Equipment & Reviews

One of the biggest concerns with winter observing and imaging is the onset of dew but there are several solutions to this problem. Blowing warm, dry air over the optics or using extendable dew-shields are some methods, but probably the most popular and convenient method is an electric dew heater.

These have been commercially available from various manufacturers since the 1980’s however can end up costing several hundred dollars for a basic setup comprising of a control box and a pair of heater-bands. There are a couple of less expensive alternatives to the commercial option which I will cover here.

The Dew Strap

Generally the dew strap is constructed from an electrically resistive circuit which generates heat as an electric current is passed through the circuit. Although everyone will have different requirements, I have found that a 4W heater was more than adequate to keep my 71mm camera lens and a 12W heater for my 90mm guide scope free of dew on the worst of evenings.

The two most popular methods of DIY dew strap construction seem to be either a parallel array of electronic resistors (see http://www.iceinspace.com.au/63-292-0-0-1-0.html) or with a continuous circuit of NiChrome resistance wire sandwiched between an insulating material, usually duct tape (see http://www.deepskywatch.com/images/articles/dew-control/instructions-nichrome-heater.jpg).
Having had success with both of these methods in the past I recently discovered an affordable alternative on an internet auction site.

 

These are a prefabricated silicone rubber strap heater band which can be found in a large range of sizes and wattage. Although most of these heater strips are rated for 12Vdc there is also a range rated at 24Vdc which can still be run at 12Vdc but as the Voltage is proportional to the square root of the Power, halving the Voltage will quarter the Power. This means a 24Vdc 40W heater will only yield 10W at 12Vdc.

The Dew Heater Controller

In most instances dew heaters won’t need to be run at full capacity, so some form of controller is useful. As the dew heater band has a fixed resistance this can be done by varying the amount of continuous current that flows through the circuit.

These days one of the simplest solutions for this is with Pulse Width Modulation (PWM). Fortunately, most DC motor controllers use PWM to vary the motor’s speed and there seem to be no shortage of these on the Internet auction sites. The units that I found for just under $10 each came equipped with a 3 digit red LED numeric display, an on/off push button switch and could handle a maximum of 60W. The LED display indicates the percentage of power delivery to the dew heater, so with a 12V 10W heater a setting of 40 would run the heater at about 4W.

One optional item in the assembly will help monitor the battery voltage and current being drawn.  Purchased online for under $5, with a dual red LED display, this volt/amp meter is a worthwhile addition for keeping track of the battery’s state of charge. To keep the package light I chose to use a RC Model 5Ahr LiPo battery which has proved to have enough capacity for several nights dew-free imaging.

The wiring is fairly straight forward for anyone with a little electrical experience and at minimum a 5A inline fuse to the battery is strongly recommended. As everyone will have their own individual requirements, if you need any further information or assistance in this project please feel free to contact me through the ASSA-Chat Forum.

Thanks to my son Andrew for the skill with his Dremel and thanks to my son Bennett for his old lunchbox.

Green laser pointers are useful astronomical tools. On public astronomy nights, the narrow beam from the laser pointer can be used to unambiguously point to celestial objects. The beam is bright and clearly visible for several reasons:

• typical green laser pointers deliver at least 5 milliwatts (mW) of power
• green light is scattered away from the beam direction by air molecules and dust particles (so you can see the beam from the side)
• the eye is particularly sensitive to the green colour of the laser pointer.

Under the South Australian Summary Offences Act 1953, hand held laser pointers with an output greater than 1 milliwatt are prohibited weapons. This Guideline has been prepared to assist ASSA members using laser pointers to understand the changes to the use of laser pointers and is provided for information only and should not be regarded as legal advice.

Note: It is the responsibility of any person using a laser pointer to confirm that their use or possession is permitted by the relevant part of the Act. The Astronomical Society of South Australia makes no warrant as to the legality of any use of a laser pointer in South Australia. If using a laser pointer in other States, astronomers are recommended to seek local advice before operating a laser pointer.

South Australian Legislation

The South Australian legislation is typical of the state based legislation dealing with laser pointers. It applies to any hand-held laser pointer >1mW regardless of the wavelength of the beam. 

The legislation provides exemptions for persons in the course of conducting his or her business or for the purpose of or in the course of his or her employment plus for members of astronomical societies engaged in Astronomy. 

The legislation specifically recognises members of the Astronomical Society of South Australia as a class of people exempt from the prohibition but only when using a laser pointer for astronomy. The legislation deals harshly with people, including astronomers that misuse a laser pointer to endanger life or cause injury. 

