This post contains a set of photographs I took of the Moon on May 19, 2010, at about 10pm. All the photos were taken at the same magnification (52x), using my SkyWatcher Explorer 130P telescope and a FujiFilm FinePix F200 EXR compact digital camera. The only variable was the exposure time.

It’s worth pointing out though that the magnification wasn’t really important. This is because the moon is so big that you don’t need to worry about magnifying it very much. The eyepiece I used gave me a magnification of 52x, but I also adjusted the magnification of the camera itself, which would have increased the magnification a bit. Then, in PaintShopPro, I played around with the magnification a bit more to get photos that were a manageable size.

So, basically, I’m not sure what the final magnification is in the photos shown below.

The astrophotography setup I use (telescope, camera and adapter) is described in the post Digital Camera Astrophotography for Absolute Beginners.

Photograph	Exposure Time in Seconds
	1 1/2
	1
	1/2
	1/3
	1/4
	1/5

You can see that a 1.5 second exposure produced a photo that was a bit over-exposed. This was also true of the 1 second exposure. Everything below that looks OK though, I think, although the 1/5 second shot looks a bit dark I guess.

My attempts to photograph Venus were, to say the least, pretty disappointing. The best shot I managed to get is shown below and this could be a photo of pretty much anything – it doesn’t really look much like Venus.

I used an exposure of 1/80th of a second for this photo. I tried various exposures but nothing seemed to work very well. Most of them were completely over exposed and looked like white, shiny blobs.

The root of the problem though was that I couldn’t get a sharp (in focus) image of Venus through the telescope, so I knew the photos would all be pretty disappointing. I used PaintShopPro to give the above image an outline – the real image looked even worse !

I have to say that I have had better results in the past, when viewing Venus, using my small (50mm) refractor.

What causes phases?

The phases of the moon and other planets are caused by the alignment of the moon or planet in the sky when viewed from Earth. 

The Moon

The Moon’s phases are produced by the alignment of the Moon and the Sun in the sky (when viewed from Earth), as shown in the following diagram.

When the Moon is in position 1, it is hidden by the glare of the Sun and is not visible at all. This is called a New Moon. At position 5, the Moon is on the opposite side of the Earth to the Sun, and appears as a Full Moon.

Mercury

Like the Moon, Mercury exhibits phases as seen from Earth. It is “new” at inferior conjunction¹ and “full” at superior conjunction¹. The planet is rendered invisible on both of these occasions because it is too close to the Sun.

The first and last quarter phases occur at greatest elongation² east and west, respectively, when Mercury’s separation from the Sun ranges anywhere from 17.9° at perihelion³ to 27.8° at aphelion⁴. At greatest elongation west, Mercury rises at its earliest before the Sun, and at greatest elongation east, it sets at its latest after the Sun.

Venus

The phases of Venus are the same as those for Mercury. It presents a “full” image when it is on the opposite side of the Sun. It shows a “quarter phase” when it is at its maximum elongation from the Sun. Venus presents a “thin crescent” in telescopic views as it comes around to the near side between the Earth and the Sun and presents its “new phase” when it is between the Earth and the Sun.

Since the planet has an atmosphere it can be seen at “new” in a telescope by the halo of light refracted around the planet. The full cycle from “new” to “full” to “new” again takes 584 days (the time it takes Venus to overtake the Earth in its orbit). The planet also changes in apparent size from 9.9 arc seconds at full (superior conjunction) up to a maximum of 68 arc seconds at new (inferior conjunction).

Venus reaches its greatest brilliancy (magnitude –4.5) when it is an intermediate crescent shape at the point in its orbit when it is 68 million kilometers away from the Earth (a combination of its closeness and the fact that it is 28% illuminated).

Mars

But what about Mars – does it have phases when viewed from Earth?

From Earth we never see Mars less than gibbous. Its orbit is outside ours, so that we can never see it from the side (relative to the Sun) or closer to the Sun. Therefore, we never see a half-Mars or a crescent Mars.

And what about Jupiter and the rest

Planets in inferior orbits undergo complete phase changes like the Moon when viewed from a planet with a superior orbit. Planets in superior orbits only go though minor changes in phase when viewed from a planet with an inferior orbit.

Mercury and Venus have inferior orbits than Earth, whereas Mars, Jupiter, Saturn, Uranus, and Neptune all have superior orbits.

