and can change the balance of your kit. If you were into Bino-viewers then they were a serious option except for the weight issue again.

Barlow lens he said were good but be aware of getting the two times as this may well just turn a 20mm into a 10mm that you already have. It's better to go for a times three product.

He said a Focal Reducer gave the effect of reducing the Focal Length of your scope which was often an advantage.

Another gadget he likes is a Field Flattener. They counteract the Petzval field curvature of an optical system. The object in designing a field flattening lens is to create a lens that shifts the focal points of the Petzval surface to lie in the same plane.

Jim finished by telling us about a Paracorr this is a universal corrector that tightens and intensifies star images on all f-ratios down to f/3. You no longer have to constantly shift a Dobsonian to keep objects centered for sharp viewing.

Mike introduced Peter for his update on:

The May Night Sky

Peter began by showing a wonderful image of aurora taken by his pal who lives in Scotland. He added that if you Google Spaceweather you could get data on when they are likely. He explained that they are caused by emissions from the Sun that come down our magnetic field lines.

He showed us a brilliant video of aurora made up from 100 30 second exposures.

He said that Jupiter was at opposition on 7th April and remained well placed at due South at midnight. He explained with the aid of a nice image the features of the planet that would be visible with a 6inch scope.

He pointed out the edges of the image were darker and said this was called limb darkening and was because the edge was further away and we were kind of looking along it rather than directly at it as with the centre of the image.

He said that when looking at Jupiter it was a good idea to ensure the scope was at ambient temperature and that you were not getting heat distortion from rooftops. Also that the instrument was correctly collimated. He suggested that a maximum magnification was 20 times per inch of aperture.

Peter mentioned the Antoniadi Scale

We saw a video clip of the Galilean moons including a shadow transit.

He told us about an amazing pro-am collaboration involving 91 sites around the world where 1000 images of Jupiter were grafted together to make a 3minute video of the planet. It really brought out the incredible differences in band speed of rotation.


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