What are the dangers of Laser Pointers?

Laser light from laser pointers can potentially burn the retina of the human eye. The danger is obviously greatest if the beam is aimed directly into the eye, rather than merely scattered from the beam and seen from the side. The danger is dependent on the wavelength of the laser light, the power of the laser pointer, the divergence of the laser beam, the distance of the person from the pointer, whether the beam is seen directly or via a reflection, how long the beam is viewed and whether the human eye's natural 'blink response' to bright light occurs.

The risk from a laser pointer is often also expressed by the 'class' of the laser pointer, although the definition is a little complicated and class definitions have changed in recent years. At one end of the scale, Class 1 laser pointers are safe for normal viewing.  Eye damage from directly viewing the beam of a Class 2 laser pointer is usually avoided by the blink response. Class 3 laser pointers can damage an eye before it has time to blink and have the potential to cause eye injury, especially in the hands of a careless or untrained operator. Class 4 lasers are even more dangerous, higher power devices.

The blink response should protect the human eye at any distance from any visible-light laser pointer beam with a power under 1 mW, typical of a Class 2 laser pointer. For laser pointers that are more powerful a greater distance is required to allow for blink response protection. For a given laser pointer beam, a quantity called the Nominal Ocular Hazard Distance (NOHD) can be calculated. Serious eye damage is probable within the NOHD and less likely at greater distances. The NOHD depends on the power, wavelength, divergence and diameter of the beam and the length of time the laser pointer beam will be viewed. For example, a 10 mW, collimated, continuous wave, green (532 nm) laser pointer beam with a diameter of 1 mm and divergence of 10-3 radian, being viewed directly for 10 seconds has a NOHD of 55 m. Clearly, this laser pointer, typical of many green laser pointers, is a potential hazard to someone close to it and should be used with care.

Damage to the eyesight of a pilot or driver is unlikely from medium power (5 mW to 20 mW) laser pointers given that the typical distances involved are considerably greater than the relevant NOHD. However, the dazzle caused by the beam scattering off dust or scratches on a windscreen, or the blink response itself could still lead to loss of control of a vehicle. A laser pointer beam could also potentially cause harm in other situations, for example by startling someone using a power tool.

Some amateur astronomers and small observatories have similar laser pointers permanently fixed to telescopes, although not always in use. While technically not hand held laser pointers, they should be used with similar safety guidelines in mind. 

Safety Guidelines

It is the view of the ASSA that the safe use of hand-held battery-operated laser pointers is possible in Astronomy by following the guidelines presented below:

1. A laser pointer must only be used in accordance with the laws of the state or territory in which it is used.
2. A laser pointer should be carried between observing sites deactivated [batteries removed].
3. Laser pointers used for astronomy must require a button to be held continuously to activate the beam. If the laser pointer is dropped, the beam will automatically switch off.
4. Before activating any laser pointer, astronomers must always check where people are located and ensure the beam is never directed in those directions.
5. Always hold laser pointers overhead in an outstretched arm before activating the switch and release the switch before lowering the pointer.
6. Never use a laser pointer to point out terrestrial objects. Aim the beam only at celestial objects. Do not aim the beam at any object on the ground, nor at aircraft, motor vehicles, any person or any animal.
7. When the laser pointer is not being used to point at celestial objects return it to its case, place it in a pocket or cover the aperture from which the beam is emitted.
8. Always cease using a laser pointer if an aircraft is heard and do not switch back on until the aircraft is clearly located and its flight path is confirmed to be well away from the patch of sky being identified with the laser pointer.
9. Store the pointer deactivated in a secure place away from the reach of children and anyone with a potential to misuse the device.

To assist the police or other enforcement Agencies, astronomers must carry evidence of their current membership of an astronomical society at all times when the laser pointer is in their possession. 

Binoculars are a great way to obtain deeper views of the heavens. They're easy for beginners to use, usually offer richer fields of view than telescopes, and can be much cheaper! However, some are better for astronomy than others.

Magnification and aperture

Binoculars are specified with two sets of numbers, such as 7 x 50 or 8 x 40. The first number is the magnifying power, while the second is the aperture of the objective lenses in millimetres.

Personally, I think of all binoculars under 40 mm aperture as daytime-only. Nevertheless, you can see a surprising amount of astronomical detail with the 8x or 10x pocket-sized models. Just avoid the ones with garish anti-glare coatings and/or highly coloured lenses. Both do weird things to star colours and magnitudes. If you have dark skies then you can still see a lot with 40 mm binoculars. For example the old 8 x 40's I use can see every star plotted in Sky Atlas 2000.

