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Telescope Buyers' FAQ - Part Eight

What are Digital Setting Circles?

This section was written by Jim Van Nuland.

Digital Setting Circles (DSCs) are a small special purpose computer mounted on or near a telescope. The scope has shaft encoders attached to sense the motion of the scope's axes, and the computer then converts these motions to the position of the telescope and displays it (for instance) in Right Ascension (RA) and Declination. An 8-conductor cable runs from the computer to the encoders, with 4 wires to each encoder. RJ-45 telephone connectors are used at the computer.

They do not move the scope. You push it by hand, and the DSCs tell you which way to move and how much.

What makes DSCs so desirable is that they work on alt/az-mounted scopes; and even with equatorial mountings it is not necessary to polar align the mount. (However, it's desirable to have the mount at least roughly polar-aligned so it follows an object.)

Additionally, most models have an internal catalogue and a 'guide' mode. One selects an object (or, in some,a planet), and the DSCs tell which way to move each axis.

They are marketed by Lumicon, Jim's Mobile, Inc., Celestron, and Orion Telescope Centers. The various brands and models differ mostly in their internal catalogues of celestial objects. All are actually manufactured by the same company, Tangent Instruments of Palo Alto, California, USA, who, however, do not sell directly to individuals. I own the NGC-MAX from JMI, so some of my statements may not apply to other versions.

Must the ground board be levelled?

No. An alt/az mount must have a fiduciary mark such that the tube can be placed accurately at 90 degrees to the elevation axis. One way to do this is to (one time only) level the ground board, then the tube. Make the mark in such a manner that it can be adjusted when something changes. Some models of DSCs allow an alt/az mount to be initialized in a vertical position. When starting the DSCs, the tube must be set horizontal (or vertical), and then two stars are used to align. The stars must be at least 20 degrees apart in the sky (90 is ideal), and the first may not be Polaris.

How does one set up an equatorial mounting?

If the mount is known to be accurately polar aligned, you may still use two stars as mentioned above. Or you may set the DSCs to take advantage of the known alignment, and it will require only one object, and no zero degree reference mark is needed.

If an equatorial mount is not polar aligned, it must have a reference mark at zero degrees declination and must use the two-star setup. For a German mount, the mark may be on either side of the scope (tube pointing east or west), and the DSCs set to correspond. The mount may be driven or undriven. As for an alt/az mount, the stars must be at least 20 degrees apart and the first may not be Polaris.

Do the DSCs support a Poncet platform?

Probably depends on the model. The NGC-MAX provides telescope type ET (equatorial table). It assumes that the table is carrying an alt/az scope, and that the scope is initialized with the tube horizontal. I believe that an equatorial mount could be used, but have not tried to simulate it.

How accurate is the device?

The position of the scope is displayed to one minute of RA and 10 minutes of dec. Guide mode displays position error to 0.1 degree of arc. The actual accuracy depends on the care with which the alignment was done, the accuracy of the mounting, accuracy with which the shaft encoders were installed, the resolution of the encoders, and a bit of luck. If the level or zero was not set accurately, the system will work poorly, and it should be re-started. If star settings were done carelessly, one can simply re-do one or both of them.

The 'luck' factor stems from the digital nature of the shaft encoders. If the encoder is on the verge of a step, you could be off by one step.

The absolute theoretical resolution is three encoder steps, assuming everything else is perfect. In practice, I get about 0.2 to 0.3 degrees, and closer near the alignment stars. If I move a long way across the sky, the error is perhaps 0.5, but then I re-align on a convenient nearby star. It's not too unusual to get 0.1 if all has gone especially well during alignment. This with 4000 step encoders.

Accuracy is best between the alignment stars, and the DSCs calculate a 'warp' so as to spread out the error. When re-aligning, only one star sighting is needed. The DSCs retain only the two most-recent star settings, provided they are at least 20 degrees apart in the sky.

What objects are in the internal catalogue?

This is the major difference between models. All have a few dozen named stars, used especially for initial alignment. Some have the planets. The Lumicon models have a catalogue of planetary nebulae, which is Dr. Jack Marling's specialty.

The NGC-MAX version 3.94 (July, 1992) has the planets; 28 user defined objects; the Messier catalogue (including M40 and M110); the full NGC, including the so-called 'non-existent' objects; about half of the IC catalogue; a catalogue of 951 interesting stars (multiple, red, variable);and a list of 367 additional deep-sky objects, many of which are very faint.

