Useful Accessories

 

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Non-optical accessories. Useful accessories that will not exist purely to gather dust in the garden shed.

1. Dew shields and Dew heaters. A vital add-on to the front of any telescope. A dew shield will help keep dirt and dust from contacting the objective lens, correcting lens or even the secondary on a Newtonian. It also helps prevent, or at least slow the onset of the formation of dew on the same optics. Today there such things as are dew heaters, a coil of insulated copper wire that is placed around the front aperture of refractors and catadioptrics, that reaches a temperature that keeps the optical surface just above the dew point (when condensation occurs). These dew heaters are extremely useful for keeping optics free of condensation during long sessions of imaging. It is peace of mind as much as anything else. For those that do not want the extra cables and a 12v supply to power them, a dew shield is an alternative. Our own experiments have shown that to be affective, a dew shield needs to have a length at least 2.5X the diameter of the aperture. Some manufacturers offer dew shields that are shorter than this, and they are rarely useful for very long into the observing period. Of course, the dew shield does not need to be much longer than 2.5X the aperture because in a few cases it can slightly increase vignetting of off-axis light. Every telescope has off-axis vignetting, it is unavoidable. Some telescope types, because of the diameter of the main tube, compared to the diameter of the objective, are a little more prone to it. Only in cases of wide-field imaging using a camera with a large sensor, and a telescope with a primary mirror with only a small clearance to the tube wall, would it be wise to have a short dew shield. One other useful aspect of a dew shield on a Newtonian is that it increases the length of the tube and can help to remove warm air out of the tube. If cool air can enter the tube below the primary mirror, the longer tube has a greater "chimney effect". It draws warmer air up and out of the tube more effectively.

2. 12v Power supply. As most complete telescopes or telescope mounts today are of the GOTO type, that is to say, they are motor driven mounts with computer assisted object location and tracking, they all require power. Most of these telescopes require 12v DC supply. Some telescopes have the facility to use AA alkaline batteries, and although these will work, they may not for very long, and in sub-zero temperatures, can sometimes fail to supply the correct power. AA batteries that are not alkaline may not work properly, and rechargeable AA batteries have a lower voltage than is required. There are many AC/DC adapters that take 230v AC and convert to 12v DC. These are usually OK as long as the maximum stated amp draw on the converter easily covers that required by the telescope. Power leads and power sockets on telescopes have Tip Positive or Tip Negative connections, it is important to ensure that both lead and socket are of the same type. Sometimes with AC/DC adapters there is an issue with "noisy mains", which can make the sensitive electronics in the telescope a little temperamental, but this is not common. Quite often, power delivery problems can occur due to the high resistance of the copper wire in the power lead, and quite often these leads are provided by the telescope manufacturer! It is a good idea to have a small voltmeter to test the resistance of the cable by measuring the OHMS. The safest method of supplying a smooth power delivery is from a 12v DC battery, of the lead acid type, used for example, as car batteries. These are commonly called Power tanks or Power stations. They can be found at electronics stores, and general hardware stores, as well as products branded by telescope manufacturers.

3. Red pocket torch or red head torch.  An essential accessory to have in the dark. It can illuminate the observing area enough to avoid tripping over tripod legs or power leads, it can be used to illuminate the eyepiece or focuser to avoid fumbling in the dark. It is also useful for reading star maps in the dark, and for finding eyepieces or accessories in an accessory case. A red light is important because the dark adapted eye is not very sensitive to long wavelength light, so red does not destroy the eye's dark adaption. Modern red lights are usually very small and use battery power very slowly. A more modern torch is the type on an elastic headband. This type is useful because it points in the same direction as the head, and leaves the hands free.

4. Prime focus Camera adapters. Essential if you wish to try astrophotography with a digital SLR. Photo adapters are in two parts - a.) t-adapter, b.) t-ring. The t-adapter fits onto or into the focuser either by screw-thread or simple push-fit depending on the telescope type. At the end of the t-adapter is a male t-thread. A t-ring fits into a specific SLR camera and so has that manufacturer's bayonet fit engineered onto the t-ring. On the inner surface of the t-ring is a female t-thread, which threads onto the male t-thread on the end of the t-adapter. So, the t-adapter fits onto the telescope, the t-ring fits onto the t-adapter and the t-ring also fits into the camera lens position. The telescope tube assembly then becomes the camera lens. This form of photography is the preferred method for wide-field deep-sky imaging with an SLR.

5. A-focal camera adapters. Another method of photography is A-focal photography. This is where the camera plus its lens receives its image from an eyepiece. This is the most common method of photography for those that wish to use compact digital cameras with a captive lens instead of an SLR without its lens. A-focal means that there is parallel light exiting the eyepiece, and into the camera lens, where it then gets focused onto the chip in the camera. Some excellent results can be achieved using this method, (see rose photo), particularly for snap shots of the moon and planets. The adapters for A-focal photography are varied. They can be specific for a particular brand or camera model, that attaches around the eyepiece at one end, and around the small camera lens at the other. They can also be a mechanical swing arm type, a bracket that holds a small camera in place over the eyepiece.