Higher magnification binoculars are harder to hand-hold steadily, resulting in a shaking image from the wobbling of their hands. The same goes for large aperture binoculars; especially if they are metal bodied instead of plastic or fibreglass. Regular arm exercises can help a lot. I find my 15 x 70's really easy nowadays, after resuming archery -- but most people will prefer to use a tripod. Big binoculars are usually sold with a tripod adapter.

Exit pupil

Exit pupil is the diameter of the light cone coming out the eyepiece. Lots of references claim that the human eye's pupil will dilate to about 7 mm when fully dark-adapted, which means that it will get the full benefit of any binoculars or a telescope that produces an exit pupil 7 mm or less in diameter. For binoculars, the exit pupil is simply the aperture divided by the magnification.

This 7 mm dilation is the basis for all those recommendations to get 7 x 50 binoculars; because 50 mm binoculars collect enough starlight to be astronomically useful (even in suburbia), and 7x is a low enough magnification to hand-hold. Their exit pupil would be 50 / 7, or about 7.1 mm.

The 7 mm exit pupil dogma is fine if you're young, but the eye's ability to dilate slowly decreases after your thirties. By your 70th birthday, the pupil dilation is typically down to about 4 mm. If your telescope or binocular's exit pupil is bigger than your dilation, then a lot of the gathered light is simply wasted against your iris.

We had an interesting demo of this during a public night a few years ago. A 7 x 50 and a 10 x 50, both on tripods, were aimed at the Pleiades and the publics were asked to describe the views. Without exception all of the older folks reported many more faint stars in the 10 x 50. The kids and Generation X'ers saw no difference apart from the magnification. Of course, both pairs of binoculars gathered exactly the same amount of light, but the 10 x 50's produced a 5 mm exit pupil. Similarly on a telescope, a higher magnification makes a dramatic difference for the senior observers.

So if you're old enough to remember the moon landings I would be recommending 10 x 50 or 12 x 50 -- not 7 x 50 unless you're buying for a kid. If you're old enough to remember the Korean War then preferably the 12 x 50; perhaps even 15 x 50 or 16 x 50.

If you can afford bigger aperture binoculars then get ones which produce an appropriate exit pupil for you for the next couple of decades. For example, 13 x 70 would produce a 5.4 mm exit pupil, 15 x 70 produce 4.7 mm, 11 x 80 produce 7.2 mm and 20 x 80 produce 4 mm.

Eye relief

Eye relief is the distance between the eyeball and the lens of an eyepiece at which you can clearly see the full field of view.

Binoculars with a short eye relief may be a problem if you wear glasses full-time, because they will keep your eye further back from the eyepiece. If your eyesight isn't too bad then, removing your glasses and re-focusing the binoculars will work fine for you. Blurry views for everyone else, of course.

If you go with this option then get a neck strap for your glasses, so that you don't drop them in the dark! If your eyes are severely different from each other and/or astigmatic, then you could only use these particular binoculars while wearing contact lenses, otherwise you probably won't get your eyeballs close enough to get the full view.

Incidentally, both of my Saxon binos (10 x 60 and 15 x 70) have good eye relief and can be used while wearing glasses. Just fold back the rubber eyecups. The 10 x 60's can be hand-held by most people but don't seem to be sold anymore. The nearest equivalent model 12 x 60's are selling for $190-240 from several Australian retailers. My 15 x 70's are selling for $250-300.

Other considerations

A problem common to the entire 'Big Cheap Binoculars' category is a less-than-perfect-view of things at the edge of the field of view. Typically, this exhibits as either spherical aberration (edge and centre have slightly different focus points) or as coma (stars on the edge have little 'tails' or 'fans'). Most of us won't care, because our attention will be in the central 90 percent of the view that looks great. Brands such as Leica, Swarovski and Fujinon put a lot of effort into eliminating all such optical defects -- and it's reflected in their retail price.

While many of us have worked out suitable ways to store, transport, assemble and pack up our portable telescopes, we are all still presented with the problem of accurately pointing the polar axis of the telescope at the South Celestial Pole (SCP). Such accurate alignment of the polar axis is necessary if you want the telescope to track celestial objects for a reasonable length of time.