For each object, the catalogue has the position, magnitude, size (diameter or separation), constellation, name (if any) and/or catalogue number, and the type of object. Some have a word or two of description. This probably varies with the brand and model.

May I add my own objects? Comets, for instance?

The NGC-MAX accepts user objects, and I presume most other high-end models do as well. I like to put in the Sun and Moon so that I can align during the day. This must be done carefully, with the Sun filter attached. This is dangerous as the filter must be removed when sighting on the Moon, and if you come back to the Sun, you must have first re-attached the filter! The moon is a poor alignment object because it has up to a degree of parallax, and it moves about 0.5 degrees per hour. But it provides a start, and it may be enough to locate some bright stars, and re-align.

What is 'identify' mode?

Identify mode is present in the NGC-MAX and probably other models. One specifies the class of object and the faintest magnitude, then the DSC selects the nearest to the telescope's position. Very nice, but in the realm of galaxies, alignment is critical and then there are too many to be certain. To check, read out the magnitude and description, and go to Guide mode and see how far away the object is.

It's especially useful in clouds, as one may point the scope into a clear spot, then ask what is nearby. One must separately search for galaxies, clusters, etc.

Identify mode runs continuously so that, as the scope is moved, the DSCs will (after a few seconds), indicate the new (or nearest) object.

Some models allow alignment on ANY catalogue object, which is helpful, but I find that accuracy is best on stars or very round objects. I find that planetary positions are especially suspect. The computer carries only the date, not the hour. (Use UT date.) I have often had poor alignments when using planets, and do so only for daylight set-ups; I re-align on stars as soon as I can find any. Open clusters are especially unreliable; galaxies are not much better.

Jim Van Nuland, San Jose (California) Astronomical Association

Can it replace star charts?

For comparatively easy objects, probably. In a crowded field, no. Some models support the Tiron Atlas 2000 and the Uranometria 2000, by indicating, for each object, the page on which it (the object) will be found. These models also indicate the chart corresponding to the position of the scope, regardless of specific object.

What other functions are present?

This varies heavily with model. The NGC-MAX (here we go again) has two that have not already been discussed.

'Timer' counts up in hours, minutes, and seconds. It can be stopped, reset, and re-started, but can't be restarted without first being reset.

"Encoder" shows the encoder positions in degrees. If an alt/az scope was pointed north when the DSC was powered up, then encoder mode will read elevation and azimuth, if the scope is also standing reasonably level.

How is it powered? How long does the battery last?

There is an internal 9-volt transistor battery. The load is 18 to 40mA (NGC-MAX), depending on how bright the display is. I suppose this might depend on the model, too. The maker claims 30 to 50 hours on an alkaline battery. They do last a good long time. There is a "low battery" indicator which would turn on at about 4.5 volts, but in practice, I get "encoder error" messages before that.

Some models have a second connector (serial port) by which external 9- 15 volts DC may be supplied. This does not require the internal battery to be removed; the two supplies are in parallel with diodes to prevent back-circuits. It does not recharge the internal battery.

How accurately should the mount be constructed?

The brief answer is, as accurately as you'd like the DSCs to operate. For an equatorial mount, there must be little flexure; the RA axis must be perpendicular to the dec axis, which in turn must be perpendicular to the optical axis of the tube.

For an alt/az mount, the ground board must be rigid, the azimuth bearing surface must be flat, dent-free and stiff; and the side bearings must be the identical height, that is, the elevation and azimuth axes must be accurately perpendicular. In addition, the optical axis of the tube must be perpendicular to the elevation axis. There is a terrible irony here: the Dobsonian mount works precisely because its kinematically stable design does not require that it be accurately constructed!

How accurately should encoders be installed?

Again, the short answer is, as accurately as you'd like the DSCs to operate. One can't do the job with a hand-held drill. On the other hand, careful work with a modest lathe and drill press is quite sufficient, especially if performed by a modest machinist. Most astronomy clubs have such a person.

Best accuracy is obtained with high-resolution encoders. Standard encoders have 2048 steps per revolution, and high-res type has 4000. One can also use gears to provide greater resolution, but see below.

If the encoder is connected directly to a shaft, the hole in the shaft must not be oversize. It must be straight, well centred, and parallel to the axis. The body of the encoder must be held so that it cannot rotate with the shaft. If it is connected by gears, the shafts must be parallel, and there must be no backlash.

Encoders are not especially delicate, but they do not like to be bent. They require very little torque and rotate continuously. The setscrew should not deform the shaft. The 4-wire connector should be looped so it does not pull on the encoder. They may be mounted such that the shaft is stationary, with the body moving, or the usual way; the direction is set in the DSCs' setup option.