<sup>1. Conjunction – An alignment of two celestial bodies such that they present the least angular separation as viewed from Earth.
2. Elongation – The angular separation of two celestial bodies. For Mercury and Venus, the greatest elongation occurs when they are at their most angular distance from the Sun as viewed from Earth.
3. The perihelion is the point in the orbit of a planet, asteroid or comet where it is nearest to the Sun.
4. The aphelion is the point furthest from the Sun of any body orbiting the Sun.</sup>

Am I pleased with the mount? well, kind of. In this post I’ll share my thoughts on what I like, and dislike, about it.

Good Points

1. Physically connecting the mount to the telescope tube and the tripod is very straightforward and can be done within a minute or so.

2. Setting up the software is also fairly easy to do. There is a one-off  task to set up the latitude for your location, which only needs to be redone if you use the telescope several miles from where the setup is done.  

3.  The setup process for each viewing session is simply a case of switching on the power supply while the telescope tube is horizontal and pointing north. 

4. Tracking objects works well so long as you have carried out steps 2 and 3 above accurately.

Bad Points

1. The battery pack does not have an on/off switch, meaning that you need to use the thumb screw to connect it to, and disconnect it from, the mount.

2. The mount has two servo motors: one for altitude adjustment and one for azimuth (horizontal) adjustment. I find that the azimuth motor seems to struggle sometimes, even when the batteries in the battery pack are new. The problem seems to be worse in the cold weather.

3. My handset broke after I had had the telescope for 11 months (see http://www.myastronomyblog.com/?p=269 for more information).

This post contains a set of photographs I took of Saturn on April 21, 2010, at about 10pm. All the photos were taken at the same magnification (52x), using my SkyWatcher Explorer 130P telescope and a FujiFilm FinePix F200 EXR compact digital camera. The only variable was the exposure time.

This was an experiment for me, as I had no idea how long an exposure to use. I started off with an exposure of 8 seconds, and gradually reduced this down to 1/10 of a second. I’ve included a selection of photos with exposure times ranging from one second to 1/10th of a second. Exposure times above one second were way too over exposed, so I haven’t bothered to include any of them.

The astrophotography setup I use (telescope, camera and adapter) is described in the post Digital Camera Astrophotography for Absolute Beginners.

Photograph	Exposure Time in Seconds
	1
	1/3
	1/4
	1/6
	1/8
	1/10

You can see that a one second exposure time produces a photo that is over exposed; the 1/3 of a second exposure is also slightly over exposed. The 1/4 and 1/6 of a second exposure times produce the best results, with the 1/8 and 1/10 of a second exposure times producing slightly under exposed photos.

(Formula to calculate magnification is: telescope focal length / eyepiece focal length)

My initial thought was that I needed to rush out and buy a 4mm or 5mm eyepiece to crank up the magnification to 260x or more (using the Barlow lens), and I have to admit I did do this – well I bought a 5mm lens anyway.

And to be honest, I’m not sure it was really worth it. Focusing at 260x magnification is quite difficult and I find that most of the time I don’t go any higher than 130x, which I can achieve with my 10mm eyepiece and the Barlow lens.

Recently I bought a 40mm eyepiece, giving me magnifications of 16.25x and 32.5x. This, I get a lot of use out of because it’s really easy to find objects using it and it gives me a wide field of view.

So, in conclusion I would think very carefully about whether you really need to buy eyepieces that give high magnifications as you’ll probably do most of your star gazing at fairly low magnifications.

Comparisons of different-sized apertures for their light-gathering power are calculated by the ratio of their diameters squared. For example, a 10-inch aperture will collect four times the amount of light of a 5-inch aperture:

(10 × 10) ÷ (5 × 5) = 4

Telescope Aperture Comparison Calculator

The following link opens a simple aperture comparison calculator that lets you compare the light gathering power of two telescopes with different apertures.

You can enter the values using any units you want to, for example, inches, mm, or cm, but you obviously need to use the same units for both values.

Aperture Comparison Calculator

What is the focal length?

The purpose of the main optical element of a telescope (primary mirror or lens) is to gather light from an object and to concentrate that light into an image. This image is a fixed size but the size depends on the focal length of the optics - the longer the focal length the larger the image will be at the focal point.  You can think of this like the distance between a slide projector and the screen – move the screen and slide projector further apart, and the image gets larger, but dimmer.

The focal length is the distance from the mirror or lens to where the image is formed. The focal length of an optical system, such as a telescope, is a measure of how strongly the system converges (focuses) or diverges (defocuses) light.

For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance.

Focal length is often expressed as an f/ratio:

f/ratio = focal length of lens/aperture of lens

Example: 130mm lens with a focal length of 650mm

f/ratio = 650/130 = f/5

What is considered a long focal length?