There are suitable techniques for aligning larger (permanently mounted) telescopes using photographic methods or mathematical analysis of the differences between the measured and known positions of stars (using the telescope's right ascension and declination displays).¹ For portable telescopes the direction of misalignment of the polar axis can be determined by observing the drift in declination for stars in various parts of the sky. Adjustment is then a matter of 'trial and error'. This method works but it is a fiddly and time-consuming process. 

The method I now use is relatively quick but involves making some careful initial settings to the telescope and learning to recognise the brighter stars within about 5° of the SCP. Also, the telescope's equatorial head should have fine adjustments in azimuth and elevation (see Figure 1).²

 

Initial Setting Up

1. The inclination of the polar axis on the telescope mount should first be set to the latitude of the observer (Figure 1). For Adelaide this is close to 35° and the mount can be left set to this value if you are only observing in and around Adelaide.

2. Next the telescope mount (usually a tripod) should be set down so that the polar axis is pointing approximately at the SCP. A popular way of locating the approximate position of the SCP is to draw an imaginary line through the long axis of the Southern Cross (Crux) and a second line bisecting, and at right angles, to the line joining the two pointers (aand bCentauri). These two lines meet at a point close to the SCP. Care should be taken during this stage to ensure that the azimuth and elevation fine adjustments are in the middle of their ranges and that the equatorial head is level.³

3. Using a bright star, the finderscope should be adjusted so that it points accurately to where the telescope is looking. Again, you can transport the telescope with the finderscope attached and already accurately aligned.⁴

Alignment with the SCP

1. First set the declination setting circle to -90° (see Figure 1).

2. Next back off the azimuth fine adjustment screws and loosen the screw underneath the equatorial head so that it can rotate in azimuth. Pan east and west until you recognise the stars in Figure 2.

   Figure 2: Stars near the SCP

   The dashed circle represents a field of view of about 5°

   The four stars,s,t,uandxOct, are just visible to the naked eye and form a quadrilateral that fits in a 5° field of view. CG Oct and HD 1348 are seventh magnitude stars forming a line withsOct. This pattern is quite distinct. The seventh magnitude star HD 110994 is about a degree away fromsOct and currently close to the SCP.

3. Now use the fine adjustments controls in elevation and azimuth to adjust the equatorial mount until the finderscope is pointing to the position marked on Figure 2 and tighten up the screw underneath the equatorial head.

This method depends upon the declination axis being at right angles to the right ascension axis and the declination scale being accurately attached.

These conditions appear to be met sufficiently well for satisfactory alignment of my EQ3 mount. With practice the method can be carried out quickly, the key to this being recognising the pattern of stars in Figure 2.

Fine Adjustment for Aligning the SCP - The Drift Method

Using this method, azimuth and altitude are adjusted separately until the effects of star drift are eliminated.

• Azimuth adjustment
  Select a star on the celestial equator and meridian (i.e., Hour Angle = 0° and Dec. = 0°).
• • If, over time, the star drifts S in the eyepiece then the southern end of the polar axis is pointing East of the SCP.
  • If the star drifts N then the southern end of the polar axis is pointing West of the SCP.
• Altitude adjustment
  Select a star in the East (and/or in the West) low on the horizon and in the southern hemisphere of the sky (e.g., ~ Hour Angle = 4 to 5 and Dec. ~ -45º).
• • If the star in the East drifts N the elevation is too low.
  • If the star drifts S the elevation is too high.
  • This is reversed for a star in the West.

¹ Details of these methods are available on request.

² The popular EQ series of equatorial mounts have such adjustments (see Figure 1).

³ The EQ mounts have a bubble level built into them. Otherwise you can buy a bubble level from a local hardware store for a few dollars.

⁴ The finder-scope should have good quality optics and about a 5° field of view.

All books on telescope making start with a comparison of the various advantages and disadvantages of different types of telescopes and then come to the conclusion that a Newtonian is the obvious way to start your telescope making activities. This is undeniably true but in the comparison the difficulties attributed to making a refractor are, I believe, greatly exaggerated. The purpose of this article is to show that making a refractor is very much easier than most people think and to encourage would-be telescope makers to "Please Consider".

Advantages

To summarise the advantages in making a lens as opposed to a mirror let me say that:

• All surfaces are spherical. A refractive surface is four times less sensitive to figure errors than a reflective one.
• You can test the lens as you go and do not need to send it away for coating. What you make is what you get and if any slight imperfection starts to bug you in a few months time you can always fine tune it a little more.
• By oiling the space between the two lens elements you can effectively eliminate small surface errors and scratches from those surfaces and any figuring need only be done on the front surface. I suppose what I am really saying is that you don't have to be a crazed obsessive to come up with a really good telescope and that by applying the same standards of accuracy to my refractor as I have to my reflectors I have come up with a result that really surprised me.