In an alt/az mount, the azimuth encoder is typically mounted atop the centre bolt. In this case, the bolt must be nicely perpendicular to the ground board, and the comments about shaft mounting (above) apply. If the rocker box has any side play, it will be nearly impossible to avoid some runout. This can be reduced by using a very long lever arm to hold the body of the encoder.

Both side bearings must be round (especially the one with the encoder), the centre must be carefully located, and the encoder shaft parallel to the elevation axis. Any runout there will cause serious inaccuracies when moving across the sky.

How accurately must the mount be constructed?

Please don't feel that only a million dollar mount can be equipped with DSCs. My 1972 Optical Craftsman (German) mount works very well, even with about 0.5 degrees of error if I shift the mounting and return to an object. This was the economy model! A machinist friend helped me drill the holes for the encoder shafts.

I used UGMA grade 10 precision gears to step up the dec shaft speed. The designer of the DSCs was amazed at that, and admitted that he used UGMA 4 with adequate results. I don't know how to calculate how much more accuracy I might be getting from my expensive gears.

My alt/az mount, crafted of wood in my shop with only hand tools, carries a 108mm f/4 scope, and *always* puts an object in a low-power field. On the other hand, if I re-collimate the scope, I must also re-position the vertical mark. I usually re-align after moving far across the sky.

If the mounting is less than perfect, it means that you will need to re-align more often. But if the mount is really sloppy, it probably will not be satisfactory.

Can I connect the DSCs to my own computer?

Yes, for some models. The NGC-MAX, and probably others, has a serial port that may be used with an external computer, so that the screen shows a dynamic star map, identifies objects, etc.

But the attached computer must take over all functions, including the prompting for 'level me', pointing at particular alignment stars, guiding, calculating the conversions for RA and Dec, etc. I understand The Sky, from Software Bisque, does all this, but I have not seen it in use nor heard from a live user.

The port is a modular telephone connector (RJ11). It has four wires: B+, data in, data out, and ground. External to the NGC-MAX, the cable must route DTR back to the attached computer as DSR, CD, and/or CTS, as needed by the attached computer. The 4th wire is +Battery, a 9 to 15 volt external power supply, which does not charge the internal battery. It is not necessary to remove the internal battery.

When the NGC-MAX is operating in 'BOX' mode, it blanks its own display and does nothing but pass the shaft encoders' values over the serial port. It multiplies them by the encoder ratios (the latter set in the NGC-MAX setup function), and scales them such that 00000 is the position at power-on, and 32767 is just under 1 rotation.

Communication is at 9600,8,N,1. When the NGC-MAX powers on, it sends a hello message such as 'V2.94'. When the attached computer sends a character (the sample program uses 'Q' but anything seems to work) down the port; and the NGC-MAX replies with 13 characters of the format '+00000t+00000' where the 't' is ASCII 9, and the 00000s are the two encoder values.

I don't use this facility, but I'm too curious not to have tried it. I used my modem program to supply the computer side. I use the NGC-MAX whenever I'm doing general observing, and I like it very well. But I don't have a portable computer to use with it, and don't too much see the need. On the other hand, if I fell into a laptop, I'd surely want to try connecting them.

Why should I start with Binoculars?

The quick answer is because you already have them, so you do not have to spend any money. Certainly going right out and buying the Fujinon 25x150 Astronomical Binocular ($11,000 list price) would be a pretty stupid thing to do, no matter how good the binoculars are.

You should also avoid the quick-focus binoculars, as they are easy to de-focus as well.

The remainder of this section was written by Paul Zander.

Based on my experience, I suggest that you start with a pair of 7x50 binoculars. This is the most popular size and hence good ones are available from many stores, even some of the discounters. Be sure to get ones that have anti-reflection coatings on the mirrors and lens. If you wear eyeglasses, you may be able to find binoculars which can focus without them (unless you have significant astigmatism). Make sure the image is sharp at the centre and edges at the same time.

'7x' is the magnification. Most people can hand hold these without needing to bother with tripods, etc. The "50" means 50mm (~2 inch) objectives (aperture). This gives light gathering ability similar to many small telescopes. Many advanced star gazers regularly use binoculars to either locate items to focus telescopes on, or just for the wider field of view.

When trying to view near the zenith, use a reclining lawn lounger: you can lie back and support your arms on the chair, giving a steadier view. You also will not get a crick in your neck.

You might also use a plastic pad to lie on.

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Author: Dennis Bishop

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