Long focal lengths are considered to be in the f/9 or greater range. A telescope of a given diameter coupled with a fairly short focal length, say f/5 produces bright images but wide fields.

This is fine for observing large deep-sky objects and star fields, but if you also want to observe planets, you’ll want a slightly longer focal length.

Speed - a bit more about focal ratio
 
Focal ratio is the ratio between an optical system’s focal length and aperture.  For example, a 100mm aperture telescope with a focal length of 1000mm would have a focal ratio of 10. 

Focal ratio determines photographic speed, so an f/5 telescope requires shorter exposure times than an f/10 telescope.  Therefore higher focal ratios are called slower, and lower focal ratios are called faster. 

Also, all things otherwise being equal, slower focal ratios produce less aberrations, but in many designs, a faster focal ratio produces a more compact telescope, meaning there is always some sort of compromise.

When I switched on the handset/motorized mount, the Set and Go Cruise buttons starting flashing, and they wouldn’t stop. This rendered the the motorized mount useless as it refused to do anything.

Having put a post on a forum, it looks as though there might be a communication problem between the handset and the mount.

I bought my telescope (a Sky-Watcher Explorer 130P) just under a year ago, so it should still be under warranty if I need to return it. I hope it doesn’t come to that as I don’t want the hassle.

So, it looks as though for the time being I’ll have to revert to my 50mm reflector telescope … oh hum …

Update – 23 February

I put another post on a different forum, again asking the same question and, yes, it looks as though it is a communications problem between the handset and the alt-azimuth SupaTrak mount.

This morning I received an email from Telescope Planet asking me to return the handset. Hopefully, it is that rather than the mount, which would cost a lot more to send through the post.

Update – 04 March

Well, I’m still without a handset, so in effect I’m still without a telescope ! We’ve had some really clear nights recently as well, so it’s all a bit frustrating …

Update – 08 March

Still no word from Telescope Planet … and we’ve had some really clear skies just recently.

Update – 13 March

Today I received a new handset from Telescope Planet … but the problem still exists. I’ve sent another emal to Telescope Planet asking them what to do next. It’s looking as though the mount will need to be replaced.

Update – 18 March

I’m still without a working telescope. I feel frustrated at the moment because, although they are always very polite, I always feel that there is a lack of information from Telescope Planet as to what they are doing to resolve my problem.

I’m also aware that the nights are becoming lighter and I’m losing valuable astronomy sessions while my telescope is broken. I’ve also started looking at alternative telescopes – just in case I end up having to buy a new one. I need this situation with my broken scope to be resolved by the end of the month (March 10).

Update – 23 March

I’ve received an email from Telescope Planet asking me to package up the mount and handset so that they can be sent to the manufacturer for testing/repair. It feels a bit like pulling teeth, but I guess we’ll get there in the end.

Update – 30 March

Today I posted the mount, power supply pack and handset off to Optical Vision (the suppliers to Telescope Planet) so that the unit can be tested and, hopefully, fixed. I guess I just have to wait now.

Update – 08 April

I contacted Optical Vision today to see if they had made any progress with my ‘mount’ problem, but it was still waiting for a technician to test it. They rang me back later though to say that they had tested the mount and it was OK. They spoke to my wife so I’m not sure what was said exactly, but they’re returning everything to me – with another new handset (see 13th March entry above – I’ve been here before).

Anyway, everything should be with me by Monday (it’s Thursday today), so we’ll see.

Update – 16 April

Success – yesterday I received my mount, handset, and battery pack back from Optical Vision, and everything now works OK. The handset has been replaced so that must have been the problem (the replacement handset that Telescope Planet sent me a while ago must have been faulty).

So, in conclusion, a successful outcome even though it has taken eight weeks to resolve the problem. Everyone I have dealt with at Telescope Planet and Optical Vision has been very polite and helpful, although I have been frustrated sometimes by the slow progress.

Orion Constellation

Here is a photo of Orion I took at about 6pm on Wednesday 17th February, 2010.

I decided to not use my telescope for this shot, but to just stick my camera on a tripod and point it at the constellation.

I’m reasonably happy with it. You can easily see the main stars in the constellation (Betelgeuse in the top left corner and Rigel in the bottom right), and you can resolve the Orion Nebula, Messier 42 (M42, NGC 1976) – the brightest diffuse nebula in the sky – which is about 2/3 of the way down the image (in the dagger), just slightly to the right of centre. It looks slightly fuzzier than the other stars in the image.

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