Design

I first started my project by contacting fellow ASSA member Ian Robinson who was able to supply me with glass blanks for a 105mm achromat lens for $120. That's the first myth debunked! (The major expense in my telescope turned out to be for the focuser.) These blanks consisted of a disk of dense flint glass about 15mm thick and a disk of crown glass of similar thickness. This crown glass is officially known as BAK 1. (It means nothing to me either!) Had I wanted to make the telescope an f/15 instead of an f/12 I could have purchased blanks for $95. Apparently BAK 1 has better refractive qualities and is more suited to shorter f-ratios and is a tad more expensive. Ian also came up with a set of radii for my f/12, generated in a few minutes on his computer. Second myth - you don't have to be a mathematician. The curves went something like this. No 1 radii = 650mm, No 2 = 430mm, No 3 = -430mm (concave) and No 4 = 0 (flat). All this added together gives, for some reason, a focal length of around 1200mm.

Grinding: I started grinding the flint on top of the crown in exactly the same way you would grind a mirror with the tool on the bottom and as the curves we were looking for were the same, (albeit one concave, one convex) in no time at all two of the four surfaces were ready to polish. In the past I have tried to make my own flats for diagonal mirrors and have been dreadfully disappointed with the results so I was a little apprehensive about this stage of the process until I found out that for a flat to be successful as part of a lens it doesn't have to be very flat at all. In fact, if you hold a straight edge against the lens, hold it to a light and you see no gaps, it's perfectly flat enough to work very well. Three down!

It's best to leave grinding the front of the lens until last because if your radii for No 2 and 3 are not spot on you can adjust No 1 to suit. This happened to me and a quick call to Ian gave me the required length to work to. This curve required that I cut a tool from plate glass 10mm thick (available from Bab's Bargains, $5 per sheet). Luckily I have access to a lapidary saw so this was quite easy but I have cut this glass in the traditional way in the past.

Again, grinding the last suffice of the lens was just like making a mirror but with tool on top, lens on bottom and paying more attention to getting the radius correct. In general, grinding lenses of this size is a very fast process and in all didn't seem to take as long as grinding my first ever 6-inch mirror. One thing to watch for in grinding is stressing the glass by fixing it too tightly or pressing too hard. Make sure it is well supported underneath as it is relatively thin and prone to distortion.

Wedge

One thing I haven't mentioned so far and which seems to loom large in any discussion on lens making is wedge. Wedge is where one side of a lens is thicker than the opposite side causing the two lens elements to be at an angle with each other and this affects the chromatic (colour) correction of the telescope. The thickness around the edge must not vary more than about 0.005mm. Having said that let me point out that eliminating wedge effect was easy and predictable, almost to the point of being a non event and should not concern anyone contemplating a similar project. To measure wedge you need the only thing remotely resembling a sophisticated testing device you will need for the entire project. I made mine with a second hand micrometer and a piece of plywood. It is very simple and I will willingly show it to anyone interested because it is beyond my threadbare ability to describe it here. Once you have decided which side of the lens is high you simply apply more pressure to that area and the wedge steadily disappears.

Polishing

I did all the initial polishing using optical polishing pads (Coburn Optical Industries, Lonsdale). These pads leave weird zones but are very fast at getting pits out which is just as well because flint glass seems to take forever to polish. The pitch lap very quickly eliminates any zoning or roughness and the combination of the two speeds the process up no end. I used the same lap for both convex surfaces, just repressing for the deeper curve and the same lap for the flat and the concave. By taping the lenses together with a thick cloth between them you can support well and avoid any stress.

Figuring and Testing

This was the most surprising part of the whole project as it turned out to be so easy and straightforward, especially considering the difficulty I have had in the past when trying to parabolise mirrors accurately. I have actually come to believe that the Foucault zone test is some sort of voodoo ritual but I had no such difficulty testing a lens because the star test images are so clear that it is obvious what needs correcting. Ian's experienced eye was a great help in this area but believe me, at least in my (limited) experience, testing and getting an accurate figure is much easier than on a short focus mirror The first moments of real satisfaction I gained from this telescope were not seeing the clearest images of Jupiter and Saturn that I had experienced or even seeing an Airy disk for the first time but when I took it up to a vantage point in the hills one day and looked out at the airport and ships at sea just like with a "proper" telescope. It's portable, gives the best images I have achieved and, with an erecting prism, can be used for anything you like.